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Encyclopedia > Asynchronous Transfer Mode

Asynchronous Transfer Mode (ATM) is a cell relay, packet switching network and data link layer protocol which encodes data traffic into small (53 bytes; 48 bytes of data and 5 bytes of header information) fixed-sized cells. ATM provides data link layer services that run over Layer 1 links. This differs from other technologies based on packet-switched networks (such as the Internet Protocol or Ethernet), in which variable sized packets (known as frames when referencing Layer 2) are used. ATM is a connection-oriented technology, in which a logical connection is established between the two endpoints before the actual data exchange begins. In computing and telecommunications, the transport layer is the second highest layer in the four and five layer TCP/IP reference models, where it responds to service requests from the application layer and issues service requests to the Internet layer. ... The Transmission Control Protocol (TCP) is one of the core protocols of the Internet protocol suite. ... User Datagram Protocol (UDP) is one of the core protocols of the Internet protocol suite. ... The Datagram Congestion Control Protocol (DCCP) is a message-oriented transport layer protocol that is currently under development in the IETF. Applications that might make use of DCCP include those with timingconstraints on the delivery of data such that reliable in-order delivery, when combined with congestion control, is likely... In the field of computer networking, the IETF Signaling Transport (SIGTRAN) working group defined the Stream Control Transmission Protocol (SCTP) as a transport layer protocol in 2000. ... The Resource ReSerVation Protocol (RSVP), described in RFC 2205, is a Transport layer protocol designed to reserve resources across a network for an integrated services Internet. ... Network congestion avoidance is a process used in computer networks to avoid congestion. ... The network layer is third layer out of seven in OSI model and it is the third layer out of five in TCP/IP model. ... The Internet Protocol (IP) is a data-oriented protocol used for communicating data across a packet-switched internetwork. ... Internet Protocol version 4 (IPv4) is the fourth iteration of the Internet Protocol (IP) and it is the first version of the protocol to be widely deployed. ... Internet Protocol version 6 (IPv6) is a network layer for packet-switched internetworks. ... The Open Shortest Path First (OSPF) protocol is a hierarchical interior gateway protocol (IGP) for routing in Internet Protocol, using a link-state in the individual areas that make up the hierarchy. ... Is Is is Yeah Yeah Yeahs third EP, to be released on July 24, 2007. ... The Border Gateway Protocol (BGP) is the core routing protocol of the Internet. ... IPsec (IP security) is a suite of protocols for securing Internet Protocol (IP) communications by authenticating and/or encrypting each IP packet in a data stream. ... In computer networking, the Address Resolution Protocol (ARP) is the standard method for finding a hosts hardware address when only its network layer address is known. ... Reverse Address Resolution Protocol (RARP) is a network layer protocol used to obtain an IP address for a given hardware address (such as an Ethernet address). ... This article is chiefly about the Routing Information Protocol (RIP) for the Internet Protocol, but also discusses some other routing information protocols. ... The Internet Control Message Protocol (ICMP) is one of the core protocols of the Internet protocol suite. ... The ICMP for IPv6 (Internet Control Message Protocol Version 6) is an integral part of the IPv6 architecture and must be completely supported by all IPv6 implementations. ... The Internet Group Management Protocol (IGMP) is a communications protocol used to manage the membership of Internet Protocol multicast groups. ... The data link layer is layer two of the seven-layer OSI model as well as of the five-layer TCP/IP reference model. ... IEEE 802. ... The IEEE 802. ... Wi-Fi (IPA: ) is the common name for a popular wireless technology used in home networks, mobile phones, video games and more. ... Official WiMax logo WiMAX, the Worldwide Interoperability for Microwave Access, is a telecommunications technology aimed at providing wireless data over long distances in a variety of ways, from point-to-point links to full mobile cellular type access. ... Dynamic synchronous Transfer