STANDARDS

STANDARDS

It has long been accepted in the communications industry that standards are

required to govern the physical, electrical, and procedural characteristics of communication

equipment. In the past, this view has not been embraced by the computer

industry. Whereas communication-equipment vendors recognize that their

equipment wiil generally interface to and communicate with other vendors' equipment,

computer vendors have traditionally attempted to lock their customers into

proprietary equipment; the proliferation of computers and distributed processing

has made that an untenable position. Computers from different vendors must communicate

with each other and, with the ongoing evolution of protocol standards,

customers will no longer accept special-purpose protocol-conversion software

development. The result is that standards now permeate all of the areas of technology

discussed in this lesson.

Throughout the lesson we will describe the most important standards that are

in use or that are being developed for various aspects of data and computer communications.

Lesson 1A looks at the key organizations involved with the development

of standards.

There are a number of advantages and disadvantages to the standards-making

process. We list here the most striking ones. The principal advantages of standards

are the following:

A standard assures that there will be a large market for a particular piece of

equipment or software. This encourages mass production and, in some cases,

the use of large-scale-integration (LSI) or very-large-scale-integration (VLSI)

techniques, resulting in lower costs.

A standard allows products from multiple vendors to communicate, giving the

purchaser more flexibility in equipment selection and use.

The principal disadvantages are these:

@ A standard tends to freeze the technology. By the time a standard is developed,

subjected to review and compromise, and promulgated, more efficient

techniques are possible.

@ There are multiple standards for the same thing. This is not a disadvantage of

standards per se, but of the current way things are done. Fortunately, in recent

years the various standards-making organizations have begun to cooperate

more closely. Nevertheless, there are still areas where multiple conflicting

standards exist.

OUTLINE OF THE LESSON

This lesson, of course, serves as an introduction to the entire lesson. A brief synopsis

of the remaining lessons follows.

Data Transmission

The principles of data transmission underlie all of the concepts and techniques presented

in this lesson. To understand the need for encoding, multiplexing, switching,

error control, and so on, the reader must understand the behavior of data signals

propagated through a transmission medium. Lesson 2 provides an understanding

of the distinction between digital and analog data and digital and analog transmission.

Concepts of attenuation and noise are also examined.

Transmission Media

Transmission media can be classified as either guided or wireless. The most commonly-

used guided transmission media are twisted pair, coaxial cable, and optical fiber.

Wireless techniques include terrestrial and satellite microwave, broadcast

radio, and infrared. Lesson 3 covers all of these topics.

Data Encoding

Data come in both analog (continuous) and digital (discrete) form. For transmission,

input data must be encoded as an electrical signal that is tailored to the characteristics

of the transmission medium. Both analog and digital data can be represented

by either analog or digital signals; each of the four cases is discussed in

Lesson 4. This lesson also covers spread-spectrum techniques.

The Data Communications Interface

In Lesson 5 the emphasis shifts from data transmission to data communications.

For two devices linked by a transmission medium to exchange digital data, a high

degree of cooperation is required. Typically, data are transmitted one bit at a time

over the medium. The timing (rate, duration, spacing) of these bits must be the

same for transmitter and receiver. Two common communication techniques-asynchronous

and synchronous-are explored. This lesson also looks at transmission

line interfaces. Typically, digital data devices do not attach to and signal across a

transmission medium directly. Rather, this process is mediated through a standardized

interface.

Data Link Control

True cooperative exchange of digital data between two devices requires some form

of data link control. Lesson 6 examines the fundamental techniques common to all

data link control protocols including flow control and error detection and correction,

and then examines the most commonly used protocol, HDLC.

Multiplexing

Transmission facilities are, by and large, expensive. It is often the case that two communication

stations will not utilize the full capacity of a data link. For efficiency, it

should be possible to share that capacity. The generic term for such sharing is multiplexing.

Lesson 7 concentrates on the three most common types of multiplexing techniques.

The first, frequency-division multiplexing (FDM), is the most widespread

and is familiar to anyone who has ever used a radio or television set. The second is

a particular case of time-division multiplexing (TDM), often known as synchronous

TDM. This is commonly used for multiplexing digitized voice streams. The third

type is another form of TDM that is more complex but potentially more efficient

than synchronous TDM; it is referred to as statistical or asynchronous TDM.

