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适用范围：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 communications 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 below illustrates a typical channel aggregation scenario. In defining the channel aggregation procedures, this International Standard 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 structure 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 communications channels at the physical interface between the CAU and the network.