Quality of Service in ATMFinal - Copy

8/8/2019 Quality of Service in ATMFinal - Copy

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By: Sunil Nakum,

M.E. CSE 2010 Batch

NITTTR,Chandigarh.

Quality Of Service In ATM

By: Sunil Nakum, NITTTR, Chandigarh1


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Topics to be covered


y What is QOS?

y Flow characteristics

y QOS User Related Attributes

y QOS Network Related Attributes

yATM Service Categories

y ATM Service Classes

y Cell Conformance Definitions

y Traffic Contract

y Traffic Control functions

y References


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What is Quality Of Service


y Industry wise set of standards and mechanisms for ensuring high

quality performance for critical applications.

y Traditionally concept of QOS meant all network traffic treated

equally. Result was poor or unacceptable performance.

y QOS concept of quality is Requirement of some applications &

users are more Critical than others. Some traffic needs preferential

treatment.


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Flow Characteristics


y Stream of packets from source to destination is called flow.

y Need of each flow can be characterized by four primary parameters:

1. Reliability: Lack of reliability means losing a packet or

acknowledgement which entails retransmission. E.g. Email , File

transfer and Internet access require reliable transmission than

telephony, audio and video.

2. Delay: Delay in data transfer from source to destination.

3. Jitter: Variation in delay for packets belonging to same flow.

4. Bandwidth: Different application needs different bandwidth.E.g. Email requires low bandwidth compared to video transfer.


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QOS User Related Attributes


y PCR(Peak Cell Rate) : Defines senders maximum cell rate.

y SCR(Sustained Cell Rate) :Defines average cell rate over a longperiod of time. Actual cell rate may be higher or lower than thisvalue but average should be equal to or less than SCR.

y MCR(Minimum Cell Rate) :Defines minimum cell rate

acceptable to sender. Network must guarantee MCR.y MBS(Maximum Burst Size):Number of cells an ATM source can

send at PCR.

y BT ( Burst Tolerance ) : BT is time interval after which an ATM

source can again send data at PCR.y MFS (Maximum Frame Size ): Maximum frame size of AAL

PDU for GFR Service Category.


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QOS Network Related Attributes


y CLR( Cell Loss Ratio) : Defines fraction of cells lost(or delivered

so late that they are considered as lost) during transmission. E.g. If

sender sends 100 cells and one of them is lost then CLR=1/100.

y CTD(Cell Transfer Delay) :Defines average time needed for a cell

to travel from source to destination. Maximum CTD and Minimum

CTD are also considered.

y CDV(Cell Delay Variation) :Defines difference between

maximum CTD and minimum CTD.

y CER(Cell Error Ratio) :Defines fraction of cells delivered in

error.

y SECBR: It is ratio of severely errored cell blocks to the total

transmitted cell blocks.


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ATM Service Categories - CBR


y Constant Bit Rate Category.

y Used by connections that request a static amount of bandwidth that

is continuously available during the connection lifetime.

y Bandwidth is characterized by PCR.

y

The source may emit cells at, or below the negotiated Peak CellRate (and may also even be silent), for periods of time.

y Intended to support real time applications requiring tightly

constrained delay variation (e.g. voice, video).

yAs delay, jitter and loss are critical CDV , CTD and CLR arespecified.


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ATM Service Categories VBR


y Variable Bit Rate.

y Intended for applications that have bursty traffic.

y Sources are expected to transmit at a rate that varies with time.

Equivalently the source can be described as bursty.

y

Bandwidth requirement is specified by SCR ,PCR and MBS.y Sensitive to Loss so CLR is Specified

y Real Time VBR requires strict bounds over CTD and CDV and

sensitive to delay and jitter. Eg. Voice and Video transfer

y Non Real time VBR No delay bounds are associated. Not sensitiveto delay and Jitter. Eg. Offline video transfer , Email, Offline

Transactions.


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ATM Service Categories- UBR


y Unspecified Bit Rate.

y Intended for applications having no requirement with regards to cell

delay, cell delay variation and cell loss.

y Does not have any negotiable parameter defined.

y

PCR is specified but have little significance .y If network is unable to carry offered load, load belonging to this

category is dropped.

y This category is intended to consume leftover capacity of network.

y E.g. Simple file transfer and email.


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ATM Service Categories -ABR


y Available Bit Rate.

y ABR category users closed loop feedback given by network.

y Feedback contains information about rate at which user can transmit

data.

y

Delivers cell at minimum rate so PCR and MCR are specified MCRmay be specified as zero.

y No strict bounds on cell delay, cell delay variation or cell loss.

y Bandwidth available from the network may vary, but shall not

become less than MCR.y Tries to utilize available bandwidth of network.


