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NBS TECHNICAL NOTE 882

U.S. DEPARTMENT OF COMMERCE/ National Bureau of Standards

Perfor

i Communicati

for Compi
NATIONAL BUREAU OF STANDARDS

The National Bureau of Standards ' was established by an act of Congress March 3, 1901. The Bureau's overall goal is to strengthen and advance the Nation's science and technology

and facilitate their effective application for public benefit. To this end, the Bureau conducts

research and provides: (1) a basis for the Nation's physical measurement system, (2) scientific and technological services for industry and government, (3) a technical basis for equity in trade, and (4) technical services to promote public safety. The Bureau consists of the Institute for Basic Standards, the Institute for Materials Research, the Institute for Applied Technology,

the Institute for Computer Sciences and Technology, and the Office for Information Programs.

THE INSTITUTE FOR BASIC STANDARDS provides the central basis within the United

States of a complete and consistent system of physical measurement; coordinates that system

with measurement systems of other nations; and furnishes essential services leading to accurate and uniform physical measurements throughout the Nation's scientific community, industry, and commerce. The Institute consists of a Center for Radiation Research, an Office of Measurement Services and the following divisions: Applied Mathematics — Electricity — Mechanics — Heat — Optical Physics — Nuclear

Sciences " — Applied Radiation " — Quantum Electronics * — Electromagnetics 3 — Time

and Frequency " — Laboratory Astrophysics " — Cryogenics "'. THE INSTITUTE FOR MATERIALS RESEARCH conducts materials research leading to improved methods of measurement, standards, and data on the properties of well-characterized materials needed by industry, commerce, educational institutions, and Government; provides

advisory and research services to other Government agencies; and develops, produces, and

distributes standard reference materials. The Institute consists of the Office of Standard Reference Materials and the following divisions: Analytical Chemistry — Polymers — Metallurgy — Inorganic Materials — Reactor

Radiation — Physical Chemistry.

THE INSTITUTE FOR APPLIED TECHNOLOGY provides technical services to promote

the use of available technology and to facilitate technological innovation in industry and Government; cooperates with public and private organizations leading to the development of

technological standards (including mandatory safety standards), codes and methods of test; and provides technical advice and services to Government agencies upon request. The Institute consists of a Center for Building Technology and the following divisions and offices: Engineering and Product Standards — Weights and Measures — Invention and Innovation — Product Evaluation Technology — Electronic Technology — Technical Analysis — Measurement Engineering — Structures, Materials, and Life Safety * — Building

Environment * — Technical Evaluation and Application 4 — Fire Technology.

THE INSTITUTE FOR COMPUTER SCIENCES AND TECHNOLOGY conducts research and provides technical services designed to aid Government agencies in improving cost effec- tiveness in the conduct of their programs through the selection, acquisition, and effective

utilization of automatic data processing equipment; and serves as the principal focus within

the executive branch for the development of Federal standards for automatic data processing

equipment, techniques, and computer languages. The Institute consists of the following

divisions: Computer Services — Systems and Software — Computer Systems Engineering — Information Technology.

THE OFFICE FOR INFORMATION PROGRAMS promotes optimum dissemination and

accessibility of scientific information generated within NBS and other agencies of the Federal Government; promotes the development of the National Standard Reference Data System and

a system of information analysis centers dealing with the broader aspects of the National Measurement System; provides appropriate services to ensure that the NBS staff has optimum

accessibility to the scientific information of the world. The Office consists of the following

organizational units: Office of Standard Reference Data — Office of Information Activities — Office of Technical

Publications — Library — Office of International Relations.

'Headquarters and Laboratories at Gaithersburg, Maryland, unless otherwise noted; mailing address Washington, DC. 20234.

3 Part of the Center for Radiation Research.

3 Located at Boulder, Colorado 80302.

* Part of the Center for Building Technology.
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Criteria for the Performance Evaluation of Data Y. , :

:./

Communications Services for Computer Networks ***'."*

Prepared December 1974

Dana S. Grubb and Ira W. Cotton

Computer Systems Engineering Division

Institute for Computer Sciences and Technology

National Bureau of Standards

Washington, D.C. 20234

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U.S. DEPARTMENT OF COMMERCE, Rogers C. B. Morton, Secrefary

\J$ NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Acting Director

III Issued September 1975
National Bureau of Standards Technical Note 882

Nat. Bur. Stand. (U.S.), Tech. Note 882, 36 pages (Sept. 1975) CODEN: NBTNAE

U.S. GOVERNMENT PRINTING OFFICE

WASHINGTON: 1975

For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402 (Order by SD Catalog No. 03.46:882). Price 85 cents. (Add 25 percent additional for other than U.S. mailing).
CONTENTS

ABSTRACT 1

INTRODUCTION 1

1 . TRANSFER RATE 2

1.1 Definition 2

1 .2 Other Definitions 2

1.3 Significance of Transfer Rate 3

1.4 Discussion.... 3

1.5 Typical Transfer Rate Requirements of

Information Processing Systems 4

1.6 Typical Transfer Rates Provided by

Telecommunications Facilities 6

2. AVAILABILITY •. 7

2.1 Definition 7

2.2 Other Definitions 7

2.3 Significance of Availability 7

2.4 Discussion 8

2.5 Typical Availability Requirements of

Information Processing Systems 9

2.6 Typical Availability Figures for

Telecommunications Facilities 9

3. RELIABILITY 11

3.1 Definition 11

3.2. Other Definitions 11

3.3 Significance of Reliability 11

3.4 Discussion 12

3.5 Typical Reliability Requirements of

Information Processing Systems 12

3.6 Typical Reliability Figures for

Telecommunications Facilities 13

4. ACCURACY 13

4.1 Definition 13

4.2 Other Definitions 13

4.3 Significance of Accuracy 14- 4.4 Discussion 14

4.5 Typical Accuracy Requirements of

Information Processing Systems 15

4.6 Typical Accuracy Figures for

Telecommunications Facilities 15

5. CHANNEL ESTABLISHMENT TIME 17

5.1 Definition 17

5.2 Other Definitions 17

5.3 Significance of Channel Establishment

Time 18

5.4 Discussion 18

5.5 Typical Channel Establishment Time

Requirements of Information Processing

Systems 19

in
5.6 Typical Channel Establishment Times

Provided by Telecommunications Facilities 19

6 . NETWORK DELAY 20

6.1 Definition 20

6.2 Other Definitions 20

6.3 Significance of Network Delay 20

6.4 Discussion 20

6.5 Typical Network Delay Requirements of

Information Processing Systems... 21

6.6 Typical Network Delays Provided by

Telecommunications Facilities 22

7 . LINE TURNAROUND DELAY 22

7.1 Definition 22

7.2 Significance 22

7.3 Discussion... 22

7.4 Typical Line Turnaround Delay

Requirements of Information Processing

Systems 23

7.5 Typical Line Turnaround Delays Provided

by Telecommunications Facilities 23

8. TRANSPARENCY 23

8.1 Definition 23

3.2 Other Definitions 23

8.3 Significance of Transparency 24

8.4 Discussion 24

8.5 Typical Transparency Requirements of

Information Processing Systems 25

8.6 Typical Transparency Characteristics of

Telecommunications Facilities 25

9. SECURITY 25

9.1 Definition 25

9.2 Significance of Security 25

9.3 Discussion 25

9.4 Security Requirements of Information

Processing Systems 26

9.5 Security Characteristics Provided by

Telecommunications Facilities 26

CONCLUSIONS 27

ACKNOWLEDGEMENTS 28

BIBLIOGRAPHY 29

Iv
Criteria for the Performance Evaluation of Data'

Communications Services for .Computer Networks

Dana S. Grubb and Ira W. Cotton

ABSTRACT

In general, when telecommunications services

are used as a means of interchanging information

between information processing systems, or between terminals and systems, a number of

parameters determine how well that interchange

is performed. This report examines the

following characteristics of telecommunications

services

1Transfer Rate

2. Availability

3. Reliability

4. Accuracy

5. Channel Establishment Time

6. Network Delay

7. Line Turnaround Delay

8. Transparency

9. Security

These terms are all defined and their

significance discussed. The effects of these

factors on data communication networks are

illustrated.

