1. Transfer of SDU's (service data units) at a constant source bit rate from source service layer to destination service layer.
   
   
2. Transfer of side information, e.g.
   • Timing information for reconstruction of voice or video.
   • Data structure (meaning of bits to service).
   • Lost data (not recovered by AAL).

1. Handle cell delay variation (in the network)
   • e.g. simulate constant bit rate phone line.
2. Handle reassembly delay variation (at the receiver).
3. Source clock recovery
   • e.g. using "residual time stamp" feature.
4. Source data structure.
5. Monitor lost or misinserted cells.
6. Monitor bit errors.
7. Report end-to-end performance quality.

1. Receive bit stream of CS.
2. Segment into 47 byte blocks.
3. Add SAR header:
   • CSI=CS Indication.
   • SN=sequence number.
   • SNP=sequence number protection (against errors in CSI or SN).

This AAL uses an interlaced Reed-Solomon code. A Reed-Solomon is able to detect and correct lost bits in a stream.

• Reed-Solomon (124, 128)

  If real data is 124 bytes, add header of 4 bytes to become 128 bytes. Receiver can detect up to 4 bytes of error from the received data.

• Interlacing

• Note that interlacing introduces additional delay.

AAL2

Same functions as AAL1 but for VBR.

One standard proposed but withdrawn!

No current pending standards.

AAL3/4

Two service models based on network layer service are:

1. AAL3 connection network layer e.g. x.25
   Must provide connection service because transport layer expects it.

   

2. AAL4 connectionless network layer e.g. IP.
   Do connection at Transport Layer if desired.

   

Takes CS-PDU, breaks into 44 Bytes SAR payload and adds 2 byte header and 2 byte trailer on each one.

SN = 0,1,....,15
• Counts cells in packet.
• Note causes limit of 16 * 44 = 704 bytes on CS-PDU, so some packets may need to be split twice (at CS and at SAR).

MID = Multiplexing ID
• Used to MUX upto 2¹⁰ higher layer connections into one ATM connection.


Example:

LI = length indication in multiples of 4 bytes (4 to 44).


10 bit CRC (for error checking).