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ATM Adaptation Layer:
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converges packets from upper layers into ATM cells
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AAL types and classes
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AAL1 - connection-oriented, constant bit rate (CBR), good for audio and video; class A
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AAL2 - connection-oriented, variable bit rate (VBR), good for compressed video; not implemented; class B
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AAL3/4 - variable bit rate (VBR), timing not required; class C or D
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AAL5 - connection-oriented, variable bit rate (VBR), timing not required; subset of AAL3/4; popular implementation; class C
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