4.2.3 ATM adaptation layer
The basic function of the ATM adaptation layer is to convert the user data supplied by higher layers into 48-byte blocks of data. The ATM adaptation layer is divided into two sub-layers – the convergence sub-layer, and the segmentation and re-assembly sub-layer. The convergence sub-layer provides services to higher layers through a set of protocols, but I do not need to describe these here. The segmentation and re-assembly sub-layer separates the messages from the convergence sub-layer into ATM cells. Each of the two sub-layers adds some protocol information, which is transported in the payload of ATM cells as illustrated in Figure 29.