HSDPA Will Transform Cellular Backhaul

by Gaby Junowicz Business Development Director

RAD Data Communications

The introduction of High Speed Downlink Packet Access (HSDPA) is expected to enable mobile operators to exploit the full potential of W-CDMA technology by offering mobile broadband services at competitive costs. With a maximum theoretical peak throughput of 14.4 Mbps per user, HSDPA can significantly reduce the time required to download rich-media files. Downloading one minute of audio from an MP3 music file, for example, takes 132 seconds with GPRS and 22.4 seconds with UMTS, but only 4.1 seconds with HSDPA. The result is an 82% saving in airtime, making it possible to serve more subscribers using the same infrastructure. By delivering speeds comparable to or better than fixed-line broadband access systems, HSDPA promises to transform cellular telephony. But high capacity performance has a price: an exponential increase in the bandwidth required to backhaul cellular traffic from Node Bs to the Radio Network Controller (RNC). Backhaul a broad term for the provision of connectivity in the service providers core network accounts for a healthy chunk of operator costs in building and maintaining a mobile network. In fact, its one of the major contributors to the high costs of building and running a mobile network. On average, transport equipment accounts for 25% of the costs of private cellular backhaul infrastructure. Transport outlays, moreover, vary between 40-60% of the total cost of leased lines, with backhaul contributing 75% of that sum. And HSDPA costs per bit are prohibitive if the mobile service has to match the fixed line offering. Alternatives to E1/T1 Lines Today, the majority of cellular networks rely on SDH or ATM transmission services with E1 access lines from the base station or Node B to the base station controller (BSC) or RNC. Although 3G data traffic is still only a relatively

small overall portion of mobile transmission, this situation will change quickly as UMTS operators take their HSDPA networks on-stream. Assuming that additional E1/T1 lines will be readily available from the landline operator, traffic will grow faster than the expected average revenue per user (ARPU). This situation is forcing mobile operators to seek alternative backhaul solutions that are cost effective and scalable and wont compromise the integrity of the voice quality. The steady deployment of IP DSLAMs on the one hand, and the successful implementation of pseudowire devices for cellular backhaul over DSL on the other, sway UMTS operators to consider lower-cost broadband networks in dealing with backhaul. As a first migration step towards packet-based transport, an interim hybrid approach is being adopted to guarantee voice quality while reducing access costs. That solution calls for the diversion of voice and real-time traffic to a TDM or ATM transport network while the HSDPA data can be carried over any packet transport media. Supporting divergent technological demands and applications Complicating the equation is the need to support simultaneously the divergent technological demands and applications of existing 2G/2.5G networks and emerging 3G operations. The easiest solution would be to compartmentalize infrastructure by building out parallel networks, using a dedicated transport network for each different mobile generation. Ultimately however, going down this route offers neither long-term proficiency or cost-effectiveness and operators would be better served by looking to integrate diverse traffic streams over a single backhaul link. Quite simply, whats required is a converged backhaul access network solution that technologically and economically delivers on all fronts. The transition from TDM circuit-switched networks to ATM and, eventually, Gigabit Ethernet/IP/MPLS packet switched networks (PSNs) raises new challenges, particularly regarding the cost and suitability of the access platform to handle and manage efficiently increased bandwidth capacity and the complexities of voice and data in a converged network. Given the drawbacks, does a converged backhaul access network solution exist thats technologically feasible, economically sound and readily available? Reducing backhaul costs One familiar method for reducing backhaul costs, traditionally implemented in high-density segments of the core network, such as the BSC or the mobile switching centre (MSC), is aggregating several E1/T1s together and utilizing statistical multiplexing to transport them over STM-1/OC-3 lines. Statistical multiplexing is also appropriate for new 3G data services.

Meanwhile, looming over the horizon, the next great challenge is the convergence of fixed and mobile networks. Convergence of fixed and mobile networks will also present a test for operator infrastructures. Some providers have already announced the convergence of mobile and fixed line services, such as BTs 21st Century Programme (21CN) and the branding of France Telecoms Internet and TV services as Orange. To succeed, convergence will have to provide the same look and feel. For that reason, services will have to be agnostic both to the access method and the devices that are being connected. The challenge for network architects, therefore, is to ensure that fixed line and mobile services, such as e-mail or Internet surfing, should be the same whether theyre being provided over a WiFi connection or a 3G mobile handset. This will require a unified transport network, which is likely to be based on IP technology. Various standards bodies are already looking to a unified IP-based transport network such as IMS (IP Multimedia Subsystem). On the other side of the equation, 3GPP, the 3rd Generation Partnership Project, has defined an all-IP approach in all its standards. Pseudowire particularly suited to cellular backhaul The task, therefore, is to provide an effective solution to connect the installed base of mobile infrastructure which has both GSM TDM-based and UMTS ATM-based network elements over IP. This requires the use of pseudowire technology. A pseudowire is an emulation of a native service (such as ATM, Frame Relay, SONET/SDH, or TDM) formed by tunneling that service's data through a packet network. Traditionally mobile networks require a high degree of synchronisation to maintain proper service quality because cellular traffic is extremely sensitive to latency and packet loss. This is achieved by distribution of a common clock to serve as a point of reference among the numerous base stations spanning the network. Data (packet-based) networks such as IP, however, are statistical-based by nature and dont provide any inherent timing information whatsoever. In a data network, problems arise as a result of Packet Loss (PL), when packets dont arrive at their destination, and Packet Delay Variation (PDV), when packets arrive with random, unpredictable delay. Anyone who has used VoIP services will be all too aware of this issue. Sophisticated clock recovery mechanisms are required to reconstruct timing and achieve the desired timing accuracy in the presence of packet delay variation and packet loss. This kind of clock recovery mechanism results in a process that negates the effect of the random PDV and captures the average rate of transmission of the original bit stream.

Pseudowire solutions, however, are particularly suited to cellular backhaul because theyre transparent to the underlying traffic. Unlike VoIP, which requires translation of signaling, pseudowires provide a transport tunnel across the statistical packet network without distortion. Deploying an HSDPA hybrid solution By applying pseudowire technologies, mobile operators will be able to speedily deploy high capacity W-CDMA services and keep HSDPA operating costs to a minimum while increasing their revenues and profitability from media-rich 3G content. As an interim solution, a hybrid solution would be to run all existing and delay-sensitive traffic over TDM links, while only the aggregation HSDPA traffic would be connected employing pseudowire technologies and packet transport networks. Mobile telephonys backhaul challenge opens a door for new solutions to be incorporated in the transport network, such as packet-based technologies. In facing the backhaul challenge, pseudowire techniques are proven technology enablers for such a migration.