presentation atm 3

8/9/2019 Presentation ATM 3

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Why Traffic Management? The ATM technology is intended to support a wide variety of services and

applications such as voice, video, and data

ATM promises to support all these different requirements with a commonnetwork

Within such a network all connections may impact on each other

ATM must manage traffic fairly and provide effective allocation of networkresources for these different applications

It is the task of ATM traffic control to:

protect the network and the end-systems from congestion in order toprovide specified and guaranteed levels of Quality of Service (QoS)

use available network resources efficiently

Traffic Management functions to prevent and control congestion acrossATM networks and provide the QoS required.


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Traffic management:- It is act of managing N/w traffic, providing serviceguarantees to user connecti

on and ensuring optimal utilization of N/W resources

Three basic Concepts:-

1. Concept of Traffic2. Concept of service

3. Optimal Utilization

1 Concept of Traffic:- The data packets exchange between communicating entities define atraffic flow. Two important traffic aspects are:-

a) Short term measure

i) Maximum allowable data rate (MADR):- This is a maximum data rate that a user isallowed at any instant of time.

ii) Burst Duration (BT):- It is time duration for which a user is allowed to send data atmaximum allowable rate.

b) Long term measure

i) Average Data Rate (ADR):- It is average data rate for the duration of data transfer.

2. Concept of Service:- It specify how offer load is serviced by N/W. Some serviceparameters are:-

i) Delay is the measure of time taken by data unit to reach one boundary from another.

ii) Delay variation (Jitter) is the variation in delay over time.

iii) Data loss ratio is ratio of total data lost to total data transmitted.

iv) Error ratio is the ratio of total erroneous data.

v) Good put is the ratio of useful data received and actual amount of data injected intoN/W


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3. Optimal Utilization of N/w Resources:-

It means element of traffic management.

Elements of traffic Management:-

1. Traffic Contracting

2. Traffic Shaping

3. Traffic policing

4. Priority Control5. Flow Control

6. Congestion Control

1. Traffic Contracting:-

Agreement between user and network across UNI regarding.It provides means to allocate N/W resources to user connection and manage N/W resources

efficiently. OR

It decide whether new connection request can be accepted or not.


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A traffic contract is defined by two set of parameters:-

1. Connection Traffic Descriptor

2. QOS Parameters or Service descriptor

1.The Connection Traffic Descriptor, which includes

a. Source Traffic Descriptor

b. Cell Delay Variation Tolerance (CDVT)

c. Conformance Definition

a. Source Traffic Descriptor

Defines the characteristics ofATM traffic coming into the network

Includes several negotiable traffic parameters: PCR, SCR, MBS, and Burst Tolerance (BT)

b. Cell Delay Variation Tolerance (CDVT)

It is the measure ofhow much cell clumping is acceptable resulting from network operations

such as cell multiplexing or the insertion of OAM cells

c. Conformance Definition

Defines what cell rates and streams will be monitored

Distinguish between a conforming and non conforming cell of a connection.

A separate traffic contract for each Virtual Path Connection (VPC) or Virtual Channel

Connection (VCC)


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Source traffic Descriptor and CDVT are explained below:-

1. Peak Cell Rate (PCR)

The maximum instantaneous rate at which the user will transmit. It is the inverse of the minimuminter cell interval.

2. Sustainable Cell Rate (SCR)

Average rate that the user can send cells to the network i.e., an average throughput.

3. Maximum Burst Size (MBS)

The maximum back-to-back cells that can be sent at the peak cell rate without violating the

sustained cell rate.

4. Burst Tolerance ( BT )The maximum burst size that can be sent at the peak rate.

5. Minimum Cell Rate (MCR)

The is the minimum cell rate that N/W must provide to a connection. Its value can be zero.


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2. QOS Parameters or Service descriptor:-

i) Maximum Cell Transfer Delay (Max CTD )

The maximum length of time allowed for N/W to transmit a cell from source to destination.

ii)Peak to Peak Cell Delay Variation (P2PCDV )

The difference of the maximum and minimum CTD experienced during the connection. Peak-to-

peak CDV and Instantaneous CDV are used.

iii)Cell Loss Ratio ( CLR )

The percentage of cells that are lost in the network due to error or congestion and are not received

by the destination

iv)Cell Error Ratio ( CER )

Ratio of total number of cells received with errors to the total number of cells delivered.

