L2TP applications on Layer 2 protocols
L2TP protocol

the level 5 ATM adaptation layer acts as a link between the software layers and the low-level layer. The AAL5 comprises 2 components: the Convergence Sub-layer (CS) and the Segmentation And Reassembly Sub-layer (SAR). The CS encapsulates the application packets in packets of different sizes, and thus manages all the control aspects of these packets (transmission times, losses, errors); the SAR, on the other hand, slices the encapsulated packets to integrate them into ATM cells (48 bytes).