Atm And Telephone Networks

Published Date: 02 Nov 2017

INTRODUCTION:

ATM services was designed with the aim to support various services with a reasonable cost.

But cost and complexity grew higher than the expectation which resulted in IP based technology.

Driven by the integration of data and telephony networks, it is called broad band integrated service vision.

-The information is divided into small packets called cells.

- These cells are transmitted asynronously.

-The ATM network is connection oriented.

-The use of fixed sized cells made it efficient.

-In a ATM, first message is sent to setup the connection.

- After the connection is made or path is setup all the cells follow the same path.

-ATM handles both constant rate traffic and variable rate traffic. It means ATM carries all kinds of traffic with end to end quality of service.

-ATM is independent of transmission medium.

-The information may be sent on a wire or fiber themselves or they may also be packaged inside the payload of other carrier systems.

-The guarantee of packet delivery is not reliable but if the packet reaches the destination the order is guaranteed.

FEATURES OF ATM:

-It uses virtual circuits.

-It uses fixed sized packets called cells which results in fast hardware switching.

-It uses statistical multiplexing.

-Different qualities of services can co-exist at a same time.

-It has good management and traffic engineering features.

-There is scalability in speed and network size.

ATM CELL

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Fig-1: ATM basic cell

Cell is the basic unit of information used by the ATM. Its size is 53 bytes. The 5 bytes contains header information and other 48 bytes contains user data or payload.

ATM and TELEPHONE NETWORKS

In telephone networks all the data rates are multiples of 64kbps whereas in ATM the data rate can be varied as per the needs of the clients. In telephone networks all the high speed circuits are manually setup but ATM allows dialing at any speed and rapid provisioning since this is done through software and signaling. ATM networks have greater advantage to telephone networks.

ATM and DATA NETWORKS

ATM is based on virtual circuit. Once the path is setup between the sender and receiver the resources are reserved for the connection before transmission whereas internet protocol is connectionless and resource reservation is not possible. ATM has fixed size cells whereas packets of internet protocol networks are of variable size.

ATM devices and interfaces:

An ATM network is made up of ATM switches and endpoints. The endpoint devices can be workstations, routers, switches, CODEC having network interface. There are two types of interfaces that connect ATM devices over point to point links.

i) User to network interface(UNI)

ii)Network to network interface(NNI)

The UNI is a connection between endpoint devices and ATM switch which may be private or public.

The NNI is a connection between two ATM switches.

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Fig-2: Components of an ATM network

WORKING OF ATM NETWORK:

ATM is connection oriented so the connection between end to end link is necessary before data transmission.

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Fig-3: Establishing a connection in a ATM network

-Check if the incoming packet can be permitted to the ATM switch.

-Schedule the cell for transmission to a destination device. By the time the cell is transmitted make the necessary changes like VPI/VCI translation, setting the Early Forward Congestion Indicator Bit, and setting the CLP bit.

-In the fig-3, Router A sends a signaling request packet switch to ATM switch 1.

-ATM switch 1 reassembles the signaling packet from Router A and then examines it.

-Every switch along the path to Router B reassembles and examines the signaling packet and then forwards it to the next switch if the QoS parameters can be supported.

-When the signaling packet arrives at Router B, Router B reassembles its and evaluates the packet. Then, an accept message is sent back to Router A, the switches set up a virtual circuit.

FUNCTIONAL LAYERS OF ATM:

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Fig-4: Layers of ATM

Physical layer: It takes care medium dependent transmission. It is further divided into two sub layers.

-Physical medium dependent layer: It synchronizes transmission and reception by sending and receiving a continuous flow of bits associated with timing information and specifies the format used by the physical medium.

-Transmission Convergence layer: This sub-layer is responsible for cell delineation and maintaining ATM cell boundaries. It generates header error control and verifies them. It also generates and checks the header error control code to ensure valid data.

ATM LAYER:

-This layer establishes connection and passes the cells through the ATM network.

-It multiplexes and de-multiplexes cells of different connections.

-It translates VPI/VCI values at the switches and at cross connections.

-It extracts and inserts the header before or after the cell is delivered to the ATM adaptation layer.

-It maintains flow control using the GFC bits of the header.

ATM adaptation layer:

-This layer communicates with the higher layer protocols. The AAL is divided into two sublayers:

=>Convergence sublayer(CS): It offers different kinds of services to the different applications. The types of services are constant bit rate-bandwidth guarantee and is suitable for real time traffic, available bit rate(ABR) which is suitable for bursty traffic and unspecified bit rate(UBR). UBR is cheaper and is suitable for bursty traffic. It also provides special AAL services at an AAL network service access point.

=>Segmentation and Reassembly sublayer: This layer segments higher layer user data into 48 byte cells at the sender's node and reassembles them in the receiver's node. This sub layer is usually implemented with ATM switch. It tears down the message passed from the upper layer and converts them into fixed size cells. To make the cells multiples of 48, padding is used.