1. A user equipment, UE, comprising: a transmitter configured to transmit, to a first access network node, a request including a resume identity for resuming, to connected mode, a connection between the UE and a network including the first access network node, wherein the resume identity includes an identifier indicating a second access network node which last served the UE in the network.

2. The UE as claimed in claim 1, wherein the request causes the first access network node to retrieve a context for the UE from the second access network node using the identifier.

3. A first access network node, comprising: a receiver configured to receive, from a user equipment, UE, a request including a resume identity for resuming, to connected mode, a connection between the UE and a network including the first access network node, wherein the resume identity includes an identifier indicating a second access network node which last served the UE in the network.

4. The first access network node as claimed in claim 3, comprising: a processor configured to retrieve a context for the UE from the second access network node using the identifier.

5. A method for a user equipment, UE, comprising: transmitting, to a first access network node, a request including a resume identity for resuming, to connected mode, a connection between the UE and a network including the first access network node, wherein the resume identity includes an identifier indicating a second access network node which last served the UE in the network. 6. The method as claimed in claim 5, wherein the request causes the first access network node to retrieve a context for the UE from the second access network node using the identifier.

7. A method for a first access network node, comprising: receiving, from a user equipment, UE, a request including a resume identity for resuming, to connected mode, a connection between the UE and a network including the first access network node, wherein the resume identity includes an identifier indicating a second access network node which last served the UE in the network.

8. The method as claimed in claim 7, comprising: retrieving a context for the UE from the second access network node using the identifier. Title of Invention : COMMUNICATION SYSTEM Technical Field [0001] The present invention relates to a communication system. The invention has particular but not exclusive relevance to wireless communication systems and devices thereof operating according to the 3rd Generation Partnership Project (3GPP) standards or equivalents or derivatives thereof (including LTE-Advanced and Next Generation or 5G networks). The invention has particular although not exclusive relevance to managing connection states for communication devices. Background Art [0002] The latest developments of the 3GPP standards are referred to as the Long Term Evolution (LTE) of Evolved Packet Core (EPC) network and Evolved UMTS Terrestrial Radio Access Network (E-UTRAN), also commonly referred as ‘4G’. In addition, the term ‘5G’ and ‘new radio’ (NR) refer to an evolving communication technology that is expected to support a variety of applications and services. Various details of 5G networks are described in, for example, the ‘NGMN 5G White Paper’ V1.0 by the Next Generation Mobile Networks (NGMN) Alliance, which document is available from https://www.ngmn.org/5g-white-paper.html. 3GPP intends to support 5G by way of the so-called 3GPP Next Generation (NextGen) radio access network (RAN) and the 3GPP NextGen core network. [0003] Under the 3GPP standards, a NodeB (or an eNB in LTE, gNB in 5G) is the base station via which communication devices (user equipment or ‘UE’) connect to a core network and communicate to other communication devices or remote servers. For simplicity, the present application will use the term base station to refer to any such base stations and use the term mobile device, user device, or UE to refer to any such communication device. The core network (i.e. the EPC in case of LTE) hosts functionality for subscriber management, mobility management, charging, security, and call/session management (amongst others), and provides connection for communication devices to external networks, such as the Internet. [0004] Communication devices might be, for example, mobile communication devices such as mobile telephones, smartphones, user equipment, personal digital assistants, laptop/tablet computers, web browsers, e-book readers and/or the like. Such mobile (or even generally stationary) devices are typically operated by a user. However, 3GPP standards also make it possible to connect so-called ‘Internet of Things’ (IoT) devices (e.g. Narrow-Band IoT (NB-IoT) devices) to the network, which typically comprise automated equipment, such as various measuring equipment, telemetry equipment, monitoring systems, tracking and tracing devices, in-vehicle safety systems, vehicle maintenance systems, road sensors, digital billboards, point of sale (POS) terminals, remote control systems and the like. Effectively, the Internet of Things is a network of devices (or "things") equipped with appropriate electronics, software, sensors, network connectivity, and/or the like, which enables these devices to collect and exchange data with each other and with other communication devices. It will be appreciated that IoT devices are sometimes also referred to as Machine-Type Communication (MTC) communication devices or Machine-to-Machine (M2M) communication devices. [0005] For simplicity, the present application refers to mobile devices in the description but it will be appreciated that the technology described can be implemented on any communication devices (mobile and/or generally stationary) that can connect to a communications network for sending/receiving data, regardless of whether such communication devices are controlled by human input or software instructions stored in memory. [0006] Communication between mobile devices and base stations is controlled using a Radio Resource Control (RRC) protocol as defined in 3GPP TS 36.331 V14.0.0. RRC handles the control plane signalling of Layer 3 between mobile telephones and the radio access network, and includes, amongst other things, functions for broadcasting system information, paging, connection establishment and release, radio bearer establishment, reconfiguration and release, mobility procedures, and power control. In accordance with the current version of the RRC protocol, at any given time, a mobile device may operate either in an ‘RRC idle mode’ (in which no data communication takes place) or an ‘RRC connected mode’ (in which data communication may take place between the mobile device and its serving base station). [0007] As mobile devices operating in the RRC connected mode move around in the area covered by the communication system, they are handed over from one cell (i.e. operated by a base station) to another cell (operated by the same or a different base station), depending on signal conditions and other requirements, such as requested quality of service, the type of service used, overall system load, and the like. Handover requires extensive signalling between the mobile device and the base stations (old and new) and also between the base stations and the core network as well. [0008] On the other hand, whilst in the RRC idle mode, mobile devices are programmed to select a ‘serving’ cell, having a good quality signal, to camp on so that when new data is to be transmitted to/from these mobile devices, they can benefit from favourable signal conditions. In the event that an idle mobile device detects a new cell with better signal quality than the current serving cell, e.g. due to the mobile device changing its location, the mobile device can perform a so-called cell reselection procedure. However, an idle mode mobile device does not inform the network about the selected new cell as long as this cell is within the same ‘tracking area’ (i.e. a larger geographic area comprising a pre-defined set of cells), because the radio network transmits system information and UE specific paging messages within the whole tracking area thus making it possible to initiate communication to/from the mobile device regardless of the current cell it camps on. [0009] In order to benefit from the lowest energy consumption and to free up valuable system resources, the mobile devices return to the RRC idle mode whenever possible and perform cell reselections (instead of handovers) as long as they remain within the same tracking area. The base station controls the transition between the various operating modes for each mobile device within its cell(s). Since the setting up and termination of an RRC connection between the base station and the mobile device requires exchanging of signalling messages and hence utilises valuable system resources, and also takes some time to complete, the transition from connected to idle mode is allowed under specific circumstances as defined in 3GPP TS 36.331. For example, the serving base station might instruct a mobile device to enter the RRC idle mode only after it has confirmed that there is no more data to be transmitted to/from the particular mobile device (e.g. both uplink (UL) and downlink (DL) buffers are empty). [0010] When it registers its current location (e.g. cell) with the core network, each mobile device also has an associated ‘S1’ connection between its serving base station and the core network. The S1 connection is either in a so-called ‘ECM-IDLE’ mode (when the mobile device is in RRC idle mode) or in an ‘ECM-CONNECTED’ mode (when the mobile device is in RRC connected mode). The S1 connection is used for transferring data (control and user data) between the mobile device and the core network (and beyond) and it is maintained as long as the mobile device remains in the RRC connected mode. On the other hand, when a mobile device enters the RRC idle mode, its associated S1 connection is also terminated (or suspended) until the mobile device has more data to send or receive in which case a new S1 connection is established to the current serving base station (or the suspended S1 connection is re-activated). [0011] When the network has data to send to an RRC idle mobile device, it triggers an appropriate paging procedure in the last known area (tracking/paging area) of the mobile device, which causes the base stations within that area to broadcast appropriate paging messages in their cells requesting that particular mobile device to enter the RRC connected state. When a previously idle mobile telephone has data to send again (or it has been paged for receiving downlink data), in order to be allocated communication resources it initiates a so called RRC connection establishment procedure by sending an appropriately formatted RRC connection request message to the base station (following a so-called Random Access Procedure which ensures that the lower layers, and in particular the Media Access Control (MAC) layer, are set up for communication with the base station). [0012] For the latest developments of the 3GPP standards, the so-called Next Generation (NG) or 5G networks, it is envisaged that mobile devices may also operate in a new RRC state, or new radio state, referred to as a ‘light-connected’ (LC) state. When a mobile device is in the LC state, the core network maintains both its control-plane and user-plane connection even after the mobile device has no more data to send or receive (and hence it is normally configured to enter the RRC idle mode). In other words, even though in the LC state the mobile device is seen as operating in idle mode from the radio access network’s (base station’s) point of view, it may still be seen as being connected from the core network’s point of view. One of the benefits of this new LC state is that mobile devices (IoT devices in particular) that have small and infrequent data transmissions do not need to perform the entire RRC connection establishment procedure every time they have data to send (or receive). Instead, an LC state capable mobile device can be configured to resume its existing RRC connection with the current serving base station whenever needed and then return to a more power efficient mode of operation until it has data to send/receive again. [0013] The mobile device can resume its RRC connection by sending to its current base station information (e.g. a resume ID) identifying the connection to be resumed. This beneficially avoids the base station and the mobile device having to go through authentication and radio bearer establishment. In order to facilitate such lightweight connection and simplified resumption of the connection between the mobile device and its serving base station, the concept of a so-called anchor base station is being considered by 3GPP. Effectively, the anchor base station is a base station responsible for storing UE Access Stratum (AS) context, caching the mobile device’s user data (UE context) and for providing the user data to other base stations as needed while terminating S1. For example, the anchor base station may be the first (or previous) base station that the mobile device registered with in a particular tracking area (or other pre-defined area). Thus, when the mobile device attempts to resume its RRC connection via a different base station (within the same area), the new base station can contact the anchor base station and retrieve the UE context along with the cached user data based on information provided by the mobile device (e.g. resume ID and/or the like). Since in the LC state the S1 connection is maintained, beneficially, the new base station can avoid having to contact the core network and/or establish a new S1 connection for the mobile device (although the new base station might need to switch the S1 connection from the anchor/previous base station to the new base station). The anchor base station concept is illustrated in Figure 8 which is reproduced from 3GPP draft no. R3-160655. [0014] The current agreement in 3GPP is that the base station maintains the S1 connection while the mobile device is lightly connected and the base station is responsible for RAN paging (within an appropriate paging/tracking area configured by the base station) when it receives the downlink data from core network. The LC state mobile device notifies the network when it moves out of its configured RAN based paging area, in which case the network can decide whether to keep the mobile device in LC mode or suspend the mobile device (e.g. request it to enter RRC idle mode). Summary of Invention Technical Problem [0015] However, the inventors have realised that since the mobile device appears to be in different states towards the RAN and the core network, this may cause a number of issues that the currently proposed systems cannot handle. Such issues include, although they are not limited to: 　　　　- how to avoid loss of DL data when RAN based paging fails; 　　　　- how to perform core network (e.g. MME) based paging as a fall-back when RAN based paging fails; 　　　　- how to inform a mobility management entity (MME) that certain operations such as load balancing, tracking area update (TAU) required, power saving mode (PSM), mobile terminated (MT) circuit switched fallback (CSFB) may be impacted due to the mobile device’s LC operation. 　　　　