chul's thinking

Today the majority of cellular networks rely on SDH/SONET or ATM transmission services with E1/T1 access lines. Although 3G data traffic is still only a relatively small overall portion of mobile transmission, this situation is expected to change quickly as UMTS operators take their HSDPA networks onstream over the next few years. Assuming that additional E1/T1 lines are readily available from the landline operator, the amount of backhaul traffic is predicted to grow faster than the expected average revenue per user (ARPU).

The challenge is that the explosive increase in backhaul waiting on the doorstep must be controlled to ensure mobile service provider revenue. For purposes of comparison, while one or two E1/T1 lines comprising the transport network would be sufficient to handle the average number of links connected to 2G cellular base stations, the impending introduction of HSDPA may require this number to increased to anywhere from eight to sixteen E1/T1s per 3G cellular site. Without mechanisms in place to control operating and capital expenses (Opex and Capex), the costs may not justify the business case.

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 transition from TDM circuit-switched networks to ATM and, eventually, Gigabit Ethernet/IP/MPLS packet switched networks 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.

Mobile operators are saddled with a bewildering choice of backhaul technologies and network interfaces as they try to anticipate which access infrastructure will best serve their current and future requirements. The easiest decision would be to build out parallel networks. Using a dedicated transport network for each different mobile generation, however, is not as efficient or potentially cost-effective as integrating diverse traffic streams over a single backhaul link.

Given the drawbacks, does a converged backhaul access network solution exist that is technologically feasible, economically sound and readily available?

One familiar method for reducing backhaul costs, traditionally implemented in high- density segments of the core network, such as the Base Station Controller (BSC) or the Mobile Switching Center (MSC), is aggregating several E1/T1s together and utilizing statistical multiplexing to transport them over STM-1 lines. Aggregation is an essential part of existing cellular network transport design because it allows for more efficient use of the transport bandwidth and simplifies network management. Statistical multiplexing, moreover, is quite appropriate for the new types of data services that 3G will introduce.

With the introduction of 3G, the mobile world is evolving into a real multimedia environment. Instead of plain voice services, a wider range of services is available to subscribers. This range embraces delay-sensitive and high quality services like video streaming, which require a reserve backhaul bandwidth (constant rate) to best effort-type services like Internet surfing, back office services, mailing, data downloads, etc., which, by nature, are statistical also in terms of the air interface and backhaul bandwidth usage. These diversified services allow the operator to design its transport network so as to maximize efficiency by employing statistical techniques.

This new era of 3G and HSDPA services presents additional challenges to network designers. Aggregation, therefore, which characterizes existing core networks, now has become an essential building block in the radio access and transport networks. In other words, it is now implemented even at cell sites.

While all of this presents a significant challenge for mobile system architects, we haven’t yet mentioned the next great challenge looming just above the horizon, which is posed by the convergence of fixed and mobile networks. This trend is now being pioneered by world-class providers such as British Telecom (BT) and France Telecom (FT), which already have announced the convergence of cellular and other services.

21CN, BT’s “21st century programme,” for example, is intended to become “an advanced communications network for the future” in which “end-users will be able to access voice messages, data or video at any time on any device” and “share personal contact directory across their home phone, PC, mobile and PDA.” Meanwhile, across the English Channel, Orange already has become the single brand for France Telecom's Internet, television and mobile offerings, the first step towards fully convergent operations.

The test here for network architects is that to succeed, convergence between fixed line and mobile services 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. From the user’s standpoint, that means that e-mail or Internet surfing must be indifferent to whether they’re being provided over a WiFi home connection or a 3G mobile handset. This requires a unified transport network, which, we probably can assume, will be based on IP technology. In fact, the various standards bodies already are defining this unified IP-based transport network. IMS (IP Multimedia Subsystem) is one such technology enabler for this convergence. On the other side of the equation, 3GPP, the 3rd Generation Partnership Project, has defined an “all-IP” approach in all its standards ever since Rev 5.

The task ahead, therefore, will be 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 pseudo-wire technology, which transports TDM/ATM circuits transparently across Ethernet, IP or MPLS packet switched networks (PSNs). Pseudo wire solutions are particularly suited to cellular backhaul because they are transparent to the underlying traffic. Unlike VoIP, which requires translation of signaling, pseudo wires provide a transport tunnel across the statistical packet network without distortion.

As a rule, mobile networks require a high degree of synchronization to maintain a 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. Packet networks, however, are statistical-based by nature and do not provide inherent timing information whatsoever. Matters are made worse in a PSN as a result of Packet Loss (PL), when packets do not arrive at their destination, and Packet Delay Variation (PDV), when packets arrive with random, unpredictable delay. 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.

By applying pseudo wire 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, those mobile operators may consider a hybrid solution that will run all their existing and delay-sensitive traffic over the deployed TDM links, while only the aggregation HSDPA traffic will be connected employing pseudo-wire technologies and packet transport networks.

Mobile telephony’s backhaul challenge opens a door for new solutions to be incorporated in the transport network, such as packet-based technologies. Pseudo-wire techniques are proven technology enablers for such a migration.