Transitioning Of A Packet-Switched Emergency
Call Between First And Second Types Of Wireless Access Networks
Background
[0001] Various wireless access technologies have been proposed or
implemented to enable mobile stations to perform communications with other mobile
stations or with wired terminals coupled to wired networks. Examples of wireless
access technologies include GSM (Global System for Mobile communications) and
UMTS (Universal Mobile Telecommunications System) technologies, defined by the
Third Generation Partnership Project (3GPP); and CDMA 2000 (Code Division
Multiple Access 2000) technologies, defined by 3GPP2. CDMA 2000 defines one
type of packet-switched wireless access network, referred to as the HRPD (High
Rate Packet Data) wireless access network.
[0002] Another more recent standard that provides packet-switched wireless
access networks is the Long Term Evolution (LTE) standard from 3GPP, which
seeks to enhance the UMTS technology. The LTE standard is also referred to as the
EUTRA (Evolved Universal Terrestrial Radio Access) standard.
Summary
[0003] In general, according to an embodiment, a method comprises detecting
that a mobile station is involved in a packet-switched emergency call, where the
mobile station is initially attached to a first type of wireless access network. Upon
detection of transitioning of the mobile station, a message is sent indicating
transitioning of the mobile station from the first type wireless access network to the
second type wireless access network to cause the packet-switched emergency call
to be performed over the second type wireless access network.
[0004] Other or alternative features will become apparent from the following
description, from the drawings, and from the claims.

Brief Description Of The Drawings
[0005] Some embodiments of the invention are described with respect to the
following figures:
Fig. 1 is a block diagram of an example arrangement that includes different
types of wireless access networks and a mechanism to transfer a packet-switched
emergency call of a mobile station between the different types of wireless access
networks, according to some embodiments;
Figs. 2-5 are message flow diagrams of processes of performing handoff of a
mobile station involved in a packet-switched emergency call session, according to
some embodiments; and
Fig. 6 is a block diagram of example components of a mobile station and a
base station/gateway, according to some embodiments.
Detailed Description
[0006] Multi-mode mobile stations are able to communicate over various different
types of wireless access networks. For example, a multi-mode mobile station can
attach to either an LTE (Long-Term Evolution) wireless access network (as defined
by 3GPP) or an HRPD (High Rate Packet Data) wireless access network (as defined
by the 3GPP2 CDMA 2000 standard). LTE is also referred to as EUTRA (Evolved
Universal Terrestrial Radio Access). Although reference is made to the EUTRA and
HRPD wireless access technologies in this discussion, in some preferred
embodiments, it is noted that other preferred embodiments can employ other types
of wireless access technologies. For example, other types of wireless access
technologies include WiMax (Worldwide Interoperability for Microwave Access)
technology (as defined by the IEEE (Institute of Electrical and Electronic Engineers)
802.16 standard), GSM (Global System for Mobile Communications, as defined by
3GPP), and so forth.
[0007] As the multi-mode mobile station reaches a boundary between different
types of wireless access networks, a decision can be made to hand off (or hand
over) the mobile station from a first type wireless access network to a second type

wireless access network such as from an EUTRA wireless access network to an
HRPD wireless access network. In the ensuing discussion, it is noted that the terms
"hand off' and "hand over" are used interchangeably.
[0008] The multi-mode mobile station may be involved in a packet-switched
emergency call during handoff between different types of wireless access networks.
An "emergency call" can refer to a emergency voice call, a non-voice emergency
call, or any other request for or notification of emergency assistance, such as a
request in response to activation of a panic button; a notification sent to emergency
personnel in response to user request (such as merely dialing 911), where the
notification includes sufficient information (e.g., name, location, etc.) for emergency
personnel to render aid without interaction from the user; and other emergency
notifications. A "packet-switched" emergency call refers to an emergency call
established over packet-switched networks (wired and/or wireless). A mobile station
is "involved" in a packet-switched emergency call if the mobile station is either (1)
actively in the emergency call, or (2) the mobile station is trying to establish the
emergency call or otherwise requesting that the emergency call be made. A packet-
switched emergency call refers to a call session made over a packet-switched
network to an emergency services entity (such as an enhanced 911 or E911 call).
An emergency call is typically directed to an emergency contact number (such as
911 in the United States).
[0009] It is desired that when a mobile station is involved in a packet-switched
emergency call, the handoff between different types of wireless access networks
should be performed as quickly as possible so that the caller does not waste
valuable time in trying to reach emergency personnel. Conventionally, a handoff of
an emergency call may require that registration procedures be performed at the
target wireless access network, which can be time consuming.
[0010] In accordance with some preferred embodiments, to reduce the amount of
delay in handing off a mobile station between source and target wireless access
networks of different types, pre-registration of the mobile station with the target
wireless access network can be performed. Such pre-registration allows the mobile

