NTIA-REPORT -78 - 4 

Digital Communication 

Performance Parameters for 

Proposed Federal Standard 1033 

VOLUME II: APPLICATION EXAMPLES 

F. G. Kimmett 

N. B. Seitz 

u.s. DEPARTMENT OF COMMERCE 

Juanita M. Kreps, Secretary 

Henry Geller. Assistant Secretary-Designate 

for Communications and Information 

May 1978 

TABLE OF CONTENTS 

LIST OF FIGURES AND TABLES 

ABSTRACT 

1. INTRODUCTION 

1.1. Purpose and Scope 

1.2. Report Organization 

1.3. Common Interface and Protocol Features 

1.4. Overview of Results 

2. NONSWITCHED PRIVATE LINE EXAMPLE 

2.1. Equipment Configuration 

2.2. Procedure Flowcharts and Transaction Profile 

2.3. Reference Events 

2.4. Parameter Values 

3. CIRCUIT-SWITCHED EXAMPLE 

Page 

iv 

1 

1 

1 

2 

3 

4 

7 

7 

7 

11 

16 

21 

3.1. Equipment Configuration 21 

3.2. Procedure Flowchart and Transaction Profile 21 

3.3. Reference Events 26 

3.4. Parameter Values 29 

4. MESSAGE SWITCHED EXAMPLE 33 

4.1. Equipment Configuration 33 

4.2. Procedure Flowchart and Transaction Profile 35 

4.3. Reference Events 37 

4.3.1. Source-to-Switch Sub-Transaction 

4.3.2. Switch-to-Destination Transaction 

4.3.3. End-to-End Transaction 

4.4. Parameter Values 

5. SUMMARY AND CONCLUSIONS 

6. REFERENCES 

APPENDIX A. USASCII COMMUNICATION CONTROL CHARACTERS 

APPENDIX B. RS-232 INTERCHANGE CIRCUITS 

iii 

38 

39 

41 

43 

49 

58 

59 

60 
LIST OF FIGURES 

Figure 2.1. Equipment configuration for the nonswitched 

private line example. 

Figure 2.2. ANSI X3.28 operating procedure flowchart 

for example 1 (master status). 

Figure 2.3. ANSI X3.28 operating procedure flowchart 

for example 1 (slave status). 

Figure 2.4. Transaction profile for the nonswitched 

private line example. 

Figure 3.1. Circuit switching overview. 

Figure 3.2. Block diagram of M37 teletypewriter (online operation). 

Figure 3.3. ANSI X3.28 operating procedure for example 2 

Page 

8 

9 

10 

12 

22 

23 

(master status). 24 

Figure 3.4. Transaction profi~e for the circuit-switched 

example. 25 

Figure 4.1. Message-switched communication system. 34 

Figure 4.2. Transaction profile for the message-switched 

example. 36 

Table 1.1. 

Table 2.1. 

Table 4.1. 

Table 5.1. 

LIST OF TABLES 

Compilation of Parameter Values 

Total Survey Character Error and Lost 

Character Statistics (AT&T, 1971a) 

Sub-transaction Performance Parameter Values 

Compilation of Parameter Values 

iv 

Page 

5 

17 

46 

50 
DIGITAL COMMUNICATION PERFORMANCE PARAMETERS 

FOR PROPOSED FEDERAL STANDARD 1033 

VOLUME II: APPLICATION EXAMPLES 

F. George Kimmett and Neal B. Seitz* 

This volume presents three examples of the use of 

proposed Federal Standard 1033, "Digital Communication 

Performance Parameters, II in specifying end-to-end telecommunication system performance. The examples illustrate, for the particular configurations assumed, the 

processes of defining user/system boundaries and interaction sequences; identifying communication functions 

and their associated outcomes; and calculating performance parameter values. 

Key words: Data communications; federal standard; performance parameters; information transfer transaction; access; user information transfer; disengagement; circuit switching; message switching 

1. INTRODUCTION 

1.1. Purpose and Scope 

The purpose of this volume is to illustrate use of the performance parameters defined in proposed Federal Standard 1033, 

"Digital Communication Performance Parameters," in specifying 

end-to-end performance of representative telecommunication 

services. Three example services are described: (1) a nonswitched private line, teletypewriter-to-teletypewriter communication service connecting only two operators; (2) a circuitswitched teletypewriter-to-teletypewriter service connecting 

selected pairs of operators through the public switched network; 

and (3) a message-switched teletypewriter-to-teletypewriter 

service providing communication between pairs of operators 

through an automated store-and-forward switching center. Each 

example application consists of four parts: 

*The authors are with the Institute for Telecommunication Sciences, 

National Telecommunications and Information Administration, U.S. 

Dept. of Commerce, Boulder, Colorado 80303. 
1. Specification of equipment configuration and 

operating protocol. 

2. Definition of a representative transaction profile. 

3. Identification of reference events, which define the 

start and end of the primary communication functions 

and signify changes in "responsibility" for their 

completion. 

4. Calculation of performance parameter values based on 

assumed performance outcomes. 

The sole purpose of specifying particular communication 

components and services in the application examples is to make 

the examples more concrete. Such specification in no way implies 

an endorsement of those products or services by the National Telecommunications and Information Administration, or any branch of 

the u.S. Government. Performance parameter values are calculated, based on assumed transaction profiles and performance 

outcomes, for the purpose of illustrating the calculation process. Although published performance data are used where available in specifying the assumed performance outcomes, it is not 

suggested that the parameter values calculated in the examples 

can be used to characterize the performance of actual offered 

services. 

1.2. Report Organization 

Volume I of this two-volume report provides a detailed 

description of the technical considerations which influenced 

development of the proposed Federal Standard; and defines the 

selected parameters in logical and mathematical terms. This 

volume (Volume II) is organized into five sections. The remainder of this section presents certain interface and protocol 

information which is common to all three selected examples; and 

provides a brief overview of key results presented in the subsequent sections. Sections 2, 3, and 4 each describe one of the 

three selected examples. Section 5 summarizes and compares the 

calculated parameter values for all three examples and presents 

2 
overall conclusions. Concise definitions of relevant USASCII 

communication control characters and RS-232-C interchange circuits are given in appendices A and B, respectively. 

1.3. Common Interface and Protocol Features 

The following interface and protocol features are common to 

all three of the selected examples: 

1. The functional interface between "the user" and "the 

telecommunication system" corresponds to the physical 

interface between a Model 37 (M37) teletypewriter station 

and the human terminal operator or unattended nonmagnetic medium (punched paper tape) on its drop side. 

2. "User information" includes the graphic subset of the 

USASCII code (ANSI, 1967) plus the following format 

control characters: space (SP); line feed (LF); horizontal tabulation (HT); vertical tabulation (VT); form 

feed (FF); carriage return (CR); and null (NUL) 1. 

3. USASCII "overhead information" characters employed 

include data link escape (DLE) , end of transmission 

(EOT) , device controls 1 and 3 (DCl and DC3), and 

enquiry (ENQ); plus any control characters generated 

and utilized within the system, e.g., acknowledge (ACK) , 

negative acknowledgement (NAK) , and synchronous/idle 

(SYN). All USASCII characters not specified above are 

assumed unused. All USASCII characters are transmitted 

in a lO-bit sequence comprising a start bit, 7 ASCII 

information bits, a parity bit, and a stop bit. Various 

changes in the states of the M37 control switches and 

indicators (e.g., ORIG button,data lamp) also represent overhead information transfers; these are identified in each specific example. 

4. Except as noted, data communication protocols follow 

the conventions established in ANSI X3.28 (ANSI, 1971). 

ITWO minor variations in the third example are explained in 

Section 4.2. 

3 
5. Interface communications between data terminal equipment 

(DTE) and data circuit-terminating equipment (DeE) 

follow RS-232-C (EIA, 1969). Events occurring at these 

interfaces are identified for completeness in the 

second and third examples, although they are systeminternal from the end user point of view. 

Specific user/system interface events are identified in each 

example. In the first example, the user message is input on-line 

at the source TTY keyboard; in the second and third examples, the 

user message is input from a source paper tape prepared off-line. 

The user message is output at the destination terminal printer 

ln each case. 

The nonprinting format control characters list~ in (2) 

above are regarded as user information on the following basis: 

(1) they do not affect the system state, i.e., all system 

components are transparent to them; and (2) they do produce an 

observable output at the destination which uniquely represents 

information input by the source. A delivered user message would 

look noticeably different if all spaces or line £eeds input by 

the source were omitted. 

1.4. Overview of Results 

Table 1.1 is a compilation of estimated performance parameter 

values for the three selected examples. It is emphasized that 

these values characterize the examples, under stated assumptions, 

and are not intended to be used in characterizing or comparing 

actual offered services. The example services are vastly different in performance, by design; and they would be appropriately 

used in meeting correspondingly different end-to-end service 

requirements. 

With the exception of the error probabilities and transfer 

rates, the bit-oriented and block-oriented user information 

transfer parameter values within each individual example are 

the same. This is true because the bits within each user 

information block (character) cross the user/system interface 

4 
Table 1.1. Compilation of Parameter Values 

NONSWITCHEO 

PERFO;tMANCE PRIVATE LINE CIRCUIT-SWITCHED MESS,~GE-SWITCHED 

PARAMETER Example E.u..'IPLE EXAMPLE 

Access Time 5.2 Seconds 37.5 Seconds 15.4 Minutes 

Incorrect Access Probabl1 i ty 0 1.1 x 10-4 0 

Access Oenl a 1 Probabi 1 i ty 1.7 x 10-3 1.1 x 10-2 6.8 x 10-4 

810ck Transfer Time 68 mi 11 i seconds 68 >:Ii 11 i seconds 16.2 minutes 

8it Transfer Time 68 milliseconds 68 ",i 11 i seconds 16.2 minutes 

Block Error Probabi 1 ity 1.5 x 10-4 1.5 x 10-4 2.5 x 10-4 

Bit Error Prababi 1 i ty .5xl0- 5< P(bl )< 1.5x10-4 - e- 1.5x10-5< P(b1 )<1.5xl0-4 - e- 2.5x10-5~ P(b1 e )2.sxl0-4 

Slock Mi sde 1 i very Probabi 1 ity 0 1.1 x 10-4 6 x 10- 7 

Bit Misdelivery Probability 0 1.1 x 10-4 6 x 10- 7 

Block Loss Probability 4.7 x 10-5 6.3 x 10-4 1.7 x 10- 3 

Bit Loss Probabil ity 4.7 x 10-5 6.8 x 10-4 1.7 x 10- 3 

Extra Block Probabil ity 0 a 7 x 10-7 

Extra Bi t Probabi 11 ty a 0 7 x 10-7 

IB10Ck Transfer Rate 5 blocks/second 14.6 blocks/second 36.2 blocks/minute 

Bit Transfer Rate 35 bi ts/ second 102.4 bits/second 4.2 bits/second 

Block Rate Efficiency 23 percent 68 percent 2.B percent 

Bit Rate Efficiency 25 percent 68 ~ercent 2.8 percent 

Oi sengagement Time 0.5 seconds 2.25 seconds 1.5 seconds 

Oi sengagement 4 x 10-4 5.7 x 10-5 1.1 x 10-4 

Deni a 1 Probabi 1 ity 

Outage Probabi 1 I ty 8 x 10-4 1.4 x 10- 3 3 x 10-3 

Service Time 

Between Outages 43 UIT hours 8.2 UIT hours 93 UIT hours 

Outage Duration 1 hour 38 ninutes 42 minutes 

User Access Ti"le Fraction 0.19 0.4 0 

User 810ck T,'ansfer 0 0 0 Time Fraction 

User Message Transfer 0.67 0.02 0 Time Fraction 

User Oi sengagement 0.8 0 0 Time Fraction 

5 
in parallel; and each block represents a fixed number of bits. 

