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ATM (Asynchronous Transfer Modus)

(look for the two documents at the end of this page)

ATM works with very short, fixed-length 53 byte cells from 2 Mbit/s up to 10 Gbit/s and more. That allows time-efficient and cost-effective hardware (switches, router etc.). Each ATM cell has a length of 53 byte, 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 important process 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 Ethernet with TCP/IP. Some units as the Centillion 100 have special high speed realtime SAR CPU´s or DSP´s designed for this particular job. At the end is a sample.


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
variablee bit-rate video
connection-oriented data
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.

One sample for the unique features of ATM :

'segmentation and reassembly' (SAR) with ATM

You know, an Ethernet packet can be 1500 bytes long and with TCP/IP a 2 Mbyte File can be sent around the world usnig the internet. There are no restrictions, how the single packets are sent and in what order they will arrive at the destination computer. TCP will check that and reassamble the file, if all packets did arrive. This may take some time, but it works. If one packet is lost or corrupted, TCP cancells all all other packets and requests a resend of the whole file. This is not a Quality of service.


With ATM its different. All incomming data will be segmented into 53 byte cells in the original incomming order, will be transmitted within the point to point connection channel protocoll in the same order and at the end will be reassambled in the same order into its origin packets. There is no waiting for lost packets or a resend procedure.


First in and first out is working here.


Here are two long artikles about modern ATM and Gigabit Ethernet.

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