Introduction to Asynchronous Transfer Mode (ATM)

© N. Ganesan, Ph.D.(Copyrights of sections as

acknowledged.)

Acknowledgment

• This presentation is based on ATM related information provided on the CISCO website, the IEC website and the ATM Forum. – Asynchronous Transfer Mode (ATM) S

witching

– ATM Fundamentals – ATM Presentations (Good set of

PowerPoint presentations).

Chapter Objectives

Chapter Modules

ATM Defined

• “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.

• It utilizes fixed length cells to carry the information

Areas of Application

Infrastucture BackbonesLAN

• LANsAsynchronous transfer mode (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.

• Infrastructure – Backbones– LAN

• Application

ATM Connections

Circuit Switching and Packet Switching

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

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

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

ATM Usage and Bandwidth

• In theory, ATM can be deployed from small LANs to very large WANs– At present, it is used mostly on

backbones, but this may change in the future with declining prices for ATM equipment

• ATM deployments can operate at speeds starting in the Mbps range scaling up to Gpbs range– Speed wise, it is very scalable

ATM’s Efficiency

• It is an asynchronous technology and it uses the links based on the need for information to be transmitted

• ATM is based on fixed length cells and the cells are small compared to many other forms of transmission such as frame relay etc.

ATM Cell Basics

• ATM carries information based on fixed length cells– Compare this to the other packet switching

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

• The length of each cell is 53 Bytes– First 5 bytes are used as the cell header– Next 48 bytes are used as the payload

carrying the data

ATM Cell Format

Header Payload (Data)

5 Bytes 48 Bytes

Fixed Length Cell Advantage

• Delay or latency is significantly reduced– ATM is therefore suited for voice and video

transmission• Fixed length cells make it easier to

switch data across multiple networks– ATM networks are built based on switches

and not routers• Fixed length cell is similar to container

based road transportation– Some parallels can be drawn with respect to

the advantage of fixed length transportation based on the benefits of container transportation

ATM Cell Header Format

ATM Cell Header—UNI Format

ATM Cell Header—NNI Format

ATM Devices

• ATM networks are built around two categories of devices– ATM Switch– ATM end-point

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

ATM End-Points

• Will contain and ATM end-point adapter

• Examples of ATM end-points– Workstations– LAN switches– Routers– DSU/CSU Units– Video Coder-Decoders (CODECs)

Devices on Which ATM Has Been Implemented

• PC, workstation, and server network interface cards

• switched-Ethernet and token-ring workgroup hubs

• workgroup and campus ATM switches • ATM enterprise network switches • ATM multiplexers • ATM–edge switches • ATM–backbone switches

Source: http://www.iec.org/online/tutorials/atm_fund/topic01.html?Next.x=33&Next.y=17

ATM Network Components

Source: CISCO

ATM Switches and Interfaces

• ATM switch supports two types of interfaces– User-Network Interface (UNI)

• Connects an ATM end-point to a switch– Network-Network Interface (NNI)

• Connects two ATM switches

Public and Private Interfaces

• UNI and NNI can further be divided to two types

• One is known as the private type and the other is known as the public type

Private and Public UNI

• Private UNI– Connects an ATM end-point to a

private ATM switch• Public UNI

– Connects an ATM end-point or a private ATM switch to a public ATM switch

Private and Public NNI

• Private NNI– Connects two ATM switches within the

same private organization• Public NNI

– Connects two ATM switches within the same public organization

A third Type of Interface

• Known as Broadband Inter-Carrier Interface (BICI)– Connects two public switches from

different service providers

Public and Private Interfaces

Source: CISCO

ATM Standards

• UNI 2.0, 3.0, 4.0• P-NNI• LAN Emulation (LANE)

– For the use of ATM over Ethernet, Token ring etc.

• Multi-protocol Over ATM

ATM Standards for Digital Lines

Area of Application

Speed Standard

T1 25 Mbps YesT3/E3 155

MbpsYes

OC-3 622 Mbps

Y

OC-12 2.4 Gbps YCheck the ATM Forum for approved standards for further information.

