A Seminar Report on

LATEST TRENDS IN ASYNCHRONOUS TRANSFER MODE

Submitted to

JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY,

ANANTAPUR

In partial fulfillment of the requirement for the award of the degree of

BACHELOR OF TECHNOLOGY

In

ELECTRONICS AND COMMUNICATION ENGINEERING

By

GR ANIL KUMAR (11691A0403)

DEPARTMENT OF ELECTRONICS AND COMMUNICATION ENGINEERING

MADANAPALLE INSTITUTE OF TECHNOLOGY AND SCIENCE

(Approved by AICTE, New Delhi, Affiliated to JNTU, Anantapur)

Madanapalle-517325, Andhra Pradesh.

MADANAPALLE INSTITUTE OF TECHNOLOGY & SCIENCE

(Approved by AICTE, New Delhi, Affiliated to JNTU, Ananthapur)

DEPARTMENT OF ELECTRONICS AND COMMUNICATION ENGINEERING

BONAFIDE CERTIFICATE

This is to certify that the technical seminar report entitled, “LATEST TRENDS IN ASYNCHRONOUS TRANSFER MODE”, is a bonafide work done by GR ANIL KUMAR (11691A0403) under our guidance and supervision, in the partial fulfillment of the requirements for the award of the degree of Bachelor of Technology in Electronics & Communication Engineering in Madanapalle institute of Technology & Science, Madanapalle, affiliated to Jawaharlal Nehru Technological University, during the academic year 2014-2015.

Seminar Guide Prof. A R REDDY, M.Tech, Ph.D.

Mr. D Balakrishna Reddy Head of the department

Assistant Professor Dept of ECE

Dept of ECE

Acknowledgement

I extend my sincere gratitude towards Mr. D Balakrishna Reddy, Asst Prof Dept. of

Electronics and Communication Engineering for giving me his valuable knowledge and

wonderful technical guidance. I also thank all the other faculty members of ECE department and

my friends for their help and support.

ABSTRACT

The paper begins with what is asynchronous transfer mode, ATM is simply a Data Link Layer

protocol. It is asynchronous in the sense that the recurrence of the cells containing from an

individual user is not necessarily periodic. It is the technology of choice for evolving B-ISDN

(Board Integrated Services Digital Network), for next generation LANs and WANs. ATM

supports transmission speeds of 155 Mbits/sec. In the future, Photonic approaches have made the

advent of ATM switches feasible, and an evolution towards an all packetized, unified, broadband

telecommunications and data communication world based on ATM is taking place.

CONTENTS

1. Introduction

2. Basic Concepts

2.1 Connectionless versus Connection-oriented

2.2 B-ISDN

2.3 ATM

3. Applications using Connectionless Communications

4. Trends

4.1 Networking is Critical

4.2 Peak of Technology Life Cycle 4.3 Standardization

5. Past Failures and Successes

6. Requirements for Success 7. Challenges

8. Summary

1 INTRODUCTION

ATM is simply a Data Link Layer protocol. It is asynchronous in the sense that the recurrence of

the cells containing from an individual user is not necessarily periodic. It is the technology of

choice for evolving B-ISDN (Board Integrated Services Digital Network),for next generation

LANs and WANs. ATM supports transmission speeds of 155 Mbits/sec. In the future, Photonic

approaches have made the advent of ATM switches feasible, and an evolution towards an all

packetized, unified, broadband telecommunications and data communication world based on

ATM is taking place.

These computers include the entire spectrum of PCs, through professional workstations upto

super-computers. As the performance of computers has increased, so too has the demand for

communication between all systems for exchanging data, or between central servers and the

associated host computer system. The replacement of copper with fiber and the advancements in

digital communication and encoding are at the heart of several developments that will change the

communication infrastructure. The former development has provided us with huge amount of

transmission bandwidth. While the latter has made the transmission of all information including

voice and video through a packet switched network possible.

With continuously work sharing over large distances, including international communication, the

systems must be interconnected via wide area networks with increasing demands for higher bit

rates. For the first time, a single communications technology meets LAN and WAN requirements

and handles a wide variety of current and emerging applications.

ATM is the first technology to provide a common format for bursts of high speed data and the

ebb and flow of the typical voice phone call. Seamless ATM networks provide desktop-to-

desktop multimedia networking over single technology, high bandwidth, low latency network,

removing the boundary between LAN WAN.

2 BASIC CONCEPTS

A lot of effort is currently being invested in the design of broadband communication systems.

The design of a system for supporting connectionless communications should be seen as an

integral part of these activities. In this chapter we review some basic concepts in B-ISDN,

discuss applications that will use a connectionless service, and survey related work. The purpose

of this chapter is to introduce the information that forms a starting point for the work presented

in the rest of the dissertation.

The concept of connectionless communication is essential to this work, and can only be

understood in relation to its opposite, i.e., connection-oriented communication. The network we

are considering for providing connectionless data communication is the B-ISDN. we will briefly

review some basic concepts of ATM, which is the network technology that will be used to

support connectionless data communication.

2.1 Connectionless versus Connection-oriented:

In this dissertation the terms connectionless and connection-oriented are used as a characteristic

of a service, i.e., the observable behavior of a communication system. A service is

connectionless if two or more users of the service can transfer data using the communication

system without first establishing and later releasing a connection.

Figure 2-1: Sequence of Service Primitives for a Connection-oriented Service

A service is connection-oriented if users must establish a connection before they can transfer

data using the communication system. The establishment of a connection is a negotiation

between the users who wish to communicate and the communication system. During the

negotiation, state information related to the connection is exchanged between the parties. The

communication system reserves resources for a connection, e.g., bandwidth.

