Selecting Primary Reference sources

All telecommunications networks require highly accurate frequency references for the effective transmission of digital signals. The quality and precision of the reference equipment has a significant impact on a network's performance. Poor synchronisation reduces the network's ability to operate effectively.

This is applicable in both TDM architectures such as PDH and SDH and also in packet based architectures utilising TCP/IP, Ethernet and WiMAX.

The principles of distribution are subtly different but the main choices are what technology you utilise to provide your primary reference source and how it is deployed in your network. Horsebridge recognise that the choice is dependant upon the operators deign preferences, network architecture and budget.
 
How do I get my Network Synchronised?

There are two interrelated issues to be resolved to satisfy this question; what sort of Primary Reference should I use? How should I distribute and regenerate synchronisation in my network?
 
We will look at the first issue.
 
Choosing a Primary Reference

All network elements should have access to a synchronisation reference derived from a very stable frequency source, referred to as the Primary Reference. The Primary Reference is the anchor for all synchronisation. The ITU have laid down a standard for Primary Reference Clocks (G.811), which states that the maximum frequency offset or error from Co-ordinated Universal Time (UTC) for a Primary Reference Clock (PRC) should be no more than 1x10-11. Therefore the maximum frequency offset between any two PRCs can be no more than 2x10-11 and consequently the maximum slip rate between two PRC synchronised elements will be no more than one slip every 72 days for E1 based traffic.
 
There are a number of methods of deriving a Primary Reference:

  • Central master clock
  • Fully distributed master clocks
  • Partially distributed master clocks
  • Fully distributed clock references from a co-operating network.
  • Partially distributed clock references from a co-operating network.

Central Master Clock

A centralized master clock synchronisation network usually has a single PRC located at the logical center of the network. Synchronisation is distributed to each traffic element, usually through the traffic network. For SDH or SONET only the aggregate signals should be used to transport synchronisation, for pure PDH, tributary signals should be used.
 
Fully Distributed Master Clocks

A distributed master clock synchronisation network has a number of active pseudo-synchronous master clocks. The network structure is composed of a number of small autonomous sub-networks or islands. This approach has the benefit of simpler distribution planning and shorter distribution paths. Although technically feasible, it would not be economically viable to deploy cesium based master clocks in such an arrangement. However, the Global Positioning System (GPS) provides a suitable synchronisation source when harnessed by modern synchronisation equipment.

Partially Distributed Master Clocks

A partially distributed master clock synchronisation network has a limited number of active pseudo-synchronous master clocks. The network structure is composed of a number of autonomous sub-networks or islands. This approach has an economic benefit over fully distributed master clocks but requires more complex distribution planning and longer distribution paths.

Fully Distributed Clock References from Co-operating Network

If a co-operating, adjacent network has master clock synchronisation, which is easily accessible at multiple points, then it is possible to use these references to synchronise the entire network. The synchronisation network structure is composed of a number of small autonomous sub-networks or islands. In this case as both networks are slaved to the same master clock then theoretically the slip rate at the network boundaries should be zero. In practice there are a number of issues associated with this strategy; can the quality and availability of synchronisation be guaranteed? Is there a cost implication of utilizing synchronisation as a service?

Partially Distributed Clock References from Co-operating Network

If a co-operating, adjacent network has master clock synchronisation, which is accessible at limited points, then it is possible to use these references to synchronise the entire network. The synchronisation network structure is composed of a number of large autonomous sub-networks or islands. This approach has the same issues as that of a fully distributed clock references from co-operating network with he addition of more complex distribution planning and longer distribution paths. The decision as to which method to choose is usually based upon a number of factors including; network size or potential, political and commercial considerations. The form of Primary Reference also dictates how synchronisation will be distributed throughout the network.