Download ATM Adaptation Layer (AAL)

ATM Adaptation Layer
• The ATM Adaptation Layer (AAL) defines
the rules for breaking up a higher level
protocol data unit (PDU) into ATM cells for
transmission on the network, and for
reassembling the PDU at the other end
• Segmentation and reassembly (SAR)
• Convergence sublayer: adds sequencing and
control information to aid SAR process
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Example: ATM Adaptation Layer
TCP
IP
AAL
ATM
ATM
NETWORK
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TCP
IP
AAL
ATM
AAL Protocols
• There are several different proposed AAL
protocols for different traffic types
• Example: AAL 1
– for real time or continuous bit rate services,
such as video traffic
– requires 1 byte of overhead per ATM cell
(i.e., within the 48 byte payload)
– 1 bit CSI, 3 bit sequence count, 3 bit CRC, and
1 bit parity
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AAL Protocols (Cont’d)
• Example: AAL 3/4
– an adaptation layer for connectionless or
connection-oriented data traffic
– 4 bytes overhead per ATM cell
– 16 bit header (2 bit type, 4 bit seq, 10 bit MID)
– 16 bit trailer (6 bit length, 10 bit CRC)
– very high overhead!!!
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Adaptation Layer : A detailed view
Higher
layer
PDU
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Adaptation Layer : A detailed view
Higher
layer
CS
layer
PDU
CS-header
PDU
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Pad CS-trailer
Adaptation Layer : A detailed view
Higher
layer
CS
layer
PDU
CS-header
PDU
SAR SAR-PDUSAR-PDUSAR-PDU
layer header payload trailer
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Pad CS-trailer
SAR-PDUSAR-PDUSAR-PDU
header payload trailer
Adaptation Layer : A detailed view
Higher
layer
CS
layer
PDU
CS-header
PDU
SAR SAR-PDUSAR-PDUSAR-PDU
layer header payload trailer
ATM
layer
Cell
Cell
header payload
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Pad CS-trailer
SAR-PDUSAR-PDUSAR-PDU
header payload trailer
Cell
Cell
header payload
AAL Protocols (Cont’d)
• Example: AAL 5
– an adaptation layer for data traffic designed by
data neworking researchers
– Simple and Efficient Adaptation Layer (SEAL)
– much lower overhead
– 8 byte trailer per PDU (i.e., last cell of a PDU)
– 1 bit of header in last cell of a PDU
– 48 bytes of data in each ATM cell
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AAL 5
User data
5 byte
header
flag
48 bytes
of data
10
8 byte
trailer
AAL 5
User data
Pad
0-47 bytes
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Control
field
Length
field
CRC
32
2 bytes
2 bytes
4 bytes
Summary
• The ATM Adaptation Layer (AAL) defines
the rules for breaking up a higher level
protocol data unit (PDU) into ATM cells for
transmission on the network, and for
reassembling the PDU at the other end
• Several AAL’s to choose from
• AAL5 is the best for data traffic
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