- how to notify the MME when a mobile device enters the LC state; 　　　　- how to exchange a RAN-based identifier (e.g. resume ID) that is used to uniquely identify a mobile device when it is in LC mode within the RAN routing area (e.g. for reducing the time it takes to resume a connection by performing context pre-fetch); 　　　　- how to expedite random access channel (RACH) processes when a mobile device transits from LC mode to regular connected mode for data transmission; 　　　　- how to synchronise RAN location update with core network location update; and 　　　　- how to enable UL data transmission when the mobile device is in LC mode. [0016] Accordingly, preferred example embodiments of the present invention aim to provide methods and apparatus which address or at least partially deal with one or more of the above issues whilst still allowing mobile devices to maintain a light connection with the network. [0017] Although for efficiency of understanding for those of skill in the art, the invention will be described in detail in the context of a 3GPP system (UMTS, LTE), the principles of the invention can be applied to other systems in which communication devices or User Equipment (UE) access a core network using a radio access technology. Solution to Problem [0018] In one aspect, the invention provides a base station for a communication network, the base station comprising: a transceiver and a controller wherein the controller is configured to: control the transceiver to receive, from a core network node, downlink data for a communication device; attempt to initiate communication with the communication device; and control the transceiver to send to the core network node, when the communication device does not respond to said attempt to initiate communication, a message to request initiation of a paging procedure for the communication device. [0019] In one aspect, the invention provides core network apparatus for a communication network, the core network apparatus comprising: a transceiver and a controller wherein the controller is configured to: control the transceiver to send, to a base station, downlink data for a communication device; control the transceiver to receive from the base station, when the communication device does not respond to an attempt by the base station to initiate communication, a message to request initiation of a paging procedure for the communication device; and initiate a paging procedure for the communication device based on said message. [0020] In one aspect, the invention provides a base station for a communication network, the base station comprising: a transceiver and a controller wherein the controller is configured to: control the transceiver to send a notification to a core network node indicating that at least one core network operation is not possible (or banned) for a given communication device. [0021] In one aspect, the invention provides a base station for a communication network, the base station comprising: a transceiver and a controller wherein the controller is configured to: maintain information identifying at least one communication device, for which the base station operates as an anchor base station, in association with respective context information for each communication device for which the base station operates as an anchor base station; and control the transceiver to provide to another base station at least one identifier to identify at least one respective communication device for which the base station operates as an anchor base station. [0022] In one aspect, the invention provides a base station for a communication network, the base station comprising: a transceiver and a controller wherein the controller is configured to: control the transceiver to receive from at least one other base station at least one identifier to identify at least one communication device for which the at least one other base station operates as an anchor base station; and fetch, from the at least one other base station, context information for each communication device for which the at least one other base station operates as an anchor base station based on said at least one identifier. [0023] In one aspect, the invention provides a base station for a communication network, the base station comprising: a transceiver and a controller wherein the controller is configured to: control the transceiver to, receive from a core network node, a message indicating that data forwarding from another base station is to happen for a communication device, together with at least one of: an identifier, associated with that communication device, for use in fetching a context relating to that communication device from the other base station; information identifying the communication device at the core network node over an S1 interface (e.g. ‘MME UE S1AP ID’); information identifying the other base station (e.g. an ‘anchor eNB ID’); a tracking area code (TAC) associated with the other base station; a globally unique ID (e.g. a GUMMEI) associated with the core network node currently serving the communication device; information identifying at least one cell identifier for at least one cell to which a handover of the communication device is prohibited; and at least one other parameter acquired by the core network node in a previously handled handover request relating to the communication device; and control the transceiver to receive data forwarded by the other base station accordingly. [0024] Aspects of the invention extend to corresponding systems, methods, and computer program products such as computer readable storage media having instructions stored thereon which are operable to program a programmable processor to carry out a method as described in the aspects and possibilities set out above or recited in the claims and/or to program a suitably adapted computer to provide the apparatus recited in any of the claims. [0025] Each feature disclosed in this specification (which term includes the claims) and/or shown in the drawings may be incorporated in the invention independently of (or in combination with) any other disclosed and/or illustrated features. In particular but without limitation the features of any of the claims dependent from a particular independent claim may be introduced into that independent claim in any combination or individually. [0026] Example embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings in which: Brief Description of Drawings [0027] [fig. 1] Figure 1 illustrates schematically a cellular telecommunication system to which example embodiments of the invention may be applied; [fig. 2] Figure 2 is a block diagram of a mobile device forming part of the system shown in Figure 1; [fig. 3] Figure 3 is a block diagram of a base station forming part of the system shown in Figure 1; [fig. 4] Figure 4 is a block diagram of a mobility management entity forming part of the system shown in Figure 1; [fig. 5] Figure 5 is a timing diagram illustrating an exemplary way in which example embodiments of the invention can be implemented in the system of Figure 1; [fig. 6] Figure 6 is a timing diagram illustrating an exemplary way in which example embodiments of the invention can be implemented in the system of Figure 1; [fig. 7] Figure 7 is a timing diagram illustrating an exemplary way in which example embodiments of the invention can be implemented in the system of Figure 1; and [fig. 8] Figure 8 illustrates schematically the anchor base station concept. Description of Embodiments [0028] Overview Figure 1 schematically illustrates a telecommunications network 1 in which mobile devices 3, mobile telephones, and other communication devices (e.g. IoT devices) can communicate with each other via E-UTRAN base stations 5 and a core network 7 using an E-UTRA radio access technology (RAT). As those skilled in the art will appreciate, whilst one mobile device 3 (denoted ‘UE’), one IoT device 3’, and three base stations 5a to 5c are shown in Figure 1 for illustration purposes, the system, when implemented, will typically include other base stations and communication devices. [0029] Each base station 5 operates one or more associated cell. In this example, the base station 5b operates ‘Cell #1’ and the base station 5c operates ‘Cell #2’. Although not shown in Figure 1, base station 5a will also typically operate one or more cells. It will also be appreciated that in some scenarios the ‘old’ base station 5a and the ‘anchor’ base station 5b might be the same. [0030] Communication devices may connect to either cell (depending on their location and possibly on other factors, e.g. signal conditions, subscription data, capability, and/or the like) by establishing a radio resource control (RRC) connection with the appropriate base station 5 operating that cell. As can be seen, the mobile device 3 is located in an area where the cells operated by the base stations 5b and 5c partially overlap. Thus, when operating in RRC idle mode (not sending/receiving data), the mobile device 3 camps on the cell having the best signal quality, and when in RRC active mode, the mobile device 3 communicates data via that cell (using e.g. the ‘Uu’ air interface). Similarly, in this example, the IoT device 3’ camps on Cell #2 (when in RRC idle mode) and communicates via the base station 5c (when in RRC active mode). [0031] When the mobile device 3 (IoT device 3’) first registers with the network (via one of the base stations 5), its serving base station 5 also establishes an associated S1 connection for relaying communications (user and control data) between the serving base station 5 and the core network 7. [0032] The base stations 5 are connected to the core network 7 via an S1 interface and to each other via an X2 interface (either directly, or via an X2 gateway). The core network 7 includes, amongst others, a mobility management entity (MME) 9, a serving gateway (S-GW) 10, and a packet data network gateway (P-GW) 11 for providing a connection between the base stations 5 and other networks (such as the Internet) and/or servers hosted outside the core network 7. [0033] The MME 9 is the network node responsible for keeping track of the locations of the mobile communication devices (mobile devices and IoT devices alike) within the telecommunications network 1 especially when a UE is RRC_IDLE mode. In particular, the MME 9 stores an identifier of the mobile communication devices’ last known cell (or tracking area) so that they can be notified when there is an incoming (voice or data) call for them and that a communication path is set up via the base station 5 currently serving the particular mobile communication device. [0034] In the following examples, the mobile device 3 connects to the network periodically (e.g. whenever one of its applications needs to communicate with the network) for sending data to a remote endpoint (e.g. a server or another communication device). The mobile device 3 is configured to operate in a light connected (LC) mode in which the network maintains an associated S1 connection even when the mobile device 3 is operating in an idle mode from the RAN’s point of view. Therefore, between its periodic re-connections, the mobile device 3 effectively enters an idle (or ‘suspended’) mode and thus avoids performing handovers as long as it remains within an area configured by its anchor base station. [0035] The serving base station 5 is responsible for configuring an appropriate RAN based paging area for the mobile device 3 (e.g. by providing a list of cells and/or paging area IDs to the mobile device 3). The RAN based paging area may be configured as one or more cells from the same or different base stations 5. For example, the RAN based paging area may be a tracking area. [0036] As illustrated by a dashed line, the mobile device 3 previously connected to the base station 5b (via Cell #1) and thus the base station 5b (acting as the anchor base station for the mobile device 3) maintains an associated UE context and terminates S1. However, it will also be appreciated that the anchor base station may be a different base station, e.g. the old base station 5a. In the present example, the anchor base station 5b configures the mobile device 3 with a RAN paging area that includes the anchor base station’s own cell (Cell #1) and any cells operated by base station 5a. [0037] As illustrated by a continuous line, the mobile device 3 is now reachable via the base station 5c (via Cell #2), e.g. due to a change in signal conditions in Cell #1 and/or movement of the mobile device 3. In this example, the mobile device 3 currently has no active connections with the radio access network (the base stations 5), thus it is configured to perform cell reselection when it moves to Cell #2 (without informing the network). [0038] As explained above, in LC state the mobile device 3 is still seen as connected (ECM-CONNECTED) from the core network’s 7 (MME 9) point of view even after the mobile device 3 has, in effect, entered an idle mode from the perspective of the base stations (and hence the mobile device 3 does not have an active data connection with its base station 5). Accordingly, when there is downlink data to send, the MME 9 does not initiate paging, for a mobile device 3 in the LC state/mode (e.g. throughout an associated tracking/paging area) because the MME 9 assumes that the mobile device 3 still has an active connection with its serving base station 5 (in this example, the anchor base station 5b). Thus, the MME 9 starts sending the downlink data to the anchor base station 5b. In response to the downlink data, the anchor base station 5b starts appropriate procedures for (RAN based) paging of the mobile device 3 within the paging area appropriate for that mobile device 3. The RAN based paging indicates to the mobile device 3 that it needs to resume its RRC connection (re-connect) via an appropriate base station for receiving downlink data. Until the mobile device 3 resumes its connection, the anchor base station 5b stores the downlink data from the MME 9 (in case of control-planes signalling or data) or from the S-GW 10 (in case of user-plane data) in its cache (memory). [0039] The base station 5b decides which cells to page when it receives the downlink data. If necessary, the anchor base station 5b may send appropriate X2 paging signalling to its neighbour base station(s) in order to reach the mobile device 3 via other than its own cell(s). However, as the mobile device 3 has moved in the meantime and it is now camping on Cell #2, it might not be able to receive (or respond to) the paging signalling. This might happen, for example, when the anchor base station 5b does not perform paging via its neighbours (even though Cell #2 may belong to the currently configured RAN paging area), when Cell #2 does not belong to the paging area configured for the mobile device 3, or when signal