The bit- and block-oriented parameter values will differ wherever 

these special conditions are not met. (Note that the term "block" 

is used here as defined in proposed Federal Standard 1033, rather 

than as defined in ANSI X3.28.) 

A summary and comparison of the estimated parameter values 

for each example is presented in Section 5. In. summarizing these 

results, two general observations are made: 

1. In each of the three example services described it was 

poss~ble, with reasonable effort, to develop representative values for all 26 specified performance parameters. 

More precise estimates of parameter values, and additional performance information such as parameter distributions, could be developed with additional analysis 

or measurement effort. 

2. With a single exception (User Block Transfer Time 

Fraction), none of the specified parameters had a value 

of zero in all three examples. The latter parameter 

will have a nonzero value in various other types of 

services, as noted in Section 5. 

These observations provide general support for the conclusion 

that the specified parameters are in fact consistent with the four 

"desired parameter attributes" listed in Section 1.1 of Volume I: 

user orientation, universal applicability, simplic~ty, and 

-completeness. It is anticipated that additional application 

examples will be developed to illustrate use of the proposed 

standard parameters in characterizing computer-to-computer and 

teleprocessing services. 

6 
2. NONSWITCHED PRIVATE LINE EXAMPLE 

2.1. Equipment Configuration 

The example presented in this section is a nonswitched 

private line,teletypewriter-to-teletypewriter communication 

service which interconnects two operators. Figure 2.1 is a block 

diagram of the assumed equipment configuration. Two primary 

communication interfaces are identified: the user/system functional interface, between the operator and the data terminal; 

and the information transfer channel interface, between the data 

terminal and dataset. The latter interface is internal to the 

system from the end user (operator) point of view. 

2.2. Procedure Flowcharts and Transaction Profile 

ANSI X3.28 operating procedure flowcharts for the nonswitched private line example are presented in Figures 2.2 and 

2.3, for master and slave stations, respectively. These flowcharts are adapted from Bell System Pub. 41713 and identify the 

ASCII control characters that may be exchanged between the user 

(operator) and half-system (terminal) entities during an information transfer transaction. 

Before message transmission can begin, the two teletypewriter terminals must be started and placed in appropriate master 

(transmit) and slave (receive) states. An operator wishing to 

transmit a message starts his local terminal, by switch action; 

starts the distant terminal, by sending the communication control 

character ENQ; and then bids for master status by sending a 

second ENQ character. A terminal which has not transmitted ENQ, 

but which has received ENQ, first inhibits the sending of ENQ so 

that its station may not bid for master status. If the nonoriginating terminal is ready to receive, it assumes slave status 

and sends the communication control character ACK as an affirmative reply. The bidding (or originating) terminal then assumes 

master status, and user information transfer proceeds. If the 

nonoriginating terminal is not ready to receive, it sends the 

7 
00 

Functional 

Interface 

Functional 

Interface 

I .. Telecommunication System .... , 

Source User 

I M37 TERMINAL /-- TELETYPE- OPERATOR WRITER 

--------

Figure 2.1. 

I ... Information Transfer Channel .... I 

I I M37 

/-- 103H DATA SET I-- VOICE GRADE I-- 103H DATA SET - TELETYPE- PRIVATE LINE WRITER 

~------~- --

Equipment configuration for the nonswitched private 

line example. 

Destination User 

I 

TERMINAL - OPERATOR 
\.0 

Local and Distant Station Start-Up 

/ \ / 

TTY Remote 

Establishment Procedure Message 

Transfer Termination Procedure 

~/\ / \ 

Depress_Motor Send_TTY Motor Send Receive __ Send __ User J EOT ~ OLE Off 

Orig. Turn-On ENQ Turn-On (ENO ~ ACK Lamp On Info \ EOT 

Delay Delay 

ContentionSend Lamp Off 

~-------J--------------------------J----------"-r Idle Line Time Out --------Off 

Out of 

Service 

f'... Receive ---Lamp--~, _-." 

NAK Flash 

Ring Bell 

Invalid or .J 

No Reply , ,..... I Idle Line OLE 

Time Out-EOT-Off 

'-- "l, ,/ Depress OLE 

Clear - EOT -Off 

Figure 2.2. ANSI X3.28 operating procedure flowchart for 

example 1 (master status). 
I-' 

o 

Start Up Procedure Establishment Procedure Message 

Transfer Termination Procedure 

/ \/ ---\/~f-- --\ 

TIV 

Receive Motor 

ENQ -TumOn 

Delay 

.,.- Receive Receive 

Lamp Send -User Inf~ ENQ On ACK EOT 

ContentionReceive Lamp Off 

Idle Line ..... ~ ___ _ ,..... _ . .L 

OLE 

EOr---Off 

----Off 

ply OLE 

Time Out~--EOT---Off 

*Receiving Terminal Out of Service, in Local or in Paper Alarm Condition. 

Figure 2.3. ANSI X3.28 operating procedure flowchart for 

example 1 (slave status). 
control character NAK. The originating operator may then reinitiate a bid for master status or may exit to a recovery position. 

When the master status operator has typed all user information he 

wishes to transfer, he may either send EOT, to give the slave 

station an opportunity to assume master (transmit) status; or 

send DLE EOT, to terminate the transaction and turn off the two 

teletypewriter motors. 

A transaction profile for the nonswitched private line 

example is presented in Figure 2.4. The profile is developed 

from Bell System Technical Reference Pub. 41713 (AT&T, 1971b) and 

its references. A single pass through the top line of the procedure flowcharts of Figures 2.2 and 2.3, and a unidirectional flow 

of user information' from the originating to the nonoriginating 

user, is assumed. The user information message is assumed to 

begin with a 2-word, 12-character heading consisting of an 

alphanumeric date-time group; no address or routing information 

is required since the service is nonswitched. The bal~nce of the 

user information transferred during the transaction consists of 

text characters selected from the USASCII "user information" 

subset defined in paragraph 1.3.1. An average text length of 100 

words and a typing speed of 50 words per minute is assumed. It 

is assumed that start of heading, start of text, and end of text 

are indicated by asterisks (*). 

2.3. Reference Events 

The following paragraphs describe the sequence of user/system 

interface events during the nonswitched private-line transaction, 

and identify the specific reference events which delimit the 

start and end of each of the five primary communication functions. 

ACCESS FUNCTION (a) 2 

The purpose of the access function is to place a source (master) user in a position to begin transferring user information 

2Symbols and notation are explained in Section 3.4.2 of Volume I. 

11 
f-J 

tv 

Access 

Phase 

User Info 

2 SECONDS 

2 S 

0.2 S 

1.0 S 

Transfer 122.4 S 

Phase 

~ Disengagement 0.5 S 

Phase 

t 

Figure 2.4. 

Source User 

(Master) Telecommunication System Destination User 

(Slave) 

DTE DCE Circuit DCE DTE 

(Depress Orig) mt Turn-On 

TTYMotor 

I Delay 

(Motor I ENO, . ENO I (TTY Motor 

to Speed) I ,I. Start) 

Y Turn-On I I TTY Motor 

I Delay 

i ENO ENO (Motor to Speed) 

(Send 

Lamp On) r 

I 

I AL (teceive 

Lamp On) ACK 

T 

I First I User Info t chararer (*) 

I 

First 

User Info 

Character (*) 

(Printed) 

I---I--... --~I-----I----l_-~----~------~I-~ (Printed) I Last Last 

User Info User Info 

, Character (*) Character (*) 

I I 

OLE 

EOT 

(Send 

Lamp Off) (Motor Off)' 

__ -+-___ (Receive 

(Motor Off) Lamp Off) 

Transaction profile for the nonswitched private 

line example. 

Cll 

E 

i= 

1 
characters. The sequence of interface events during the access 

phase is as follows: 

1. The operator of the station desiring master status 

activates a ~ontrol switch (ORIG) to start the teletypewriter motor at the local station. This action 

represents the reference event start of access (a). 

2. An elapsed time interval of 2 seconds is required to 

allow the teletypewriter motor to attain operating 

speed. 

3. The operator of the station desiring master status depresses the ASCII control character ENQ and this action 

transmits a wake-up signal over the private line to 

start the teletypewriter motor at the distant terminal. 

4. The remote station receives ENQ and its teletypewriter 

motor is turned on. A time interval of 2 seconds is 

required to allow the teletypewriter motor to attain 

operating speed. 

5. The operator of the originating station makes a bid 

for master status by transmitting a second ENQ. 

6. The nonoriginating terminal receives the transmitted 

ENQ. The terminal assumes slave status; illuminates 

the receive condition lamp; and responds with the 

control character ACK. 

7. The terminal bidding for master status acquires that 

status upon receipt of the ACK character, and its 

send condition lamp illuminates. 

8. The operator of the master status teletypewriter types 

the first user information character (*). This action 

represents the reference event end of access (a*). 

USER INFORMATION TRANSFER FUNCTIONS (bl, b2, and b3) 

During the user information transfer phase, the master 

station operator types the message and heading text and the 

slave teletypewriter prints the corresponding received characters. Note that the heading and delimiters are considered user· 

information here, since they do not affect the communication 

13 
system state. Reference events for the three UIT functions are 

defined as follows: 

1. Start of bit (and block) transfer events (bl, b2) occur 

each time the master operator types a user information 

character. Each character is regarded as a block, 

containing seven user information bits; and each typing 

action thus initiates seven bit transfer functions and 

one block transfer function. 

2. End of bit (and block) transfer events (bl*, b2*) occur 

each time the slave terminal prints a user information 

character. Again, one block transfer function and 

seven bit transfer functions are ended by each character printing. 

3. For simplicity, the message transfer function is 

defined to comprise 4284 bits; i.e., the number of 

user information bits transferred during a single 

(average) information transfer transaction. The event 

start of message transfer (b3) thus occurs on typing of 

the first user information character of each information transfer transaction; ~nd the event end of message 

transfer (b3*) occurs on printing of the last user 

information character of each transaction3 . 

DISENGAGEMENT FUNCTIONS (d) 

The disengagement functions nullify the master/slave 

relationship established during an information transfer transaction, and turn off the teletypewriter motors. Two distinct 

disengagement functions are considered during each transaction, 

one associated with each user interface. The sequence of disengagement events is as follows: 

1. The master operator types the control characters DLEEOT. These characters are interpreted as a disengagement request signal by the M37 teletypewriters; and 

typing of the DLE character thus represents the 

3In practice, a somewhat longer message might be required to 

attain more confidence in the measured parameter values. 

14 
reference event start of disengagement (d) for both 

user interfaces. 

2. About 500 milliseconds after start of typing of the 

DLE-EOT character sequence at the master terminal, this 

terminal detects the shutdown request and responds by 

turning off the send lamp and teletypewriter motor. 

This event represents the end of the disengagement 

function (d*) at the master terminal. The same event 

occurs approximately one millisecond later at the 

slave terminal. 

RESPONSIBILITY TRANSFER EVENTS 

The timing of the nonswitched private line example transaction is such that there are no periods of "split" responsibility; either the master station operator or the system is 

always responsible for producing the "next" interface event in 

the transaction sequence. Referring to Figure 2.4, during the 

access phase: 

1. The system is responsible between ORIG and ACK 

(assuming the ENQ's are sent at 2-second intervals); 

and 

2. The user is responsible between ACK and first block 

transfer (*). 

The periods between ORIG and the first ENQ, and between the first 

and second ENQ's, are attributable to the system since the 

originating operator is precluded from typing characters during 

the 2-second warm-up period. Any delay beyond 2 seconds would 

be attributable to the user. Expiration of the 2-second warm-up 

period is an example of a "referred event" of the type described 

in Section 2.5 of Volume I. 

