Page 359 - From GMS to LTE

VoLTE, VoWifi and Mission Critical Communication 345

UE P-CSCF S-CSCF I-CSCF S-CSCF P-CSCF UE
180 (Ringing)
180 (Ringing)
180 (Ringing)
180 (Ringing)
180 (Ringing)

Answer
200 (ok)
200 (ok)
200 (ok)
200 (ok)
200 (ok)
ACK
ACK

Figure 5.8 VoLTE call establishment part 2.

separate the two IMS networks from each other. Once the message arrives at the S‐CSCF
of the destination subscriber, the P‐CSCF responsible for the user’s device is determined
and the message is then forwarded. The P‐CSCF then tries to forward the Invite message
to the subscriber. If successful the device returns a SIP ‘Session Progress’ message to the
P‐CSCF and from there via all other IMS servers back to the originating device.
Among other information the SIP ‘Invite’ and ‘Session Progress’ messages contain the
aforementioned Session Description Protocol (SDP) section in which all voice codecs
that are supported by the device are listed. This is required to select a suitable speech
codec and to establish a dedicated bearer with suitable Quality of Service settings on the
air interface. As described before it is the task of the P‐CSCF to establish the dedicated
bearer as it is the component in the IMS network that communicates with the underlying
LTE network via the PCRF. Such interaction is possible as the P‐CSCF not only forwards
the SIP messages but also parses them for the information it requires for bearer handling.
Once resources have been assigned for the speech channel the destination device
alerts the user and returns a SIP ‘180 Ringing’ message to the originator, again via all
IMS components involved in forwarding signaling messages. IP packets containing the
speech data, however, can be exchanged directly between the two devices. If both
subscribers are in the same network, the speech packets can thus be sent to the PDN‐
Gateway and are looped back into the network straight away instead of traversing the
SGi interface to another network (cp. Figure 5.5). It should be noted at this point that in
practice speech packets usually traverse a gateway in the network to implement a quick
transfer of the voice call from LTE to a circuit‐switched UMTS or GSM channel in case
the user roams out of the LTE coverage area. This is part of the 3GPP Release 10 Single
Radio Voice Call Continuity (SRVCC) functionality described further below.
As the 3GPP IMS specifications contain too many implementation options, network
operators decided to agree on a common set of options to use for their Voice over LTE
systems. This ensures that IMS speech services are compatible between networks and
that software for devices can be developed that is usable in all IMS networks and
between all mobile devices supporting the agreement. This led to the GSMA IR.92