SIP - Enabled Framework for Multi-Domain Roaming Control Plane in a
WiMAX Access Network
RELATED APPLICATION
This application claims the benefit under 35 U.S.C. 119(e) of U.S. Provisional Patent Application Serial No. 60/931,355 filed on May 23,2007 which is hereby incorporated by reference in its entirety.
FIELD OF THE INVENTION
The present invention relates generally to the field of network access, and particularly to mobility and roaming in wireless networks.
BACKGROUND OF THE INVENTION
In the past, mobile devices such as cell phones were limited to voice service and rudimentary data services. But mobile devices are quickly evolving into full fledged multi-media devices capable of voice service, e-mail service, text messaging, and even video-on-demand. New broadband network access services must be employed to provide the bandwidth and service quality required by these rapidly evolving services.
One such broadband network access service is referred to as '*WiMAX". WiMAX is defined as Worldwide Interoperability for Microwave Access by the WiMAX Forum, formed in June 2001 to promote conformance and interoperability of the IEEE 802.16 standard, officially known as WirelessMAN.
The WiMAX Forum NWG (network working group) has defined a multi-domain mobility architecture including many functional entities (FEs) to support full mobility and multi-domain roaming within a WiMAX network. But the WiMAX NWG has not yet defined a solution for implementing the communications between FEs that must occur to implement full mobility. One current proposal suggests defining relay, routing, and session management at the FE functional layer. This means IP routing infrastructure and protocols must be reinvented at the application layer, so this is not a practical solution. Another proposal suggests implementing IP

multicasting in all nodes. But the gateway and base station portions of an access service node in a WiMAX network are peripheral devices that do not typically implement IP transport functionality such as IP multicast. In order to require such would lead to significantly increased cost for these devices.
What is needed is a communication protocol for carrying WiMAX control plane traffic, which preferably provides a low-cost, scalable, interoper^le and standard-compliant solution.
SUMMARY OF THE INVENTION
In accordance with the principles of the invention, middleware is provided as a control plane for WiMAX control messaging. Accordingly, A WiMAX system includes a plurality of Access Service Networks (ASNs). Each ASN associated with a Session Initiation Protocol (SIP) server. A plurality of Functional Entities (FEs) are distributed across the ASNs, wherein each FE is associated with a WiMAX control function. Each FE is also associated with a SIP agent. Each FE is operable to control a function associated vvith a subscriber station (SS). The SS has a fiinction controlled by a first FE. Logic for the transferring the SS function controlled by the first FE to a second FE includes: logic cn:q)loyed by the SIP server associated with the ASN associated with the first FE, and by the SIP server associated with the ASN associated with the second FE, and by the SIP agents in the first and second FEs for establishing a SIP session between the first and second FEs; and, logic for exdianging WiMAX control mess^es over the established session to transfer the SS function from the first FE to the second FE.
Furthermore, the invention is particularly applicable where the SS is a mobile SS. The first and second FEs may be in the same ASN - i.e. the SS mobility is intra-domain - or, the first and second FEs may be in different ASNs - i.e. the SS mobility is inter-domain.
BRIEF DESCRIPTION OF THE DRAWINGS
' In order to facilitate a fuller understanding of the present invention, reference
is now made to the appended drawings. These drawings should not be construed as limiting the present invention, but are intended to be exemplary only.

Figure 1 is a representation of a multi-domain WiMAX network in which the invention can be employed.
Figure 2 is a reference model of a WiMAX network as defined by the WiMAX NWG.
Figure 3 shows two ASN profiles as defined by the WiMAX NWG.
Figure 4 is a representation of a multi-domain WiMAX network incorporating the invention.
Figure 5 is a flow diagram of the invention.
Figure 6 is a representation of a first embodiment of the invention as embothed inter-domain and intra-domain, wherein an MS roams between BS.
Figure 7 is a second embodiment of the invention, wherein an MS is paged.
Figure 8 is a working flow diagram of the embodiment of Figure 7.
DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS
In Figure 1 there is shown an exemplary WiMAX network 10 in which the invention can be implemented. The network includes a master access services network (ASN) gateway 12, which could also be a customer service network (CSN), coupled to multiple access services networks (ASNs) 14,16,18, also referred to as "domains". Each ASN 14,16,18 includes an ASN gateway 20,22,24. Multiple base stations (BS) 26a..n, 28a..n, 30a..n are coupled to the respective ASN gateways 20, 22,24. A mobile station (MS) 32, such as a cell phone in a car, or a laptop traveling on a train, etc., is coupled to a base station on an ASN, for example the base station 26a on the ASN 14. The base station 26a maintains the radio connectivity and data exchange operations between the MS 32 and the ASN 14. As the MS 32 moves, a hand-off of radio control operations may occur between the BS 26a and another BS 26 in the ASN 14. This may be a "fast handover" between BS, as shown by line R8, or it may be a handover involving the ASN gateway 20 via network R6. Depending on how long, or in what direction, the MS continues to move, a handofif may need to occur fi-om a BS in one ASN to a BS in another ASN. For instance, a handover from the BS 26a to the BS 28a may occur involving the ASN gateway 20 and 22 over network R4. Also, a handoff may need to occur from a BS in one ASN to a BS in another ASN involving the master ASN gateway 12. For instance, a handover from