During the UIT phase, responsibility alternates between the 

system and the master operator. The first 67 milliseconds (ms) 

after typing of each user information character are attributable 

to the system, since the M37 terminal has a maximum character 

input rate of 15 characters per second. All time between 

expiration of the 67 millisecond delay (another "referred event") 

15 
and initiation of the next user information character is attributable to the master operator. The average duration of user 

responsibility between characters is 133 milliseconds, since a 

5 characters-per-second user typing rate is assumed. 

During the disengagement phase, the user typing time for 

DLE-EOT (assumed to be 200 ms in excess of the minimum typing 

time) is attributable to the user; and the remaining time is 

attributable to the system. 

2.4. Parameter Values 

Probable performance parameter values for the nonswitched 

private line example are derived on the basis of the procedure 

flowcharts of Figures 2.2 and 2.3; the transaction profile of 

Figure 2.4; and the following assumptions. 

1. Information from Table 2.1, which is reproduced from 

the Bell System Telecommunications Network Connection 

Survey, 1969-70 (AT&T, "197la) is used in estimating bit 

and block (character) error and loss probabilities 4 . 

2.. Path length is taken to be 20 miles and the telephone 

line wiring and loadi~g is considered to result in a 

propagation velocity of 20,400 miles per second. 

3. An average of 48 independent and separate messages 

are assumed to be transmitted each day by the source 

terminal operator. As described above, each message 

consists of a heading (date-time group) equivalent to 

2 words; and a message text with an average of 100 

words. Messages are transmitted "on-line" by the 

source operator at an average typing rate of 50 words 

per minute (5 characters per second). 

4Strictly speaking, these statistics are applicable only to 

switched data communications. The reported Character Error Rate 

is used directly in estimating private line Block Error Probability, since the data indicate this statistic was not dominated 

by a few bad calls. The observed L2~t Character Rate is reduced 

by a factor of 14.5 to 1 (to 4.7xlO ) in estimating private line 

Block Loss Probability, to eliminate the effect of the eight worst 

calls observed in the survey. 

16 
~alls 

Table 2.1. Total Survey Character Error and Lost 

Character Statistics (AT&T, 1971a) 

Mileage Band 

All Calls Short Medium Long 

534 171 186 177 

tharacters 6 7.00xl0 6 7.48xl0 6 6 Transmitted 21. 31xlO 6.83xlO 

Character Errors 3,110 751 1,053 1,306 

lLost Characters 14,511 9,581 1,476 3,454 

!character Error -4 -4 -4 -4 Rate 1.46xlO 1.07xlO 1. 42 xlO 1.90xIO 

Lost Character -4 -4 -4 -4 Rate 6.81xlO 13.7xlO 1. 98 xlO 5.03xlO 

4. Two types of phenomena are assumed to affect the availability of the nonswitched private line service: total 

failures in essential transmission equipment (e.g., 

power supply failures, broken wires); and degraded 

performance (e.g., adverse weather conditions impairing 

microwave transmission). Total failures are assumed 

to occur at an average rate of twice per year and are 

assumed to last an average of four hours. These 

failures are assumed to be uncorrelated with service 

usage. If they begin during a period of usage, they 

will result in an observed outage; if they begin during 

a period of non-usage, they will result in a series of 

access denials. It is assumed that the operator retries unsuccessful access attempts three times initially, and once each half hour thereafter, producing 

a total of 10 access denials during a 4-hour outage. 

Degraded performance events are assumed to cause belowthreshold values for one or more supported primary 

parameters (outages) in the case o£ one message per 

month. 

17 
5. Loss or incorrect transmission of either the DLE or EOT 

control character will res~lt in a disengagement fail~re 

at the nonoriginating user; the probability of this 

event is derived from the data presented in Table 2.1, 

under the assumption of independent errors. 

PRIMARY PARAMETER VALUES, NONSWITCHED PRIVATE LINE EXAMPLE 

(1) Access Time W(a s ) = 5.2 seconds 

See Figure 2.4. 

(2) Incorrect Access Probability p(am) = 0 

Example used is for a nonswitched private line service. 

(3) Access Denial probability P(a~) = 1.7 XlO- 3 

30 denials/17S20 messages = 1.7xlO-3 

year year 

Total failures produce 20 denials per year (10 during 

each of two 4-ho~r outages, as noted in (4) above). 

Combined character error or loss probability is about 

2xlO- 4 , (1.5 xlO- 4+4.7xlO-S). Three characters must be 

successfully transferred to achieve successful access. 

6XlO- 4 denials x (17S20_20)messages = 10 denials per year 

message year due to character 

loss or error. 

(4a) Block Transfer Time -W(b2 s ) = 68 milliseconds 

Signal Element (Baud) Time: 

6.67 milliseconds/bit x 10 bits/character 

= 66.7 milliseconds. 

" 1 secondsx20 '1 Propagat10n T1me: 20,400 mile m1 e~4 

= 9.8 xlO seconds. 

(4b) Bit Transfer Time W(bls ) = 68 milliseconds 

See 4a. 

(Sa) Block Error Probability P(b2 ) = 1.SXlO-4 

e 

See Table 2.1. 

(5b) Bit Error Probability: 1.SXlO-S $ P(ble ) $ 1.SXlO-4 

Error in a single bit or all bits produces a single 

character error. See Table 2.1. 

(6a) Block Misdelivery probability P{b2 ) = 0 

m 

Example used is for a nonswitched private line service. 

18 
(6b) Bit Misdelivery Probability P(blm} = 0 

See 6a. 

(7a) Block Loss Probability P(b2 t } = 4.7xlO-5 

See assumption 1, Section 2.4, and associated footnote 4. 

-5 (7b) Bit Loss Probability P(blt } = 4.7xlO 

See assumptions. Loss of a block loses all bits in the 

block. 

(8_) Extra Block Probability P(b2x } = 0 

Transmit protocol has no retransmission or buffering. 

(8b) Extra Bit Probability P(blx ) = 0 

See 8a. 

(9a) Block Transfer Rate R(b2 s ) ~ 5 blocks/second 

-4 good bloc~s 

6l2(1-(0.5+l.5}10 ) message = 4 999 good blocks 

122.4 UIT seconds • second 

message 

(9b) Bit Transfer Rate R(bls } ~ 35 bits/second 

7 user bits/block x 4.999 good blocks/second 

= 34.99 bits/second. 

(lOa}Block Rate Efficiency Q(b2 ) = 23 percent s 

5 blocks x 7 bits(user)/150 bits 0 233 

second block second = . . 

(lOb)Bit Rate Efficiency Q(bls ) = 23 percent 

35 bits /150 bits : 0.233. 

second second 

(11) Disengagement Time = 0.5 seconds 

See Figure 2.4. Master and slave disengagement functions are assumed concurrent. 

(12) Disengagement Denial Probability P(dt } = 4xlO-4 

2(2XlO- 4 ) = 4XlO- 4 

Loss or error in either OLE or EOT will cause denial. 

Combined character error or loss probability is 2XlO-4 ; 

see (3) above. 

SECONDARY PARAMETER VALUES, ~ONSWITCHED PRIVATE LINE 

(1) Outage Probability P(b3 } = 8XlO-4 

z 

19 
14 outages/year ~ 17520 

Two total failures plus 

messages = 8xO-4 

year 

twelve degraded performance 

periods per year (one per month) . 

(2) Service Time Between Outages W(y*) = 43 UIT hours 

596 UIT hours. 14 outages = 42.6 UIT hours 

year year outage 

(3 ) Outage Duration W(z*) = 1 hour 

(12 outages) (1/2 hour)+(2 outages) (4 hours) 

14 outages 

Resulting availability value would be 0.98. 

ANCILLARY PARAMETER VALUES, NONSWITCHED PRIVATE LINE 

(1) User Access Time Fraction: 

p(a) = 0.19 

Access Time W(a ) = 5.2 seconds 

s 

Operator responsibility W (a ) = 1.0 seconds 

u s 

Split respopsibility Wr(as ) = 0 seconds 

W (a )+0.5 W (a ) u s v s 

W (a ) s 

p(a) = 1+0.5(0) = 5.2 = 0.19. 

(2) User Block Transfer Time Fraction: 

p(b2) = 0 

Block Transfer Time W(b2 ) = 68 milliseconds 

s 

= 1 hour. 

Operator responsibility W b2 } = 0.0 millLseconds 

u s 

Split responsibility W (b2 ) = 0.0 milliseconds. 

v s 

(3) User Message Transfer Time Fraction: 

p(b3) = 0.67 

Message Transfer Time W(b3 ) = 122.4 seconds 

y 

Operator responsibility W (b3 ): u y 

0.133 s/character x 612 characters = 81.6 seconds 

Split responsibility W (b3 ) = 0.0 seconds 

v y 

User is typing at one-third the maximum input rate. 

(4) User Disengagement Time Fraction: 

p(d) = 0.8 

Disengagement Time W(d ) = 0.5 seconds 

s 

Operator responsibility W (d ) = 0.4 seconds 

u s 

Split responsibility W d ) = 0.0 seconds. 

v s 

20 
3. CIRCUIT-SWITCHED EXAMPLE 

3.1. Equipment Configuration 

The example presented in this section is a circuit-switched 

teletypewriter-to-teletypewriter service connecting selected 

pairs of users through the public switched network. A typical 

circuit-switching arrangement is shown in Figure 3.1, which is an 

example of a basic single-stage relay matrix. To expand the 

capability of the system, several matrices are typically interconnected into a grid network, forming more than one switching 

stage. The end-to-end equipment configuration for the circuitswitched example is identical to the one shown in Figure 2.1, 

except that the public switched network replaces the private line 

block. A block diagram of the M37 teletypewriter is shown in 

Figure 3.2, which also identifies the relevant interchange circuit connections. This diagram is adapted from Teletype (1969). 

3.2. Procedure Flowchart and Transaction Profile 

An ANSI X.28 master station operating procedure flowchart 

for the circuit-switched example is presented in Figure 3.3. 

The procedure is similar to that specified for the private-line 

example, except for addition of the physical connection and 

disconnection activities. These activities establish and terminate a conventional circuit-switched telephone connection between 

the master and slave terminal operators. 

A transaction profile for the circuit-switched example is 

presented in Figure 3.4. The profile is developed from information in Bell System Technical References (AT&T, 1968ai AT&T, 

1968b); and represents a single pass through the top iine of the 

procedure flowchart of Figure 3.3, exiting via the "depress 

clear" route. The example assumes that the user information 

message to be transferred is prepared off line in paper tape 

formi and that the completed message tape is mounted in the M37 

paper tape reader at the master station prior to the start of the 

transaction. This enables message transfer at the full M37 

operating speed of 150 words per minute. The same message format 

21 
IV 

IV 

Central Office Network 

--7 

L ./ __ 

COMMON 

CONTROL 

Source Stations I r III Destination Stations 

Local Loops Local Loops r---------------------------------~ L ________________________________ ~ 

Figure 3.1. Cireui t swi tehing overview. 
N 

W 

2 "c 

::> 

Cl 

c: 

"S: 

"~ 

Q) 

a: 

2 "c 

::> 

Cl 

c: 

E "E 

(J) 

c: 

co 

t= 

PAGE PRINTER 

PUNCH 

ANSWER BACK 

KEYBOARD" 

READER 

f-.--

Serial Data 

RECEIVE -b 

Frame Ground AA 

~ CONTROL 

~ 

- Serial Data 

BB 

CHANNEL 

Data Terminal Ready c::r:: 

CD 0 

CONTROL -- Data Set Ready 

le..:> 

CC UJ 

-- Ring Indicator z: 

z: 

Signal Ground 

CE 0 

e..:> 

-b- AB UJ 

e..:> 103H 

- Lt 

c::r:: ~ DATA SET UJ 

II-- z: 

e..:> 

~ MODE 

C'-..J 

M 

C'-..J I 

CONTROL 1 (/) 0::: 

~ SEND Request to Send 

I---

"~ CA 

CONTROL _ Clear to Send CB 

_" Ring Indicator CE 

1 -- DISTRIBUTOR Serial Data ~ BA 

I Paral'loel 

Data 

TELEPHONE 

l CONTROL 1 HANDSET AND 

PANEL 

ATIENDANT 

SET 

Figure 3.2. Block diagram of M37 teletypewriter (on-line) 

operation. 

