The ATM Adaptation Layer (AAL) relays ATM cells between ATM Layer and higher layer. When relaying information received from the higher layers, it segments the data into ATM cells. When relaying information received from the ATM Layer, it must reassemble the payloads into a format the higher layers can understand. This operation, which is called Segmentation and Reassembly (SAR), is the main task of AAL. Different AALs were defined in supporting different traffic or service expected to be used. The service classes and the corresponding types of AALs were as follows:

Class A - Constant Bit Rate (CBR) service: AAL1 supports a connection-oriented service in which the bit rate is constant. Examples of this service include 64 Kbit/sec voice, fixed-rate uncompressed video and leased lines for private data networks.

Class B - Variable Bit Rate (VBR) service: AAL2 supports a connection-oriented service in which the bit rate is variable but requires a bounded delay for delivery. Examples of this service include compressed packetized voice or video. The requirement on bounded delay for delivery is necessary for the receiver to reconstruct the original uncompressed voice or video.

Class C - Connection-oriented data service: For connection-oriented file transfer and in general, data network applications where a connection is set up before data is transferred, this type of service has variable bit rate and does not require bounded delay for delivery. Two AAL protocols were defined to support this service class, and have been merged into a single type, called AAL3/4. But with its high complexity, the AAL5 protocol is often used to support this class of service.

Class D - Connectionless data service: Examples of this service include datagram traffic and in general, data network applications where no connection is set up before data is transferred. Either AAL3/4 or AAL5 can be used to support this class of service.

Operation Administration and Maintenance (OA&M) - OA&M is defined for supervision, testing, and performance monitoring. It uses loop-back for maintenance and ITU TS standard CMIP, with organization into 5 hierarchical levels: Virtual Channel (F5 - Between VC endpoints), Virtual Path (F4- Between VP endpoints), Transmission Path (F3- Between elements that perform assembling, disassembling of payload, header, or control), Digital Section (F2 Between section end-points, performs frame synchronization) and Regenerator Section (F1- Between regeneration sections).

AAL0 PDU:

AAL0 payload consists of 48 bytes without special field, is also referred to as raw cells.

AAL1 PDU:

• SN- Sequence number. Numbers the stream of SAR PDUs of a CPCS PDU (modulo 16).
• CSI- Convergence sublayer indicator. Used for residual time stamp for clocking.
• SC- Sequence court.
• NP- Sequence number protection.
• CRC- Cyclic redundancy check calculated over the SAR header.
• P- Parity calculated over the CRC.
• SAR PDU payload- 47-byte user information field.

AAL2 PDU : AAL2 is perfect for low-rate voice traffic, with compression, silent and idle channel suppression. AAL type 2 is subdivided into the Common Part Sublayer (CPS ) and the Service Specific Convergence Sublayer (SSCS ).

AAL2 CPS Packet

The CPS packet consists of a 3 octet header followed by a payload. The structure of the AAL2 CPS packet is shown in the following illustration.

• CID- Channel identification.
• LI- Length indicator: the length of the packet payload associated with each individual user. Value is one less than the packet payload and has a default value of 45 bytes (may be set to 64 bytes).
• UUI- User-to-user indication. Provides a link between the CPS and an appropriate SSCS that satisfies the higher layer application.
• HEC- Header error control.
• Information payload- Contains the CPS/SSCS PDU.

AAL2 CPS PDU

The structure of the AAL2 CPS PDU is shown as follows:

• OSF - Offset field. Identifies the location of the start of the next CPS packet within the CPS-PDU.
• SN - Sequence number. Protects data integrity.
• P - Parity. Protects the start field from errors.
• SAR PDU payload - Information field of the SAR PDU.
• PAD - Padding.

AAL2 SSCS Packet

The SSCS conveys narrowband calls consisting of voice, voiceband data or circuit mode data. SSCS packets are transported as CPS packets over AAL2 connections. The CPS packet contains a SSCS payload. There are 3 SSCS packet types: Type 1 Unprotected; this is used by default; Type 2 Partially protected; and Type 3 Fully protected: the entire payload is protected by a 10-bit CRC which is computed as for OAM cells. The remaining 2 bits of the 2-octet trailer consist of the message type field.

AAL2 SSCS Type 3 Packets:

The AAL2 type 3 packets are used for Dialled digits, Channel associated signalling bits, Facsimile demodulated control data, Alarms and User state control operations. The general sturcture of AAL2 SSCS Type 3 PDUs is shown as follows. The format varies according to the actual message type.

• Redundancy - Packets are sent 3 times to ensure error correction. The value in this field signifies the transmission number.
• Time stamp - Counters packet delay variation and allows a receiver to accurately reproduce the relative timing of successive events separated by a short interval.
• Message dependant information
  Packet content that varies, depending on the message type.
• Message type - The message type code.
• CRC-10 - The 10-bit CRC.

AAL3/4 PDU:

• ST -Segment Type: BOM (Begin of Message), COM (Continuation of Message), EOM (End of Message), SSM (Single Segment Message).
• SN -Sequence number. Numbers the stream of SAR PDUs of a CPCS PDU (modulo 16).
• MID - Multiplexing Indication
• PDU payload -44-byte user information field.
• LI -Length indicator.
• CRC -Cyclic redundancy check calculated over the SAR header.

 AAL3/4 CS PDU:

• CPI - Common Part Indication
• BTag -Beginning Tag
• BAsize -Buffer Allocation Size
• PDU payload -Variable length user information field up to 40 Bytes
• PAD -Padding (up to 3 bytes) used to cell align the trailer.
• AL -Alignment. A filling byte coded with zero
• ETag -End Tag.
• LEN -Length of Information Field

AAL5 CS PDU:

AAL5 is the simple and efficient AAL (SEAL) which is the one used most for data traffic; it has no per-cell length nor per-cell CRC fields.

• PDU payload -Variable length user information field
• PAD -Padding used to cell align the trailer which may be between 0 and 47 bytes long.
• UU -CPCS user-to-user indication to transfer one byte of user information
• CPI - Common Part Indication
• LI -Length indicator.

For OA&M cells, there are pre-defined (reserved) VPI/VCI numbers:

• 0/0 Unassigned or Idle0/1 Meta-signaling
• 0/3 Segment F4 Flow0/4 End-to-end F4 flow
• 0/5 Signaling  0/15 SMDS
• 0/16 Interim Layer Management Interface (ILMI)

F4/F5 OA&M PDU format:

• OAM type / Function type- The possible values for OAM type and function type are defined for Fault, Performance, Activation/Deactivation
• CRC-10 -Cyclic redundancy check calculated over the SAR header. G(x) = x¹⁰+x⁹+x⁵+x⁴+x+1