the BS 26a to the BS 30a may occur involving the ASN gateway 20 and the master ASN gateway 12 over to the ASN gateway 24 to the BS 30a over network R3.
The WiMAX network described above is a "flat" network. That is, the network is non-hierarchical and all network entities are IP entities. But as can be seen, the control plane in the WiMAX network must be dynamically adjustable in order to acconunodate mobile stations. That is, when the MS 32 moves from the ASN 14 domain to the ASN 18 domain, certain control functions must move with it. WiMAX defines control messages to be exchanged between the various control entities in the network to support continuous data connectivity for the MS 32, but WiMAX does not define the manner in which these control messages should be transferred between these entities. In accordance with the invention, there is provided middleware for establishing a control plane for the exchange of these control messages.
In Figure 2 there is shown a reference model of a WiMAX network 100 as defined by the WiMAX N WG. A WiMAX network 100 includes a NAP 102 having ASNs 104a.. 104n interfaced via an "R4" defined network interface. Each ASN 104a..n includes BS 106a..n coupled via defined network interface "R8". The BS are coupled to an ASN gateway 108 via defined network interface "R6". Each ASN 104 is coupled to a home network service point (NSP) 110 or a visited NSP 110. Each NSP includes a customer service network (CSN) 112. ASNs 104 are coupled to CSNs 112 via defined network interface "R3". CSNs 112 are coupled to each other via defined network interface "R5". CSNs may be further coupled to the Internet 114.
When the mobile station moves, it may move via a "fast hand-over" between BS 106 within a single ASN 104 via network interface "R8". Or, it may move "roam" between BS in a single ASN 104 via a radio control anchorpoint in an ASN gateway 108 via network interface "R6". Or, it may move via different ASNs 104, but still in the same CSN i 12, via network interfeces "R4, R6". ". Or, it may move via different ASNs 104, via the CSN 112, via network mterfaces "R3, R4, R6". Or, it may move to a different BS 106 on a different ASN on a different CSN, via network interfaces "R5, R3, R4,R6". The invention described herein operates on network interfaces R3, R4, R5, and R6.
Also shown in Figure 2 are "Functional Entities" (FEs) 150. WiMAX FEs 150 are distributed amongst the various network elements (e.g. BS, ASN, ASN

gateway, CSN) based on their role assignment (controller / agent, anchor / peer, serving / target) and location (intra-domain or inter-domain). Examples of FEs include security FE 150a, paging FE 150b, radio resource FE 150c, MIP FE 150d, Name/Address FE 150e, Service Flow FE 150f, Location FE 150g, Handover FE 150h, policy/QoS FE 150i, AAA FE 150j, and Context FE 150k.
Referring now to Figure 3, there are shown ASN profiles as defined by the WiMAX specification. Profile A (200) and Profile C (202). The Profile C ASN 202 includes an ASN gateway 204 and three BS 206a, 206b, 206c. In Profile C, radio resource management is distributed. Each BS a, b, c includes all the FEs of a radio resource controller 208 a, b, c and agent 210 a, b, c respectively. This results in complexity of implementation, and thus expense, for the BS. But &st handovers can be performed between the BS when an MS roams between them. However, when an MS needs to roam to a BS attached to another ASN, the handover will involve the gateway 204.
The Profile A ASN 200 includes an ASN gateway 220 and three BS 222a, 222b. 222c. The .\SN gateway 220 includes RRC (radio resource control) Anchor FE 224, PG (paging control) Anchor 226, and HO (handover) Anchor 228. The BS 222a includes a corresponding RRC agent 230a, a PG agent 232a, and an HO agent 234a. Likewise, the BS 222b includes a corresponding RRC agent 230b, a PG agent 232b, and an HO agent 234b. The BS 222c includes a corresponding RRC agent 230c, a PG agent 232c, and an HO agent 234c. This arrangement makes the BS easier and cheaper to implement and is adapted more easily to a multi-vendor environment In this arrangement, when an MS roams between a BS in the ASN 200, it must do so via the ASN anchor gateway 220.
In the Profile A case, as a mobile station moves between intra-domain BS, control functions need to move with the mobile station to dififerent FEs in the same ASN. In the Profile C case, as a mobile station moves between BS inter-domain, control fiinctions need to move with the mobile station to different FEs in different ASN. In either case, the target FEs are non-deterministk: due to the randomness of the mobile station's movement, and the movement may occur across multiple domains (i.e. ASNs). Thus, in accordance with the invention, there is provided a communication mechanism between FEs over which WiMAX control messages may be exchanged. It functions between inter- and Intra-domain FEs. It is able to

