Asynchronous Transfer Mode (ATM) is a "high-speed transmission protocol in

which data blocks are broken into small cells that are transmitted individually

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and possibly via different routes in a manner similar to packet-switching

technology". In other words, it is a form of data transmission that allows

voice, 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 the

phone, video over cable networks and data over an internetwork. ATM is a cell-

switching and multiplexing technology designed to be a fast, general purpose

transfer mode for multiple services. It is asynchronous because cells are not

transferred periodically. Cells are given time slots on demand. What seperates

ATMs is its capability to support multimedia and integrate these services along

with data over a signal type of transmission method. The ATM cell is the data

unit used to transmit the data. The data is broken into 48-byte data packets for

transmission. Five bytes of control data are appended to the 48-byte data

packets, forming a 53-byte transmission frame. These frames are then transmitted

to the recipient, where the 5-byte control data (or Header) is removed and the

message is put back together for use by the system In an ATM network, all data

is switched and multiplexed in these cells. Each ATM cell sent into the network

contains addressing information that achieves a virtual connection from

origination to destination. All cells are then transferred, in sequence, over

this virtual connection. Asynchronous Transfer Mode: The header includes

information about the contents of the payload and about the method of

transmission. The header contains only 5 octets. It was shortened as much as

possible, containing the minimum address and control functions for a working

system. The sections in the header are a series of bits which are recognized and

processed by the ATM layer. Sections included in the header are Generic Flow

Control (GFC), Cell Loss Priority (CLP), Payload Type, Header Error Control, the

Virtual Path Identifier and the Virtual Channel Identifier. The Header is the

information field that contains the revenue bearing payload. A GFC is a 4-bit

field intended to support simple implementations of multiplexing. The GFC is

intended to support flow control. The CLP bit is a 1-bit field that indicates

the loss priority of an individual cell. Cells are assigned a binary code to

indicate either high or low priority. A cell loss priority value of zero

indicates that the cell contents are of high priority. High priority cells are

least likely to be discarded during periods of congestion. Those cells with a

high priority will only be discarded after all low priority cells have been

discarded. Cell loss is more detrimental to data transmission than it is to

voice or video transmission. Cell loss in data transmission results in corrupted

files. The Payload Type section is a 3-bit field that discriminates between a

cell payload carrying user data or one carrying management information. User

data is data of any traffic type that has been packaged into an ATM cell. An

example of management Asynchronous Transfer Mode: information is information

involved in call set-up. This section also notes whether the cell experienced

congestion. The Header Error Control field consists of error checking bits. The

Header Error Control field is an 8-bit Cyclic Redundancy Code to check for

single bit and some multi-bit errors. It provides error checking of the header

for use by the Transmission Convergence (TC) sublayer of the Physical layer. The

Virtual Path Identifier in the cell header identifies a bundle of one or more

VCs(virtual channels).The Birtual Channel Identifier (VCI) in the cel header

identifies a single VC on a paricular Virtual Path. The path is divided into

channels. The choice of the 48 byte payload was made as a compromise to

accommodate multiple forms of traffic. The two candidate payload sizes were

initially 32 and 64 bytes. The size of the cell has and effect on both

transmission efficiency and packetization delay. A long payload is more

efficient than a small payload since, with a large payload, more data can be

transmitted per cell with the same amount of overhead (header). For data

transmission alone, a large payload is desirable. The longer the payload is,

however, the more time is spent packaging. Certain traffic types are sensitive

to time such as voice. If packaging time is too long, and the cells are not sent

off quickly, the quality of the voice transmission will decrease. The 48 byte

payload size was the result of a compromise that had to be reached between the

64 byte payload which would provide efficient data transfer but poor quality

voice and the 32 byte payload which could Asynchronous Transfer Mode: transmit

voice without echo but provided inefficient data transfer. The 48 byte payload

size allows ATM to carry multiple forms of traffic. Both time-sensitive traffic

(voice) and time-insensitive traffic can be carried with the best possible

balance between efficiency and packetization delay. ATM Advantages: 1. ATM

supports voice, video and data allowing multimedia and mixed services over a

single network. 2. High evolution potential, works with existing, legacy

technologies 3. Provides the best multiple service support 4. Supports delay

close to that of dedicated services 5. QoS (Quality of Service)classes 6.

Provides the capability to support both connection-oriented and connectionless

traffic using AALs(ATM Adaptation Layers) 7. Able to use all common physical

transmission paths (DS1,SONET) 8. Cable can be twisted-pair, coaxial or

fiber-optic 9. Ability to connect LAN to WAN 10. Legacy LAN emulation 11.

Efficient bandwidth use by statistical multiplexing 12. Scalability 13. Higher

aggregate bandwidth 14. High speed Mbps and possibly Gbps Asynchronous Transfer

Mode: ATM disadvantages: 1. Flexible to efficiency's expense, at present, for

any one application it is usually possible to find a more optimized 2.

Technology 3. Cost, although it will decrease with time 4. New customer premises

hardware and software are required 5. Competition from other technologies -100

Mbps FDDI, 100 Mbps Ethernet and fast ethernet 6. Presently the applications

that can benefit from ATM such as multimedia are rare 7. The wait, with all the

promise of ATM's capabilities many details are still in the standards process

Asynchronous Transfer Mode:


1. Freeman, Roger L. ((1996). Telecommunication System Engineering: Third

Edition. City: New York, John Wiley & Sons, INC. 2. Spohn, Darren L. (1997).

Data Network Design. City: McGraw-Hill Company. 3. Taylor, D. Edgar (1995). The

McGraw-Hill Internetworking Handbook. City: New York, McGraw-Hill Company.

Internet: 1. Quigley, David (1997). A Technical View of ATMs. [online],