"Introduction The ATM adaptation layer (AAL) enhances the services provided by the ATM Layer to support the functions required by the next higher Layer. The AAL performs functions required by the user@ control and management planes and supports the mapping between the ATM Layer and the next higher Layer. The functions performed in the AAL depend upon the higher Layer requirements. The AAL supports multiple protocols to fit the needs of the different AAL service users. The AAL is therefore service dependent. Scope of the Recommendation The scope of this Recommendation is the interaction between different user@ control and management requirements on one side and the ATM Layer on the other side. Architecturally the AAL is a layer between the ATM Layer and the next higher Layer in each of the user plane@ the control plane and the management plane. The B-ISDN Protocol Reference Model is given in Recommendation I.321. Examples of services provided by the AAL include: handling of transmission errors; handling quantization effect due to cell information field size; handling of the lost and misinserted cell condition; flow control and timing control. Adaptation layer functions for the control and management planes require further study. Objective of this Recommendation The objective of this Recommendation is to provide a classification of services1) (based on the attributes of timing relation between source and destination@ bit rate and connection mode) which may require AAL capabilities accessed through different service access points (SAPs). It also gives an introduction to the services provided by the AAL and the functions performed in the AAL. The service classification and AAL functional organization indicated in this Recommendation are provided to assist in the development and selection of suitable methods to support a wide range of services. The AAL specifications described in Recommendation I.363 are recommended where they are considered to be appropriate@ taking into account all the service and network considerations. It is intended that Recommendation I.363 should not preclude standardization of other AAL protocols. 1) The term ""service"" in this Recommendation is used in two different meanings. In one case@ it is used in the sense of a Layer service and in the other case in the sense of a telecommunication service@ e.g. voice service@ data transmission service."