Content area
Full text
In order for the network to support the burgeoning era of new high speed digital services, transmission systems based on the synchronous digital hierarchy (SDH) must be implemented successfully. This raises a number of synchronization problems and issues in the network that are a result of the new SDH ring and chain topologies. It is worthwhile, therefore, to analyse the synchronization network with special attention to distribution architectures that will enable timing distribution to migrate to a higher level of reliability in support of the new topologies and services.
The goal of network synchronization is to get the best possible source of timing to all nodes of the network. The two fundamental requirements that must be in place to ensure network synchronization are:
* an accurate source of timing;
* a 'reliable messenger' that distributes timing to all of the nodes.
The architecture employed to date to achieve network synchronization has been based on hierarchical timing distribution, which involves the establishment of primary reference clock (PRC) locations which then feed subtending nodes of either transit node clock (TNC) or local node clock (LNC) quality. These PRC locations fill the first fundamental requirement of the hierarchical network for an accurate source. These locations typically contain replicated caesium clocks manually calibrated to universal coordinated time (UTC).
The second requirement of hierarchical timing distribution is filled by qualified landline connections that serve as reliable messengers for transporting timing from one node to the next. These landlines are typically engineered and maintained to minimize the effects of network rearrangements on synchronization distribution. The deployment of SDH introduces timing complexities as a result of ring and chain transmission architectures. Now, the network is no longer made up of simple point-to-point connections, but has been replaced with a transport network of 'intelligent' and 'survivable' connections. These intelligent connections result in paths that are subject to change for the reliable messenger. In addition to the changes, these routes are no longer through paths but are subject to perturbations introduced by serial SDH network elements. The subsequent administration and maintenance of this variable path timing network must also be carefully planned in order to ensure network integrity.
* As the SDH network continues to grow, the role of landline connections, as the...