The essential interactions between a communication system and its users can be described by

means of the sequence of service primitives, which are exchanged at the boundaries between the

system and its users. Figure 2-1 shows a likely sequence of service primitives in case of a

connection-oriented service, where two users want to communicate. Service primitives are

denoted by double arrows, to indicate an interaction in which both the user and the

communication system are involved. First, the initiating users request the system to establish a

connection (Connection Request). In this request at least the address of the required destination

and often parameters regarding the characteristics of the traffic and the required Quality of

Service (QoS) are passed to the system. The system analyses the parameters of the request and

determines to what extent it can support the requested connection. If the connection can be

supported, a Connection Indication primitive occurs between the system and the called user. This

user checks whether and under what conditions it wants to accept the connection. The acceptance

of the connection is indicated to the system by another interaction (ConnectionResponse).

Finally the initiating user is informed of the successful establishment of the connection

(ConnectionConfirm). From now on, the users can transfer data. The data is passed from the

sending user to the system by means of a DataRequest primitive, transported by the system, and

passed from

Figure 2-2: Sequence of Service Primitives for a Connectionless Service

The system to the receiving user by means of a Data Indication primitive. After all data has been

transferred, the connection should be closed again. Therefore, one of the users, not necessarily

the initiating user, informs the system that it wants to release the connection (Release Request).

The system releases the connection and informs the other user (Release Indication).

For a connectionless service, the sequence of service primitives is much simpler (Figure 2-2). No

primitives for establishing or releasing a connection are needed. Each data unit is transferred

individually. The sending user passes the data to the system by means of a Data Request

primitive. The system transports the data and passes it to the receiving user by means of a Data

Indication primitive. All information regarding the transfer of the data unit, e.g., required

destination address, should be passed between the users and the network in these primitives.

An essential characteristic of a connectionless service is that the system does not have any

advance knowledge about the data that should be transferred (e.g., destination, or rate at which

data units are offered to the system). In case of a connection-oriented service this information is

agreed upon during the connection establishment, so that the system can reserve resources for the

transfer of the data units.

For both types of service there are classes of applications that they support best. Applications

that require a direct association between the users, such as telephony, are best served by a

connection-oriented service. Other applications, such as those which involve the transfer of only

a single unit of information between a source and a destination are better served by a

connectionless service. Moreover, there are a huge number of applications that use the TCP/IP

protocol suite. These applications require a connectionless service at the network layer, also

when they will be supported by the B-ISDN . It is possible to use different types of services at

different layers in the network. In [82] the use of connectionless protocols at the network layer,

to support all types of applications is advocated. Others prefer to use a single connection-

oriented protocol, i.e., ATM, to support all applications. Since ATM has been standardized for

future integrated networks, we assume ATM as the basic method to serve all applications.

However, we believe that additional provisions have to be made in the network to accommodate

applications that are connectionless in nature.

2.2 B-ISDN:

The Broadband Integrated Services Digital Network (B-ISDN) is expected to be the major future

telecommunications network for the wide area. It will provide a wider range of possible

applications, and support much higher throughputs than present telecommunication networks.

Applications, which can be very diverse in nature, will all be supported by a single network.

Moreover, a user can access the network via a single standardized interface, the User-Network

Interface (UNI).

The applications that should be supported by the B-ISDN are not only very diverse; they also put

very diverse requirements on the network. The network should support:

• point-to-point as well as multi-point communications;

• single-medium as well as multimedia communications;

• connectionless as well as a connection-oriented communications;

• narrowband as well as broadband communications;

• communications involving constant bit rate as well as variable bit rate traffic; and

• communications for applications with a very diverse range of QoS requirements.

Some examples of applications that are foreseen for the B-ISDN are video-conferencing, High

Definition Television (HDTV) distribution, video-on-demand, telephony, and applications that

are currently supported by the Internet.

In order to support all these applications, the Asynchronous Transfer Mode (ATM) has been

adopted for multiplexing and switching in the network ([16]). ATM is a technique, where all user

information is transferred in Protocol Data Units (PDUs) of fixed size, called cells. Cells are

identified by means of a header

Figure 2-3: The B-ISDN Protocol Reference Model

so that they can be identified for (de)multiplexing and switching. ATM should also be used at the

UNI of the B-ISDN. Two different UNIs have been defined ([78]); one with a bit rate of 155.520

Mbits/s, and one with a bit rate of 622.080 Mbits/s.

A Protocol Reference Model (PRM) has been defined for the B-ISDN in [18] (see Figure 2-3).

The PRM is divided into planes and layers. The division into (vertical) planes is done to identify

different types of functionality, i.e., for the transfer of user information, for the control of calls

and connections, and for management. The division into (horizontal) layers is done to create a

stepwise independency between the medium, used for the transmission of signals, and the

applications. A layer uses the next lower layer to provide a certain, less medium dependent, and

more application dependent, service to the next higher layer.

The B-ISDN PRM distinguishes between three planes. The user plane contains the functions that

deal with the user information. The control plane contains functions for the control of calls and

connections, and the transport of information on behalf of these functions (signaling). Finally,

the management plane provides for the coordination between user and control plane, and for the

management of individual entities and the overall network. The management plane has been

subdivided into layer management, performing management functions for specific protocol

layers and their entities, and plane management, performing coordination between the planes

and management of the system as a whole.