Key words: Computer communications; computer

networking; data communications; networks;

performance requirements; telecommunications,

INTRODUCTION

In general, when telecommunications services are used as

a means of interchanging information between information

processing systems, or between terminals and systems, a number of parameters determine how well that interchange is

performed. This report examines the following

characteristics of telecommunications services:

1

)

Transfer Rate

2) Availability

3) Reliability

4) Accuracy

5) Channel Establishment Time

6) Network Delay

7) Line Turnaround Delay

8) Transparency

9) Security
These criteria are all user-oriented performance

parameters. Thus, they differ from such operational

measures as signal-to-noise ratio or signaling rate, which

have traditionally been used to evaluate communications

services. Users of data communications services are only

interested in the external manifestations of network

parameters in order to select the service offering the best

performance for their particular needs and at a price they

can afford to pay. For this purpose the criteria listed

above are superior.

In order to design a system with the best overall

performance, it is necessary to consider the relationships

between all of these parameters. These parameters are not

all independent and attempts to improve the performance of

one parameter may degrade one or more of the others. Thus,

an understanding of the inter-relationships is necessary for

effective trade-offs to be made.

In this report the meanings of these communications

service parameters are clarified and' their impact and

significance with regard to information processing systems

are discussed. There is a particular attempt to demonstrate

how these parameters, which were established for traditional

communications services, may be applied to newer services,

such as the packet-switched networks now coming into

existence. Examples of the values of these parameters have

been selected from both new and traditional communications

services

.

These parameters are not felt to be an exhaustive list

for evaluating and comparing data communications services.

However, they are felt to be among the most significant

parameters, and the evaluation of different services on the

basis of these parameters can provide a means for the

comparison of quite different communications services.

Differences in performance between services, according to

these parameters, will often be reflected in price

differences. A parametric analysis of alternative services

should enable users to identify the true cost of different

levels of performance, and, thereby, help them to select the

service which is most cost-effective for their particular

requirements

.

1 . TRANSFER RATE

1 .

1

Definition

The number of information bits transferred by means of

the telecommunications channel divided by the time required

for their transfer and acceptance.

1.2 Other Definitions

Transfer rate is defined in ANSI X3.44 - 1974 (for data

communication systems using the X3.28 - 1971 character-
oriented control procedure) as Transfer Rate of Information

Bits (TRIB). (Ref. 1 & 2.) TRIB is defined as "The ratio of

the number of Information Bits accepted by the receiving

Terminal Configuration during a single Information Transfer

Phase (Phase 3) to the duration of the Information Transfer

Phase. TRIB -is expressed in bits per second." (The

capitalized terms have specific meanings that are defined in

the standard .

1.3 Significance of Transfer Rate

Signaling rate is the data rate of the

telecommunications service in bits per second. However, the

computer user is concerned with the number of information

bits that are transferred in a unit of time. The transfer

rate is reduced below that of the telecommunication

service's signaling rate by such factors as the bits used

for the header and the error detection code, and the

retransmission of message blocks or frames containing

errors. Thus, the user may specify a particular signaling

rate from the common carrier, but the actual transfer rate

will usually be substantially less. Increasing the

signaling rate on a given telecommunications facility will

usually (though not always) increase the transfer rate, but

the increase in the transfer rate is often less than the

increase in the signaling rate due to the increased effect

of the factors cited above.

1 . 4 Discussion

Signaling rates are generally offered by the common

carriers or by the vendors of equipment for use on common

carrier facilities in increments specified in the ANSI

standard X3.1 - 1969 (ref. 3) for synchronous transmission

and at lower speeds for asynchronous transmission. Line

quality is the concern of the common carriers. It affects

the user by the degree to which it reduces the transfer rate

by requiring the retransmission of information blocks or frames containing errors caused by poor line quality.

Control procedures (hardware and software protocol used to

transfer data and control information) are the concern of

the information processing system staff. The choice of the

control procedure to be used involves software and hardware

considerations, but also affects the transfer rate by the

amount of overhead bits that must be allocated for control

and for error detection and retransmission purposes. As

discussed in section 4.3, the undetected error rate may be

made arbitrarily small by increasing the number of bits used

for error detection, but at the cost of increasing the

overhead and reducing the transfer rate.

Propagation delay and line turnaround (if applicable)

are factors that affect the transfer rate and, with line

quality, affect the choice of block or frame length to be

used for an optimal transfer rate. (Block or frame length

will be limited by the nature of the usage of the system and

by the available storage for buffering.
Where sophisticated control procedures are employed for

routing or guaranteeing the integrity of data, measuring the

transfer rate may result in some differing opinions. To the

communications engineer, control procedures which are

implemented as headers and trailers on message blocks or frames are part of the data. However, to the applicationsoriented computer specialist, they are overhead. In a sophisticated computer network (such as, ARPANET) with many

levels of control protocol, all implemented in software,

even the computer specialists may not agree on the specific

criteria to be used to measure the effective transfer rate.

It is essential that a consistent interpretation be followed

when comparisons are made between services.

1.5 Typical Transfer Rate Requirements of Information

Processing Systems

Typical transfer rate experience with interactive

terminals on various computer networks are shown in the

table below, with all speeds shown in bits per second (ref.

4). The user's rates are limited by the user's think time

and typing speed and by the transfer rate of the

telecommunications facility. The system's rates are limited

by the speed of the terminal, by the transfer rate, and by

the ability of the computer to process data for output as rapidly as it can be accepted. (Signaling speed includes

the start bit, 1 or 2 stop bits and the parity bit, while

user and system rates include only the 7 information bits.)

Speed in Bits per Second

Average

Signaling of

Speed User System both

TYMNET (ref. 5) 110. 3-5 35.0

GE Information Services (ref. 5) 110. - 49.

300. - 147.

Jackson, Stubbs and Fuchs of

Bell Telephone Labs (ref. 6 & 7)

moderately loaded scientific 110. 3-4 61.6 25.2

heavily loaded scientific 110. 1.9 14.7 10.5

moderately loaded business 150. 5.6 58.1 28.0

It may be concluded from this data that terminals and

computer systems do not have the same transfer rate
requirements. People type much more slowly than computer

systems, so that an asymmetric communications link (with

respect to transfer rate) may be appropriate for interactive

computing.

Even on computer output, the average transfer rate did

not approach the maximum achievable. Since computer output

over a communication line usually does take place at the

maximum achievable rate, we may conclude that computer

output is rather bursty. That is, high data rate transfers

are separated by lengthy intervals when no data is

transferred. It would seem to be desirable to utilize some

form of multiplexing technique, in order to achieve higher

line utilization.

In view of these conclusions, transfer rate does not

appear to be the limiting variable for interactive computer

applications. However, it is true that interactive systems

in use today were designed for the rather low transfer rates

that were commonly available when they were designed.

Furthermore, interactive applications generally require much

higher burst transfer rates than average transfer rates. In

such systems, excess line capacity is frequently provided to

accommodate these burst throughput requirements during peak

periods.

For RJE (remote job entry) terminals, the transfer rates

needed are limited by the speed of the input/output devices

used. The following are typical maximum transfer rates:

Maximum transfer rate

in bits per second

Line printer:

120 characters per line

300 lines per minute 4200

Card reader:

80 characters per card

600 cards per minute

Card punch:

80 characters per card

91 cards per minute

5600

840

For corcputer-to-computer traffic, transfer rates

approaching channel speeds for local peripherals (such as,

magnetic tape units or rotating storage) are desirable.

Typical speeds for such devices are in the range of one million bps and up, though such transfers tend to be rather

bursty. Most existing computer-to-computer traffic operates

at much lower data rates, with 50 Kbps being the highest

rate commonly in use today.
1.6 Typical Transfer Rates Provided by Telecommunications

Facilities

Most common carrier offerings utilize analog circuits.

