Essays

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 Internet work.

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 separates ATM 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.

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.

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) sub-layer of the Physical layer. The Virtual Path Identifier in the cell header identifies a bundle of one or more VCs (virtual channels). 

The Virtual Channel Identifier (VCI) in the cell header identifies a single VC on a particular 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 an effect on both c transmission efficiency and packetisation delay. A large 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 larger 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 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 packetisation delay. There are many advantages of ATM:

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.  (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 fibre-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

There are some disadvantages of ATM:              

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.