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ATM endpoints. The user adaptation layer deals with the flow of user data
information into the ATM layer.
To accommodate different user data applications, ATM supports five different data
adaptation layers. Each AAL service has two functions: segmentation and
reassemble (SAR) and common part convergence sublayer (CPCS). The SAR
segments the ULP user data into the 48-byte cells needed for transmission and
extracts the cell data to reconstruct the UPL data as the cells are delivered. The
CPCS "preps" the ULP data by inserting additional buffer and segment tags so the
ULP data can be segmented and reassembled cleanly by the SAR.
AAL1
AAL1 is a CBR connection-oriented adaptation service. It is used for applications
that have (PVC) connections between the source and destination endpoints that
require traditional dedicated circuit transport, such as voice and video applications.
AAL1 requires that synchronization timing be shared between the two ATM
endpoints. It uses a 47-Byte Protocol Data Unit (PDU) and a 1-Byte header that
contains a 4-bit Sequence Number (SN), which is used to provide timing and cell
order information, and a 4-bit Sequence Number Protection (SNP), which is a CRC of
the SN. The SNP is used to perform integrity checks on the SN field.
AAL2
AAL2 is a VBR connection-oriented adaptation service. AAL2 has never been fully
developed by the ITU-T standard and is not supported in current ATM
implementations.
AAL3/4
AAL3/4 is a VBR connection-oriented and connectionless service. AAL3/4 does not
require a (PVC) connection between source and destination endpoints. AAL3/4 uses
a 44-byte PDU combined with a 2-byte SAR header and a 2-byte SAR footer. The
SAR header contains three fields:
• Segment type—Indicates what part of the ULP message is contained in the
PDU-payload (beginning, middle, or end)
• Sequence number—Indicates the PDU-payload's placement in the ULP
message reassembly
• Multiplexing identification number—Indicates the source of the PDU-payload
so the data can be discerned from other ULP messages that are contained in
the multiplexed VC stream