
ATM Overview
1-5
The CCITT (now ITU-T) has defined different types of AALs to handle different
kinds of traffic. Bay Networks routers support AAL 3/4 and AAL 5. The next two
sections describe the AAL process.
AAL 3/4
In the AAL 3/4 process, a Convergence Sublayer (CS) header and trailer envelop
the data from the higher layer protocols, creating a CS protocol data unit (PDU).
AAL 3/4 divides the PDU into 44-octet segments. Each segment is encapsulated
with a segmentation and reassembly (SAR) header and trailer, creating a 48-octet
SAR PDU. This 48-octet SAR PDU is the payload of the ATM cell (Figure 1-4).
Figure 1-4. ATM Adaptation Layer 3/4
Service Data Unit (SDU)
SAR PDU
ATM Cell
SDU
SAR PDU
SAR PDU
ATM Cell
ATM Cell
AAL 3/4
Convergence
Sublayer
AAL 3/4
SAR
Layer
ATM
Layer
AAL
Layer
Service
Class
Header
Trailer
Trailer
Header
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