Circuit Switching

Any treatment of the technology and architecture of circuit-switched networks

must of necessity focus on the internal operation of a single switch. This is in contrast

to packet-switched networks, which are best explained by the collective behavior

of the set of switches that make up a network. Thus, Lesson 8 begins by examining

digital-switching concepts, including space- and time-division switching. Then,

the concepts of a multinode circuit-switched network are discussed; here, we are

primarily concerned with the topics of routing and control signaling.

Packet Switching

There are two main technical problems associated with a packet-switched network,

and each is examined in Lesson 9:

Routing. Because the source and destination stations are not directly connected,

the network must route each packet, from node to node, through the

network.

Congestion control. The amount of traffic entering and transiting the network

must be regulated for efficient, stable, and fair performance.

The key design issues in both of these areas are presented and analyzed; the

discussion is supported by examples from specific networks. In addition, a key

packet-switching interface standard, X.25, is described.

Frame Relay

Lesson 10 examines the most important innovation to come out of the work on

ISDN: frame relay. Frame relay provides a more efficient means of supporting

packet switching than X.25 and is enjoying widespread use, not only in ISDN but in

other networking contexts. This lesson looks at the data-transfer protocol and callcontrol

protocol for frame relay and also looks at the related data link control protocol,

LAPF.

A critical component for frame relay is congestion control. The lesson

explains the nature of congestion in frame relay networks and both the importance

and difficulty of controlling congestion. The lesson then describes a range of congestion

control techniques that have been specified for use in frame relay networks.

Asynchronous Transfer Mode (ATM)

Lesson 11 focuses on the transmission technology that is the foundation of broadband

ISDN: asynchronous transfer mode (ATM). As with frame relay, ATM is

finding widespread application beyond its use as part of broadband. This lesson

begins with a description of the ATM protocol and format. Then the physical layer

issues relating to the transmission of ATM cells and the ATM Adaptation Layer

(AAL) are discussed.

Again, as with frame relay, congestion control is a vital component of ATM.

This area, referred to as ATM traffic and congestion control, is one of the most

complex aspects of ATM and is the subject of intensive ongoing research. This

lesson surveys those techniques that have been accepted as having broad utility in

ATM environments.

LAN Technology

The essential technology underlying all forms of local area networks comprises

topology, transmission medium, and medium access control technique. Lesson 12

examines the first two of these elements. Four topologies are in common use: bus,

tree, ring, star. The most common transmission media for local networking are

twisted pair (unshielded and shielded), coaxial cable (baseband and broadband),

and optical fiber. These topologies and transmission media are discussed, and the

most promising combinations are described.

LAN Systems

Lesson 13 looks in detail at the topologies, transmission media, and MAC protocols

of the most important LAN systems in current use; all of these have been

defined in standards documents. The discussion opens with what might be called

traditional LANs, which typically operate at data rates of up to 10 Mbps and which

have been in use for over a decade. These include Ethernet and related LANs and

two token-passing schemes, token ring and FDDI (fiber distributed data interface).

Then, more recent high-speed LAN systems are examined, including ATM LANs.

Finally, the lesson looks at wireless LANs.

Bridges

The increasing deployment of LANs has led to an increased need to interconnect

LANs with each other and with wide-area networks. Lesson 14 focuses on a key

device used in interconnection LANs: the bridges. Bridge operation involves two

types of protocols: protocols for forwarding packets and protocols for exchanging

routing information.

This lesson also returns to the topic of ATM LANs to look at the important

concept of ATM LAN emulation, which relates to connecting other types of LANs

to ATM networks.

Protocols and Architecture

Lesson 15 introduces the subject of protocol architecture and motivates the need

for a layered architecture with protocols defined at each layer. The concept of protocol

is defined, and the important features of protocols are discussed.

The two most important communications architectures are introduced in this

lesson. The open systems interconnection (OSI) model is described in some detail.

Next, the TCPIIP model is examined. Although the OSI model is almost universally

accepted as the framework for discourse in this area, it is the TCPIIP protocol suite

that is the basis for most commercially available interoperable products.