Within the user and control plane, protocol layers are identified. The lower layers are common to

both planes. The Physical Layer provides for the convergence of ATM cells to signals that can

be transferred over a physical medium. The ATM Layer provides for the end-to-end transfer of

cells along a connection. It performs switching and multiplexing of cells from different

connections. The ATM Adaptation Layer (AAL) adapts the common service provided by the

ATM Layer to a service that can better support specific classes of applications. It provides for

instance for segmentation and reassembly and for synchronization of source and destination.

Different views exist for relating the layering adopted in the B-ISDN PRM and the layering of

the Reference Model for Open Systems Interconnection (OSI-RM) ([65], [110], [111]). In the

context of this dissertation, it is convenient to situate the AAL in the OSI Data link Layer. The

ATM Layer can be considered as an upper sub layer of the OSI Physical Layer, or as a lower sub

layer of the Data link Layer.

The applications to be supported by the B-ISDN have very diverse communication requirements.

The ATM Layer is common to all applications. The purpose of the AAL is to adapt the ATM

service to the service required for the specific applications requirements. In order to come up

with a limited number of AAL services, applications have been classified according to the

following communication requirements:

• timing relation between source and destination (required or not required);

• bit rate (constant or variable); and

• connection mode (connection-oriented or connectionless).

Since not all combination of the above requirements are foreseen, only four service classes are

distinguished according to Table 2-1. Clearly, the class of AAL service we are interested in for

the support of connectionless data communications over ATM is service class D. This class does

not provide a timing relation between source and destination, supports variable bit rate traffic,

and is connectionless.

Table 2-1: AAL Service Classes

For each of the service classes, one or more protocols have been defined that can provide the

service. For class D, two protocols, called AAL 3/4 and AAL 5 have been defined ([20], [75])1.

2.3 ATM :

The Asynchronous Transfer Mode (ATM) is a technique that is used for switching and

multiplexing in the B-ISDN. It can route fixed-size data units, called cells, through a network of

switches interconnected by links, from source to destination. ATM is connection-oriented, i.e.,

prior to the transfer of cells, a connection is established through the network, and cells are

forwarded along the connection subsequently.

Multiplexing of cells from different connections on an ATM link is done using asynchronous

time division. Time is divided into slots, in which a single ATM cell fits. Slots can be assigned to

connections asynchronously, i.e., whenever a slot is needed for a connection it can be used.

Identification of the connection a cell belongs to is done by means of a label in the header of the

cell. Figure 2-4 gives an example of a series of cells, which are transmitted on a link

consecutively. All cells with the same label, e.g., 7, are identified as belonging to a certain

connection.

Figure 2-4: Asynchronous Time Division Multiplexing

The alternative to asynchronous time division multiplexing is synchronous time

division multiplexing, which is used in present telecommunication networks.

Time is again divided into slots, and slots are grouped into frames. Slots are

assigned to a connection synchronously, i.e., a connection is assigned the same

number of slots in each frame. Identification of the connection a slot belongs to is

done by means of the position of a slot in the frame. Figure 2-5 gives an example

of a series of consecutive cells in this case. All cells which are the third one of a

frame are identified as belonging to a certain connection.

An important advantage of asynchronous over synchronous time division multiplexing

is that the transmission capacity assigned to a connection can vary in

time, depending on the needs of the application. Furthermore, unlike with

synchronous time division multiplexing, where the assigned capacity is an

integer multiple of the smallest possible capacity (one slot assigned per frame),

any capacity can be assigned to a connection.

ATM switches receive cells on a number of incoming links. The connection an

incoming cell belongs to is uniquely defined by the incoming link, and the label

carried in the cell header. In the switch a routing table is maintained, in which the

outgoing ATM link, and the required label for the connection on that ATM link is

given for each connection. Using this table, the switch can replace the label of a

cell, and forward it to the proper outgoing link. Figure 2-6 gives an example of

ATM switching. The shaded entry in the routing table indicates that all cells that

arrive on link 2 with label 7 should be forwarded to link 4, while the label should

be modified to 3. ATM cells are drawn as in Figure 2-4, i.e., from left to right, first

the header, which contains the label, and then the payload.

As stated before, ATM is a connection-oriented technique. The header of an ATM

cell relates the cell to a previously established ATM connection. Two types of

ATM connections are identified, Virtual Path Connections (VPCs) and Virtual

Channel Connections (VCCs). Therefore, also two identifiers can be found in the

header of the cell, a Virtual Path Identifier (VPI) and a Virtual Channel Identifier

(VCI). The combination of the two, referred to as VPI/VCI, determines the ATM

connection a cell belongs to.

A physical link between two switches carries a number of Virtual Path Links

(VPLs), each of them is identified by a VPI. The concatenation of a number of

VPLs forms a Virtual Path Connection (VPC). Within a VPC, a number of Virtual

Channel Links (VCLs) can be identified. Each VCL is identified by a VCI, which is

Figure 2-6: ATM Switchingli

unique for that VPC. The concatenation of a number of VCLs forms a Virtual

Channel Connection.

Figure 2-7 shows the relationship between the different types of links and connections.

In the figure, a series of 7 ATM switches is shown. On a link between a pair

of switches, a number of VP links can be identified. A concatenation of VP links

forms a VP connection, e.g., from the first to the third switch. All switches operate

on VPs; only some of them operate also on VCs. These are also visible at the VC

level. Within a VP connection (between two VC switches), a number of VC links

can be identified. The concatenation of a number of these links forms a VC

connection, e.g., between the first and the seventh switch.