Mode , or DTM for short, is a network protocol. ... Token-Ring local area network (LAN) technology was developed and promoted by IBM in the early 1980s and standardised as IEEE 802. ... Ethernet is a large, diverse family of frame-based computer networking technologies that operate at many speeds for local area networks (LANs). ... In computer networking, fiber-distributed data interface (FDDI) is a standard for data transmission in a local area network that can extend in range up to 200 km (124 miles). ... In the context of computer networking, frame relay consists of an efficient data transmission technique used to send digital information quickly and cheaply in a relay of frames to one or many destinations from one or many end-points. ... General Packet Radio Service (GPRS) is a Mobile Data Service available to users of Global System for Mobile Communications (GSM) and IS-136 mobile phones. ... Evolution-Data Optimized or Evolution-Data only, abbreviated as EV-DO or EVDO and often EV, is one telecommunications standard for the wireless transmission of data through radio signals, typically for broadband Internet access. ... High-Speed Packet Access (HSPA) is a collection of mobile telephony protocols that extend and improve the performance of existing UMTS protocols. ... High-Level Data Link Control (HDLC) is a bit-oriented synchronous data link layer protocol developed by the International Organization for Standardization (ISO). ... In computing, the Point-to-Point Protocol, or PPP, is commonly used to establish a direct connection between two nodes. ... The Point-to-Point Tunneling Protocol (PPTP) is a method for implementing virtual private networks. ... In computer networking, the Layer 2 Tunneling Protocol (L2TP) is a tunneling protocol used to support virtual private networks (VPNs). ... ISDN redirects here. ... ARCNET (also CamelCased as ARCnet, an acronym from Attached Resource Computer NETwork) is a local area network (LAN) protocol, similar in purpose to Ethernet or Token Ring. ... Link Layer Topology Discovery (LLTD) is a licensed data link layer protocol for network topology discovery and quality of service diagnostics, developed by Microsoft as part of their Windows Rally set of technologies. ... This article does not cite any references or sources. ... The Ethernet physical layer is the physical layer component of the Ethernet standard. ... RS-232 (also referred to as EIA RS-232C or V.24) is a standard for serial binary data interchange between a DTE (Data terminal equipment) and a DCE (Data communication equipment). ... Synchronous optical networking, is a method for communicating digital information using lasers or light-emitting diodes (LEDs) over optical fiber. ... There are very few or no other articles that link to this one. ... Optical fibers An optical fiber (or fibre) is a glass or plastic fiber designed to guide light along its length. ... Coaxial Cable For the weapon, see coaxial weapon. ... 25 Pair Color Code Chart 10BASE-T UTP Cable Twisted pair cabling is a common form of wiring in which two conductors are wound around each other for the purposes of cancelling out electromagnetic interference known as crosstalk. ... In telecommunications, cell relay refers to a method of statistically multiplexing fixed-length packets, i. ... In computer networking and telecommunications, packet switching is a communications paradigm in which packets (messages or fragments of messages) are individually routed between nodes, with no previously established communication path. ... The data link layer is layer two of the seven-layer OSI model as well as of the five-layer TCP/IP reference model. ... In networking, a communications protocol or network protocol is the specification of a set of rules for a particular type of communication. ... In computer networking and telecommunications, packet switching is a communications paradigm in which packets (messages or fragments of messages) are individually routed between nodes, with no previously established communication path. ... The Internet Protocol (IP) is a data-oriented protocol used for communicating data across a packet-switched internetwork. ... Ethernet is a large, diverse family of frame-based computer networking technologies that operate at many speeds for local area networks (LANs). ... A connection-oriented networking protocol is one which identifies traffic flows by some connection identifier rather than by explicitly listing source and destination addresses. ...