dynamically create sessions between FEs in order to respond to the non-deterministic motion of mobile stations. It provides multicast capability for paging, handover, and radio resource management, since multiple BS must be notified of the possible need to support an incoming mobile station. In accordance with advantages of the invention, the commimication solution is a middleware solution that supports auto discovery, session management, and call relay. The middleware is scalable, standard-compliant, and infrastructure transparent.
Referring to Figure 4, there is shown an embodiment of the invention. A Light-weight SIP protocol is used for session communication between FEs. Depicted are two ASNs, ASN 300 and ASN 350. ASN 300 includes a Sff server 302 and three BS, BS 304, 306, and 308. The BS 304 includes an FE or group of FEs, herein shown as an RRC, with a SIP agent (also known as a SIP client) 310. The BS 306 likewise includes an RRC with a SIP agent 312, and the BS 308 includes an RRC with a SIP agent 314. The SIP agents 310,312,314 communicate with the SIP server 302.
Likewise, ASN 350 includes a SIP server 352 and three BS, BS 354,356, and 358. The BS 354 includes an FE or group of FEs, herein shown as an RRC, with a SIP agent 360. The BS 356 likewise mcludes an RRC with a SIP agent 362, and the BS 358 includes an RRC with a SIP agent 364. The SIP agents 360,362,364 communicate with the SIP server 352.
Interdomain, within ASN, SIP agents 310,312, 314 register with the SIP server 302, or an auto-discovery algorithm is used by the SIP server 302 to identify agents coupled to it. Within ASN 350, SIP agents 360,362, and 364 register with the SIP server 352 in the same manner. Intra-domain, ASN SIP servers 302 and 352 can set up and tear down SIP sessions between themselves.
Now an FE in one domain can exchange WiMAX control messages with an FE in another domain, for instance in anticipation of a roaming event. For example, the RRC 310 in the BS 304 in the ASN 300 can send WiMAX control mrasages to the RRC 364 in the BS 358 in the ASN 350, as shown via the heavy line from BS 304 to BS 358, by sending WiMAX control messages over aTCP/UDP connection between the IP RRCs. The WiMAX control messages are sent via a SIP session that has been set up between SIP agent 310, SIP server 302, SIP server 352, and SIP agent 358. When the RRCs 310 and 364 communication is complete, the SIP session can be torn down.

