ATM Adaptation Layer

The purpose of the ATM Adaptation Layer (AAL) is to adapt the PDUs passed down from the higher layer onto ATM cells. As the higher level PDUs may in general be of an arbitrary size, so one of the two sublayers in the AAL is responsible for segmentation and reassembly (SAR) of the higher layer PDUs. The other sublayer, the convergence sublayer (CS), is responsible for packaging the higher layer PDU with any additional information required for the adaptation necessary and offering an interface to the B-ISDN user. As mentioned previously, there are many and varied requirements for applications using the B-ISDN and as there are several AAL protocols defined. The classification of these protocols was guided by considering the following parameters:
· Timing relationship. The (non-)requirement for synchronisation between the
receiver and sender.
· Bit rate. Constant or variable.
· Connection mode. Connection oriented or connectionless.

A summary of the (sensible) combinations of these parameters is given in Figure . Each of these classes may be looseley associated with the the ATM Forum classes described previously: Class A, CBR; Class B, VBR; Class C, ABR(ish); Class D, UBR. This mapping also give example use of the various classes. The various AAL classes are provided by several AAL protocols identified by a type number. The use of a particular AAL type for the provision of a particular service is not wholy well defined.
Figure: AAL service classification
AAL Type 0. This is effectively a NULL AAL. It is not really an official AAL type but is mentioned for completeness.
AAL Type 1. This AAL type is normally used by Class A (CBR) services. The function performed by this AAL are:
· Segmentation and reassembly of user information.
· Handling of cell delay variation (jitter).
· Handling of cell reassembly variation.
· Handling of lost and misinserted cells.
· Source clock frequency recovery at receiver.
· Source data structure recovery at receiver.
· Monitoring and handling of AAL-PCI bit errors.
· Monitoring and (possibly) correcting the bit errors in the user information field.
· For circuit emulation, monitoring and maintenance of end-to-end QoS.
AAL Type 2. This AAL type would be used with Class B (VBR). This type is not well defined and it seems possible that the it may be merged with AAL Type 1 in the future. Some of its functions are similar to ALL Type 1:
· Segmentation and reassembly of user information.
· Handling of cell delay variation (jitter).
· Handling of lost and misinserted cells.
· Source clock frequency recovery at receiver.
· Monitoring and handling of AAL-PCI bit errors.
· Monitoring and (possibly) correcting the bit errors in the user information field.
It also has the additional functions:
· Handle SDUs from a variable bit rate source.
· Transfer timing information between source and destination.
· Notify the higher layers of uncorrectable errors in AAL.
AAL Type 3/4. There was once separate Type 3 and Type 4 AALs, but they have now been merged. This AAL is now intended to support both Class C (ABR) and Class D (UBR) services.
In this AAL, the convergence sublayer is split into two (Figure ), the service specific convergence subslayer (SSCS) and the common part convergence sublayer (CPCS). The SSCS is application dependent, i.e. it could be for a VBR video application. The CPCS is responsible for constructing PDUs that can be sent to the other end user. There are two modes of operation of ALL Type 3/4; message mode and streaming mode.
Figure: AAL Type 3/4 sub-layering
The message mode is intended for use framed data where the AAL-SDU is a logical unit of data with respect to the B-ISDN user (Figure ). It allows the the transport of a single AAL-SDU in one or (optionally) more than one CS-PDU. The CS-PDU may be then further spilt into several SAR-PDUs. The AAL-SDU can be of an arbitrary size.
Figure: AAL Type 3/4 message mode service
In streaming mode, the AAL-SDUs are of fixed size and one or more of them may be transported in a single CS-PDU (Figure ). Each AAL-SDU is delivered in a separate SAR-PDU.
Figure: AAL Type 3/4 streaming mode service
In both cases, the SAR sublayer provides error detection and both these modes can offer the following operational procedures:
· Assured operation. Flow control and retransmission of missing or erroneous
AAL-SDUs. Flow control restricted to point-to-point connections at the ATM
layer and point-to-multipoint flow control possible.
· Non-assured operation. No retransmission of missing or erroneous SAR-PDUs.
Optionally deliver erroneous PDUs to user. Allow flow control for point-to-point
connections but not point-to-multipoint.
This AAL type also provides multiplexing at the SAR sublayer. AAL Type 5.
This AAL type provides similar services as AAL Type 3/4, but has a reduced overhead when compared to AAL Type 3/4. It is intended for use by VBR sources with timing relationship between source and destination. It has identical modes and operational procedures as AAL Type 3/4. The difference is that this AAL does not provide the AAL Type 3/4 multiplexing capability. The SAR sublayer accepts only AAL-SDUs that are an integer multiple of 48 octets. So, it would be possible for this AAL to offer an efficient cell based interface to the B-ISDN user.
AAL Types 1 and Type 2 are not used much.
AAL Type 3/4 has been chosen for the provision of the Bellcore switched multimegabit data service (SMDS), the European version of which is the connectionless broadband data service (CBDS).
AAL Type 5 has been selected by the IETF to provide IP services over ATM.

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