Page 378 - From GMS to LTE

364 From GSM to LTE-Advanced Pro and 5G

device is no longer able to change supplementary service settings as the non‐VoLTE
device does not support the XCAP interface.

5.3.16 VoLTE Emergency Calls
As VoLTE is not an over‐the‐top voice service, network operators are required to sup-
port emergency calls not only in GSM and UMTS but also in LTE and over VoLTE.
Emergency calls are automatically connected by the network to a Public Safety
Answering Point (PSAP), i.e. an emergency call center that is closest to the user’s cur-
rent location based on the cell the call originates from. In traditional GSM and UMTS
networks a number of short codes such as 112 and 911 are recognized by all mobile
devices as emergency numbers and a special emergency call setup procedure is invoked
when the user dials such numbers. While the two numbers above are globally usable,
SIM cards can contain additional numbers that also have to be treated by mobile devices
as emergency numbers. In this way it is possible to implement country‐specific emer-
gency call short codes. When a mobile device establishes an emergency call in GSM or
UMTS it does not include the number dialed but only informs the network that this is
an emergency call. In this way, a short code known by a user in one country also works
when they roam abroad and are unaware of the local emergency call short codes.
Furthermore, the network gives emergency calls the highest priority and even preempts
other ongoing non‐emergency calls in case of network overload. Also, any mobile device
for GSM or UMTS voice telephony shall be allowed to make emergency calls independ-
ent of whether the user is allowed to use the network for non‐emergency services or
not. This is to ensure that emergency calls can be made even in locations where only
networks to which the user has no access are available, e.g. in rural areas where only a
competitor’s network is available. For VoLTE the same rules apply and are implemented
as follows.
In practice, VoLTE‐capable LTE networks can offer emergency calls in two ways.
Especially in early VoLTE networks, emergency calling is offered via the CS‐Fallback
(CSFB) mechanism to GSM or UMTS as described in Chapter 4. In other words, such
VoLTE networks are not capable of handling emergency calls at all. Whether VoLTE
emergency calls are supported or CSFB has to be used is announced by the network in
the System Information Broadcast (SIB) 1 message that LTE devices receive independ-
ent of their current registration status. If VoLTE emergency calls are supported, SIB 1
includes the ‘ims‐EmergencySupport’ parameter, set to ‘true’. If the parameter is not
present, a device that is not registered to the network has to use a GSM or UMTS net-
work for emergency calls.
In addition, an LTE network announces VoLTE emergency call support at the end of
the LTE Attach or Tracking Area Update in two ways. If VoLTE and VoLTE emergency
calling are supported, the network includes the ‘EPS Network Feature Support’ infor-
mation element and sets the bits for VoLTE support (IMS VoPS: IMS voice over PS
session in S1 mode) and VoLTE emergency support (EMC BS: emergency bearer ser-
vices in S1 mode supported) to 1. In the parameter description S1 refers to the LTE
network interface between the eNodeB and the MME.
Optionally, the network can include a list of emergency call numbers that are valid in
the country and information as to which emergency service, such as police, ambulance,
fire brigade, etc., they connect to. This way, local emergency numbers can be specified