conditions at the edge of Cell #2 prevent the mobile device 3 from successfully receiving (or responding to) paging signalling in that cell. [0040] Beneficially, if the anchor base station 5b does not receive any response from the mobile device 3 within a predefined time interval (after start of paging), the base station 5b requests the MME 9 to initiate S1 based (legacy) paging procedures. However, before the MME 9 is able to provide assistance in this respect, the MME 9 needs to move the mobile device 3 from connected state to idle state (e.g. ECM-CONNECTED to ECM-IDLE). In order to allow this, the base station 5b is configured to use legacy signalling in order to tear down the existing S1 connection. For example, the base station 5b may be configured to send an appropriately formatted message (such as a ‘UE Context Release Request’ and/or the like) including information indicating that S1 based paging is required for the mobile device 3 in its last recorded tracking area. Beneficially, the base station’s request allows the MME 9 to move the mobile device 3 to idle state (ECM-IDLE) allowing the MME 9 to trigger a legacy S1-based paging. Advantageously, the message from the base station 5b may also indicate that the paging is requested without triggering a release of the mobile device’s 3 bearer via the S-GW 10 (i.e. unlike the conventional procedure upon receipt of a UE Context Release Request for a UE in legacy RRC idle state). [0041] The MME 9 triggers S1 based paging and waits for an appropriate response from the mobile device 3. In this example, the mobile device 3 responds to the MME’s paging request via the new base station 5c by performing appropriate RRC and S1 connection setup procedures (via the new base station 5c / Cell #2). This is possible when a new base station 5 does not know or understand as to how to locate the associated UE context using the UE identifier (e.g. Resume ID) received from the mobile device 3. The MME 9 registers Cell #2 as the mobile device’s 3 new location. [0042] When the new base station 5c establishes a new RRC and S1 connection for the mobile device 3, the MME 9 is beneficially able to indicate to the new base station 5c that data forwarding from the old (anchor) base station 5b is to happen. In this example, the MME 9 requests data forwarding in its response to the anchor base station 5b (e.g. a ‘UE Context Release Response’ and/or the like). [0043] Moreover, the MME 9 may also be configured to monitor (preceding) handovers performed by the mobile device 3 and obtain context information and other parameters relating to the mobile device 3. Therefore, the MME’s 9 message to the new base station 5c may also include information (previously obtained or held by the MME 9) that may be used by the new base station 5c to fetch the UE context from the anchor base station 5b. Similarly, the MME 9 is able to provide information relevant to the anchor base station 5b in order to perform data forwarding. [0044] On the other hand, in case the mobile device 3 is not found (i.e. it does not respond to the network-wide S1 based paging either), then the MME 9 can follow appropriate legacy procedures in order to release the UE context (held at the anchor base station 5b and the MME 9) and trigger a release of the access bearer towards the S-GW 10. After the UE context is released, the mobile device 3 is considered to be in RRC idle state (instead of LC state) and the mobile device 3 is able to communicate with the network again by moving to RRC connected state, e.g. by triggering an appropriate RRC connection establishment operation (instead of a simple resume operation). [0045] In a particularly beneficial example, the base stations 5 may be configured to notify the MME 9 that certain operations are not possible for the mobile device 3 because it is currently operating in LC state. Such MME operations may include, for example, load balancing, TAU required, Power Saving Mode, MT CSFB, and/or the like. Thus, even if the MME 9 continues to see the mobile device 3 as being in a connected state (S1 connected / ECM-CONNECTED), the MME 9 is able to avoid such procedures. [0046] Beneficially, the serving/anchor base station 5 (in this example, the base station 5b) is configured to send, to the MME 9, information identifying whether the mobile device 3 is operating in LC state and/or whether certain MME operations are to be avoided (or banned). Based on the received information, the MME 9 takes different handling for certain operations such as CSFB or will not release S1 with load balancing TAU required. As a result, when overloaded, the MME 9 may be configured to release S1 connections that do not belong to those UEs that are in LIGHT-CONNECTED mode. [0047] Beneficially, the above procedures allow the MME 9 to avoid performing and/or requesting procedures that would otherwise be inappropriate for the current operating state of the mobile device 3. [0048] In order to facilitate swift resuming of an RRC connection for mobile devices 3 via a different based station (in this example, resuming the mobile device’s connection via base station 5c and Cell #2), the base stations 5 of this system may also be configured to exchange information with each other relating to the mobile devices 3 (users) for which they act as an anchor base station. [0049] Specifically, the base stations 5 (at least the ones which operate as anchor base stations) may be configured to provide, to their neighbour base station(s) over the X2 interface, information identifying any UE AS context, security data, suspended bearers at that particular base station 5, and/or the mobile devices 3 associated with such suspended bearers. [0050] Using such exchanged information (e.g. identifiers of suspended bearers and/or mobile devices associated with such bearers), neighbour base stations 5 are beneficially able to pre-fetch the associated UE context for bearers suspended at another (anchor) base station. This allows them to minimise latency at the time of handover (or idle mode mobility) of a particular mobile device 3 to a cell operated by the new (neighbour) base station 5 (because the UE context is already available locally at the new base station 5). This may be useful for ultra-reliable low latency communications. [0051] In summary, in this network it is possible to provide better service continuity and more efficient resource usage at user equipment (the mobile device and/or the IoT device), base stations, and the MME when the user equipment (which may be operating in LC state) moves to and/or camps on a new cell operated by a different base station. [0052] Mobile Device Figure 2 is a block diagram illustrating the main components of the mobile device 3 shown in Figure 1 (e.g. a mobile telephone or an IoT device). As shown, the mobile device 3 has a transceiver circuit 31 that is operable to transmit signals to and to receive signals from a base station 5 via one or more antenna 33. The mobile device 3 has a controller 37 to control the operation of the mobile device 3. The controller 37 is associated with a memory 39 and is coupled to the transceiver circuit 31. Although not necessarily required for its operation, the mobile device 3 might of course have all the usual functionality of a conventional mobile telephone (such as a user interface 35) and this may be provided by any one or any combination of hardware, software and firmware, as appropriate. Software may be pre-installed in the memory 39 and/or may be downloaded via the telecommunications network or from a removable data storage device (RMD), for example. [0053] The controller 37 is configured to control overall operation of the mobile device 3 by, in this example, program instructions or software instructions stored within the memory 39. As shown, these software instructions include, among other things, an operating system 41, a communications control module 43, a UE state module 45, an RRC module 46, and a NAS module 49. [0054] The communications control module 43 is operable to control the communication between the mobile device 3 and its serving base station 5 (and other communication devices connected to the serving base station 5, such as further mobile devices, IoT devices, core network nodes, etc.). [0055] The UE state module 45 is responsible for managing an operating state of the mobile device 3 (e.g. by obtaining appropriate configuration (such as state transition timer(s), information identifying a RAN paging area) and information identifying UE location, cell information, etc.) and control the other modules (e.g. the RRC module 46 and the NAS module 49) according to the current operating state of the mobile device 3. [0056] The RRC module 46 is operable to generate, send and receive signalling messages formatted according to the RRC standard. For example, such messages are exchanged between the mobile device 3 and its serving base station 5. The RRC messages may include, for example, messages relating to establishing / resuming an RRC connection with an appropriate base station 5. [0057] The NAS module 49 is operable to generate, send and receive signalling messages formatted according to the NAS standard. For example, such messages are exchanged between the mobile device 3 and the MME 9 (via the serving base station 5, using the RRC module 46). The NAS messages may include, for example, messages relating to registering and/or updating a tracking area (or cell) where the mobile device 3 is currently located. [0058] Base Station Figure 3 is a block diagram illustrating the main components of a base station 5 shown in Figure 1. As shown, the base station 5 has a transceiver circuit 51 for transmitting signals to and for receiving signals from user equipment (such as the mobile device 3 / IoT device 3’) via one or more antenna 53, a core network interface 55 (e.g. an S1 interface, NG-C interface, and/or the like) for transmitting signals to and for receiving signals from the core network 7, and a base station interface 56 (e.g. an X2 interface, Xn interface, and/or the like) for transmitting signals to and for receiving signals from neighbouring base stations. The base station 5 has a controller 57 to control the operation of the base station 5. The controller 57 is associated with a memory 59. Although not necessarily shown in Figure 3, the base station 5 will of course have all the usual functionality of a cellular telephone network base station and this may be provided by any one or any combination of hardware, software and firmware, as appropriate. Software may be pre-installed in the memory 59 and/or may be downloaded via the telecommunications network 1 or from a removable data storage device (RMD), for example. The controller 57 is configured to control the overall operation of the base station 5 by, in this example, program instructions or software instructions stored within the memory 59. As shown, these software instructions include, among other things, an operating system 61, a communications control module 63, a UE state module 65, an RRC module 66, an X2 module 67, and an S1AP module 68. [0059] The communications control module 63 is operable to control the communication between the base station 5 and user equipment (mobile device 3 / IoT device 3’) and other network entities that are connected to the base station 5. The communications control module 63 also controls the separate flows of downlink user traffic (via associated data radio bearers) and control data to be transmitted to the communication devices associated with this base station 5 including, for example, control data for managing operation of the mobile device 3 and/or the IoT device 3’. [0060] The UE state module 65 is responsible for managing and monitoring the operating states of mobile devices 3 served by the base station 5 (e.g. by generating and sending appropriate configuration, such as state transition timer(s), information identifying a RAN paging area, etc.) and, when appropriate, providing information on the current operating state of a particular mobile device 3 to other nodes (e.g. the MME 9). [0061] The RRC module 66 is operable to generate, send and receive signalling messages formatted according to the RRC standard. For example, such messages are exchanged between the base station 5 and the mobile device 3 (and other user equipment within the cell of the base station 5). The RRC messages may include, for example, messages relating to establishing / resuming an RRC connection for a particular mobile device 3. [0062] The X2 module 67 is operable to generate, send and receive signalling messages (X2/Xn messages) formatted according to the X2AP (or XnAP) standard. The X2/Xn messages may include, for example, messages relating to paging a mobile device 3, data forwarding, transferring/fetching of UE context (and other information relating to the mobile device 3) between neighbouring base stations. [0063] The S1AP module 68 is operable to generate, send and receive signalling messages formatted according to the S1AP (or NG-C AP) standard. For example, such messages are exchanged between the base station 5 and the mobility management entity (MME) 9. The S1AP messages may include, for example, messages relating to registering the location and/or operating state (e.g. LC) of user equipment in a cell of the base station and/or associated responses. [0064] Mobility Management Entity Figure 4 is a block diagram illustrating the main components of the mobility management entity (MME) 9 shown in Figure 1. As shown, the mobility management entity 9 has a transceiver circuit 71 for transmitting signals to and for receiving signals from the base stations 5 (and/or communication devices connected to the base stations 5) via a base station interface 75 (e.g. an S1 interface). The mobility management entity 9 has a controller 77 to control the operation of the mobility management entity 9. The controller 77 is associated with a memory 79. Although not necessarily shown in Figure 4, the mobility management entity 9 will of course have all the usual functionality of a cellular telephone network mobility management entity and this may be provided by any one or any combination of hardware, software and firmware, as appropriate. Software may be pre-installed in the memory 79 and/or may be downloaded via the telecommunications network 1 or from a removable data storage device (RMD), for example. The controller 77 is configured to control the overall operation of the mobility management entity 9 by, in this example, program instructions or software instructions stored within the memory 79. As shown, these software instructions include, among other things, an operating system 81, a communications control module 83, a UE state/location registration module 85, an S1AP module 88, and a NAS module 89. [0065] The communications control module 83 is operable to control the communication between the mobility management entity 9 and the base stations 5, mobile devices 3, IoT devices, and other network entities that are connected to the mobility management entity 9. [0066] The UE state/location