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ATM Adaptation Layer. Rizwan Rehman, CCS. 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 - PowerPoint PPT Presentation
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1Rizwan Rehman, CCS
ATM Adaptation Layer
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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
TCP
IP
AAL
ATM ATMNETWORK
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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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PDUHigherlayer
Adaptation Layer : A detailed view
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PDU
PDUCS-header Pad CS-trailer
Higherlayer
CSlayer
Adaptation Layer : A detailed view
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PDU
PDUCS-header Pad CS-trailer
SAR-PDUheader
SAR-PDUpayload
SAR-PDUtrailer
SAR-PDUheader
SAR-PDUpayload
SAR-PDUtrailer
Higherlayer
CSlayer
SARlayer
Adaptation Layer : A detailed view
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PDU
PDUCS-header Pad CS-trailer
Cellheader
Cellpayload
SAR-PDUheader
SAR-PDUpayload
SAR-PDUtrailer
SAR-PDUheader
SAR-PDUpayload
SAR-PDUtrailer
Higherlayer
CSlayer
SARlayer
ATMlayer
Cellheader
Cellpayload
Adaptation Layer : A detailed view
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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
48 bytes of data
5 byteheader
8 bytetrailer
flag
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AAL 5
CRC32
Controlfield
PadUser dataLength
field
0-47 bytes 2 bytes 4 bytes2 bytes
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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