Internetworking

With the proliferation of networks, internetworking facilities have become essential

components of network design. Lesson 16 begins with an examination of the

requirements for an internetworking facility and the various design approaches that

can be taken to satisfy those requirements. The remainder of the lesson explores

the use of routers for internetworking. The internet protocol (IP) and the new IPv6,

also known as IPng, are examined. Various routing protocols are also described,

including the widely used OSPF and BGP.

Transport Protocols

The transport protocol is the keystone of the whole concept of a computer communications

architecture. It can also be one of the most complex of protocols. Lesson

17 examines in detail transport protocol mechanisms and then introduces two

important examples: TCP and UDP.

Network Security

Network security has become increasingly important with the growth in the number

and importance of networks. Lesson 18 provides a survey of security techniques

and services. The lesson begins with a look at encryption techniques for insuring

privacy, which include the use of conventional and public-key encryption. Then, the

area of authentication and digital signatures is explored. The two most important

encryption algorithms, DES and RSA, are examined, as well as MD5, a one-way

hash function important in a number of security applications.

Distributed Applications

The purpose of a communications architecture is to support distributed applications.

Lesson 19 examines three of the most important of these applications; in

each case, general principles are discussed and are followed by a specific example.

The applications discussed are network management, world-wide web (WWW)

exchanges, and electronic mail. The corresponding examples are SNMPv2, HTTP,

and SMTPIMIME. Before getting to these examples, the lesson opens with an

examination of Abstract Syntax Notation One (ASN.l), which is the standardized

language for defining distributed applications.

ISDN and Broadband ISDN

The integrated-services digital network (ISDN) is a projected worldwide public

telecommunications network that is designed to service a variety of user needs.

Broadband ISDN is an enhancement of ISDN that can support very high data rates.

Lesson A looks at the architecture, design principles, and standards for ISDN

and broadband ISDN.

Standard Organization

THROUGHOUTTH IS LESSON, we describe the most important standards in use or being

developed for various aspects of data and computer communications. Various organizations

have been involved in the development or promotion of these standards. This lesson provides

a brief description of the most important (in the current context) of these organizations:

IETF

IS0

ITU-T

Internet Standards and the IETF

Many of the protocols that make up the TCPIIP protocol suite have been standardized or are

in the process of standardization. By universal agreement, an organization known as the

Internet Architecture Board (IAB) is responsible for the development and publication of

these standards, which are published in a series of documents called Requests for Comments

(RFCs).

This section provides a brief description of the way in which standards for the TCPIIP

protocol suite are developed.

The Internet and Internet Standards

The Internet is a large collection of interconnected networks, all of which use the TCPIIP

protocol suite. The Internet began with the development of ARPANET and the subsequent

support by the Defense Advanced Research Projects Agency (DARPA) for the development

of additional networks to support military users and government contractors.

The IAB is the coordinating committee for Internet design, engineering, and management.

Areas covered include the operation of the Internet itself and the standardization of

protocols used by end systems on the Internet for interoperability. The IAB has two principle

subsidiary task forces:

Internet Engineering Task Force (IETF)

Internet Research Task Force (IRTF)

The actual work of these task forces is carried out by working groups. Membership in

a working group is voluntary; any interested party may participate.

It is the IETF that is responsible for publishing the RFCs. The KFCs are the working

notes of the Internet research and development community. A document in this series may

be on essentially any topic related to computer communications, and may be anything from

a meeting report to the specification of a standard.

The final decision of which RFCs become Internet standards is made by the IAB, on

the recommendation of the IETF. To become a standard, a specification must meet the following

criteria:

Be stable and well-understood

Be technically competent

Have multiple, independent, and interoperable implementations with operational

experience

Enjoy significant public support

Be recognizably useful in some or all parts of the Internet

The key difference between these criteria and those used for international standards is

the emphasis here on operational experience.

The Standardization Process

Figure 1.12 shows the series of steps, called the standards track, that a specification goes

through to become a standard. The steps involve increasing amounts of scrutiny and testing.

At each step, the IETF must make a recommendation for advancement of the protocol, and

the IAB must ratify it.

The white boxes in the diagram represent temporary states, which should be occupied

for the minimum practical time. However, a document must remain a proposed standard for

at least six months and a draft standard for at least four months to allow time for review and

comment. The gray boxes represent long-term states that may be occupied for years.

