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EE 4105 Communication Engg-IIDr. Mostafa Zaman Chowdhury

Slide # 6Introduction to Asynchronous Transfer Mode

(ATM)

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ATM Defined

Ø“ATM is a cell-switching and multiplexing technology that combines the benefits of circuit switching (guaranteed capacity and constant transmission delay) with those of packet switching (flexibility and efficiency for intermittent traffic)” –CISCO.

Ø It utilizes fixed length cells to carry the information

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Areas of ApplicationInfrastucture

BackbonesLAN

Ø LANs ATM is a high-performance, cell-oriented switching and multiplexing technology that utilizes fixed-length packets to carry different types of traffic. ATM is a technology that will enable carriers to capitalize on a number of revenue opportunities through multiple ATM classes of services; high-speed local-area network (LAN) interconnection; voice, video, and future multimedia applications in business markets in the short term; and in community and residential markets in the longer term.

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ATM Connections

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Circuit Switching and Packet Switching

Ø ATM is circuit switched because it establishes virtual circuits for communication

Ø At the same time, the virtual circuits are established over packet switched networks

Ø As such, it combines the benefits of circuit switched and packet switched technologies

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ATM Usage and BandwidthØ In theory, ATM can be deployed from small LANs to very

large WANsØ ATM deployments can operate at speeds starting in the

Mbps range scaling up to Gpbs rangev Speed wise, it is very scalable

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ATM Cell Basics

Ø ATM carries information based on fixed length cellsv Compare this to the other packet switching technologies such as

Frame Relay etc. where each packet may be of a different length

Ø The length of each cell is 53 Bytesv First 5 bytes are used as the cell headerv Next 48 bytes are used as the payload carrying the data

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Header Payload (Data)

5 Bytes 48 Bytes

Fixed Length Cell Advantage

ØDelay or latency is significantly reducedvATM is therefore suited for voice and video

transmissionØFixed length cells make it easier to switch data

across multiple networksvATM networks are built based on switches and not

routers

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ATM Devices

Ø ATM networks are built around two categories of devicesv ATM Switchv ATM end-point

Ø An ATM switch can be connected to either another ATM switch or and ATM end-point

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ATM End-PointsØ Will contain and ATM end-point adapterØ Examples of ATM end-points

v Workstationsv LAN switchesv Routersv Video Coder-Decoders (CODECs)

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ATM Switches and Interfaces

Ø ATM switch supports two types of interfacesv User-Network Interface (UNI)

− Connects an ATM end-point to a switchv Network-Network Interface (NNI)

− Connects two ATM switches

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Public and Private Interfaces

Ø UNI and NNI can further be divided to two typesØ One is known as the private type and the other is known as

the public typeØ Private UNI

v Connects an ATM end-point to a private ATM switch

Ø Public UNIv Connects an ATM end-point or a private ATM switch to a public

ATM switch

Ø Private NNIv Connects two ATM switches within the same private organization

Ø Public NNIv Connects two ATM switches within the same public organization

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ATM Standards for Digital Lines

Area of Application Speed

T1 25 Mbps

T3/E3 155 Mbps

OC-3 622 Mbps

OC-12 2.4 Gbps

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ATM Services

Ø There are basically three types of ATM services v Permanent Virtual Circuits (PVC)v Switched Virtual Circuits (SVC)v Connectionless Service

Ø SVC is the most widely used serviceØ ATM Virtual Connections

v Two types of connections existv One is known as the Virtual Path (VP)

− There can be a number of virtual paths along a physical connection and there can be a number of virtual circuits within a virtual path

v The other is known as the Virtual Circuit (VC)

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Virtual Circuits and Paths

Source: CISCO

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ISO-OSI and ATM Models

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ATM Quality of Service (QoS)

Ø ATM’s advantage over competing technologies is that it is able to offer a specified QoS

Ø For example, ATM providers can guarantee to their customers that the end-to-end latency will not exceed a specified level

Ø QoS Offered by ATMv Constant Bit Rate (CBR)v Variable Bit Rate (VBR)v Unspecified Bit Rate (UBR)v Available Bit Rate (ABR)

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Constant Bit Rate (CBR)

Ø Highest grade service (Grade A)Ø A constant bandwidth is available for the virtual channelØ Suitable for voice-over-IP (Transfer fixed rate

uncompressed video), and other traffic that requires steady, guaranteed throughput

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Variable Bit Rate (VBR)

Ø Second in the level of service (Grade B)Ø It is divided into to categories

v Variable Bit Rate Non-Real Time (VBR-NRT)− Provides bandwidth only as needed− Traffic that is not very sensitive to cell-delay or latency is handled by

this service− An example use would be to send multimedia e-mail

v Variable Bit Rate Real Time (VBR-RT) − Again, this is similar to VBR-RT in providing bandwidth as needed− But VBR-RT is designed for applications where cell-delay or latency

cannot be accommodated

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Available and Unspecified Bit Rate (ABR)

Ø Available Bit Rate (ABR)v Third in the level of service (Grade C)v Bandwidth is adjusted based on the amount of traffic in the

network v Suitable for data traffic such as file transfer and Email that are

basically not time sensitive

Ø Unspecified Bit Rate (UBR)v Fourth in the level of service (Grade D)v Dose not offer any fixed values for the traffic parameters

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