Abstract: In a synchronous digital communications system a network of SDH (or SONET) equipments (SE) are arranged to exchange synchronisation signals via ports on the SEs, each port being allocated a source identifier (SID). These SIDs are used to identify the source of each synchronisation signal input at a particular SE. When the SE selects a synchronisation signal to transmit out of its ports, the SID associated with that selected signal is compared with SID of each port. If a match is found the synchronisation status message (SSM) associated with the synchronisation signal sent out at that port is set to “Do Not Use” (DNU) to warn the destination SE that the synchronisation signal has been looped back. Each SE is allocated a unique equipment identifier. The SID of each port is set to the unique SE identifier of SE to which it is directly connected.

Abstract: A communications system utilizes, inverse multiplexing to transfer wide band signals over a plurality of narrow band links. To enable efficient use of the available links, incoming data whether or not cell or packet based, is inverse multiplexed in a byte format regardless of packet boundaries into virtual containers. Overhead signals are generated to enable the original data to be reassembled at the receiving end.

Abstract: In a network of SDH equipments (e.g. synchonous multiplexers add-drop or SMA) for the communication of signals comprising data, a synchronization signal for synchronizing the data and a synchronization status message (SSM) indicating the quality of the source of the synchronization signal, when a synchronization signal is output via a sychronization signal output (SSO) to a stand-alone synchronization equipment (SASE) for conditioning and subsequently input via a synchronization signal input (SSI), the SSM information may be lost. The invention provides means for associating, within the SDH equipment, the revelant SSM with the synchronization signal input at the SSI.

Abstract: A retiming arrangement for use in a demultiplexer in an SDH data transmission system uses Bit Justification data, and not Pointer data, to modify a recovered clock signal and generate a clock signal for retiming purposes. The invention is especially for use in enabling third party users to carry primary rate timing data across an SDH network.

Abstract: In an SDH telecommunications system, a buffer overfill/underfill problem, which is consequent upon either a synchronization failure or an excessive amount of wander, is overcome by detecting such failure or wander and temporarily changing the mode of operation of a de-synchronizer at a terminating end of the SDH path. A Synchronization Status Messaging Byte (SSMB) is used as an indicator of the quality of the incoming bearer timing.