The ATM Adaptation Layer AAL

The basic purpose of the AAL is to isolate the higher layers from the specific characteristics of the ATM layer by mapping the higher-layer protocol data units (PDUs) into the payload of the ATM cell and vice versa. Sublayering of the AAL. To support services above the AAL, some independent functions are required of the AAL. These functions are organized in two logical sublayers: the convergence sublayer (CS) and the segmentation and reassembly (SAR) sublayer. The primary functions of these layers are:

• SAR—The segmentation of higher-layer information into a size suitable for the information field of an ATM cell. Reassembly of the contents of ATM cell information fields into higher-layer information.

• CS—Here the prime function is to provide the AAL service at the AAL-SAP (service access point). This sublayer is service dependent. Service Classification of the AAL. Service classification is based on the following parameters:

• Timing relation between source and destination (this refers to urgency of traffic): required or not required.

• Bit rate: constant or variable.

• Connection mode: connection-oriented or connectionless.

When we combine these parameters, four service classes emerge as shown in Figure 20.8. Examples of the services in the classes shown in Figure 20.8 are as follows.

• Class A: constant bit rate such as uncompressed voice or video.

• Class B: variable bit rate video and audio, connection-oriented synchronous traffic.

Service Parameters

Class A

Class B

Class C



Not Required

Bit Rate



Connection Mode



AAL Types



AAL3/4 or AAL5

AAL3/4 or AAL5


DS1.E1, n x 64-kbps emulation

Packet video, audio

Frame relay X.25


Figure 20.8 Services classifications of AAL. (Based on Refs. 2, 8, and 10.)

Figure 20.8 Services classifications of AAL. (Based on Refs. 2, 8, and 10.)

• Class C: connection-oriented data transfer, variable bit rate, asynchronous traffic.

• Class D: connectionless data transfer, asynchronous traffic such as SMDS.

Note that SMDS stands for switched multimegabit data service. It is espoused by Bellcore (Telcordia) and is designed primarily for LAN interconnect. AAL Categories or Types. There are five different AAL categories. The simplest of AAL-0. It just transmits cells down a pipe. That pipe is commonly a fiber-optic link. Ideally, it would be attractive that the bit rate here be some multiple of 53 x 8 bits or 424 bits. For example, 424 Mbps could handle 1 million cells per second.

AAL-1. AAL-1 is used to provide transport for synchronous bit streams. Its primary application is to adapt ATM cell transmission to typically E1/DS1 and SDH/SONET circuits. AAL-1 is specifically used for voice communications (POTS—plain old telephone service). AAL-1 robs one octet from the payload and adds it to the header, leaving only a 47-octet payload. This additional octet in the header contains two major fields: sequence number (SN) and sequence number protection (SNP). The principal purpose of these two fields is to check that mis-sequencing of information does not occur by verifying a 3-bit sequence counter. It also allows for the original clock timing of the data received at the far end of the link. The SAR-PDU format of AAL-1 is shown in Figure 20.9.8 The 4-bit sequence number (SN) is broken down into a 1-bit CSI (convergence sublayer indicator) and a sequence count. The SNP contains a 3-bit CRC and a parity bit. End-to-end synchronization is an important function for the type of traffic carried on AAL-1. With one mode of operation, clock recovery is via a synchronous residual time stamp (SRTS) and common network clock by means of a 4-bit residual time stamp extracted from CSI from cells with odd sequence numbers. The residual time stamp is transmitted over eight cells. It supports DS1, DS3, and E1 digital streams. Another mode of operation is structured data transfer (SDT). SDT supports an octet-structured nXDS0 service.

AAL-2. AAL-2 handles the variable bit rate (VBR) scenario such as MPEG (motion picture experts group) video. It is still in the ITU-T organization definitive stages.

8SAR-PDU stands for segmentation and reassembly—protocol data unit.

■ Cell header



SAR-PDU payload

4 bits

4 bits

47 octets

SAR-PDU header

SAR-PDU header


SN Sequence number (4 bits); to detect lost or misinserted cells. A specific value of the sequence number may indicate a special purpose, e.g. the existence of convergence sublayer functions. The exact counting scheme is for further study.1

SNP Sequence number protection (4 bits). The SNP field may provide error detection and correction capabilities. The polynomial to be used is for further study.

Figure 20.9 SAR-PDU format for AAL-1. This figure shows the content of a cell that contains an SAR-PDU. [From CCITT Rec. I.363, Figure 1/I.363, page 3 (Ref. 10).]