The standards for ATM were first developed in the mid 1980s. The goal was to design a single networking strategy that could transport real-time video and audio as well as image files, text and email. Two groups, the International Telecommunications Union and the ATM Forum were involved in the creation of the standards. ATM has been used primarily with telephone and IP networks. The International Telecommunication Union (ITU) is an international organization established to standardize and regulate international radio and telecommunications. ... The ATM Forum was founded in 1991 to be the industry consortium for Asynchronous Transfer Mode. ...

Contents

ATM Addressing

A Virtual Channel (VC) denotes the transport of ATM cells which have the same unique identifier, called the Virtual Channel Identifier (VCI). This identifier is encoded in the cell header. A virtual channel represents the basic means of communication between two end-points, and is analogous to an X.25 virtual circuit.[1] A Virtual Channel Identifier (VCI) is a unique identifier which indicates a particular virtual circuit on a network. ... X.25 is an ITU-T standard protocol suite for wide area networks using leased lines, the phone or ISDN system as the networking hardware. ... In telecommunications and computer networks, a virtual circuit (VC), synonymous to virtual connection and virtual channel, is a connection oriented communication service that is delivered by means of packet mode communication. ...


A Virtual Path (VP) denotes the transport of ATM cells belonging to virtual channels which share a common identifier, called the Virtual Path Identifier (VPI), which is also encoded in the cell header. A virtual path, in other words, is a grouping of virtual channels which connect the same end-points, and which share a traffic allocation. This two layer approach can be used to separate the management of routes and bandwidth from the setup of individual connections. Virtual Path Identifier refers to an 8-bit (user to network packets) or 12-bit (network-network packets) field within the header of an Asynchronous Transfer Mode packet. ...


Successes and failures of ATM technology

ATM has proven very successful in the WAN scenario and numerous telecommunication providers have implemented ATM in their wide-area network cores. Many ADSL implementations also use ATM. However, ATM has failed to gain wide use as a LAN technology, and its complexity has held back its full deployment as the single integrating network technology in the way that its inventors originally intended. Since there will always be both brand-new and obsolescent link-layer technologies, particularly in the LAN area, not all of them will fit neatly into the synchronous optical networking model for which ATM was designed. Therefore, a protocol is needed to provide a unifying layer over both ATM and non-ATM link layers, as ATM itself cannot fill that role. IP already does that; therefore, there is often no point in implementing ATM at the network layer. Wide Area Network (WAN) is a computer network that covers a broad area (i. ... Asymmetric Digital Subscriber Line (ADSL) is a form of DSL, a data communications technology that enables faster data transmission over copper telephone lines than a conventional voiceband modem can provide. ... LAN redirects here. ... The data link layer is layer two of the seven-layer OSI model as well as of the five-layer TCP/IP reference model. ... Synchronous optical networking, is a method for communicating digital information using lasers or light-emitting diodes (LEDs) over optical fiber. ... The Internet Protocol (IP) is a data-oriented protocol used for communicating data across a packet-switched internetwork. ... The network layer is third layer out of seven in OSI model and it is the third layer out of five in TCP/IP model. ...


In addition, the need for cells to reduce jitter has declined as transport speeds increased (see below), and improvements in Voice over IP (VoIP) have made the integration of speech and data possible at the IP layer, again removing the incentive for ubiquitous deployment of ATM. Most Telcos are now planning to integrate their voice network activities into their IP networks, rather than their IP networks into the voice infrastructure. In telecommunication, jitter is an abrupt and unwanted variation of one or more signal characteristics, such as the interval between successive pulses, the amplitude of successive cycles, or the frequency or phase of successive cycles. ... An overview of how VoIP works A typical analog telephone adapter for connecting an ordinary phone to a VoIP network Ciscos implementation of VoIP - IP Phone Voice over Internet Protocol, also called VoIP (pronounced voyp), IP Telephony, Internet telephony, Broadband telephony, Broadband Phone and Voice over Broadband is the... A telephone company (or telco) provides telecommunications services such as telephony and data communications. ...


Many technically sound ideas from ATM were adopted by MPLS, a generic Layer 2 packet switching protocol. ATM remains widely deployed, and is used as a multiplexing service in DSL networks, where its compromises fit DSL's low-data-rate needs well. In turn, DSL networks support IP (and IP services such as VoIP) via PPP over ATM and Ethernet over ATM (RFC 2684). MPLS is a common abbreviation for Multiprotocol Label Switching. ... The Open Systems Interconnection Reference Model (OSI Model or OSI Reference Model for short) is a layered abstract description for communications and computer network protocol design, developed as part of the Open Systems Interconnect initiative. ... In telecommunications, multiplexing (also muxing or MUXing) is the combining of two or more information channels onto a common transmission medium using hardware called a multiplexer or (MUX). ... DSL redirects here. ... PPPOA or PPPoA, Point-to-Point Protocol (PPP) over ATM, is a network protocol for encapsulating PPP frames in ATM AAL5. ...


