ByKauleshwarnath Pandey

Maravar Kannan

ATM

Asynchronous Transfer Mode

• ATM is the cell relay protocol designed by ATM Forum and adopted by ITU-T.

• The combination of ATM & B-ISDN will allow high-speed interconnection of all the world’s networks.

• In fact, ATM can be thought of as the “highway” of the information superhighway.

• ATM operates at a speed of 155 & 622 Mbps & is a connection oriented network

Design Goals• Need for a transmission system to optimize the use

of high-data-rate transmission media, in particular optical fiber.

• A system that can interface with the existing systems, such as various packet networks, & to provide wide area interconnectivity between them without lowering their effectiveness or requiring their replacement.

• design that can be implemented inexpensively• able to work with & support the existing

telecommunications hierarchies• must be connection oriented to ensure accurate &

predictable delivery.• to move as many of the functions to hardware as

possible and eliminate as many software functions as possible

Packet Networks• Data communications today are

based on packet switching & packet networks. A packet is a combination of data & overhead bits that can be passed through the network as a self contained unit. The overhead bits, in the form of a header & trailer, act as an envelope that provides identification & addressing information as well as the data required for routing, flow control, error control, …

ATM LANs Issues to be resolved• Connectionless Vs Connection-

oriented• Physical Addresses Vs Virtual

Connection Identifier• Multicasting & Broadcasting

delivery• varying size & intricacy• To improve utilization

Cell Networks• Many of the packet internetworking problems are

solved by adopting a concept called cell networking.

• A cell is a small data unit of fixed size. In a cell network, all data are loaded into identical cells that can be transmitted with complete predictability & uniformity.

• As packets of different sizes & formats reach the cell network from a tributory network, they are split into multiple small data units of equal length & loaded into cells.

• Because of the same size, the problems associated with multiplexing different sized packets are avoided.

Advantages of Cells• High speed of the link coupled with

the small size of the cells• A cell network can handle real time

data• To a cell network, the smallest unit

is a cell, not a bit. • Switching & multiplexing can be

implemented in hardware rather than software

Asynchronous TDM• ATM uses asynchronous TDM (that’s why

called as ATM) to multiplex cells coming from different channels. It uses fixed size slots.

• ATM mux fill a slot with a cell from any input channel that has a cell, the slot is empty if none of the channels has a cell to send.

• At the first tick of the clock, the multiplexer fills the slot with a cell from each channel. When all the cells from all the channels are multiplexed. The output slots are empty.

ATM Switching:Contentionless Time Division

• Advantages

• Non-blocking

• Deterministic performance—probability of cell loss = 0

• Flexible port speeds – (DS-1, E-1, DS-3, E-3, 100M, 155M, 622M)

• Hardware multicast without increasing fabric cell traffic

• Low transit delays

• Disadvantages

• Limited scalability on single TDM fabric – Use time-space-time to expand

ATM Architecture ATM is a cell

switched network. The user access

devices, called the end points, are connected through a user-to-network interface (UNI) to the switches inside the network.

The switches are connected through network-to-network interfaces.

ATM Protocol Stack

Upper Layers

ATM Adaptation Layer

ATM Layer

Physical Layer

The ATM Layer

header payload

Fixed length packet = cell

ATM Layers in end-point devices & switchesThe end points use all three layers while theswitches use only the two bottom layers.

ATM Process

Traffic Type

TransmissionUnit

Switching

Connection Type

Delivery

Access

Rate & Media

Data

VariablePacket

Packet

Connectionless

Best Effort

Shared

Protocol Dependent

Voice

FixedFrame

Circuit

Connection-oriented

Guaranteed

Dedicated

Channel Dependent

Data, Voice, Video

FixedCell

Cell

Connection-oriented

Defined Classes

Dedicated

Application Dependent

ATM ConventionalLAN

ConventionalTelecom

ATM Process

Traffic Type

TransmissionUnit

Switching

Connection Type

Delivery

Access

Rate & Media

Data

VariablePacket

Packet

Connectionless

Best Effort

Shared

Protocol Dependent

Voice

FixedFrame

Circuit

Connection-oriented

Guaranteed

Dedicated

Channel Dependent

Data, Voice, Video

FixedCell

Cell

Connection-oriented

Defined Classes

Dedicated

Application Dependent

ATM ConventionalLAN

ConventionalTelecom

ATM is packet switching!

• Switched or permanent connections

• Traffic type independent (voice, data, interactive video)

• Fixed length packet - 53 bytes (cell)

header payload

Fixed length packet = cell

Anatomy of an ATM Cell

VPI: Virtual Path Identifier CLP: Cell Loss PriorityVCI: Virtual Channel Identifier HEC: Header Error CheckPTI: Payload Type Indicator GFC: Generic Flow Control

GFC (UNI) or VPI (NNI) VPI VPI VCI

VCI VCI PTI CLP

HEC

8 7 6 5 4 3 2 1Byte 1Byte 2Byte 3Byte 4Byte 5

Header

Payload48 Bytes

Thank You