An ATM cell has a payload field of 48 octets. The header of the cell is 5 octets

long. The major fields in the header are the VPI and VCI field. Furthermore, the

header contains a Header Error Control (HEC) field to protect the header against

bit errors. A Cell Loss Priority (CLP) field can be used to indicate the relative

importance of the cell in the connection. The Payload Type (PT) field carries additional

information, e.g., an ATM-user-to-ATM-user indication that is transported

transparently by the network. At the UNI, also a Generic Flow Control (GFC)

field is present in the header in order to control the access to a shared link.

Figure 2-7: ATM Connections

Error control should be performed in layers above ATM on an end-to-end basis.

Only errors in the header are detected or corrected (depending on the mode of

operation) by means of the 8 bit HEC field, if possible.

In order to protect an ATM network against overload and guarantee the

requested QoS to the users, bandwidth is reserved for each connection. A

Connection Admission Control (CAC) mechanism checks whether or not bandwidth

for a newly requested connection is available. If not, the connection is

refused. If the bandwidth is available, the connection is established, and the

bandwidth is reserved. In order to check if the user does not violate the agreed

bandwidth another mechanism, called Usage Parameter Control (UPC), has to be

implemented at the border of (the public part of) the network, directly after the

UNI. Cells that cause a violation of the agreed bandwidth are either discarded

directly, or given a low cell loss priority, so that they are the first ones to be

discarded in case of congestion.

3 Applications using Connectionless Communications

The emphasis in this dissertation is on connectionless communications over the

wide area, with throughputs that have not been available up to now. In order to

provide insight into the requirements on the system, we will explore the characteristics

of applications that have been identified in a number of publications, e.g.,

[5], [39], [95], [96], [106], [128], [146]:

• file transfer,

• terminal access,

• information retrieval (e.g., World Wide Web),

• computer graphics,

• distributed supercomputing,

• remote procedure call (RPC),

• virtual memory page swapping and paging, and

• electronic mail.

Although not all applications listed here need necessarily be supported by a

connectionless network, most of them are connectionless in nature. This means

that the applications need to transfer one or more individual pieces of information,

not a stream of related pieces of information. Electronic mail is an example

of an application that is inherently connectionless. Its purpose is to transfer individual

messages from source to destination. There is no need for an association

between source and destination before the message is transferred.

The end-systems that implement the applications can be attached directly to the

B-ISDN. However, most applications have been first introduced in a local environment,

so that a lot of existing end-systems have been attached to Local Area

Networks (LANs). Another reason for attaching end-systems to LANs is the

locality of a large share of the generated traffic.

However, there is a growing demand for communications over a wider area. This

can be accommodated by interconnecting LANs by means of networks that span

a wider geographical area. Most LANs use connectionless protocols. In order to

avoid complicated and costly protocol conversion, it is often desirable to interconnect

these LANs by means of connectionless networks. A first stage into this

Figure 3-1 Connection of End-systems, LANs, and MANs to a WAN

development is the interconnection of LANs by Metropolitan Area Networks

(MANs), which typically have a geographic span of about 50 kilometers. Many of

these MANs are based on the Distributed Queue Dual Bus (DQDB) technique,

defined in the IEEE 802.6 standard ([63]).

If a Wide Area Network (WAN) supporting broadband connectionless communications

becomes available, it can be used to interconnect these MANs, and to

interconnect LANs directly. Furthermore, end-systems with high communication

needs can be attached directly to the WAN. This will lead to a scenario where

end-systems are attached to either a LAN, a MAN, or a WAN, with LANs being

connected to either a MAN or a WAN, and the MANs connected to the WAN (see

Figure 3-1).

We refer to the systems that interconnect different networks, and perform the

needed conversion, as Interworking Units (IWUs). These IWUs can function as

routers, interconnecting the different networks at the network layer, e.g., using

the Internet Protocol (IP) ([107]) or the ISO Connectionless Network Protocol

(CLNP) ([66]). The IWU can also function as a bridge, interconnecting the

different networks at the medium access control (MAC) sub layer ([128]).

It is expected that most traffic to be transported by the wide area connectionless

network will initially come from end-systems attached to LANs. Interconnection

of these LAN is often referred to as the most important application of the

network. In fact, the real applications to be supported are those listed above.

From the point of view of the WAN, the LAN only aggregates the traffic generated

by these applications.

In order to get more insight in the traffic generated by the individual applications,

we consider the requirements on the transfer of an single unit of information. [39]

and [95] list for a number of applications the expected size of an information unit.

Furthermore, they give an indication of the required response time for these

applications. Figure 3-2graphically represents these requirements. For each

application an area of requirements is given, expressing both the uncertainty and

the variation in requirements. Note that the information units sizes and response

time requirements are given at the application level. An information unit may

very well be transported in a number of smaller packets at the network level

Figure 3-2 Applications Requirements

Figure3-3 Throughput Requirements of Applications

In Figure 3-2 a number of diagonal lines have been drawn that give an indication

of the throughput required to transfer information units of a certain size within

the response time. Figure 3-3summarizes these throughput requirements. It can

be seen from the figure that throughput requirements range from less than a

kilobit per second for electronic mail and terminal access to a gigabit per second

for computer graphics.

In principle, the given applications do not tolerate any loss or corruption of data.

This does not imply that no loss or corruption can be tolerated from the connectionless

service we are designing. End-to-end protocols used on top of the

connectionless service (e.g., a transport protocol) can enhance the provided QoS.

4 .1 Networking is Critical :

Networking has become the most critical part of computing. Today, computers are used

mostly for transferring information from one peripheral to another, from network to the

disk, from disk to the video screen, from keyboard to the disk, and so on. Mail, _le transfer,

information browsing using World Wide Web, Gopher, and WAIS takes up more time of the

computing resources than computing per se. Initially, when the computers were designed,

the performance was measured by the \add" instruction time. Today, it is the \move"

instruction that is the key to the perceived performance of a system. This means that the

bus performance is more important than the arithmetic logical unit (ALU) performance.

