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of Commerce

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of Standards

Institute For Computer

Sciences and Technology

A 11 IDS L554A7

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Sinology

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PUBLICATIONS

NBSIR 87-3536

A Methodology for Broadband Token

Passing Bus S nteroperabi I ity Testing

Robert Rosenthal

Daniel R Stokesberry

Computer Science

and Technology , ItCl?

? 4

NBSIR 87-3536

A Methodology for Broadband Token

Passing Bus Interoperability Testing

Robert Rosenthal

Daniel P. Stokesberry

Institute for Computer Sciences and Technology

National Bureau of Standards

Gaithersburg, MD 20899

Issued April 1987

U.S. DEPARTMENT OF COMMERCE

Malcolm Baldrige, Secretary

National Bureau of Standards

Ernest Ambler, Director

TABLE OF CONTENTS

ABSTRACT , 1

INTRODUCTION 2

IEEE 802.4 OPTIONS 3

TESTBED PARAMETRIC ASSUMPTIONS 4

CHARACTERISTICS TO BE TESTED 5

TEST FRAMES 6

TEST FRAME TRANSMITTERS AND RECEIVERS 7

TEST FRAME A and A* (PRIME) 7

TEST FRAME B AND B

J (PRIME) 8

CONTENTION GENERATOR . 9

ESTABLISH A LOGICAL RING 9

TEST DEFINITIONS 10

BASELINE 10

SHORT FRAME TEST 15

LONG FRAME TEST 21

REMODULATOR RESPONSE TO SIMULATED CONTENTION ...... 23

TOKEN BUS TEST 32

TABLES USED TO CONDUCT THE TESTING 36

SHORT FRAME TABLES 3 6

LONG FRAME TABLES 36

CONCLUSIONS 40

iii
FIGURES

Figure 1. IEEE 802.4 TEST METHODOLOGY 4

Figure 2. BROADBAND BUS OPTION SELECTION 4

Figure 3. PARAMETER VALUES FOR INTEROPERABILITY TESTS ... 5

Figure 4. EQUIPMENT CHARACTERISTICS 6

Figure 5. TESTING CAPABILITY OF VENDOR EQUIPMENT 7

Figure 6a. TEST FRAME A DATA 8

Figure 6b. TEST FRAME A' (PRIME) DATA 8

Figure 7a. TEST FRAME B DATA 9

Figure 7b. TEST FRAME B' (PRIME) DATA 9

Figure 8. MAC PARAMETERS 10

Figure 9a SCHEMATIC DIAGRAM OF CABLE PLANT 12

Figure 9b HEADEND CONFIGURATION 12

Figure 10a CABLE PLANT SWEEP 13

Figure 10b FORWARD CHANNEL 13

Figure 10c REVERSE CHANNEL 13

Figure 11. PSEUDO SILENCE 14

Figure 12. TEST FRAME A 16

Figure 13. TEST FRAME A' (PRIME) 19

Figure 14. END OF TEST FRAME A' (PRIME) 20

Figure 15. TEST FRAME B 22

Figure 16. TEST FRAME B ' (PRIME) 22

Figure 17. UPLINK PSEUDO SILENCE TEST 25

Figure 18. UPLINK FRAME FRAGMENT 26

Figure 19. UPLINK CW WITH PAD IDLE 27

Figure 20. UPLINK CW THEN SILENCE . 28

TABLES

Table 1. Summary of Short Frame Tests 17

Table 2. Summary of Long Frame Tests 23

Table 3. Modem Addresses and Transmit Power Levels 32

Table 4. Summary of Ring Test (No Induced Noise) 34

Table 5. Summary of Ring Tests (With Induced Noise) .... 35

Table 6. Test Completion Matrix 3 6

Table 7. SHORT FRAME TEST SHEET 37

Table 8. LONG FRAME TEST SHEET 38

Table 9. RING TEST SHEET 39

iv
A METHODOLOGY FOR BROADBAND TOKEN PASSING BUS

INTEROPERABILITY TESTING

Robert Rosenthal and Daniel Stokesberry

National Bureau of Standards.

ABSTRACT

A method for testing broadband token passing bus interoperability

is described. The method was tested in a controlled laboratory

environment at the National Bureau of Standards with four modern

manufacturers' and three headend remodulator manufacturers'

equipment. The tested equipment implemented specific token

passing bus options found in commercially available products.

