December 2001

BY RAVI RAVISHANKAR

This column, which looks at evolving the packet-switched domain to deliver voice and multimedia services, is the second in a three-part series that explores the evolution of the wireless network.

An inherent limitation of the circuit-switched, wireless network is its inability to efficiently deliver high-speed data services. This limitation has spurred the development of overlay packet-switched (PS) data networks like general packet radio service (GPRS) to deliver data applications. The PS overlays are best suited for �bursty� data services like e-mail and mobile Internet browsing. However, they will require further evolution to deliver fully integrated voice and multimedia services.

This evolution likely will occur in several phases. From a service perspective, the first phase, known as 2.5G, will add support for wireless data such as WAP and wireless Web browsing. The second phase will expand to include voice support. From a network implementation perspective, 2.5G will include the deployment of an overlay core network. This phase will require minimum modification to the existing 2G radio access infrastructure and will reuse the spectrum that is already available to 2G operators. The second phase will require deploying a completely new 3G radio access network (RAN) architecture, which moves packet-switched technology to the edge of the network. The 3G RAN will also provide increased bandwidth to the edge devices utilizing new spectrum. In all these cases, new handsets will be required to access the wireless data service.

GETTING TO THE CORE
GPRS is the most popular core network technology being deployed to support wireless data. GPRS serving nodes (GSN) are the heart of the packet-switched fabric; they are comparable to the mobile switching center (MSC)/visitors� location register (VLR) in today�s circuit-switched, wireless network. Serving GSN (SGSN) nodes support mobility management functions such as registration of the mobile devices to the network and roaming. A backbone IP network known as the GPRS core provides packet transport between the GSN nodes. The gateway GSN (GGSN) acts as a doorway between the GPRS core IP network and external data networks such as the Internet or an intranet. The overlay GPRS network uses SS7 signaling to interact with 2G GSM network elements. The 2G GSM signaling network provides most of the signaling required for GPRS.
The 2G radio access network interfaces to the SGSN using the frame-relay-based gigabit interface, which is defined in GSM specifications. As 3G RANs are deployed, a new interface defined by 3GPP, IU-PS, is required from 3G RAN to the SGSN.

BEYOND DATA
The wireless network is evolving along a path that is similar to that of wireline networks. Like digital subscriber lines and cable in the wireline world, GPRS provides �always on� access to the network with data rates that are higher than those achievable with dial access services. Though the initial driver for these alternate, high-speed access technologies is data, it is clear that voice still remains the �killer app.� The rate of penetration of mobile phones in developed and developing countries is clear proof of this. Providing traditional voice services using a GPRS data backbone is as important as utilizing packet transmission in traditional GSM voice networks.
GPRS provides a �content-neutral data pipe� between any two data devices, where at least one is a wireless device. Supporting voice as another form of data becomes a natural extension of the GPRS network. However, there are real challenges in the actual implementation � providing seamless roaming between 3G network and 2G networks, establishing peering arrangements needed for GPRS roaming between carriers, and providing service transparency between 2G and 3G voice. And, bridging the PSTN paradigm to the Internet paradigm also presents challenges. It should be possible for subscribers to make voice calls to a PSTN-like phone number, URL address, or an e-mail-like address with total transparency. In some cases, using a PSTN signaling protocol such as SS7 may be the appropriate technology to provide service transparency with 2G. Other cases may require using an Internet-based protocol such as session initiation protocol (SIP) to take advantage of the Internet-style services. This requires a new network entity � a signaling router � that can provide the protocol and address mediation and find the service and route to the service with total transparency to the bearer network.

ENHANCED SERVICE CREATION
Wireless operators have made a huge investment for 3G spectrum. Neither voice or data services alone can guarantee a profitable return on this investment. Operators are looking at developing a new breed of multimedia services to drive revenues and profitability to offset the expenditure. These services will emerge from the true convergence of voice and data with enhancements from wireless-specific services such as location-based service. True service revolution can only be achieved when a significant number of third-party application providers and content providers can independently develop services, which can then be delivered to the end subscriber using carriers networks as a distribution channel. An open service creation environment is critical to create this value chain of carriers, application service providers, and the content providers. The 3GPP open service architecture (OSA) is an attempt to migrate in this direction, though it is too premature to call it a success.

CONCLUSION
It is a challenging time for the entire communications industry. However, wireless carriers have an advantage � they can benefit from the experiences gained in the VoIP revolution that has already begun in the wireline segment. The new breed of VoIP-based services deployed and proven in the enterprise wireline segments will become the cornerstone for next-generation wireless network services. c

Mr. Ravi Ravishankar is director, Advanced Technology Planning, Tekelec. His focus is on defining signaling solutions and products for the next-generation packet telephony and 3G wireless networks. Tekelec is a leading developer of telecommunications signaling infrastructure, softswitches, testing and diagnostic solutions, and service applications. Please visit their Web site at www.tekelec.com.

[ Return To The December 2001 Table Of Contents ]