Asynchronous Transfer Mode: Difference between revisions

'''Asynchronous Transfer Mode''' ('''ATM''') is a [[telecommunications]] standard defined by the [[American National Standards Institute]] and [[International Telecommunication Union Telecommunication Standardization Sector]] (ITU-T, formerly CCITT) for digital transmission of multiple types of traffic. ATM was developed to meet the needs of the [[Broadband Integrated Services Digital Network]] as defined in the late 1980s,<ref name="bisdn" /> and designed to integrate telecommunication networks. It can handle both traditional high-throughput data traffic and [[Real-time computing|real-time]], [[low-latency]] content such as [[telephony]] (voice) and video.<ref>Telcordia Technologies, ''Telcordia Notes on the Network'', Publication SR-2275 (October 2000)</ref><ref name="ATMF-INTRO">ATM Forum, The User Network Interface (UNI), v. 3.1, {{ISBN|0-13-393828-X}}, Prentice Hall PTR, 1995, page 2.</ref> ATM is a [[cell switching]] technology,<ref>{{cite journal |title=Asynchronous Transfer Mode: An Emerging Network Standard for High-Speed Communications |author=Ronald J. Vetter |journal=Advances in Computers |volume=44 |year=1997|pages=285–330 | quote=ATM is based on the concept of cell switching. ATM combines the benefits of traditional packet switching (used in today’s data networks) and circuit switching (used in the telephone network). |doi=10.1016/S0065-2458(08)60341-1 |isbn=978-0-12-012144-1 }}</ref><ref>{{cite web |url=https://www.ibm.com/docs/en/aix/7.1?topic=adapters-atm-technology |title=ATM technology |date=27 August 2024 |publisher=[[IBM]] |quote=Asynchronous Transfer Mode (ATM) is a cell-switching, connection-oriented technology.}}</ref> providing functionality that combines features of [[circuit switching]] and [[packet switching]] networks by using [[asynchronous communication|asynchronous]] [[time-division multiplexing]].<ref>{{cite web|url= http://www.itu.int/rec/dologin_pub.asp?lang=e&id=T-REC-I.150-199902-I!!PDF-E&type=items|title=Recommendation I.150, B-ISDN Asynchronous Transfer Mode functional characteristics |publisher=ITU}}</ref><ref name="McDysan 1999 287">McDysan (1999), p. 287.</ref> ATM was seen in the 1990s as a competitor to [[Ethernet]] and networks carrying IP traffic, as it was faster and, unlike Ethernet, designed with quality-of-service in mind, but it fell out of favor once Ethernet reached speeds of 1 gigabit per second.<ref>{{cite book | url=https://books.google.com/books?id=ghy9BOw6svMC&dq=atm+network&pg=PP1 | isbn=978-0-471-49827-8 | title=An Introduction to ATM Networks | date=28 November 2001 | publisher=John Wiley & Sons }}</ref>

In the [[OSI model|Open Systems Interconnection (OSI) reference model]] [[data link layer]] (layer 2), the basic transfer units are called ''[[Frame (networking)|frames]]''. In ATM, these frames are of a fixed length (53 [[Octet (computing)|octets]]) called ''cells''. This differs from approaches such as [[Internet Protocol]] (IP) (OSI layer 3) or [[Ethernet]] (also layer 2) that use variable-sized packets or frames. ATM uses a [[connection-oriented model]] in which a [[virtual circuit]] must be established between two endpoints before the data exchange begins.<ref name="McDysan 1999 287"/> These virtual circuits may be either permanent (dedicated connections that are usually preconfigured by the service provider), or switched (set up on a per-call basis using [[Signaling (telecommunications)|signaling]] and disconnected when the call is terminated).

The ATM network reference model approximately maps to the three lowest layers of the OSI model: [[physical layer]], data link layer, and [[network layer]].<ref name="McDysan-Spohn">McDysan, David E. and Spohn, Darrel L., ''[[iarchive:atmtheoryapplica00mcdy/page/n9/mode/2up|ATM: Theory and Application]]'', {{ISBN|0-07-060362-6}}, McGraw-Hill series on computer communications, 1995, page 563.</ref> ATM is a core protocol used in the [[SONET/SDH|synchronous optical networking and synchronous digital hierarchy]] (SONET/SDH) backbone of the [[public switched telephone network]] and in the [[Integrated Services Digital Network]] (ISDN) but has largely been superseded in favor of [[next-generation network]]s based on IP technology. Wireless and mobile ATM never established a significant foothold.