Generally, SIP is used to manage unicast and multicast sessions between FEs in order to provide a "control plane for the control plane". SIP sessions are set up and torn down between FEs as a mobile station moves. The SIP sessions serve as the control plane over which the FEs send their WiMAX control messages. Thus, control between FEs can be adjusted as the mobile station moves. Intra-domain, in a Profile C type or in a Profile A type ASN wherein Anchor FEs are used, control can be transferred firom one FE in a BS to another FE in a BS over a TCPAJDP connection between the FEs over an R6 network interfece. WiMAX control messages are relayed via SIP sessions maintained between the FEs and the SIP server. Inter-domain, an FE in one domain (ASN) can exchange WiMAX control messages with an FE in another domain (As shown in Fig. 4 via line from BS 302 to BS 358) by sending the control messages over a TCPAJDP connection over an R4 or R3 interface.
Light Weight SIP is a convenient protocol for use in that it is a standard compliant protocol; thus it offers ease of mteroperability. SIP is implemented as middleware; thus, it is ASN infiastnicture transparent. That is, the SIP sessions are agnostic to the underlying ASN access technology, be it IP, Ethernet, MPLS, etc.
In particular, in order to implement the mvention, each FE (or FE group) is built with an embedded SIP agent (Fig. 4, RRCs with SIP agents 310,312,314,360, 362, 364). A SIP proxy server is installed for each ASN control plane - i.e. for each domain. "Lightweight" SIP - that supporting text only, without support for multimedia streams - is sufficient for use for purposes of the invention. It will be helpful to extend SDP (IETF RFC 3312/3313) to include bandwidth, tunnel-end address, and tunnel ID for tunnel management (e.g., IP GRE, E-VLAN, MPLS PW). The FE (or FE group) registers to the SIP server with a URL address and an IP address. As shown in Figure 4, each SIP agent has an API associated with it. APIs 370, 372, 374, 375,377, and 379 are associated with SIP ^ents 310, 312,314,360, 362, 364 respectively. The APIs are provided by the SIP agents and used by FEs to communicate with each other. Each FE calls its SIP agent via its SIP agent's respective API. In turn, the SIP agents use a SIP stack to create, change, and maintain unicast or multicast point-to-point sessions dynamically. SIP multi-party sessions are used for multicasting control traffic to support anchoring functions such as paging and RRM spare capacity request. The session can be pre-configured or set up on demand. All WiMAX control messages can be implemented in the known manner using

HTTP, Servlet, CGI, or SOAP over the sessions, and can be transmitted over TCP/UDP ports.
In Figure 5 there is shown a flow diagram of the steps of the invention. It may be determined that F£s need to exchange WiMAX control messages - because an MS is moving and radio resources must be adjusted, or because an MS has moved while it was sleepmg and must be located for data forwarding, etc. When control messages need to be exchanged between FEs, (step 380), the SIP servers and agents in the ASNs associated with the FEs establish SIP sessions (stq> 382). In some cases, these SIP sessions may aheady be established. In others, they may need to be established on demand. Once a SIP session is established between the FEs, the WiMAX control messages are exchanged via TCP or UDP messaging over IP (step 384). Once the message exchange is complete, the session may be torn down (step 386).
Referring to Figure 6, there is shown an example WiMAX network wherein SIP sessions are set up between FEs for WiMAX control message exchange. An ASN 400 and ASN 450 are shown. ASN 400 includes gateway (GW) 402 and BS 404, 406, and 408. BS 404 includes radio resource management agents and controllers 410,412. Likewise, BS 406 includes radio resource management agents and controllers 414,416, and BS 408 includes radio resource management agents and controllers 418,420. The GW includes a radio resource control anchor 422, designated as anchor for inter-domain mobility. ASN 400 further includes SIP server 424. All of the FEs in the ASN 400, including the RRCs 416,418, and 422, have registered their URLs and IP names with the SIP server 424.
ASN 450 is configured similarly to ASN 400. ASN 450 includes gateway (GW) 452 and BS 454,456, and 458. BS 454 includes radio resource management agents and controllers 460,462. Likewise, BS 456 includes radio resource management agents and controllers 464,466, and BS 458 mcludes radio resource management agents and controllers 468,470. The GW includes a radio resource control anchor 472, designated as anchor for inter-domain mobility. ASN 450 fiirther includes SIP server 474. All of the FEs in the ASN 450, including the RRCs 466, 468, and 472, have registered their URLs and IP names with the SIP server 474.
An MS 480 is shown as its network connectivity moves from BS 406 to BS 408 in ASN 400, and then over to BS 454 in ASN 450. When the MS moves from BS 406 to BS 408, or when such a move is indicated or anticipated, a SIP session request