station to more quickly acquire a traffic channel at the target wireless access network
without having to perform time-consuming registration during the handover.
Alternatively, instead of performing pre-registration, a predefined emergency call
session can be used instead, where the context (including registration) for the
emergency call session is already known to both the mobile station and the target
wireless access network, such that establishment of such context can be avoided
during the handover for the packet-switched emergency call.
[0011] Generally, according to some preferred embodiments, the procedure for
handing off a mobile station that is involved in an emergency call includes detecting
that the mobile station is to be handed off from a source wireless access network to
a target wireless access network, where the source and target wireless access
networks are of different types {e.g., the source wireless access network is the
EUTRA network and the target wireless access network is the HRPD access
network). In response to detecting that the mobile station is to be handed off, a
message is sent indicating handoff of the mobile station from the source wireless
access network to the target wireless access network to cause the packet-switched
emergency call to be performed over the target wireless access network.
[0012] More generally, the mobile station that is involved in a packet-switched
emergency call can be transitioned between the source and target wireless access
networks without a handoff being performed. For example, the mobile station may
initially be attached to the source wireless access network. However, the mobile
station can enter a region (e.g., a tunnel), where the link with the source wireless
access network is lost. Once the mobile station exits the region, the mobile station
may re-attach to the target wireless access network. In this scenario, no handoff is
performed. However, in accordance with some embodiments, the emergency call is
still transitioned from the source to target wireless access network. Thus, a mobile
station "transitioning" from a source wireless access network to a target wireless
access network can refer to the mobile station being handed off from the source
wireless access network to the target wireless access network, or otherwise moving
from the source wireless access network to the target wireless access network
(without explicit handoff).

[0013] Although embodiments are discussed in the context of handing off a
mobile station from a source wireless access network to a target wireless access
network, it is noted that techniques as discussed can also be applied when the
mobile station performs, any other type of transitioning from the source wireless
access network to the target wireless access network.
[0014] Fig. 1 illustrates an example arrangement that has a source wireless
access network 100 (e.g., EUTRA network) and a target wireless access network
102 (e.g., HRPD access network), where the source and target wireless access
networks are of different types. In the example of Fig. 1, it is assumed that a multi-
mode mobile station 104 is to be handed off (as indicated by dash arrow 106) from
the source wireless access network 100 to the target wireless access network 102.
[0015] As used here, reference to an EUTRA wireless access network refers to a
wireless access network that conforms to the requirements of the EUTRA standard
developed by 3GPP, as that standard currently exists or as the standard evolves
over time. Note that EUTRA can refer to the current EUTRA standard, or to
modifications of the EUTRA standard that are made over time. It is expected that in
the future a standard that has evolved from EUTRA may be referred to by another
name. Thus, it is contemplated that the term "EUTRA" as used here is intended to
cover such future standards as well. Similarly, reference to other standards, such as
HRPD, is also intended to refer to the current wireless access standard or to a
standard that is to be developed in the future.
[0016] In the source wireless access network 100, a base station 108 is provided
to perform wireless communication with the mobile station 104. In the EUTRA
context, the base station 108 is referred to as an enhanced node B ("eNode B"). The
target access network 102 also includes a base station 110, which conforms to the
HRPD standard, for example. The base station 108 or 110 is able to perform one or
more of the following tasks: radio resource management, mobility management for
managing mobility of mobile stations, routing of traffic, and so forth. More generally,
the term "base station" can refer to a cellular network base station, an access point
used in any type of wireless network, or any type of wireless transmitter to