~cal Loop 
Message . Station Start-Up and Switching Establishment Procedure Transfer Termination Procedure 

I \1 \i\ I \ 

Oepress Modem 

Data-Dialing -Switching --Hand 

Key Shakin 

N 

,j:>. 

9 

Send~Receive_Send 

ENQ ACK Lamp On 

Outot 

Service '- Receive __ Lamp 

NAK Flash 

Ring Bell 

'- Invalid or 

No Reply 

User EOT Info 

Contentic 

SendLam~ 

II " T 

0 

OLE EOT'--- .. Off 

ne Out,----Off 

e OLE 

ut-EOr-Off 

3-Second 

s Continuous Disconnectl 

Clear - Spacing - TIY Off 

Signal 

Figure 3.3. ANSI X3.28 operating procedure for example 2 

(master status). 
2 SECONDS 

Access 

Phase 

I 

lOS 

12 S 

10 S 

1.5 S 

0.85 S 

1 S 

0.15 S 

1 S 

41.2 S 

User Info 

Transfer 

Phase----

-h;sDisengagement 

Phase 

J 1.5 S 

User I DTE I 

.'. (Data Key Depressed) 

(Data TTY On CC On 

DCE Switched 

Network 

Lamp I 

On) __ --~--..---D~ia~I"~O~n~e~---+_-~ 

(Dialing) 

(Ringback) 

~;..-- Dialing Signals;~-~ 

Switching 

Call Set-Up 

F2M 

(F2M --+----:--::'-::----,,---t""- Heard) CFOn 

I "Handshaking" CBOn 

F1M 

F2 

---F1 

----

DCE DTE User 

CE On (Rlnlglng) 

Answer (D~ta Key Depressed) J 

..-Ol-ff-H .. O ....O-k---;C~C:::'";:o::n----T:;:T:;:Yj-;::O::n-,;1,Data 

Li\mp 

Onl 

CFOn 

CBOn 

, Com,Plete 

,Receive i.. __ -1E~N~Q~ ____ ~~~~ ______ ~ __________ r-__ ~ __ IRIE~N~Q~~,-____ t-~Lamp 

BA Data B ata On) 

I I ACK ACK 

(Send ~---+--------~B~B~D~a~ta~~----~----------r---"--fB~A~a~tia--~ Lamp (Tape Reader On) 

On) ~":""":"'FiLrs""t--"':"':-";B;XA-1D=::a::'-ta::-"----+------+----fB=iiB'itiD;:;ati;;a:---FF=tir:Ssi't ~,Prlnted) 

, 

I Character User Info (*) c~:,~~:~~o"'1 

~-"'LJa-s""t ---"'B~A~D~a:-:ta:--ol----+-----+----FBiFBrlD'i:a:iit;a---IL;as~t-(Prlilted) User Info User Info I Character, ' I Character (*) (Clear Key Depressed) 

