Asynchronous Transfer Mode

Asynchronous Transfer Mode, or ATM for short, is a cell relay network protocol which encodes data traffic into small fixed sized (53 byte) cells instead of variable sized packets as in packet-switched networks (such as the Internet protocol or Ethernet).

Introduction

ATM was intended to provide a single unified 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.

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 fixed-size '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.

In 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. The full suite of ATM standards provides definitions for layer 1 (physical connections), layer 2 (data link layer) and layer 3 (network) of the classical OSI seven-layer networking model. The ATM standards drew on concepts drawn from the telecommunications community, rather than the computer networking community. For this reason, extensive provision was made for integration of most existing telco technologies and conventions into ATM.

As a result, ATM provides a highly complex technology, with features intended for applications ranging from global telco networks to private local area computer networks.

Numerous telcoss have implemented wide-area ATM networks, and many ADSL implementations utilise ATM. However, ATM has failed to gain wide use as a LAN technology, and its great complexity has held back its full deployment as the single integrating network technology in the way that its inventors originally intended.

(Many people, particularly in the Internet protcol-design community, considered this vision chimerical in any case. Their argument went something like this: We know that there will always be both brand-new and obsolescent link-layer technologies, particularly in the LAN area, and it is fair to assume that not all of them will fit neatly into the SDH model that ATM was designed for. Therefore, some sort of protocol is needed to provide a unifying layer over both ATM and non-ATM link layers, and ATM itself can't fill that role. Conveniently, we have this protcol called "IP" which already does that. Ergo, there is no point in implementing ATM at the network layer.)

Most of the good ideas from ATM migrated into MPLS, a generic layer 2 packet switching protocol. ATM remains useful and widely deployed as a multiplexing layer in DSL networks, where its compromises fit the needs of the application well.

ATM will probably also survive for some time in higher-speed interconnects where carriers have already committed themselves to existing ATM deployments as a way of combining PDH/SDH traffic and packet-switched traffic into a single infrastructure.

ATM Concepts

Traffic contracts are usually maintained by the use of "Shaping", a combination of queuing and marking of cells, and enforced by "Policing".

Traffic Shaping

This is usually done at the entry point to an ATM network and attempts to ensure that the cell flow will meet its traffic contract.

Traffic Policing

To maintain network performance it is possible to police virtual circuits against their traffic contracts. If a circuit is exceeding its traffic contract the network can either drop the cells or mark the Cell Loss Priority (CLP) bit to identify a cell as discardable further down the line. Basic policing works on a cell by cell basis but this is sub-optimal for encapsulated packet traffic as discarding a single cell will invalidate the whole packet anyway. As a result schemes such as Partial Packet Discard (PPD) and Early Packet Discard (EPD) have been created that will discard a whole series of cells until the next frame starts. This reduces the number of redundent cells in the network saving bandwidth for full frames. EPD and PPD work with AAL5 conections as they use the frame end bit to detect the end of packets.

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, obtained by a simple averaging of the US proposal of 64 bytes and European proposal of 32, said by some to be motivated by a European desire not to need echo-cancellers on national trunks.

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

ASCII diagram of a UNI ATM cell

Fields not mentioned above:
PT = Payload Type (3 bits)
GFC = Generic Flow Control (4 bits)
HEC = Header Error Correction (checksum of header only)

 8           4           0
- - - - - - - - - - - - - -
|  GFC       |  VPI       |
- - - - - - - - - - - - - -
|  VPI       |  VCI       |
- - - - - - - - - - - - - -
|  VCI                    |
- - - - - - - - - - - - - -
|  VCI       | PT     |CLP|
- - - - - - - - - - - - - -
|  HEC                    |
- - - - - - - - - - - - - -
|                         |
|  48 bytes of payload    |
|                         |
           ...
|                         |
|                         |
- - - - - - - - - - - - - -

The PT field is used to designate various special kinds of cells for Operation and Management (OAM) purposes, and to delineate packet boundaries in some AALs.

In a UNI cell the GFC field is reserved for an (as yet undefined) local flow control/submultiplexing system between network and user. All four GFC bits must be zero by default.