The methodology specifies seven environmental and electrical

parameters; for the first set of tests, nominal parameter

settings were selected. The methodology uses specific test

frames developed for their unique bit patterns. These frames

were transmitted, received and counted to exercise the broadband

modems and headend remodulators. The methodology organizes the

frame transmissions into five test definitions. Each test

definition is repeated for each headend remodulator.

Keywords: Local area Networks; physical layer testing; protocol

testing; test methodology; token bus.

1
1 . INTRODUCTION

The manufacturing automation community is developing a

Manufacturing Automation Protocol (MAP) specification 1 derived

from specific International Standards Organization (ISO) data

communication protocols. These protocols, developed in a

layered framework or model known as the ISO Open Systems

Interconnection (OSI) Basic Reference Model include

contributions from the American National Standards Institute

(ANSI). ANSI accredits the Institute of Electrical and

Electronic Engineers (IEEE) to develop technical standards. The

IEEE has developed a family of local area networking standards

for use in office, factory and process control applications.

These standards are published with both an ANSI/IEEE reference

and an ISO reference. The MAP specification includes "ANSI/IEEE

802.4-1985 ISO/DIS 8804 Token Passing Bus," 3 a document often

referred to simply as 802.4.

802.4 specifies protocols and interfaces at the lowest layers of

the OSI Basic Reference Model -- the physical layer and a portion

of the link layer called the media access control sublayer or

MAC. The physical layer and MAC sublayer, like most protocol

specifications, identify many options for the myriad of

applications envisioned. With these standards in place, user and

vendor communities are able to select appropriate options for

specific applications.

The manufacturing automation community has not only identified

802.4 for use in the MAP specifications, it has worked closely

with the National Bureau of Standards’ Implementors Workshop to

develop a set of options for use in products that implement OSI

protocols. An "agreements" document ^ defines the specific

options selected for 802.4 as well as the specific options for

other OSI protocols selected for use in the upper layers. These

workshops are expected to expedite the development of OSI

protocols and promote interoperability of independently

manufactured data communications equipment.

End users and original equipment manufacturers or OEM users

expect this equipment to interoperate. Larger automated

machinery and smaller mini and micro systems and controllers from

different manufactures must work together on the factory floor

providing the user with cost effective, off-the-shelf solutions

for distributed computer applications. To demonstrate token

passing bus interoperability, a test methodology has been

designed and tested by 802.4 equipment vendors working closely

with the National Bureau of Standards.

The test methodology demonstrates token passing bus physical

layer interoperability by systematically testing the modem's

capabilities to transmit and receive data frames through

different headend remodulator equipment and by systematically

2
testing the remodulator’s capability to respond appropriately to

noise bursts and other media specific phenomena and physical

layer protocols.

This methodology describes a set of five tests designed to

demonstrate the electrical and functional compatibility of

different 802.4 broadband implementations. In test 1, a baseline

cable plant facility that meets or exceeds the 802.4 broadband

cable plant specification is established. Each remodulator is

switched, one at a time, into the cable plant facility where the

assigned forward channel is checked for the presence of pseudo

silence -- the normal remodulator output when no input signal is

present. Test 2 demonstrates the transmission and reception of

short frames. Good and bad frames are counted and recorded; in

test 3, long test frames are transmitted and received. In test

4, frame fragments and other combinations of simulated

contentions are sent to the remodulator. Finally, in test 5, a

ring is established and link protocol data units are sent between

token passing bus stations.

The methodology is not complete. An interoperability test of all

MAP physical layer products in every network configuration is

impossible. Rather, the methodology demonstrates with reasonable

assurance that MAP specifications for IEEE 802.4 broadband

physical layer protocols are adequate to insure interoperability

among modem and headend equipment.

2. IEEE 802.4 OPTIONS

While the test methodology developed at the National Bureau of

Standards is based on ANSI/IEEE Std 802.4-1985 ISO/DIS 8802/4, it

reflects recent changes made and approved by the IEEE 802.4

working committee. A new "change document," developed by the

working committee, has neither been approved by the IEEE

Standards Board nor considered by the ISO. This change document

corrects known errors in the 802.4 Standard and these changes are

reflected in all known emerging vendor implementations. These

changes most likely will be reflected in future approved

ANSI/IEEE and ISO 802.4 standards.