communicate with mobile stations. The term "base station" can also encompass an
associated controller, such as a base station controller or a radio network controller.
It is also contemplated that the term "base station" also refers to a femto base station
or access point, a micro base station or access point, or a pico base station or
access point. A "mobile station" can refer to a telephone handset, a portable
computer, a personal digital assistant (PDA), or an embedded device such as a
health monitor, attack alarm, and so forth.
[0017] In the EUTRA context, the base station 108 in the source wireless access
network 100 is connected to a serving and/or packet data network or gateway 112,
which terminates the user plane interface towards the enhanced node B and
assumes the responsibility for packet routing and transfer towards an external
network 114. The base station 110 of the target access network 102 is similarly
connected to a serving and/or packet data network or gateway 116, which in the
HRPD context is referred to as a packet data serving node (PDSN).
[0018] The external network 114 can include a packet-switched network (e.g.,
the Internet) and/or a circuit-switched network, such as the PSTN (public switched
telephone network). An emergency entity 118 is connected to the external network
114, where the emergency entity 118 can be a 911 call center, a police department,
a fire department, and so forth.
[0019] Fig. 2 is a message flow diagram of a process according to an
embodiment of the invention. It is assumed that the mobile station is initially
attached (at 202) with the source wireless access network 100 (in other words, the
mobile station is in a connected state with the source wireless access network 100.
In accordance with some embodiments, the mobile station 104 is also pre-registered
(at 202) with the target wireless access network 102. In this scenario, the mobile
station 104 is able to perform pre-registration with the target wireless access network
102 while the mobile station is camped on the source wireless access network 100k.
Pre-registration refers to session set up, performing binding, and any other
procedure(s) that is required for the mobile station 104 to establish an active traffic
session with the target access network. However, the mobile station 104 while

attached to the source wireless access network is not communicating traffic with the
target wireless access network even though the mobile station 104 is pre-registered
with the target wireless access network.
[0020] Pre-registration allows the mobile station 104 and target wireless access
network 102 to have sufficient information (stored in the mobile station 104 and
target wireless access network 102) about a session such that a traffic channel can
be quickly assigned to the mobile station 104 to direct switching of the emergency
call to the target wireless access network 104.
[0021] Fig. 2 also shows that the mobile station is currently active in a packet-
switched emergency call (at 204) over the source wireless access network 100. This
means that the mobile station 104 is currently in a voice call with the emergency
entity 118.
[0022] While the packet-switched emergency call is progressing, the mobile
station makes measurements regarding wireless channel conditions with respect to
wireless links between the mobile station 104 and each of the source and target
wireless access networks 100 and 102. For example, the measurements can be of
pilot channels transmitted by base stations. As the mobile station approaches a
boundary between the source and target wireless access networks, signaling with
the source wireless access network 100 may become weaker while signaling with
the target wireless access network 102 may become stronger. The mobile station .
sends (at 206) the measurements to the source wireless access network 100.
[0023] Based on the received measurements, the source wireless access
network 100 can make a handover decision (at 208) to initiate handoff of the mobile
station 104 to the target wireless access network 102. Once the source wireless
access network 100 makes the decision to perform the handoff, the source wireless
access network 100 sends a handover command (at 210) to the mobile station 104.
[0024] In response to the handover command from the source wireless access
network 100, the mobile station 104 sends (at 212) one or more messages indicating
that the handoff is to be performed. As a result of the pre-registration performed by

the mobile station 104 with respect to the target wireless access network 102, the
mobile station 104 has sufficient information to connect to the target wireless access
network 102 when handoff to the target wireless access network occurs.
[0025] The one or more messages sent by the mobile station 104 can be in the
format of the standard corresponding to the target wireless access network 102 (e.g.,
HRPD standard). For example, the one or more messages can include an HRPD
Route Update message, which indicates the potential handoff target. Another
message that can be sent by the mobile station 104 for performing the handoff is an
HRPD Connection Request for establishing a connection with the target wireless
access network 102. As shown in the example of Fig. 2, the Route Update and
Connection Request messages are sent through the source wireless access network
100 to the target wireless access network 102. For example, these messages could
be encapsulated within another message according to the standard corresponding to
the source wireless access network 100 (e.g., EUTRA standard).
[0026] To ensure that the handoff is performed as quickly as possible, an
emergency indication can be provided in the one or more messages sent (at 212) by
the mobile station 104. For example, the emergency indication can be provided as
an emergency indicator in the Connection Request message. Alternatively, the
source wireless access network 100 can provide an emergency indication as part of
interworking messages sent from the source wireless access network 100 to the
target wireless access network 102 to perform handoff of the mobile station 104.
[0027] Alternatively, the emergency indication can be included in a header of a
message according to a format of the source wireless access network 100 (e.g.,
EUTRA format). In this case, the source wireless access network 102 will add the
emergency indication to a message forwarded from the source wireless access
network 100 to the target wireless access network 102.
[0028] In response to receiving the one or more messages sent at 212, the
source wireless access network 102 sends (at 214) various response messages to
the mobile station 104 (through the source wireless access network 100). The
messages sent can include a TCA (Traffic Channel Assignment) message according

