True 4G mobile backhaul: Key ingredients

For service providers, revenue growth is increasingly driven by 3G and eventually 4G mobile services that provide access to compelling smartphone and tablet-based applications. The challenge in designing true 4G mobile backhaul is to build a foundation that protects existing investments in 2G and 3G services, provides a superior subscriber experience, prepares for future 4G service requirements, and minimizes costs. This challenge can be met by deploying a true 4G mobile backhaul network with three key ingredients: resiliency, synchronization and performance.

To understand the importance of each of these elements, it is important to understand the basic architecture of a mobile network. At a high level, the mobile network consists of the cell site, cell site aggregation and the mobile core.

The cell site includes 2G, 3G and 4G radios that connect to thousands of mobile devices. cell site routers are deployed at the cell site, collecting T1/E1 and Ethernet traffic from these radio towers for handoff to the mobile backhaul network over fiber or microwave Ethernet backhaul (See Figure 1.) cell site aggregation routers aggregate traffic from multiple cell site routers for handoff to the mobile core where calls are terminated and data services originate. cell site aggregation routers are connected with resilient synchronous gigabit Ethernet rings to the aggregation hub. Driven by the requirement for the lowest cost per bit transported, resilient synchronous gigabit Ethernet provides the key ingredients for true 4G mobile backhaul.

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Figure 1 As the transport network connecting multiple cell sites to the mobile core, the mobile backhaul network plays a crucial role in the delivery of services

Resiliency

True 4G mobile backhaul provides superior subscriber experience by providing carrier-grade resiliency based on ring architectures. Legacy backhaul networks based on SONET/SDH fiber rings provide carrier-grade resiliency through Automatic Protection Switching (APS), providing 50 ms recovery times from network faults. For Ethernet-based mobile backhaul, two packet ring resiliency protocols enable carrier grade resiliency for a superior subscriber experience -- RFC 3619 Ethernet Automatic Protection Switching (EAPS) and the emerging International Telecommunications Union (ITU) G.8032 standard for Ethernet Ring Protection Switching.

Outside the United States, Ethernet backhaul from the cell site to the cell site aggregation network is predominantly over microwave because fiber is too expensive. Although microwave radios only support point-to-point connectivity, building highly resilient microwave access rings is now possible with cell site Routers that support EAPS or G.8032 as illustrated in Figure 2.

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Figure 2 EAPS and ITU-T G.8032 enables highly resilient microwave access rings

Synchronization

2G and 3G voice services were designed to rely on T1/E1 TDM for backhaul, which were inherently synchronous. True 4G mobile backhaul enables low-cost packet-based alternatives to TDM backhaul, but will require synchronization to protect the investment in 2G and 3G services.

Two protocols have emerged for packet network synchronization -- ITU-T G.8262 Synchronous Ethernet (SyncE) and IEEE 1588v2 Precision Time Protocol. SyncE uses the physical layer of the Ethernet link to distribute the clock between nodes and provides the benefit of deterministic frequency distribution, independent of data traffic performance. IEEE 1588v2 operates at a higher layer, utilizing time stamps to distribute both time and frequency. IEEE 1588v2 timing accuracy is more dependent on data traffic performance than SyncE. Both play a role in true 4G mobile backhaul, ensuring that legacy TDM traffic encapsulated in TDM psuedowires is appropriately synchronized. Synchronization helps to ensure clean hand-off between towers as a subscriber roams with their mobile device.

Performance

ITU standards for 4G mobile services call download speeds of up to 1 Gbps to a mobile device. As 3G and 4G services proliferate, the amount of data traffic at the cell site increases significantly. To enable the price-performance required for 3G services and unleash true 4G mobile services, true 4G mobile backhaul provides high-density line-rate gigabit Ethernet with a clear path to line-rate 10GE without fork-lift upgrades.

Conclusion

As service providers deploy mobile backhaul for the growing wave of 3G services, it is important to keep requirements for emerging 4G services in mind. True 4G mobile backhaul includes three key ingredients – resiliency, synchronization and performance. Deploying true 4G mobile backhaul today protects the service provider’s investment in legacy 2G services while providing cost-effective backhaul for 3G services and laying the groundwork for the performance required to unleash 4G services.

Mark Showalter, Senior Director of Service Provider Marketing for Extreme Networks, has over 20 years of experience in the data communications industry focusing on carrier networks. Prior to joining Extreme, Showalter held positions at Infonetics Research, Matisse Networks, Procket Networks, CoSine Communications, and Ascend Communications.

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