The NNI cell format is almost identical to the UNI format, except that the 4-bit GFC field is re-allocated to the VPI field, extending the VPI to 12 bits. Thus, a single NNI ATM interconnection is capable of addressing almost 2¹² VPs of up to almost 2¹² VCs each (in practice some of the VP and VC numbers are reserved).

ATM-specific signalling and routing protocols

Q.2931,etc.

To be written...

Layer 3 networking over ATM switched virtual circuits

NSAP addressing, ATMARP, IP over ATM SVCs, etc...

To be written...

Domain	Definition
Computing	Asynchronous Transfer Mode (ATM, or "fast packet") A method for the dynamic allocation of bandwidth using a fixed-size packet (called a cell). See also ATM Forum, Wideband ATM. ATM acronyms (http://www.atmforum.com/atmforum/acronym_index.html). Indiana acronyms (http://cell-relay.indiana.edu/cell-relay/FAQ/ATM-Acronyms.html). [More detail? Data rate(s)?] (1996-04-01). Source: The Free On-line Dictionary of Computing.
Census	A process that increases the amount of information that can be electronically transferred at one time between sites. (references)
Source: compiled by the editor from various references; see credits.

Domain	Title
References	Asynchronous Transfer Mode Production in Australia: A Strategic Entry Report, 1998 (reference) (more reference examples)

Books	Asynchronous Transfer Mode (Atm): Technical Overview (reference) ATM: Asynchronous Transfer Mode User's Guide (reference) (more book examples)
Source: compiled by the editor from various references; see credits.

Subject	Topic	Quote
Economic History	Switzerland	It is actively developing value-added services, including private virtual networks, and design and operation (with partners Cisco, Siemens, Alcatel, Ascom/Ericsson) of the ATM (Asynchronous Transfer Mode) infrastructure. (references)
	Malaysia	The Government's vision is to turn Malaysia into the technology hub of the Southeast Asian region, primarily through their establishment of the Multimedia Super Corridor (MSC). The MSC consists of high-tech companies that will enjoy state-of-the-art telecommunication facilities involving fiber optics, Asynchronous Transfer Mode (ATM), and Asymmetrical Digital Subscriber Line (ADSL). (references)
Source: compiled by the editor from ICON Group International, Inc.; see credits.

Expression	Frequency per Day
asynchronous transfer mode	75
asynchronous transfer mode services	11
asynchronous transfer mode atm	5
Source: compiled by the editor from various references; see credits.

Language		Translations for "asynchronous transfer mode"; alternative meanings/domain in parentheses.
Chinese		非同步傳輸模式 (ATM), 異步傳輸模式 (ATM). (various references)

Danish		asynkron overførelsesmodus. (various references)

Dutch		ATM (standard atmosphere), Asynchronous Transfer Mode, asynchrone transfermodus. (various references)

Finnish		ATM, asynkroninen toimintamuoto, asynkroninen siirtomuoto, asynkroninen pakettivälitys. (various references)

French		MTA, mode de transfert ATM, mode de transfert asynchrone. (various references)

German		ATM (air traffic management, standard atmosphere), asynchroner Transfer-Modus, asynchroner Transfermodus, asynchroner Übertragungsmodus. (various references)

Greek		ασύχρονος τρόπος μεταφοράς. (various references)

Italian		modo di trasferimento asincrono, modalit� di trasferimento asincrona. (various references)

Pig Latin		asynchronousay ansfertray odemay

Portuguese		ATM (ATM), Modo de Transferência Assíncrono. (various references)

Spanish		MTA, modo de transferencia asíncrono. (various references)

Swedish		ATM, asynkront överföringsmod. (various references)
Source: compiled by the editor from various translation references.

Expression using "asynchronous transfer mode": asynchronous transfer mode adaptation layer. Additional references.
Source: compiled by the editor from various references; see credits.

Misspellings
	"Asynchronous Transfer Mode" is suggested in spellcheckers for the following: asyncronous transfer mode. (additional references)
Source: compiled by the editor, based on several corpora (additional references).

Asynchronous Transfer Mode

Definition: Asynchronous Transfer Mode