ATM Services

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

• Note: SVC is the most widely used service

ATM Virtual Connections

• Two types of connections exist• One is known as the Virtual Path

(VP)• The other is known as the Virtual

Circuit (VC)

Virtual Connection Composition

• 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

Virtual Circuits and Paths

Source: CISCO

Transmission Path (Physical Media)

• Physical medium standards to carry ATM include the following:– Synchronous Digital Hierarchy/Synchronous

Optical Network (SDH/SONET)– DS3/E3– Over Multi-Mode Fiber (MMF) at 155 Mbps– Over Shielded Twisted Pair (STP) at 155

Mbps

ISO-OSI and ATM Models

LAN Emulation (LANE)

• Defined by the ATM forum to emulate popular LAN protocols such as Ethernet and Token Ring

• In other words, “the LANE protocol makes an ATM network look and behave like either the Ethernet or the Token Ring network” – CISCO

• An emulated LAN is known as ELAN

Multi-Protocol Over ATM (MPOA)

• Enables the transmission of data between ELANs without having to continuously go through certain routers

• After the initial flow of data through routers known as Multi-Protocol Servers (MPS), it would be directed along the path of ATM switches (See next slide and CISCO documentation)

Data Flow Between ELANs with and without MPOA

Without MPOA

With MPOA

ATM Quality of Service (QoS)

• ATM’s advantage over competing technologies is that it is able to offer a specified QOS

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

QoS Offered by ATM

• Constant Bit Rate (CBR)• Variable Bit Rate (VBR)• Unspecified Bit Rate (UBR)• Available Bit Rate (ABR)

Constant Bit Rate (CBR)

• Highest grade service (Grade A)• A constant bandwidth is available

for the virtual channel• Suitable for voice-over-IP (Transfer

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

Variable Bit Rate (VBR)

• Second in the level of service (Grade B)

• It is divide into to categories– Variable Bit Rate Non-Real Time (VBR-

NRT)– Variable Bit Rate Real Time (VBR-RT)

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

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

Sample Applications for VBR-RT

• Voice with speech activity detection– Telephony

• Compressed interactive video– Video conferencing

Available Bit Rate (ABR)

• Third in the level of service (Grade C)

• Bandwidth is adjusted based on the amount of traffic in the network

• Suitable for data traffic such as file transfer and Email that are basically not time sensitive

Unspecified Bit Rate (UBR)

• Fourth in the level of service (Grade D)

• Dose not offer any fixed values for the traffic parameters

QoS Enforcement

• Traffic contract• Traffic shaping• Traffic policing• (See CISCO document for more

details)

Competing Technologies

• Frame Relay competes at the backbone– Eventually, ATM will be the choice for

backbone technology• Gigabit Ethernet

– ATM’s competition on the desktop area and LANs will be challenged effectively by high speed and proven Ethernet technologies

End of Module

Practical ATM Networks

© N. Ganesan, Ph.D.

Chapter Objectives

ATM Interfaces

ATM and other technologies.

ATM

Source: CISCO

ATM Network Building Block

• The main building block of ATM networks is the ATM switch– Workgroup switch– Campus switch– Enterprise switch– Multi-service access switch

ATM Network Hierarchy

Source: CISCO

Work Group Switch

• These switches will have Ethernet and ATM ports

• Used for connecting Ethernet networks to ATM switches

• Note: At the desktop level Ethernet still remains as the leading network technology

Campus Switch

• Used for designing small-scale ATM backbones

• Can be used for connecting various campus backbone technologies – FDDI– Gigabit Ethernet etc.

Move from Traditional to New Backbones

Enterprise ATM Switch

• Used in building large enterprise backbones

• Can be used for integrating different services and technologies on the backbone

Multi-Service Access Switch

• A common ATM infrastructure can be used in a public network to support disparate services– Frame relay– LAN interconnection– Public ATM services

• Their use usually extends beyond a private network to a public network

• Used by service providers to connect MANs and WANs for example

ATM Interface Types

• User-to-Network Interface (UNI)• Network-to-Network Interface (NNI)

Interface Types in a Network

Source: CISCO

ATM Interfaces

• LAN interfaces• Backbone interfaces

ATM to LAN Interface

Source: http://www.cisco.com/univercd/cc/td/doc/cisintwk/idg4/nd2008.htm#28807

LAN Emulation Issues

• Emulate an Ethernet LANs• Map traditional LAN addresses to

ATM addresses

ATM to WAN Interface

Source: CISCODXI – Data Exchange InterfaceHSSI – High Speed Serial InterfaceDSU – Data Service Unit

Load Balancing with ATM Switches

Source: CISCO

Summary

• ATM networks are basically built around ATM switches

• ATM interfaces can be used for integrating disparate WAN technologies

• At the desktop LAN, ATM can be used for emulating the traditional Ethernet environment for example– But, in general, the Ethernet technology is very popular

in terms of ease of use and speed– Therefore, ATM at the LAN front is likely to be used for

connecting Ethernet LANs to the backbone if the backbone is based on ATM

• For the backbone and large public backbones, ATM is bound to become the technology of choice

References

• At CISCO– http://www.cisco.com/univercd/cc/td/

doc/cisintwk/idg4/nd2008.htm

– http://www.cisco.com/univercd/cc/td/doc/cisintwk/idg4/nd2012.htm

• ATM Forum – http://www.atmforum.org/