CER = Errored Cells______________

Successfully Transferred Cells + Errored Cells

v) Severely Errored Cell Block Ratio ( SECBR )SECBR = Severely Errored Cell Blocks

Total Transmitted Cell Blocks

vi)Cell Misinsertion Ratio ( CMR )

CMR = Misinserted Cells

Time Interval


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Cell Transfer Delay Probability Density Model


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Mechanism of Contract Management:-

Connection Admission Control (CAC)

Responsible for determining whether a connection request is admitted or denied

For each connection request, CAC obtain the following information from the traffic

contract

Values of parameters in the source traffic descriptor

The requested and acceptable values of each QoS parameter

The value of the CDVT

The requested conformance definition

Based on that information and the networks definition of a compliant connection

to determine

Whether the connection can be accepted or not

Allocation of network resource


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CONNECTION ADMISSION CONTROL (CAC)


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2. Traffic Shaping:-

A mechanism that alters the traffic characteristics of a cell stream on a connection to achieve

better network efficiency or to ensure conformance to the traffic parameters in the traffic contract

Traffic shaping examples:

Peak cell rate reduction

Burst length limiting

Spacing cells in time to reduce CDV

Cell scheduling policy

Mechanism of Traffic Shaping:- Traffic shaping is implemented using leaky bucketalgorithm. In this scheme, bursty data generated by source is stored in buffer and sent out at

lower rate, thereby transforming bursty traffic withhigh PCR to more stable traffic with alower PCR. This mechanism is referred to as peak cell rate mechanism.

PCRPCR

Da

ta

R

at

e

Da

ta

Rat

e

Time Time

Before Traffic Shaping AfterTraffic Shaping


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3. Traffic Policing- Set of action taken by N/W to monitor and ensure that traffic contract is respected in terms of

traffic offered, at user access and N/W access. The mechanism to monitor the traffic and enforce

the traffic contract is termed as Usage parameter Control (UPC).

What is UPC?

Commonly known as Traffic Policing

A network traffic control mechanism

Required at the public UNI

Detects and stops user traffic violations

Ensures QoS for other connections

UPC Functions? Monitors cells submitted at the UNI

Checks for connection compliance

Is the user sending data too quickly?

Is the user obeying the traffic contract?

Checks validity of VPI/VCI values

Is the user using the correct VPI/VCI?

UPC Action:-

1. For non-conforming cells:-

Tagging or Marking:- It refers to changing the priority of packets from high to low. Tagging

is first measure that may be adopted further packets belonging to non-conforming

connection. In case of overloading tagged packet are first to be discarded.

Packet Discarding:- It refers to discarding packets belonging to non-conforming


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. 2. For conforming cells:-

Transparently pass or

Traffic shape

Generic Cell Rate Algorithm:- Several algorithm are defined to monitorATMTraffic and to

enforce traffic contract . The most commonly used algorithm is GCRA. Which is

Used to define conformance with respect to the traffic contract

For each cell arrival, GCRA determines whether the cell conforms to the traffic contract of the

connection

The UPC function may implement GCRA to enforce conformance

GCRA defined two parameters:-

1. Increment (I) which specifies increment value when next cell is expected.

2. Limit(L) which specifies permissible deviation from expected arrival time.

GCRA checks conformance of traffic contract, using two concept of expected and actual cell

arrival time. For incoming cell, the expected arrival time is TAT (Theoretical arrival time).

The conformance of cell is decided on the basis of its TAT. The cell is said to be conforming,

if it arrives at time (TAT-L). All cells arriving before time (TAT-L) are marked as non-

conforming cells.


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Equivalent representations of the GCRA

Continuous-State Leaky Bucket Algorithm

Virtual Scheduling algorithm

TAT: Theoretical Arrival Time

ta(k): Time of arrival of a cell

Yes

Yes

No

X < 0

?

TAT > ta(k) + L

?

TAT < ta(k)

?

No

Yes

No

Non

Conforming

Cell Non

Conforming

Cell

Yes

X = X - (t a(k) - LCT)

X = 0

X > L

?

X = X + I

LCT = ta(k)Conforming Cell

No TAT = ta (k)

TAT = TAT + I

Conforming Cell

Arrival of a cell k at time ta(k)

Virtual SchedulingAlgorithmContinuous-State Leaky Bucket Algorithm


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Traffic Policing of ATM Service Categories-

The ATM Forum specifies the following five categories of services:

CBR: Constant Bit Rate

rt-VBR: Real-Time Variable Bit Rate

nrt-VBR: Non-Real-Time Variable Bit Rate

UBR: Unspecified Bit Rate

ABR: Available bit Rate

presentation atm 3

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