2937.ATM Traffic and Service Parameterization

8/12/2019 2937.ATM Traffic and Service Parameterization

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Mapping ATM cells into STS-3c Frame


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Section overhead:A-bytesused for framing. Pattern present in A1 andA2 indicate the beginning of the frame

C1unique id of each STS-1

B1 bytefor section error monitoring Line overhead:

H-bytesused to indicate the offset the first byte ofthe beginning of the SPE

B2 bytefor line error monitoring K2Automatic Switching protection byte, used for

signaling between line level entities Growth bytefor performance monitoring


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Path overhead

J1 bytestate path trace field, to verify pathconnection

B3 bytefor error monitoring

C2 byteindicate mapping of the ATM withinthe payload field

G1 byteused to convey to the source someinformation to enable it to monitor the totalpath.


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ATM traffic & serviceparameterization


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Overview

2 types of parameters

Traffic parameters/descriptors

users perspective

provides quantitative measurements of the trafficcharacteristics

Service parameters/descriptors

network perspectiveDescribe the quality with which the network is

carrying user traffic


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ATM Traffic Parameters

Source traffic descriptor set of traffic descriptors belonging to the ATM source

capture intrinsic traffic characteristics of theconnection requested by source

It includes following parameters Peak Cell Rate (PCR)

Sustainable Cell Rate (SCR)

Maximum Burst size (MBS) Burst Tolerance (BT)

Maximum Cell Rate (MCR)

Maximum Frame Size (MFS)


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PCR

PCR is the maximum rate at which a user isallowed to inject data into the network.

It defines an upper bound on the traffic that can

be submitted by an ATM source. The inverse of PCR gives the minimum inter-

arrival time of cells for a given connection.

The maximum value of PCR is bounded by the

link rate.


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SCR

Sustainable Cell Rate is a measure of long-termaverage of user traffic.

SCR is an upper bound on the long-term

average of conforming cells for an ATMconnection.

An ATM source may, at times, send at a rategreater than SCR.

However, the average rate of the connectionmust remain less than SCR


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MBS

Maximum Burst Size is the amount of data thatan ATM source can send at its peak cell rate.

It is the number of cells an ATM source can send

at PCR. If MBS and PCR are known, then the maximum

burst duration is determined using the relation

TMBS = MBS/PCR, where TMBS is theduration for which a user can send data at thepeak cell rate.


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BT

Burst Tolerance is a measure of the intervalbetween consecutive bursts during which cellsare sent at PCR.

It is the time interval after which an ATM sourcecan again send data at PCR without violating thelong-term average of SCR.

The value of BT is given by the formula:

BT = (MBS-1)((1/SCR)(1/PCR))


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MCR

Minimum cell rate parameter is defined forlow-priority applications involving best-effort transfer.

It is the minimum cell rate that thenetwork must provide to a connection.

Its value can even be zero.


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MFS

Maximum Frame Size:

This parameter specifies the maximumsize of AAL PDU for the guaranteed FrameRate service category.


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ATM Service Parameters


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CLR

Cell Loss Ratio

It is the fraction of cells that are either notdelivered to the destination or deliveredafter pre-specified time.

Formula : - ((Cells lost)/Total cellstransmitted))

Approximate range : - 1* 10-7


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maxCTD

Maximum Cell Transfer Delay

Approximate Range : - 1-1000 msec


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P2P-CDV

Peak-to-Peak Cell Delay Variation

It is the difference between maxCTD andthe fixed CTD that could be experiencedby any delivered cell during the entireconnection holding time.

Approximate Range : - 1-130 msec


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CER

Cell Error Ratio

It is the ratio of the total number of cellsdelivered with error to the total number ofcells delivered.



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CMR

Cell Mis-insertion Ratio

It is the number of cells, meant for someother destination, inserted per second.

Approximate range : - 1cell/day


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SECBR

Severely Errored Cell Block Ratio

It is the ratio of severely errored cellblocks to the total transmitted cell blocks.


Table 7.2


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All 6 QoS parameters cover followingcategories of assessment:

Speed (CTD, CDV)

Accuracy (CER, CMR, SECBR)

Dependability (CLR)


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Factors effecting QoS Parameters

Propagation delay

Represents the delay in transporting bits overthe physical media.