These consist of subvoice-grade channels (for teletype-speed

devices and other very low speed signaling), voice-grade

channels, and wideband channels. The sub voice-grade

channels provide signaling rates in the 30 to 150 bit per

second range. Voice-grade channels provide a nominal three

kilohertz channel that permits data signaling rates from 110

to 4800 bits per second (9600 if leased lines are used),

depending upon the type of modem used. Wideband group

channels currently permit signaling rates to 50 Kbps, with

higher data rates being technically feasible.

Recently, communications services based on digital

technology have been offered. These have signaling rates in

the 2.4 to 56 Kbps range. However, most computer data

transmission is still done via voice-grade, analog channels.

Transfer rates for the voice-grade dial network tend to

be somewhat lower than for comparable leased lines using the

same signaling rates. This is due to the higher error rates

caused by the increased impulse noise on the dial network

from the switching equipment and, in the case of some

modems, from the effect of variations in the amplitude and

delay distortion from one connection to the next.

The Bell System's 1969-70 Connection Survey (ref. 8)

contains data on several hundred dialed (voice-grade)

telephone connections between numerous cities at data

signaling rates of from 150 bps to 4800 bps. These are used

in the NBS Technical Note 779 (ref. 9) to derive transfer

rates using the ANSI X3.44 - 1974 term, TRIB, and the Basic

Mode Control procedure of X3.28 - 1971 for signaling rates

from 1200 to 4800 bps. (At the time of the Connection

Survey, 4800 bps was the practical upper limit for data

communication on the dial network, while up to 9600 bps was

possible using conditioned leased lines.) For a near

optimal block size of 10,000 bits, the following TRIB values

were found:

TRIB

90th percentile

750

1550

2500

2700

Signaling Rate TRIB

bps 50th percentile

1200 800

2000 1600

3600 2800

4800 3600
In a packet-switched computer network (such as the

ARPANET), the transfer rate for actual user data is likely

to be significantly less than the signaling rate for the

backbone communications system. In the ARPANET, for

example, the internode signaling rate is 50 Kbps. This is

degraded to an effective rate of about 40 Kbps by the

overhead of the internode control procedures, but

experiments have shown that rates as high as 60 Kbps can be

achieved because of the opportunity for the parallel

transfer of data through alternative routing (ref. 37).

However, for bulk file transfers run using the file transfer

protocol, rates in the range of 4 to 10 Kbps were

experienced by NBS users. These lower rates might be

attributed to end-to-end protocol overhead within the

communications subnet, end-to-end protocol overhead between

hosts, the inability of the heavily loaded hosts to keep up

with higher data rates, or other factors.

2. AVAILABILITY

2.Definition

The portion of a selected time interval during which the

telecommunications facility is capable of performing its

assigned function.

2.2 Other Definitions

Availability is defined in ANSI X3.44 - 1974 (for data

communication systems using the X3.28 - 1971 characteroriented control procedure) as "The portion of a selected

time interval during which the Information Path is capable

of performing its assigned data communication function.

Availability is expressed as a percentage."

Availability is defined by the Bell System to be the

percentage of a long period of time that a channel is

operating within its transmission parameter limits (ref.

10).

2.3 Significance of Availability

It is not possible to guarantee 100 percent availability

of the telecommunications facility. Thus, an information

processing system may lose part or all of its

telecommunications service. The significance of this varies

with system usage. For some real-time systems, the loss of

even part of the telecommunications facility is quite

serious. For others, it may be merely an inconvenience,

especially if local computing work may be done in the

interim.
2.4 Discussion

There are various reasons why a telecommunications

facility may be unavailable to the user. Generally . these

concern equipment failures or an overloading of the

facility. However, the concept, of availability can also

refer to non-temporal factors, such as, geographic coverage,

as in the case of some of the newer common carrier

offerings, which are initially available only in selected

cities. Availability will also be affected if the

telecommunications facility is not scheduled for use during

certain hours of the day or days of the week.

The users of the switched telephone network may be

unable to complete calls due to trunk busy conditions,

transmission outages or failures in the switching equipment.

Trunk busy conditions occur during times of heavy business

usage of the facility, when it may require numerous attempts

to establish the desired connection. Transmission outages

and failures in the switching equipment may increase the

probability of receiving a trunk busy indication, but do not

normally prevent all use of the facility, due to the

existence of alternative routings that may be automatically

used by the telecommunications facility. Total

unavailability of the user's lines is relatively rare due to

the passive "nature of the local user loops and to the use of

alternative power sources by most common carriers during

power black outs.

Leased line users are not concerned with trunk busy

problems, but do have a very real problem with circuit

failures. Since their circuits are dedicated, the failure of

an electronic component anywhere in the circuit is likely to

cause total unavailability of the circuit. The repair or substitution of alternative circuits may require a few

hours, so users with the need for a high availability will

usually provide access to the switched network or some

alternative channel. In the case of using the switched

network, there may be some degradation of the performance

due to the somewhat lower signaling rates possible on the

switched network. Also, since leased and switched lines

would normally have local loops in the same multi-wire

cable, a local loop problem might well affect both services.

Since modern computer communications systems employ

components beyond the basic communications circuits provided

by the common carriers, there are more sources of problems.

Failure of the node computer (switching computer), the

interface to the computer, or a software failure in a host

computer, in addition to simple line failure, may make the

system unavailable. Also, failures can be either hardware

or software in nature, thus complicating the problem of

diagnosis and repair.

The availability characteristics of modern packet

switching systems is strongly affected by the way in which

flow control mechanisms are implemented. Packet switching
systems commonly refuse to accept input at times in order to

prevent overload, thus denying service to some users while

others are being serviced. Such refusals may be relatively

short term, such as the refusal to accept data for an already established conversations until buffers are cleared,

or relatively longer term, such as could result from

restrictions on the number of (virtual) connections

permitted between particular points on the network.

The principal approach to be employed in planned data

networks for maintaining the availability of the complex

system (with many different components, all subject to

individual failure) is redundancy. A partial degree of

redundancy exists in some current networks, which provide

alternate routing of messages. However, this only protects

against the failure of one part of the system: the

communication circuit itself. Planned networks are including

redundant node computers and redundant links from the nodes

_to the host computers in order to provide full redundancy

for the entire path from originator to recipient.

The concept of partial availability may be applied to

large networks, since failures may occur which prevent

certain communications paths from being used, while others

are not affected at all.

2.5 Typical Availability Requirements of Information

Processing Systems

The National* Library of Medicine (ref. 11) specified an availability of no worse than 95 percent non-busy during

peak hours of operation as the requirement for their

bibliographic retrieval system. Since the availability of

computers is, typically, in the range of from 0.95 to 0.99,

this is a fairly typical requirement. However, for some

uses this availability is not adequate. Applications, such

as, airline reservations or stock brokerage order, entry

simply cannot tolerate system unavailability. Such

installations often use duplexed computers with an availability thai? may be as high as 0.996. In these cases,

the availability of the telecommunications plant should, if

possible, be raised to a similiarly high value.

2.6 Typical Availability Figures for Telecommunications

Facilities

Relatively little has been published regarding the

availability of voice-grade leased lines. However,

customer-reported outages provide a source of relevant

information. It is not possible to determine from these

figures how long the line was out prior to the customer

observing the outage and reporting it to the telephone

company. Also, short outages are less likely to be reported

than long outages. Intrastate and local lines are likely to

have different outage characteristics than interstate lines.

Loss of availability due to transmission parameters being

Q
out of limits is likely to be more of a problem at the

higher signaling rates.

The Bell System reports the following customer-reported

outages for l'eased two-point interstate data service. Since

the service includes the use of Bell-supplied modems, the

availability of the leased circuits themselves is somewhat

higher than the figures shown. 72 percent per month with no reported outage; the average number of reported outages per

channel per month is . 0.56; the average outage time per

channel per month is 68 minutes; where at least one outage

does occur, the average outage time per channel per month is

241 minutes; and the percentage of outages exceeding 2.0

hours is 23 percent (ref. 10).

The probability of users of the switched telephone

network not receiving a trunk busy signal during the peak

busy hour has been estimated at 0.94, with 90 percent

confidence intervals of plus or minus 0.02 (ref. 12).