(Data 

.Lamp-..~TT~Y~O~II~~~~C~C~O~II~--~-rO~n~HoOk 

Off) I I I 

.......... F2S 

(Data 

~~~--~~~---~~~Lamp On Hook CC Oil TTY Off Off) 

CFOff 

CBO" 

I 

Figure 3.4. Transaction profile for the circuit~switched example. 

25 
and length used in the private line example are assumed, except 

that the heading is defined to include a one-word originator ID 

in addition to the date-time group. The call is assumed to be 

answered manually (in data mode) by an operator at the receiving 

terminal. For completeness, events at both the user/PTE and 

DTE/DCE interfaces are indicated; but the latter events are still 

viewed as system-internal. 

In order to estimate total access time, it is necessary to 

make some assumptions about the duration of the physical dialing 

process. The dial speed of a typical rotary dial is 10 pulses 

per second. Total dialing time for a 7-digit telephone number 

is estimated at 10 seconds: 4 seconds operator time (for 

positioning the dialed digits) and 6 seconds system time (for 

dial rotation back to stop). Total switching time is estimated 

at 12 seconds, and answering time is estimated at 10 seconds. 

3.3. Reference Events 

The following paragraphs describe the sequence of user/ 

system interface events during the circuit-switched transaction; 

and identify the specific reference events which delimit the 

start and end of each of the five primary communication functions. 

1. The originating operator initiates the call by 

depressing the M37 data key (with the handset on-hook). 

The calling DCE goes off-hook, and turns on interchange 

circuit CC (Data Set Ready); and the teletypewriter 

motor starts. Depression of the data key corresponds 

to the reference event start of access (a). 

2. The local central office extends a dial tone signal 

to the calling terminal. The si~nal is transmitted to 

the calling operator via loudspeaker. 

3. The normal rotary dialing process takes place. 

4. The network establishes a circuit switched connection 

to the called station and initiates ringing (CE on). 

A ringback signal is heard at the calling station. 

5. The called terminal operator answers the call by 

depressing the M37 data key. The called DCE goes 

26 
off-hook, and turns on interchange circuit CC (Data 

Set Ready); and the teletypewriter motor starts. 

6. The called DCE transmits an F2M (mark) line signal to 

the calling station. The signal is heard by the calling 

operator; and the calling DeE turns interchange circuit 

CF (Received Line Signal Detector) on. 

7. The calling DCE responds with an FIM line signal. This 

signal causes the called DCE to turn on interchange 

circuits CF (Received Line Signal Detector), and CB 

(Clear to Send) • 

8. After a timed delay period, interchange circuit CB 

(Clear to Send) is turned ort by the calling data set. 

Modem "handshaking" is now complete; and the two M37 

terminals are in a position to communicate directly. 

9. The calling operator bids for master status by sending 

the ASCII control character ENQ. 

10. The called terminal receives the transmitted ENQ. The 

terminal assumes slave status; illuminates the receive 

condition lamp; and responds with the control character 

ACK. 

11. Receipt of the ACK character causes the tape reader at 

the calling terminal to turn on; and the first user 

information character (*) is read and transmitted. 

This event represents the end of access (a*). 

USER INFORMATION TRANSFER FUNCTIONS (bI', b2, and b3) 

During the user information transfer phase the master (originating) terminal automatically reads the paper tape containing 

the user information message; and the slave terminal prints the 

corresponding received message. Reference events for the UIT 

functions are the same as those defined for the private-line 

example, except that the starting events occur·on reading of user 

information characters by the paper tape reader, rather than on 

typing of characters at the keyboard. 

DISENGAGEMENT FUNCTIONS (d) 

The disengagement functions nullify the master/slave 

27 
relationship established during an information transfer transaction; turn off the teletypewriter motors; and disconnect the 

circuit-switched connection between the communicating stations. 

Both stations are returned to the idle state to await subsequent 

transactions. The sequence of disengagement events ~s as follows: 

1. The calling operator initiates disconnection by 

depressing the M37 clear key. This action represents 

the reference event start of disengagement (d) for 

both user interfaces. 

2. The originating data set turns interchange circuit CB 

(Clear to Send) off; and transmits a 3-second 

continuous-spacing (F2S) signal to the called data set. 

3. The called data set detects the continuous-spacing 

signal and disconnects (goes on-hook) after a nominal 

1.S-second delay. The called DCE turns off interchange 

circuits CC (Data Set Ready), CF (Received Line Signal 

Detector) and CB (Clear to Send). The called DCE data 

lamp goes out. This event co~responds to the reference 

event end of disengagement for the called station. 

The called DCE's on-hook action also causes interchange circuit CF (Received Line Signal Detector) to 

go off at the calling station. 

4. After transmitting the continuous-spacing signal for 

three seconds, the calling data set goes on-hook; and 

turns interchange circuit CC (Data Set Ready), and the 

data lamp, off. This event corresponds to the reference event end of disengagement for the calling station. 

RESPONSIBILITY TRANSFER EVENTS 

The timing of the circuit-switched teletypewriter-toteletypewriter transaction is such that there are no periods of 

"split" responsibility; either a user or system entity is always 

responsible for advancing the overall transaction to successful 

completion. Referring to Figure 3.4, during the access phase: 

1. The system is responsible between depression of the 

data key and dial tone; 

28 
2. Responsibility alternates between the system and the 

originating operator during the dialing period, with 

the user responsible (on the average) 40% of the time; 

3. The system is responsible between end of dialing and 

start of ringing; 

4. The nonoriginating user is responsible between start of 

ringing and answer; 

5. The system is responsible between answer and completion 

of modem "handshaking;" 

6. The originating user is responsible between completion 

of "handshaking" and typing of the ENQ character; and 

7. The system is responsible between typing of ENQ and 

the end of the phase. 

During the UIT phase, the system has responsibility until the 

last user information character is read (and printed); and the 

originating user has responsibility from that point until the 

end of the phase (when the clear key is depressed). The system 

has responsibility for both disengagement functions throughout 

the disengagement phase. 

3.4. Parameter Values 

Probable performance parameter values for the circuitswitched example are derived on the-basis of the procedure flowchart of Figure 3.3; the transac-tion profile of Figure 3.4; and 

the following assumptions. 

1. Assumptions 1 through 4 of Section 2.4 apply, except 

that in defining availability (assumption 4), "total 

failure" events are assumed to occur once rather than 

twice per year; and "degraded performance" events are 

assumed to affect two messages (rather than one message) 

per month. 

2. Messages are prepared "off-line" on separate teletypewriter equipment by the terminal operator; and are 

transmitted, using the paper tape reader, at a rate of 

150 words per minute. 

29 
3. It is assumed that two complete message transmissions 

per year are misde1ivered (to other compatible terminals 

in a subnetwork within the public-switched network) 

as a result of system switching errors. 

4. It is assumed that lout of 100 calls will be blocked 

within the switching netwprk, resulting in Access 

Denial. This corresponds to a Grade of Service of 1%. 

5. Loss or error in the continuous-spacing (disconnect) 

signal can cause Disengagement Failure at the nonoriginating user. This event is assumed to occur once 

per year .. 

PRIMARY PARAMETER VALUES, CIRCUIT .... SWITCHED EXAMPLE 

(1) 

(2 ) 

Access Time W(a ) = 37.5 seconds 

s 

See Figure 3.4. Includes the t.ime required during 

·physica1·connection (including dialing, set-up, and 

ringing); during modem handshaking; and during terminal 

handshaking. 

Incorrect Access 

2 messag es/17520 

year 

Probability Pea ·-4 ) = 1.lx10 m . 

messages = 1.14X10-4 

year 

Incorrect access is a result of a system switching error 

which connects the originating terminal to an un-. 

intended (but compatible) terminal in the subnetwork. 

(3) Access Denial Probability P(al ) ~ 1.lx10-2 

1,85 denials -;- 17520 messages =1.06X10-2 

year year ' 

One total failure produces 10 denials per year; (note 4, 

Section 2.4); 1% grade of service produces 175 denials 

per year. 

(4a) Block Transfer Time W(b2 ) = 68 milliseconds 

. s . . 

Signal Element (Baud) Time: 

6.67 milliseconds/bit x 10 bits/character 

= 66.7 milliseconds. 

1 Propagation Time: 20,400 

30 

seconds x 20 '1 

ml. '1 ml. es e . 

= 9.8 X10-4 seconds. 
(4b) Bit Transfer Time W(bl ) = 68 milliseconds 

s 

See 4a. 

(Sa) Block Error Probability P(b2e) = 1.5xlO-4 

See Table 2.1. 

(5b) Bit Error probability P(bl ) e 

-5 

= l.5xlO ~ P(ble ) 

-4 

(6a) 

(6b) 

(7 a) 

~ 1.5xlO • 

Error in a single bit or all bits produces a single 

character error. See Table 2.1. 

Block Misdelivery Probability P(b2 ) = 1.lxlO-4 

m 

2 messag es/17520 messages =1.14XIO-4. 

year year 

Bit Misdelivery Probability P(blm) = 1.lxlO-4 

Same as Block Misdelivery Probability_ 

alock Loss Probability P(b2!) = 6.8XIO~4 

See Table 2.1. 

(7b) Bit Loss Probability P(bl!) = 6.8xlO-4 

See Table 2.1. 

(8a) Extra Block Probability = 0 

Transmit protocol has no retransmission or buffering. 

(8b) Extra Bit Probability = 0 

See 8a. 

(9a) Block Transfer Rate R(b2 s ) = 14.6 blocks/second 

-4 good blocks 

618(1-(6.8+1.5)10 ) message = 14 63 good blocks 

U1T seconds • message 

42.2 message 

User clear response adds 1 second to U1'1' time. 

(9b) Bit Transfer Rate R(bls ) = 102.4 bits per second 

7 user bits/block x 14.63 good blocks/second 

= 102.41 bits/second. 

(lOa)Block Rate Efficiency Q(b2s) = 68 percent 

14.63 blocks x 7 bits (user)/150 bits = 

second block second 0.683. 

(lOb)Bit Rate Efficiency Q(bls ) = 68 percent 

102.41 bits. /150 bits 

second second = 0.683. 

31 
(11) Disengagement Time = 2.25 seconds 

See Figure 3.4. Originating user disengagement time 

(3 s.) and nonoriginating user disengagement time 

(1.5 s.) are averaged in determining the Disengagement Time value. 

(12) Disengagement Denial Probability P(d£) = 5.75XIO-5 

1 failur~/year f 17520 messages/year = 5.7XIO-5 

SECONDARY 

(1) 

(2) 

(3 ) 

PARAMETER VALUES, CIRCUIT-SWITCHED EXAMPLE 

Outage Probability P(b3 ) = 1.4XIO-3 

. z. -3 

25 outages/year f 17520 messages/year = 1.43xlO . 

Service Time Between Outages W(y*) = 8.2 UIT 

205 UIThours . 25 outages = 8.2 UIT hours 

year year outage 

Outage Duration W(z*) = 38 minutes 

(24 outages) (1/2 hour)+(l outage) (4 hours) 

25 outages 

= 8.2 UIT hours 

outage 

Note that blocking does not influence the secondary 

parameter values. 

ANCILLARY PARAMETER VALUES, CIRCUIT-SWITCHED EXAMPLE 

(1) User Acces~ Time Fraction: 

p(a) = 0.4 

Access time W(a ) = 37.5 seconds 

s 

Operator responsibility W (a ) = 15 seconds 

u s 

Split responsibility W (a ) = 0 seconds 

v s 

p (a) 

W (a )+W (a ) u s v s 

W (a) s 

15+0.5(0) = 37.5 

(2) User Block Transfer Time Fraction: 

p(b2) = 0 

0.4. 

Block Transfer Time W(b2 s ) = 68 milliseconds 

Operator responsibility W(b2 ) = 0 seeonds 

u s 

Spli t responsibility W .(b2 ) = 0 seconds. 

v s 

(3) User Message Transfer Time Fraction: 

p(b3) = .0237 

32 
Message Transfer Time W(b3 ) = 42.2 seconds 

y 

Operator responsibility W (b3 ) = 1 second 

u y 

Spl£t responsibility W (b3 ) = 0 seconds. 

v y 

(4) User Disengagement Time Fraction: 

p(d) = 0 

Disengagement Time Wed ) = 2.25 seconds 

s 

Operator responsibility W (d ) = 0 seconds 

u s 

Split responsibility W (d ) = 0 seconds. 

v s 

4. MESSAGE SWITCHED EXAMPLE 

4.1. Equipment· Configuration 

The example presented in this. section is a message-switched 

teletypewriter-to-teletypewriter service providing communication 

between pairs of users through an automated store-and-forward 

switching center. The s\Ali tching center is assumed to be implemented in a general-purpose digital computer supported by appropriate local peripherals. The center is connected with remote 

. (user) M37 teletypewriters via conventional Data-Phone ® service 

supported by auto-dial and auto-answer equipment (Fig. 4.1). 

The switching center automatically calls each remote user 

terminal in sequence; reads any previously prepared messages 

awaiting transfer to other users, and stores these messages for 

later delivery; and delivers any previously stored messages 

addressed to the called user. Each end-to-end message-switched 

transaction thus consists of two circuit-switched sub-transactions, separated by a period of message storage in the switch: 

one transaction between the source terminal and the switch, and 

one transaction between the switch and the destination terminal. 

This method of end-to-end communication is similar to that 

illustrated by the message telegram example in Section 2.5 of / 

Volume I. Store-and-forward switching centers of this type are 

commercially available; e.g., see Teleswitcher Corporation (1974). 

33 
103H 

AUTO-ANSWER 

DCE 

Remote M37 Terminal 

103H 

AUTO-ANSWER 

DCE 

Remote M37 Terminal 

Control Console 

(Typical 

User-3.o-User 

, Into7ation Flow) 

103H!801C 

AUTO-DIAL 

DCE 

Digital Computer 

(With Storage Peripherals) 

Control Station 

.. Message Switching Center 

103H 

AUTO-ANSWER 

DCE 

Remote M37 Terminal 

l03H 

AUTO-ANSWER 

DCE 

Local M37 Terminal 

103H 

AUTO-ANSWER 

DCE 

Remote M37 Terminal 

Figure 4.1. Message-switched communication system. 

34 
4.2. Procedure Flowchart and Transacton Profile 

The operating procedures for each circuit-switched subtransaction. can be represented by the flowchart illustrated in 

Figure 3.3. In each sub-transaction, the switch initially assumes 

master status and confirms correct connection by eliciting and 

checking the user terminal answerbaCk (a sequence of 20 preset 

characters ending with ACK). The switch then relinquishes master 

status to receive any waiting messages; and reassumes master 

status to deliver any outgoing messages and terminate the call. 