I/O performance is more important than the SPEC marks.

There are several other reasons for communications and networking becoming critical. First,

the users have been moving away from the computer. In the sixties, computer users went

to computer rooms to use them. In the seventies, they moved to terminal rooms away from

the computer rooms. In the eighties, the users moved to their desktop. In the nineties,

they are mobile and can be anywhere. This distance between the users and the computers

has lead to a natural need for communication between the user and the computer or the user

interface device (which may be a portable computer) and the servers.

Second, the system extent has been growing continuously. Up until eighties, the computers

consisted of one node spread within 10 meters. In nineties, the systems consists of hundreds

of nodes within a campus. The increasing extent leads to increasing needs for computing.

Figure 4-1 Shaped technology curve

from timesharing to personal computing. Now we need ways to work together with

other users. So, in the next ten years, emphasis will be on cooperative computing. This will

further lead to increase in communications.

In the last decade, we were busy developing corporate networks, and campus networks. In

the next decade, we need to develop intercorporate networks, national information infras-

tructures, and international information infrastructures. All these developments will lead to

more growth in the field of networking and more demands for the personnel with skills in

networking.

The increasing role of communications in computing has lead to the merger of the telecom-

munications and computing industries. The line between voice and data communications is

fading away. Data communication is expected to take over voice communication in terms of

volume.

4.2 Peak of Technology Life Cycle

Most technologies follow an S-shaped curve shown in Figure 1, where the number of problems

solved is plotted against time. There are three distinct stages in the life of a technology.

In the beginning, all problems are hard and it takes a lot of resources and time to solve

a few problems. At this stage, a lot of money is spent in research but there is very little

revenue. Most of this research is funded by the traditional government funding agencies,

such as, National Science Foundation and Advanced Research Project Agency (ARPA) in

the United States.

After some of the key problems have been solved, a lot of other problems can be solved by

spending little money. At this stage, the curve takes an upturn. The amount of revenues to

Figure 4-2 The exponential growth of Internet

be made from the technology is much more than the investments. It is at this stage, that

the industries take over technology development. Numerous small companies are formed and

quickly grow to become large corporations.

Finally, when all the easy problems have been solved, the remaining problems are hard and

would require a lot of resources. At this stage, the researchers usually move on to some other

technology and a new S-shaped curve is born.

The computing industry in general and the networking sector in particular is currently going

through the middle fast growing region of the technology life cycle curve. The number of

problems solved is indicated by the deployment of the technology. In case of networks, one

can plot the number of hosts on the networks, bytes per host, number of networks on the

internet, total capacity (in MIPS) of hosts on the network, total memory or total disk space

of the hosts on the network. In each case, one would see a sudden exponential up turn in

the last few years.

Figure 2 shows the famous Internet growth curve. The figure shows the number of hosts

on the Internet in the last 20 years. The data before July 1988, although plotted is hardly

visible. Since 1988, growth has defied all predictions.

4.3 Standardization

When a technology reaches the middle fast growing region, it becomes necessary to standard-

ize it to make it usable for the masses. The computing industry in general and networking

in particular is undergoing through this phase. Even if people use different computers, it

is necessary that the networking interfaces be standardized so that these diverse computers

can communicate with one another.

The standardization requires a change in the way business is done. Before standardization,

a majority of the market is vertical. The only way for users to maintain compatibility is to

buy the complete system from one manufacturer. System vendors make more money than

component vendors. IBM, DEC, and Sun Microsystems are examples of such system vendors.

After standardization, the business situation changes. Users can and do buy components

from different vendors. The market becomes horizontal. Companies specializing in specific

components and fields take prominence. Intel for processors, Microsoft for operating systems,

Novell for networking are examples of this trend.

To survive in this post-standardization era, invention alone is not sufficient. Only those new

ideas that are backed by a number of vendors become standardized and are adopted. It,

thus, becomes necessary to form technology partnerships.

5 Past Failures and Successes

In the last fifteen years, we have seen a number of networking technologies that were very

promising during their life time but were not successful in the long run. A sample of such

technologies is listed in Table 5-1. In each case, the technology listed in the second column was

more promising than the one in the third column until the year shown in the first column.

In early 80's, when Ethernet was being introduced, some argued that broadband Ethernet,

which allows voice, video, and data to share a single cable would be more popular than

baseband Ethernet. As we all know, today there are a few broadband installations. Most

installations of Ethernet are baseband. The cost of combining the three services was just

too high. The analog circuits required for frequency multiplexing were not as reliable and

economical as digital circuits with separate wiring.

Around the same time, when computer companies were trying to sell Ethernets, PBX man-

ufacturers were presenting PBX as the better alternative, again because it was already there

and it could handle voice as well as data. However, PBX was not accepted by the customers

simply because it did not provide enough bandwidth.

The Integrated Service Digital Network (ISDN) was standardized in 1984 and was very

promising then. However it's deployment has been much too slow. Even after ten years, it

is not possible to get an ISDN connection at most places. Even at those places where it is

available, the 64 kbps bandwidth provided by it is not sufficient for most data applications.

For low bandwidth applications, modems on analog lines provide a better alternative. Mo-

dem technology has advanced much beyond expectation. Today, one can get 28.8 kbps and

56 kbps modems that work with all pervasive analog lines and do not have monthly charges

associated with the extra ISDN line.