To minimize [[queuing delay]] and [[packet delay variation]] (PDV), all ATM cells are the same small size. Reduction of PDV is particularly important when carrying voice traffic, because the conversion of digitized voice into an analog audio signal is an inherently [[real-time computing|real-time]] process. The [[codec|decoder]] needs an evenly spaced stream of data items.

The NNI cell format replicates the UNI format almost exactly, 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 2¹⁶ VCs each.{{efn|In practice, some of the VP and VC numbers are reserved.}}

Following the initial design of ATM, networks have become much faster. A 1500-byte (12000-bit) full-size [[Ethernet frame]] takes only 1.2&nbsp;μs to transmit on a {{nowrap|10 Gbit/s}} network, reducing the motivation for small cells to reduce jitter due to contention. The increased link speeds by themselves do not eliminate jitter due to queuing.

An ATM network must establish a connection before two parties can send cells to each other. This is called a [[virtual circuit]] (VC). It can be a permanent virtual circuit (PVC), which is created administratively on the endpoints, or a switched virtual circuit (SVC), which is created as needed by the communicating parties. SVC creation is managed by [[signaling (telecommunications)|signaling]], in which the requesting party indicates the address of the receiving party, the type of service requested, and whatever traffic parameters may be applicable to the selected service. ''[[Admission control|Call admission]]'' is then performed by the network to confirm that the requested resources are available and that a route exists for the connection.

Another advantage of the use of virtual circuits comes with the ability to use them as a multiplexing layer, allowing different services (such as voice, Frame Relay, IP). The VPI is useful for reducing the switching table of some virtual circuits that have common paths.<ref>{{cite web |title=What is VPI and VCI settings of broadband connections? |url=http://www.techlineinfo.com/what-is-vpi-and-vci-settings-of-broadband-connections/ |website=Tech Line Info |publisher=Sujith |access-date=1 July 2010}}</ref>

ATM can build virtual circuits and virtual paths either statically or dynamically. Static circuits (permanent virtual circuits or PVCs) or paths (permanent virtual paths or PVPs) require that the circuit be composed of a series of segments, one for each pair of interfaces through which it passes.

ATM networks create and remove switched virtual circuits (SVCs) on demand when requested by an [[end station]]. One application for SVCs is to carry individual telephone calls when a network of telephone switches is interconnected using ATM. SVCs were also used in attempts to replace [[local area network]]s with ATM.

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), each have a set of parameters describing the connection.

If the traffic on a virtual circuit exceeds its traffic contract, as determined by the GCRA, the network can either drop the cells or set the Cell Loss Priority (CLP) bit, allowing the cells to be dropped at a congestion point. 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 a packet's worth of cells. As a result, schemes such as partial packet discard (PPD) and early packet discard (EPD) have been developed to discard a whole packet's cells. This reduces the number of useless cells in the network, saving bandwidth for full packets. EPD and PPD work with AAL5 connections as they use the end-of-packet marker: the ATM user-to-ATM user (AUU) indication bit in the payload-type field of the header, which is set in the last cell of a SAR-SDU.

[[Traffic shaping]] usually takes place in the [[network interface controller]] (NIC) in user equipment, and attempts to ensure that the cell flow on a VC will meet its traffic contract, i.e., cells will not be dropped or reduced in priority at the UNI. Since the reference model given for traffic policing in the network is the GCRA, this algorithm is normally used for shaping as well, and single and dual [[leaky bucket]] implementations may be used as appropriate.

Wireless ATM,<ref name="watm1">{{Cite web|url=http://connectedplanetonline.com/wireless/mag/wireless_wireless_atm_debate/|title=The Wireless ATM Debate|date=15 June 2013|website=archive.ph|archive-url=https://archive.today/20130615202256/http://connectedplanetonline.com/wireless/mag/wireless_wireless_atm_debate/|archive-date=2013-06-15}}</ref> or mobile ATM, consists of an ATM core network with a wireless access network. ATM cells are transmitted from base stations to mobile terminals. Mobility functions are performed at an ATM switch in the core network, known as a ''crossover switch'',<ref name="watm5">[http://www.barnesandnoble.com/w/wireless-atm-and-ad-hoc-networks-c-k-toh/1112737100?ean=9780792398226 Book on Wireless ATM Networks] - [[Chai Keong Toh]], Kluwer Academic Press 1997</ref> which is similar to the [[mobile switching center]] of GSM networks.

|title=A Textbook on ATM Telecommunications, Principles and Implementation