A protocol or other specification that is not considered ready for standardization may

be published as an experimental RFC. After further work, the specification may be resubmitted.

If the specification is generally stable, has resolved known design choices, is believed

to be well-understood, has received significant community review, and appears to enjoy

enough community interest to be considered valuable, then the RFC will be designated a

proposed standard.

For a specification to be advanced to draft-standard status, there must be at least two

independent and interoperable implementations from which adequate operational experience

has been obtained.

After significant implementation and operational experience has been obtained, a

specification may be elevated to standard. At this point, the specification is assigned an STD

number as well as an RFC number.

Finally, when a protocol becomes obsolete, it is assigned to the historic state.

The International Organization for Standardization (ISO)

IS0 is an international agency for the development of standards on a wide range of subjects.

It is a voluntary, nontreaty organization whose members are designated standards bodies of

participating nations, plus nonvoting observer organizations. Although IS0 is not a governmental

body, more than 70 percent of IS0 member bodies are governmental standards institutions

or organizations incorporated by public law. Most of the remainder have close links

with the public administrations in their own countries. The United States member body is the

American National Standards Institute.

IS0 was founded in 1946 and has issued more than 5000 standards in a broad range of

areas. Its purpose is to promote the development of standardization and related activities to

facilitate international exchange of goods and services and to develop cooperation in the

sphere of intellectual, scientific, technological, and economic activity. Standards have been

issued to cover everything from screw threads to solar energy. One important area of standardization

deals with the open systems interconnection (OSI) communications architecture

and the standards at each layer of the OSI architecture.

In the areas of interest in this lesson, IS0 standards are actually developed in a joint

effort with ,another standards body, the International Electrotechnical Commission (IEC).

IEC is primarily concerned with electrical and electronic engineering standards. In the area

of information technology, the interests of the two groups overlap, with IEC emphasizing

hardware and IS0 focusing on software. In 1987, the two groups formed the Joint Technical

Committee 1 (JTC 1). This committee has the responsibility of developing the documents

that ultimately become IS0 (and IEC) standards in the area of information technology.

The development of an IS0 standard from first proposal to actual publication of the

standard follows a seven-step process. The objective is to ensure that the final result is acceptable

to as many countries as possible. The steps are briefly described here. (Time limits are

the minimum time in which voting could be accomplished, and amendments require

extended time.)

1. A new work item is assigned to the appropriate technical committee, and within that

technical committee, to the appropriate working group. The working group prepares

the technical specifications for the proposed standard and publishes these as a draft

proposal (DP). The DP is circulated among interested members for balloting and technical

comment. At least three months are allowed, and there may be iterations. When

there is substantial agreement, the DP is sent to the administrative arm of ISO, known

as the Central Secretariat.

2. The DP is registered at the Central Secretariat within two months of its final approval

by the technical committee.

3. The Central Secretariat edits the document to ensure conformity with IS0 practices;

no technical changes are made. The edited document is then issued as a draft international

standard (DIS).

4. The DIS is circulated for a six-month balloting period. For approval, the DIS must

receive a majority, approval by the technical committee members and 75 percent

approval of all voting members. Revisions may occur to resolve any negative vote. If

more than two negative votes remain, it is unlikely that the DIS will be published as a

final standard.

5. The approved, possibly revised, DIS is returned within three months to the Central

Secretariat for submission to the IS0 Council, which acts as the board of directors of

ISO.

6. The DIS is accepted by the Council as an international standard (IS).

7. The IS is published by ISO.

As can be seen, the process of issuing a standard is a slow one. Certainly, it would be

desirable to issue standards as quickly as the technical details can be worked out, but IS0

must ensure that the standard will receive widespread support.

ITU Telecommunications Standardization Sector

The ITU Telecommunications Standardization Sector (ITU-T) is a permanent organ of the

International Telecommunication Union (ITU), which is itself a United Nations specialized

agency. Hence, the members of ITU-T are governments. The U.S. representation is housed

in the Department of State. The charter of the ITU is that it "is responsible for studying technical,

operating, and tariff questions and issuing Recommendations on them with a view to

standardizing telecommunications on a worldwide basis." Its primary objective is to standardize,

to the extent necessary, techniques and operations in telecommunications to achieve

end-to-end compatibility of international telecommunication connections, regardless of the

countries of origin and destination.

