ATM Adaptation Layer

1
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

2
Example ATM Adaptation Layer
ATM NETWORK
3
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

4
AAL Protocols (Contd)

• 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!!!

5
Adaptation Layer A detailed view
PDU
Higher layer
6
Adaptation Layer A detailed view
PDU
Higher layer
CS layer
PDU
CS-header
Pad
CS-trailer
7
Adaptation Layer A detailed view
PDU
Higher layer
CS layer
PDU
CS-header
Pad
CS-trailer
SAR layer
SAR-PDU header
SAR-PDU payload
SAR-PDU trailer
SAR-PDU header
SAR-PDU payload
SAR-PDU trailer
8
Adaptation Layer A detailed view
9
AAL Protocols (Contd)

• 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

10
AAL 5
User data
8 byte trailer
flag
48 bytes of data
5 byte header
11
AAL 5
CRC 32
Control field
Pad
User data
Length field
12
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 AALs to choose from
• AAL5 is the best for data traffic