Asynchronous Transfer Mode Essays (1139 words) - Network Protocols

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