Understanding the network.pdf - Back to Home

The SAR footer contains two fields:
• Length indication—Displays the size of the actual data content in the PDUpayload
• CRC—A CRC value for the entire PDU for error checking
AAL5
AAL5 is also a VBR connection-oriented adaptation service. AAL5 does not have the
SAR overhead that AAL3/4 has. The AAL5 PDU contains a 40-Byte PDU-payload and
an 8-Byte CPCS footer. The CPCS footer contains user and cell boundary information,
the size of the actual data portion (versus buffer) contained in the PDU-payload, and
a CRC for integrity checking. The lack of SAR processing and processing information
makes AAL5 attractive compared to AAL3/4 because it provides a payload savings
and reduces the processing overhead and requirements on the ATM switch. AAL5 is
used for most ATM LAN transport applications, such as LAN emulation and classical
IP over ATM.
ATM Signaling
Signaling AAL (SALL) provides transport of SVC call setup and teardown messages.
Due to the importance of signaling messages, SALL provides a robust transport
mechanism, and uses the AAL5 CPCS and SAR functions. The SALL PDU consists of
a 40-Byte PDU-payload with an 8-Byte CPCS footer. SALL uses two sublayers to
process ULP signaling information: the Service-Specific Coordination Function
(SSCF) and the Service-Specific Connection-Oriented Protocol (SSCOP). The SSCF
provides the mapping service from the ULP application to the SSCOP. The SSCOP
provides the connection-oriented reliable transport mechanism for the signaling
messages. In addition to the error detection capabilities provided by AAL5, SSCOP
provides frame sequencing, frame recovery, and error correction capabilities.
ATM VCs are established manually or dynamically. Manual VCs are employed with
ATM PVC-type circuits. Dynamic VCs are used for SVC applications. ATM connection
setup uses a hop-to-hop signaling method. This approach sets up calls by cascading
the call setup request from switch to switch in the call path. This establishes the VCC
path between each switch along the way to the destination. When the destination
endpoint gets the request, it then accepts or denies the call.
The SVC call dialog for establishing a bidirectional point-to-point connection
between two ATM endpoints goes like this: