Asynchronous Transfer Mode

ATM for short, a cell-based network layer which switches on fixed sized (53 byte) cells instead of variable sized packets as in Internet protocol

ATM was intended to provide a single wide-area-networking standard that could support both synchronous channel networking (PDH, SDH) and packet-based networking (IP, Frame Relay, etc), whilst supporting multiple levels of quality of service for packet traffic. It its original conception, ATM was to be the enabling technology of the 'Broadband Integrated Services Digital Network' (B-ISDN) that would replace the existing PSTN.

ATM sought to resolve the conflict between circuit-switched networks and packet-switched networks by mapping both bitstreams and packet-streams onto a stream of small ATM cells tagged with virtual circuit identifiers.

The cells are typically sent on demand within a synchronous time-slot pattern in a synchronous bit-stream: what is asynchronous here is the sending of the cells, not the low-level bitstream that carries them.

Numerous telcos have implemented wide-area ATM networks, and ATM is used in many ADSL implementations.

However, ATM has failed to be widely adopted as a LAN technology, and its great complexity has prevented it from being fully deployed as a native network technology in the way that its inventors originally intended.

ATM Concepts

ATM is a channel based transport layer. This is encompessed in the concept of Virtual Paths (VP's) and Virtual Circuits (VC's). Every ATM cell has a VP/VC pair defined in its header. As these cells traverse an ATM network switching is achieved by changing the VP/VC values. Although the VP/VC values are not constent from one end of the connection to the other the concept of a circuit is (unlike IP where any given packet could get to its destination by a different route to preceeding and following packets).

this could be explained better

Another key ATM concept is that of the traffic contract. When an ATM circuit is set up each switch is informed of the traffic class of the connection.

Traffic Classes

ATM traffic contracts are part of the mechanism by which "Quality of Service" (QoS) is ensured. There are three basic types (and several varients) which each have a set of parameters decribing the connection.

UBR - Unspecified Bit Rate, you get whats left after all other traffic has had its bandwidth

CBR - Constant Bit rate, you specify a Peak Cell Rate (PCR) which is what you get (has realtime and non-realtime varients)

VBR - Variable Bit Rate, you specify an average cell rate which can peak at a certain level for a maxium time (used for "bursty" traffic).

Most traffic classes also introduce the concept of Cell Delay variation Time (CDVT) which defines the "clumping" of cells .

Traffic contracts are usually maintened by the use of "Shapping" and enforced by "Policing".

Structure of an ATM Cell

An ATM cell consists of a 5 byte header and a 48 byte payload. The payload size of 48 bytes was a compromise between the needs of voice telephony and packet networks.

ASCII diagram of a header?