registration module 85 is responsible for keeping track of current location and state (e.g. idle or connected) of user equipment connected to the MME 9. [0067] The S1AP module 88 is operable to generate, send and receive signalling messages formatted according to the S1AP (or NG-C AP) standard. For example, such messages are exchanged between the mobility management entity 9 and connected base stations 5. The S1AP messages may include, for example, messages relating to registering the location and/or operating state (e.g. LC) of user equipment in a cell of the base station, requesting data forwarding, requesting path switching, and/or associated responses. [0068] The NAS module 89 is operable to generate, send and receive signalling messages formatted according to the NAS standard. For example, such messages are exchanged between the MME 9 and the mobile device 3 (via a base station 5, using the S1AP module 88). The NAS messages may include, for example, messages relating to registering and/or updating a tracking area (or cell) where the mobile device 3 is currently located. [0069] In the above description, the mobile device 3, the base station 5, and the mobility management entity 9 are described for ease of understanding as having a number of discrete modules (such as the communications control modules and the UE state modules). Whilst these modules may be provided in this way for certain applications, for example where an existing system has been modified to implement the invention, in other applications, for example in systems designed with the inventive features in mind from the outset, these modules may be built into the overall operating system or code and so these modules may not be discernible as discrete entities. These modules may also be implemented in software, hardware, firmware or a mix of these. [0070] A more detailed description will now be given (with reference to Figures 5 to 7) of the scenario discussed above where a mobile device operating in LC state moves between cells operated by different base stations. [0071] Operation - first example Figure 5 is a timing diagram (message sequence chart) illustrating an example process performed by components of the network 1 when performing a paging operation for a mobile device (UE or IoT device) operating in LC state. [0072] It will be appreciated that in this example, the new base station 5c is a base station configured in accordance with Rel-14 standards (or later) and hence it supports user equipment operating in LC mode. [0073] Initially, the mobile device 3 operates in LC state and its previous serving base station 5b acts as an anchor base station for the mobile device 3. Accordingly, the anchor base station 5b (using its UE state module 65) stores the UE context associated with the mobile device 3 and configures an appropriate RAN based paging area for the mobile device 3 (e.g. by providing a list of cells and/or paging area IDs to the mobile device 3) while terminating S1. When the mobile device 3 leaves the area (Cell #1) served by the anchor base station 5b (and/or the signal conditions in Cell #1 deteriorate), the mobile device 3 selects a new cell to camp on that has more favourable signal conditions. In this example the mobile device 3 selects and camps on Cell #2 which is operated by the base station 5c. Since the mobile device 3 has no active connections with the radio access network, the anchor base station 5b is not aware of the mobile device’s 3 current location (Cell #2). From the perspective of the anchor base station 5b, therefore, the mobile device 3 is deemed to be idle (in LC state). [0074] Whilst the mobile device 3 is deemed to be operating in the LC state, the MME 9 (using its UE state/location registration module 85) maintains an associated S1 connection (ECM-CONNECTED state) even when the mobile device 3 is operating in an idle mode from the RAN’s point of view (which may not be visible by the MME 9). Accordingly, as generally shown in steps S501a and S501b, when the MME 9 has downlink data to send to the mobile device 3 (e.g. control-plane signalling / user-plane data), the MME 9 (using its S1AP module 88) starts transmitting the downlink data to the anchor base station 5b without performing paging. It will also be appreciated that in some cases, a different core network node, e.g. the S-GW 10 may transmit downlink data to the anchor base station 5b. [0075] In response to receiving the downlink data from the MME 9, the controller 57 of the anchor base station 5b triggers, in step S502, RAN-based paging, which typically involves transmitting paging signalling via the Uu interface, via the X2 interface, or both. In step S503, the anchor base station 5b performs RAN based paging of the mobile device 3 within the paging area that is appropriate for that mobile device 3. Although not shown in Figure 5, the anchor base station 5b may also send appropriate X2 paging signalling to its neighbour base station(s) 5 in order to attempt to reach the mobile device 3 via neighbour cells. Effectively, the RAN based paging indicates to the mobile device 3 that it needs to resume its RRC connection (re-connect) via the anchor base station 5b or another appropriate base station for receiving the downlink data. [0076] At the start of the paging, the anchor base station 5b starts an appropriate timer (e.g. a paging timer) and stores the downlink data from the MME 9 (or S-GW 10) in its local cache (memory 59). [0077] In this example, the mobile device 3 is unable to receive (or respond to) the paging signalling. This might happen, for example, when the anchor base station 5b does not perform paging via its neighbours (even though Cell #2 may belong to the currently configured RAN paging area), when Cell #2 does not belong to the paging area configured for the mobile device 3, or when signal conditions at the edge of Cell #2 prevent the mobile device 3 from successfully receiving (or responding to) paging signalling in that cell. [0078] As generally shown in step S504, if the anchor base station 5b does not receive any response from the mobile device 3 within a predefined time interval (e.g. at expiry of the paging timer), the base station 5b proceeds to requesting help from the MME 9. [0079] Specifically, the anchor base station 5b generates and sends, in step S505, and appropriately formatted S1 signalling message (such as a ‘UE Context Release Request’ and/or the like) requesting the MME 9 to initiate S1 based (legacy) paging procedures. As shown in Figure 5, the message from the base station 5b may also indicate (or it may be interpreted such) that the MME 9 does not need to trigger a release of the mobile device’s 3 bearer via the S-GW 10. [0080] However, before the MME 9 can proceed to assist the base station 5b (e.g. by initiating a network-wide paging procedure), the MME 9 needs to align the UE state from connected to idle (ECM-CONNECTED to ECM-IDLE). Therefore, upon receipt of the message at S505, the MME 9 (using its UE state/location registration module 85) moves the S1 connection for the mobile device 3 into idle (e.g. ECM-IDLE) and then it proceeds to initiating legacy paging procedures in the last know tracking area of the mobile device 3 in step S506. Therefore, the MME 9 generates (using its S1AP module 88) and sends, in step S507, and appropriately formatted paging request to the base stations 5 operating cells that belong to the last know tracking area (including the new base station 5c). [0081] As shown, the MME’s paging request triggers the new base station 5c to carry out, in step S508, appropriate paging for the mobile device 3 over the Uu interface (in Cell #2). However, as shown in Figure 5, the MME 9 and the S-GW 10 do not carry out a ‘Release Access Bearer’ procedure, i.e. the MME 9 maintains the existing S1 bearer for the mobile device 3 via the S-GW 10 despite having received a UE Context Release Request from the base station 5a. [0082] After triggering the S1 based paging, the MME 9 waits for a response from the mobile device 3. In this example, as generally shown in step S509, the mobile device 3 responds to the Uu paging request by the new base station 5c by performing appropriate RRC and S1 connection setup procedures (via Cell #2). Typically, step S509 involves the mobile device 3 (using its NAS module 49) generating and sending an appropriately formatted handover request to the MME 9 (via the new base station 5c). Using its UE state/location registration module 85, the MME 9 registers Cell #2 as the mobile device’s 3 new location. As part of step S509, the MME 9 may also indicate (for example, in an appropriately formatted ‘Initial Context Setup request’ and/or the like) to the new base station 5c that data forwarding from the anchor base station 5b to the new base station 5c is imminent so that the new base station 5c is able to prepare appropriate resources for receiving the forwarded data. [0083] Beneficially, the MME 9 may be configured to monitor (preceding) handovers performed by the mobile device 3 and obtain (hold in memory 79) context information and other parameters relating to the mobile device 3. Therefore, the MME’s 9 message to the new base station 5c may also include one or more of the following information held by the MME 9: an appropriate identifier associated with the mobile device 3 (e.g. a Resume Id) that the new base station 5c may use for context fetching; information identifying the mobile device 3 at the MME 9 over the S1 interface (e.g. ‘MME UE S1AP ID’); information identifying the anchor base station 5b (e.g. an ‘anchor eNB ID’); a TAC of the anchor base station 5b; a Globally Unique MME ID (GUMMEI) associated with the MME 9 currently serving the mobile device 3, a handover restriction list (e.g. a list of cell identifier for cells to which a handover of the mobile device 3 is not allowed), and other parameter found in previously handled handover requests relating to the mobile device 3. [0084] Similarly, the MME 9 is able to provide information relevant to the anchor base station 5b in order to perform data forwarding. Thus, in step S510, the MME 9 requests the anchor base station 5b to perform data forwarding by generating and sending an appropriate S1 signalling message to the anchor base station 5b (e.g. a ‘UE Context Release Command’ and/or the like). In this example, the MME 9 sends the UE Context Release Command to the anchor base station 5b and includes information identifying the new base station 5c (e.g. an eNB ID / gNB ID) and information identifying an appropriate forwarding address for the new base station 5c (e.g. a tunnel endpoint identifier (TEID) and/or the like). The new base station 5c is thus able to trigger an appropriate procedure (e.g. by generating and sending a ‘Retrieve UE Context Request’ to the anchor base station 5b) in order to fetch the context relating to the mobile device 3 from the anchor base station 5b. Although not shown in detail in Figure 5, the anchor base station 5b generates and transmits an appropriately formatted ‘Retrieve UE Context Response’ to the new base station 5c including the requested UE context. [0085] After a successful X2-based (or S1-based) context retrieval, the base stations 5 proceed to SN Status transfer and data forwarding. Specifically, the anchor base station 5b (using its X2 module 67) generates and sends, in step S511, and appropriate ‘SN Status transfer’ message in order to transfer the status of the transceiver (uplink receiver status / downlink transmitter status) relating to the mobile device 3. It will be appreciated that the status of the transceiver may include respective Packet Data Convergence Protocol (PDCP) sequence numbers (SNs) used in the uplink and downlink direction which allow the new base station 5c to preserve the status following the mobile device 3 resuming its connection with the network via Cell #2. [0086] In step S512, the anchor base station 5b (using its X2 module 67) starts forwarding, to the new base station 5c using the forwarding address provided by the MME 9, any cached downlink data, and the new base station 5c relays the forwarded data (not shown in Figure 5) to the mobile device 3 via the Uu interface. [0087] As generally shown in step S513, the MME 9 and the new base station 5c also initiates an appropriate path switch procedure (comprising a ‘Path Switch Request’ including information identifying the path to be switched (e.g. in an ‘MME UE S1AP ID’ information element) and an associated acknowledgement). In response to the path switch procedure the MME 9 also requests, in step S514, the S-GW 10 to modify the bearer associated with the mobile device 3 (i.e. to tunnel data to the new base station 5c instead of the anchor base station 5b). Once the bearer has been modified, there is no need for the anchor base station 5b to forward downlink data to the new base station 5c. [0088] After the path switch triggered by new base station 5c (in step S513), the new base station 5c is able to trigger UE context release towards the old (anchor) base station 5b. In more detail, after switching the path (S1 connection) associated with the mobile device 3 from the anchor base station 5b to the new base station 5c, the new base station 5c generates and sends, in step S515, a UE Context release message indicating to the anchor base station 5b that the anchor base station 5b no longer needs to store the UE context associated with the mobile device 3. Effectively, this message prompts the anchor base station 5b to delete the UE context associated with the mobile device 3 (e.g. after transferring the UE context to the new base station 5c) and thus the new base station 5c becomes the new anchor base station for the mobile device 3. In step S516, the former anchor base station 5b confirms that the UE context release has been completed by generating and sending an appropriate S1 acknowledgement to the MME’s 9 command. [0089] Operation - second example Figure 6 is a timing diagram (message sequence chart) illustrating another example process performed by components of the network 1 when performing a paging operation for a mobile device (UE or IoT device) operating in LC state. In this example, which is a modification of the example described with reference to Figure 5, the new base station 5c is a base station configured in accordance with pre-Rel-14 standards (and hence it is not capable of receiving forwarded data from the anchor base station 5b). Therefore, in this example, the anchor base station 5b is configured to forward any cached data to the S-GW 10. [0090] Steps S601a to S608 correspond to steps S501a to S508 described above hence their description is omitted herein for brevity.

We claim:
1. A user equipment, UE, comprising:
a transmitter configured to transmit, to a first access network node, a request
including a resume identity for resuming, to connected mode, a connection
between the UE and a network including the first access network node, wherein
the resume identity includes an identifier indicating a second access network
node which last served the UE in the network.
