Abstract: A packet switched communications system and method for transmitting synchronous data from a source module (4) to a terminating module (8) over a network comprising plurality of modules (4, 5, 7, 8) interconnected via transmission links (2, 6, 9). Each module operates with a clock of nominal frequency that is not synchronized with the clocks of the other module(s) and has a single input and one or more outputs. The method includes determining the phase difference between the input clock and the output clock of each module, and transmitting the phase difference to the terminating module (8) in the network. The received accumulated phase difference at the terminating module (8) is used to lock the output clock at the terminating module to the input clock at the source module.

Abstract: A communications system operator provides to each user of a multi-user communications system a committed information rate (CIR). When a user requests bandwidth for transmission, a system controller queues the user in a high-priority subclass A. The system controller allocates bandwidth to the user at the head of the queue as requested but if the user's bandwidth, measured as an average over a sampling time, exceeds its CIR, it is moved into a low-priority subclass B. The user can then only obtain further bandwidth after other users in the high-priority subclass have been allocated bandwidth up to their CIRs. If a user in the low-priority subclass only receives allocation of a small amount of bandwidth, so that its average inforamtion rate falls below its CIR, it is moved back in to the queue in the high-priority subclass.

Abstract: A packet switched communications network comprises a controller (11) and a plurality of controlled units (12, 13, 14) arranged in a ring. Each of the controlled units comprises a switch (31) at its input, each switch including control means (44) that are responsive to control instructions transmitted by the controller (11). Depending on the instructions the switch (33) will either be configured to pass control instructions and data into or out of the controlled unit (12, 13, 14), or to forward the control instructions and data to a further controlled unit (12, 13, 14) that is connected to the controller (11) or back to the controller (11). In order for units in the ring, that are not registered with the controller (11), to acquire an address, the controller (11) transmits periodically broadcast packets, which contain a unique registration address to be used by an unregistered unit. Once registered, the unit can receive control instructions from the controller (11) as well as data.