
Configuring ATM DXI Services
2-16 117375-A Rev. A
The AAL is divided into two sublayers: the convergence sublayer (CS) and the
segmentation and reassembly (SAR) sublayer. These two sublayers convert
variable-length messages into 48-byte segments, while ensuring the integrity of
the data.
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 2-10).
Figure 2-10. 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
ATM0004A
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