2. The UE as claimed in claim 1, wherein
the request causes the first access network node to retrieve a context for the UE
from the second access network node using the identifier.
3. A first access network node, comprising:
a receiver configured to receive, from a user equipment, UE, a request including
a resume identity for resuming, to connected mode, a connection between the
UE and a network including the first access network node, wherein
the resume identity includes an identifier indicating a second access network
node which last served the UE in the network.
4. The first access network node as claimed in claim 3, comprising:
a processor configured to retrieve a context for the UE from the second access network node using the identifier.
5. A method for a user equipment, UE, comprising:
transmitting, to a first access network node, a request including a resume identity
for resuming, to connected mode, a connection between the UE and a network
including the first access network node, wherein
the resume identity includes an identifier indicating a second access network
node which last served the UE in the network. 6. The method as claimed in claim 5, wherein
the request causes the first access network node to retrieve a context for the UE
from the second access network node using the identifier.
7. A method for a first access network node, comprising:
receiving, from a user equipment, UE, a request including a resume identity for
resuming, to connected mode, a connection between the UE and a network
including the first access network node, wherein
the resume identity includes an identifier indicating a second access network
node which last served the UE in the network.
8. The method as claimed in claim 7, comprising:
retrieving a context for the UE from the second access network node using the
identifier.

Title of Invention : COMMUNICATION SYSTEM
Technical Field
[0001]
The present invention relates to a communication system. The invention has particular but not exclusive relevance to wireless communication systems and devices thereof operating according to the 3rd Generation Partnership Project (3GPP) standards or equivalents or derivatives thereof (including LTE-Advanced and Next Generation or 5G networks). The invention has particular although not exclusive relevance to managing connection states for communication devices.
Background Art
[0002]
The latest developments of the 3GPP standards are referred to as the Long Term Evolution (LTE) of Evolved Packet Core (EPC) network and Evolved UMTS Terrestrial Radio Access Network (E-UTRAN), also commonly referred as ‘4G’. In addition, the term ‘5G’ and ‘new radio’ (NR) refer to an evolving communication technology that is expected to support a variety of applications and services. Various details of 5G networks are described in, for example, the ‘NGMN 5G White Paper’ V1.0 by the Next Generation Mobile Networks (NGMN) Alliance, which document is available from https://www.ngmn.org/5g-white-paper.html. 3GPP intends to support 5G by way of the so-called 3GPP Next Generation (NextGen) radio access network (RAN) and the 3GPP NextGen core network.
[0003]
Under the 3GPP standards, a NodeB (or an eNB in LTE, gNB in 5G) is the base station via which communication devices (user equipment or ‘UE’) connect to a core network and communicate to other communication devices or remote servers. For simplicity, the present application will use the term base station to refer to any such base stations and use the term mobile device, user device, or UE to refer to any such communication device. The core network (i.e. the EPC in case of LTE) hosts functionality for subscriber management, mobility management, charging, security, and call/session management (amongst others), and provides connection for communication devices to external networks, such as the Internet.
[0004]
Communication devices might be, for example, mobile communication devices such as mobile telephones, smartphones, user equipment, personal digital assistants, laptop/tablet computers, web browsers, e-book readers and/or the like. Such mobile (or even generally stationary) devices are typically operated by a user. However, 3GPP standards also make it possible to connect so-called ‘Internet of Things’ (IoT) devices (e.g. Narrow-Band IoT (NB-IoT) devices) to the network, which typically comprise automated equipment, such as various measuring equipment, telemetry equipment, monitoring systems, tracking and tracing devices, in-vehicle safety systems, vehicle maintenance systems, road sensors, digital billboards, point of sale (POS) terminals, remote control systems and the like. Effectively, the Internet of Things is a network of devices (or "things") equipped with appropriate electronics, software, sensors, network connectivity, and/or the like, which enables these devices to collect and exchange data with each other and with other communication devices. It will be appreciated that IoT devices are sometimes also referred to as Machine-Type Communication (MTC) communication devices or Machine-to-Machine (M2M) communication devices.
[0005]
For simplicity, the present application refers to mobile devices in the description but it will be appreciated that the technology described can be implemented on any communication devices (mobile and/or generally stationary) that can connect to a communications network for sending/receiving data, regardless of whether such communication devices are controlled by human input or software instructions stored in memory.
[0006]
Communication between mobile devices and base stations is controlled using a Radio Resource Control (RRC) protocol as defined in 3GPP TS 36.331 V14.0.0. RRC handles the control plane signalling of Layer 3 between mobile telephones and the radio access network, and includes, amongst other things, functions for broadcasting system information, paging, connection establishment and release, radio bearer establishment, reconfiguration and release, mobility procedures, and power control. In accordance with the current version of the RRC protocol, at any given time, a mobile device may operate either in an ‘RRC idle mode’ (in which no data communication takes place) or an ‘RRC connected mode’ (in which data communication may take place between the mobile device and its serving base station).
[0007]
As mobile devices operating in the RRC connected mode move around in the area covered by the communication system, they are handed over from one cell (i.e. operated by a base station) to another cell (operated by the same or a different base station), depending on signal conditions and other requirements, such as requested quality of service, the type of service used, overall system load, and the like. Handover requires extensive signalling between the mobile device and the base stations (old and new) and also between the base stations and the core network as well.
[0008]
On the other hand, whilst in the RRC idle mode, mobile devices are programmed to select a ‘serving’ cell, having a good quality signal, to camp on so that when new data is to be transmitted to/from these mobile devices, they can benefit from favourable signal conditions. In the event that an idle mobile device detects a new cell with better signal quality than the current serving cell, e.g. due to the mobile device changing its location, the mobile device can perform a so-called cell reselection procedure. However, an idle mode mobile device does not inform the network about the selected new cell as long as this cell is within the same ‘tracking area’ (i.e. a larger geographic area comprising a pre-defined set of cells), because the radio network transmits system information and UE specific paging messages within the whole tracking area thus making it possible to initiate communication to/from the mobile device regardless of the current cell it camps on.
[0009]
In order to benefit from the lowest energy consumption and to free up valuable system resources, the mobile devices return to the RRC idle mode whenever possible and perform cell reselections (instead of handovers) as long as they remain within the same tracking area. The base station controls the transition between the various operating modes for each mobile device within its cell(s). Since the setting up and termination of an RRC connection between the base station and the mobile device requires exchanging of signalling messages and hence utilises valuable system resources, and also takes some time to complete, the transition from connected to idle mode is allowed under specific circumstances as defined in 3GPP TS 36.331. For example, the serving base station might instruct a mobile device to enter the RRC idle mode only after it has confirmed that there is no more data to be transmitted to/from the particular mobile device (e.g. both uplink (UL) and downlink (DL) buffers are empty).
[0010]
When it registers its current location (e.g. cell) with the core network, each mobile device also has an associated ‘S1’ connection between its serving base station and the core network. The S1 connection is either in a so-called ‘ECM-IDLE’ mode (when the mobile device is in RRC idle mode) or in an ‘ECM-CONNECTED’ mode (when the mobile device is in RRC connected mode). The S1 connection is used for transferring data (control and user data) between the mobile device and the core network (and beyond) and it is maintained as long as the mobile device remains in the RRC connected mode. On the other hand, when a mobile device enters the RRC idle mode, its associated S1 connection is also terminated (or suspended) until the mobile device has more data to send or receive in which case a new S1 connection is established to the current serving base station (or the suspended S1 connection is re-activated).
[0011]
When the network has data to send to an RRC idle mobile device, it triggers an appropriate paging procedure in the last known area (tracking/paging area) of the mobile device, which causes the base stations within that area to broadcast appropriate paging messages in their cells requesting that particular mobile device to enter the RRC connected state. When a previously idle mobile telephone has data to send again (or it has been paged for receiving downlink data), in order to be allocated communication resources it initiates a so called RRC connection establishment procedure by sending an appropriately formatted RRC connection request message to the base station (following a so-called Random Access Procedure which ensures that the lower layers, and in particular the Media Access Control (MAC) layer, are set up for communication with the base station).
[0012]
For the latest developments of the 3GPP standards, the so-called Next Generation (NG) or 5G networks, it is envisaged that mobile devices may also operate in a new RRC state, or new radio state, referred to as a ‘light-connected’ (LC) state. When a mobile device is in the LC state, the core network maintains both its control-plane and user-plane connection even after the mobile device has no more data to send or receive (and hence it is normally configured to enter the RRC idle mode). In other words, even though in the LC state the mobile device is seen as operating in idle mode from the radio access network’s (base station’s) point of view, it may still be seen as being connected from the core network’s point of view. One of the benefits of this new LC state is that mobile devices (IoT devices in particular) that have small and infrequent data transmissions do not need to perform the entire RRC connection establishment procedure every time they have data to send (or receive). Instead, an LC state capable mobile device can be configured to resume its existing RRC connection with the current serving base station whenever needed and then return to a more power efficient mode of operation until it has data to send/receive again.
[0013]
The mobile device can resume its RRC connection by sending to its current base station information (e.g. a resume ID) identifying the connection to be resumed. This beneficially avoids the base station and the mobile device having to go through authentication and radio bearer establishment. In order to facilitate such lightweight connection and simplified resumption of the connection between the mobile device and its serving base station, the concept of a so-called anchor base station is being considered by 3GPP. Effectively, the anchor base station is a base station responsible for storing UE Access Stratum (AS) context, caching the mobile device’s user data (UE context) and for providing the user data to other base stations as needed while terminating S1. For example, the anchor base station may be the first (or previous) base station that the mobile device registered with in a particular tracking area (or other pre-defined area). Thus, when the mobile device attempts to resume its RRC connection via a different base station (within the same area), the new base station can contact the anchor base station and retrieve the UE context along with the cached user data based on information provided by the mobile device (e.g. resume ID and/or the like). Since in the LC state the S1 connection is maintained, beneficially, the new base station can avoid having to contact the core network and/or establish a new S1 connection for the mobile device (although the new base station might need to switch the S1 connection from the anchor/previous base station to the new base station). The anchor base station concept is illustrated in Figure 8 which is reproduced from 3GPP draft no. R3-160655.
[0014]
The current agreement in 3GPP is that the base station maintains the S1 connection while the mobile device is lightly connected and the base station is responsible for RAN paging (within an appropriate paging/tracking area configured by the base station) when it receives the downlink data from core network. The LC state mobile device notifies the network when it moves out of its configured RAN based paging area, in which case the network can decide whether to keep the mobile device in LC mode or suspend the mobile device (e.g. request it to enter RRC idle mode).
Summary of Invention
Technical Problem
[0015]
However, the inventors have realised that since the mobile device appears to be in different states towards the RAN and the core network, this may cause a number of issues that the currently proposed systems cannot handle. Such issues include, although they are not limited to:
　　　　- how to avoid loss of DL data when RAN based paging fails;
　　　　- how to perform core network (e.g. MME) based paging as a fall-back when RAN based paging fails;
　　　　- how to inform a mobility management entity (MME) that certain operations such as load balancing, tracking area update (TAU) required, power saving mode (PSM), mobile terminated (MT) circuit switched fallback (CSFB) may be impacted due to the mobile device’s LC operation.
　　　　- how to notify the MME when a mobile device enters the LC state;
　　　　- how to exchange a RAN-based identifier (e.g. resume ID) that is used to uniquely identify a mobile device when it is in LC mode within the RAN routing area (e.g. for reducing the time it takes to resume a connection by performing context pre-fetch);
　　　　- how to expedite random access channel (RACH) processes when a mobile device transits from LC mode to regular connected mode for data transmission;
　　　　- how to synchronise RAN location update with core network location update; and
　　　　- how to enable UL data transmission when the mobile device is in LC mode.