If an incorrect answerback is received, the switch will disconnect the call and will attempt to redial the proper terminal. 

If repeated redialing still results in unsuccessful connection, 

the switch will bypass that terminal in the polling sequence; if 

a message is being held for that terminal, it will be retained or 

alternately routed to another previously selected terminal for 

delivery. 

A transaction profile for the message-switched example is 

presented in Figure 4.2. The profile is developed from the same 

sources listed (for the circuit switched transaction) in Section 

3.2. Various general information from Teleswitcher Corporation 

(1974) and other message~switching literature is also used. The 

example follows the progress of a single message through a complete end-to-end message-switched transaction, from activation of 

the source terminal through completion of printing at the destination. The example assumes that the message is prepared offline in paper tape form; and that the completed message tape is 

mounted in the M37 paper tape reader prior to the start of the 

transaction. This enables message transfer at 150 wpm during the 

UIT phases of both sub-transactions. The same message format and 

length used in the circuit-switched example are used, with the 

following exceptions: 

1. A one-word destination ID is added to the heading field, 

This ID enables the switch to route the message to the 

intended destination. It is viewed as overhead rather 

than user information (based on the criteria defined in 

35 
End-to-End Sub-Transaction Transaction 

Activities 

Access 

Phase 

(15.4 min) 

First 

DIsengagement 

Function 

T 

EMd-to-End 

User Information Transfer Phase (16.9 min) 

Second 

Disengagement 

Function 

Activities 

ng 

15 MIN 

2S 

4S 

12 S 

2.5 S 

Source 

Access 

to Switch 

5S 

Source 

to$witch 

UIT 42 S 

t 1.5 S Source! 

Switch Disengage- menl __ 

t 1.5 S 

Polling 15 MIN Delay 

2 S 

4S 

12 S 

2.5 S 

Switch 

Access to 

Destination 

5S 

Switch to 

Destination UIT t 42 S 

Figure 4.2. 

ay 

(Data 

lamp 

On) 

(R ..... 

On) 

IData 

Lamp 

Off) 

r 

DTE DCE Sw"l:t DCE/ACU -. DCE/ACU 

Switch Network' 

R 

PoINng Datay 

I 1011 :- C~ 

I 

, DIal Tone I 

I (Auto DialIng) 

I SwItchlng'" 

I RlnaIno ~Sel-Up 

~ AutoAnswer 

"'- ,un tl&,.. ..... F2M ..... 

R .-F1M/ 

C ~un I 

~ I I 

- CBpn I 

I EN EiQ 

~rt>ack AnTI 1K 

6)!~ ~r 

.J:w~ng Headl SOH: • MESSAGE STX 

Text STXng ~EO Text DISK ETX ETX. 

DC3 DC3 

~2S/ CD 011 

CCOff On Hoot< CBOff 

On Hoot< CC .Off • 

PotllnQ Delay 

me Of! 

Dial Tone . (Auto Dialing) 

F I 

I 

+ I CE10n 

." 

1_~rbaCk ACK 

~; 

l~ 

1~~~Jg READ (~!:!) 

t (ETX) 

co;Off 

(End) CCOII 

I 

PublIc DCE DTE 

I 

U Switched 

Network 

Terminal Ena bled 

(Auto Answer ) 

00. I 

Switching 

Call Set-U!: I 

(Auto Answer T Data 

Lamp 

On) 

Off 

V2M Hook 

"'FIM '-, ~ 

-.£L 

i"--F2S,-

On Hook 

I 

CCOn 

Tn 

E7Ql 

Answ:eroack 

ACK 

51 I 

l5 3 

CCOff 

CBOff 

I 

-~f1~g 'i'~,t 

(ETX) 

Wrinted 

Message) 

(Data 

Lamp 

Off) 

Transaction profile for the message-switched 

example. 

36 
Section 2.3 of Volume I), and is not printed at the 

destination terminal. 

2. The heading delimiter characters (*) are replaced with 

their ANSI X3.4 control equivalents: i.e., start of 

heading (SOH), start of text (STX) , and end of text 

(ETX). These nonprinting characters are assumed to 

affect the format of the printed message in a defined 

way, and are thus regarded as user informationS. 

Both sub-transactions (calls) are initiated and terminated 

by the switching center; and in fact, the only user participation 

in the entire transaction is the initiating event (activation of 

the paper tape reader at the source terminal). It is assumed that 

the switch initiates a call to each user terminal in the subnetwork at 30-minute intervals; the average delay between user 

activation of the tape reader and the start of the next call is 

thus 15 minutes. The remote M37 data terminals are assumed to be 

arranged to provide a 20-character automatic answerback sequence 

ending with ACK, in response to ENQ. Message input to the switch 

is assumed to preceed message output to the user terminal if both 

directions of transfer are required during an individual subtransaction. For simplicity, it is assumed that only one message 

is transferred between the participating users during the example 

transaction. It is assumed that the destination user notifies 

the switch when he has no user information to transfer by leaving 

an "EOT tape" in the paper tape reader. 

4.3. Reference Events 

The following paragraphs describe the sequence of user/ 

system interface events during the message-switched transaction; 

and identify the specific reference events which delimit the 

start and end of each of the five primary communication functions. 

SAs an example, the control character ETX might cause the 

switching center to generate, and transmit to the destination 

terminal, a specified number of line feeds followed by a carriage 

return. 

37 
For c~mpleteness, reference events are defined for each of three 

separate transactions: 

Source-to-Switch Sub-Transaction 

switch-to-Destination Sub-Transaction 

End-to~End Transaction. 

Only the latter events would actually be required in specifying 

end-to-end performance, since the switch (like the telegraph 

operator in the message telegram example) is internal to the 

system from the end user point of view. 

4.3.1. Source-to-Switch Sub-Transaction 

ACCESS PHASE, SOURCE TO SWITCH 

The sequence of interface events during the access phase of 

the source-to-switch sub-transaction is as follows: 

1. The operator at the source terminal station places a 

message (previously prepared in paper tape form) on the 

paper tape reader of the teletypewriter, and sets the 

reader auto switch on to enable automatic message transmission. This action represents the reference event 

start of access for the source-to-switch sub-transaction. 

2. After a polling delay averaging 15 minutes, the computer 

at the message switching center initiates a circuitswitched call to the source terminal by activating the 

Call Request (CRQ) signal to the 801C automatic calling 

unit. 

3. Automatic dialing, answering, modem handshaking, and 

answerback take place in the conventional manner, 

ending with the switch having master status. 

4. The switch relinquishes master status and turns on the 

paper tape rea~er at the source terminal by sending the 

control sequence EOT DCl. 

5. The source terminal reads and transmits the first user 

information character (SOH). This.event represents 

the reference event end of access for the source-toswitch sub-transaction. 

38 
USER INFORMATION TRANSFER PHASE, SOURCE TO SWITCH 

During the source-to-switch user information transfer phase 

the source terminal reads and transmits the paper tape message; 

and the message is received and stored at the switching center. 

Reference events for the three UIT functions within the source-toswitch sub-transaction are the same as those defined for the 

private-line example, except that the starting events occur on 

reading of user information characters by the paper tape reader; 

and the ending events occur on storage of user information characters on disk at the switching center. The source-to-switch UIT 

phase ends upon reading of a DC3 character by the source terminal. 

This character would be punched on the source tape by the 

operator during off-line message preparation. 

DISENGAGEMENT PHASE, SOURCE TO SWITCH 

Source-to-switch disengagement terminates the circuitswitched connection between the message switching center and the 

source terminal, and turns the source paper tape reader off. 

The sequence of disengagement events is as follows: 

1. The source terminal reads the DC3 character from the 

source paper tape. This character notifies the switch 

that the source has nothing further to transmit; and 

turns off the source paper tape reader. Reading of 

the DC3 character represents the start of source-toswitch disengagement. 

2. The switch responds to the DC3 signal by turning off 

interchange circuit CD (Data Terminal Ready). This 

event causes source-to-switch disengagement Via a 3-

second continuous spacing signal, in the same manner 

described in Section 3.3. 

4.3.2. switch-to-Destination Transaction 

ACCESS PHASE, SWITCH TO DESTINATION 

Completion of storage of the source message at the switch 

is presumed to result in queueing of a transmit request (speci39 
fying the intended destination) within the computer's message 

output software. This action programs the switch to output 

the message during its next call to the destination terminal 

(assumed to occur during the normal polling sequence, after a 

delay averaging 15 minutes). The sequence of interface events 

during the "access phase" of the switch-to-destination subtransaction is as follows: 

1. Queueing of the transmit request represents the reference event start of access for the switch-to-destination sub-transaction. 

2. After a polling delay averaging 15 minutes, the 

computer at the message switching center initiates a 

circuit-switched call to the destination terminal, in 

the same manner as described above. 

3. Automatic dialing, answer, modem handshaking, and 

answerback take place in the conventional manner, ending 

with the switch having master status. 

4. The switch determines that the destination terminal has 

no message ready for output via an exchange of EOT 

signals. 

5. The switch reads a record containing the first user 

information character from disk storage, and begins 

message transmission. Reading of the first user 

information character from storage represents the 

reference event end of access for the switch-todestination sub-transaction. 

USER INFORMATION TRANSFER PHASE, SWITCH TO DESTINATION 

During the switch-to-destination user information transfer 

phase, the switch reads and transmits the source message characters, and the destination terminal prints the corresponding 

received characters. Reference events for the UIT functions are 

the same as those defined above, except that the starting events 

occur on reading of user information characters from disk 

storage; and the ending events occur on printing of user informa40 
tion characters at the destination. The switch-to-destination 

UIT phase ends on initiation of circuit disengagement by the 

message switching computer, as described below. 

DISENGAGEMENT PHASE, SWITCH TO DESTINATION 

Switch-to-destination disengagement terminates the circuitswitched connection between the message switching -center and the 

destination terminal. The sequence of disengagement events 

is as follows: 

1. The switching computer initiates disengagement by 

turning off interchange circuit CD (Data Terminal 

Ready). This event represents the start of switch-todestination disengagement. 

2. Switch-to-destination disengagement is then effected 

via a 3-second continuous spacing signal, in the same 

manner described in Section 3.3. 

4.3.3. End-to-End Transaction 

To facilitate understanding of reference events for the 

end-to-end (source-to-destination) message-switched transaction, 

essential elements of the access and disengagement definitions 

are repeated below: 

Access: relates only to the source-to-switch portion of 

the end-to-end message-switched transaction. 

Disengagement: is a general function referring to disconnection and clearing at either user 

interface. Source user disengagement and 

destination user disengagement are thus 

regarded as separate "trials" of a general 

disengagement function; and each complete 

end-to-end transaction will include two 

disengagement trials. 

A more complete presentation of these definitions is provided 

in Section 3.2 of Volume I. 

ACCESS FUNCTION (a) 

·The access function starts when the source terminal operator 

turns on the M37 reader auto switch (with the completed 

41 
message tape mounted in the tape reader). A system with 

zero access time would begin reading the message immediately 

after this action. 

"The access function ends when the source terminal reads the 

first user information character off the source tape. 

USER INFORMATION TRANSFER FUNCTIONS (bl, b2, and b3) 

-Each function starts with reading of a specified user information character (off the source tape) by the source terminal. 

-Each function ends with printing of a specified received 

character by the destination terminal. 

The block and message information units are specified in the same 

way as in Section 2.3. 

DISENGAGEMENT FUNCTIONS (d) 

Two disengagement functions take place during the end-toend transaction: source user disengagement and destination user 

disengagement. 

-The start (d) of the source user disengagement function 

occurs when the source termination reads the DC3 control 

character. 

-The end (d*) of the source user disengagement function 

occurs when circuit CC (and the data lamp) go off at the 

source terminal. At this point, the source user and terminal are free to participate in subsequent transactions 6 

-The start (d) of the destination user disengagement function 

occurs when the destination user receives the last character of the programmed ETX sequence (Sec. 4.2). 

-The end (d*) of the destination user disengagement function 

occurs when circuit CC (and the data lamp) go off at the 

destination terminal. 

6As an example, the station might provide circuit-switched access 

to a population of users within a different subnetwork, as 

described in Section 3. 

42 
RESPONSIBILITY TRANSFER EVENTS 

The system has unilateral responsibility throughout the 

assumed end-to-end transaction; hence there are no responsibility 

transfer events. 

4.4. Parameter Values 

Probable performance parameter values for the messageswitched example are derived on the basis of the transaction 

profile of Figure 4.3; and the following assumptions. 

1. Assumptions 1 through 4 of Section 2.4 apply, except 

that in defining availability (assumption 4), "total 

failure" events are assumed to occur once per year, 

and degraded performance events are assumed to affect 

two messages per month, on each user-switch circuit. 

2. Messages are prepared "off-line" on separate teletypewriter equipment by the source terminal operator; and 

are transmitted over both circuit-switched connections 

at 150 wpm. 

3. The switch polls each user terminal every half hour, 

in a continuous round-robin cycle, independent of the 

destinations of locally stored messages. 

