Two elements can be used to describe a reference configuration of the User-Network access of B-ISDN. They are 1)Functional groups 2) Reference points. The following figure gives the reference configuration.
B-NT1 functions are similar to Layer1 of the OSI Reference model and some of the functions are 1)Line Transmission Termination 2)Transmission Interface handling 3)OAM functions.
B-NT2 functions are similar to layer1 and higher layers of the OSI model. Some functions of B-NT2 are 1) Adaptation functions for different interface media and topology 2) Multiplexing & demultiplexing and concentration of traffic 3) Buffering of ATM cells 4) Resource allocation & Usage parameter control 5) Signalling protocol handling 6) Interface handling 7)Switching of internal connections. SB and TB indicate reference points between the terminal and the B-NT2 and between B-NT2 and B-NT1 respectively.
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