AAL-3/4. Initially, in ITU-T Rec I.363 (Ref. 10), there were two separate AALs, one for connection-oriented variable bit rate data services (AAL-3) and one for connectionless service. As the specifications evolved, the same procedures turned out to be necessary for both of these services, and the specifications were merged to become the AAL-3/4 standard. AAL-3/4 is used for ATM transport of SMDS, CBDS (connectionless broadband data services, an ETSI initiative), IP (Internet protocol) and frame relay.

AAL-3/4 has been designed to take variable-length frames/packets and segment them into cells. The segmentation is done in a way that protects the transmitted data from corruption if cells are lost or mis-sequenced. Figure 20.10 shows the cell format of an AAL-3/4 cell. These types of cells have only a 44-octet payload, and additional overhead fields are added to the header and trailer.9 These carry, for example, the BOM, COM, and EOM indicators (carried in segment type [ST]) as well as a MID (multiplexing identifier) so that the original message, as set up in the convergence sublayer PDU (CS_PDU),

! Cell header




SAR-PDU payload



44 octets


2 octets

2 octets u

SAR-PDU header SAR-PDU trailer

SAR-PDU header SAR-PDU trailer


ST Segment type (2 bits)

SN Sequence number (4 bits)

MID Multiplexing identification (10 bits)

LI Length indicator (6 bits)

CRC Cyclic redundancy check code (10 bits)

Figure 20.10 SAR-PDU format for AAL-3/4. [From ITU-T Rec. I.363, page 13, Figure 6/I.363 (Ref. 10).]

9Trailer consists of overhead fields added to the end of a data frame or cell. A typical trailer is the CRC parity field appended at the end of a frame.

can be delineated. The header also includes a sequence number for protection against misordered delivery. There is the MID (multiplexing identification) subfield which is used to identify the CPCS (common part convergence sublayer) connection on a single ATM layer connection. This allows for more than one CPCS connection for a single ATM-layer connection. The SAR sublayer, therefore, provides the means for the transfer of multiple, variable-length CS-PDUs concurrently over a single ATM layer connection between AAL entities. The SAR_PDU trailer contains a length indicator (LI) to identify how much of the cell payload is filled. The CRC field is a 10-bit sequence used to detect errors across the whole SAR_PDU. A complete CS_PDU message is broken down into one BOM cell, a number of COM cells and one EOM cell. If an entire message can fit into one cell, it is called a single segment message (SSM), where the CS_PDU is 44 or less octets long.

AAL-3/4 has several measures to ensure the integrity of the data which has been segmented and transmitted as cells. The contents of the cell are protected by the CRC-10; sequence numbers protect against misordering. Still another measure to ensure against corrupted PDUs being delivered is EOM/BOM protection. If the EOM of one CPCS_PDU and the BOM of the next are dropped for some reason, the resulting cell stream could be interpreted as a valid PDU. To protect against these kinds of errors, the BEtag numeric values in the CPCS_PDU headers and trailers are compared, to ensure that they match. Two modes of service are defined for AAL-3/4:

1. Message Mode Service. This provides for the transport of one or more fixed-size AAL service data units in one or more CS-PDUs.

2. Streaming Mode Service. Here the AAL service data unit is passed across the AAL interface in one or more AAL interface data units (IDUs). The transfer of these AAL-IDUs across the AAL interface may occur separated in time, and this service provides the transport of variable-length AAL-SDUs. The streaming mode service includes an abort service, by which the discarding of an AAL-SDU partially transferred across the AAL interface can be requested. In other words, in the streaming mode, a single packet is passed to the AAL layer and transmitted in multiple CPCS-PDUs, when and as pieces of the packet are received. Streaming mode may be used in intermediate switches or ATM-to-SMDS routers so they can begin retransmitting a packet being received before the entire packet has arrived. This reduces the latency experienced by the entire packet.

AAL-5. This type of AAL was designed specifically to carry data traffic typically found in today's LANs. AAL-5 evolved after AAL-3/4, which was found to be too complex and inefficient for LAN traffic. Thus, AAL-5 got the name "SEAL" for simple and efficient AAL layer. Only a small amount of overhead is added to the CPCS_PDU. There is no AAL level cell multiplexing. In AAL-5 all cells belonging to an AAL-5 CPCS_PDU are sent sequentially. To simplify still further, the CPCS_PDUs are padded10 to become integral multiples of 48 octets, ensuring that there never will be a need to send partially filled cells after segmentation.

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