is sent from RRC 416 to SIP server 424 via link 482, SIP server 424 then forwards it to RRC anchor 422 via link 488, thus a session is established between the RRC FE 416 in the BS 406 and the RRC anchor 422 in the ASN GW 402. Accordingly, a second SIP session 484-is established between the RRC anchor 422 and the RRC FE 420 in the BS 408 via link 488 and 484, and a third SIP session is established between RRC 416 and RRC 420 via link 482 and 484. Once these SIP sessions are established among the various RRCs, th^ can exchange control messages for radio resource management. For example, after the SIP session between the RRC 416 and RRC 420 is established, WiMAX TCPAJDP control messages 486 are sent from the RRC 416 to the RRC 420, with the coordination from the RRC anchor 422 via the network interface R6 to ultimately switch radio resource control for the MS 480 to the BS 408.
When the MS moves from the BS 408 to the BS 454. WiMAX control messages are sent to transfer radio resource control from the RRC 420 in the BS 408 to the RRC 462 in the BS 454. In order to establish a control path for these messages, a SIP session 496 is established (if not already maintained) between the RRC FE 420 in the BS 408 located at ASN 400 and RRC 462 in the BS 454 located at ASN 450. The SIP signaling procedures involve RRC 420 as source entity, RRC 462 as destination entity, and SIP server 424 in ASN 400 and SIP server 474 in ASN 450 to relay the SIP messages, via the link 484 in ASN 400, the link 494 between SIP server , 424 and 474, and the link 490 in ASN 450. In this manner, a control plane is established and maintained so that WiMAX control messages over session 496 can be sent via TCP/UDP from the RRC 420 in the BS 408 in the ASN 400 to the RRC 462 in the BS 454 in the ASN 450 with the coordination of the RRC anchors 422,472 via the network interfiice R4.
Meanwhile, as the MS 480 moves across domains, the RRC anchor 422 sends WiMAX control messages containing MS 480 information to the neighboring ASN 450, and sends WiMAX control messages inquiring of the neighboring ASN 450's radio channel availability via the R4 network interface. These WiMAX control messages are sent via the SIP session that is maintained between the RRC anchor 422 and the RRC anchor 472 in the GW 452. One such message may be an "RRM spare capacity request" message. This is a multicast message that addresses many BS to see which BS are available to service an incoming MS. In order to transport this message, a multi-party SIP session is set up by the SIP server 474 in the ASN 450

among all the RRCs 472,462,466,470 so that the RRC anchor 472 can send the RRM spare capacity request multicast message to all the RRCs 462,466, and 470 in the respective BS 454,456, and 458.
Referring now to Figure 7, there is shown another WiMAX network example wherein WiMAX control messages are exchanged between FEs via SIP sessions. In this example, data needs to be sent from a home network (not shown) to a sleeping mobile subscriber station. Again, in a WiMAX network, the location of a mobile subscriber station is non-deterministic because the station can move between BS, ASNs, and/or CSNs while asleep. When data arrives for a sleeping MS, WiMAX buffers the data and uses a paging mechanism to discover the location of the sleeping MS.
In Figure 7 there is shown a master ASN GW 502 coupled via R3 network interfaces to multiple ASNs 504,506, and 508, each including ASN GW 510, 512, and 514 respectively. The ASN GWs 510,512, and 514 are peers coupled via R4 network interfaces. Each ASN GW 510,512, and 514 is coupled via R6 network interfaces to a set of BS. ASN GW 510 is coupled to BS 516, 518. ASN GW 512 is coupled to BS 520, 522. ASN GW is coupled to BS 524, 526. BS 520 is shown coupled to multiple MS 528a, b, c. Likewise, BS 522 is shown coupled to multiple MS 530 a, b, c.
The master ASN GW 502 is shown to include a SIP server 534. The ASN GW 510 includes a SIP server 536. The ASN GW 512 includes a SIP server 538. The ASN GW 540 includes a SIP server 540. Each BS is also shown to include an FE having a SIP agent. Each BS may include more than one FE/SIP agent, one being shown here for clarity of description. The BS 516 includes SIP agent 542. TheBS 518 includes SIP agent 544. The BS 520 includes SIP agent 546. The BS 522 includes SIP agent 548. The BS 524 includes SIP agent 550. The BS 526 includes SIP agent 552.
The SIP agents 542 and 544 have registered a URL and IP name with the SIP server 536 in the ASN GW 510. Likewise, the SIP agents 546 wid 548 have registered a URL and IP name with the SIP server 538 in the ASN GW 512, and the SIP agents 550 and 552 have registered a URL and IP name with the SIP server 540 in the ASN GW 514. In turn, each ASN SIP server 510,512, and 514 has registered its URL and IP name with the SIP server 534 in the Anchor ASN 502. SIP sessions

