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ATM (Asynchronous Transfer Mode)

Asynchronous Transfer Mode works with very short, fixed-length cells @ 44.7Mbps to 2,4Gbps and more. That allows for time-efficient and cost-effective hardware (switches, etc.). ATM uses 53 byte cells, consisting of a 5 byte header and a 48 byte payload. Because ATM is connection-oriented the cells can have such a short adress space and the cells are not used for establishing the circuit and maintaining it. Once a circuit is set up the bandwidth can be used entirely for data transport. After the circuit is set up, ATM associates each cell with the virtual connection between origin and destination. This can be a virtual channel or path. The 40 bit header holds 8 bits for the virtual path (256 max), and 16 bits for the virtual channel (65536 max). Having both virtual paths and channels make it easy for the switch to handle many connections with the same origin and destination.

The proces that segments a longer entity of data into 53 byte cells is called 'segmentation and reassembly' (SAR). The data that goes into these cells comes from different native mode protocols, such as TCP/IP. The ATM Adaptation Layer (AAL) takes care of the differences between the different sources. The AAL adapts the protocols to an ATM intermediate format. It uses socalled 'classes' to do so. AAL type 3 and 4 handle transmissions of connectionless data, AAL type 5 is intended for connection-oriented services.

ATM relies on different classes of service to accomodate different applications (voice, video, data). They define the bits and bytes that are actually transmitted, as well as the required bandwidth, allowable error rates, and so forth. Class A and B, have timing compensation, for applications that cannot tolerate variable delays. Class C and D, no timing compensation, for data applications like LAN interconnect. Class D also simulates connectionless communicaations, comonly found on LANs.

Class A B C D
Timing yes yes no no
Bit rate constant variable variable variable
Mode Connection-oriented, circuit emulation Connection-oriented, variablee bit-rate video Connection-oriented, connection-oriented data Connectionless, connectionless data
AAL Type 1 Type 2 Type 3/4 Type 3/4
      Type 5  
AAL 1:
for isochronous, constaant bit-rate services, such as audio and video. This adaption layer corresponds to fractional and full T1 and T3, but with a greater range of choices for data rates.
AAL 2:
for isochronous variale bit-rate services, such as compressed video.
AAL 3/4:
for variable bi-rate data, such as LAN applications. Originally designed as two different layers, one for connetion-oriented services (like frame relay) and one for connectionles services (like SMDS). both can be done by the same AAL though.
AAL 5:
for vriable bit-rate data that must be formatted into 53-byte cells. Similar to AAL 3/4, easier to implement, less features.

The service-specific convergence sublayer (SSCS) maps (converts) the data to the ATM layer. The convergence sublayer (CS) then compensates for the various interfaces (copper and fiber) that may be used on an ATM network. The ATM network can use Sonet, T1, E1, T3, E3, E4, FDDI, pure cells, Sonet SDH, block-encoded fiber, etc.

ATM Networks: A Series of Tutorials by Raj Jain
Allied Telesyn ATM Tech Briefs