Page 379 - From GMS to LTE

VoLTE, VoWifi and Mission Critical Communication 365

in addition to the internationally standardized numbers and numbers stored on the SIM
card. Also, it is possible this way to route emergency calls to different emergency call
centers as will be shown below. This is not possible in GSM and UMTS where all emer-
gency calls made in one geographical location are routed to the same emergency center
independent of the number dialed.
When the user dials an emergency number and the LTE network is VoLTE emergency‐
call‐capable the mobile device does not use the existing VoLTE SIP connectivity over
the IMS default bearer but establishes a separate default bearer for the emergency call.
No APN name need be given. Instead, the request type has to be set to ‘emergency’ in
the PDN Connectivity Request message and all RRC messages below it. This way, the
network recognizes that an IMS emergency call default bearer is to be established and
will give it precedence over all other traffic in the network. Once the bearer is estab-
lished and an emergency registration has been performed the mobile device sends a
SIP emergency Invite to the network over this bearer. If the subscriber was already
registered, the network can validate the subscriber’s identity. If the subscriber was not
registered, e.g. because only a competitor’s network was available at a location, the iden-
tity of the subscriber cannot be validated. In most countries the call is nevertheless
allowed to proceed.
In the SIP Invite message two header parameters give the network more information
about the type of emergency call that the user wants to establish. The generic case looks
as follows:
INVITE urn:service:sos SIP/2.0
To: “112” <urn:service:sos>
If the network has defined further numbers for more specific emergency services, for
example 909 for the fire brigade, the Universal Resource Name (URN) would be
extended as follows:
To: “909” <urn:service:sos.fire>
This is different to GSM and UMTS emergency calls today in which neither the number
dialed nor the type of emergency service center can be given.
In addition, the mobile device includes a P‐ANI (P‐Access Network Identifier) parameter
with the cell‐ID where the subscriber is currently located. This is not emergency call‐specific
but is also done for every established VoLTE call.
In some countries, such as the US, national regulation requires additional location
information to be sent as part of the emergency call. In VoLTE this is done by including
Secure User Plane Location (SUPL) data in the emergency call establishment, which
contains the current GPS location of the user.

5.4 VoLTE Roaming

While VoLTE is used in many networks around the world today, voice telephony
when subscribers roam abroad is still mostly based on circuit‐switched technology.
When a VoLTE device today detects that it registers to a VPLMN (Visited Public
Land Mobile Network) abroad it typically deactivates its VoLTE capabilities and
behaves like a device that only supports circuit‐switched voice services in GSM/