Reducing the Cost of Wireless Backhauling Through Circuit Emulation

Data rate requirements of backhaul connections for wireless base transceiver stations (BTSs) continue to increase, while the cost of available Gigabit Ethernet connections decreases. As a result, IP/Ethernet backhauling has become a prime choice for new installments. However, for the hundreds of thousands of basestations deployed with time-division multiplexed (TDM) connections (E1/T1), carriers must pay much more for TDM lease lines, but can cut their costs by using circuit emulation service (CES) to transport the signals over a less expensive Ethernet network. For any CES application, clock recovery at the far end of the link represents a major challenge, as the interworking function (IWF) blocks need to support differential and adaptive clock recovery. For more demanding applications, such as synchronization of wireless basestations, carriers can apply a hybrid timing generator (HTG). This paper discusses the protocol details of CES, issues around clock recovery, and the network, node, and device architectures of CES solutions. The paper will also cover the benefits of FPGA-based implementations. Introduction As an increasing number of applications demand higher bandwidth, and Internet usage continues to expand, telecom operators need to adapt their networks. Most of the networks used by telecom operators today are based on TDM systems. These are ideal for transporting voice traffic and supporting leased-line applications, but with data traffic having become more dominant over the last few years, operators are migrating their networks to packet-based systems. In fact, some operators are even planning all-packet or all-Internet protocol (IP) networks as their next-generation solutions. For a smooth migration from a TDM-based to a packet-switched network (PSN), operators need to create a homogeneous network that uses packet technology in its core backbone, the metro, and the access networks. With reduced capital and operational expenses, a single network can be a substantial commercial advantage. However, even as operators plan all-IP networks, they still need to allow transport of TDM traffic over a PSN, since the TDM equipment used by business and government is not likely to be replaced at the same time. What’s more, network providers generate much more revenue from leased-line services than they do from data networks (Figure 1). Heavy competition from new entrants in the data services market forces established companies to lower their rates to keep up. Within the existing laws of Europe and the United States, even when telecom operators migrate to an all-IP network, they are legally bound to provide TDM access points for third-party resellers and wholesalers. Figure 1. Comparison of TDM Leased Line vs. Ethernet Services Revenue (Source: IDC, 2003) January 2008, ver. 1.0 1 WP-01049-1.0 Reducing the Cost of Wireless Backhauling Through Circuit Emulation Altera Corporation For TDM and leased line services, pricing pressure is less of an issue. In fact, many established network providers are seeing an increase in leased-line revenues due to the growth of networks for wireless backhaul. With the revenue difference between data and leased-line services, CES has emerged as a popular method for established players to maintain their revenue stream while the core of the network switches from native TDM lines to a packet-based infrastructure. This approach only works, of course, if end users are guaranteed the quality and performance levels they expect. Impact on the Wireless Network Many countries are experiencing stability in the number of voice calls, but an exponential increase in data services usage. This trend is also showing up on wireless backhaul networks. As data usage increases, the technology for high-speed data access on mobile networks is rapidly evolving to support the demand. However, the demand is causing increasingly visible strain on wireless backhaul networks. A typical radio basestation (RBS) can support a few hundred voice calls in parallel. On the backhaul link to a mobile switching center (MSC), a bandwidth of about four E1 or T1 lines is reserved per RBS. While a moderate mobile data connection today provides 384 Kbps, the next-generation networks already offering downlink speeds ranging from 1.8 to 14.4 Mbps (UMTS HSDPA phase 1). In the following planned phases (HSPA Evolved and Long Term Evolution [LTE]), the downlink data rate ultimately reaches 200 Mbps, while the uplink may provide up to 100 Mbps. To support this exponential growth in access link speeds, wireless backhaul networks need to be rebuilt using bigger pipes, such as those built with Ethernet technology. Continued bundling of E1 or T1 TDM pipes for transport simply will not be adequate to support these increases in data traffic. While an Ethernet-based PSN is ideal to transport data services, voice calls are TDM-based and need special handling on a PSN. Furthermore, a wireless basestation needs a high-quality clock reference to ensure its radio signal does not disturb neighboring basestations and to allow hitless handover of voice calls as mobile users move from station to station. Most basestations currently use a TDM network interface to transport voice and data traffic, as well as distribute network synchronization from a central source to all stations. Synchronization distribution is not supported by current Ethernet networks. CES and Transport Protocols A CES IWF allows a smooth migration from TDM traffic to packet traffic and vice versa. Instead of using fixed bandwidth multiplexed TDM circuits, the TDM traffic is packetized and aggregated with other TDM or IP traffic onto a single network. Traditional TDM networks have an implicit dual purpose: to transport data, and to maintain network wide signal synchronization. Today’s PSNs, however, do not support network synchronization. To compensate for this, network operators have created overlay networks for synchronization, or use expensive, GPS-based reference clocks. For any CES application, clock recovery at the far end of the link represents a major challenge. The IWF blocks support differential and adaptive clock recovery, but for more demanding applications, such as synchronization of wireless basestations, design engineers can apply an HTG. A CES IWF supports several independent E1/DS1 TDM-over-packet data engines. The TDM data from each input port is processed and forwarded as a packet stream. At the remote end, the packets are reassembled into a TDM data stream. Each of the TDM ports may be independently timed on ingress, with the reassembly function supporting independent clock recovery to retime the TDM data on egress. Figure 2 shows one method for encapsulating the TDM data over the PSN, where the implementation uses the popular and standardized SAToP and CESoPSN variants. (Both standards are compatible with those described by the Metro Ethernet Forum (MEF-8)).