ATM (Asynchronous Transfer Mode)

• “Asynchronous transfer mode (ATM) is a high performance, cell-oriented switching and multiplexing technology that utilizes fixed-length packets to carry different types of traffic.”– It is a connection oriented network where before

transmission, connection is established and then data transmission takes place only through that connected route.

• The transmitter and receiver does not need to be connected to the network at the same time. They do not use the same clock signals. Here each data word is accompanied by stop(1) and start(0) bits, which identify the beginning and end of the word. When no data is being transmitted, the communication line is always high. This type of communication is proved to be inefficient when there is large amount of data, extra start and stop bits slow down the data transmission.

• Connection established by ATM Reference Model are often referred to as Virtual Circuit. This is what most of us encountered on our home phones. We place a call and either get our destination part or encounter a busy signal, we can not transmit any message. A single circuit is used for duration of the call.

Virtual Circuit(1) A temporary communications path created between devices in a switched communications system. For example, a message from New York to Los Angeles may actually be routed through Atlanta and St. Louis. Within a smaller geography, such as a building or campus, the virtual circuit traverses some number of switches, hubs and other network devices.

(2) A logical circuit within a physical network. The actual lines may be shared by other users at the same time, but the virtual circuit appears exclusive to the users who are communicating with each other. The terms "permanent virtual circuit" and "virtual private network" also describe this kind of logical circuit.

• ATM consists of fixed-size packets called cells. Each cell is of size 53 bytes. The first 5 bytes of the cell corresponds to header and last 48 bytes corresponds to payload information. The ATM header consists of the following information:- a) Virtual Path Identifier (VPI)b) Virtual Channel Identifier (VCI)c) Header Error Control (HEC)d) Payload Type (PT)e) Cell Loss Priority (CLP)

VIRTUAL PATH IDENTIFIER (VPI)

VIRTUAL CHANNEL IDENTIFIER (VCI)

HEADER ERROR CONTROL (HEC)

PAYLOAD TYPE (PT)PT= 1; HEAVY TRAFFICPT=0; NO CONGESTION

CELL LOSS PRIORITY (CLP)CLP=1; DISCARD ATM CELLCLP=0; DO NOT DISCARD ATM CELL

Fig. ATM Header Part

• Both VPI and VCI together identify next switch on the way to its destination host. These identifiers ensure that which cell belongs to which connection. HEC (Header Error Control) contains the information like Checksum, Parity bytes for error free delivery of data. The PT (Payload Type) field indicates whether there is congestion (i.e. heavy traffic) on the line or not.

• ATM can ensure the sequential delivery of data but not the error free delivery because the HEC part of header ensure that if there is one bit error in the payload that will be detected as checksum is also performed on first for bytes and parity byte is also capable of detecting one byte error. If there is heavy congestion on the line, PT is set to one end else zero and the CLP indicates whether the cell should be discarded due to heavy congestion or not. If CLP=1, the cell should be discarded else not.

• The ATM architecture uses a logical model to describe the functionality it supports. It consists of 3 layers namely :- the physical layer at bottom, ATM layer in the middle, the ATM Adaptation Layer at the Upper. At the higher level user can define its own layer according to requirements and plus 3 planes named as control plane, user plane, and management plane.

ATM Architecture

The ATM Reference Model.

(1) The Physical Layer• The Physical Layer of ATM Reference model

corresponds to physical layer of OSI reference model. It is concerned with the decisions like- How many volts are required to represent 0 or 1, what transmission medium (guided or un-guided) to use for transferring data? ATM physical layer has the responsibility of converting the ATM cell into bit stream and the packaging it into the frame which can be carried away by the physical medium. The ATM Physical Layer is divided into 2 sub-layers:-

1. The ATM Physical Medium Dependent (PMD) sub-layer.

2. The Transmission Convergence (TC) sub-layer.

1. The PMD deals with the type of physical medium to use and also manages the synchronization i.e. transmission is associated with timings.

2. The TC sub-layer has the duty of converting bit streams into ATM cell, so that the ATM layer can accept it. It also maintains information regarding HEC for the error free delivery of cell. It manages the cell boundaries.

(2) The ATM Layer

• The ATM layer manages the virtual circuit establishment and release between the nodes. The flow control and buffer management is done here. It multiplexes and de-multiplexes cells of different connections and extracts and inserts the header before or after the cell is delivered to AAL (ATM Adaptation Layer).

(3) ATM Adaptation Layer (AAL)

• Most of the layers do not accept ATM cells, these need to be converted into a specific frame packets larger than a cell for higher level layers defined by user. This job is done by ATM Adaptation Layer (AAL). AAL is divided into 2 sub-layers: -

1.The Convergence sub-layer (CS).2.The Segmentation and re-assembly sub-

layer(SAR).

Fig. Parts of AAL (ATM Adaptation Layer)

• CS: - The CS accepts the frame, divides and packs it into 53 bytes different cells, and sends these cells to the destination and the at the other end these cells are reassembled to make it a frame.

• SAR: - The SAL divides data frames into ATM cells at the transmitting end and re-assembles them into their original format at the receiving end.

The Higher Layer accepts user data, arrange itinto packets, and hand it to the AAL.

PlanesThe ATM Reference model is composed of following planes which span all layers.1. The Control Plane: - It is concerned with connection

management between nodes.2. The User Plane: - It is responsible for data transfer,

error control, flow control and buffer management.3. The Layer Management Plane: - It manages layer

specific functions such as detection of failures and protocol problems.

4. The Plane Management : - It manages and co-ordinates functions related to complete system.

ATM ApplicationsATM is used in both LAN’s and WAN’s.

1. ATM WAN’s: - ATM is basically a WAN technology that delivers cells over a long distance. In this type of application, ATM is mainly used to connect LAN’s or other WAN’s together. A Router between the ATM network and the other network serves as an end point. The router has two stacks of protocols: one belonging to the ATM and the other belonging to the other protocol.

2. ATM LAN’s :- ATM was originally as a WAN technology. However, the high data rate of the technology (155 and 622 Mbps) has attracted the attention of designers who are looking for more and more speed in LAN’s. at the surface level, the use of ATM technology in LAN’s seems very natural.

Advantages 1. ATM supports Voice, Video and Data allowing

multimedia and mixed services over a single network.2. Scalable bandwidth at affordable costs provides

support for widely different traffic characteristics.3. High evolution potential, works with existing, legacy

technologies.4. Provides the best multiple service support.5. Quality of service(QOS) guarantees provides for

emerging classes of application.6. Ability to connect LAN to WAN.7. Scalability8. High speed Mbps and possibly Gbps.

Disadvantages

1. Flexible to efficiency’s expense, at present, for any one application it is usually to find a more optimized technology.

2. High Cost.3. New Customer premises hardware and

software are required.4. Overhead of cell header (5bytes per cell).5. Complex mechanisms for achieving QOS.