In 1986, IEEE 802.4 (token bus) was touted as a better alternative than IEEE 802.3/Ethernet

Table 5-1 Networking Failures and Successes of the Past

for real time environment. It was said that Ethernet could not provide the delay

guaranteesrequired for manufacturing and industrial environments. Manufacturing Automation

Proto-col (MAP/TOP) was seen as the right solution. Today, IEEE 802.3/Ethernet is used in

allsuch environments. Token buses are practically nonexistant.Up until 1988, ISO/OSI protocol

stack was seen as the leading contender for networking

everywhere. Networking researchers in most countries were implementing ISO/OSI proto-

cols. The United States Government Open Systems Interconnection Prfile (GOSIP) even

made ISO/OSI a mandatory requirement for government purchases. Today, TCP/IP pro-

tocol stack dominates instead. The OSI protocols suffered from the common problems of

standards: it had too many features. Any feature required by any application in the world

needed to be supported by the standard. The protocols took too long to standardize and

were quite complex. The \build before you standardize" philosophy of the TCP/IP protocol

stack helped in its success.

Up until 1991, IEEE 802.6 Dual Queue Dual Bus (DQDB) was seen as a promising candidate

for metropolitan area networks. It is no longer considered viable. The unfairness problem

and general problems of bus architectures have made it undesirable.

Xpress Transfer Protocol (XTP) was designed as the high-performance alternative to TCP/IP.

Protocol Engines { the company leading the design of XTP declared bankruptcy in 1992.

6 Requirements for Success

There are several lessons to learn from the list presented in Table 1. First, all technologies

appear very promising when first proposed. However, not all survive. Those that survive

meet all the following requirements.

1. Low cost

2. High performance

3. Killer application

4. Timely completion

5. Interoperability among various implementations of the same technology

6. Coexistence with existing (legacy) technology.

After a brief overview of ATM networks in the next section, we discuss some of these issues

as they apply to ATM networks.

7 Challenges

In this section, we discuss a few of the requirements identified earlier and see

what needs to be done to ensure success of the ATM technology.

7.1 Economy of Scale

Today, networking technology seems to be far ahead of the applications. High-speed fibers

have been installed but there is not enough video traffic to fill them. Invention is becoming

the mother of necessity. We need to create a need for the high bandwidth. In such a

situation, generally low cost is the primary motivator. When the high-speed technology

is proportionately cheaper than lower-speed alternatives, the buyer's considerations change

from \Why should I buy high speed?" to \Why should I not buy high speed?"

Today, there is diseconomy of scale. Higher speed networks cost more in per-bit than lower

speed networks. Ten Mbps Ethernet cards can be had for $50. However, 100 Mbps FDDI

cards cost closer to $1000. This diseconomy of scale has a significant impact on user adoption.

We have seen this happen in other areas of computing. Today ten 100-MIPS computers cost

much less than one 1000-MIPS computer. Therefore, we see more distributed computing

than supercomputing. Applying the same logic, it appears that unless there is economy of

scale, people may divide their networking applications among multiple low-speed links than

one high-speed link. Of course, there are a few applications that will not work on speeds in

the range of 10 Mbps. For these, the users have no choice but to use higher speed links.

This diseconomy of scale a_ects all high speed technologies including ATM. However, ATM

has a bigger uphill battle due to its newness. In a recent ATM Forum user survey conducted

Dr. John McQuillan, the users were asked that given the same price, which 100 Mbps network

they would buy: ATM or 100-Mbps Ethernet. The answer was Ethernet because it is

something the users feel very comfortable with. We ATM designers will have to work hard

to get the ATM equipment prices below the 100-Mbps Ethernet to get acceptance.

7.2 Tariff

Tariffing the ATM tra_c is another problem. Today's telecommunications tari_s are de-

signed for low bandwidth high cost voice traffic. It costs $25/month for a simple 64-kbps

analog phone line. At this rate, the phone company has the potential of making $211k/year

on a 45 Mbps link. A coast to coast T3 link does cost $180k to $240k per year. A 155 Mbps

link would cost three times as much. However, 155 Mbps ATM circuits are being tari_ed at

$13k to $45k per year. That's 10 to 50 times cheaper than today's rates. The situation is

similar to that of the computer industry. When the computer prices started going down, the

old established companies designed to sell expensive mini and supercomputers had trouble

keeping their overheads down to be able to sell personal computers. While new compa-

nies designed to make money selling personal computers cheaply ourished, old companies

tried hard to maintain their existing business almost to the point of bankruptcy. Success of

cheap telecommunications using ATM technology has the potential of doing the same to the

telecommunications industry. The telecommunications companies with overheads designed

for expensive voice services will have a tough time selling cheap data services. The danger

of decreasing revenues may prove to be a hinderence to the success of ATM at least in the

wide area networking (WAN) market.

7.3 Performance

Figure 3 shows a layered model of people involved in designing a high speed network. At

the bottom are the physicists, who work on _bers and lasers. They are working today with

10 Gbps lasers and their challenge is to design 100 Gbps lasers. The next level up are

the LAN designers who are designing FDDI and ATM networks using these _bers. The

LAN designers are working at 100 to 155 Mbps { two orders of magnitude lower than the

physicists. Then there are LAN adapter designers who take the 100 Mbps protocol standard

and design adapters which run barely at 20 Mbps. Although some users aren't aware of this,

many FDDI adapters cannot transmit or receive more than 20 Mbps. Thus, we loose the

performance by a factor of 5 at this layer.

