1

Scope

This International Standard defines a set of procedures collectively called 'Digital Channel Aggregation', which are used in the provision of an aggregated bearer service. Digital channel aggregation involves a means for the combination of multiple switched or unswitched 56 kbit/s or 64 kbit/s digital commun ications channels across one or more public and/or private networks into higher bandwidth digital bi-directional channels between Channel Aggregation Units (CAUs) serving terminal applications at network endpoints. Figure 1 illustrates a typical channel aggregation scenario.

In defining the channel aggregation procedures, this International St andard sets out the means used by a CAU to

- establish the parameters to be used for the aggregated connection; - synchronise and align multiple communications channels to maintain the bit sequence integrity of the aggregated bandwidth across the network(s); - optionally monitor data transfer throughout a call to detect failure modes; - institute failure recovery procedures; - optionally dynamically vary the bandwidth on demand during a call.

The channel aggregation procedures above are defined at the interface between the CAU and the network in terms of the frame structur e and information messages applied to individual communications channels. Not defined by this International Standard are

- the call control procedures used to establish and disconnect the individual communications channels that make up the higher bandwidth connections; - the higher level protocols or applications that use the channel aggregation services; - the frame structure on the individual communications channels at the physical interface between the CAU and the network as required for 'normal' 56 kbit/s or 64 kbit/s bearer services as opposed to aggregated bearer services; - the electrical characteristics of the individual communica tions channels at the physical interface between the CAU and the network.