AT & T cites a preliminary design objective of an average of at least 99-96 percent availability for its new

Dataphone Digital Service (ref. 13).

Datran (Data Transmission Company) cites an average

availability of all customers' circuits on their new digital

data network of 0.9879 for the time interval from April 1,

1974 to July 24, 1974. Since most of the outages involved

subscriber loop problems that were being corrected, the

availability was said to be improving drastically (ref. 14).

Datran claims that the availability of the high speed

channels over the "backbone" of their network (thus,

excluding customer loops which are often leased from other

carriers) is 0.9998 (ref. 15).

Early availability statistics for the ARPANET (ref. 16)

showed backbone circuit outages that had a weighted average

of 1.64 percent down time (98.36 percent availability) for

the calendar year 1971, though individual lines varied from

zero to 29-43 percent. Statistics for June 1972 through

December 1972 for the switching computers (IMPS) showed that

these processors were out an average of 1.39 percent of the

time due to hardware or software failures and 2.42 percent

overall (includes preventative maintenance, site

environmental problems, etc.). Again, the variability of the

performance of individual machines was large, ranging from

zero to 8.69 percent for hardware and software failures and

from zero to 9.37 percent overall. Mean time between failure

(MTBF) was 228 hours overall, and 411 hours for

hardware/sof tware failures. Mean time to repair (MTTR) was 5

hours 29 minutes overall and 5 hours 41 minutes for

hardware/software failures.

Recent availability statistics for the ARPANET (ref. 17)

show a significant improvement. For the first 8 months of

1974, circuit outages had an average of O.38 percent (99.62

percent availability). For the same time period, the

10
switching computers were out an average of 0.80 percent due

to hardware and software failures and 1.33 percent overall . The MTBF was 403 hours and the MTTH was 3-02 hours for the

hardware and software failures. While these data are not

necessarily valid for other computer networks and the level

of availability is achieved at a relatively high cost, they

are indicative of what can be done. The improvements in

ARPANET'S performance are indicative of similar improvements

with new systems which take some time to reach their best

level of performance.

3. RELIABILITY

3.

Definition

The ability to maintain the use of the

telecommunications facility until the information transfer

has been successfully completed.

3.2 Other Definitions

Reliability is defined in ANSI X3.12 - 1970 (ref. 1-8) as "The probability that a device will perform without failure

for a specified time period or amount of usage."

3-3 Significance of Reliability

The concept of reliability is somewhat similar to that

of availability. The failure of any component which

prevents messages from being sent from an originator to a recipient affects availability if the system is not in use

at the time of the failure and reliability if it is in use. The effect of the failure, however, is the same in either

case

Reliability is distinct from availability, however, in

that it describes the performance of a system after it has

accepted traffic for delivery. A system which refuses input

due to malfunctions may have poor availability

characteristics, but a quite acceptable level of

reliability.

The significance of the reliability of the

telecommunications facility to the information processing

system is related to the purpose for which the system is

used and to the system's own reliability. For some uses, the

failure of the telecommunications facility during the

information transfer process is a fairly minor

inconvenience, provided that the facility is able to re- establish the communication. (If this failure is due to a condition that prevents further use of the

telecommunications facility, then the situation also becomes

a matter of availability.

However, for applications having a time urgency, for

lengthy transactions requiring the user to repeat work

already done, or for frequent failures, poor reliability

11
is quite significant. To some extent, system design can compensate for the effects of poor reliability, but the

effect of such failures can be very significant if they are

a common occurance.

Poor reliability can also affect the cost of using a particular service. A high failure rate implies high

maintenance costs, which, directly or indirectly, must be

paid by the user.

3.4 Discussion

There are various reasons why telecommunications

facilities have less than perfect reliability. The causes

may be grouped into two classes: 1) those involving the

temporary disruption of the information transfer process,

and 2) those involving a longer disruption of service, but

where a readily available alternative telecommunications

facility may be used. (There is also the case where there

is a long disruption of service without readily available

alternative service, but this becomes more a matter of

availability.) Generally, reliability is much less of a problem for most users than availability.

Thus, the approach taken in large networks with respect

to reliability is the same as that for availability:

redundancy of components to assure successful operation.

Large networks also generally provide sophisticated

diagnostic capabilities and repair procedures in order to

minimize the mean time to repair (MTTR).

3.5 Typical Reliability Requirements of Information

Processing Systems

There is no simple numerical expression which can be

given for the reliability requirements of information

processing systems, as systems vary widely in their response

to interruptions of service due to failure of the

telecommunications facility. Some systems suspend the

disconnected program for a fixed or variable length of time

and permit the user to resume processing when communications

have been restored. At the other extreme, some systems

simply abort the job, causing the processing, and possibly

even the files, to be lost.

Frequently, ...the applications on the information

processing system will dictate the reliability requirements.

For each particular application, it might be possible to

determine, a numerical requirement based on the length of

time to complete a transaction. Applications, such as,

stock brokerage or airlines reservations, which are

inherently real-time, will have much more stringent

requirements than, for example, interactive program

debugging.

12
3.6 Typical Reliability Figures for Telecommunications

Facilities

For the dial telephone network the probability of being

able to complete a ten minute call without being erroneously

disconnected by the switching network is estimated to be

better than 0.998 (ref. 12). The reliability of switched

or leased telecommunications channels tends to vary with the

length and routing of the circuit through the

telecommunications facility, with reliability being reduced

somewhat as the number of circuit components is increased.

4. ACCURACY

4.1 Definition

The correctness and completeness of the information

accepted by the receiving terminal.

4.2 Other Definitions

Accuracy is defined in ANSI X3.44 - 1974 (for data

communication systems using the X3.28 - 1971 characteroriented control procedure) in terms of Residual Error Rate

(RER). RER is defined as "the ratio of the sum of (1)

erroneous Information Characters accepted by the receiving

Terminal Configuration and (2) Information Characters

transmitted by the sending Terminal Configuration but not

delivered to the receiving Terminal Configuration, to the

total number of Information Characters contained in the

source data. Within (1) above, erroneous Information

Characters may 'be either undetected, or detected but

uncorrected, within the Information Path." (The capitalized

terms have specific meanings that are defined in the

standard.

This is computed:

RER = (Ce. + Cu + Cd) divided by Ct, where

Ce = Erroneous Information Characters accepted by the

receiving Terminal Configuration.

Cu = Information Characters transmitted and assumed

by the sending Terminal Configuration to be

accepted by the receiving Terminal Configuration

(undelivered)

Cd = Information Characters accepted in. duplicate by the

receiving Terminal Configuration which were not

intended for duplication (duplicates).

Ct = Information Characters contained in the source data.

Accuracy is defined in the Office of Telecommunications

report OTR 75-54 "Performance Criteria for Digital Data

Networks" (ref. 19) as having at least four accuracy

13
performance parameters. These are designated as bit error

ratio, false bit ratio, erasure bit ratio and duplicate bit

ratio. These are defined in that report as:

Bit Error Ratio - the ratio of the number of incorrect bits

in non-duplicated messages, accepted by the user at a correct destination, to the total number of user originated

information bits.

False Bit Ratio - the ratio of the number of correct and

incorrect bits accepted by the user at incorrect

destinations, to the total number of user originated

information bits.

Erasure Bit Ratio - the ratio of the number of bits which

are not accepted by the user at a correct destination to the

total number of user originated information bits.

Duplicate Bit Ratio - the ratio of the number of correct and

incorrect but duplicated bits, accepted* by the user at a correct destination, to the total number of user originated

information bits.

4.3 Significance of Accuracy

The significance of undetected errors varies greatly

with the usage of the information processing system. For

example, systems used for banking and electronic funds

transfer require undetected 'error rates vastly better than

thdse required by interactive terminals used for

bibliographic retrieval, where the high redundancy of the

English language reduces the significance of errors.

However, inaccurate systems tend to have reduced transfer

rates, due to the need to retransmit information which is

received with detected errors.

