Avaya Configuring ATM Services Manual de usuario Pagina 35

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Understanding ATM, ATM Router Redundancy, and OAM
308612-14.00 Rev 00
1-9
The CCITT (now ITU-T) has defined different types of AALs to handle different
kinds of traffic. Nortel Networks ATM routers support a CS function compatible
with AAL 5, as defined in Section 1 of the CCITT (now ITU-T) B-ISDN Protocol
Reference Model (PRM).
In AAL 5, only a trailer attaches to the data from the upper-layer protocols to
create a CS PDU. AAL 5 divides the protocol data unit (PDU) into a 48-octet SAR
PDU; however, it does not add an SAR header and trailer. This 48-octet SAR PDU
becomes the payload of the ATM cell (Figure 1-5
).
Figure 1-5. ATM Adaptation Layer 5
Note: AAL 5 does not attach an SAR header or trailer to the PDU.
Service data unit (SDU)
SAR PDU
ATM cell
SDU
SAR PDU
SAR PDU
ATM cell
ATM cell
AAL 5
convergence
sublayer
AAL 5
SAR
layer
ATM
layer
AAL
layer
Service
class
Trailer
ATM0005A
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