Asynchronous Transfer Mode or ATM is a high-speed networking standard designed to support both voice and data communications. ATM is normally utilized by Internet service providers on their private long-distance networks. ATM operates at the data link layer (Layer 2 in the OSI) over either fiber or twisted-pair cable. ATM is a statistical time−division multiplexed (TDMed) form of traffic that is designed to carry any form of traffic and enables the traffic to be delivered asynchronously to the network. The system also uses preassigned slots for transmission for the traffic flow. It is also a connection-oriented protocol but uses virtual circuits like permanent virtual circuits and switched virtual circuits.

ATM can also be used across group of networks and also any other network like LAN, CAN, WAN and MAN. ATM also emerged as the basis for future broadband communications with high speed data rate and larger bandwidths. For basis of speed and performance a table is shown below.

ATM differs from more common data link technologies like Ethernet in several ways. For example, ATM utilizes no routing. Hardware devices known as ATM switches establish point-to-point connections between endpoints and data flows directly from source to destination. Additionally, instead of using variable-length packets as Ethernet does, ATM utilizes fixed-sized cells. ATM cells are 53 bytes in length, that includes 48 bytes of data and five (5) bytes of header information.

Why cells?

When speech, data and video pushed through into one network they encounter queuing or jitter that would produce traffic. Because packet size are not constant, this produces jitters or traffic. That is why all packets, or "cells," should have the same small size. In addition the fixed cell structure means that ATM can be readily switched by hardware without the inherent delays introduced by software switched and routed frames.

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

ATM traffic contracts form part of the mechanism by which "quality of service" (QoS) is ensured. There are four basic types (and several variants) which each have a set of parameters describing the connection.

1.     CBR - Constant bit rate: a Peak Cell Rate (PCR) is specified, which is constant.

2.     VBR - Variable bit rate: an average cell rate is specified, which can peak at a certain level for a maximum interval before being problematic.

3.     ABR - Available bit rate: a minimum guaranteed rate is specified.

4.     UBR - Unspecified bit rate: traffic is allocated to all remaining transmission capacity.

ATM defines two different cell formats: UNI (User-Network Interface) and NNI (Network-Network Interface). Most ATM links use UNI cell format.

                   UNI ATM CELL                                                                         NNI ATM CELL

  

ATM Layered Architecture

ATM layered architecture falls into the 1^st two OSI layers which are the DATA link layer and the physical link layer. The Physical Layer defines the medium for transmission, any medium-dependent parameters (e.g., rate, quality of service required), and framing used to find the data contained within the medium. The ATM Layer provides the basic 53-byte cell format, by defining the 5-byte ATM header for each 48-byte payload segment handed down by the AAL. The ATM Adaptation Layer (AAL) adapts the higher-level data into formats compatible with the ATM Layer requirements, i.e., this layer segments the data and adds appropriate error control information as necessary. It is dependent on the type of services (voice, data, etc.) being transported by the higher layer. Several AAL protocols have been defined for specific types of data. These are loosely associated with various classes of data. However, no AAL is restricted to a specific data class or type; all types of data could conceivable be handled by any of the AALs.

AAL 1

Constant bit rate, connection-oriented, synchronous traffic (e.g., uncompressed voice)

AAL 2

Definition never completed undefined, but envisioned to be for variable bit rate, connection-oriented, synchronous traffic (e.g., compressed video)

AAL 3/4

Variable bit rate, 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

AAL 5

Similar to AAL 3/4 with a simplified information header scheme that requires only one header per data unit and uses the PTI bit (see below) to indicate the last cell in a transmission Examples of services that use AAL 5 are Classic IP over ATM, and LAN Emulation (LANE). AAL 5 is the most widely used ATM Adaptation Layer Protocol.

ATM Applications

ATM is an enabler of basic and advanced applications such as remote sensing, 3-D interactive simulations, tele-instruction, biological teleresearch, and medical teleconsultations. Edge devices at the boundary of an ATM network convert non-ATM traffic streams into standard ATM cells. ATM technology is implemented in backbone, enterprise, and edge switches as well as hubs, routers, bridges, multiplexer, servers, servers farms, and NICs (Network Interface Cards) in high-end Internet appliances. The ATM data Exchange Interface(DXI) enables fast acces to public network services. A flexible and extendible networking solution. 

Sources:

http://www.techfest.com/networking/atm/atm.htm

http://en.wikipedia.org/wiki/Asynchronous_Transfer_Mode

http://compnetworking.about.com/od/networkprotocols/g/bldef_atm.htm

http://forums.techarena.in/guides-tutorials/5186.htm