The IEEE 802.4-1985 ISO/DIS 8802/4 document specifies three

physical layer technologies: 1) Phase Continuous-FSK, 2) Phase

Coherent-FSK, and 3) Broadband Bus AM/PSK. Phase Continuous FSK

has been deleted from the change document and Phase Coherent FSK

has undergone considerable technical change. Minor technical

changes and clarifications to the Broadband Bus AM/PSK

documentation are reflected in the change document. Figure 1

identifies Broadband Bus as an option selected for MAP and the

focus of this interoperability test methodology.

3
Phase Phase Broadband

Continuous Coherent Bus

FSK FSK AM/PSK

OPTION *

Figure 1. IEEE 802.4 TEST METHODOLOGY

Broadband Bus AM/PSK is defined in chapters 8, 9, 14 and 15 of

the IEEE specification. Chapter 8 describes the media access

control (MAC) to physical layer interface. Chapter 9 describes

generic station management of the physical layer. Chapter 14 is

the broadband bus physical layer specification and chapter 15 is

the specification for the broadband medium which includes the

regenerative headend specification. These chapters enumerate the

various options for broadband bus. The options are summarized in

Figure 2 where an asterisk indicates options selected for use in

this test methodology.

REPEATERS ENCODING

active regenerative * duobinary *

MEDIUM DATA RATE

single coaxial cable * 1 MBit/s

dual coaxial cable 5 MBit/s

10 MBit/s *

CHANNEL ASSIGNMENTS TRUNK CABLE

Forward Reverse RG-6 * RG-11

P 3

* Serai Rigid

Q

4' * R 4A

S 5'

T 6

U FM1

Figure 2. BROADBAND BUS OPTION SELECTION

3. TESTBED PARAMETRIC ASSUMPTIONS

A wide variety of environmental and electrical parameters are

possible in a token bus network. Seven factors identified in

Figure 3 characterize the network being tested. The values

4
selected were used during the first set of tests conducted at the

National Bureau of Standards.

FACTOR LEVEL

LOW MEDIUM HIGH

noise > 46 dB S/N NONE 30 dB S/N

transmit level NONE +43 dBmV NONE

receive level -7 dBmV + 3 dBmV +13 dBmV

temperature NONE ambient NONE

humidity NONE ambient NONE

channel EVERY CHANNEL OF INTEREST

cable length EVERY LENGTH OF INTEREST

Figure 3. PARAMETER VALUES FOR INTEROPERABILITY TESTS

The levels chosen seem a reasonable choice. The noise level

specified is easily achieved with short 50 foot RG-6 cable.

Transmit and receive levels are also easily obtained without

additional amplification. While temperature and humidity level

changes may impact performance, ambient level settings seem

adequate to test the test methodology.

4. CHARACTERISTICS TO BE TESTED

IEEE 802-. 4 physical layer implementations may differ considerably

from vendor to vendor; yet each implementation must exhibit

similar electrical and functional characteristics. The approach

is to first identify and categorize these characteristics. Then,

an appropriate stimulus is defined to test the characteristic.

For example, the characteristic of a modem to scramble and

descramble data is tested using test frames A and A' . Figure 4

associates a stimulus with each characteristic considered.

5
r CHARACTERISTIC STIMULUS

scrambler /descrambler

encoding

headend capture

timing recovery

minimum preamble

{0} and {4} kickers

Test Frame A and A*

AGC stability

phase recovery

Test Frame B and B'

abort sequence

contention

Contention Generator

headend AGC/phase

dynamic variation

turnoff level — Establish a Token Ring

variable length frames using MAC Protocols

turn off time

Figure 4. EQUIPMENT CHARACTERISTICS

5 . TEST FRAMES

The test methodology requires equipment capable of sending,

receiving and counting the four test stimuli identified in figure

4 above. Figure 5 below indicates the capabilities required of

each modem. An "x" indicates that all modems (or controllers)

must exhibit the capability while an M o" indicates specialized

equipment. In particular, test frame generator A and A' must be

available in each modem (or controller) while test frame

generator B and B' and a contention generator are specialized

equipment not required by each vendor; but, each vendor must be

capable of receiving, and counting these generated frames. All

6
vendor modems or MAC controllers connected to these modems must

be capable of forming and maintaining a logical ring by executing

portions of the media access control protocol.

