Primary Reference Source Product Overview

The deployment of many PRSs throughout the network provides the following benefits over the traditional single PRS architecture:

• Enables migration from TDM to Ethernet
• Simplifies network timing administration and record keeping
• Reduces the complex task of distributing clock signals
• Prevents clock errors from propagating across the network
• Ensures high Quality of Service (QoS)
• Limits timing loops
• Reduces alarms especially for 3E SSU offices
• Increases security
• Flattens the sync distribution hierarchy
• Lowers Operation, Administration, Maintenance & Provisioning costs
• Reduces the number of network recovery clocks (TSG/SSU) operating in tandem
• Minimizes SONET/SDH pointer adjustments
• Prevents up-stream network clock errors from propagating across the network
• Provides total control of the network synchronization sources

Symmetricom offers a wide variety of Primary Reference Sources (PRSs). These products are categorized by the source that is used to derive their timing outputs. There are three generally accepted PRS sources:

• Cesium
• GPS
• Packet PRS (IEEE 1588 Precision Time Protocol)

Cesium PRSs are self-contained and autonomous providing the core synchronization reference for all networks. GPS systems use a roof top, window, or wall mounted antenna. GPS signals cannot penetrate the building, so access must be ensured using an antenna via a view to the sky. Packet PRS employs carrier class IEEE 1588 Precision Time Protocol to deliver PRS quality synchronization over Carrier Ethernet interoffice connections providing antenna-less installation well suited to applications where access to GPS signals is difficult or impossible.

Symmetricom recommends a mix of each type of PRS for most networks. For large and important offices, dual GPS and Cesium PRSs providing source redundancy and diversity, high reliability and security is suggested. This type of application is also referred to as a Primary Reference Clock (PRC).

In other offices, we recommend a mix of GPS, cesium and Packet PRSs deployed throughout the network. This is to gain the benefits of local PRS deployment and to mitigate the inherent risks associated with depending on a single source. The exact mix will be contingent on local Service Provider policies and the unique site specifics of each office. Site-specific factors to consider include:

• the type and location of the office
• ownership of the building
• access to a view to the sky via the roof top, windows, or outside walls.

These factors impact the cost and complexity of installing a GPS antenna.

For simple antenna installations, GPS systems are the most economical. Complex antenna installations and in cases where there is no view to the sky, Cesium Packet PRS is often the most economical solution. CDMA systems are recommended at network edge facilities and for emergency restoration purposes.