PDF of this tutorial
Most WSPs will migrate to Release 4 via Release 99. Release 4 specifies the migration of the circuit switched voice network to an ATM or IP core network. Other functional areas are also covered by Release 4, such as broadcast services and network-assisted location services.
Figure 6 shows an overview of a Release 4 network.
Figure 6. UMTS Release 4 Network
Moving to a packet-switched core voice network will allow WSPs to gain from the inherent efficiencies of a Voice over Packet (VoP) network. In circuit-switched voice, a dedicated channel of fixed size (e.g. 64kbps) is assigned to carry the voice. In a normal voice call, this channel is under-utilized. In VoP, only the resources required are utilized.
In R4, the MSC functionality is split into two logical functions, which will typically be provided by physically separate network elements, namely:
- Media Gateway (MGW)
- MSC Server
The bearer switching functions of the MSC are now provided by the MGW.
Figure 7 shows this split of MSC functionality.
Figure 7. Split of MSC Functionality in Release 4
UMTS Media Gateway
The MGW is responsible for switching the bearer, i.e. user, traffic. The MGW can also be utilized to convert bearer traffic between two different formats. For example, PCM circuit voice to VoP. The MGW will contain transcoders and echo canceling equipment.
UMTS MSC Server
In standard VoP terminology, this is known as a Media Gateway Controller. 3GPP have specified two instances of a Media Gateway Controller, namely the MSC Server and the Gateway MSC Server. The Gateway MSC Server is an MSC Server that controls the connections to other networks, e.g. the PSTN.
The MSC Server provides all the call control capabilities required by the MGW. The MSC Server is also responsible for Mobility Management. Typically, the MSC Server will also contain the Visitor Location Register (VLR) functionality.
A single MSC Server can control multiple Media Gateways. This provides major scalability benefits. That is, when extra capacity is needed, it may be possible to add only a new MGW and utilize an existing MSC Server.
New Interfaces
The new interfaces added as part of R4 are:
- Mc
- The Mc interface is used by the MSC Server to control the Media Gateway(s). The protocol used on this interface is a derivation of the IETF H.248/MEGACO standard. The variations to the H.248/MEGACO standard are implemented using the H.248/MEGACO standard extension mechanism.
- Nc
- The Nc interface is used to share signaling information between multiple MSC Servers involved in a session (e.g. a voice call). This in turn allows the relevant Media Gateways to be effectively controlled for the duration of the call. The call control protocol suggested for this interface is BICC (Bearer Independent Call Control). The actual bearer (i.e. Nb) transport used is transparent to the BICC signaling protocol, hence the term "Bearer Independent".
- Nb
- The Nb interface is used to transport data (e.g. voice packets) between Media Gateways. For example, one MGW may be connected to the UTRAN and another MGW is connected to the PSTN, with the voice packets being transferred between them using Nb.