4. A 1% Grade of Service is assumed for each circuitswitched sub-transaction. The message switch will 

attempt to redial blocked calls three times, and will 

then skip the subject terminal in the polling sequence, 

resulting in an Access Denial. The probability of 

three successive blocked calls is assumed to be 10-4 

for each circuit-switched sub-transaction. 

5. Incorrect connection (connection of the message switch 

to an unintended, but compatible terminal) is assumed 

to have the same probability of occurrence specified 

for Incorrect Access in Section 3; i.e., 1.lXIO- 4 . 

- Checking of the user terminal answerback sequence will 

reduce the sub-transaction Incorrect Access Probabilities by at least a factor of 10-6 , to essentially 

negligible proportions. Detected incorrect answer43 
backs will result in automatic redialing by the m,essage 

switching center, contributing (negligibly) to Access 

Denial Probability. 

6. Loss or error in the continuous spacing disconnect 

signal can cause Disengagement Failure at either user 

terminal. This event is assumed to occur once per year 

at each terminal. 

7. It is assumed that the message switch will discard one 

received source message a month as a result of detection of "substantial" (i.e., multiple) character errors 

or losses. In all such cases, the switch will notify 

the source user of the message non-delivery via a 

control printout. This feature provides a rudimentary 

form of retransmission error control. 

8. In addition to outages in the circuit-switched network, 

outages may occur (independently) at the switching 

center. Power failure 'is assumed to be the dominant 

cause of switching center outages. Such outages are 

assumed to have the same frequency and average duration as circuit outages, i.e., one 4-hour outage per 

year. The switch will assume that any message being 

transmitted at the time of onset of a power outage is 

lost, and will retransmit the message in its entirety. 

This feature will result in duplicate delivery of a 

message fragment averaging half a message (300 characters) in length about every 3.6 years. 

9. Misdelivery will occur (in the end-to-end transaction) 

whenever an undetected source-to-switch line error 

changes a message destination address to the valid 

address of another user in the subnetwork. The undetected address error rate is assumed to be 10-6 ; and 

the probability of an address word error producing an 

assigned but unintended address is assumed to be 10-3 . 

Values are developed independently for each of the three 

communication transactions defined in Section 4.3. Parameter 

values for the source-to-switch and switch-to-destination sub44 
transactions are presented in Table 4.1. These values are based 

on the same calculations used in the circuit-switched example, 

with the following exceptions: 

1. Access Times include the assumed 15 minute average 

polling delay in each case. 

2. Source-to-switch block and bit loss probabilities are 

increased from the "raw channel" values by the assumption that the switch discards messages received with 

"substantial" detected errors. The source-to-switch 

block and bit error probabili ti·es are correspondingly 

reduced. 

3. Extra bits and blocks occur as a result of switch 

outage during switch-to-destination transmission, as 

described above. 

4. Ancillary parameter values are not calculated for the 

sub-transactions. Values for these parameters could be 

determined, by defining an "intermediate user" (of the 

public switched network) within the switch; but these 

values would not be useful in characterizing the endto-end transaction since the switch is not a user from 

the end-to-end viewpoint. 

5. Source messages discarded by the switch as a result of 

"substantial" errors or losses are counted as outages 

in characterizing the source-to-switch service. 

PRIMARY PARAMETER VALUES, END-TO-END MESSAGE-SWITCHED EXAMPLE 

(1) Access Time W(a ) = 15.4 minutes 

s 

(2) 

See Figure 4.3. The access function relates only to 

the source-to-switch portion of the end-to-end messageswitched transaction. A IS-minute average polling 

delay is included. 

Incorrect Access probability Pea ) = 0 

m 

Incorrect access cannot occur in message-switched 

systems since user information transfer begins before 

the intended destination user is contacted. 

45 
.1:>0 

0"1 

Table 4.1. Sub-Transaction Performance Parameter Values 

Parameter 

Access Time 

Incorrect Access Probability 

Access Denial Probability 

Block Transfer Time 

Bit Transfer Time 

Block Error Probability 

Bit Error Probability 

Block Misdelivery Probability 

Bit Misdelivery Probability 

Block Loss Probability 

Bit Loss Probability 

Extra Block probability 

Extra Bit Probability 

Block Transfer Rate 

Bit Transfer Rate 

Block Rate Efficiency 

Bit Rate Efficiency 

Disengagement Time 

Disengagement Denial Probability 

Outage Probability 

Service Time Between Outages 

Outage Duration 

Source-to-Switch 

Sub-Transaction 

15.4 minutes 

o 

6.7xlO- 4 

68 milliseconds 

68 milliseconds 

10-4 

lO-5$p(b1 )$10-4 

e 

o 

o 

10-3 

lO-j 

o 

o 

14.8 blocks/ 

second 

104 bits/second 

69% 

69% 

2.25 seconds 

5.7xlO-5 

-3 1.4xlO 

8.2 UIT hours 

38 minutes 

Note: See text for assumptions and explanation. 

Switch-to-Destination 

Sub-'l'r ansact ion 

15.4 minutes 

o 

-4 6.7xlO 

68 milliseconds 

68 milliseconds 

1. 5xlO- 4 

1.5xlO-5 $P(bl )$1.5xlO-4 

e 

o 

o 

-4 6.8xlO 

-4 6.8xlO 

7.7xlO-6 

7.7xlO- 6 

14.8 blocks/second 

104 bits/second 

69% 

69% 

2.25 seconds 

-5 5.7xlO 

-3 1. 4xlO 

8.2 UIT hours 

38 minutes 
(3) Access Denial Probability p(a!)=6.8 xIO-4 

12 denials ~ 17520 messages = 6.8XIO-4 

(4a) 

year year 

One total failure produces 10 denials per year (see 

note 4, Section 2.4); 10-4 3-call blocking probability 

produces about 2 denials per year. 

Block Transfer Time W(b2 ) = 16.2 minutes 

s 

See Figure 4.2. Includes all time between start of 

source-to-switch UIT and start of switch-to-destination 

UIT. 

(4b) Bit Transfer Time W(bl ) =16.2 minutes 

s 

(5a) 

(5b) 

See 4a. 

Block Error Probability P(b2 ) = 2.5XIO-4 

e 

See Table 4.1. Errors in the source-to-switch link 

are reduced (by a factor of 30%) by the switch's 

error detection feature. 

Bit Error Probability: 2.5xlO-5 ~ P(bl ) ~ 2.5xlO-4 

e 

Error in a single bit or all bits produces a single 

character error. 

(6a) Block Misdelivery Probability P(b2 ) = 6XIO-7 

m 

10-6 undetected address errors x 10-3 valid addresses 

message error 

x 600 misdelivered blocks = 6XIO-7 . valid address 

(6b) Bit Misdelivery Probability P(bl ) = 6XIO-7 

m 

See 6a. Both numerator and denominator are multiplied 

by 7 in the bit misdelivery case. 

(7a) Block Loss Probability P(b2!) = 1.7xIO-3 

See Table 4.1. The separate loss probabilities for each 

sub-transaction are added to produce the end-to-end 

loss probability. 

(7b) Bit Loss Probability P(bl!) = 1.7xIO-3 

See 7a. 

47 
(8a) Added Block Probability P(b2x ) = 7X10-7 

switch outages 

(during transmission 

to given destination) x 300 extra blocks 

year outage 

= 7 extra blocks 

year 

One switch outage is assumed per year; probability that 

the switch is transmitting to a given destination at 

outage onset is 84 seconds/hr transmission time 

~3600 seconds/hr total time=2.3%. 

7 extra blocks ~ lX10 7 blocks/year = 7X10- 7 • 

year 

(8b) Added Bit Probability P(blx ) = 7X10-7 

See 8a. 

(9a) Bit Transfer Rate R(b2 s ) = 36.2 blocks/minute 

-3 good blocks 

618(1-10) message = 36.2 blocks/minute 

16.9 UIT minutes/message 

15 minutes (average) message storage is counted as 

UIT time. 

(9b) Bit Transfer Rate R(bls ) = 4.2 bits/second 

7 bits (information)/block x 0.6 blocks/second 

= 4.2 bits/second. 

(lOa)Block Rate Efficiency Q(b2s) = 2.8 percent 

0.6 blocks x 7 bits (user)/150 bits = 0.028. 

second block second 

(lOb)Bit Rate Efficiency Q(bls ) = 2.8 percent 

4.2 bits /150 bits = 0.028 

second second • 

(11) Disengagement Time W(ds ) = 1.5 seconds 

See Figure 4.3. Both user terminals are cleared 1.5 

seconds after request issuance. 

(12) Disengagement Denial Probability = 1.lX10-4 

1 failure/terminal year x 2 terminals ~ 17520 messages -4 year = 1.14xlO ." 

48 
SECONDARY 

(1) 

(2) 

(3) 

PARAMETER VALUES, END-TO-END MESSAGE SWITCHED EXAMPLE 

Outage Probability = P(b3 ) = 3X10-3 

z 

51 outages/year 7 17520 messages/year = 2.9XIO-3 . 

Service Time Between Outages W(y*) = 96 hours 

4.9xl0 3 UIT hours 7 51 outages = 96 UIT hours 

year year outage 

Outage Duration W(z*) = 42 minutes 

(48 outages) (1/2 hour)+(3 outages) (4 hours) - 0 7 h 

51 t - . ours. ou ages . 

1 outage/month/linkx2 linksx12 months/year=48 outages/ 

year. 

ANCILLARY PARAMETER VALUES, END-TO-END MESSAGE-SWITCHED EXAMPLE 

All user performance time fractions are zero since the 

system has unilateral responsibility throughout the end-to-end 

transaction. 

5. SUMMARY AND CONCLUSIONS 

This volume has illustrated application of the performance 

parameters specified in proposed Federal Standard 1033 to the 

characterization of three example telecommunication services. 

The selected examples are similar in having a common data 

terminal (the M37 teletypewriter) as the user interface point; 

but differ widely in system transmission and switching facilities 

and user/system interaction sequences. 

Table 5.1 is a compilation of performance parameter values 

for the three selected examples. It is emphasized that these 

values characterized the examples, under the stated assumptions, 

and are not intended to be used in characterizing or comparing 

actual offered services. The example services are vastly 

different in performance, by design; and they would be appropriately used in meeting correspondingly different end-to-end 

service requirements. A brief summary and comparison of the 

estimated parameter values for each example follows. 

Access Time varies widely between the three examples, as 

would be expected from their differing approaches to the funda49 
Table 5.1. Compilation of Parameter Values 

NONSWITCHED 

PERfORMANCE PRIVATE LINE CIRCUIT -SWITCHED MESSI\GE-SLHTCHED 

PARA.~!::TER Example EXAMPLE EXf-MPLE 

Access Time 5.2 Seconds 37.5 Seconds 15.4 Mi nutes 

Incorrect Access Probabi 1 i ty 0 1.1 x 10-4 0 

Access Oeni a 1 Probabil ity 1.7 x 10- 3 1.1 x 10-2 6.8 x 10-4 

Block Transfer Time 68 milliseconds 68 mi 11 i seconds 16.2 minutes 

Bit Transfer Time 68 mi 11 i seconds 68 mi 11 i seconds 16.2 minutes 

Block Error Probabi 1 i ty 1.5 x 10-4 1.5 x 10-4 2.5 x 10-4 

Bit Error Probability .5x10- 5< P(bl )< 1.5x10-4 - e- 1.5x10-5< P(b1 )<1.5x10-4 - e- 2.5x10-5.:;. P(b1 e )2.5xlO- 4 

Block Mi sde 1 i very Probabil i t~ 0 1.1 x 10-4 6 x 10- 7 

Bit Misdelivery Probability 0 1.1 x 10-4 6 x 10- 7 

Block Loss Probability 4.7 x 10- 5 6.8 x 10-4 1.7 x 10- 3 

Bit Loss Probabil ity 4.7 x 10-5 6.8 x 10-4 1.7 x 10- 3 

Extra Block Probabi 1 ity 0 0 7 x 10- 7 

Extra ai t Probabil i ty 0 0 7 x 10- 7 

Block Tran<;fer Rate 5 blocks/second 14.6 blocks/second 36.2 blocks/minute 

Bit Transfer Rate 35 bits/second 102.4 bits/second 4.2 bits/second 

Block Rate Efficiency 23 percent 68 percent 2.8 percent 

ili t Rate Efficiency 25 percent 68 percent 2.8 percent 

01 sengagement Time 0.5 seconds 2.25 seconds 1.5 seconds 

Oi sengagement 4 x 10-4 5.7 x 10- 5 1.1 x 10-4 

Denial Probability 

Outage Probabil ity 8 x 10-4 1.4 x 10-3 3 x 10- 3 

Service Time 

Between Outages 43 UIT hours 8.2 UIT hours 93 UIT hours 

Outage Duration 1 hour 38 minutes 42 mi nutes 

User Access Ti:'le Fraction 0.19 0.4 0 

User Block Transfer 0 0 0 Time Fraction 

User Message Transfer 0.67 0.02 0 Time fraction 

User Oi sengagement 0.8 0 0 Time fraction 

50 
mental issue of resource sharing. Access Time is shortest in 

the nonswitched example, where no resource sharing is attempted; 

is intermediate in the circuit-switched example, where an endto-end information path is established between user terminals 

on request, for each individual transaction; and is longest in 

the message-switched example, where user access opportunities are 

afforded at relatively infrequent intervals, independent of user 

demand. 

Incorrect Access Probability is nonzero only in the circuitswitched case. There is no switching in the private line case; 

and in the message-switched case, the first system contact with 

the destination user takes place after the end of access. 

Access Denial Probability values also vary as a function of 

the "priority" given individual users by the resource sharing 

approach. Access Denial Probability is highest in the circuitswitched case, as a result of the assumed 1% Grade of Service; 

is intermediate in the nonswitched case, which is nonblocking 

but relatively vulnerable to errors in the Access Request characters; and is lowest in the message-switched case, which 

provides access via circuit switching but allows automatic redialing to reduce call blocking probability. 

With the exception of the error probabilities and transfer 

rates, the bit-oriented and block-orientedUIT parameter values 

within each individual example are the same. This is true because the bits within each user information block (character) 

cross the user/system interface in parallel; and each block 

contains (represents) a fixed number of bits. In this special 

case, separate values for the corresponding bit- and blockoriented UIT parameters are not really required; but they will be 

required in general, since the bits within a block may cross the 

user/system interface serially and may vary in number. 

Block (and Bit) Transfer Times are extremely low (and 

essentially identical) in the nonswitdhed and circuit-switched 

examples, reflecting the fact that each service provides an 

essentially real-time information path between users. The 

51 
corresponding times are substantially longer in the messageswitched case as a result of the relatively long polling delay 

assumed. While the message-switched Block Transfer Time does 

seem excessive in comparison with the values for the other 

examples, it is substantially shorter than that provided by 

many alternative message transfer services (e.g., u.s. mail); 

and a l6-minute delay can be quite acceptable in the type of 

non-real time applications to which message switching has 

traditionally been applied. This delay could of course be 

shortened substantially by a traffic-sensitive polling algorithm, 

at some cost in circuit and switch utilization. 

Block (and Bit) Error Probabilities are estimated to be 

essentially the same in each case, since none of the example 

systems has any significant error control capability. Error 

