Timing and Synchronization in Digital Telephone Exchange System
5.35.1 National Interworking
The CO digital switching system shall allow the introduction of a fully synchronized digital network employing a master slave configuration.
At the highest level of the network hierarchy, a National Master clock of very high accuracy and stability shall be used as a reference for network synchronization purpose. Timing signals shall be distributed through the Integrated Digital Network (IDN), and until IDN is completed, the CO digital telephone exchange shall allow plesiochronous operation with the free run mode of its own exchange clock.
The second level of network hierarchy, i.e., the zone switching center and/or the group switching center, shall be equipped with clocks of lower standards than the master clock. These exchanges shall be synchronized to the international telephone exchange on a master slave configuration. In the event of failure or loss of the synchronization link for a particular exchange, synchronization shall automatically switchover to a stand-by synchronization link, which can be either from the international telephone exchange or from a zone switching center or a group switching center, acting as secondary master clock.
The third level of the network hierarchy, i.e., the local digital exchanges shall operate as a slave to their corresponding upper hierarchy levels.
5.35.2 International Interworking
The synchronized network is requested to operate, on a plesiochronous mode with other national digital networks in accordance with ITU-T G.811. The manufacturer shall ensure that, the design of the proposed digital exchange(s) for Saudi Aramco, are in compliance with ITU-T G.811, which states that, all clocks at network notes that terminate international links will have a long-term frequency departure of not greater than ± 1 X 10-11. This is the minimum accuracy of stratum one (1) clock requirements. The National Master Clock (NMC) shall be collocated with the digital international telephone exchange.
5.35.3 Exchange Timing Distribution
The distribution of the timing within each exchange shall be such that the exchange maintains synchronism through the exchange. Furthermore, whenever it is required, the Remote Satellite Unit (RSU) and the Remote Concentrator Unit (RCU) shall operate synchronously with their parent exchange or RSU. The RSU shall be able to operate in free run when isolated from the parent exchange.
5.35.4 System Tolerance
The system shall accept phase deviations without the introduction of the slips and/or errors, within specified limits of “jitter” and “wander” according to ITU-T G.821, and ITU-T G.822. In the event where, plesiochronous operation is introduced during the phase at which IDN is not completed, the system shall be equipped in such a manner as to accept phase deviations without the introduction of slips and or errors. The manufacturer shall state the accepted limits, and also shall state clearly the long-term accuracy and stability data of the proposed clocks at all levels of the network hierarchy. The average life expectancy of the clock oscillators, regarding system clock reliability, is 20 years.
5.35.5 Synchronization Loop
The CO digital exchange shall check and report any phase differences between the master and local clock signals, and shall detect and report slips, bipolar violations, and lost synchronization.
The CO digital exchange clocks shall present at least 1+1 redundancy with automatic changeover and without loss of synchronization in the event of failure. The manufacturer shall state the type of redundancy offered.