The use of Asynchronous Transfer Mode (ATM) technology and services creates the need for an adaptation layer in order to support information transfer protocols, which are not based on ATM. This adaptation layer defines how to segment and reassemble higher-layer packets into ATM cells, and how to handle various transmission aspects in the ATM layer. Asynchronous Transfer Mode, or ATM for short, is a cell relay network protocol which encodes data traffic into small fixed sized (53 byte; 48 bytes of data and 5 bytes of header information) cells instead of variable sized packets as in packet-switched networks (such as the Internet Protocol or...

Examples of services that need adaptations are Gigabit Ethernet, IP, Frame Relay, SONET/SDH, UMT/Wireless, etc. Gigabit Ethernet (GbE) is a term describing various technologies for implementing Ethernet networking at a nominal speed of one gigabit per second. ... The Internet Protocol (IP) is a data-oriented protocol used by source and destination hosts for communicating data across a packet-switched internetwork. ... Frame relay, also found written as frame-relay, is an efficient data transmission technique used to send digital information quickly and cheaply to one or many destinations from one or many end-points. ... Synchronous Optical Networking, commonly known as SONET, is a standard for communicating digital information over optical fiber. ... Synchronous Optical Networking, commonly known as SONET, is a standard for communicating digital information over optical fiber. ... UMT can refer to: Universal Military Training This page concerning a three-letter acronym or abbreviation is a disambiguation page — a navigational aid which lists other pages that might otherwise share the same title. ...

The main services provided by AAL are:

• Segmentation and reassembly
• Handling of transmission errors
• Handling of lost and misinserted cell conditions
• Timing and flow control

The following ATM Adaptation Layer protocols (AALs) have been defined by the ITU-T. It is meant that these AALs will meet a variety of needs. The classification is based on whether a timing relationship must be maintained between source and destination, whether the application requires a constant bit rate, and whether the transfer is connection oriented or connectionless. Segmentation and Reassembly refers to the process used to divide and reformat data so as to allow it to be transported as the 48 byte payload in multiple ATM cells, followed by its reconstitution at the destination of the ATM Permanent virtual circuit through which it has been carried. ... The ITU Telecommunication Standardization Sector (ITU-T) coordinates standards for telecommunications on behalf of the International Telecommunication Union (ITU) and is based in Geneva, Switzerland. ...

• AAL Type 1 supports constant bit rate (CBR), synchronous, connection oriented traffic. Examples include T1 (DS1), E1, and x64 kbit/s emulation.
• AAL Type 2 supports time-dependent Variable Bit Rate (VBR-RT) of connection-oriented, synchronous traffic. Examples include Voice over ATM. AAL2 is also widely used in wireless applications due to the capability of multiplexing voice packets from different users on a single ATM connection.
• AAL Type 3/4 supports VBR, data traffic, connection-oriented, asynchronous traffic (e.g. X.25 data) or connectionless packet data (e.g. SMDS traffic) with an additional 4-byte header in the information payload of the cell. Examples include Frame Relay and X.25.
• AAL Type 5 is similar to AAL 3/4 with a simplified information header scheme. This AAL assumes that the data is sequential from the end user and uses the Payload Type Indicator (PTI) bit to indicate the last cell in a transmission. Examples of services that use AAL 5 are classic IP over ATM, Ethernet Over ATM, SMDS, and LAN Emulation (LANE). AAL 5 is a widely used ATM adaptation layer protocol. This protocol was intended to provide a streamlined transport facility for higher-layer protocols that are connection oriented.

AAL 5 was introduced to: ATM Adaptation Layer 5 Computers use ATM Adaptation Layer 5 (AAL5) to send data across an ATM network. ...

• reduce protocol processing overhead.
• reduce transmission overhead.
• ensure adaptability to existing transport protocols.

The AAL 5 was designed to accommodate the same variable bit rate, connection-oriented asynchronous traffic or connectionless packet data supported by AAL 3/4, but without the segment tracking and error correction requirements.

External links

• ATM forum
• International Telecommunication Union