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ATM Service Categories- GFR


y Intended to support non-real-time applications.

y

It is designed for applications that may require a minimum rateguarantee and can benefit from accessing additional bandwidth

dynamically available in the network.

y The service guarantee is based on AAL-5 PDUs (frames) and, under

congestion conditions, the network attempts to discard completePDUs instead of discarding cells without reference to frame

boundaries.

y PCR,MCR,MBS,MFS is specified.

yThe user may always send cells at a rate up to PCR, but the networkonly commits to carry cells in complete frames at MCR. Traffic

beyond MCR will be delivered within the limits of available

resources.


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ATM Service Classes


y Class 1 : Specifies QOS parameters required for constant bit rate

voice and video.y Class 2 : Specifies QOS parameters required for variable bit rate

voice and video and other multimedia traffic.

y Class 3 : Specifies QOS parameters for interoperations of

connection oriented protocols such as Frame Relay.y Class 4: Specifies QOS Parameters for connection less services such

as IP and SMDS services.

y Unspecified QOS class : Do not specify any QOS parameters. No

explicit commitment by network. This QOS class is typically usedby UBR traffic.


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Cell Conformance Definitions


y Each cell can be declared as conforming or non-conforming

depending upon conformance definition.y For each service category conformance definition in defined.

y Two categories of cell flows are defined:

y CLPTransparent (CLP=0+1) : Cell flow in which CLP bit isignored by the network. Cells with CLP=0 and CLP=1 are given

identical treatment. Cell tagging option is not applicable.

y CLPsignificant(CLP=0) : Cell flow in which CLP bit is notignored. CLP objectives apply only to CLP=0.


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Cell Conformance Definitions


Conformance

Definitions

PCR

Flow

SCR

Flow

Tagging

Option Active

MCR CLR on

CBR.1 0+1 ns n/a ns 0+1

VBR.1 0+1 0+1 n/a ns 0+1

VBR.2 0+1 0 No ns 0

VBR.3 0+1 0 Yes ns 0

ABR 0 ns n/a Yes 0

GFR.1 0+1 ns No Yes 0

GFR.2 0+1 ns Yes Yes 0

UBR.1 0+1 ns No ns Unspecified

UBR.2 0+1 ns Yes ns Unspecified


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Traffic Contract


y Agreement between user and network across UNI (User Network

Interface) regarding:

y The QOS that a network is expected to provide.

y The ConnectionTraffic Descriptor, which includes,

y SourceTraffic Descriptor

y Cell DelayVariationTolerance (CDVT)

y Conformance Definition

y SourceTraffic Descriptor

y

Defines the characteristics of ATM traffic coming into thenetwork .

y Includes several negotiable traffic parameters: PCR, SCR, MBS,

and BurstTolerance (BT).


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Traffic Control Functions


y Connection Admission Control (CAC)

y Usage Parameter Control (UPC)

y Selective cell discarding

y Explicit Forward Congestion Indication (EFCI)

yFrame Discard

y ABR Flow Control

y RM cell


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Connection Admission Control


y Responsible for determining whether a connection request is admitted

or denied.

y Acts as preventive congestion control mechanism.

y Two basic categories:

y Non-statistical or peak bandwidth allocation : Allocates resources at

peak cell rate. Simple to implement but lead to underutilization ofnetwork.

y Statistical allocation: Provides better resource utilization by taking

into account the statistical nature of connections.

y Statistical gain: Measures advantage of using statistical algorithm overnon statistical algorithm.


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Connection Admission Control (Cont.)


y CBR:

y

CBR connection is characterized by its PCR only, so peak bandwidthallocation scheme is sufficient. No statistical gain to be achieved.

y VBR

y VBR connection is bursty in nature. Burstiness = SCR/PCR.

yIf burstiness tends towards 1 , peak bandwidth allocation issufficient.

y In extreme case burstiness


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Selective Cell Discard and EFCI


y Selective Cell discard

y A congested network may selectively discard cells which meeteither or both the following condition:

y Cells which have CLP = 1

y This is to protect the CLP = 0 flow as much as possible

y Explicit Forward congestion Indication (EFCI)

y A network element in an impending congested state or a

congested state may set an EFCI in the cell header.

y This indication may be examined by the destination end-system.

y The end-system may adaptively lower the cell rate of the

connection.


