asynhronous transfer mode

8/14/2019 Asynhronous Transfer Mode

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Asynchronous Transfer Mode(ATM)

Mathews VergisTEL 660

Winter trimester 2005 - 2006


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Topics to be discussed:

Need for network convergence

Introduction to ATM

ATM Interfaces and Service categories Basic ATM Concepts and Operation

ATM Cell Structure and Addressing

ATM Layers


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Popular misconceptions even among

Engineers

The phasor was invented by Captain Kirk of

Star Trek

Armature reaction is a chemical reaction

ATM stands for Automated Teller Machines


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Need for Network

Convergence PSTN sometimes used as a data network

backbone but since it is circuit switched

(voice optimized) not very WAN efficient

Delay sensitive traffic such as voice not

possible on data networks since there is no

guarantee of QoS


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Types of Traffic and demand on a

communication channelVoice

Its generation is asynchronous (a speaker may speakanytime)

Its transmission must be synchronous (once themessage starts, it must flow continuously as it isspoken)

The bandwidth required for a voice conversation indigital communication is relatively small and constant

(64K) The signals may contain a high degree of error and

the information can still be retrieved correctly


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communication channel

Video

The generation is synchronous (continuous)

Its transmission is synchronous. Thebandwidth required is variable and it couldrange from under 64 Kbps to several Mbps inthe same session.

Error control should be tight - otherwise thewrong information on the monitor may triggersevere wrongful actions


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communication channel

Data

Its generation could be either asynchronous (text) or

synchronous (telemetry)

Its transmission in general can be asynchronous (data

typically can wait patiently in buffers)

The information is extremely error-sensitive, so

extreme caution must be exercised in transmissionand error control must be very tight.


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How can we combine voice , data

and video on the same link?

Fixed and relatively short packets

Delays associated with each packet are going

to be short and fixed predictable

transmission

If Voice and Video can be given priority

handling then mixing is possible without any

diminishing in quality


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Introduction

ATM Asynchronous Transfer Mode

It is a high speed, connection oriented

switching and multiplexing technology

capable of transmitting voice, video and data

and interconnecting LANs

ATM is asynchronous because information

streams can be sent independently without the

need of a common clock


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History of ATM

Developed in the early 80s as a switching technologyfor Broadband Integrated Services Digital Network

Anchorage Accord in 1996 declares availability of

specs required to implement a multi-service ATMnetwork


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Market Segments of ATM


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Features and Benefits of ATM

Convergence of Voice , Video and Data onone network

High speed switching at hardware level

Bandwidth on demand

Predefined and guaranteed QoS and CoS

Superior Management features

Scalability in network size and speed

Ease of integration with other technologies


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ATM Applications


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ATM Fast Packet Standards and

Services

Handles traffic through fast

packet switching

technique

Must be able to handle both

circuit and packet switching

Must also be able to

accommodate the different

bit rates variable (packet

switching) andconstant

(circuit switching)

Uses Cell relay technology


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Important terms relevant to ATM

Quality of Service (QoS) :A broadly used term that refers tothe performance attributes of an end-to-end connection. A QoSdefinition for data would address attributes such as error rates,lost packet rates, throughput, and delay

Class of Service (CoS) :It is a way of managing traffic in anetwork by grouping similar types of traffic together andtreating each type as a class with its own level of service

priority

Fast Packet Switching :A packet switching technique thatincreases the throughput by eliminating overhead. Overhead

reduction is accomplished by allocating flow control and errorcorrection functions to either the user applications or thenetwork nodes that interface with the user. Cell relay is animplementation of this.


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Cells and Cell relay

A Cell is a formatted packet that uses a fixed lengthdata unit

Cell relay is the process of moving these cellsthrough switching elements

Fixed size cells can be switched at a very high speedand add predictability to data transmissions

Variable length frames produce unpredictablepatterns and performances as the buffer time cannot

be determined Cell tax overhead imposed by ATM cells which can

cut into amount of data that can be transferred


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ATM Interfaces

ATM is a connectionoriented technique designedto transport both connectionand connection-less services

Operations at the boundaryof a network are connectionoriented

Within the network theoperation is connectionless

User to Network Interface (UNI) :Connection existing between theuser equipment and ATMequipment.

Network to Network Interface(NNI) : Connection via which

traffic travels between ATMdevices in the same network.

Intercarrier Interface (ICI) : Usedto send traffic across intermediatenetworks

Data Exchange Interface (DXI) :

Used to transmit packets ratherthan cells to the ATM interfacewhen non ATM equipment isused


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ATM Service Categories

Allow for traffic to be buffered and queued for later

transmission

Can permit loose timing and asynchronous operations between

sender and receiver


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Fundamental ATM Operations

Concept

A virtual or logical connection is established

ATM forms a packet of fixed length 53 octets ( 5octet header and 48 octet information field )

Cells are placed in a queue, on reaching ATM switch Cells are then multiplexed asynchronously with other

cells for transmission

Switch adapts the incoming bit rate to match the

transmit channel bit rate Switch inserts dummy cells to meet the aggregate bit

stream rate of 155.52 Mbps


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ATM Virtual paths and

ChannelsATM Virtual Circuit Terminology

Virtual Channel (VC) provides afixed pathway or route between 2points. Setup across an ATMnetwork whenever data transferbegins.