can now be set up and torn down between any client server pair in order that WiMAX control messages can be passed between FEs.
In Figure 7, there is also shown DP/FA controller 568 and anchor PC/LR controller in Master ASN GW 502; PC controller 570, 554 and 572 in ASN 510, ASN 512, and ASN 514; and PC agents 574,576,578 and 580 in BS 516,518,520 and 522. In Figure 7, DP/FA controller 568 is for payload data management and forwarding, and all PC controllers and agents are for paging management.
Now, referring to Figures 8 and 7, the WiMAX control message flow is shown. Downlink data is received from a home agent (label "1"), destined for the MS 530b, while the MS 530b is asleep. The anchor datapath FE 568 in the master ASN GW 502 receives and buffers the data, recognizes that the MS 530b is in Idle mode, and then sends an MS info request to the Paging controller FE 556 to locate the MS (label "2"). The paging controller FE 556 in the ASN GW 502 confirms the request indicating that the MS is authorized for service (label "3"). Paging controller 556 figures out which paging group the MS 530b is in (in this example the paging group may include ASN 504 and ASN 506), and then retrieves the MS paging info (comprising PGID, paging cycle, paging offeet, a relay PCID, or a set of BSIDs including the last reported one) and constructs a Paging Announce message. The paging controller FE 556 in the ASN GW 502 then sends a multicast paging message to all the paging controllers in both ASN 504 and ASN 506 based on its knowledge of the topology of the Paging region. For the received paging announce message, in turn, paging controller 570 in ASN GW 510 and paging controller 554 further multicast the message to all the BS (paging agent 574,576,578 and 580 at BS 516, 518, 520, 522) in the paging group (label "4"). The MS is then notified by the BS to which it is attached that there is data waiting for it (label "5").
In accordance with the invention, in order for the datapath FE 568 to send the MS info Request to the paging controller FE 556, a SIP session is established between the FEs 554 and 556.
Once the session setup is complete, the MS mfo Request message is relayed via TCP/UDP accordance with the invention from the data path FE 568 to the paging controller FE 556 in the master ASN GW 502 (Fig. 8 label "2" and label "3") indicating which paging group the MS is in and if the MS is authorized.

As well, to enable master anchorPC controller 556 to send multicasting paging announcement messages to all the PC agents in the paging group, multi-party sessions need to be established from master anchor PC controller to all relay PC controllers such as PC FE 570 and PC FE 554. In turn, PC FE 570 and 554 will also need to establish the multi-party session to all the PC agents in each BS.
In order for the Paging controller FE 556 in the ASN GW 502 to send the multicast paging announcement message to all BS included in the paging group that includes the MS, a muhi-party SIP session is established among the master paging controller 556 and relay PC controllers 570 and 554 via SIP server 534,536 and 538. In turn, PC FE 570 and 554 will also need to establish the multi-party session to all the PC agents in each sub-pagjng-group such as 574,576,578 and 580 in the BS 516, 518, 520 and 522 via SIP server 536 and 538, respectively (Fig. 7 564 and Fig 7 566).
After all the sessions are established (if they did not exist before), the paging controller FE 556 in the ASN GW 502 can then send a multicast TCP paging message to all the SIP agents (546,548) in the paging group (label "4"). The BS 522 to which the targeted MS is attached can then broadcast page messages to its attached MS 530 a,b,c.
The present invention is not to be limited in scope by the specific embodiments described herein. Various modifications of the present mvention, in addition to those described herein, will be apparent to those of ordinary skill in the art from the foregoing description and accompanying drawings. One skilled in the art will understand that many specific implementations can be employed to achieve the logical functionality of the invention. All such modifications are intended to fall within the scope of the invention. Further, although aspects of the present invention have been described herein in the context of a particular implementation in a particular environment for a particular purpose, those of ordinary skill in the art will recognize that its usefulness is not limited thereto and that the present invention can be beneficially implemented in any number of environments for any number of purposes.

