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Acknowledgment

I have taken efforts in this project. However, it would

not have been possible without the kind support and help of many

individuals and organizations. I would like to extend our sincere

thanks to all of them.

I am highly indebted to Ms. Rakhi Channa Mam and Mr.

Nandan Sujati Sir for their guidance and constant supervision as well

as for providing necessary information regarding the project & also

for their support in completing the project.

I would like to express our gratitude towards our parents for

their kind co-operation and encouragement which help me in

completion of this project.

I would like to express my special gratitude and thanks to industry

persons for giving me such attention and time.

My thanks and appreciations also go to our colleague in

developing the project and people who have willingly helped me out

with their abilities.

Harsh


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ABSTRACT:-

The goal is first to introduce performance monitoring aspects of

asynchronous transfer mode (ATM) networks and then to focus on traffic andcongestion control schemes. To deal with this performance monitoring

management, a framework for defining a generic intelligent and integrated

model for network management is described. As an example of the efficiency of

this intelligent management architecture, we measure the performance of a new

congestion control scheme. This scheme uses the cell loss priority (CLP) bit, the

explicit forward congestion indicator and the explicit backward congestionindicator. The intelligent management uses different parameters and builds a

complex but efficient control scheme. We show that this new control scheme

allows performance to be increased by an order of magnitude.

The primary objective in the present term paper is to gain fundamental

understanding of the performance achievable in ATM networkd as a function of

various system characteristics. We derive limit theorems that characterize the

achievable performance in terms of offered traffic, the admissable region, and

the revenue measure. The insightsobtained allow for substantial simplifications

in the design of the real time connection admission control algorithms. In

particular, we describe how the boundaries of admissable region with convex

complements may be linearizedthus reducing the admissable regionso as to

obtain a convenient loss network representation. The asymptotic results for theachievable performance suggest that the potential reduction in revenue is

immaterial in high capacity networks. Numerical experiments confirm that the

actual reduction is typically negligible, even in network of moderate capacity.


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ATM Network:-

Asynchronous Transfer Mode (ATM) is a switching technique for tele -

communication networks. It uses asynchronoustime-division multiplexingand

encodes data into small, fixed-sizedcells. This differs from other protocols such

as theInternet Protocol SuiteorEthernetthat use variable sized packets

orframes. ATM has similarity with bothcircuitandpacketswitched

networking. This makes it a good choice for a network that must handle both

traditional high-throughput data traffic, and real-time,low-latencycontent such

as voice and video. ATM uses aconnection-orientedmodel in which avirtual

circuitmust be established between two endpoints before the actual data

exchange begins.

While the role of ATM is diminishing in favor ofnext-generation

networks, it still plays a role in thelast mile, which is the connection between

anInternet service providerand the home user. For an interesting write-up of

the technologies involved, including the deep stacking of communications

protocols used.

One reason ATM works well with disparate kinds of data is that it's a

connection-oriented. A sender and receiver on a network set up a fixed path

between each other before sending data, and the information arrives in the order

it was sent.

Other protocols, such as TCP/IP, are connectionless. That is, they

don't have fixed connections, so individual data packets may go to different

destinations and they may be delayed or arrive in the wrong order.
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Performance monitoring of ATM Networks

Application performance depends on factors such as: hardware, protocols,

network design, other users, and the applications design.

Traditional networks are designed with no traffic differentiation; all traffic

time-critical and non-time-critical is treated equally. Hence, a user transfer a file

and a user executing some real time application tasks such as videoconference

are treated in the same way. With unlimited bandwidth, this scenario poses no

problems. However, as bandwidth becomes increasingly limited, there is a

higher degree of contention amongst these applications. In this situation, it

becomes important to ensure that time-critical applications do not suffer. A

network that can provide different levels service is often said to support quality

of service.

ATM is well known for providing a rich set of QoS capabilities and in many

respects, these schemes are similar to those provided in an IP network, however,

the ATM networks have some special features of their own.

How ATM Works

ATM network uses fixed-length cells to transmit information. The cell

consists of 48 bytes of payload and 5 bytes of header. Transmitting the

necessary number of cells per unit time provides the flexibility needed to

support variable transmission rates.

ATM network is connection-oriented. It sets up virtual channel connection

(VCC) going through one or more virtual paths (VP) and virtual channels

(VC) before transmitting information. The cells is switched according to the

VP or VC identifier (VPI/VCI) value in the cell head, which is originally set

at the connection setup and is translated into new VPI/VCI value while the

cell passes each switch.


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ATM resources such as bandwidth and buffers are shared among users, they

are allocated to the user only when they have something to transmit. The

bandwidth is allocated according to the application traffic and QoS request at

the signaling phase. So the network uses statistical multiplexing to improve

the effective throughput.