ATM will remain deployed for some time in higher-speed interconnects where carriers have already committed themselves to existing ATM deployments; ATM is used here as a way of unifying PDH/SDH traffic and packet-switched traffic under a single infrastructure. However, ATM is increasingly challenged by speed and traffic shaping requirements of converged networks. In particular, the complexity of SAR imposes a performance bottleneck, as the fastest SARs known run at 10 Gbit/s and have limited traffic shaping capabilities. Currently it seems likely that gigabit Ethernet implementations (10Gbit-Ethernet, Metro Ethernet) will replace ATM as a technology of choice in new WAN implementions. The Plesiochronous Digital Hierarchy (PDH) is a technology used in telecommunications networks to transport large quantities of data over digital transport equipment such as fibre optic and microwave radio systems. ... Synchronous optical networking, is a method for communicating digital information using lasers or light-emitting diodes (LEDs) over optical fiber. ... Segmentation and Reassembly refers to the process used to divide and reformat data so as to allow it to be transported as the 48 byte payload in multiple ATM cells, followed by its reconstitution at the destination of the ATM Permanent virtual circuit through which it has been carried. ... Ethernet is a large, diverse family of frame-based computer networking technologies that operate at many speeds for local area networks (LANs). ... A Metro Ethernet is a computer network based on the Ethernet standard and which covers a metropolitan area. ...


Recent developments

Interest in using native ATM for carrying live video and audio has increased recently. In these environments, low latency and very high quality of service are required to handle linear audio and video streams. Towards this goal standards are being developed such as AES47 (IEC 62365), which provides a standard for professional uncompressed audio transport over ATM. This is worth comparing with professional video over IP. Background This article describes a standardised method of interconnecting digital audio over a telecommunication standard network. ... IEC 62365 is a standard that specifies a method for packing AES3 professional digital audio streams over Asynchronous Transfer Mode newtorks. ...


ATM concepts

IBM Turboways ATM 155 PCI network interface card
IBM Turboways ATM 155 PCI network interface card

Image File history File linksMetadata Download high-resolution version (1395x808, 296 KB) IBM Turboways ATM 155 PCI network interface card. ... Image File history File linksMetadata Download high-resolution version (1395x808, 296 KB) IBM Turboways ATM 155 PCI network interface card. ... For other uses, see IBM (disambiguation) and Big Blue. ... This article is about the computer bus type. ...

Why cells?

The motivation for the use of small data cells was the reduction of jitter (delay variance, in this case) in the multiplexing of data streams; reduction of this (and also end-to-end round-trip delays) is particularly important when carrying voice traffic. In telecommunication, jitter is an abrupt and unwanted variation of one or more signal characteristics, such as the interval between successive pulses, the amplitude of successive cycles, or the frequency or phase of successive cycles. ...


This is because the conversion of digitized voice back into an analog audio signal is an inherently real-time process, and to do a good job, the codec that does this needs an evenly spaced (in time) stream of data items. If the next data item is not available when it is needed, the codec has no choice but to produce silence or guess - and if the data is late, it is useless, because the time period when it should have been converted to a signal has already passed. In computer science, real-time computing (RTC) is the study of hardware and software systems which are subject to a real-time constraint—i. ... A codec is a device or program capable of encoding and/or decoding a digital data stream or signal. ...


Now consider a speech signal reduced to packets, and forced to share a link with bursty data traffic (i.e. some of the data packets will be large). No matter how small the speech packets could be made, they would always encounter full-size data packets, and under normal queuing conditions, might experience maximum queuing delays.


At the time ATM was designed, 155 Mbit/s SDH (135 Mbit/s payload) was considered a fast optical network link, and many PDH links in the digital network were considerably slower, ranging from 1.544 to 45 Mbit/s in the USA (2 to 34 Mbit/s in Europe). Synchronous Optical Networking, commonly known as SONET, is a standard for communicating digital information over optical fiber. ... The Plesiochronous Digital Hierarchy (PDH) is a technology used in telecommunications networks to transport large quantities of data over digital transport equipment such as fibre optic and microwave radio systems. ...