CLAIMS
1. A broadband network access system comprising:
one or more Access Service Networks (ASNs), each ASN associated with a Session Initiation Protocol (SIP) server;
a plurality of Functional Entities (FEs) distributed across the ASNs, wherein each FE is associated with a broadband network access system control function, each FE operable to control a function associated with a subscriber station (SS);
a plurality of SIP agents, each SIP agent associated with an FE or a group of FEs;
a SS having a function controlled by a first FE;
logic for transferring the SS function controlled by the first FE to a second FE comprising:
logic employed by the SIP server associated with the ASN associated
with the first FE, and by the SIP server associated with the ASN associated
with the second FE, and by the SIP agents associated with the first and second
FEs for establishing a SIP session between the first and second FEs; logic for exchanging broadband network access system control
messages over the established session to transfer the SS function from the first
FE to the second FE.
2. The broadband network access system of claim 1 wherein each SIP agent is
associated with an API, and wherein the logic for establishing a SIP session between
the first and second FEs employs the APIs in each of the SIP agents associated with
the first and second FEs to establish the SIP session, and wherem the logic for
exchanging broadband network access system control messages enq)loys the APIs in
each of the SIP agents associated with the first and second FEs to transfer SS fiinction
fi-om the first FE to the second FE.
3. The broadband network access system of claim I wherein the SS is a mobile
SS, and wherein the first and second FEs are in the same ASN, and wherein the SIP
server associated with the ASN associated with the first FE is the same as the SIP
server associated with the ASN associated with the second FE.

4. The broadband network access system of claim 1 wherein the SS is a mobile SS, and wherein the first and second FEs are in different ASNs, and wherein the SIP server associated with the ASN associated with the first FE is different from the SIP server associated with the ASN associated with the second FE.
5. The broadband network access system of claim 1 wherein the first and second FEs are radio resource controllers.
6. The broadband network access system of claun 1 wherein the first and second FEs are handover controllers.
7. The broadband network access system of claim 1 wherein the first and second FEs are paging controllers.
8. The broadband network access system of claim 1 wherein the first and second FEs are context exchange controllers.
9. The broadband network access system of claim 1 wherein the first and second FEs are security controllers.
10. The broadband network access system of claim 1 wherein the first and second FEs are service flow and data forwarding controllers.
11. The broadband network access system of claim 1 wherein the logic for establishing a SP session between the first and second FEs comprises logic for establishing a point-to-point SIP session between the first and second FEs.
12. The broadband network access system of claim 11 wherein the logic for exchanging broadband netwoik access system control messages over the established session to transfer SS function from the first FE to the second FE exchanges control messages over a session established between the first FE and the second FE in a first ASN.

13. The broadband network access system of claim 11 wherein the logic for exchanging broadband network access system control messages over the established session to transfer SS function from the first FE to the second F£ exchanges control messages over a session established between the first FE in a first ASN and the second FE in a second ASN.
14. The broadband network access system of claim 1 wherein the logic for establishing a SIP session between the first and second FEs comprises logic for establishing a multiparty SIP session between an ASN and a group of FEs including the second FE.
15. The broadband network access system of claim 14 wherein the logic for exchanging broadband network access system control messages over the established session to transfer SS function from the first FE to the second FE exchanges control messages over a session established between the first FE in a first ASN and exchanges multicast control messages over a multiparty SIP session between a second ASN and a group of FEs including the second FE.
16. A program product comprising a computer readable medium having embothed therein a computer program for storing data, the computer program operable in a broadband network access system comprising one or more Access Service Networks (ASNs) having a plurality of Functional Entities (FEs) distributed thereacross, wherein each FE is associated with a broadband network access system control function associated with subscriber stations (SS), the computer program comprising:
logic for the transferring an SS function controlled by a first FE to a second FE comprising:
logic employed by a SIP server associated with the ASN associated with the first FE, and by a SIP server associated with the ASN associated with the second FE, and by SIP agents associated with the first and second FEs, for establishing a SIP session between the fu^t and second FEs;
logic for exchanging broadband network access system control messages over the established SIP session to transfer the SS function from the first FE to the second FE.
17. The program product of claim 16 wherein each SIP agent is associated with an
API, and wherein the logic for establishing a SIP session between the first and second
FEs employs the APIs in each of the SIP agents associated with the first and second