[0016]
Accordingly, preferred example embodiments of the present invention aim to provide methods and apparatus which address or at least partially deal with one or more of the above issues whilst still allowing mobile devices to maintain a light connection with the network.
[0017]
Although for efficiency of understanding for those of skill in the art, the invention will be described in detail in the context of a 3GPP system (UMTS, LTE), the principles of the invention can be applied to other systems in which communication devices or User Equipment (UE) access a core network using a radio access technology.
Solution to Problem
[0018]
In one aspect, the invention provides a base station for a communication network, the base station comprising: a transceiver and a controller wherein the controller is configured to: control the transceiver to receive, from a core network node, downlink data for a communication device; attempt to initiate communication with the communication device; and control the transceiver to send to the core network node, when the communication device does not respond to said attempt to initiate communication, a message to request initiation of a paging procedure for the communication device.
[0019]
In one aspect, the invention provides core network apparatus for a communication network, the core network apparatus comprising: a transceiver and a controller wherein the controller is configured to: control the transceiver to send, to a base station, downlink data for a communication device; control the transceiver to receive from the base station, when the communication device does not respond to an attempt by the base station to initiate communication, a message to request initiation of a paging procedure for the communication device; and initiate a paging procedure for the communication device based on said message.
[0020]
In one aspect, the invention provides a base station for a communication network, the base station comprising: a transceiver and a controller wherein the controller is configured to: control the transceiver to send a notification to a core network node indicating that at least one core network operation is not possible (or banned) for a given communication device.
[0021]
In one aspect, the invention provides a base station for a communication network, the base station comprising: a transceiver and a controller wherein the controller is configured to: maintain information identifying at least one communication device, for which the base station operates as an anchor base station, in association with respective context information for each communication device for which the base station operates as an anchor base station; and control the transceiver to provide to another base station at least one identifier to identify at least one respective communication device for which the base station operates as an anchor base station.
[0022]
In one aspect, the invention provides a base station for a communication network, the base station comprising: a transceiver and a controller wherein the controller is configured to: control the transceiver to receive from at least one other base station at least one identifier to identify at least one communication device for which the at least one other base station operates as an anchor base station; and fetch, from the at least one other base station, context information for each communication device for which the at least one other base station operates as an anchor base station based on said at least one identifier.
[0023]
In one aspect, the invention provides a base station for a communication network, the base station comprising: a transceiver and a controller wherein the controller is configured to: control the transceiver to, receive from a core network node, a message indicating that data forwarding from another base station is to happen for a communication device, together with at least one of: an identifier, associated with that communication device, for use in fetching a context relating to that communication device from the other base station; information identifying the communication device at the core network node over an S1 interface (e.g. ‘MME UE S1AP ID’); information identifying the other base station (e.g. an ‘anchor eNB ID’); a tracking area code (TAC) associated with the other base station; a globally unique ID (e.g. a GUMMEI) associated with the core network node currently serving the communication device; information identifying at least one cell identifier for at least one cell to which a handover of the communication device is prohibited; and at least one other parameter acquired by the core network node in a previously handled handover request relating to the communication device; and control the transceiver to receive data forwarded by the other base station accordingly.
[0024]
Aspects of the invention extend to corresponding systems, methods, and computer program products such as computer readable storage media having instructions stored thereon which are operable to program a programmable processor to carry out a method as described in the aspects and possibilities set out above or recited in the claims and/or to program a suitably adapted computer to provide the apparatus recited in any of the claims.
[0025]
Each feature disclosed in this specification (which term includes the claims) and/or shown in the drawings may be incorporated in the invention independently of (or in combination with) any other disclosed and/or illustrated features. In particular but without limitation the features of any of the claims dependent from a particular independent claim may be introduced into that independent claim in any combination or individually.
[0026]
Example embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings in which:
Brief Description of Drawings
[0027]
[fig. 1] Figure 1 illustrates schematically a cellular telecommunication system to which example embodiments of the invention may be applied;
[fig. 2] Figure 2 is a block diagram of a mobile device forming part of the system shown in Figure 1;
[fig. 3] Figure 3 is a block diagram of a base station forming part of the system shown in Figure 1;
[fig. 4] Figure 4 is a block diagram of a mobility management entity forming part of the system shown in Figure 1;
[fig. 5] Figure 5 is a timing diagram illustrating an exemplary way in which example embodiments of the invention can be implemented in the system of Figure 1;
[fig. 6] Figure 6 is a timing diagram illustrating an exemplary way in which example embodiments of the invention can be implemented in the system of Figure 1;
[fig. 7] Figure 7 is a timing diagram illustrating an exemplary way in which example embodiments of the invention can be implemented in the system of Figure 1; and
[fig. 8] Figure 8 illustrates schematically the anchor base station concept.
Description of Embodiments
[0028]
Overview
Figure 1 schematically illustrates a telecommunications network 1 in which mobile devices 3, mobile telephones, and other communication devices (e.g. IoT devices) can communicate with each other via E-UTRAN base stations 5 and a core network 7 using an E-UTRA radio access technology (RAT). As those skilled in the art will appreciate, whilst one mobile device 3 (denoted ‘UE’), one IoT device 3’, and three base stations 5a to 5c are shown in Figure 1 for illustration purposes, the system, when implemented, will typically include other base stations and communication devices.
[0029]
Each base station 5 operates one or more associated cell. In this example, the base station 5b operates ‘Cell #1’ and the base station 5c operates ‘Cell #2’. Although not shown in Figure 1, base station 5a will also typically operate one or more cells. It will also be appreciated that in some scenarios the ‘old’ base station 5a and the ‘anchor’ base station 5b might be the same.
[0030]
Communication devices may connect to either cell (depending on their location and possibly on other factors, e.g. signal conditions, subscription data, capability, and/or the like) by establishing a radio resource control (RRC) connection with the appropriate base station 5 operating that cell. As can be seen, the mobile device 3 is located in an area where the cells operated by the base stations 5b and 5c partially overlap. Thus, when operating in RRC idle mode (not sending/receiving data), the mobile device 3 camps on the cell having the best signal quality, and when in RRC active mode, the mobile device 3 communicates data via that cell (using e.g. the ‘Uu’ air interface). Similarly, in this example, the IoT device 3’ camps on Cell #2 (when in RRC idle mode) and communicates via the base station 5c (when in RRC active mode).
[0031]
When the mobile device 3 (IoT device 3’) first registers with the network (via one of the base stations 5), its serving base station 5 also establishes an associated S1 connection for relaying communications (user and control data) between the serving base station 5 and the core network 7.
[0032]
The base stations 5 are connected to the core network 7 via an S1 interface and to each other via an X2 interface (either directly, or via an X2 gateway). The core network 7 includes, amongst others, a mobility management entity (MME) 9, a serving gateway (S-GW) 10, and a packet data network gateway (P-GW) 11 for providing a connection between the base stations 5 and other networks (such as the Internet) and/or servers hosted outside the core network 7.
[0033]
The MME 9 is the network node responsible for keeping track of the locations of the mobile communication devices (mobile devices and IoT devices alike) within the telecommunications network 1 especially when a UE is RRC_IDLE mode. In particular, the MME 9 stores an identifier of the mobile communication devices’ last known cell (or tracking area) so that they can be notified when there is an incoming (voice or data) call for them and that a communication path is set up via the base station 5 currently serving the particular mobile communication device.
[0034]
In the following examples, the mobile device 3 connects to the network periodically (e.g. whenever one of its applications needs to communicate with the network) for sending data to a remote endpoint (e.g. a server or another communication device). The mobile device 3 is configured to operate in a light connected (LC) mode in which the network maintains an associated S1 connection even when the mobile device 3 is operating in an idle mode from the RAN’s point of view. Therefore, between its periodic re-connections, the mobile device 3 effectively enters an idle (or ‘suspended’) mode and thus avoids performing handovers as long as it remains within an area configured by its anchor base station.
[0035]
The serving base station 5 is responsible for configuring an appropriate RAN based paging area for the mobile device 3 (e.g. by providing a list of cells and/or paging area IDs to the mobile device 3). The RAN based paging area may be configured as one or more cells from the same or different base stations 5. For example, the RAN based paging area may be a tracking area.
[0036]
As illustrated by a dashed line, the mobile device 3 previously connected to the base station 5b (via Cell #1) and thus the base station 5b (acting as the anchor base station for the mobile device 3) maintains an associated UE context and terminates S1. However, it will also be appreciated that the anchor base station may be a different base station, e.g. the old base station 5a. In the present example, the anchor base station 5b configures the mobile device 3 with a RAN paging area that includes the anchor base station’s own cell (Cell #1) and any cells operated by base station 5a.
[0037]
As illustrated by a continuous line, the mobile device 3 is now reachable via the base station 5c (via Cell #2), e.g. due to a change in signal conditions in Cell #1 and/or movement of the mobile device 3. In this example, the mobile device 3 currently has no active connections with the radio access network (the base stations 5), thus it is configured to perform cell reselection when it moves to Cell #2 (without informing the network).
[0038]
As explained above, in LC state the mobile device 3 is still seen as connected (ECM-CONNECTED) from the core network’s 7 (MME 9) point of view even after the mobile device 3 has, in effect, entered an idle mode from the perspective of the base stations (and hence the mobile device 3 does not have an active data connection with its base station 5). Accordingly, when there is downlink data to send, the MME 9 does not initiate paging, for a mobile device 3 in the LC state/mode (e.g. throughout an associated tracking/paging area) because the MME 9 assumes that the mobile device 3 still has an active connection with its serving base station 5 (in this example, the anchor base station 5b). Thus, the MME 9 starts sending the downlink data to the anchor base station 5b. In response to the downlink data, the anchor base station 5b starts appropriate procedures for (RAN based) paging of the mobile device 3 within the paging area appropriate for that mobile device 3. The RAN based paging indicates to the mobile device 3 that it needs to resume its RRC connection (re-connect) via an appropriate base station for receiving downlink data. Until the mobile device 3 resumes its connection, the anchor base station 5b stores the downlink data from the MME 9 (in case of control-planes signalling or data) or from the S-GW 10 (in case of user-plane data) in its cache (memory).
[0039]
The base station 5b decides which cells to page when it receives the downlink data. If necessary, the anchor base station 5b may send appropriate X2 paging signalling to its neighbour base station(s) in order to reach the mobile device 3 via other than its own cell(s). However, as the mobile device 3 has moved in the meantime and it is now camping on Cell #2, it might not be able to receive (or respond to) the paging signalling. This might happen, for example, when the anchor base station 5b does not perform paging via its neighbours (even though Cell #2 may belong to the currently configured RAN paging area), when Cell #2 does not belong to the paging area configured for the mobile device 3, or when signal conditions at the edge of Cell #2 prevent the mobile device 3 from successfully receiving (or responding to) paging signalling in that cell.
[0040]
Beneficially, if the anchor base station 5b does not receive any response from the mobile device 3 within a predefined time interval (after start of paging), the base station 5b requests the MME 9 to initiate S1 based (legacy) paging procedures. However, before the MME 9 is able to provide assistance in this respect, the MME 9 needs to move the mobile device 3 from connected state to idle state (e.g. ECM-CONNECTED to ECM-IDLE). In order to allow this, the base station 5b is configured to use legacy signalling in order to tear down the existing S1 connection. For example, the base station 5b may be configured to send an appropriately formatted message (such as a ‘UE Context Release Request’ and/or the like) including information indicating that S1 based paging is required for the mobile device 3 in its last recorded tracking area. Beneficially, the base station’s request allows the MME 9 to move the mobile device 3 to idle state (ECM-IDLE) allowing the MME 9 to trigger a legacy S1-based paging. Advantageously, the message from the base station 5b may also indicate that the paging is requested without triggering a release of the mobile device’s 3 bearer via the S-GW 10 (i.e. unlike the conventional procedure upon receipt of a UE Context Release Request for a UE in legacy RRC idle state).
