SUBJECT G2)

Can AAL5 be used for connection-less protocols?

This thread started with questions about whether AAL5 supports connection oriented or connection-less protocols. Check the November and December 1993 archives for the subject: "AAL Type 5 question".

First some background

Officially, AAL 5 provides support for adaption of higher layer connection- oriented protocols to the connection-oriented ATM protocol. There was, however, a debate going on, claiming, that AAL 5 could also be used to adapt higher layer connectionless protocols to the connection-oriented ATM protocol.

The whole debate is grounded in a systematical approach of the ITU-T, which states, that all AALs should be classified into four different classes, to minimise the number of AALs required for supporting any imaginable service.

The classification of the ITU-T is as follows:

+------------------------+-----------+-----------+-----------+-----------+
|                        |  Class A  |  Class B  |  Class C  |  Class D  |
+------------------------+-----------+-----------+-----------------------+
|Timing relation between |        Required       |   Not Required        |
|source and destination  |                       |                       |
+------------------------+-----------+-----------+-----------------------+
| Bit Rate               |  Constant |          Variable                 |
+------------------------+-----------+-----------------------+-----------+
| Connection Mode        |     Connection-Oriented           |Connection-|
|                        |                                   | less      |
+------------------------+-----------------------------------+-----------+
AAL 5 is currently agreed to be in Class C. Some parties at the standardisation bodies claim that it could be as well in Class D.

At the moment the following mapping between AALs and classes applies:

The reason for AAL3/4 in classes C and D is the follwing: The ITU-T started to define AAL3 for Class C and AAL 4 for Class D. They turned out to be identical after long debates.

Reality Check

The real issue is how to run a connection-less service over ATM which is inherently connection-oriented. AALs themselfs merely transport higher layer packets across an ATM virtual circuit. Connection-less services are actually provided by higher layer protocols such as CLNAP. Given that, there is nothing to prevent folks from using AAL5 to implement a connection-less communication mode. This is exactly what the IETF is doing with IP over ATM, and what the ATM Forum is also doing with LAN Emulation.

The reality is that these folks expect that AAL5 will be largely used for connection-less upper layer protocols such as CLNP and IP. So some find it strange to have AAL5 classified as an AAL for connection- oriented services only.

However, from an ITU-T service Class perspective, you must stick strictly to the view that to call an AAL "Class D" it must support each and every posssible connection-less protocol. The current agreement in the ITU-T is that AAL5 can not claim this and so is officially considered a "Class C" AAL.

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