Overview of the 3GPP Release 5 All-IP Network Architecture

The next step in the UMTS evolution is the introduction of an all-IP multimedia
network architecture (see Figure 4-6). This step in the evolution represents
a change in the overall call model. Specifically, both voice and data
are largely handled in the same manner all the way from the user terminal
to the ultimate destination. This architecture can be considered the ultimate
convergence of voice and data.

As we can see from Figure 4-6, voice and data no longer need separate
interfaces; just a single Iu interface can carry all the media.Within the core network, that interface terminates at the SGSN—there is no separate
media gateway.

We also find a number of new network elements, notably the Call State
Control Function (CSCF), the Multimedia Resource Function (MRF), the
Media Gateway Control Function (MGCF), the Transport Signaling Gateway
(T-SGW), and the Roaming Signaling Gateway (R-SGW).
An important aspect of the all-IP architecture is the fact that the user
equipment is greatly enhanced. Significant logic is placed within the UE. In
fact, the UE supports the Session Initiation Protocol (SIP), which is
described in Chapter 8, “Voice over IP Technology.” The UE effectively
becomes a SIP user agent. As such, the UE has far greater control of services
than previously.

The CSCF manages the establishment, maintenance, and release of multimedia
sessions to and from user devices. This includes functions such as
translation and routing. The CSCF acts like a proxy server/registrar, as
defined in the SIP architecture described in Chapter 8.
The SGSN and GGSN are enhanced versions of the same nodes used in
GPRS and UMTS Release 1999 and Release 4. The difference is that these
nodes, in addition to data services, now support services that have traditionally
been circuit-switched—such as voice. Consequently, appropriate
Quality of Service (QOS) capabilities need to be supported either within the
SGSN and GGSN or, at a minimum, in the routers immediately connected
to them.

The Multimedia Resource Function (MRF) is a conference bridging function
used to support features such as multi-party calling and meet-me conference
service.

The Transport Signaling Gateway (T-SGW) is an SS7 gateway that provides
SS7 interworking with standard external networks such as the
PSTN. The T-SGW will support Sigtran protocols. The Roaming Signaling
Gateway (R-SGW) is a node that provides signaling interworking with
legacy mobile networks that use standard SS7. In many cases, the T-SGW
and R-SGW will exist within the same platform.

The media gateway (MGW) performs interworking with external networks
at the media path level. The MGW in the 3GPP Release 5 network
architecture is the same as the equivalent function within the 3GPP
Release 4 architecture. The MGW is controlled by a Media Gateway Control
Function (MGCF). The control protocol between these entities is ITU-T
H.248. The MGCF also communicates with the CSCF. The protocol of choice
for that interface is SIP.

It should be noted that the Release 5 all-IP architecture is an enhancement
to an existing Release 1999 or Release 4 network. It is effectively the addition of a new domain in the core network—the IP-Multimedia (IM)
domain. This new domain, which enables both voice and data to be carried
over IP all the way from the handset, uses the services of the PS domain for
transport purposes. That is, it uses the SGSN, GGSN, Gn, Gi, etc.—nodes
and interfaces that belong to the PS domain.