At this rate, a typical full-length 1500 byte (12000-bit) data packet would take 77.42 µs to transmit. In a lower-speed link, such as a 1.544 Mbit/s T1 link, a 1500 byte packet would take up to 7.8 milliseconds. To help compare orders of magnitude of different times this page lists times between 10−6 seconds and 10−5 seconds (1. ...


A queueing delay induced by several such data packets might be several times the figure of 7.8 ms, in addition to any packet generation delay in the shorter speech packet. This was clearly unacceptable for speech traffic, which needs to have low jitter in the data stream being fed into the codec if it is to produce good-quality sound. A packet voice system can produce this in a number of ways: In computer engineering, a queueing delay is the time a job waits in a queue until it can be executed. ...

  • Have a playback buffer between the network and the codec, one large enough to tide the codec over almost all the jitter in the data. This allows smoothing out the jitter, but the delay introduced by passage through the buffer would be such that echo cancellers would be required even in local networks; this was considered too expensive at the time. Also, it would have increased the delay across the channel, and conversation is difficult over high-delay channels.
  • Build a system which can inherently provide low jitter (and minimal overall delay) to traffic which needs it.
  • Operate on a 1:1 user basis (i.e., a dedicated pipe).

ATM was designed to implement a low-jitter network interface. However, to be able to provide short queueing delays, but also be able to carry large datagrams, it had to have cells. ATM broke up all packets, data, and voice streams into 48-byte chunks, adding a 5-byte routing header to each one so that they could be reassembled later. The choice of 48 bytes was, as is all too often the case, political instead of technical.[2] When the CCITT was standardizing ATM, parties from the United States wanted a 64-byte payload because having the size be a power of 2 made working with the data easier and this size was felt to be a good compromise between larger payloads optimized for data transmission and shorter payloads optimized for real-time applications like voice; parties from Europe wanted 32-byte payloads because the small size (and therefore short transmission times) simplify voice applications with respect to echo cancellation. Most of the interested European parties eventually came around to the arguments made by the Americans, but France and a few allies held out until the bitter end. With 32 bytes, France would have been able to implement an ATM-based voice network with calls from one end of France to the other requiring no echo cancellation. 48 bytes (plus 5 header bytes = 53) was chosen as a compromise between the two sides, but it was ideal for neither and everybody has had to live with it ever since. 5-byte headers were chosen because it was thought that 10% of the payload was the maximum price to pay for routing information. ATM multiplexed these 53-byte cells instead of packets. Doing so reduced the worst-case queuing jitter by a factor of almost 30, removing the need for echo cancellers. The ITU Telecommunication Standardization Sector (ITU-T) coordinates standards for telecommunications on behalf of the International Telecommunication Union (ITU) and is based in Geneva, Switzerland. ...


Cells in practice

Different types of services are supported by ATM via ATM Adaptation Layers (AAL). Standardized AALs include AAL1, AAL2, and AAL5, and the rarely used AAL3 and AAL4. AAL1 is used for constant bit rate (CBR) services and circuit emulation. AAL2 through AAL4 are used for variable bit rate (VBR) services, and AAL5 for data. Which AAL is in use for a given cell is not encoded in the cell. Instead, it is negotiated by or configured at the endpoints on a per-virtual-connection basis. The use of Asynchronous Transfer Mode (ATM) technology and services creates the need for an adaptation layer in order to support information transfer protocols, which are not based on ATM. This adaptation layer defines how to segment and reassemble higher-layer packets into ATM cells, and how to handle various...


