Overview of the 3GPP Release 4 Network Architecture

Overview of the 3GPP
Release 4 Network Architecture
Figure 4-5 shows the basic network architecture for 3GPP Release 4. The
main difference between the Release 1999 architecture and the Release 4
architecture is that the core network becomes a distributed network.
Rather than having traditional circuit-switched MSCs, as has been the case
in previous network architectures, a distributed switch architecture is
introduced.

Basically, the MSC is divided into an MSC server and a media gateway
(MGW). The MSC server contains all of the mobility management and call
control logic that would be contained in a standard MSC. It does not, however,
contain a switching matrix. The switching matrix is contained within the MGW, which is controlled by the MSC server and can be placed
remotely from the MSC.

Control signaling for circuit-switched calls is between the RNC and the
MSC server. The media path for circuit-switched calls is between the RNC
and the MG. Typically, an MG will take calls from the RNC and routes those
calls towards their destinations over a packet backbone. In many cases, that
packet backbone will use the Real-Time Transport Protocol (RTP) over the
Internet Protocol (IP). As can be seen from Figure 4-5, packet data traffic
from the RNC is passed to the SGSN and from the SGSN to the GGSN over
an IP backbone. Given that data and voice can both use IP transport within
the core network, a single backbone can be constructed to support both
types of service. This can mean significant capital and operating expenses
compared to the construction and operation of separate packet and circuitswitched
backbone networks.

At the remote end, where a call needs to be handed off to another network,
such as the PSTN, another media gateway (MGW) is controlled by a
Gateway MSC server (GMSC server). This MGW will convert the packetized
voice to standard PCM for delivery to the PSTN. It is only at this point
that transcoding needs to take place. Assuming, for example, that speech over the air interface is carried at 12.2 Kbps, then the voice does not need
to be converted up to 64 Kbps until it reaches the MGW that interfaces with
the PSTN. This packetized transport can mean significant bandwidth savings
on the backbone network, particularly if the two MGWs are some significant
distance apart.

The control protocol between the MSC server or GMSC server and the
MGW is the ITU H.248 protocol. This protocol was developed jointly by
the ITU and the Internet Engineering Task Force (IETF). It also goes by the
name media gateway control (MEGACO). The call control protocol between
the MSC server and the GMSC server can be any suitable call control protocol.
The 3GPP standards suggest but do not mandate the Bearer Independent
Call Control (BICC) protocol, which is based on the ITU-T
recommendation, Q.1902.

In many cases, an MSC server will also support the functions of a GMSC
server.Moreover, one MGW may have the capability to interface both with
the RAN and with the PSTN. In that case, calls to or from the PSTN can be
handed off locally. This can represent another major saving.

Consider, for example, a scenario where an RNC is located in one city
(City A) and is controlled by an MSC in another city (City B). Let’s assume
that a subscriber in City A makes a local phone call.Without a distributed
architecture, the call needs to travel from City A to City B (where the MSC
is), only to be connected back to a local PSTN number in City A.With a distributed
architecture, the call can be controlled by an MSC server in City B,
but the actual media path can remain within City A, thereby reducing
transmission requirements and reducing network operations costs.

One will notice that, in Figure 4-5, the HLR may also be known as a Home
Subscriber Server (HSS). The HSS and HLR are functionally equivalent,
with the exception that interfaces to an HSS will use packet-based transports
such as IP, whereas an HLR is likely to use standard Signaling System
7 (SS7)-based interfaces. Although not shown, a logical interface exists
between the SGSN and HLR/HSS and between the GSN and HLR/HSS.

Many of the protocols used within the core network are packet-based,
using either IP or ATM. The network must, however, interface with traditional
networks—through the use of media gateways. Moreover, the network
must also interface with standard SS7 networks. This interface is
achieved through the use of an SS7 gateway (SS7 GW). This is a gateway
that on one side supports the transport of a SS7 message over a standard
SS7 transport. On the other side, it transports SS7 application messages
over a packet network such as IP. Entities such as the MSC server, the
GMSC server, and HSS communicate with the SS7 gateway using a set of transport protocols specially designed for carrying SS7 messages in an IP
network. This suite of protocols is known as Sigtran.