to HRPD for assigning one or more traffic channels to the mobile station 104. Other
messages are also included in the responses sent from the target wireless access
network 102 through the source wireless access network 100 to the mobile station
104. ,*.;«-..
[0029] The exchange of messages at 212 and 214 effectively involves tunneling
of first format messages (e.g., HRPD messages) associated with the target wireless
access network 100 over the link between the source wireless access network 100
and the mobile station 104. For example, the HRPD messages are tunneled
(encapsulated) within EUTRA messages sent between the source wireless access
network 100 and the mobile station 104.
[0030] In response to receipt of the response messages (sent at 214), the mobile
station 104 initiates (at 216) connection to the target wireless access network 102
according to the received TCA message. The mobile station 104, after successfully
connecting to the target wireless access network 102, sends (at 218) an HRPD
Traffic Channel Complete message to acknowledge the TCA message received by
the mobile station 104.
[0031] At this point, the mobile station is connected (at 220) with the target
wireless access network 102. The emergency call is re-established (at 222)
between the mobile station 104 and the target wireless access network 102, using
the packet-based voice call profile (e.g., voice-over-Internet protocol (VoIP) profile)
of the call that was performed through the source wireless access network 100.
[0032] Fig. 2 assumes that the mobile station 104 is already actively in an
emergency call while the mobile station 104 is attached to the source wireless
access network 100. Alternatively, the mobile station 104 can be in an idle mode
(not actively engaged in the emergency call) while the mobile station is initially
attached to the source wireless access network 100. This scenario is depicted in
Fig. 3, which shows the mobile station being in an idle mode while attached to the
source wireless access network 100 (at 302). Moreover, as with the Fig. 2 example,
the mobile station 104 has pre-registered (at 302) with the target wireless access
network 102.

[0033] While in idle mode, the mobile station 104 requests (at 304) a packet-
switched emergency call with the source wireless access network 100. The mobile
station 104 also sends (at 306) signal measurements (e.g., pilot channel
measurements) to the source wireless access-network 100. Based on the received
measurements, the source wireless access network 100 can make a decision (at
308) to handover the emergency call (that is being requested by the mobile station
104) to the target wireless access network 102. Based on this decision to handover
the emergency call, the source wireless access network 100 sends (at 310) a
message to the mobile station 104 to command the mobile station 104 to hand over
to the target wireless access network 102. The handover message sent at 310 can
include an emergency indicator to show its priority and to serve as an internal
indicator between the EUTRA and HRPD protocol stacks.
[0034] Based on the handover command, the mobile station initiates (at 312) a
connection to the target wireless access network 102 based on handover information
contained in the handover command. Since the mobile station 104 knows that the
handover to the target wireless access network 102 is for making an emergency call,
the mobile station 104 sends (at 314) one or more messages with the emergency
call indication. As discussed above, such messages sent from the mobile station
104 to the target wireless access network 102 for performing the handover can
include a Route Update message and a Connection Request message, which can
be tunneled through the source wireless access network 100 to the target wireless
access network 102. A procedure {e.g., HRPD procedure) for establishing the
emergency call is then employed (at 316) to make the emergency call through the
target wireless access network 102.
[0035] Figs. 2 and 3 assume that the mobile station 104 has performed pre-
registration with the target wireless access network 102. However, in some cases,
the pre-registration may not be enabled, or pre-registration is enabled but the mobile
station 104 failed to complete the pre-registration. As a result, the mobile station 104
has not pre-established a specific session between the mobile station 104 and the
target wireless access network 102 for an emergency call.

