System information

Troubleshooting ATM Switching Environments 21-439
The ATM Layer
The AAL is divided into two sublayers: the convergence sublayer (CS) and the segmentation and
reassembly sublayer (SAR; see Figure 21-12).
Figure 21-12 Information Flow Through AAL
The convergence sublayer (CS) accepts higher-layer traffic for transmission across the network.
Depending on the AAL type, header and/or trailer fields are added to the packet. The packet is then
segmented by the SAR sublayer to form 48-byte payloads (also known collectively as SAR-PDUs).
Upon receipt of cell payloads, the AAL removes any AAL-specific information from each payload
and reassembles the entire packet before passing it to a higher layer (see Figure 21-13).
Figure 21-13 The SAR Portion of the AAL Process
The ATM Layer
The ATM layer accepts the 48-byte SAR-PDUs from the SAR process, adds a 5-byte header to each,
and produces ATM cells for transfer to the physical layer (see Figure 21-14).
Figure 21-14 The ATM Layer Process
CS-PDU CS-PDU SAR-PDU SAR-PDUCS-PDU
AAL processes
Convergence sublayer (CS):
user data is broken up into
packets (CS-PDUs)
Segmentation and reassembly (SAR)
sublayer: each CS-PDU is broken up
into SAR-PDUs
User data To ATM layer
48-byte payload
CS-PDU CS-PDUCS-PDUCS process
SAR-PDUSAR process SAR-PDU SAR-PDU SAR-PDU
User data stream
SAR-PDU SAR-PDU
From SAR sublayer
of AAL
SAR-PDU
5-byte
header
5-byte
header
SAR-PDU
To ATM layer To physical layer
ATM process
ATM cell ATM cell