Asynchronous Transfer Mode 3393

Asynchronous Transfer Mode (ATM) is a "high-speed transmission protocol inwhich data blocks are broken into small cells that are transmitted individuallyand possibly via different routes in a manner similar to packet-switchingtechnology". In other words, it is a form of data transmission that allowsvoice, video and data to be sent along the same network. In the past, voice,video and data were transferred using separate networks: voice traffic over thephone, video over cable networks and data over an internetwork. ATM is a cell-switching and multiplexing technology designed to be a fast, general purposetransfer mode for multiple services. It is asynchronous because cells are nottransferred periodically.

Cells are given time slots on demand. What seperatesATMs is its capability to support multimedia and integrate these services alongwith data over a signal type of transmission method. The ATM cell is the dataunit used to transmit the data. The data is broken into 48-byte data packets fortransmission. Five bytes of control data are appended to the 48-byte datapackets, forming a 53-byte transmission frame.

These frames are then transmittedto the recipient, where the 5-byte control data (or Header) is removed and themessage is put back together for use by the system In an ATM network, all datais switched and multiplexed in these cells. Each ATM cell sent into the networkcontains addressing information that achieves a virtual connection fromorigination to destination. All cells are then transferred, in sequence, overthis virtual connection. Asynchronous Transfer Mode: The header includesinformation about the contents of the payload and about the method oftransmission.

The header contains only 5 octets. It was shortened as much aspossible, containing the minimum address and control functions for a workingsystem. The sections in the header are a series of bits which are recognized andprocessed by the ATM layer. Sections included in the header are Generic FlowControl (GFC), Cell Loss Priority (CLP), Payload Type, Header Error Control, theVirtual Path Identifier and the Virtual Channel Identifier. The Header is theinformation field that contains the revenue bearing payload. A GFC is a 4-bitfield intended to support simple implementations of multiplexing.

The GFC isintended to support flow control. The CLP bit is a 1-bit field that indicatesthe loss priority of an individual cell. Cells are assigned a binary code toindicate either high or low priority. A cell loss priority value of zeroindicates that the cell contents are of high priority. High priority cells areleast likely to be discarded during periods of congestion.

Those cells with ahigh priority will only be discarded after all low priority cells have beendiscarded. Cell loss is more detrimental to data transmission than it is tovoice or video transmission. Cell loss in data transmission results in corruptedfiles. The Payload Type section is a 3-bit field that discriminates between acell payload carrying user data or one carrying management information. Userdata is data of any traffic type that has been packaged into an ATM cell. Anexample of management Asynchronous Transfer Mode: information is informationinvolved in call set-up.

This section also notes whether the cell experiencedcongestion. The Header Error Control field consists of error checking bits. TheHeader Error Control field is an 8-bit Cyclic Redundancy Code to check forsingle bit and some multi-bit errors. It provides error checking of the headerfor use by the Transmission Convergence (TC) sublayer of the Physical layer.

TheVirtual Path Identifier in the cell header identifies a bundle of one or moreVCs(virtual channels).The Birtual Channel Identifier (VCI) in the cel headeridentifies a single VC on a paricular Virtual Path. The path is divided intochannels. The choice of the 48 byte payload was made as a compromise toaccommodate multiple forms of traffic. The two candidate payload sizes wereinitially 32 and 64 bytes.

The size of the cell has and effect on bothtransmission efficiency and packetization delay. A long payload is moreefficient than a small payload since, with a large payload, more data can betransmitted per cell with the same amount of overhead (header). For datatransmission alone, a large payload is desirable. The longer the payload is,however, the more time is spent packaging.

Certain traffic types are sensitiveto time such as voice. If packaging time is too long, and the cells are not sentoff quickly, the quality of the voice transmission will decrease. The 48 bytepayload size was the result of a compromise that had to be reached between the64 byte payload which would provide efficient data transfer but poor qualityvoice and the 32 byte payload which could Asynchronous Transfer Mode: transmitvoice without echo but provided inefficient data transfer. The 48 byte payloadsize allows ATM to carry multiple forms of traffic. Both time-sensitive traffic(voice) and time-insensitive traffic can be carried with the best possiblebalance between efficiency and packetization delay.

ATM Advantages: 1. ATMsupports voice, video and data allowing multimedia and mixed services over asingle network. 2. High evolution potential, works with existing, legacytechnologies 3. Provides the best multiple service support 4. Supports delayclose to that of dedicated services 5.

QoS (Quality of Service)classes 6.Provides the capability to support both connection-oriented and connectionlesstraffic using AALs(ATM Adaptation Layers) 7. Able to use all common physicaltransmission paths (DS1,SONET) 8. Cable can be twisted-pair, coaxial orfiber-optic 9. Ability to connect LAN to WAN 10. Legacy LAN emulation 11.

Efficient bandwidth use by statistical multiplexing 12. Scalability 13. Higheraggregate bandwidth 14. High speed Mbps and possibly Gbps Asynchronous TransferMode: ATM disadvantages: 1.

Flexible to efficiency's expense, at present, forany one application it is usually possible to find a more optimized 2.Technology 3. Cost, although it will decrease with time 4. New customer premiseshardware and software are required 5. Competition from other technologies -100Mbps FDDI, 100 Mbps Ethernet and fast ethernet 6.

Presently the applicationsthat can benefit from ATM such as multimedia are rare 7. The wait, with all thepromise of ATM's capabilities many details are still in the standards processAsynchronous Transfer Mode: Bibliography1. Freeman, Roger L. ((1996).

Telecommunication System Engineering: ThirdEdition. City: New York, John Wiley & Sons, INC. 2. Spohn, Darren L. (1997).

Data Network Design. City: McGraw-Hill Company. 3. Taylor, D.

Edgar (1995). TheMcGraw-Hill Internetworking Handbook. City: New York, McGraw-Hill Company.Internet: 1.

Quigley, David (1997). A Technical View of ATMs. [online],Available: http://www.mathcs.carleton.edu/students/quigleyd/atmtech.html.