Depends upon

Distance b/w Source and Destination

Speeds of the links

Affects only CTD


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Media Error Statistics

Represents the error characteristics introducedby the physical media.

Errors may be

Single-bit Error Burst Error

OFC is preferred as it is less susceptible toexternal interference and error seldom occur in

bursts. This factor affects all parameters, barring CTD

and CDV


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Switch Architecture

It has a significant impact on theperformance measures of ATM connection.

Most important design issues like bufferingand scheduling strategies affectparameters like CTD, CDV and CLR


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Buffer Capacity

Very large and small buffer sizes havetheir own disadvantages.

Very small buffer size leads to frequentbuffer overflows during load conditions.

Very large buffer leads to high delays.

This factor affects CTD, CDV, CLR andSECBR.


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Traffic Load

Fluctuating traffic load implies variableamount of buffering from source todestination.

High traffic load may cause bufferoverflows.

As the traffic load increases, CLR, CTD

and CDV value also increases.


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Number of intermediate nodes

This affects all of the QoS parameters.

The number of nodes affect delays in theform of additional processing at each ofthe intermediate node.

With the increase in number of nodes theloss probability due to buffer overflow at

an intermediate node also increases.


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Resource Allocation

Allocating buffer space on per connectionbasis.

Both bandwidth and buffer allocation haveimpact on both the CTD and CDV.


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Failures

Failures can be in the form of Linkbreakdown

Switch crashing

Port failure

Others

A malfunctioning switch also corrupts the

outgoing cells so affects CER and SECBR.


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Service Characteristics

ATM service offerings are classified intothe following categories:

CBR

rt-VBR

nrt-VBR Table 7.4

UBR ATM service categories

ABR

GFR


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CBR

Constant Bit Rate

CBR is used by applications that require aconstant bandwidth allocated to them

throughout the life of the connection. This service category places strict upper

bounds on cell delay variation.

Applications using CBR are voice, video etc. The bandwidth requirement of the CBR traffic

is characterized by PCR only.


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rt-VBR

Real-Time Variable Bit Rate

This category is intended for applications thathave bursty traffic and require strict bounds

over CTD and CDV. Bandwidth requirements are specified by SCR

and PCR.

Applications using VBR are compressedvideo, voice and other real-time applicationswith bursty traffic.


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nrt-VBR

Non Real-Time Variable Bit Rate

This service category is for applicationshaving bursty traffic, but which are not

sensitive to cell delay and delay variation. Bandwidth requirements is characterized by

PCR, SCR and MBS.

Typical applications are off-line transfer ofvideo, multimedia e-mail etc.


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UBR

Unspecified Bit Rate

It is intended for applications having norequirement, whatsoever, with regards to cell

delay, cell delay variation and cell loss. This service category is intended to consume

the leftover capacity of the network.

Example: simple file transfer and e-mail.


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ABR

Available Bit RateABR service category is used by applications

that can adapt to closed-loop feedback given

by the network. The feedback information contains

information about the rate at which the usercan transmit data.

The traffic is characterized by PCR and MCR. Example applications of ABR service category

include data transfer and email.


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GFR

Guaranteed Frame Rate

The GFR service category, like ABR, attemptsto dynamically utilize the residual bandwidth

in the network. However, unlike ABR, there isno complex flow control mechanism.

GFR service category is defined to carry databelonging to non-real time applications.


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QoS Classes

There are two categories of QoS Classes:

Specified QoS Class

Unspecified QoS Class


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Specified QoS Class

ATM forum has defined four specifiedclasses

Class 1: specifies the QoS parameters

typically required for circuit emulation andconstant bit rate voice and video.

Class 2: specifies the QoS parameters

typically required for variable bit rate voiceand video and other multimedia traffic.


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Class 3: specifies QoS parameters for interoperationof connection-oriented protocols, such as frameRelay.

Class 4: specifies QoS parameters for connectionless

services such as LAN emulation, IP or SMDS services.

Unspecified QoS Class: This Class doesnt specifyany of the QoS parameters. The service offered

by the network is just best effort. Usedtypically by the UBR traffic.

2937.ATM Traffic and Service Parameterization

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