4.4 Discussion

Errors on telecommunications circuits tend to be in

error bursts or groups, rather than being randomly

distributed, since the causes for the errors tend to be

conditions that are present for a time period covering the

transmission of several bits. For example, impulse noise

often occurs as a series of noise spikes of several

milliseconds duration each. This grouping of errors tends

to reduce the effectiveness of errpr detection codes below

that for randomly occurring errors.

Early control procedures up through the Basic Mode

Control Procedure of X3.28 - 1971 and the IBM Bisync

procedure use a combination of vertical and longitudinal

parity checking that is reasonably effective for most users.

More recently, polynomial error detection codes have

been developed that have much lower undetected error rates

and a reduced number of overhead bits required for the error

detection process. These are used with the newer control

14
procedures and are placed* at the end of the transmission

block. It is possible to reduce the undetected error rate

to any arbitrarily low value by using codes of greater

leng,th. While this is true for other types of codes as well, polynomial codes are particularly efficient in this

regard.

4.5 Typical Accuracy Requirements of Information

Processing Systems

The accuracy requirements of information processing

systems are often not precisely known. The system may have

programs for widely different uses, each of which has its

own inherent accuracy requirements. fSven the individual

user may not be able to assess the value of accuracy or the

cost of inaccuracy. Also, the effect of undetected errors

may involve intangibles, such as, customer dissatisfaction.

In a study done for the U. S. Postal Service, the consultant

recommended a desired bit error probability of one in 10**12

(ref. 20). In general, the error detection codes used, are

sufficiently effective relative to the error rates

encountered on most telecommunications facilities that the

accuracy requirements of most users are satisfied.

4.6 Typical Accuracy Figures for Telecommunications

Facilities

Accuracy using vertical and longitudinal parity checking

is dependent on the character size (bits per vertical parity

bit), block length (bits per longitudinal parity bit) and

the error burst characteristics of the telecommunications

circuit used. For an error to be undetected, there must be an even number of error bits in each character and an even

number of error bits in each bit position of the characters

(per longitudinal parity bit). In the simplest case, there

would be the same two bits in error in each of two

characters.

Due to the error burst tendencies of most

telecommunications circuits, it is difficult to obtain good

estimates of the undetected error rate using vertical and

longitudinal parity checking. According to Martin (ref.

21), the residual error rate on switched telephone

facilities having an error rate of one in 10**5 (in this

report double asterisks denote exponentiation) might be in

the range of one bit error in 10**7 to one in 10**9 bits.

Since there is one parity bit for every seven information

bits plus a parity bit in every track, the efficiency of

this error detection process is less than 0.875.

With the polynomial error detection codes, both the

undetected error rate and the efficiency can be improved.

The undetected error rate obtained with a polynomial code is

dependent on the length of the code, with the longer codes

providing vastly better detection capabilities, and on the

error burst characteristics of the telecommunications

circuit used. The detection capabilities of polynomial

15
codes are defined by Peterson (ref. 22 and 23). The

efficiency of polynomial codes is quite good. A 16 bit code

used with a block length of 1000 bits has an efficiency of

98.4 percent, while providing very effective checking of the

data.

A properly chosen polynomial code will detect 100

percent of: single bits in error, two bits in error, an odd

number of bits in error, and all error bursts equal to or less than the number of code bits. It will also detect a very high percentage of longer bursts up to the length of

the code. The length of a polynomial code is defined to be

2**(m-1), where m is the number of code bits used.

Using theorem 8.3 (page 152) of Peterson's book (ref.

22), a 16 bit polynomial code is shown to be capable of

detecting all errors due to error bursts less than 17 bits

long and to have an undetected error probability of 2**-15

for 17 bit error bursts and 2**-l6 for bursts that are more

than 17 bits long, up to the code length of 32,768 bits.

(Other considerations limit block lengths to much less than

32,000 bits.)

To relate this to a typical system application, error

burst statistics are needed. Using the Bell System survey

for dial telephone connections (ref. 8), the 50th percentile

connection for a 2000 bits per second modem was found to

have an error burst rate of about 5. x 10**-7. Using the

Burst Length Distributions chart from the same report, about

one percent of the error bursts were found to be 17 bits

long and about 14 percent were found to be longer than 17

bits.

Thus, the theoretical probability of an undetected error

in this particular situation would be about:

U = (5. x 10**-7)(0.01 x 3.1 + 0.14 x 1.5)(10**-5)

or about one bit in 10**12. At 2000 bits per second this

would mean more than 138,000 hours (15.7 years) between

undetected errors.

Packet switched networks with much greater transmission

rates tend to use somewhat longer polynomial error detection

codes to achieve even greater accuracies. For example, the

ARPANET uses a 24-bit polynomial code, for which an undetected bit error rate of one in 10**12 is claimed, or about one undetected error per year in the network (ref.

35).

The Bell System supports a leased line minimum performance

of one 1000-bit block in error per 100 blocks transmitted

when certain Bell synchronous modems are used and when some

other specified conditions are met. (Support indicates a performance goal of the Bell System that is not included in

the tariff, as being legally obligatory.) This applies to the

Bell 201 A modem at a signaling rate of 2000 bps; the Bell

16
201C at 2400 bps; and the Bell 208A at 4800 bps. The Bell

System also supports a minimum performance of 1 bit error

per 100,000 bits transmitted when certain Bell asynchronous

modems are used and some other specified conditions are met.

This applies to the Bell 202C, D, E and R modems at 1200,

1400 and 1 800 bps for basic, CI and C2 channel conditioning,

respectively. At least 80 percent of all leased lines

should meet these performance goals (ref. 10).

The digital data offerings of the common carriers have

defined accuracy in terms of average error-free seconds.

Datran guarantees 99.95 percent average error-free seconds

during customer transmissions.

Datran also cites an error performance measured on a circuit from Dallas, Texas to St. Louis, Missouri for a six

day period in July 1974 as providing an "extrapolated

probability of error free seconds" of 99.98 percent. They

state that this level of service will be provided on switched circuits, since there should be virtually no additional errors introduced by the switch itself (ref. 14).

AT & T cites a preliminary design objective of 99.5

percent error-free seconds at 56 Kbps and better performance

at the lower rates of 9.6, 4.8 and 2.4 Kbps for their

Dataphone Digital Service (ref. 13).

5. CHANNEL ESTABLISHMENT TIME

5.1 Definition

The time required for a switched telecommunications

facility to provide a circuit connection between a calling

terminal and a called terminal.

5.2 Other Definitions

Channel establishment time is contained within the

Transfer Overhead Time (TOT) terra of ANSI X3.44 - 1974 for

data communication systems using the X3.28 - 1971 characteroriented control procedure. Transfer Overhead Time is

defined as "A measure of the individual communication phase

times required to establish and dis-establish an Information

Path in order to effect an information transfer."

17
5.3 Significance of Channel Establishment Time

Channel establishment time includes both the time to

operate , the dialing mechanism and the time for the network

to complete the connection. This time is probably not too

significant to most people who are using the "network for its

intended purpose of making voice calls. Remote terminal

users that have lengthy dialogues with the computer may not

find this time too significant either, especially if the

sign-on procedure is lengthy. However, a terminal user who

must make a great many brief accesses to the computer each

day could find the channel establishment time to be quite an annoyance.

In the case of computer-initiated calls (using an automatic calling unit), the several seconds required for

switching time, does impose a quite significant delay before

the computer can begin its transaction with the remote

terminal. While the computer is not idle during* this time,

it may be a significant time factor to the system. In

general, if this time is commensurate with the time required

for the data transfer to take place, then there is at least

the implication of a significant time delay.

For some information processing applications, the

channel establishment time and its fixed cost is the most

significant factor in the evaluation of a communications

system. Applications that must perform continuous polling

of remote terminals tend to be used with leased

communication lines that are routed to each terminal,

thereby reducing the channel establishment time to that

required for modem re-equalization. However, the

availability of switched telecommunications service with

minimal channel establishment time and common carrier

charges computed on a measured time basis with a minimum

time of no more than a few seconds (e.g. Datran's Datadial

service) will probably cause many users to change from

leased line service to switched service.

