Asynchronous Tranfer Mode (ATM)

Outline

IntroductionDesign goalsATM protocol architectureATM logical connectionsATM CellsATM Application

Introduction

ATM : Switching techniques Can support data rates. Advantage is reliability and fidelity of modern facilities

provide faster packet switching.Was developed as part of the work on the broadband ISDNCombination of ATM with B-ISDN will allow high speed

interconnections of all the world’s network.

Design goals1. To optimize the use of the high data rate transmission

media,in particular optical fiber.2. It can interface with the existing system and to provide wide

area interconnectivity between them without lowering their effectiveness.

3. It can be implemented inexpensively.4. The new system must able to work and support the existing

telecommunication hierarchies.5. It must be connection oriented to ensure accurate and

predictable delivery .6. To move as many of the functions to hardware and eliminate

as many software functions as possible

continued… The information flow on each logical connection is organized

into fixed-size packets called cells.ATM is streamlined protocol with minimal error and flow control capabilities .It reduces the overhead of processing ATM cells and reduces the number of overhead bits to operate at high data rates.

Functions:1.It involves the transfer of data in discrete chunks.2.It allows multiple logical connections to be multiplexed over a single physical interface.

ATM PROTOCOL ARCHITECTURE

Continued…

Physical layer: Involves the specification of transmission medium and the

signal encoding scheme. Data rate specified in this layer 155.52Mbps and 622.08Mbps.

ATM layer: Provide packet transfer capabilities. Defines transmission of data in fixed size cells. Define the use of the logical connection.

Continued…

ATM Adaptation Layer(AAL): Service dependent. Maps higher layer information into ATM cells to be

transported over an ATM network . Collects the information from ATM cells for deliver to higher

layers.

It consists of three separate planes:

1.User plane

2.Control plane

3.Management plane

Continued…User plane :provides user information transfer along with

associated controls.Control plane:performs call control and connection control

function.Management plane:1.Plane management • it performs mangement functions related to a system.• Provides coordination between all the planes.2.Layer management • It performs management functions relating to resources and

parameters residing in its protocol entities.

ATM LOGICAL CONNECTION

Continued… ATM connections are identified by a virtual channel identifier

(VCI) and a virtual path identifier (VPI). VCI and VPI are combined to be used in a switch to route a

cell. The identity of a "physical" link is identified by two "logical"

links: virtual channel (VC) and virtual path (VP). When a connection is set up, the values of these identifiers

remain unchanged for the lifetime of the ATM connection.

FUNCTIONS

Virtual channel:

• Setup between two end users through the network.

• Leads to control signaling and user-network exchange, network-network exchange.

Virtual path:

• Helps to contain the control cost by grouping connections sharing common paths through the network into a single unit.

• Network management actions can be applied to a small number of groups of connections instead of a large number of individual connection.

Continued…

Advantages of virtual path:

1.Simplified network architecture

2.Increased network performance and reliability.

3.Reduced processing and short connection setup

4.Enhanced network services.

CALL ESTABLISHMENT

Continued…

A VCC can be established between end users, between end user and switch, or between two switches.

• Between end users: to carry user data/control signaling. VPC provides the users with an overall capacity. The set of VCCs should not exceed the VPC capacity.

• Between end user and switch: for user to network control information.

• Between two switches: for traffic management and routing functions.

Continued…Characteristics of virtual channel: 1.Quality of service: Quality of service is specified by cell loss

ratio and cell delay variation. 2.Switched and semi permanent virtual channel connections can

be established. 3.Cell sequence integrity4.Traffic parameter negotiation and usage monitoring.

The strategies can be: • Deny request for new VCCs (the best choice) • Discard cells • Terminate existing connections (worst)

ATM CELLS

The ATM cell has 53 bytes, containing 5 bytes of header and 48 bytes of information.

The advantages of this small and fixed cell size are:

• Cells can be switched more efficiently , leading to high-speed data transfer.

• Since the cell size is fixed, hardware implementation is easier.

• Queuing delay is reduced.

Therefore it allows support for multimedia services.

ATM cell: user–network interface.

ATM cell: network–network interface.

Continued…The UNI header consists of the following fields:

Generic flow control - used to provide local functions, such as identifying multiple stations that share a single ATM interface. The GFC field is typically not used and is set to a default value.

Virtual path identifier – used in conjunction with the VCI, to identify the next destination of a cell as it passes through a series of ATM switches

Virtual channel identifier - used, in conjunction with the VPI, to identify the next destination of a cell as it passes through a series of ATM switches on its way to its destination

Continued…Payload type - the first bit indicates whether the cell contains

user data or control data. If the cell contains user data, the second bit indicates congestion, and the third bit indicates whether the cell is the last in a series of cells that represent a single AAL5 frame.

Congestion loss priority - indicates whether the cell should be discarded if it encounters extreme congestion as it moves through the network.

Header error control - is a checksum contains error control information.

Header error control-flow chart

ATM ApplicationsATM is used for applications such as audio/video transmission in

real time it is necessary that at the receiving end, the cells are received without variable delay. the cells should be received at a constant speed. Example,

PCM-coded speech produces a continuous stream of bits at the rate of 64kbps. To transmit PCM over ATM, the PCM data stream has to be assembled into cells and transmitted over the network; at the receiver, the cells should be received with constant delay.

Continued…

To achieve this, special protocols are required.

ATM adaptation layer (AAL) has been defined to provide the necessary services to handle such applications.

The AAL provides the following services:

• Segmentation and reassembly

• Flow control and timing control

• Handling of transmission errors

• Handling of lost cells