November 2001

BY RAVI RAVISHANKAR

Wireless carriers face a number of issues as they consider the move to a packet-based network. They have made a very significant investment in the existing networks, and it is not economically feasible to abandon that infrastructure. The large geographic footprints of many global carriers may not be covered by the new 3G spectrum. Carriers will need to continue to support a large base of existing customers with 2G services as they transition to packet-based networks. And, the compatibility and acceptance of new handsets must be considered.

The move from 2G to 3G will not take place quickly. 2G technology will be in place for a long while, with 3G developments taking place in parallel with the existing network. The shift from 2G to 3G likely will be a phased migration -- one that will allow carriers to leverage their existing networks and maintain reliability and quality of existing services.

This column, which explores the evolution of the circuit-switched domain to packetized voice, is the first in a three-part series. The subsequent columns will look at evolving the packet-switched, data services network to support voice and multimedia services and the evolution of the signaling domain to handle GPRS and 3G.

Getting to the Core
Mobile switching centers (MSCs) are the heart of the switching fabric in today's wireless networks. The MSCs, which interface to the radio system and the public switched telephone network, perform critical functions including:

• Switching voice traffic from the wireless network to the PSTN if the call is mobile-to-landline or to another MSC within the wireless network if the call is mobile-to-mobile;
   
• Delivering short message service (SMS); and
   
• Providing subscriber mobility management.

Typically, the circuit-switched wireless network is a fully-meshed architecture; each MSC is connected to every other MSC in the network by time division multiplexed (TDM) trunks. Mesh networks lack the flexibility to scale easily or economically to accommodate network expansion.

A Better Strategy
Wireless carriers can accommodate increasing traffic loads today and create the foundation for a pure-packet network by employing next-generation technology to create a common packet infrastructure to interconnect MSCs. This is accomplished by deploying packet tandems comprised of centralized media gateway controllers (MGCs) or softswitches controlling an overlay of distributed media gateways (MGs) collocated with MSCs.

TDM trunks from each MSC are terminated on a stand-alone MGs. The media gateways perform the IP or ATM conversion under the control of the softswitch, which can be located in a centralized server farm.

This architecture dramatically simplifies the network and reduces bandwidth requirements by eliminating the point-to-point connections between MSCs. As network traffic increases, new MSCs can be added to accommodate the growth with a single connection to the softswitch and without any interconnection to other MSCs. The use of media gateways also allows operators to deploy next-generation services without doing software upgrades at every MSC in the network. And, by terminating TDM trunks from the PSTN at the media gateway rather than at the MSC, valuable mobile switching center resources are freed.

Packet-based technology also improves routing within the wireless network. The softswitch can handle several of the MSC's functions including routing and home location register (HLR) look-ups. In today's 2G network, calls entering the wireless network can require extensive routing before they can be completed. Each call has to be routed back to the subscriber's home MSC or gateway MSC, which launches a look-up in a HLR to determine the subscriber's location. The call is then routed to the serving MSC, a trip that may traverse multiple switches. In a packet-based architecture, gateway intelligence can be added to the centrally located softswitch, which performs the look-up and then routes the call directly to the serving MSC.

On To 3G
With a common, core packet network in place, operators create a fabric that is optimized for advanced data services and can also carry wireless voice and data traffic. The evolution to full-blown 3G is greatly simplified with the major changes and additions taking place in the radio access network (RAN). A new interface defined by 3GPP, IU-CS, is required from the 3G RAN to the MSC server (the 3G equivalent of a MSC). While it's possible to upgrade the existing MSCs to support IU-CS and MSC server functions, it's an expensive and time-consuming proposition. The MSC server software can be added to the centralized softswitch and the physical interface can be terminated on the distributed MGs. With this approach, carriers do not have to undertake the expensive and cumbersome task of upgrading the hardware and software at each and every MSC in the network.

Next month's column will look at the network evolution from a different perspective -- how to evolve the packet switched data networks such as GPRS to deliver integrated voice and multimedia service.

Mr. Ravi Ravishankar is director, Advanced Technogy Planning, Tekelec, and has over 19 years of telecom and network experience. He is a director for the International Softswitch Consortium's Technical Advisory Committee (TAC), and a key architect in several first-to-market products providing solution engineering to large customers. Tekelec is a leading developer of telecommunications signaling infrastructure, softswitches, testing and diagnostic solutions, and service applications.

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