ATM QoS parameter

Primary objective of ATM is to provide QoS guarantees while

transferring cells across the network. There are mainly three QoS parameters

specified for ATM and they are indicators of the performance of the network

Cell Transfer Delay (CTD):

The delay experienced by a cell between the first bit of the cell is transmitted

by the source and the last bit of the cell is received by the destination. This

includes propagation delay, processing delay and queuing delays at switches.

Maximum Cell Transfer Delay (Max CTD) and Mean Cell Transfer Delay

(Mean CTD) are used.

Peak-to-peak Cell Delay Variation (CDV):

The difference of the maximum and minimum CTD experienced during the

connection. Peak-to-peak CDV and Instantaneous CDV are used.

Cell Loss Ratio (CLR):

The percentage of cells lost in the network due to error or congestion that are

not received by the destination. CLR value is negotiated between user andnetwork during call set up process and is usually in the range of 10

-1to 10

-15.

ATM Service Categories

Providing desired QoS for different applications is very complex. For example,

voice is delay-sensitive but not loss-sensitive, data is loss- sensitive but not

delay-sensitive, while some other applications may be both delay-sensitive and

loss-sensitive.


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To make it easier to manage, the traffic in ATM is divided into five service

classes accorcing to various combination requested QoS:

CBR: Constant Bit Rate

CBR is the service category for traffic with rigorous timing requirements

like voice, and certain types of video. CBR traffic needs a constant cell

transmission rate throughout the duration of the connection.

rt-VBR: Real-Time Variable Bit Rate

This is intended for variable bit rate traffic for e.g. certain types of video

with stringent timing requirements.

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

This is for bursty sources such as data transfer, which do not have strict time

or delay requirements.

UBR: Unspecified Bit Rate

This is ATMs best-effort service, which does not provide any QoS

guarantees. This is suitable for non-critical applications that can tolerate or

quickly adjust to loss of cells.

ABR: Available Bit Rate

ABR is commonly used for data transmissions that require a guaranteed

QoS, such as low probability of loss and error. Small delay is also required

for some application, but is not as strict as the requirement of loss and error.

Due to the burstiness, unpredictability and huge amount of the data traffic,

sources implement a congestion control algorithm to adjust their rate of cell


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generation. Connections that adjust their rate in response to feedback may

expect a lower CLR and a fair share of available bandwidth.

The available bandwidth at an ABR source at any point of time is dependant

on how much bandwidth is remaining after the CBR and VBR traffic have

been allocated their share of bandwidth. Figure 1 explains this concept.

VBR

CBR

ABRTotal

Bandwidth

Figure 1 -- ATM bandwidth allocation to different service

t

ATM QoS Priority Scheme

Each service category in ATM has its own queue. There are mainly

two schemes for queue service. In round-robin scheme, all queues have the

same priority and therefore have the same chance of being serviced. The links

bandwidth is equally divided amongst the queues being serviced. Another

scheme is weighted round-robin scheme, which is somehow similar to WFQ in

IP networks: queues are serviced depending on the weights assigned to them.

Weights are determined according to the Minimum Guaranteed Bandwidth

attribute of each queue parameter in each ATM switch. This scheme ensures

that the guaranteed bandwidth is reserved for important application such as

CBR service category.


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ATM Traffic Control

An ATM network needs Traffic Control capabilities to cope with the

variousservice classesand to cope with potential errors within the network at any

time (eg. a problem with thephysical layer) . The network should have the

following traffic control capabilities .

Traffic Control Procedures and their Impact on Resourse

Managment

Traffic control procedures for ATM networks are currently not fully

standardised . But the goal of these procedures are to ,

acheive good ATM network efficency .

meet the user's quality of service requirements .

with a method that is gennerally applicable . Hence , more sophisticated traffic

control and resourse management actions are being taken into account .

The basic problem of ATM networks is the statistical behaviour of the cell

arival process (eg. at a buffer where cells generated at several different sources

are multiplexed together). It has been found that the quality of service prameters

, such as jitter and loss probability , are very sensitive to the assumed sourse

characteristic . Therefore , it is necessary to use detailed source traffic models

for performance evaluation .

Network Resource Management

A tool of network resource management which can be used for Traffic Control is

thevirtual pathtechnique . By grouping several virtual channels together into a

virtual path other forms of control can be simplified (ie. connection admission
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10/19

controlandusage parameter control and network parameter control) only the

collective traffic of an entire virtual path has to be handled . Priorty control can be

supported by re aggreting traffic types requiring different qualities of service

through virtual paths . Messages for the operation of traffic control can be more

easly distributed , a single message refering to all thevirtual channelswithin

avirtual pathwill do .

Connection Admission Control

Connection Admission Control is the set of actions taken by the network during the

call set-up (or reneogation ) phase to establish if avirtual path or virtualchannelcan be accepted by the network .