[0036] However, in accordance with this alternative embodiment, a predefined
emergency call session is used, where information about the predefined emergency
call session is stored in the mobile station 104 and the target wireless access
network 104. The predefined emergency call session establishes the context for the
emergency call after handoff, such that the emergency call can more quickly be re-
established with the target wireless access network 102 after handoff. In yet another
alternative embodiment, a predefined emergency call session is not required.
[0037] As depicted in Fig. 4, the mobile station 104 is initially attached (at 402) to
the source wireless access network 100, but without a pre-registration with the target
wireless access network 102. As with the example of Fig. 2, the mobile station 104
is in an active packet-switched emergency call (at 404) through the source wireless
access network 100.
[0038] In the example of Fig. 4, the mobile station 104 sends (at 406) signal
measurements (e.g., pilot measurements) to the source wireless access network
100, which can make a handover decision (at 408) based on the measurements. In
response to making the handover decision, the source wireless access network 100
sends (at 410) a handover command to the mobile station 104.
[0039] In an alternative embodiment, the mobile station 104 is configured to
perform handover to the target wireless access network autonomously; in other
words, the mobile station 104 can perform the switch from the source wireless
access network 100 to the target wireless access network 102 without first
performing a handover procedure with the source wireless access network 100. The
autonomous handover performed by the mobile station 104 can be based on signal
measurements (of signals with base stations in the respective source and target
wireless access networks 100 and 102). However, even though the mobile station
104 is able to autonomously perform handover, the mobile station 104 may still send
some type of an indication to the source wireless access network 100 indicating that
the mobile station 104 is switching to a different system.
[0040] As further depicted in Fig. 4, the mobile station 104 initiates (at 412) a
connection to the target wireless access network 102, based on the handover

information from the source wireless access network 100 (in the handover command
received at 410) or based on autonomous handover of the mobile station 104.
[0041] The mobile station 104 then requests (at 414) a connection with the target
wireless access network 102, including an emergency indication in the one or more
request messages sent at 414. Next, the packet-switched emergency call is re-
established with the target wireless access network 102 with the emergency profile
of the predefined emergency session. This emergency profile is known to both the
access network 102 and the mobile station 104. The predefined emergency call
session allows for quicker establishment of an emergency call over the target
wireless access network 102 after the handover, since the emergency context has
already been established so that context negotiation (including registration) between
the mobile station 104 and the target wireless access network 102 does not have to
be performed.
[0042] In an alternative scenario, as depicted in Fig. 5, instead of actively being
in an emergency call as depicted in Fig. 4, the mobile station 104 can be in idle
mode (similar to the scenario of Fig. 3) when attached to the source wireless access
network 102, and the mobile station is not pre-registered with the target wireless
access network 102 (502 in Fig. 5). In this case, the Fig. 4 procedure is modified by
adding another message from the idle mode mobile station 104 to the source
wireless access network 100 for requesting (at 504) to make a packet-switched
emergency call (similar to message 304 in Fig. 3). The remaining procedure of this
modified flow is similar to the procedure of Fig. 4.
[0043] Fig. 6 is a block diagram depicting example components of the mobile
station (104 in Fig. 1) and a base station and/or gateway, which can include any of
the entities 108,110,112, and 116 in Fig. 1. Each of the mobile station and base
station and/or gateway is considered a communications node. The mobile station
includes handover logic 602, which can be software executable on one or more
processors 604. The one or more processors 604 are connected to a
communications interface 606 and storage media 608. The communications

interface 606 allows the mobile station 104 to communicate over an air interface with
the base station.
[0044] The base station/gateway includes a communications interface 610, which
is connected to one or more processors 612 that are in turn connected to storage
media 614. The base station/gateway also includes handover logic 616 executable
on the one or more processors 612.
[0045] Instructions of software described above (handover logic 602 and 616)
are loaded for execution on the one or more processors 604 or 612. Each processor

While the invention has been disclosed with respect to a limited number of
embodiments, those skilled in the art will appreciate numerous modifications and
variations therefrom. It is intended that the appended claims cover such
modifications and variations as fall within the true spirit and scope of the invention.

What is claimed is:
1. A method comprising:
detecting, by one or more processors, that a mobile station is involved in a
packet-switched emergency call, wherein the mobile station is initially attached to a
first type of wireless access network;
detecting, by the one or more processors, transitioning of the mobile station
from the first type wireless access network to a second, different type of wireless
access network; and
in response to detecting transitioning of the mobile station, the one or more
processors sending a message indicating transitioning of the mobile station from the
first type wireless access network to the second type wireless access network to
cause the packet-switched emergency call to be performed over the second type
wireless access network after the transitioning.
2. The method of claim 1, wherein the one or more processors that perform the
detecting tasks and the sending task are in the mobile station.
3. The method of claim 1, wherein the one or more processors that perform the
detecting tasks and the sending task are part of a base station in the first type
wireless access network.
4. The method of claim 1, further comprising:
pre-registering the mobile station with the second type wireless access
network when the mobile station is initially attached to the first type wireless access
network.
5. The method of claim 4, wherein pre-registering the mobile station includes
establishing a session between the mobile station and the second type wireless
access network.

