Reader Forum: Mobile backhaul transition requires new testing

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As data delivery moves toward next generation of networks providing data, video, and voice services encompassing wired and wireless networks, one of the major challenges for wireless service providers is moving from a time division multiplexing (TDM)-based infrastructure to Carrier Ethernet.
Infonetics Research estimates there are more than 4 billion cellular subscribers, and by end of year 2009 more than 350 million cellular mobile subscribers used broadband connections. The subscriber count of cellular mobile broadband access will exceed wireline broadband access in 2010. Worldwide wireless broadband subscribers will have an annual growth rate (CAGR) of 37% from 2009 to 2014.
Technologies such as high speed packet access (HSPA) and evolved HSPA (HSPA+) are improvements that lead to better utilization of the existing radio spectrum. HSPA improves the end-user experience by increasing peak data rates to 14 Mbit/s in the downlink and 5.8 Mbit/s in the uplink. With 240 commercial deployments in over 110 countries worldwide, HSPA implementations are well under way. Long-term evolution (LTE), as defined by the Third Generation Partnership Project (3GPP), is widely acknowledged as the next-generation technology for both voice and data wireless transmission.
Mobile backhaul is the network for transporting mobile traffic between cell sites (BTS/NodeB’s) and radio controllers (BSC/RNCs). TDM circuits have historically inter-connected base stations to regional network controllers. However, the rapid growth in mobile broadband traffic has overloaded TDM circuits and they are unable to keep up with the uptick in wireless traffic growth. Using Carrier Ethernet for wireless backhaul allows operators to support large bandwidth increases from cell sites, while only incurring small increases in operational costs.
It is essential that wired and wireless components now be tested with the same types and scale of traffic seen in actual network deployments. The iPhone is the perfect example of a high performance, multimedia-capable device. Networks, both wireless and wired, must forward and shape handheld-driven traffic so as to ensure balanced quality of experience (QoE) for all network users.
Such increases require providers to understand their network’s ability to deliver services to millions of customers in a timely and consistent fashion. Providers must employ subscriber modeling. Subscriber modeling emulates real-world traffic patterns and data levels. It’s not enough to just simulate users and applications — service providers must be able to test network reliability down to the behavior and experience of each individual subscriber.
The Metro Ethernet Forum (MEF) is the leading organization developing technical specifications and implementation agreements to promote interoperability and deployment of Carrier Ethernet and its use in mobile backhaul. The MEF 22 Mobile Backhaul Implementation Agreement defines:
• Use cases and migration strategy
• Services (classes of service, service types, service performance)
• Generic interworking function
• Synchronization and clock recovery
MEF 22 outlines a phased implementation approach (see figure below). The first phase is supplying packet offload of data services from TDM links to IP/Ethernet using Carrier Ethernet services. In this phase voice traffic remains on a separate TDM network.
A minority of operators worldwide have started to take the plunge to a single IP/Ethernet network carrying integrated data, voice, and video traffic. Carriers must have confidence, however, that the proposed backhaul approach will match TDM characteristics in terms of reliability, voice quality, and timing/synchronization.
The MEF addresses reliability and failover by defining a new set of operation, administration, and maintenance (OAM) protocols and procedures. It is important to test that a device or system can support Ethernet Virtual Circuits (EVCs) with defined service performance in terms of frame delay, frame delay variation, and frame loss ratio.
Conquering timing issues is one of the fundamental challenges of using Carrier Ethernet for mobile backhaul. Cell sites must be synchronized in order for inter-base station handovers to function properly and reliably. To address the issue, MEF 22 recommends the use of IEEE specification 1588v2 and ITU-T Sync-E.
IEEE 1588v2 defines a protocol for achieving highly accurate timing and synchronization in a packet network. Clock information is distributed using dedicated timing packets between a master and slave node. Synchronization packets are always exchanged between a master and slave regardless of the amount of data traffic. 1588v2 delivers both frequency and phase, and it can be used for FDD and TDD system timing solutions. The IEEE 1588v2 packets are fully Ethernet and IP standards compliant and backward compatible with all existing Ethernet and IP routing and switching equipment.
Scalability and stress tests go beyond functional testing and measure how well a synchronization scheme scales in a real network. For example, it is important to test the capability and capacity of a boundary clock or PTP enabled network in achieving synchronization across thousands of network nodes. Equally important is testing devices or networks under high data rates and a high number of ordinary clocks achieving synchronization while measuring both data rate and synchronization performance. Data measurements should include packet delay variation (minimum delay, maximum delay, jitter, and distribution). A critical aspect of synchronization performance is PTP message prioritization under the maximum traffic load data rate.
As mobile broadband data demands continue to explode, the cost of TDM scales exponentially in relation to traffic growth. The ARPU carriers can charge for advanced services are relatively flat. IP/Ethernet backhaul has the potential to solve the ARPU-traffic disconnect for 3G HSPA today and 4G LTE in the future.