ATM Adaptation Layer

ATM provides transport of packets from higher layer protocols which are variable in length and longer than the 48 payload section of the ATM cell. To have ATM work in the communications layers, it must segment the larger packets into multiple ATM cells and later reassemble the multiple cells back into the original packet. This process is called Segmentation and Reassembly or SAR for short.

The ATM layer responsible for the SAR process is the ATM Adaptation Layer or AAL. There are three specified ways to do this SAR function and they are called AAL-1, AAL-3/4, and AAL-5.

AAL-1

AAL-1, a connection-oriented service, is suitable for handling circuit-emulation applications, such as voice and video conferencing. Circuit-emulation service also accommodates the attachment of equipment currently using leased lines to an ATM backbone network. AAL-1 requires timing synchronization between the source and destination. For this reason, AAL-1 depends on a medium, such as SONET, that supports clocking.

The AAL-1 process prepares a cell for transmission in three steps. First, synchronous samples (for example, 1 byte of data at a sampling rate of 125 microseconds) are inserted into the Payload field. Second, Sequence Number (SN) and Sequence Number Protection (SNP) fields are added to provide information that the receiving AAL-1 uses to verify that it has received cells in the correct order. Third, the remainder of the Payload field is filled with enough single bytes to equal 48 bytes.

To ensure the cells are reassembled properly, a sequence number is used. The structure of the AAL-1 PDU is given in the following illustration.

Figure 1. AAL-1 SAR

Where:

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.
SNP - Sequence number protection.
CRC - Cyclic redundancy check calculated over the SAR header.
Parity - Parity calculated over the CRC.
SAR PDU - payload 47-byte user information field.

AAL-3/4

AAL-3/4 supports both connection-oriented and connectionless data. It was designed for network service providers and is closely aligned with Switched Multimegabit Data Service (SMDS). AAL-3/4 will be used to transmit SMDS packets over an ATM network.

AAL-3/4 prepares a cell for transmission in four steps:

1. The convergence sublayer (CS) creates a protocol data unit (PDU) by prepending a beginning/end tag header to the frame and appending a length field as a trailer.

2. The segmentation and reassembly (SAR) sublayer fragments the PDU and prepends a header to it.

3. The SAR sublayer appends a CRC-10 trailer to each PDU fragment for error control.

4. The completed SAR PDU becomes the Payload field of an ATM cell to which the ATM layer prepends the standard ATM header.

An AAL-3/4 SAR PDU header consists of type, sequence number, and multiplexing identifier fields. Type fields identify whether a cell is the beginning, continuation, or end of a message. Sequence number fields identify the order in which cells should be reassembled. The multiplexing identifier determines which cells from different traffic sources are interleaved on the same VCC so that the correct cells are reassembled at the destination.

Figure 2. AAL-3/4 SAR

AAL-3/4 CS PDU

The basis of AAL-3 is the packet which is shown in the middle of Figure 2 where:

CPI - Common part indicator. Represents the units of the BAsize and length field. A value of only zero indicates bytes
Btag - Beginning tag. This field must have the same value as Etag for same CPCS-PDUs and a different value as the Btag/Etag preceding and successive CPCS-PDUs.
BAsize - Buffer allocation size. In message mode this is equal to the length field. In streaming mode this is equal to or greater than the length field.
PAD - Up to 3 bytes of padding to achieve 32-bit alignment in the information field
AL - Alignment. A filling byte coded with zero.
ETAG - End tag. Refer to Btag.
Length - Length of the Information field. This value is used to indicate information loss or gain.

Functions of AAL-3/4 SAR include identification of SAR SDUs; error indication and handling; SAR SDU sequence continuity; and multiplexing and demultiplexing.

AAL-3/4 SAR PDU

The structure of the AAL-3/4 SAR PDU is shown at the bottom of Figure 2 where:

ST - Segment type. Values 10 = Beginning of message; 00 = Continuation of message; 01 = End of message; 11 = Single segment message
SN - Sequence number. Numbers the stream of SAR PDUs of a CPCS PDU (modulo 16).
MID Multiplexing identification. This is used for multiplexing several AAL-3/4 connections over one ATM link.
LI - Length indication. Contains the length of the SAR SDU in bytes
CRC - Cyclic redundancy check calculated over the SAR header.

AAL-5 SAR PDU

AAL-5 is the primary AAL for data and supports both connection-oriented and connectionless data. It is used to transfer most non-SMDS data, such as classical IP, over ATM. AAL-5 also is known as the simple and efficient adaptation layer (SEAL) because the SAR sublayer simply accepts the CS-PDU and segments it into 48-octet SAR-PDUs without adding any additional fields.

AAL-5 prepares a cell for transmission in three steps:

1. The CS sublayer appends a variable-length pad and an 8-byte trailer to a frame. The pad ensures that the resulting PDU falls on the 48-byte boundary of an ATM cell. The trailer includes the length of the frame and a 32-bit cyclic redundancy check (CRC) computed across the entire PDU. This allows the AAL-5 receiving process to detect bit errors, lost cells, or cells that are out of sequence.

2. The SAR sublayer segments the CS PDU into 48-byte blocks. A header and trailer are not added (as is in AAL-3/4), so messages cannot be interleaved.

3. The ATM layer places each block into the Payload field of an ATM cell. For all cells except the last, a bit in the Payload Type (PT) field is set to zero to indicate that the cell is not the last cell in a series that represents a single frame. For the last cell, the bit in the PT field is set to one.
   

Figure 3. AAL-5 Formats

AAL-5 is used to carry computer data such as TCP/IP. It is the most popular AAL and is sometimes referred to as SEAL (simple and easy adaptation layer). The basis of AAL-5 is the packet (or SAR PDU) which is composed of the following fields:

AAL-5 CS PDU

The structure of the AAL-5 CS PDU is illustrated below in the middle of Figure 3 where:

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 indicator is a filling byte (of value 0). This field is to be used in the future for layer management message indication.
Length - Length of the Information field.
CRC-32 - Cyclic redundancy check computed from the Information field, PAD, UU, CPI and Length fields. It is a 32-generator polynomial.

AAL-5 SAR PDU

The structure of the AAL-5 SAR PDU consists of a 48 byte payload and shown at the bottom of Figure 3 which is just a standard ATM cell.

More Information

The ATMForum has specifications available at: www.atmforum.com.

Additional ATM seminars:

An Introduction to ATM - An overview of ATM, the types of connections, and a description of the various cell formats.

ATM Connection Management - A description of how connections are established, Quality of Service (QoS), and Virtual Connections (PVCs and SVCs).

In Summary:

• The ATM Adaptation Layer has several ways to segment and reassemble packets.

• The type of AAL used depends on the type of information needing to be sent.

• The AAL used also depends on the type of physical layer transport.