Since the time ATM was designed, networks have become much faster. A 1500 byte (12000-bit) full-size Ethernet packet takes only 1.2 µs to transmit on a 10 Gbit/s optical network, removing the need for small cells to reduce jitter. Some consider that this removes the need for ATM in the network backbone. Additionally, the hardware for implementing the service adaptation for IP packets is expensive at very high speeds. Specifically, the cost of segmentation and reassembly (SAR) hardware at OC-3 and above speeds makes ATM less competitive for IP than Packet Over SONET (POS). SAR performance limits mean that the fastest IP router ATM interfaces are OC12 - OC48 (STM4 - STM16), while (as of 2004) POS can operate at OC-192 (STM64) with higher speeds expected in the future. Optical Carrier levels are used for the categories of bandwidth in a SONET fiber optic network. ... Packet over SONET/SDH, abbreviated POS, is communications protocol for transmitting packets over SDH or SONET, which are both circuit switched protocols. ...


On slow links (2 Mbit/s and below). ATM still makes sense, and this is why so many ADSL systems use ATM as an intermediate layer between the physical link layer and a Layer 2 protocol like PPP or Ethernet. Asymmetric Digital Subscriber Line (ADSL) is a form of DSL, a data communications technology that enables faster data transmission over copper telephone lines than a conventional voiceband modem can provide. ... In computing, the Point-to-Point Protocol, or PPP, is commonly used to establish a direct connection between two nodes. ...


At these lower speeds, ATM's ability to carry multiple logical circuits on a single physical or virtual medium is useful, although other techniques exist, such as PPP and Ethernet VLANs, which are optional in VDSL implementations. DSL can be used as an access method for an ATM network, allowing a DSL termination point in a telephone central office to connect to many internet service providers across a wide-area ATM network. In the United States, at least, this has allowed DSL providers to provide DSL access to the customers of many internet service providers. Since one DSL termination point can support multiple ISPs, the economic feasibility of DSL is substantially improved. Ethernet is a large, diverse family of frame-based computer networking technologies that operate at many speeds for local area networks (LANs). ... A virtual LAN, commonly known as a VLAN, is a logically segmented network mapped over physical hardware. ... It has been suggested that VDSL2 be merged into this article or section. ...


Why virtual circuits?

ATM is a channel-based transport layer, using Virtual circuits (VCs). This is encompassed in the concept of the Virtual Paths (VP) and Virtual Channels. Every ATM cell has an 8- or 12-bit Virtual Path Identifier (VPI) and 16-bit Virtual Channel Identifier (VCI) pair defined in its header. Together, these identify the virtual circuit used by the connection. The length of the VPI varies according to whether the cell is sent on the user-network interface (on the edge of the network), or if it is sent on the network-network interface (inside the network). A virtual circuit (VC) is a communications arrangement in which data from a source user is passed to a destination user over more than one real communications circuit during a single period of communication, and the switching is hidden from the users. ... Virtual Path Identifier refers to an 8-bit (user to network packets) or 12-bit (network-network packets) field within the header of an Asynchronous Transfer Mode packet. ... A Virtual Channel Identifier (VCI) is a unique identifier which indicates a particular virtual circuit on a network. ...


As these cells traverse an ATM network, switching is achieved by changing the VPI/VCI values. Although the VPI/VCI values are not necessarily consistent from one end of the connection to the other, the concept of a circuit is consistent (unlike IP, where any given packet could get to its destination by a different route than the others).


Another advantage of the use of virtual circuits is the ability to use them as a multiplexing layer, allowing different services (such as voice, Frame Relay, n*64 channels , IP). In the context of computer networking, frame relay consists of an efficient data transmission technique used to send digital information quickly and cheaply in a relay of frames to one or many destinations from one or many end-points. ...


Using cells and virtual circuits for traffic engineering

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. If a service (or application) wishes to use a broadband network (an ATM network in particular) to transport a particular kind of traffic, it must first inform the network about what kind of traffic is to be transported, and the performance requirements of that traffic[1]. The application presents this...


ATM traffic contracts are 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. In the fields of packet-switched networks and computer networking, the traffic engineering term Quality of Service, abbreviated QoS, refers to resource reservation control mechanisms. ...

  • CBR - Constant bit rate: a Peak Cell Rate (PCR) is specified, which is constant.
  • VBR - Variable bit rate: an average cell rate is specified, which can peak at a certain level for a maximum interval before being problematic.
  • ABR - Available bit rate: a minimum guaranteed rate is specified.
  • UBR - Unspecified bit rate: traffic is allocated to all remaining transmission capacity.