The ITU-T was created on March 1, 1993, as one consequence of a reform process

within the ITU. It replaces the International Telegraph and Telephone Consultative Committee

(CCITT), which had essentially the same charter and objectives as the new ITU-T.

ITU-T is organized into 15 study groups that prepare Recommendations:

I. Service Description

2. Network Operation

3. Tariff and Accounting Principles

4. Network Maintenance

5. Protection Against Electromagnetic Environment Effects

6. Outside Plant

7. Data Network and Open Systems Communications

8. Terminal Equipment and Protocols for Telematic Services

9. Television and Sound Transmission

10. Languages for Telecommunication Applications

11. Switching and Signalling

12. End-to-End Transmission Performance

13. General Network Aspects

14. Modems and Transmission Techniques for Data, Telegraph, and Telematic Services

15. Transmission Systems and Equipment

Work within ITU-T is conducted in four-year cycles. Every four years, a World

Telecommunications Standardization Conference is held. The work program for the next

four years is established at the assembly in the form of questions submitted by the various

study groups, based on requests made to the study groups by their members. The conference

assesses the questions, reviews the scope of the study groups, creates new or abolishes existing

study groups, and allocates questions to these groups.

Based on these questions, each study group prepares draft Recommendations. A draft

Recommendation may be submitted to the next conference, four years hence, for approval.

Increasingly, however, Recommendations are approved when they are ready, without having

to wait for the end of the four-year Study Period. This accelerated procedure was adopted

after the study period that ended in 1988. Thus, 1988 was the last time that a large batch of

documents was published at one time as a set of Recommendations.

THERE ARE A number of resources available on the Internet for keeping up with

developments in this field.

USENET Newsgroups

A number of USENET newsgroups are devoted to some aspect of data communications and

networking. As with virtually all USENET groups, there is a high noise-to-signal ratio, but it

is worth experimenting to see if any meet your needs. Here is a sample:

comp.dcom.lans, comp.dcom.1ans.misc: General discussions of LANs.

comp.std.wireless: General discussion of wireless networks, including wireless LANs.

comp.security.misc: Computer security and encryption.

comp.dcom.ce11-relay: Covers ATM and ATM LANs.

comp.dcom.frame-relay: Covers frame-relay networks.

comp.dcom.net-management: Discussion of network-management applications, protocols,

and standards.

comp.protocols.tcp-ip: The TCPIIP protocol suite.

Web Sites for This lesson

A special web page has been set up for this lesson at http://www.shore.net/-wdDCC5e.html

The site includes the following:

Links to other web sites, including the sites listed in this lesson, provide a gateway to relevant

resources on the web.

Links to papers and reports available via the Internet provide additional, up-to-date

material for study.

We also hope to include links to home pages for courses based on the lesson; these

pages may be useful to other instructors in providing ideas about how to structure the

course.

Additional problems, exercises, and other activities for classroom use are also planned.

As soon as any typos or other errors are discovered, an errata list for this lesson will be

available at http://www.shore.net/-ws/welcome.htmlT.h e file will be updated as needed.

Please email any errors that you spot to ws@shore.net. Errata sheets for other lessons are at

the same web site, as well as discount ordering information for the lessons.

Other Web Sites

There are numerous web sites that provide some sort of information related to the topics of

this lesson. Here is a sample:

http://www.soc.hawaii.edu/conlcom-resources.html: Information and links to resources

about data communications and networking.

http://www.internic.net/ds/dspgOl.htmlM: aintains archives that relate to the Internet

and IETF activities. Includes keyword-indexed library of RFCs and draft documents as

well as many other documents related to the Internet and related protocols.

http://www.ronin.com/SBA: Links to over 1500 hardware and software vendors who

currently have WWW sites, as well as a list of thousands of computer and networking

companies in a Phone Directory.

http:/lliinwww.ira.uka.de/bibliography/index.html: The Computer Science Bibliography

Collection, a collection of hundreds of bibliographies with hundreds of thousands

of references.