A connection can only be established if sufficent network resources are available

to establish the connection end-to-end with the required quality of service . The

agreed quality of service for any of the existing channels must not be affected by

the new connection .

Two classes of prameters are forseen to support connection admission control :

a set of prameters describing the source traffic characteristics .

another set of prameters to identify the quality of service required .

Usage Parameter Control and Network Parameter Control

Usage Parameter Control (UPC) and Network Parameter Control (NPC) do the

same job at different interfaces . The UPC function is performed at the user

network interface , while the NPC function is performed at the network node

interface .
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The main purpose of UPC/NPC is to protect the network resources from malicious

as well as unintentional misbehaviour which can effect the quality of service of

other already established connections.

Usage prameter monitoring includes the following functions :

Checking the validity ofVPI/ VCIvalues.

Monitoring the traffic volume entering the network from all active VP and

VC connections to ensure that the agreed prameters are not violated .

Monitoring the total volume of the accepted traffic on the access link.

What is actually carried out depends on the access network configuration.

Usage parameter control can simply disgardcells that violate the neogiated traffic

parameters . In addition a 'guilty'connection may be released . A less rigorous

measure would be to 'tag ' the cells and let them through if they do not cause harm

to the network.

Priority Control

ATM cells have an explicit cell loss priority bit in theheaderso at least two

different ATM priority classes can be distinguished . A single ATM connection

can be have both priority classes when the information to be transmitted is

classified into more and less important parts .

Traffic Shaping

Traffic shaping actively alters the traffic characteristics of a stream of cells on

aVPC or VCCin order to reduce the peak cell rate , limit the burst length or

reduce the cell delay vairation by suitablyspacing cells in time . This of course

must be within the limits of the cell sequence integrity of an ATM connection.
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Traffic shaping is an option for both network operators and users , and for a

network opreator , traffic shaping may assist in dimensioning the network more

cost-effectivley.

Congestion Control In ATM

What is Expected from Congestion Control

Objectives

The objectives of traffic control and congestion control for ATM are: Support a set

of QoS parameters and classes for all ATM services and minimize network and

end-system complexity while maximizing network utilization.

Selection Criteria

To design a congestion control scheme is appropriate for ATM network and non-

ATM networks as well, the following guidances are of general interest.

Scalability

The scheme should not be limited to a particular range of speed, distance,

number of switches, or number of VCs. The scheme should be appliable for

both local area networks (LAN) and wide area networks (WAN).

Fairness

In a shared environment, the throughput for a source depends upon the

demands by other sources. There are several proposed criterion for what is

the correct share of bandwidth for a source in a network environment. And

there are ways to evaluate a bandwidth allocation scheme by comparing its

results with a optimal result.


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Robustness

The scheme should be insensitive to minor deviations such as slight

mistuning of parameters or loss of control messages. It should also isolate

misbehaving users and protect other users from them.

Implementability

The scheme should not dictate a particular switch architecture. It also should

not be too complex both in term of time or space it uses.

Generic Functions

It is observed that events responsible for congestion in broadband networks have

time constants that differ by orders of magnitude, and multiple controls with

approciate time constants are necessary to manage network congestion.

We can classify the congestion control schemes by the time scale they operate

upon: network design, connection admission control (CAC), routing (static or

dymanic), traffic shaping, end-to-end feedback control, hop-by-hop feedback

control, buffering. The different schemes are functions on different severity of

congestion as well as different duration of congestion.

Another classification of congestion control schemes is by the stage that the

operation is performed: congestion prevention, congestion avoidance and

congestion recovery. Congestion prevention is the method that make congestion

impossible. Congestion avoidance is that the congestion may happen, but the

method avoid it by get the network state always in balance. Congestion recovery is

the remedy steps to take to pull the system out of the congestion state as soon as

possible and make it less damaging when the congestion already happened.

No matter what kind of scheme is used, the following outstanding problems are the

main diffculties that need to be treated carefully: the burstiness of the data traffic,


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the unpredictability of the resource demand and the large propagation delay versas

the large bandwidth.

To meet the objectives of traffic control and congestion control in ATM networks,

the following funtions and procedures are suggested by the ATM Forum Tecnical

Committee .

Connection Admission Control

Connection Admission Control (CAC) is defined as the set of actions taken by the

network during the call set-up phase in order to determine whether a connection

request can be acceted or should be rejected.

Based on the CAC algorithm, a connection request is progressed only when

sufficient resources such as bandwidth and buffer space are available along the

path of a connection. The decision is made based on the service category, QoS

desired and the state of the network which means that the number and conditions

of existing connections.

Routing and resource allocation are part of CAC when a call is accepted.

Usage Parameter Control

Usage Parameter Control (UPC) is defined as the set of actions taken by the

network to monitor and control traffic at the end-system access. Its main purpose is

to protect network resources from user misbehavior, which can affect the QoS of

other connections, by detecting violations of negotiated parameters and taking

appropriate actions.