6. The method of claim 4, wherein after the transitioning from the first type
wireless access network to the second type wireless access network, the mobile
station does not have to perform a registration procedure with the second type
wireless access network to perform the packet-switched emergency call since the
mobile station is already pre-registered with the second type wireless access
network.
7. The method of claim 1, further comprising:
storing information relating to a predefined emergency call session to be used
with the second type wireless access network.
8. The method of claim 7, further comprising:
after the handoff of the mobile station to the second type wireless access
network, using the information of the predefined emergency call session to avoid
having to negotiate a context of the packet-switched emergency call with the second
type wireless access network.
9. The method of claim 1, further comprising:
as part of the handoff, including in one or more messages sent to the second
type wireless access network an emergency indication.
10. The method of claim 1, wherein the first type wireless access network is an.
Evolved Universal Terrestrial Radio Access (EUTRA) network.
11. The method of claim 10, wherein the second type wireless access network is
a High Rate Packet Data (HRPD) access network.

12. A communications node comprising:
one or more processors configured to:
store information regarding a session for a packet-switched emergency
call, where in the session is between a mobile station and a target wireless access
network;
communicate messaging to allow transitioning of the mobile station
from a source wireless access network to the target wireless access network,
wherein the source and target wireless access networks are different types of
networks; and
after the transitioning, using the stored information to establish the
packet-switched emergency call over the target wireless access network.
13. The communications node of claim 12, comprising the mobile station.
14. The communications node of claim 12, comprising a node in the target
wireless access network.
15. The communications node of ciaim 12, wherein the information is stored in
response to pre-registering the mobile station with the target wireless access
network while the mobile station is attached to the source wireless access network.
16. The communications node of claim 12, wherein the stored information is
related to a predefined emergency call session.
17. The communications node of claim 12, wherein the source wireless access
network is an Evolved Universal Terrestrial Radio Access (EUTRA) network.
18. The communications node of claim 12, wherein the messaging comprises
messaging according to a standard of the target wireless access network tunneled
through a link between the mobile station and the source wireless access network.

19. An article comprising at least one computer-readable storage medium storing
instructions that upon execution cause by one or more processors to:
detect that a mobile station is involved in a packet-switched emergency call,
wherein the mobile station is initially attached to a first type of wireless access
network;
detect transitioning of the mobile station from the first type wireless access
network to a second, different type of wireless access network; and
in response to detecting transitioning of the mobile station, send a message
indicating transitioning of the mobile station from the first type wireless access
network to the second type wireless access network to cause the packet-switched
emergency call to be performed over the second type wireless access network after
the transitioning.
20. The article of claim 19, wherein the instructions upon execution cause the one
or more processors to further perform one of:
pre-registering a session with the second type wireless access network while
the mobile station is attached to the first type wireless access network; and
store information relating to a predefined emergency call session for use after
transitioning of the mobile station to the second type wireless access network.
21. A method comprising:
storing information relating to a predefined emergency call session to be used
between a mobile station and a wireless access network; and
establishing, by one or more processors, a packet-switched emergency call
using the stored information relating to the predefined emergency call session.
22. The method of claim 21, wherein the one or more processors are part of the
mobile station or part of a base station in the wireless access network.

23. A communications node comprising:
a computer-readable storage medium to store information relating to a
predefined emergency call session to be used between a mobile station and a
wireless access network; and
one or more processors configured to establish a packet-switched emergency
call using the stored information relating to the predefined emergency call session.

24. The communications node of claim 23, comprising the mobile station or a
base station in the wireless access network.

A mobile station that is initially attached to a first type of wireless access network is involved in a packet-switched
emergency call Upon detection of transitioning of the mobile station, a message is sent indicating transitioning of the mobile station
from the first type wireless access network to the second type wireless access network to cause the packet-switched emergency
call to be performed over the second type wireless access network.