Asynchronous Transfer Mode (ATM) and QoS

1CSIT560 by M. Hamdi

Asynchronous Transfer Mode (ATM)

and QoS


ATM Era : Multiservice NetworksDeparture from Service Specialization

Multiservice Network

video

voice

bulkdata

interactivedata


Why ATM Did Not Make it the Way it was Initially Envisioned

Advantages of ATM

Disadvantages of ATM

Commercial Factors

• Single network optimized for everything (Data, phone, TV)

• Same technology for WAN, MAN, LAN (Seamless integration)

• QoS oriented and high-speed oriented

• Fast Hardware• Tremendous amount of research has been done

• Large overhead for packets

• QoS is a bit complicated from the applications point of view and network management point of view

• Not that great from web browsing (which is one killer application)

• Millions of networks already installed

• Lack of applications

• Expensive at the LAN (where it really matters)

• No strong business incentive for QoS (even up to now)

• Can achieve similar speed with an IP router as compared to an ATM switch (May be)


What is ATM?

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


ATM Cell Relay:The Underlying Technology

Cell Features Benefit

Small Low latency to support real-time services like audio and video (What is an appropriate size?)

Fixed Length Fast hardware switching and scalability

Standardized Usable in all networks (LAN and WAN)

Cells

Voice

Data

Video


Without Short Cells

Data packet(2000)

Voicepacket (50)

Router

Input

ports

Output

ports

A voice packet waits behind a large data packet


With Short Cells

Data #1(50)

Voice(50)

Data #40(50)

Router

Input

ports

Output

ports

Voice packet to be transmitted after Data #1

Data #2(50)

•Voice packet can go immediately after data packet #1

•Waiting for voice is reduced significantly


Virtual Paths & Virtual Channels

• Unique on a link-by-link basis• Virtual channels are contained within virtual paths• Interpreted at each switch to:

• determine output link• determine outgoing VPI/VCI

• Two-level structure:• allows “trunking” of virtual channels as one virtual path• virtual path can be switched• both used to route cells through network

PhysicalTransmission

LinkVCs VP

VCs VP

VCsVP

VCsVP

A Virtual Path (VP) describes the semi-permanent route between two end points. A Virtual Channel (VC) describes a cell transmission channel inside a virtual path


Connection Identifiers


ATM switch routing

ATMSwitch

ATMSwitch

ATMSwitch

ATMSwitch

ATMSwitch

VirtualPaths

VirtualCircuits


ATM Switches

• ATM switches translate VPI/VCI values

• VPI/VCI value unique only per interface—eg: locally significant and may be re-used elsewhere in network

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11

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6464

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VPI/VCIPort VPI/VCIPort

Input Output


ATM Switching

Connections (routes) set up by software• Routing (path through multiple-switch network)

and resource allocation is performed once per connection by switch control CPU

Cells are switched by hardware• Hardware (table lookup + switching fabric)

switches each incoming cell to appropriate output port

• Once a connection is established, cells are not touched by software


VP and VC Switch

• Two types of ATM switch• VP switch does not look at VCIs, switching is based

on VPIs only• VCI does not change when passing through a VP

switch; VPI may change• VC switch looks at both VPI and VCI• VCI (as well as VPI) may change when passing

through a VC switch


Routing with a VP Switch


A Conceptual View of a VP Switch


Routing with a VC Switch


A Conceptual View of a VC Switch


ATM Protocol Stack

Upper LayersUpper Layers

ATM Adaptation LayerATM Adaptation Layer

ATM LayerATM Layer

Physical LayerPhysical Layer


ATM Architecture

Presentation

Session

Network

Data Link

PhysicalTransmission-convergencephysical medium dependent

Transport

Application

ATM Layer

ATM Adaptation Layer

Upper Layer Protocols


Adaptation Layers: Service Classes


Service Classes and Capacity of Network


QUEUES and PRIORITY

Classifier

CBR TrafficCBR Traffic

VBR TrafficVBR Traffic

UBR TrafficUBR Traffic

ABR TrafficABR Traffic

Ou

tpu

t

Priority 1

Priority 2

Priority 3

Priority 4


ATM Adaptation Layer: Summary

ServiceCategories

Class

ATM ATM Adaptation LayerAdaptation Layer

(AAL)(AAL)

ATM LayerATM Layer

Physical LayerPhysical Layer

Bit Rate ConnectionMode

TimingConcern

ApplicationExamples

• Bandwidth andBandwidth andthroughput guaranteedthroughput guaranteed

• Good for voice Good for voice and videoand video

AAL1A CBR(Constant)

Connection-Oriented

Yes

• Best effort bandwidth Best effort bandwidth and throughputand throughput

• Good for live video,Good for live video,multimedia, multimedia, LAN-to-LANLAN-to-LAN

AAL2B VBR(Variable)VBR-RT

andVBR-NRT

Connection-Oriented

Yes

• Best effort withBest effort withcongestion feedbackcongestion feedback

• Reliable delivery Reliable delivery of bursty traffic ifof bursty traffic iflatency okaylatency okay

AAL5C ABR(Available)

Connection-Oriented

No

• No guaranteeNo guarantee• For SMDS/LANFor SMDS/LAN

AAL3/4D UBR(Un-

specified)

Connection-less

No


QUALITY OF SERVICE

– Max CDT, Mean CTD, CDV, CLR, CER, SECBR, CMR

CBR rt-VBR nrt-VBR ABR UBR

PCR Yes Yes Yes Yes Yes

SCR N/A Yes Yes N/A N/A

MBS N/A Yes Yes N/A N/A

MCR No No No Yes No

CDVT(PCR) Yes Yes Yes Yes Yes

CDVT(SCR) N/A Yes Yes N/A N/A


Application Requirements

BandwidthBandwidth

DelayDelay

ReliabilityReliability

Cost ($ or Admin)Cost ($ or Admin)

• Peak Cell Rate (PCR)• Sustained Cell Rate (SCR)• Minimum Cell Rate (MCR)

• Peak Cell Rate (PCR)• Sustained Cell Rate (SCR)• Minimum Cell Rate (MCR)

• Cell Transfer Delay (CTD)• Cell Delay Variation (CDV)• Cell Transfer Delay (CTD)• Cell Delay Variation (CDV)

• Cell Loss Ratio (CLR)• Cell Loss Ratio (CLR)

• Link Weighting• Link Weighting

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Asynchronous Transfer Mode (ATM) and QoS