Page 354 - From GMS to LTE

340 From GSM to LTE-Advanced Pro and 5G

Figure 5.4 List of codecs in the SDP section of a SIP Invite message.

message that is then sent to a voice mail system in the network. SIP proxies can thus
offer services similar to those of the MSC architecture for circuit‐switched networks
described in Chapter 1. To communicate with subscribers on circuit‐switched networks,
SIP proxies can interact with signaling and media gateways. The signaling gateway
translates the SIP messages into classic SS‐7 messages and vice versa (cp. Chapter 1)
while the media gateway translates an IP‐based media stream into an E‐1‐based stream
and vice versa.

5.3 The IP Multimedia Subsystem (IMS) and VoLTE

5.3.1 Architecture Overview
For mobile networks, the SIP system has been significantly extended by 3GPP and is
referred to as the IP Multimedia Subsystem (IMS). Figure 5.5 shows the central compo-
nents of the IMS. The core of the system is the Serving Call Session Control Function
(S‐CSCF), which implements the role of SIP Registrar and SIP proxy. To communicate
with the central database, the Home Subscriber Server (HSS), the S‐CSCF uses the Cx
interface and the Diameter Protocol, as described in RFC 6733 [6].
As there can be several S‐CSCF in a large IMS system, a distribution function is
required for incoming requests. This is performed by the Interrogating‐CSCF (I‐CSCF),
which also has a connection to the HSS via the Cx interface to be able to retrieve sub-
scriber information relevant for the routing of the signaling messages to the responsible