ATM Adaptation Layers (AAL)
AAL1: constant bit-rate CBR service
AAL3/4: VBR service, checksums on each cell, detects loss/corruption; 4 bytes of 48 byte payload used for overhead [see book, p 183]
AAL5: VBR service, checksums on each frame, detects loss/corruption; simpler header, less overhead than AAL3/4