probabilities are estimated to be slightly higher in the messageswitched case, since this service requires transmission of each 

message character over two (tandem) links. Delivered error 

probabilities are improved slightly in the latter case by the 

switch's provision for discarding source messages received with 

substantial detected errors; but this provision increases the 

loss (nondelivery) probabilities correspondingly. It would be 

advantageous to distinguish nondelivery with notification from 

loss in describing the performance of systems with this feature. 

Block (and Bit) Misdelivery Probabilities are nonzero in 

both the circuit-switched and message-switched services. In 

each case, the estimated values are highly dependent on two 

factors: the probability of address errors, and the probability 

of connection (or delivery) to an unintended but compatible 

terminal, given an address error. Misdelivery is assumed to be 

more likely in the circuit-switched case, since the public 

switched network has relatively little provision for detecting 

signalling errors. 

Block (and Bit) Loss Probabilities are lowest in the nonswitched case, as a result of the less variable transmission 

quality of private line services; are intermediate in the 

52 
circuit-switched case; and are highest in the message switched 

case, for two reasons: 

1. The switch intentionally discards messages with 

substantial detected errors, and the blocks in such 

messages are counted as lost; 

2. Each block must traverse two tandem links in transit 

between end users. 

Extra Block (and Bit) Probability is estimated to be nonzero only in the message-switched case, since neither of the other 

services has provision for "remembering" 9reviously transferred 

blocks. In principle, bit errors in the Disengagement Request 

characters (DLE EOT and the continuous spacing signal) can 

result in printing of extra characters in the nonswitched and 

circuit-switched services; we disregard this possibility here 

since these characters are most likely to be transformed into 

other (nonprinting) control characters. The message-switching 

service will deliver a substantial message fragment containing 

many Extra Blocks to the destination user whenever switch operation is interrupted by a power outage, since the switch retransmits such interrupted messages in their entirety. 

Block (and Bit) Transfer Rates and Rate Efficiencies differ 

widely between the three examples as a result of the same 

design differences discussed above in connection with Access 

Time and Block Transfer Time. The differences between the nonswitched and circuit-switched values are primarily a result of 

differences in user input delay, as can be demonstrated by 

comparing the corresponding user-independent values (see Sec. 

5.3 of Volume I). The message-switched rate and rate efficiency 

values are low as a result of (1) storage in the switch, and 

(2) the absence of "pipelining" in traditional message-switched 

systems (Kleinrock, 1976). 

Disengagement Time values are essentially similar in the 

three examples, and are relatively insignificant under the 

assumed loading conditions. Disengagement Time values become 

more significant as service utilization increases, since dis53 
engagement delays postpone subsequent access whenever the queue 

of user messages awaiting transmission is not empty. 

Disengagement Denial Probability estimates reflect differences in the disengagement procedures employed in each example 

service. Disengagement Denial is least likely in the circuitswitched case, since the 3-second continuous spacing signal 

represents a highly redundant (and hence reliable) disconnect 

signal. Disengagement Denial is roughly twice as likely in the 

message-switched case, since there are two possibilities for 

transmission errors in the continuous spacing signal - one 

associated with each user terminal. The nonswitched service is 

most vulnerable to disengagement failure, since the Disengagement 

Request is transmitted via a single (DLE-EOT) character p~ir: 

all the "eggs" are in one "basket." 

Outage Probability estimates are affected by the assumption 

that two distinct phenomena, "total" failures and degraded 

performance, can produce observed outages. Outage Probability 

is lowest in the nonswitched case, where performance is assumed 

to be least variable; is intermediate in the circuit-switched 

case, where more frequent instances of degraded performance are 

assumed; and is highest in the message-switched case, for two 

reasons: 

1. The tandem link effect described previously; 

2. The fact that the switch itself can fail. 

Service Time Between Outage estimates are sensitive to two 

factors: the number of assumed outages per measurement period; 

and the UIT time required to obtain a given "increment" of 

service (e.g., complete message transfer) from each example 

system. Service Time Between Outages is lowest in the circuitswitched case, since the number of assumed outages is relatively 

high and the UIT time required to transfer a message is low. 

Service Time Between Outages is intermediate in the nonswitched 

case, where the number of "degraded performance" outages is 

lower and the UIT time per message longer; and is highest in 

the message switched case, which has a much longer UIT time per 

54 
unit of service than either of the other examples as a result of 

storage time in the switch. In effect, the message storage time 

adds to the end-to-end UIT time without providing any UIT 

service (or risking any outages); and this time "inflates" the 

W(y*) parameter value. This points up a fundamental difference 

between Service Time Between Outages and Outage Probability: the 

former expresses the frequency of outages in units of time, 

irrespective of service (UIT) rate; whereas the latter expresses 

the frequency of outages in units of service provided (messages 

transferred), irrespective of time. Separate W(y*) values can 

only be directly compared between services having the same UIT 

rate; whereas Outage Probability values can be directly compared 

irrespective of service rate. 

Outage Duration estimates are determined simply by the mix 

of short and long outages assumed. Outage Duration is lowest in 

the circuit-switched example, which has the highest proportion 

of short (degraded performance) to long (total failure) outages; 

1S intermediate in the message-switched example; and is highest 

in the nonswitched example, which has the lowest proportion of 

short to long outages. This relationship is consistent with 

intuitive expectation, since switched services will (in general) 

transfer information over a different physical path on each 

transaction, thus producing more variable performance. 

User Performance Time Fraction values reflect differences in 

the user component of primary function performance time between 

the example services. User Access Time Fraction is zero in the 

message-switched case, since this is a polling application in 

which the system has unilateral access responsibility. User 

Access Time Fraction is relatively low in the nonswitched case, 

where the only user delay is his response to the "clear to send" 

indication; and is highest in the circuit-switched case, 

reflecting the users' dialing and answer responsibilities. User 

Block Transfer Time Fraction is zero in all three examples, since 

the system has unilateral responsibility for block transfer in 

each case; but this parameter will have a nonzero value in any 

service where the user takes an active role during the block 

55 
transfer performance period7 . User Message Transfer Time 

Fraction is zero in the message-switched example, again reflecting 

the system's unilateral responsibility; is low in the circuitswitched example, where the only user responsibility during 'the 

UIT phase is to terminate the phase after the last message 

character has been read; and is highest in the nonswitched case, 

where the source user's input (typing) rate reduces the UIT rate 

by about a factor of 3. User Disengagement Time Fraction is nonzero only in the nonswitched case, where a small amount of 

excess DLE EOT typing time is technically attributable to the 

source user. There is no instance of "split" responsibility in 

the examples chosen, but this situation ~an occur in more 

sophisticated services, as was described in Section 5.2 of 

Volume I. 

In surveying the results summarized in Table 4.1, two 

general observations can be made: 

1. In each of the three example services described it 

was possible, with reasonable effort, to develop 

representative values for all 26 specified performance 

parameters. More precise estimates of parameter 

values, and additional performance information such as 

parameter distributions, could be developed with 

additional analysis (or measurement) effort. 

2. With a single exception (User Block Transfer Time 

Fraction), none of the specified parameters had a 

value of zero in all three examples. The latter parameter will have a nonzero value in various other types 

of services, as was noted above. 

These observations provide general support for the conclusion that the specified parameters are in fact consistent with 

7"Mailbox" services provide one example of this situation: a 

destination user must place a call to a remote message storage 

facility (within the system) to "read his mail" from other 

(source) users. One well-known computer network which provides 

this service to subscribers is the ARPA network. 

56 
the four "desired parameter attributes" listed in Section 1.1 

of Volume I. 

It is anticipated that additional application examples will 

be developed to illustrate use of the proposed standard parameters in characterizing computer-to-computer and teleprocessing 

services8 . Several of the specified parameters have already been 

measured on the ARPA network (Payne, 1978); and measurement plans 

for the remaining parameters are under development. 

80ne such example (communication between remote "user processes" 

on the ARPA network) is outlined in Appendix B of the published 

draft standard (National Communications System, 1977). 

57 
6. REFERENCES 

ANSI (1967), USA standard code for information interchange (USAS 

X3.4-l967) . 

ANSI (1971), American national standard code for information 

interchange in specified data communication links (ANSI 

X3.28-l97l). 

AT&T (1968a), Bell System Technical Reference Pub. 41102, Data 

set 103E/103G/103H interface specification, October. 

AT&T (1968b), Bell System Technical Reference, model 37 teletypewriter stations for Data-Phone® service, September. 

AT&T (197la), Bell System Technical Reference Pub. 41007, 1969-70 

telecommunications network connection survey, April. 

AT&T (197lb), Bell System Technical Reference Pub. 41713, model 

33, 35, and 37 stations for point-to-point private line 

service, August. 

EIA (1969), EIA Standard RS-232-C, Interface between data terminal 

equipment and data communication equipment employing serial 

binary data interchange, August. 

Kleinrock (1976), Queueing Systems, Volume 2: Computer Applications (John Wiley and Sons, Inc., New York, NY). 

National Communications System (1977), Proposed federal standard 

1033, Federal Register, Vol. 42, No. 35, February 22. 

Payne, J.A. (1978), ARPANET host to host access and disengagement 

measurements, Office of Telecommunications Report No. 78-

U.S. Dept. of Commerce, Boulder, CO 80303. 

Teleswitcher Corporation (1974), Specification, teleswitcher® 

model 510/24 message switching system, Dec. 16. 

® Teletype (1969), Teletype model 37 product catalog, April. 

58 
APPENDIX A. USASCII COMMUNICATION CONTROL CHARACTERS 

ENQ Enquiry - A communication control character used in data 

communication systems as a request for a response from a 

remote station. 

ACK Acknowledgment - A communication control character transmitted by a receiver as an affirmative response to a 

sender. 

NAK Negative Acknowledgment - A communication control character 

transmitted by a receiver as a negative response to the 

sender. 

SOH Start of Heading - A communication control character used 

at the beginning of a sequence of characters which constitute a machine-sensible address or routing information. 

Such a sequence is referred to as the heading. 

STX Start of Text - A communication control character which 

precedes a sequence of characters that is to be treated 

as an entity and entirely transmitted through to the 

ultimate destination. Such a sequence is referred to as 

text. 

ETX End of Text - A communication control character used to 

terminate a sequence of characters started with STX and 

transmitted as an entity. 

EOT End of Transmission - A communication control character 

used to indicate the conclusion of a transmission, which 

may have contained one or more texts and associated 

headings. 

DLE Used as a prefix to additionalUSASCII characters to form 

a sequence representing a communication control function 

not directly represented by a single control character. 

NOTE 

For a more complete description of USASCII control character 

functions, refer to ANSI (1967). 

59 
APPENDIX B. RS-232 INTERCHANGE CIRCUITS 

Interface between Data Terminal Equipment and Data Communication Equipment employing Serial Binary Data Interchange. 

CIRCUIT AA - Protective Ground 

Ground conductor that is electrically bonded to the 

machine or equipment frame. 

CIRCUIT AB - Signal Ground or Common Return 

Establishes common ground reference potential for all 

interchange circuits except circuit AA (protective ground). 

CIRCUIT BA - Transmitted Data 

Generated by the Data Terminal Equipment and transferred 

to the local transmitting signal converter for transmission 

to remote data terminal equipment. 

CIRCUIT BB - Received Data 

Generated by the receiving signal converter in response 

to data signals received from remote data terminal 

equipment via the remote transmitting signal converter. 

CIRCUIT CA - Request to Send 

Conditions the local data communication equipment for 

data transmission. 

CIRCUIT CB - Clear to Send 

Data set is ready to transmit data. 

CIRCUIT CC - Data Set Ready 

Indicates the status of the local data set. 

CIRCUIT CD - Data Terminal Ready 

Used to control switching of the data communication equipment to the communication channel. 

CIRCUIT CE - Ring Indicator 

Ringing signal being received. 

CIRCUIT CF - Received Line Signal Detector 

Suitability criteria being met. 

NOTE 

For a more complete description of RS-232-C interchange circuits, 

refer to EIA (1969). 

60