5. "4 Discussion

The concept of channel establishment time as a one-time

overhead delay to set up a transmission path for the

duration of the communication does not apply for the new

data networks based on packet switching technology. In these

cases, all messages are delivered with equal average

rapidity from sender to receiver. In one view, there is no

channel establishment delay; in the other view, this delay

is imposed on every communication between sender and

receiver, not just the first. However, the delay, as measured for the physical task of establishing a communication path, is considerably less for packet switched

networks than for the present circuit switched networks.

Note, however, that for networks of computers there may

be a different sort of channel establishment delay

introduced by the software in the receiving computer which

18
must condition that computer to communicate according to

specifications indicated by the sender. This process of

establishing communications on mutually agreeable terms may

take several data exchanges in each direction before all

procedural matters are resolved.

5.5 Typical Channel Establishment Time Requirements

of Information Processing Systems

While not generally stated in system specifications, a terminal user should not have to wait for more than several

seconds for the channel to be established.

5.6 Typical Channel Establishment Times Provided by

Telecommunications Facilities

For telephone calls to subscribers served by the same

telephone end office, the channel establishment time is

determined by three factors: 1) the number of digits

required; 2) the type of dialing mechanism (rotary or multitone pushbutton); and 3) the telephone end office switching

machine type. For a typical requirement of 7 digits, rotary

dialing time is about 10 seconds and multi-tone pushbuttons

about 4 seconds. An automatic (computer-controlled) calling

unit needs about the same time, as the fixed dial tone time

delay tends to cancel any speed advantage in the dialing

process. Telephone office switching time is generally 2

seconds or less.

For telephone calls to subscribers not served by the

same end office, the channel establishment time includes

about 50 percent more dialing time (if an area code is

required) and the time for switching between the various

telephone offices and centers. The telephone network is a structured hierarchy with 5 ranks or classes of switching

centers. The ranks proceed from end office through toll

center, primary center, and sectional center to regional

center. There are numerous trunks between offices and

centers. Calls entering the network are always routed over

the most direct available cross trunk to the destination.

However, it is possible for a call to be routed up through

the hierarchy to the regional office, across to the regional

office for the destination, and down through its hierarchy.

In this case, about 25 seconds would be added to the channel

establishment time. This is a rare occurrence even for

transcontinental calls (ref. 24).

Long distance calls typically have 3 to 5 trunks. The

average connect time for calls up to 180 miles is 11.1

seconds; for 180 to 725 miles it is 15.6 seconds; and for

725 miles to 2900 miles it is 17.6 seconds. (The standard

deviations are 4.6, 5.0 and 6.6 seconds, respectively. Ref.

12.)

Datran cites a switched network response time of less

than 3 seconds 99 percent of the time and a mean response

time of less than 1 second (ref. 14).

19
6. NETWORK DELAY

6.

Definition

The time required for information to travel through the

telecommunications channel to the receiving terminal or computer.

6.2 Other Definitions

The ANSI X3S3 Task Group 5 is considering the following

definition: Message Transfer Time (MTT) "is the time

required for a message to be transmitted from a source and

accepted at the designated sink".

6.3 Significance of Network Delay

Depending upon the capabilities of the terminal

facilities (particularly with respect to buffer capacity),

the application's information interchange requirements

(volume), the response characteristics (real-time or other),

the mode of channel operation (half or full-duplex), and the

reliability of the channel, the delay of the channel can be

of varying degrees of significance. For example, an interactive application involving short bursts of

conversational information interchange, operated in echoplex mode, (full-duplex with echo checking of the input)

requires a minimum network delay. Otherwise, the operator's

striking of the keys and the data input process must be

retarded by the character echo response time. Similarly,

large network delays can seriously impede the information

interchange process when substantial quantities of data must

be transferred in a half-duplex channel. In the case of very

long delays, the system may have to provide buffer storage

for two or more blocks of information, as the negative

acknowledgement may not be received until one or more

additional blocks have been sent.

6.4 Discussion

Telephone circuit delays are usually not very

significant. However, when satellite links are used, the

network delay becomes much longer and can be a problem. In

the case of full duplex- echoing, a delay of a few hundred

milliseconds can be quite annoying for terminal users. In

the case of half duplex operation, long network delays will

seriously reduce the transfer rate with the shorter block

lengths (due to the greatly increased time required for

acknowledgements) and will tend to force the system to use a longer block length than would otherwise be optimal. Even

with optimally chosen block lengths, propagation delay tends

to reduce the transfer rate somewhat, as the longer block

lengths will increase the probability of a block containing

an error, thereby, increasing the percentage of blocks that

must be retransmitted.

20
Message and packet switched communication systems

introduce additional network delay because of the time spent

processing the message or packet at each node through which

it passes. In the case of a packet-switched system, which

breaks up the longer messages into packets for transmission

through the network and then re-assembles them prior to

delivery, any time spent in message dis-assembly/re-'assembly

(including time spent waiting for constituent packets at the

re-assembly point) is part of the network delay. Packetswitched systems with adaptive routing are affected not only

by the network delay, but by variations in that delay, as the re-assembly of the packets is affected by . such

variations. This may become critical in extreme cases.

Thus it may be necessary to specify the standard deviation

of the network delay in these cases.

If the definition of network delay suggested above is to

be used for computer networks, the time interval of concern

must be carefully identified. Is it from the time of

transmission for the first character of the input to the

receipt of that character on the output, or is the last

character a better starting point? For packet switched

systems, the first-to-first measure might be unrealistic, if

intermediate packets in a message were subject to internal

delays. Measuring from last to last may not be fair either.

If the input to the system was large and quite bursty, the

system may have had the opportunity to forward the first

parts of the message before the last parts were entered.

Thus, it is necessary to compensate for irregularities in

entering data, perhaps by taking an average of the two types

of measures. It may also be desirable to adjust for the

delays in data entry or exit, as these may or may not be

considered to be a part of the network delay.

6.5 Typical Network Delay Requirements of

Information Processing Systems

The bibliographic retrieval system of the National

Library of Medicine (ref. 11) specified the round trip

network delay as not being in excess of 2.0 seconds. (This

requirement was due to the terminal user's need for a reasonable response time.)

For applications requiring echoing of the input data,

the overall response (network delay in both directions, line

turnaround delay and computer response) should not exceed

0.1 to 0.2 seconds. For applications involving users engaged

in high-intensity "brainstorming", requiring the- ready

access of data from his own "short-term memory", the

response delay should not exceed 2 seconds. However, some

uses may be able to tolerate response delays of up to 15

seconds or more (ref. 25).

The goal for the ARPANET (ref. 26) was a maximum round

trip delay of 0.5 seconds for the transmission of a 1000-bit

packet

21
6.6 Typical Network Delays Provided by

Telecommunications Facilities

Terrestrial telephone circuit network delays are

typically in the range of from 2 to 15 milliseconds. When

satellite circuits are used the delays are typically 300

milliseconds long. ARPANET network delays vary with the

queueing delays at each node computer and with the number of

node computers through which the messages must pass. Delays

are in the 20 to 120 millisecond range (ref. 26).

7. LINE TURNAROUND DELAY

7.1 Definition

The time delay required by half-duplex circuits to

reverse the direction of transmission.

7.2 Significance

The line turnaround delay reduces the transfer rate of

half-duplex circuits by increasing the time required for

acknowledgements. It may necessitate a longer block length

than would otherwise be optimal, since turnaround delay,

being a fixed constant, is less significant for longer block

lengths. (This delay does not apply to full-duplex circuits,

since telecommunications channels are permanently

established in both directions.)

7.3 Discussion

Leased telephone circuits are usually available as fourwire, full -duplex circuits. Dial telephone circuits are

normally two-wire, unless the user is willing to double the

line costs by using two telephone circuits. At the lower

signaling rates, the modems usually are designed for two-way

simultaneous operation by using half of the available

bandwidth for transmission in each direction. However, at

the higher signaling rates this is not possible and most

high speed modems used on the dial network use two-way

alternate operation. (There are some modems that have a very

slow speed reverse channel by using a small portion of the

bandwidth, but they have their own performance problem with

the time required for sending the acknowledgement message.)