VBR has real-time and non-real-time variants, and is used for "bursty" traffic. Non-real-time is usually abbreviated to vbr-nrt.


Most traffic classes also introduce the concept of Cell Delay Variation Tolerance (CDVT) which defines the "clumping" of cells in time.


Traffic contracts are usually maintained by the use of "Shaping", a combination of queuing and marking of cells, and enforced by "Policing". Traffic shaping (also known as packet shaping) is an attempt to control computer network traffic in order to optimize or guarantee performance, low latency, and/or bandwidth by delaying packets[1]. Traffic shaping deals with concepts of classification, queue disciplines, enforcing policies, congestion management, quality of service (QoS), and fairness. ...


Traffic shaping

Traffic shaping 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 shaping (also known as packet shaping) is an attempt to control computer network traffic in order to optimize or guarantee performance, low latency, and/or bandwidth by delaying packets[1]. Traffic shaping deals with concepts of classification, queue disciplines, enforcing policies, congestion management, quality of service (QoS), and fairness. ...


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 farther 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). 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 redundant cells in the network, saving bandwidth for full frames. EPD and PPD work with AAL5 connections as they use the frame end bit to detect the end of packets.


Types of virtual circuits and paths

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


PVPs and PVCs are conceptually simple, but require significant effort in large networks. They also do not support the re-routing of service in the event of a failure. Dynamically built PVPs (soft PVPs or SPVPs) and PVCs (soft PVCs or SPVCs), in contrast, are built by specifying the characteristics of the circuit (the service "contract") and the two endpoints.


Finally, switched virtual circuits (SVCs) are built and torn down on demand when requested by an end piece of equipment. One application for SVCs is to carry individual telephone calls when a network of telephone switches are inter-connected by ATM. SVCs were also used in attempts to replace local area networks with ATM.


Virtual circuit routing

Most ATM networks supporting SPVPs, SPVCs, and SVCs use the Private Network Node Interface or Private Network-to-Network Interface (PNNI) protocol. PNNI uses the same shortest path first algorithm used by OSPF and IS-IS to route IP packets to share topology information between switches and select a route through a network. PNNI also includes a very powerful summarization mechanism to allow construction of very large networks, as well as a call admission control (CAC) algorithm that determines whether sufficient bandwidth is available on a proposed route through a network to satisfy the service requirements of a VC or VP. The Open Shortest Path First (OSPF) protocol is a hierarchical interior gateway protocol (IGP) for routing in Internet Protocol, using a link-state in the individual areas that make up the hierarchy. ... Is Is is Yeah Yeah Yeahs third EP, to be released on July 24, 2007. ... Admission control is a network Quality of Service (QoS) procedure[1]. Admission control determines how bandwidth and latency are allocated to streams with various requirements[2]. Admission control schemes therefore need to be implemented between network edges and core to control the traffic entering the network. ...


Call admission and connection establishment

A connection has to be established for two parties to be able to send cells to each other. In ATM this is called a VC ("Virtual Connection"). It can be a PVC ("Permanent Virtual Connection"), which is created administratively, or an SVC("Switched Virtual Connection"), which is created as needed by the communicating parties. SVC creation is done by "signaling" in which the requesting party indicates the address of the receiving party, the type of service requested, and traffic parameters if applicable to the selected service. "Call admission" is then done by the network to confirm that the requested resources are available, and that a route exists for the connection.


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 chosen as described above ("Why Cells?").


ATM defines two different cell formats: NNI (Network-Network Interface) and UNI (User-Network Interface). Most ATM links use UNI cell format. Network to Network Interface, or NNI for short, is an interface which specifies signaling and management functions between two networks. ... User Network Interface (UNI) is a demarcation point between the responsibility of the service provider and the responsibility of the subscriber. ...