[0041]
The MME 9 triggers S1 based paging and waits for an appropriate response from the mobile device 3. In this example, the mobile device 3 responds to the MME’s paging request via the new base station 5c by performing appropriate RRC and S1 connection setup procedures (via the new base station 5c / Cell #2). This is possible when a new base station 5 does not know or understand as to how to locate the associated UE context using the UE identifier (e.g. Resume ID) received from the mobile device 3. The MME 9 registers Cell #2 as the mobile device’s 3 new location.
[0042]
When the new base station 5c establishes a new RRC and S1 connection for the mobile device 3, the MME 9 is beneficially able to indicate to the new base station 5c that data forwarding from the old (anchor) base station 5b is to happen. In this example, the MME 9 requests data forwarding in its response to the anchor base station 5b (e.g. a ‘UE Context Release Response’ and/or the like).
[0043]
Moreover, the MME 9 may also be configured to monitor (preceding) handovers performed by the mobile device 3 and obtain context information and other parameters relating to the mobile device 3. Therefore, the MME’s 9 message to the new base station 5c may also include information (previously obtained or held by the MME 9) that may be used by the new base station 5c to fetch the UE context from the anchor base station 5b. Similarly, the MME 9 is able to provide information relevant to the anchor base station 5b in order to perform data forwarding.
[0044]
On the other hand, in case the mobile device 3 is not found (i.e. it does not respond to the network-wide S1 based paging either), then the MME 9 can follow appropriate legacy procedures in order to release the UE context (held at the anchor base station 5b and the MME 9) and trigger a release of the access bearer towards the S-GW 10. After the UE context is released, the mobile device 3 is considered to be in RRC idle state (instead of LC state) and the mobile device 3 is able to communicate with the network again by moving to RRC connected state, e.g. by triggering an appropriate RRC connection establishment operation (instead of a simple resume operation).
[0045]
In a particularly beneficial example, the base stations 5 may be configured to notify the MME 9 that certain operations are not possible for the mobile device 3 because it is currently operating in LC state. Such MME operations may include, for example, load balancing, TAU required, Power Saving Mode, MT CSFB, and/or the like. Thus, even if the MME 9 continues to see the mobile device 3 as being in a connected state (S1 connected / ECM-CONNECTED), the MME 9 is able to avoid such procedures.
[0046]
Beneficially, the serving/anchor base station 5 (in this example, the base station 5b) is configured to send, to the MME 9, information identifying whether the mobile device 3 is operating in LC state and/or whether certain MME operations are to be avoided (or banned). Based on the received information, the MME 9 takes different handling for certain operations such as CSFB or will not release S1 with load balancing TAU required. As a result, when overloaded, the MME 9 may be configured to release S1 connections that do not belong to those UEs that are in LIGHT-CONNECTED mode.
[0047]
Beneficially, the above procedures allow the MME 9 to avoid performing and/or requesting procedures that would otherwise be inappropriate for the current operating state of the mobile device 3.
[0048]
In order to facilitate swift resuming of an RRC connection for mobile devices 3 via a different based station (in this example, resuming the mobile device’s connection via base station 5c and Cell #2), the base stations 5 of this system may also be configured to exchange information with each other relating to the mobile devices 3 (users) for which they act as an anchor base station.
[0049]
Specifically, the base stations 5 (at least the ones which operate as anchor base stations) may be configured to provide, to their neighbour base station(s) over the X2 interface, information identifying any UE AS context, security data, suspended bearers at that particular base station 5, and/or the mobile devices 3 associated with such suspended bearers.
[0050]
Using such exchanged information (e.g. identifiers of suspended bearers and/or mobile devices associated with such bearers), neighbour base stations 5 are beneficially able to pre-fetch the associated UE context for bearers suspended at another (anchor) base station. This allows them to minimise latency at the time of handover (or idle mode mobility) of a particular mobile device 3 to a cell operated by the new (neighbour) base station 5 (because the UE context is already available locally at the new base station 5). This may be useful for ultra-reliable low latency communications.
[0051]
In summary, in this network it is possible to provide better service continuity and more efficient resource usage at user equipment (the mobile device and/or the IoT device), base stations, and the MME when the user equipment (which may be operating in LC state) moves to and/or camps on a new cell operated by a different base station.
[0052]
Mobile Device
Figure 2 is a block diagram illustrating the main components of the mobile device 3 shown in Figure 1 (e.g. a mobile telephone or an IoT device). As shown, the mobile device 3 has a transceiver circuit 31 that is operable to transmit signals to and to receive signals from a base station 5 via one or more antenna 33. The mobile device 3 has a controller 37 to control the operation of the mobile device 3. The controller 37 is associated with a memory 39 and is coupled to the transceiver circuit 31. Although not necessarily required for its operation, the mobile device 3 might of course have all the usual functionality of a conventional mobile telephone (such as a user interface 35) and this may be provided by any one or any combination of hardware, software and firmware, as appropriate. Software may be pre-installed in the memory 39 and/or may be downloaded via the telecommunications network or from a removable data storage device (RMD), for example.
[0053]
The controller 37 is configured to control overall operation of the mobile device 3 by, in this example, program instructions or software instructions stored within the memory 39. As shown, these software instructions include, among other things, an operating system 41, a communications control module 43, a UE state module 45, an RRC module 46, and a NAS module 49.
[0054]
The communications control module 43 is operable to control the communication between the mobile device 3 and its serving base station 5 (and other communication devices connected to the serving base station 5, such as further mobile devices, IoT devices, core network nodes, etc.).
[0055]
The UE state module 45 is responsible for managing an operating state of the mobile device 3 (e.g. by obtaining appropriate configuration (such as state transition timer(s), information identifying a RAN paging area) and information identifying UE location, cell information, etc.) and control the other modules (e.g. the RRC module 46 and the NAS module 49) according to the current operating state of the mobile device 3.
[0056]
The RRC module 46 is operable to generate, send and receive signalling messages formatted according to the RRC standard. For example, such messages are exchanged between the mobile device 3 and its serving base station 5. The RRC messages may include, for example, messages relating to establishing / resuming an RRC connection with an appropriate base station 5.
[0057]
The NAS module 49 is operable to generate, send and receive signalling messages formatted according to the NAS standard. For example, such messages are exchanged between the mobile device 3 and the MME 9 (via the serving base station 5, using the RRC module 46). The NAS messages may include, for example, messages relating to registering and/or updating a tracking area (or cell) where the mobile device 3 is currently located.
[0058]
Base Station
Figure 3 is a block diagram illustrating the main components of a base station 5 shown in Figure 1. As shown, the base station 5 has a transceiver circuit 51 for transmitting signals to and for receiving signals from user equipment (such as the mobile device 3 / IoT device 3’) via one or more antenna 53, a core network interface 55 (e.g. an S1 interface, NG-C interface, and/or the like) for transmitting signals to and for receiving signals from the core network 7, and a base station interface 56 (e.g. an X2 interface, Xn interface, and/or the like) for transmitting signals to and for receiving signals from neighbouring base stations. The base station 5 has a controller 57 to control the operation of the base station 5. The controller 57 is associated with a memory 59. Although not necessarily shown in Figure 3, the base station 5 will of course have all the usual functionality of a cellular telephone network base station and this may be provided by any one or any combination of hardware, software and firmware, as appropriate. Software may be pre-installed in the memory 59 and/or may be downloaded via the telecommunications network 1 or from a removable data storage device (RMD), for example. The controller 57 is configured to control the overall operation of the base station 5 by, in this example, program instructions or software instructions stored within the memory 59. As shown, these software instructions include, among other things, an operating system 61, a communications control module 63, a UE state module 65, an RRC module 66, an X2 module 67, and an S1AP module 68.
[0059]
The communications control module 63 is operable to control the communication between the base station 5 and user equipment (mobile device 3 / IoT device 3’) and other network entities that are connected to the base station 5. The communications control module 63 also controls the separate flows of downlink user traffic (via associated data radio bearers) and control data to be transmitted to the communication devices associated with this base station 5 including, for example, control data for managing operation of the mobile device 3 and/or the IoT device 3’.
[0060]
The UE state module 65 is responsible for managing and monitoring the operating states of mobile devices 3 served by the base station 5 (e.g. by generating and sending appropriate configuration, such as state transition timer(s), information identifying a RAN paging area, etc.) and, when appropriate, providing information on the current operating state of a particular mobile device 3 to other nodes (e.g. the MME 9).
[0061]
The RRC module 66 is operable to generate, send and receive signalling messages formatted according to the RRC standard. For example, such messages are exchanged between the base station 5 and the mobile device 3 (and other user equipment within the cell of the base station 5). The RRC messages may include, for example, messages relating to establishing / resuming an RRC connection for a particular mobile device 3.
[0062]
The X2 module 67 is operable to generate, send and receive signalling messages (X2/Xn messages) formatted according to the X2AP (or XnAP) standard. The X2/Xn messages may include, for example, messages relating to paging a mobile device 3, data forwarding, transferring/fetching of UE context (and other information relating to the mobile device 3) between neighbouring base stations.
[0063]
The S1AP module 68 is operable to generate, send and receive signalling messages formatted according to the S1AP (or NG-C AP) standard. For example, such messages are exchanged between the base station 5 and the mobility management entity (MME) 9. The S1AP messages may include, for example, messages relating to registering the location and/or operating state (e.g. LC) of user equipment in a cell of the base station and/or associated responses.
[0064]
Mobility Management Entity
Figure 4 is a block diagram illustrating the main components of the mobility management entity (MME) 9 shown in Figure 1. As shown, the mobility management entity 9 has a transceiver circuit 71 for transmitting signals to and for receiving signals from the base stations 5 (and/or communication devices connected to the base stations 5) via a base station interface 75 (e.g. an S1 interface). The mobility management entity 9 has a controller 77 to control the operation of the mobility management entity 9. The controller 77 is associated with a memory 79. Although not necessarily shown in Figure 4, the mobility management entity 9 will of course have all the usual functionality of a cellular telephone network mobility management entity and this may be provided by any one or any combination of hardware, software and firmware, as appropriate. Software may be pre-installed in the memory 79 and/or may be downloaded via the telecommunications network 1 or from a removable data storage device (RMD), for example. The controller 77 is configured to control the overall operation of the mobility management entity 9 by, in this example, program instructions or software instructions stored within the memory 79. As shown, these software instructions include, among other things, an operating system 81, a communications control module 83, a UE state/location registration module 85, an S1AP module 88, and a NAS module 89.
[0065]
The communications control module 83 is operable to control the communication between the mobility management entity 9 and the base stations 5, mobile devices 3, IoT devices, and other network entities that are connected to the mobility management entity 9.
[0066]
The UE state/location registration module 85 is responsible for keeping track of current location and state (e.g. idle or connected) of user equipment connected to the MME 9.
[0067]
The S1AP module 88 is operable to generate, send and receive signalling messages formatted according to the S1AP (or NG-C AP) standard. For example, such messages are exchanged between the mobility management entity 9 and connected base stations 5. The S1AP messages may include, for example, messages relating to registering the location and/or operating state (e.g. LC) of user equipment in a cell of the base station, requesting data forwarding, requesting path switching, and/or associated responses.