Figure 7-1 A layered view of people who affect the performance seen by the user

The next (fourth) layer consists of processor designers. Unless designed carefully, a processor

may not be able to keep up with the high speed LAN adapters. The fifth layer of operating

system reduces the performance further. The sixth layer is that of network protocols{ some

of which are not able to cope with high speed links. Finally the top (seventh) layer is the

application, which sees a usable bandwidth of a few megabits per second.

Unless higher layers are improved, changing the lower layers will not result in any higher

performance for the user. Today's problem is not so much in networking protocols as it is in

proper I/O designs for the processors and operating systems. We can provide the user with

155 Mbps or 622 Mbps links, but they will not be able to use it unless, operating system

and processor designs are improved accordingly. The only exception is the backbone where

specialized hardware and software are used. The backbone components (switches, routers, or

bridges) are designed specially for high communication speeds. Thus, the _rst place where

high-speed is will be used is in the backbone. The desktop market will have to wait for

better operating systems and processors.

Another lesson to learn from this layered model is that for high speed networks to become

a reality, all seven layers have to be improved. Bad performance even in one layer can delay the

introduction of the High-speed Networking.

7.4 Application

It is well known that future applications will be multimedia including data, voice, image, and

video. What is less well known is that the voice traffic will be a negligible part of the traffic.

This can be seen by considering what happens on telephone networks today. On the AT&T

network, approximately 125 to 130 million calls are made per day and an average call lasts

around 5 minutes. Each call requires a bandwidth of 64 kbps. Thus, the total bandwidth

used by voice in AT&T network is approximately 28.8 Gbps, which is only one thousandth

of the potential bandwidth of a fiber. Even if all 200 million people of the United States were

to talk 24 hours a day, the total bandwidth required will be only 12.8 Tbps, which again

is less than the potential bandwidth of a fiber. In a survey of private networks reported in

the August 1992 issue of IEEE Spectrum, it was found that in 1985, 75% of the traffic was

voice. In 1990, the voice percentage dropped to 56% and in 1995, it is projected to be 39%.

If we were to make a forecast based on this trend, we would conclude that the voice traffic

will be zero or negligible by the year 2010. The reason for this decline is not that people are

not talking enough but that while voice traffic is limited by the population, the data traffic

is not. Computers have no limit on the speed at which they can transmit and so there is

no limit to the value to which data traffic can grow. It is this exponentially growing data

business that most telecommunications companies want to get into.

Next, let us consider characteristics of video traffic. One hour of uncompressed HDTV

requires 540 GB of storage. At today's storage prices of $1/MB, this works out to approx-

imately $150 per second of video. This is somewhat expensive and only researchers funded

on government grant can a_ord to store the video at this price. If compressed, the storage

requirements drop by a factor of 60 to 200 and the price becomes $2.50 to $0.75 per second,

which is more reasonable. The conclusion is that most video will be in compressed form sim-

ply for storage. Compression means that the bandwidth requirements vary, and therefore,

variable bit rate (VBR) service rather than constant bit rate (CBR) is likely to be used more

often.

Also, at high speeds, the connection holding times become shorter. At 1 Gbps, it takes

only 10 seconds to transmit 1 hour of compressed VHS movie. It takes even smaller time at

higher speeds. Thus, unless the bandwidth is free, most users of high speed will start and

shutdown the connection after 10-20 seconds. In other words, the traffic will be short-lived

and bursty. This is closer to today's data traffic than voice traffic. For ATM networks to

succeed, they should be able to handle the bursty traffic eficiently.

In 1984, when the ATM cell size was being decided in CCITT, they were thinking about 64

kbps voice. At that speed, 32-byte cells need 4 ms. If larger cells were used, the time to

collect the voice would become too large and would require echo cancellation. The Europeans,

therefore, wanted 32-byte cells, while US position was that the cells be at least 64 bytes long.

The limit of 48 bytes was chosen as an average of 32 and 64. In other words, the cell size

was chosen not for high speed applications but for 64-kbps voice applications. Several other

design and implementation decisions for ATM networks were similarly done as if they were

being designed for voice. One example of such design philosophy is simply dropping cells

on congestion. Requiring users to indicate which cells aren't important by the congestion-

loss priority (CLP) bit is another example. For voice, some cells can be dropped without

significant impact. However, this is not true for data. Every single bit is important and all

dropped bits have to be retransmitted.

The cell size is not suitable for high-speed applications in general and for video traffic in

particular. A single HDTV frame requires 50,000 cells. Switching 50,000 times for each

frame is a not the optimal way.

Prior to the formation of the ATM Forum in October 1991, most of the ATM networks

design decisions were made as if the network was being designed for voice. It is only in 1994

that the importance of data traffic was realized and the available bit rate (ABR) service was

introduced. It is now well understood that the key to ATM technology success is its support

of data traffic. If ATM fails to support data, it will not be able to stay around for video

traffic.

7.5 Scalability

Queueing theory tells us that the variance of response time in a queue depends upon the

variability of the service time (cell time) and square of the cell time:

Variance(response time) = Variance(cell time) + Cell time2

Making the cells same size makes the first term zero. Making the cell time small reduces the

variance in response time. For delay sensitive real time applications, smaller cells provide

reduced delay variation.

It is important to note that we used time in the above equation and not size. The time

requirements of an application do not change as the bandwidth of the network changes.

For example, 30 frames per second video will need one frame every 33 ms regardless of

the speed of the link. Even at Gigabit per second or Terabit per second speeds, the video

will need a response time variation in milliseconds. The cell time of 6 ms would satisfy

most delay sensitive applications. Although 6 ms is 48 bytes at 64 kbps, it is 900 kB at 1.2

Gbps. By using smaller cells at higher speeds we get micro- and nano-second delay variation.

