Page 358 - From GMS to LTE

344 From GSM to LTE-Advanced Pro and 5G

network changes the event state, e.g. when a server is shutting down and has to end the
connection. A registration event is also sent to the mobile device to inform it if another
device has also registered for the same public user identity, i.e. the same telephone num-
ber. This way, it is possible to implement Multi‐SIM services so customers can use
several devices with the same subscription.

5.3.3 VoLTE Call Establishment
Once registered, voice calls can be initiated or received. This section gives a general
overview while further sections then go into the details of particular parts of the overall
process. Figures 5.7 and 5.8 show which messages are sent to establish a session between
two IMS devices. The role of the MMTEL server is not shown in the example, as in this
basic scenario no messages are modified by the application server (e.g. there is no mes-
sage modification to forward a call to the voice mail system).
In the first step the originating device sends a SIP Invite message to the SIP proxy, in
this case to the P‐CSCF. The P‐CSCF in turn confirms the reception with a SIP ‘100
Trying’ message and forwards the Invite message to the S‐CSCF. Here, the message is
analyzed and forwarded to an I‐CSCF whose task is to find the S‐CSCF of the destination
subscriber, to which the message is then forwarded. The S‐CSCF of the destination sub-
scriber can be located either in the same network or in a different network in the case
where the destination subscriber belongs to a different network operator. In the latter
case Border Gateway Controllers (BGCs) are part of the transmission chain to properly

UE P-CSCF S-CSCF I-CSCF S-CSCF P-CSCF UE
Invite
100 (Trying)
100 (Trying) Invite
100 (Trying) 100 (Trying) 100 (Trying) 100 (Trying)
183
(Session Prgs.)
183
(Session Prgs.)

PRACK
PRACK
Resource Res. 200 (ok)
200 (ok) Resource Res.

Update
Update
200 (ok)

200 (ok)

Figure 5.7 VoLTE call establishment part 1.