[0068]
The NAS module 89 is operable to generate, send and receive signalling messages formatted according to the NAS standard. For example, such messages are exchanged between the MME 9 and the mobile device 3 (via a base station 5, using the S1AP module 88). The NAS messages may include, for example, messages relating to registering and/or updating a tracking area (or cell) where the mobile device 3 is currently located.
[0069]
In the above description, the mobile device 3, the base station 5, and the mobility management entity 9 are described for ease of understanding as having a number of discrete modules (such as the communications control modules and the UE state modules). Whilst these modules may be provided in this way for certain applications, for example where an existing system has been modified to implement the invention, in other applications, for example in systems designed with the inventive features in mind from the outset, these modules may be built into the overall operating system or code and so these modules may not be discernible as discrete entities. These modules may also be implemented in software, hardware, firmware or a mix of these.
[0070]
A more detailed description will now be given (with reference to Figures 5 to 7) of the scenario discussed above where a mobile device operating in LC state moves between cells operated by different base stations.
[0071]
Operation - first example
Figure 5 is a timing diagram (message sequence chart) illustrating an example process performed by components of the network 1 when performing a paging operation for a mobile device (UE or IoT device) operating in LC state.
[0072]
It will be appreciated that in this example, the new base station 5c is a base station configured in accordance with Rel-14 standards (or later) and hence it supports user equipment operating in LC mode.
[0073]
Initially, the mobile device 3 operates in LC state and its previous serving base station 5b acts as an anchor base station for the mobile device 3. Accordingly, the anchor base station 5b (using its UE state module 65) stores the UE context associated with the mobile device 3 and configures an appropriate RAN based paging area for the mobile device 3 (e.g. by providing a list of cells and/or paging area IDs to the mobile device 3) while terminating S1. When the mobile device 3 leaves the area (Cell #1) served by the anchor base station 5b (and/or the signal conditions in Cell #1 deteriorate), the mobile device 3 selects a new cell to camp on that has more favourable signal conditions. In this example the mobile device 3 selects and camps on Cell #2 which is operated by the base station 5c. Since the mobile device 3 has no active connections with the radio access network, the anchor base station 5b is not aware of the mobile device’s 3 current location (Cell #2). From the perspective of the anchor base station 5b, therefore, the mobile device 3 is deemed to be idle (in LC state).
[0074]
Whilst the mobile device 3 is deemed to be operating in the LC state, the MME 9 (using its UE state/location registration module 85) maintains an associated S1 connection (ECM-CONNECTED state) even when the mobile device 3 is operating in an idle mode from the RAN’s point of view (which may not be visible by the MME 9). Accordingly, as generally shown in steps S501a and S501b, when the MME 9 has downlink data to send to the mobile device 3 (e.g. control-plane signalling / user-plane data), the MME 9 (using its S1AP module 88) starts transmitting the downlink data to the anchor base station 5b without performing paging. It will also be appreciated that in some cases, a different core network node, e.g. the S-GW 10 may transmit downlink data to the anchor base station 5b.
[0075]
In response to receiving the downlink data from the MME 9, the controller 57 of the anchor base station 5b triggers, in step S502, RAN-based paging, which typically involves transmitting paging signalling via the Uu interface, via the X2 interface, or both. In step S503, the anchor base station 5b performs RAN based paging of the mobile device 3 within the paging area that is appropriate for that mobile device 3. Although not shown in Figure 5, the anchor base station 5b may also send appropriate X2 paging signalling to its neighbour base station(s) 5 in order to attempt to reach the mobile device 3 via neighbour cells. Effectively, the RAN based paging indicates to the mobile device 3 that it needs to resume its RRC connection (re-connect) via the anchor base station 5b or another appropriate base station for receiving the downlink data.
[0076]
At the start of the paging, the anchor base station 5b starts an appropriate timer (e.g. a paging timer) and stores the downlink data from the MME 9 (or S-GW 10) in its local cache (memory 59).
[0077]
In this example, the mobile device 3 is unable to receive (or respond to) the paging signalling. This might happen, for example, when the anchor base station 5b does not perform paging via its neighbours (even though Cell #2 may belong to the currently configured RAN paging area), when Cell #2 does not belong to the paging area configured for the mobile device 3, or when signal conditions at the edge of Cell #2 prevent the mobile device 3 from successfully receiving (or responding to) paging signalling in that cell.
[0078]
As generally shown in step S504, if the anchor base station 5b does not receive any response from the mobile device 3 within a predefined time interval (e.g. at expiry of the paging timer), the base station 5b proceeds to requesting help from the MME 9.
[0079]
Specifically, the anchor base station 5b generates and sends, in step S505, and appropriately formatted S1 signalling message (such as a ‘UE Context Release Request’ and/or the like) requesting the MME 9 to initiate S1 based (legacy) paging procedures. As shown in Figure 5, the message from the base station 5b may also indicate (or it may be interpreted such) that the MME 9 does not need to trigger a release of the mobile device’s 3 bearer via the S-GW 10.
[0080]
However, before the MME 9 can proceed to assist the base station 5b (e.g. by initiating a network-wide paging procedure), the MME 9 needs to align the UE state from connected to idle (ECM-CONNECTED to ECM-IDLE). Therefore, upon receipt of the message at S505, the MME 9 (using its UE state/location registration module 85) moves the S1 connection for the mobile device 3 into idle (e.g. ECM-IDLE) and then it proceeds to initiating legacy paging procedures in the last know tracking area of the mobile device 3 in step S506. Therefore, the MME 9 generates (using its S1AP module 88) and sends, in step S507, and appropriately formatted paging request to the base stations 5 operating cells that belong to the last know tracking area (including the new base station 5c).
[0081]
As shown, the MME’s paging request triggers the new base station 5c to carry out, in step S508, appropriate paging for the mobile device 3 over the Uu interface (in Cell #2). However, as shown in Figure 5, the MME 9 and the S-GW 10 do not carry out a ‘Release Access Bearer’ procedure, i.e. the MME 9 maintains the existing S1 bearer for the mobile device 3 via the S-GW 10 despite having received a UE Context Release Request from the base station 5a.
[0082]
After triggering the S1 based paging, the MME 9 waits for a response from the mobile device 3. In this example, as generally shown in step S509, the mobile device 3 responds to the Uu paging request by the new base station 5c by performing appropriate RRC and S1 connection setup procedures (via Cell #2). Typically, step S509 involves the mobile device 3 (using its NAS module 49) generating and sending an appropriately formatted handover request to the MME 9 (via the new base station 5c). Using its UE state/location registration module 85, the MME 9 registers Cell #2 as the mobile device’s 3 new location. As part of step S509, the MME 9 may also indicate (for example, in an appropriately formatted ‘Initial Context Setup request’ and/or the like) to the new base station 5c that data forwarding from the anchor base station 5b to the new base station 5c is imminent so that the new base station 5c is able to prepare appropriate resources for receiving the forwarded data.
[0083]
Beneficially, the MME 9 may be configured to monitor (preceding) handovers performed by the mobile device 3 and obtain (hold in memory 79) context information and other parameters relating to the mobile device 3. Therefore, the MME’s 9 message to the new base station 5c may also include one or more of the following information held by the MME 9: an appropriate identifier associated with the mobile device 3 (e.g. a Resume Id) that the new base station 5c may use for context fetching; information identifying the mobile device 3 at the MME 9 over the S1 interface (e.g. ‘MME UE S1AP ID’); information identifying the anchor base station 5b (e.g. an ‘anchor eNB ID’); a TAC of the anchor base station 5b; a Globally Unique MME ID (GUMMEI) associated with the MME 9 currently serving the mobile device 3, a handover restriction list (e.g. a list of cell identifier for cells to which a handover of the mobile device 3 is not allowed), and other parameter found in previously handled handover requests relating to the mobile device 3.
[0084]
Similarly, the MME 9 is able to provide information relevant to the anchor base station 5b in order to perform data forwarding. Thus, in step S510, the MME 9 requests the anchor base station 5b to perform data forwarding by generating and sending an appropriate S1 signalling message to the anchor base station 5b (e.g. a ‘UE Context Release Command’ and/or the like). In this example, the MME 9 sends the UE Context Release Command to the anchor base station 5b and includes information identifying the new base station 5c (e.g. an eNB ID / gNB ID) and information identifying an appropriate forwarding address for the new base station 5c (e.g. a tunnel endpoint identifier (TEID) and/or the like). The new base station 5c is thus able to trigger an appropriate procedure (e.g. by generating and sending a ‘Retrieve UE Context Request’ to the anchor base station 5b) in order to fetch the context relating to the mobile device 3 from the anchor base station 5b. Although not shown in detail in Figure 5, the anchor base station 5b generates and transmits an appropriately formatted ‘Retrieve UE Context Response’ to the new base station 5c including the requested UE context.
[0085]
After a successful X2-based (or S1-based) context retrieval, the base stations 5 proceed to SN Status transfer and data forwarding. Specifically, the anchor base station 5b (using its X2 module 67) generates and sends, in step S511, and appropriate ‘SN Status transfer’ message in order to transfer the status of the transceiver (uplink receiver status / downlink transmitter status) relating to the mobile device 3. It will be appreciated that the status of the transceiver may include respective Packet Data Convergence Protocol (PDCP) sequence numbers (SNs) used in the uplink and downlink direction which allow the new base station 5c to preserve the status following the mobile device 3 resuming its connection with the network via Cell #2.
[0086]
In step S512, the anchor base station 5b (using its X2 module 67) starts forwarding, to the new base station 5c using the forwarding address provided by the MME 9, any cached downlink data, and the new base station 5c relays the forwarded data (not shown in Figure 5) to the mobile device 3 via the Uu interface.
[0087]
As generally shown in step S513, the MME 9 and the new base station 5c also initiates an appropriate path switch procedure (comprising a ‘Path Switch Request’ including information identifying the path to be switched (e.g. in an ‘MME UE S1AP ID’ information element) and an associated acknowledgement). In response to the path switch procedure the MME 9 also requests, in step S514, the S-GW 10 to modify the bearer associated with the mobile device 3 (i.e. to tunnel data to the new base station 5c instead of the anchor base station 5b). Once the bearer has been modified, there is no need for the anchor base station 5b to forward downlink data to the new base station 5c.
[0088]
After the path switch triggered by new base station 5c (in step S513), the new base station 5c is able to trigger UE context release towards the old (anchor) base station 5b. In more detail, after switching the path (S1 connection) associated with the mobile device 3 from the anchor base station 5b to the new base station 5c, the new base station 5c generates and sends, in step S515, a UE Context release message indicating to the anchor base station 5b that the anchor base station 5b no longer needs to store the UE context associated with the mobile device 3. Effectively, this message prompts the anchor base station 5b to delete the UE context associated with the mobile device 3 (e.g. after transferring the UE context to the new base station 5c) and thus the new base station 5c becomes the new anchor base station for the mobile device 3. In step S516, the former anchor base station 5b confirms that the UE context release has been completed by generating and sending an appropriate S1 acknowledgement to the MME’s 9 command.
[0089]
Operation - second example
Figure 6 is a timing diagram (message sequence chart) illustrating another example process performed by components of the network 1 when performing a paging operation for a mobile device (UE or IoT device) operating in LC state. In this example, which is a modification of the example described with reference to Figure 5, the new base station 5c is a base station configured in accordance with pre-Rel-14 standards (and hence it is not capable of receiving forwarded data from the anchor base station 5b). Therefore, in this example, the anchor base station 5b is configured to forward any cached data to the S-GW 10.
[0090]
Steps S601a to S608 correspond to steps S501a to S508 described above hence their description is omitted herein for brevity.