AAL (ATM Adaptation Layer) is a third example message level protocol. A software layer that accepts user data, such as digitized voice, video or computer data, and converts to and from cells for transmission over an ASYNCHRONOUS TRANSFER MODE network. AAL software mostly runs at the end-points of a connection, though in a few circumstances AAL software is run inside an ATM switch. AAL includes facilities to carry traffic that uses other network protocols, such as TCP/IP, over ATM.
This is the adaptation layer ATM, which takes care of the interface with upper layers. This floor is itself subdivided into two levels, one taking into account the problems directly related to interworking with the top layer and the other those concerning the fragmentation and reassembly of cells in messages.
The role of this layer is to transport from one end of the messages whose format is specified, the maximum size can not exceed 64 KB, as in the Internet. This block must be cut into small fragments of 48 bytes to get into the ATM cell. this breakdown may actually fall below 48 bytes, for example 47 or 44 bytes to retrieve bytes of supervision in the given part.
In AAL, four classes of service, 1, 2, 3 and 4 have been defined, which correspond four classes protocols. This subdivision was amended in 1993 by the consolidation of classes 3 and 4 and by adding a new class of protocols, Class 5, which defines a simplified data transport. Finally, in 2000, the class 2 has been transformed, which has led to the definition of three classes 1, 2 and 5:
• Class 1. Matches a circuit emulation, that is to say, the establishment of a virtual circuit capable of transporting that would come from a circuit and restore output the same circuit. We use this class to carry uncompressed telephone speech. Conventional telecom operators have such protocols to serve all circuit applications.
• Class 2. Corresponds to the transport of information that would type circuit start but that would have compressed so that the flow rate becomes variable.
This class carries applications with timing constraints, such as class A, but with a variable flow. It contains all the applications of telephone speech and compressed video. UMTS in particular, chose this solution for the transport of its speech channels.
• Class 5. Allows to transit without adding any additional function, except possibly an error check.