In two-way alternate operation the echo suppressors must

either be disabled at all times or they must be reversed

each time that the direction of transmission is reversed.

When echo suppressors are not used, time must be allowed for

echos to subside before transmitting in the reverse

direction.

22
7.4 Typical Line Turnaround Delay Requirements of

Information Processing Systems

Line turnaround delay is more ' significant for short

block length transmissions than for longer block lengths.

For systems operating asynchronously in "echo-plex" mode,

where transmission is a character at a time and all

characters are echoed for verification and/or printing, very

short turnaround delays are essential. For longer block

lengths, somewhat longer turnaround delays are tolerable.

The turnaround delay requirements will also be closely

coupled to the propagation delay requirements described in

section 6.5.

7.5 Typical Line Turnaround Delays Provided by

Telecommunications Facilities

The time required for reversing the echo suppressors

varies somewhat, so modems that do not disable the echo

suppressors should be adjusted for the longest reversal

delay that they are likely to encounter. A time of 150

milliseconds is usually considered to be safe. On short

distance circuits, the common carrier may omit the echo

suppressors. In this case, a delay of 50 milliseconds is

generally adequate for the echos to subside.

Modems that disable the echo supressors may still have

substantial line turnaround delay, as high speed modems also

have a minimum turnaround delay required for re-training the

equalizer. Modern modems typically need 40 to 50

milliseconds to this.

Systems that cannot tolerate these turnaround times can generally specify full-duplex circuits, thereby, avoiding

the delay altogether. Modern packet networks, for example,

all specify full-duplex circuits.

8. TRANSPARENCY

8.1 Definition

The absence of code or procedural constraints imposed by

the telecommunication facility.

8.2 Other Definitions

The IBM Corporation in manual GA27-3093-0, "IBM

Synchronous Data Link Control, General Information", defines

transparency as follows: "In data communications, the

quality of information data when there is no possibility of

interference with data link control, regardless of format or content. The transparent information is unrecognized by data

link controls."

23
8.3 Significance of TransparencyTransparency is a qualitative term that refers to the

absence of code or procedural constraints being imposed on the information processing system. Its significance is

relative to the degree that system changes or user education

are required to compensate for lack of transparency by the

telecommunications plant.

8.4 Discussion

Code transparency may be impaired by the modem's

inability to transmit long strings of ones or long strings

of zeros. It may also be impaired if certain bit code

combinations are not permitted. For example, if the Basic

Mode Control Procedure of X3-28 - 1971 is used, then an EOT

(end of transmission) code cannot be used within the text of

a message, as it may cause a disconnect.

In the case where the link control procedure is part of

the telecommunications service provided, additional

transparency problems exist. Frequently, control procedure

transparency is achieved by modifying the user's text before

and after transmission in order to remove any illegal bit

sequences. This approach is taken both with the characteroriented and the newer bit-oriented link control procedures,

and may be accomplished either in software or hardware. The

result of the modification, however, is a longer message

than was originally specified to be sent. In a worst case

situation the message actually transmitted might be nearly

twice as long as the originally specified message. Thus,

poor transparency characteristics for a control procedure

can be overcome, but at the expense of some reduction in the

transfer rate.

Transparency is a complex concept when applied to a network of computers. Packet networks undertake to deliver

data as supplied to them; in that sense the network is

transparent, since the data put in is delivered without

examination or alteration. At a higher level, computer

networks impose severe procedural restrictions on their

usage. Protocol conventions are usually established for both

terminal users and computer programs that communicate via

the network. Thus, present computer networks would have to

be judged to be quite opaque.

24
8.5 Typical Transparency Requirements of

Information Processing Systems

The bibliographic retrieval system of the National

Library of Medicine (ref. 11) specified that "The proposed

configuration must in no way require that NLM applications

programs be redesigned to accommodate any peculiarities of

such configurations. The configuration must be transparent

to all transmissions between the user and the NLM and SDC

(System Development Corp.) computers once the link has been

established.

8.6 Typical Transparency Characteristics of

Telecommunications Facilities

Code transparency is not a problem with modern modems.

The transparency characteristics of computer networks were

discussed in 8.4

9. SECURITY

9.1 Definition

The ability to protect the information being transferred

by the telecommunications facility from deliberate or inadvertent disclosure, modification or loss.

9.2 Significance of Security

The importance of security in an information processing

system is dependent directly upon the usage of the system.

For some government and military systems, security is

considered to be an extremely significant factor. For many

commercial systems, the potential threat of industrial

espionage or malicious damage is taken very seriously.

9.3 Discussion

Protection of information involves the entire system.

This includes the computer, the telecommunications facility,

the terminal, the terminal user's authorization to use the

system and the protection of user's files from each other.

The telecommunications facility is vulnerable to the

unauthorized disclosure of information being transmitted

through it. For this reason, users who feel that they have

security needs are likely to seek methods of improving

telecommunications security.

The most effective method of improving

telecommunications security is encryption. Encryption is the

substitution of other characters or bits for the data

characters or bits being transmitted, or the transposition

of the data characters or bits within the message for

purposes of secrecy. This is done according to a predetermined method, thus permitting the receiving end to

25
decrypt the characters or bits back to the original data.

Most encryption methods use a "key", which consists of

several characters or bits used to specify the substitution

or transposition process to be used. Security is achieved

by the secrecy of either the encryption method or the key

used.

Packet switched networks may be viewed as providing some

minor degree of security by virtue of the alternative

routing of successive packets of a message. Thus,

eavesdropping on any single .circuit may not provide access

to the complete message. Encryption techniques may also be

used in such networks, both on an end-to-end and link-bylink basis. If end-to-end encryption is used, the message

will also be protected even while it is in the switching

computers.

Packet and message switching systems can also implement

"closed user groups" as an aid to overall security. This

facility defines a logical network between a particular

group of systems and prevents unauthorized user modes from

even establishing connections with those systems.

9.4 Security Requirements of Information Processing

Systems

Most systems do not yet impose security requirements on the telecommunications facility. Systems that are felt to

have such needs normally achieve the necessary security by

means of physical security for the facility and encryption

of the data passing through the facility. The National

Bureau of Standards has published an encryption algorithm

that is being proposed as a Federal Information Processing

Standard (ref. 36).

9.5 Security Characteristics Provided by

Telecommunications Facilities

Wiretapping of telephone facilities is difficult for

outsiders, except where the wiring enters the user's

premises. Wiring within the user's premises may be protected

by physical means and by periodic inspections. Further

protection may be obtained by the use of some form of

burglar alarm, possibly monitored by the computer. The

complexity of the cable wiring outside of the user's

premises and the use of multiplexing equipment within the

telephone company premises are generally sufficient to make

wiretapping unlikely, unless there is collusion with

telephone company personnel (ref. 27).

Packet networks, such as the ARPANET, may be criticized

on the grounds that the switching computers reside on customer premises and, thus, could be tampered with by

unauthorized personnel. Planned commercial packet switching

systems do not intend to operate in this manner, but rather

will locate the switching computers off customer premises.
CONCLUSIONS

This report has identified and discussed nine parameters

to be used in the evaluation and comparison of alternative

data - communications services which are used by information

processing systems. While this list of parameters • is not

exhaustive, it is felt to represent the key factors.

The number of alternatives available today for data

communications is not small, and it is growing as more and

more specialized carriers begin to offer service. Many of

these service offerings are quite different, ranging from

"raw" communications, as in the leasing of a. circuit, up

through "packaged" or "value-added" service, where the

vendor undertakes to provide an improved communications

service above mere circuit capacity.

The parameters which have been described enable a prospective user to compare these diverse offerings.

Differences between services will show up in the values of

the various parameters, and the user will be able to

identify what is being purchased in terms of specific

performance characteristics. For example, the difference in

price between circuit leasing and value-added packetswitched service should show up in improved availability,

accuracy and channel establishment time. With differences

identified, the user will be able to decide, along with' the

price of the each of the different offerings, which service

is best suited for the particular application.

