ATM Adaptation layers

ATM adaptation layers

SUBJECT D1-1) Question:What are the various ATM Adaptation layers?

Answer: In order for ATM to support many kinds of services with different traffic characteristics and system requirements, it is necessary to adapt the different classes of applications to the ATM layer. This function is performed by the AAL, which is service-dependent. Four types of AAL were originally recommended by CCITT. Two of these have now been merged into one.

Briefly the four ATM adaptation layers (AAL) have been defined:

AAL1 - Supports connection-oriented services that require constant bit rates and have specific timing and delay requirements. Example are constant bit rate services like DS1 or DS3 transport.
AAL2 - This adaptation is a method for carrying voice over ATM. It consists of variable size packets (max : 64 bytes) encapsulated within the ATM payload. This was previously known as Composite ATM or AAL-CU. The ITU spec which describes this is called ITU-T I.363.2.
AAL3/4 - This AAL is intended for both connectionless and connection oriented variable bit rate services. Originally two distinct adaptation layers AAL3 and 4, they have been merged into a single AAL which name is AAL3/4 for historical reasons.
AAL5 - Supports connection-oriented variable bit rate data services. It is a substantially lean AAL compaired with AAL3/4 at the expense of error recovery and built in retransmission. This tradeoff provides a smaller bandwidth overhead, simpler processing requirements, and reduced implementation complexity. Some organizations have proposed AAL5 for use with both connection-oriented and connectionless services.

Note that some folks talk about an "AAL0" which normally refers to a 'null' AAL, i.e the case where the payload is directly inserted into a cell. This typically requires that the payload can always be fitted into a single cell so that the AAL is not needed for upper layer PDU delineation when the upper layer PDU bridges several cells.

SUBJECT D1-2) Question: How does AAL5 work?

Answer: Here is is a very simplified view of AAL5 and AALs in general. AAL5 is a mechanism for segmentation and reassembly of packets. That is, it is a rulebook which sender and receiver agree upon for taking a long packet and dividing it up into cells. The sender's job is to segment the packet and build the set of cells to be sent. The receiver's job is to verify that the packet has been received intact without errors and to put it back together again.

AAL5 (like any other AAL) is composed of a common part (CPCS) and a service specific part (SSCS). The common part is further composed of a convergence sublayer (CS) and a segmentation and reassembly (SAR) sublayer.

+--------------------+
|                    | SSCS
+--------------------+
|        CS          |
| ------------------ | CPCS
|       SAR          |
+--------------------+

SAR segments higher a layer PDU into 48 byte chunks that are fed into the ATM layer to generate 53 byte cells (carried on the same VCI). The payload type in the last cell (i.e., wherever the AAL5 trailer is) is marked to indicate that this is the last cell in a packet. (The receiver may assume that the next cell received on that VCI is the beginning of a new packet.)

CS provides services such as padding and CRC checking. It takes an SSCS PDU, adds padding if needed, and then adds an 8-byte trailer such that the total length of the resultant PDU is a multiple of 48. The trailer consist of a 2 bytes reserved, 2 bytes of packet length, and 4 bytes of CRC.

SSCS is service dependent and may provide services such as assured data transmission based on retransmissions. One example is the SAAL developed for signalling. This consists of the following:

+--------------------+
|       SSCF         |
| ------------------ | SSCS
|       SSCOP        |
+--------------------+
|        CS          |
| ------------------ | CPCS
|       SAR          |
+--------------------+

SSCOP is a general purpose data transfer layer providing, among other things, assured data transfer.

SSCF is a coordination function that maps SSCOP services into those primitives needed specifically for signalling (by Q.2931). Different SSCFs may be prescribed for different services using the same SSCOP.

The SSCS may be null as well (e.g. IP-over-ATM or LAN Emulation).

There are two problems that can happen during transit. First, a cell could be lost. In that case, the receiver can detect the problem either because the length does not correspond with the number of cells received, or because the CRC does not match what is calculated. Second, a bit error can occur within the payload. Since cells do not have any explicit error correction/detection mechanism, this cannot be detected except through the CRC mismatch.

Note that it is up to higher layer protocols to deal with lost and corrupted packets. This can be done by using a SSCS which supports assured data transfer, as discussed above.

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Last Changed 24 November 2002