A problem occurs for legacy GSM base stations that have relied on recovering synchronization from traditional T1/E1 backhaul lines provided by the incumbent local exchange carrier/public telephone and telegraph (ILEC/PTTs) to maintain their 50 ppb frequency accuracy requirement. Changing the backhaul to circuit emulation services (CES), such as the satellite transport as shown in Figure 2, requires that a local source of synchronization be placed at the base station to deliver an accurate clock reference since the satellite network cannot support the 50 ppb requirement on its own. There are two ways to achieve this: 1) install an external GPS clock to externally time the base station equipment, or 2) use a GPS-based retimer as shown in Figure 2.

A GPS-based retimer buffers incoming traffic and clocks it back out with PRS level accuracy (Figure 3). A retimer can be transparently introduced to an existing base station as the retimer is placed on the back-haul feed directly before the base station. The timing signal the base station receives is reclocked to be precise and stable, enabling accurate synchronization between base stations.

Note that the retimer itself requires access to a PRS-based clock. Typically, this is achieved using an integrated GPS receiver. Such a retimer should implement a cut-through mechanism to preserve communications when the GPS signal is not available, eliminating the retimer as a point of failure. When cut-through is enabled, the base station will revert to the original backhaul timing scheme and its original dropped call rate without the retimer. In this way, retimers can only improve quality, never reduce it.

Retimers can result in a dramatic reduction in dropped calls. A five-cell field trial conducted in September 2004 with a major GSM operator resulted in a 25.5% reduction in dropped calls when the backhaul synchronization signal was retimed (Table 3). The five base stations involved in this test were experiencing relatively high dropped call rates, and synchronization impairments on the backhaul lines were suspected as the root cause. Note that these measurements include call hand-offs with base stations not using retimer synchronization feeds. While there was still significant improvement in these cases, the most substantial improvement was realized when both base stations involved in the call hand-off were retimed.

Certainly, the future is moving toward IP and lower operating costs. To achieve this without an external synchronization source such as a global positioning system (GPS) retimer, however, carriers must find a way to reliably tunnel timing signals through the IP network. One of the more promising technologies currently under study is the Institute of Electrical and Electronics Engineers (IEEE) 1588 precision time protocol. The IEEE 1588 standard was developed to support Ethernet local area network (LAN) environments for applications such as factory automation where distributed motors and servos need to be accurately time synchronized. At this time, the IEEE has opened a working group to study the feasibility of enhancing the IEEE 1588 protocol for use in telecom wide area network (WAN) applications. The IEEE 1588 precision time protocol is based on a master clock exchanging two-way timing packets over Ethernet with slave clocks embedded in the equipment requiring synchronization. For example, carriers would place master clocks in their network, which would serve multiple local base stations, typically within a few hops from the master clock.

To support IEEE 1588, all base stations would have a client that would calibrate itself to the master clock using a two-way protocol. The master clock could be implemented as a stand-alone server located in the wireless network, or within a metro Ethernet network.

Primary technical challenges faced by IEEE 1588 for applicability in telecom networks include whether carriers will be able to constrain the number of network hops between master clocks and slave clocks, as well as whether delay variation can be sufficiently contained as too much jitter and wander would compromise IEEE 1588 accuracy. Realistically, standardization activity has just begun and it may take considerable time for the standard to emerge. In addition, the performance of IEEE 1588 over a wide area network in various traffic conditions and backhaul network bandwidth/speeds is yet to be studied or understood. In the mean-time, carriers will still need to address increasing instability in their backhaul connections.

The future of the telecom industry is IP. However, in order to keep customers long enough to enjoy infrastructure savings, carriers must implement mechanisms for maintaining quality connectivity through synchronization accuracy. Accurate synchronization is the Achilles' heel of today's mobile cellular networks. The cost of customer churn as a result of poor QoS from dropped calls and other sources can dwarf the savings achieved by moving to IP. While efforts are in motion to increase the reliability of clock signals transported over IP, carriers need mechanisms they can deploy today. Stand-alone compact rubidium oscillators, and GPS-based PRS feeds and retimers restore stability to critical timing signals essential for accurate synchronization in a manner completely independent of whichever backhaul flavor a particular base station employs. For base stations that have not yet moved to IP, retimers can effectively compensate for the creeping degradation of the TDM network.

Barry Dropping joined Symmetricom in October 1999 following Symmetricom's acquisition of Hewlett-Packard's communication synchronization business as director of wireless OEM products. Before coming to Symmetricom, Dropping spent 15 years at Hewlett-Packard where he served in a variety of management positions across engineering, marketing and operations. Dropping holds a BS in Electrical Engineering Technology from the DeVry Institute. He can be reached at BDropping@symmetricom.com.

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