Generic Cell Rate Algorithm

The Generic Cell Rate Algorithm (GCRA) is used to define conformance with

respect to the traffic contract. For each cell arrival, the GCRA determines whether

the cells conforms to traffic contract of the connection. The UPC fuction may

implement GCRA, or one or more equivalent algorithms to enforce conformance.


15/19

GCRA is a virtual scheduling algorithm or a continuous-state Leaky Bucket

Algorithm as difined by the flowchart in Figure 2 and Figure 3 It is defined with

two parameters: the Increment (I) and the Limit (L). The notation GCRA(I,L) is

often used.

Figure 2: Virtual Scheduling Algorithm


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Figure 3: Continuous-State Leaky Bucket Algorithm

The GCRA is used to define the relationship between PCR and CDVT, and

relationship between SCR and BT. The GCRA is also used to specify the

conformance of the declared values of and the above parameters.

Priority Control

The end-system may generate traffic flows of different priority using the Cell Loss

Priority (CLP) bit. The network may selectively discard cells with low priority if

necessary such as in congestion to protect, as far as possible, the network

performance for cells with high priority.


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Traffic Shaping

Traffic shaping is a mechanism that alters the traffic characteristics of a stream of

cells on a connection to achieve better network efficiency whilst meeting the QoS

objectives, or to ensure conformance at a subsequent interface.

Examples of traffic shaping are peak cell rate reduction, burst length limiting,

reduction of CDV by suitably spacing cells in time, and queue service schemes.

Traffic shaping may be performed in conjuntion with suitable UPC functions.

Leaky Bucket Algorithm

The most famous algorithm for traffic shaping is leaky bucket algorithm. This

method provides a pseudo-buffer(Figure 4). Whenever a user sends a cell, the

queue in the pseudo-buffer is increased by one. The pseudo-server serves the queue

and the service-time distribution is constant. Thus there are two control parameters

in the algorithm: the service rate of the pseudo-server and the pseudo-buffer size.

Figure 4: Leaky Bucket Method


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As long as the queue is not empty, the cells are transmitted with the constant rate

of the service rate. So the algorithm can receive a bursty traffic and control the

output rate. If excess traffic makes the pseudo-buffer overflow, the algorithm can

choose discarding the cells or tagging them with CLP=1 and transmitting them.

PCR or SCR can be controlled by choosing appropritiate values of service rate and

buffer size. In addition, PCR and SCR can both be controlled by combining two

buckets with one for each of the parameters. And there are many variances of the

original scheme.

Network Resource ManagementIn Network Resource Management (NRM) is reponsible for the allocation of

network resources in order to seperate traffic flows according to different service

characteristics, to maintain network performance and to optimise resource

utilisation. Thie function is mainly concerned with the management of virtual paths

in order to meet QoS requirements.

Frame Discard

If a congested network needs to discard cells, it may be better to drop all cells of

one frame than to randomly drop cells belonging to different frames, because one

cell loss may cause the retransmission of the whole frame, which may cause more

traffic when congestion already happened. Thus, frame discard may help avoid

congestion collapse and can increase throughput. If done selectively, frame discard

may also improve fairness.

Feedback Control

Feedback controls are defined as the set of actions taken by the network and by the

end-systems to regulate the traffic submitted on ATM connections according to the

state of network elements.

Feedback mechanisms are specified for ABR service class by ATM Forum

Technical Committee. We will discuss it in detail later.


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ABR Flow Control

As we have discussed before, the ABR service category uses the link capacity that

is left over and is applied to transmit critical data that is sensitive to cell loss. That

makes traffic management for this class the most charllenging by the fluation ofthe network load condition, the burstiness of the data traffic itself, and the CLR

requirement.

The ATM Forum Technical Committee Traffic Management Working Group have

worked hard on this topic, and here are some of the main issues and the current

progress of this area.

Some Early Debates

Congestion management in ATM is a hotly debated topic, many contradictory

beliefs exist on most issues. These beliefs lead to different approaches in the

congestion control schemes. Some of the issues have been closed after a long

debate and the ATM Forum Technical Committee final adopted one of them, and

others are still open and the debates are continuing.

REFERENCES:-

1. www.cse.wustl.edu

2. www.slideshare.net

3. www.wikipedia.com

4. www.portal.acm.org

5. www.ieeexplore.ieee.org

6. ATM Concepts and Protocols By Sumit Kesra

7. Larry L.Peterson, Bruce S.Davie , Computer Networks, a

systems approach, second edition.
http://www.slideshare.net/http://www.slideshare.net/http://www.wikipedia.com/http://www.wikipedia.com/http://www.portal.acm.org/http://www.portal.acm.org/http://www.portal.acm.org/http://www.wikipedia.com/http://www.slideshare.net/

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