Unfortunately our eyes cannot even feel the difference between millisecond and microsecond

delays, and therefore, we are wasting switching resources.

The cost of a switch depends partly upon its switching speed in cells per second. Given a

switch design, it is possible to make a higher speed switch by simply increasing the cell size

without any significant increase in cost.

What we need are \constant-time" cells and not \constant-size" cells for scalability to high

speed. With constant size cells, the cell time decreases as the speed increases and it becomes

necessary to switch more and more cells per second thereby increasing the cost.

The telecommunications industry claims SONET to be scalable in bandwidth. SONET uses

constant-time frames. All SONET frames are 125 microseconds long. As the speed increases,

the number of bytes in the frame increases proportionately.

Many of today's ATM networks use SONET links. In these networks, the large video image is

broken down in small cells, which are then packed into a large SONET frame and transmitted.

At the receiver, the SONET frame is unloaded, the information is switched cell by cell { all

of whom are probably going to the same destination. After switching, the cells are loaded

into another SONET frame and forwarded to the next switch. This process of unloading

SONET frames and switching cell by cell is clearly unnecessary, given that at high speed,

the amount of information to be transmitted is also generally higher. We could just switch

SONET frames or use a technology which makes use of the best features of SONET and

ATM.

7.6 Simplicity

The final challenge that ATM technology faces is that of keeping it simple. During the

design of IEEE 802.3/Ethernet standard, there was fierce competition between CSMA/CD

and token ring camp - both trying their best to keep their design more cost effective than

the other. This competition was good and did help keep the scope of both standards limited.

For ATM, unfortunately, there is no equal competition at this time. Thus, any thing that

needs to be done by networks has to be done by ATM networks. The design is becoming

too complex. Too many options are being added. ATM networks have to work for constant

bit rate (CBR) traffic as well as variable (VBR) and available bit rate (ABR) traffic. They

have to work for local area (LAN) as well as for wide area (WAN). They have to work for

low-speed as well as for high speed. They have to work for private networks as well as for

public networks. All these options add to the complexity. The situation is similar to that of

OSI.

One of the advantages of switches over routers was that switches were supposed to simple.

They no longer are. For ATM switches, switching is only a negligible part of their responsibil-

ity. A large part of switch resources is consumed by connection set up, route determination,

address translation, multicasting, anycasting, ow control, congestion control, and so on.

Another element adding to the complexity of ATM is the fact that it is being developed at

multiple standards bodies: ITU and ATM Forum. Strictly speaking, ATM forum is not a

standards body. ITU is supposed to develop the standards and ATM Forum is supposed to

select a subset of options provided by ITU. But in reality, ITU is too slow. ATM Forum

cannot wait for ITU to finalize its standard and so it is taking a leading role in developing

it in parallel. A considerable amount of time at both bodies is used in reconciling the

agreements made at the other body. Vendors will end up implementing both ITU and ATM

Forum versions of the standards and users will have to bear the cost even though just one

set would have been fine.

8 Summary

Networking is a critical part of computing today and is growing exponentially. Networking

is in the mid fast-growing region of the technology curve.

High speed networking will succeed if and only if there is economy of scale so that using

higher speed links results in cost savings. Unfortunately, this is not case right now. We face

the danger of users dividing their applications and using several low-speed links.

Considerable amount of resources are being put on ATM networks. However, its success

will depend upon our being able to transfer data at a lower cost and higher performance

than legacy LANs. Also, we will have to control the desire to incorporate all options at once

otherwise it will become too complex.

BIBLIOGRAPHY

[1] M. Ajmone Marsan, G. Conte, G. Balbo, “A Class of Generalized Stochastic Petri Nets for

the Performance Evaluation of Multiprocessor Systems”, ACM Transactions on Computer

Systems, Vol. 2, No. 2, May 1984, pp. 93 - 122.

[2] G.J. Armitage, K.M. Adams, “Packet Reassembly During Cell Loss”, IEEE Network, Vol. 7,

No. 5, September 1993, pp. 26 - 34.

[3] R. Atkinson, “ Default IP MTU for use over ATM AAL5”, RFC 1626, May 1994.

[4] ATM Forum, “ATM User-Network Interface Specification- Version 3.0”, ISBN 0-13-

225863-3, PTR Prentice Hall, Englewood Cliffs, NJ, USA, September 1993.

[5] J.J. Barrett, E.F. Wunderlich, “LAN Interconnect Using X.25 Network Services”, IEEE

Network, Vol. 5, No. 5, September 1991, pp. 12 - 16.

[6] Bellcore TA-TSY-000772, “Generic System Requirements in Support of Switched Multi-

megabit Data Service”, Bellcore, Issue 3, October 1989, Supplement 1, December 1990.

[7] C. Blondia, O. Casals, “Statistical Multiplexing of VBR Sources: A MatrixAnalytic

Approach”, Proceedings of the workshop on Performance Aspects of ATM, PTT Research,

Leidschendam, The Netherlands, October 1991.

[8] G. Boiocchi, P. Crocetti, L. Fratta, M. Gerla, M.A. Marsiglia, “ATM Connectionless Server:

Performance Evaluation”, Proceedings IFIP WG6.4 International Workshop on Performance of

Communication Systems, Martinique, French Caribbean Island, January 1993, pp. 185 - 195.

[9] G. Boiocchi, P. Crocett, L. Fratta, M. Gerla, “Performance Evaluation of a Connectionless

Multicast Service”, Elsevier, Teletraffic Science and Engineering, Vol. 1, Proceedings of the

14th International Teletraffic Congress - ITC 14,