Virtual Path (VP) groups of VCsused to tell a switch how to forwardan ATM cell through an ATMnetwork

Virtual Path Identifier (VPI) inthe ATM header used to identifyroute established in the ATM Switch

Virtual Channel Identifier (VCI) -in the ATM header used to identify achannel within a VP


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ATM Virtual paths (contd)

Transmission path physicalmedia transporting the cells

Virtual Channel Connection(VCC) connection from sourceend user VCI to destination userVCI

Virtual path Connection (VPC) connection from source end userVPI to the destination end userVPI

Operation : Cell is receivedacross a link on a known VPI or

VCI value - their values areremapped as necessary as allVCIs and VPIs are onlysignificant to the local link


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ATM VCI and VPI Swapping

Logical ID Swapping : This is the technique by which the Logical IDof one link is changed to another one as the cell passes through it. ATMswitch changes the cell header VPI/VCI fields to reflect a new VPI andVCI for the outgoing cell. This can be done in one of 2 ways : VP is

predefined in the switch or VP is set up dynamically when cell reachesthe switch.

Forwarding process depends on 2 lookup tables within the switch: VP table record of VPs on each link

VC table maintain the output VP and VC to send the cell Connection Admission Control (CAC) - procedure used to decide if a

request for an ATM connection can be accepted based on the attributesof both the requested connection and the existing connections


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VC and VP Swapping


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ATM Cell Structure


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ATM Cell Structure


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ATM Addressing

Uses addressing similar tonumerical addressing for telephonenumbers

Uses E.164 addresses for publicATM (B- ISDN) networks

Extended ATM addressing to

include private networks Overlaymodel ATM layer maps networklayer (IP) addresses to ATMaddresses. Address format uses OSInetwork service access point(NSAP) addresses

NSAP address providing thelogical point between the networkand transport layers of the OSImodel the location of this point isidentified by network serviceprovider


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ATM Address formats

Currently 4 formats used for ATM networks

The fields are divided into 2 sections: network and user

Network prefixes : fields as needed by the network side of the

UNI User prefixes : Fields as needed by the user side of the UNI


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ATM address formats (contd)


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ATM Routing Domains and Areas

ATM address 20 byte string that has the following fields:

Country code

Administrative authority

Routing domain Area identifier

End system identifier

NSAP

International code

ISDN telephone number


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Authority and Format Identifier (AFI): This identifier ispart of the network level address header. Value for 1 st AFI fieldcan be : DCC (hex 39),E.164 (hex 45), ICD (hex 47)

Routing domains : used for traffic management and

allocating bandwidth capacity. Defined in the lookup tables inthe switch.

End System Identifier (ESI) : identifies an end system( computer or LAN) within an area

Selector Field not used by the ATM network

Purpose of the ATM address format is identify ATM devicesin an ATM network


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ATM Layers

ATM architecture uses a logical reference modelto describe its functions

ATM functions correspond to physical layer and

part of the Data Link layer of the OSI model On its own ATM has function at layers 1and 2 of

the OSI model, but today TCP/IP is routed overATM networks which means it can also function

at layers 3 and 4 of the OSI model.


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ATM reference model

The ATM reference model constructed as 3 planes

which span all the layers : Control generates and manages the signaling requests User manages the transfer of data

Management contains 2 parts: Layer management : manages layer specific functions

Plane management : manages and co ordinates functions related to

the whole system


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ATM and OSI model


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ATM Adaptation layer

Has 2 sublayers:

Convergence Sublayer ( CS )

Determines the Class of service (CoS) for the incoming traffic

Provides a specific AAL service at an AAL network serviceaccess point (NSAP)

Segmentation and Reassembly Sublayer (SAR)

Segments higher level user data into 48 byte cells plus

necessary overhead at the sending node and reassembles cells

at the receiving node


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AAL Types and Class of Service

(CoS)

Depending on data type, the AAL protocol provides 5 AAL

types to accommodate a particular service class

AAL 5 is the most popular AAL type

For IP, LAN frames , signaling messages, frame relay , video


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ATM Layer

Performs the framing , multiplexing /demultiplexing of cells and also does theswitching

Generates cell headers on transmitting node based on information from higher layers

Generic flow control

VCI /VPI translation Extracts cell headers on a receiving node and

passes cell payload to higher layers


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Physical Layer

Transports ATM cells on a communications channeland defines mechanical specifications ( connectorsetc.)

2 sublayers: Transmission Convergence sublayer

Maps cells into the physical layer frame format on transmit anddelineates ATM cells in the received bit stream

Generates HEC on transmit

Generates idle cells for cell rate decoupling or speed matching Physical medium sublayer

Medium dependent function like bit transfer, bit alignment


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References:

http://www.techfest.com/networking/atm/atm.htm

http://www.dit.upm.es/snh/arhelp/glossaries/atmf/gloss-a.html

http://www.rhyshaden.com/atm.htm

http://www.cisco.com/univercd/cc/td/doc/cisintwk/ito_doc/atm.htm

Trivedi, Carol, Wide Area Networks; EMCParadigm 2004

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