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Asynchronous Transfer Mode Essay, Research Paper

Asynchronous Transfer Mode

Asynchronous Transfer Mode: Asynchronous Transfer Mode By Gene

Bandy State Technical Institute Asynchronous Transfer Mode: Asynchronous

Transfer Mode(ATM) is a “high-speed transmission protocol in which data

blocks are broken into small cells that are transmitted individually 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

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