Mobile operators are racing to deploy high-speed data services in order to acquire and retain lucrative mobile professional users. Because high-speed data services require increased backhaul capacity, mobile operators are seeking alternative, lower-cost backhaul methods in order to meet increasing data demands. At the same time, cost-reduction measures must not sacrifice consistent and high-quality service. As the network shifts to an Internet protocol (IP) backhaul, maintaining precise frequency distribution throughout the network is essential for maintaining service level assurance. The quality of synchronization mobile operators put into their network directly impacts the quality of service (QoS) that comes out of their network.

The transmission of voice, video and data through any communication network requires a stable frequency reference, and precise frequency synchronization is especially critical in mobile networks for the successful call signal hand-off between base stations as well as for the transport of real-time services. Global system for mobile communications (GSM) and universal mobile telecommunications system (UMTS) base stations must hold a carrier frequency accuracy of ±50 parts per billion (ppb) over the 10-year service life of the equipment. If individual base stations drift outside the specified 50 ppb limit, mobile hand-off performance decays, resulting in high dropped-call rates, impaired data services, and, ultimately, lost customers. The problem is, as more networks transition to an IP-centric backhaul, these changes in the backhaul also impact how the network derives an accurate sync feed.

GSM base stations have traditionally derived their long-term frequency accuracy from locking a relatively low-performance quartz oscillator embedded in the base station to a recovered clock signal from a T1/E1 leased line backhaul facility. Timing signals based on a primary reference source (PRS) transmitted over the backhaul keep the embedded oscillator calibrated to within sufficient accuracy. Without a well-synchronized backhaul feed to lock to, the oscillator frequency would drift out of specification in a matter of months, requiring regular and costly service calls to manually calibrate oscillators across base stations throughout the network. Thus, a reliable and accurate clock source is required for accurate synchronization.

It is important to note that even base stations that still make use of a T1/E1 time-division multiplexing (TDM) backhaul are beginning to experience levels of degradation that reduce QoS to the point that customers notice. Latency, jitter and wander were fairly consistent in the past, given how well-timed T1 lines were. However, as many backhaul providers increase the use of circuit emulation or IP encapsulation technologies to reduce their infrastructure costs, consistency of synchronization suffers. In addition, many providers are now transporting T1/E1s over a synchronous optical network (SONET) that introduces large phase deviations due to SONET pointer adjustments. SONET pointer adjustments can significantly degrade the stability of synchronization seen at the base station. The introduction of such errors can be large enough to isolate a base station, effectively cutting it off from its source of synchronization. As synchronization instability increases, so does the number of dropped calls, as does the number of customers that consider migrating to the competition.

Until recently, synchronization of GSM base stations has been taken for granted. As long as the T1/E1s were well timed, the base station could hold the 50 ppb requirement indefinitely. However, as backhaul transport evolves toward IP, base stations can no longer rely on recovering synchronization from the network side. In order to maintain consistent, quality connectivity, base station equipment manufacturers and backhaul service providers must take timing into consideration. New methods of synchronization are required to meet rising expectations of next-generation mobile users (Table 1).

When base stations carried just voice traffic, a single T1/E1 connection typically provided enough bandwidth for the backhaul connection. The rollout of third-generation (3G) data services, however, has increased the bandwidth needs for the backhaul connection significantly and moving to T3/E3 connections is simply too expensive.

Transport networks are rapidly evolving to IP-rich topologies. This offers mobile operators the increased backhaul capacity they require for deployment of high-bandwidth data services and the cost advantage of IP transport. However, the move to Ethernet backhaul will eliminate the option for base station clock recovery from the backhaul facility. Operators will need to move to an independent source of synchronization at the base station to meet the UMTS 50 ppb requirement (Figure 1).

In addition to traditional span line clock recovery where the base station recovers an accurate clock from the T1/E1 backhaul feed, UMTS Node B infrastructure suppliers are introducing high-quality embedded clock options to be ready for IP backhaul. Many of these options mirror code-division multiple access (CDMA) 2000 designs where GPS clocks are embedded into the base stations to provide a time-of-day reference needed for call hand-offs. CDMA networks have always relied on embedded GPS-based clocks with precision rubidium or quartz oscillators, making them inherently prepared for the evolution to IP backhaul from a sync quality point of view. Table 2 provides a summary of base station clock options for UMTS.

Using rubidium-based oscillators is the most robust solution for independent synchronization of UMTS base stations, as rubidium oscillators are proven to meet the 50 ppb requirement over the full service life of the equipment. Quartz oscillators, on the other hand, are subject to higher native aging rates and warm-up/restabilization characteristics that make it difficult to assure compliance to the 50 ppb requirement for more than a few years. This exposes network operators to QoS degradation and potentially high maintenance costs associated with manually calibrating quartz oscillators to bring them back on frequency after only a few years in the field. The danger to the operator is that this type of failure is undetectable until QoS issues reach a critical threshold.

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