Aal5 - 3Com CoreBuilder 9000 User Manual

ATM Adaptation Layer (AAL) 443

AAL5

The AAL5 is the data-oriented adaptation layer. The CRC is appended to
the end of the frame to protect the user information against bit and cell
errors. It is padded to align to 48 bytes (Figure 35). To indicate frame
boundaries and determine when to reassemble and when to stop
reassembling, the space bit for Payload Type (PT) in the cell header is zero
except for the last cell in the packet (when it is a one). The cells are
reassembled by looking at the VPI/VCI and, for a given VPI/VCI,
reassembling them into a larger packet (Figure 36). Multiple
conversations may be interleaved on a given connection. An example of
an application that uses the AAL5 is LAN Emulation (See Figure 34 for a
summary of the applications used by the various AALs.)
Figure 34 Service Classes and AAL Types