FEs to establish the SIP session, and wherein the logic for exchanging broadband network access system control messages enploys the APIs in each of the SIP agents associated with the first and second FEs to transfer SS function from the first FE to the second FE.
18. The program product of claim 16 wherein the SS function is associated with a mobile SS, and wherein the first and second FEs are in the same ASN, and wherein the SIP server associated with the ASN associated with the first FE is the same as the SIP server associated with the ASN associated with the second FE.
19. The program product of claim 16 wherein the SS is a mobile SS, and wherein the first and second FEs are in different ASNs, and wherein the SIP server associated with the ASN associated with the first FE is different from the SIP server associated with the ASN associated with the second FE.
20. The program product of claim 16 wherein the first and second FEs are radio resource controllers.
21. The program product of claim 16 wherein the first and second FEs are handover controllers.
22. The program product of claim 16 wherein the first and second FEs are paging controllers.
23. The program product of claim 16 wherem the first and second FEs are context exchange controllers.
24. The program product of claim 16 wherein the first and second FEs are security controllers.
25. The program product of claim 16 wherein the first and second FEs are service flow and data forwarding controllers.
26. The program product of claim 16 wherein the logic for establishing a SIP session between the first and second FEs comprises logic for establishing a point-to-point SIP session between the first and second FEs.
27. The program product of claim 26 wherein the logic for exchanging broadband network access system control messages over the established session to transfer SS function from the first FE to the second FE exchanges control messages over a session established between the first FE and the second FE in a first ASN.
28. The program product of claim 26 wherein the logic for exchanging broadband network access system control messages over the established session to transfer SS

function from the first FE to the second FE exchanges control messages over a session established between the first FE in a first ASN and the second FE in a second ASN.
29. The program product of claim 16 wherein the logic for establishing a SIP session between the first and second FEs comprises logic for establishing a multiparty SIP session between an ASN and a group of FEs including the second FE.
30. The program product of claim 29 wherein the logic for exchanging broadband network access system control messages over the established session to transfer SS function from the first FE to the second FE exchanges control messages over a session established between the first FE in a first ASN and exchanges multicast control messages over a multiparty SIP session between a second ASN and a group of FEs including the second FE.
31. A method for use in a broadband network access system comprising one or more Access Service Networks (ASNs) having a plurality of Functional Entities (FEs) distributed thereacross, wherein each FE is associated with a broadband network access system control function associated with subscriber stations (SS), the method comprising the steps of:
establishing a SIP session between a first FE and a second FE, the session being established by logic employed by a SIP server associated with the ASN associated with the first FE, and by a SIP server associated with the ASN associated with the second FE, and by SIP agents associated with the first and second FEs;
exchanging broadband network access system control messages over the established SIP session to transfer an SS function controlled by the first FE to the second FE.
32. The method of claim 31 wherein each SIP agent is associated with an API, and
wherein the step of establishing a SIP session between the first and second FEs
employs the APIs in each of the SIP agents associated with the first and second FEs to
establish the SIP session, and wherein the step of exchanging broadband network
access system control messages employs the APIs in each of the SIP agents associated
with the first and second FEs to transfer SS function from the first FE to the second
FE.
33. The method of claim 31 wherein the SS function is associated with a mobile
SS. and wherein the first and second FEs are in the same ASN, and wherein the SIP

server associated with the ASN associated with the first FE is the same as the SIP server associated with the ASN associated with the second FE.
34. The method of claim 31 wherein the SS is a mobile SS, and wherein the first and second FEs are in different ASNs, and wherein the SIP server associated with the ASN associated with the first FE is different from the SIP server associated with the ASN associated with the second FE.
35. The method of claim 31 wherein the first and second FEs are radio resource controllers.
36. The method of claun 31 wherein the first and second FEs are handover controllers.
37. The method of claim 31 wherein the first and second FEs are paging controllers.
38. The method of claim 31 wherein the first and second FEs are context exchange controllers.
39. The method of claim 31 wherein the first and second FEs are security controllers.
40. The method of claim 31 wherein the first and second FEs are service flow and data forwarding controllers.
41. The method of claim 31 wherein the step of establishing a SIP session between the first and second FEs comprises establishing a point-to-point SIP session between the first and second FEs.
42. The method of claim 41 wherein the step of exchanging broadband network access system control messages over the established session to transfer SS function fix)m the first FE to the second FE comprises exchanging control messages over a session established between the first FE and the second FE in a first ASN.
43. The method of claim 41 wherein the step of exchanging broadband network access system control messages over the established session to transfer SS function from the first FE to the second FE comprises exchangmg control messages over a session established between the first FE in a first ASN and the second FE in a second ASN.
44. The method of 31 wherein the step of establishing a SIP session between the first and second FEs comprises establishing a multiparty SIP session between an ASN and a group of FEs including the second FE.

45. The method of claim 44 wherein the step of exchanging broadband network
access system control messages over the established session to transfer SS function
from the first FE to the second FE comprises exchanging control messages over a
session established between the first FE in a first ASN and exchanges multicast
control messages over a multiparty SIP session between a second ASN and a group of
FEs including the second FE.