Diagram of the UNI ATM Cell

7

4
3


0
GFC VPI
VPI
VCI
VCI
VCI PT CLP
HEC




Payload (48 bytes)



Diagram of the NNI ATM Cell

7

4
3


0
VPI
VPI
VCI
VCI
VCI PT CLP
HEC




Payload (48 bytes)



GFC = Generic Flow Control (4 bits) (default: 4-zero bits)
VPI = Virtual Path Identifier (8 bits UNI) or (12 bits NNI)
VCI = Virtual channel identifier (16 bits)
PT = Payload Type (3 bits)
CLP = Cell Loss Priority (1-bit)
HEC = Header Error Correction (8-bit CRC, polynomial = X8 + X2 + X + 1)

The PT field is used to designate various special kinds of cells for operations, administration, and maintenance (OAM) purposes, and to delineate packet boundaries in some AALs. Virtual Path Identifier refers to an 8-bit (user to network packets) or 12-bit (network-network packets) field within the header of an Asynchronous Transfer Mode packet. ... A Virtual Channel Identifier (VCI) is a unique identifier which indicates a particular virtual circuit on a network. ... edit ... The Header Error Correction (HEC) is a field in the Asynchronous Transfer Mode cell consisting of an 8-bit checksum of the cells header only. ...


Several of ATM's link protocols use the HEC field to drive a CRC-Based Framing algorithm, which allows the position of the ATM cells to be found with no overhead required beyond what is otherwise needed for header protection. The 8-bit CRC is used to correct single-bit header errors and detect multi-bit header errors. When multi-bit header errors are detected, the current and subsequent cells are dropped until a cell with no header errors is found. CRC-Based Framing The concept of CRC-Based Framing was developed in StrataCom, Inc. ...


In a UNI cell the GFC field is reserved for a local flow control/submultiplexing system between users. This was intended to allow several terminals to share a single network connection, in the same way that two ISDN phones can share a single basic rate ISDN connection. All four GFC bits must be zero by default. The flow control mechanism is used for controlling the flow of data in a network under well-defined conditions, while congestion control is used for controlling the flow of data when congestion has actually occurred . ... ISDN redirects here. ...


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 216 VCs each (in practice some of the VP and VC numbers are reserved).


References

  1. P.S. Neelakanta; A Textbook on ATM Telecommunications, Principles and implementation. CRC Press. 2000 ISBN 0-8493-1805-X.

Further reading

  • Amos E. Joel, Jr., Asynchronous Transfer Mode (IEEE Press, 1993)
  • Martin De Prycker, Asynchronous Transfer Mode. Solutions for Broadband ISDN (Prentice-Hall, 1993)
  • Tom Golway, Planning and Managing ATM Networks. New York: Manning, 1997. ISBN 132621894.
  • P.S. Neelakanta A Textbook on ATM Telecommunications, Principles and implementation. CRC Press. 2000 ISBN 0-8493-1805-X.

Amos Edward Joel, Jr. ... In the 1970s the telecommunications industry conceived that digital services would follow much the same pattern as voice services, and conceived a grandiose vision of end_to_end circuit switched services, known as the Broadband Integrated Services Digital Network (B-ISDN). ... Prentice Hall is a leading educational publisher. ...

External links


  Results from FactBites:
 
Asynchronous Transfer Mode (5328 words)
ATM is based on the switching of 53-byte cells, in which each cell consists of a 5-byte header and a payload of 48 bytes of information.
Thus, the ATM layer is responsible for relaying cells from the AAL to the Physical layer for transmission, and in the opposite direction from the Physical layer to the AAL for use in an endpoint.
ATM Adaptation layers are distinguished from one another based on the method by which the 48-byte cell payload constructed as a data stream generated by a higher-level protocol is passed to the ATM layer.
Asynchronous transfer mode - Hill2dot0 (1704 words)
Asynchronous transfer mode (ATM) derives its name from the fact that the transmission system can accommodate traffic arriving at fixed time intervals, which is demanded by voice and video applications, as well as bursty traffic that arrives at no preset rate.
ATM is akin to an escalator serving two types of traffic: people with infants who we assume should have priority access to the moving stairs and people who are able to wait.
ATM is a transmission scheme that combines aspects of statistical multiplexing and synchronous transfer mode.
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