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Frame Discarding


y Number of techniques used to discard cells on frame basis.

y

Partial Packet Discard : Once a cell of frame is dropped allremaining cells of that frame are also drop excluding end-of-frame

cell. For AAL5, third bit PTI of an AAL5 PDU is 1 for last cell and

0 for remaining cells. This bit is used to identify start and end of

AAL5 PDU.

y Early Packet Discard : In PPD some dead cells are carried till

destination. To avoid this drawback EPD is used. Entire incoming

frame is discarded to prevent any possibility of buffer overflow. To

use EPD buffers are monitored and occupancy levels maintained

within threshold. Once occupancy level crosses given threshold, a

complete frame is discarded.


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Usage Parameter Control


y Mechanism to monitor traffic and enforce traffic contract is termed

as UPC.

y Detects and stops user traffic violations.

y Checks whether user is obeying traffic contract or not.

y

UPC Action for non conforming cells discarding or tag cell as lowpriority cell.

y UPC action for conforming cell is cell passing.

y GCRA Algorithm is used. For each cell arrival, GCRA determines

whether the cell conforms to the traffic contract of the connection.y Equivalent representations of the GCRA,

y Continuous-State Leaky Bucket Algorithm

y Virtual Scheduling algorithm


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Equivalent Versions of GCRA


ForF

irst Cell ofconnection TAT=ta(1)

For First Cell of

connection X=0,

LCT=ta(1)

ContinuousState Leaky Bucket Algorithm

X=Value ofleaky bucket counter

X=Auxiliary Variable

LCT=Last Conformance Time

Virtual Scheduling AlgorithmTAT=Theoretical Arrival Time

ta(k)=Time ofarrival for cell k

I=Increment

L=Limit


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GCRA for Various Services


Service GCRA(I,L)

CBR GCRA(1/PCR,CDVT)

VBR GCRA(1/PCR,CDVT)

GCRA(1/SCR,CDVT + BT)

UBR GCRA(1/PCR,CDVT)

GFR GCRA(1/PCR,CDVT)

Policing function also discard cells in access of

maximum frame size.


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ABR Flow Control


y Source controls its data injection rates according to loading

conditions of network .

y The current load of network is conveyed to source through

Resource Management Cells.

y ABR service doesnt provide any QOS guarantee, it provides

efficient means to access available bandwidth in network.y Two flow control Approaches are there:

1. Credit Based Flow Control

2. Rate Based Flow Control


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ABR Flow Control (Cont.)


y Credit Based Flow Control (Hop by HopFeedback):

y

Source periodically receives credit from next hop using which itsends data.

y For every cells sent sender decrements credit available. Once

credit is zero sender is prohibited to send more data.

y

Each node in path maintains separate queue for each VC.y It requires per link per VC monitoring so switch design is

complex.

y Rate Based Flow Control:

yEnd to end feedback flow control.

y One feedback loop from source to destination.

y Destination or any intermediate node sends back control

information towards source.


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ABR Flow Control Model


ATMforum adopted rate base flow control mechanism.

y Source generates RM cells called forward RM cell which is looped

backed by destination and sent back to source called backward RM

cell.

End System

(Source)

End System

(Destination)ATM Network

Forward RM Cell User Data cells

Backward RM Cell


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ABR Flow Control Model


y Intermediate node has three options:

y

Node can directly insert information in forward or backward RMcell.

y Node can set congestion bit of ATM cell header to convey

congestion notification. Destination in this case updates backward

RM cell according to received information.

y Node can directly generate a backward RM cell. In this case

communication no more remains end to end.


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RM Cell Information


y Minimum Cell Rate (MCR) and Current Cell Rate (CCR) of source.

y

NI (No Increase ) Bit : If NI=1 source cant increase its data rate. IfNI=0 source can increase its data rate by Rate Increase Factor (RIF)

negotiated during connection set up.

y CI (Congestion Indication) Bit: If CI=1 means there is congestion

somewhere between source and destination. Decrease data rate byRate Decrease Factor (RDF).

y ER (Explicit Rate) : Explicit Rate at which source is meant to transfer

data.

y

DIR Bit: If DIR=0 travelling from source to destination and If DIR=1looped back.

y BN (Backward Notification) Bit: If BN=0 source generated RM cell. If

BN=1 Intermediate node or destination generated RM cell.


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References


Books:

y

ATM Networks Concepts and Protocols By Sumit Kasera and PankajSethi, Tata McGraw-Hill

y ATM Networks Concepts, Protocols & Applications By Rainer,

Manfred N Huber, Stefan, Pearson Education

Websites :y http://en.wikipedia.org/wiki/Traffic_contract

y http://broadband-forum.org/ftp/pub/approved-specs/af-tm-

0121.000.pdf

y technet.microsoft.com/en-us/library/cc757120(WS.10).aspx


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Documents

Quality of Service in ATMFinal - Copy