Page 389 - From GMS to LTE

VoLTE, VoWifi and Mission Critical Communication 375

5.6 VoLTE Compared to Fixed‐Line IMS in Practice

It is an interesting historical twist that SIP as part of the wireless IP Multimedia
Subsystem (IMS) was initially a relatively simple protocol and designed for
fixed‐line IP networks. Once development was well underway in 3GPP for the
mobile world it was decided to also base the next generation of carrier fixed‐line
voice telephony on the 3GPP IMS specification. When comparing the SIP call
establishment signaling of a fixed‐line IMS device, shown in Figure 5.18, things look
quite similar to the establishment of a mobile VoLTE call. While the SIP ‘Invite’, ‘100
Trying’, ‘183 Session Progress’ and ‘200 OK’ messages are also used in VoLTE, quite
a number of other things are missing or slightly different. Major differences are
that there is no IPSec tunnel, no use of non‐standard UDP ports, no precondition
and bandwidth negotiations, no early‐media negotiation and no a‐SRVCC
announcements.
One message that is different compared to VoLTE rather than missing is the 407
Proxy Authentication Required after the first SIP Invite message. This is required as
IPSec is not used to create an authenticated and secure tunnel between the SIP User
Agent and the fixed‐line IMS in the network during registration.
Furthermore, fixed‐line IMS networks use a different set of speech codecs compared
to the mobile world. The excerpt below shows the protocols announced in the SDP part
of the SIP Invite message. Unfortunately, none of the announced codecs are compatible
with those used in the mobile world. As a consequence, a speech path transcoder needs
to be put between a fixed and a mobile IMS system.
m=audio 7078 RTP/AVP 9 8 0 2 102 100 99 101 97 120 121
a=sendrecv
a=rtpmap:2 G726–32/8000
a=rtpmap:102 G726–32/8000
a=rtpmap:100 G726–40/8000
a=rtpmap:99 G726–24/8000
a=rtpmap:101 telephone-event/8000
a=fmtp:101 0–15
a=rtpmap:97 iLBC/8000
a=fmtp:97 mode=30
a=rtpmap:120 PCMA/16000
a=rtpmap:121 PCMU/16000
a=rtcp:7079
a=ptime:20

Figure 5.18 Fixed‐line IMS call establishment. Source: Gerald Combs / Wireshark.