The task of selection is not an easy one, however.

Users must exercise considerable caution in comparing data

from different services. It is particularly important to be

sure that the data are expressed in the same way and based

on comparable sets of assumptions. For example, certain

parameters may vary according to the load on the system or the peak to average ratios might be quite different. Users

should seek to obtain these parameters for the same

conditions under which they would expect to use the system.

One entire area which has not been discussed in this

report, but which may be very important in the selection

between alternatives is that of network management. The

inclusion of such capabilities as a centralized trouble

shooting desk, regional support personnel, sophisticated

diagnostic routines, and complete statistical reports

showing usage, traffic patterns and load, may be quite

important to prospective users. Of course, many of these

factors will show up in the various performance measures,

such as, availability, or, on the other side, in the cost of

the service. Users must decide for themselves the value of

any management services which do not directly affect

performance.

27
ACKNOWLEDGEMENTS

The authors would like to express their appreciation to

Dr. Peter McManamon of the Office of Telecommunications and

to Mr. George Sugar of the National Bureau of Standards for

their help in numerous discussions. Also, to Mr. William

Combs of Tymshare Corp., Dr. Dixon Doll of DMW

Telecommunications Corp. , Mr. Lynn Hopewell of Network

Analysis Corp., Dr. Barry Wessler of Telenet Communications

Corp. and Dr. Dennis Branstad of NBS for their assistance in

reviewing the manuscript for this report.

28
BIBLIOGRAPHY

(1) American National Standard X3.44 - 1974,

"Determination of Performance of Data Communications

Systems"

(2) American National Standard X3-28 - 1971, "Procedures

for the Use of the Communication Control Characters

of the American National Standard Code for Information

Interchange in Specified Data Communication Links".

(3) American National Standard X3.1 - 1969, "Signaling

Speeds for Data Transmission".

(4) Blanc, R. P., "Review of Computer Networking

Technology", NBS Technical Note 804, Jan. 1974.

(5) Schwartz, M. , Boorstyn, R.,and Pickholtz, R.,

"Terminal-Oriented Computer Communications Networks",

Proc. IEEE, Nov. 1972, pp. 1408-1422.

(6) Jackson, P. and Stubbs, C. , "A Study of Multi-Access

Computer Communications", SJCC, 1969, pp. 491-504.

(7) Fuchs, E. and Jackson P., "Estimating Distributions

of Random Variables for Certain Computer

Communications Traffic Modes", CACM, Vol. 13, No. 12,

Dec. 1972, pp. 752-757.

(8) "1969-70 Switched Telecommunications Network

Connection Survey", Bell System Technical Reference

PUB 41007, April 1971 .

(9) Grubb, D. S. , "Data Communications System Throughput

Performance Using High Speed Terminals on the Dial

Telephone Network", NBS Technical Note 779, May 1973.

(10) "Data Communications Using Voiceband Private Line

Channels", Bell System Technical Reference PUB 41004,

Oct. 1973.

(11) "Request for Proposal NLM 72-102 ADP/Telecommuncations Support for a Remote-Access Bibliographic

Information Retrieval System", National Library

of Medicine, 1972.

(12) "Data Communications Using the Switched Telecommunications Network", Bell Systems Technical Reference

PUB 41005, May 1971

(13) "Digital Data System Channel Interference Specifications", Bell System Technical Reference PUB 41021,

March 1973-

29
(14) Farinholt, E. V., "Datran's Switched Digital

Network", IEEE Northeast Electronics Research and

Engineering Meeting, Boston, Mass.., Oct. 1974.

(15) "High Speed Private Channel Data Service", Data

Transmission Company.

(16) Network Analysis Corp., "ARPANET: Design, Operations,

Management and Performance", Report to Rome Air

Development Center, April 1973-

(17) Bolt, Beranek and Newman Inc., report to the

Advanced Research Projects Agency, Sept. 1974.

(18) American National Standard X3.12 - 1970, "Vocabulary

for Information Processing".

(19) McManamon, P., Rosich, R. , Payne, J. and Miles, M.

,

"Performance Criteria for Digital Data Networks",

Office of Telecommunications OTR 75-54, Jan. 1975.

(20) McManamon, P., "Satellite Communications for the U. S

Postal Service", Second Annual Telecommunications

for Government Conference, Boulder, Colorado,

Oct. 1973-

(21) Martin, J., "Teleprocesing Network Organization",

Prentice-Hall, 1970.

(22) Peterson, W. W. , "Error-Correcting Codes", Wiley,

1961.

(23) Peterson, W. W. and Welden, E. J., "Error-Correcting

Codes", MIT Press, 1972.

(24) AT & T Long Lines Division, information provided to

the Federal Telecommunications Standards Committee,

June 1974.

(25) Miller, R. B. , "Response Time in Man-Computer

Conversational Transactions", FJCC 1968, pp. 267-277.

(26) Heart, F., Kahn, R., Ornstein, S. , Crowther, W. and

Walden, D., "The Interface Message Processor for the

ARPA Computer Network", SJCC, 1970, pp. 551-567.

(27) Martin, J., "Security, Accuracy and Privacy in

Computer Systems", Prentice-Hall, 1973.

(28) Renninger, C. and Branstad, D., "Government Looks at

Privacy and Security in Computer Systems", NBS

Technical Note 809, Feb. 1974.

(29) Renninger, C. , "Approaches to Privacy and Security in

Computer Systems", NBS Special Publication 404,

Sept. 1974.

30
(30) Federal Information Processing Standards Publication

31, "Guidelines for Physical Security and Risk

Management", 1974.

(31) Boustead, C. and Mehta, K., "Getting Peak Performance

on a Data Channel", Data Communications, July 1974.

(32) Doll, D., "Where to Spend Money to Improve System

Availability", Data Communications, Nov. 1974,

pp. 57-61.

(33) Frank, H. and Hopewell, L., "Network Reliability",

Datamation, August 1974, pp. 85-91.

(34) Martin, J., "Systems Analysis for Data Transmission",

Prentice-Hall, 1972.

(35) Roberts, L. G. and Wessler, B. D., "Computer Network

Development to Achieve Resource Sharing", Proc. SJCC,

1970, pp. 543-549.

(36) National Bureau of Standards, "Encryption

Algorithm for Computer Data Protection", Federal

Register, Volume 40, Number 52, March 17, 1975.

(37) Cole, Gerald D., "Performance Measurements on the

ARPA Computer Network", Proc, Second Symposium on Problems in the Optimization of Data Communications

Systems, October 20-22, 1971, pp. 39-45.

31
NBS-114A (REV. 7-73)

U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO. NBS TN-8822. Gov't Accession

No.

3. Recipient's Accession No.

4. TITLE AND SUBTITLE

Criteria for the Performance Evaluation

of Data Communications Services

for Computer Networks

5. Publication Date September 1975

6. Performing Organization Code 650.01

7. AUTHOR(S)

Dana S. Grubb and Ira W. Cotton

8. Performing Organ. Report No.

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Project/Task/Work Unit No. Project 6502131

11. Contract/Grant No. STRS

12. Sponsoring Organization Name and Complete Address (Street, City, State, ZIP)

Computer Systems Engineering Division

Institute for Computer Sciences and Technology

National Bureau of Standards

Washington, D. C. 20234

13. Type of Report & Period Covered Interim Report

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

16. ABSTRACT (A 200-word or less (actual summary of most significant information. If document includes a significant

bibliography or literature survey, mention it here.) In general, when telecommunications services are used as a means of interchanging

information between information processing systems, or between terminals and systems,

a number of parameters determine how well that interchange is performed. This report

examines the following characteristics of telecommunications services:

1. Transfer Rate

2. Availability

3. Reliability

4. Accuracy

5. Channel Establishment Time

6. Network Delay

7. Line Turnaround Delay

8. Transparency

9. Security

These terms are all defined and their significance discussed. The effects of these

factors on data communication networks are illustrated.

17. KEY WORDS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper name; separated by semicolons)

Computer communications; computer networking; data communications; networks;

performance requirements; telecommunications.

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