The mobile broadband tidal wave is upon us. At the end of 2009, more than 350 million cellular mobile subscribers used broadband connections. This number is expected to reach 1.7 billion by 2014. Growth in the number of mobile broadband subscribers equals growth in mobile data traffic.

In December 2009, mobile data surpassed voice traffic on a global basis for the first time in history. The crossover occurred at approximately 140,000 terabytes per month in both voice and data traffic. Data traffic grew globally at a rate of 280% during each of the last two years, and it’s forecasted to double annually over the next five years, reaching 1.8 exabytes per month by 2017 (Fig. 1).

Growth Strains Backhaul Networks

Mobile backhaul is the network for transporting mobile traffic between cell sites (BTS/NodeBs) and radio controllers (BSC/RNCs). Because backhaul accounts for an estimated 25% to 30% of total operating expenses, mobile operators need to optimize their networks with the most cost-efficient backhaul technologies.

Traditionally, time domain multiplexing (TDM) circuits have interconnected basestations to regional network controllers — a fine solution for voice-only systems or low-bandwidth data traffic. Rapid growth in mobile broadband traffic, however, has overloaded TDM circuits. Providers are therefore unable to keep up with the growth in wireless traffic.

Adding more TDM circuits to address this challenge is not a viable option for two reasons. First, recurring monthly costs for legacy backhaul technologies (PDH, ATM over PDH, and SONET/SDH) increase linearly with traffic. Second, operators can only charge a relatively flat average revenue per user (ARPU) for enhanced services, and are thus prevented from passing increased expenses on to consumers.

Operators are instead moving to packet-based backhaul technologies, using Internet Protocol (IP) and Ethernet to gain a lower cost per bit. Using Carrier Ethernet for wireless backhaul allows operators to support large bandwidth increases from cell sites, while keeping operational costs in check. Operators can significantly reduce their cost per connection by moving from TDM to Ethernet (Fig. 2).

Moving From TDM To IP/Ethernet Backhaul

Recent improvements such as 802.1Q virtual local-area network (VLAN) tagging and Ethernet operations, administration, and maintenance (OAM) make Ethernet a viable technology for transporting services over the mobile backhaul. In a recent survey of global service providers by Infonetics, 100% of respondents claimed to be deploying IP/Ethernet backhaul in 2010. This “phased” migration is outlined in the Metro Ethernet Foundation MEF 22 Mobile Backhaul Implementation Agreement (Fig. 3).

Mobile Backhaul Migration

Phase 1 is a hybrid implementation where Carrier Ethernet is used for packet offload of data services and TDM is retained for voice. TDM provides the necessary clock synchronization across the network for call setup and handover. This approach is not an ideal solution, as it forces carriers to maintain and pay for two separate networks.

The ultimate goal is Phase 2, in which a single Carrier Ethernet network is used to backhaul all services. The Infonetics survey indicated that 65% of service providers plan to eventually move to a single IP/Ethernet backhaul.

Before pursuing this final stage of migration, however, carriers must have confidence that timing over packet (ToP) technologies can satisfy the strict clock synchronization requirements of wireless standards. Furthermore, as carriers implement these changes, design engineers must quickly master the new technologies and standards associated with the transition.