STIMULUS

CAPABILITY A A’ B B' CONTENTION RING

Send Test Frame X X o o o X

Count Total Sent X X o o X

Receive Test Frame X X X X X X

Count Total Received X X X X X X

Count Total Received X X X X X X

in Error

Figure 5. TESTING _CAPABILITY OF VENDOR EQUIPMENT'

5.1. TEST FRAME TRANSMITTERS AND RECEIVERS

The IEEE 802.4 physical layer specification requires scrambling

the output data bits at the transmitter and descrambling at the

receiver, providing a pseudo random distribution of energy on the

coaxial cable during packet transmission. When long strings of

similar scrambled symbols are transmitted, they are modified or

"kicked" to provide suitable energy levels for the receiver

detectors

Three symbols are used during packet transmission: {0}, {2} and

{4}. The {0} represents a zero bit; the {4} represents a one

bit; and, the {2} represents a code violation used in start

delimiters, stop delimiters, abort sequences and kickers. The

rules for scrambling and kicking are found in the 802.4 standard.

The register used to hold the scrambler output data is said to be

seeded when it is set to all ones and deseeded when set to all

zeros. Each transmitted frame includes a 32 bit frame check

sequence (PCS) derived from a cyclic redundance check (CRC)

calculation. The register used in this calculation is also said

to be seeded when set to all ones and deseed when set to all

zeros

5.2. TEST FRAME A and A' (PRIME)

Each modem has the capability to transmit test frame A and A* and

to count the total frames sent. The interframe time for test

frame A and A' was set at 500 microseconds. Test frame A and A'

are defined in figure 6a and 6b below. The IEEE 802.4 frame

format descriptor is above the hexadecimal digits. The frame

7
control is abbreviated "FC," the destination address is

abbreviated "DA," the source address is abbreviated "SA," and the

frame control sequence is abbreviated "FCS."

Test frame A is a well formed token consisting of four octets of

preamble, a valid start delimiter, the FC, DA, SA and FCS fields

defined in figure 6a and an end delimiter.

FC DA SA FCS

10 0a 00 00 00 00 00 j 12 bb 3f af bl 00 ff ff ff ff

Figure 6a. TEST FRAME A DATA

The bit pattern in the destination address (DA) was chosen to

cycle the scrambler to zero, exercising the {0} kicker mechanism.

The source address (SA) zeros or deseeds the CRC shift register

forcing each CRC bit to zero. The resultant FCS (the negation of

the CRC) is thus all ones (ff ff ff ff).

Test frame A' is an IEEE 802.2 ^ Logical Link Control data frame

consisting of four octets of preamble, a valid start delimiter,

the FC, DA, SA DATA and FCS fields defined in figure 6b below,

and a valid end delimiter.

FC DA SA DATA FCS

02 12 00 00 00 00 00 22 00 00 00 00 00 ff ff ff ff|

68 db 90 3a 00 00 00 00

Figure 6b. TEST FRAME A' (PRIME) DATA

The data portion is used to zero or deseed the CRC at the same

time the scrambler is seeded or set to all ones. The scrambler

seeds before an octet alignment providing a check for octet

aligned kicker action just before the FCS. The seeded scrambler

results in an FCS of all ones with a scrambler of all ones thus

checking {4} kickers at the very end of the frame. This last

kicker results in a {0} {2} {2} just before the end delimiter.

The first two symbols of the end delimiter are {2} {2} providing

four {2} symbols in a row -- the only way four {2} symbols can

ever be generated in a legal 802.4 frame.

5.3. TEST FRAME B AND B' (PRIME)

8
Test frames B and B' are IEEE 802.2 Logical Link Control data

frames. They consist of four octets of preamble, a valid start

delimiter, the fields defined in figures 7a and 7b below, and a valid end delimiter. The data field contains repeating octets of

00 or ff to fill the frames. The frames are 4000 total octets

each.

FC DA SA DATA FCS

02 67 00 00 00 00 00 00 00 00 00 00 00 00 ... 00 0c dl e9 ef

Figure 7a. TEST FRAME B DATA

FC DA SA DATA FCS

02 d8 ff ff ff ff ff ff ff ff ff ff ff ff ... ff ed ed 5c a4

Figure 7b. TEST FRAME B' (PRIME) DATA

A specialized generator is required for test frame B and B'

This generator has the capability to transmit a sequence of these

test frames with an interframe gap of 500 microseconds and to

count the total frames sent. Test frame B and B* tests the

automatic gain control (AGC) stability and phase recovery of the

modems and the remodulator while checking proper insertion and

removal of zero {0} and one {4} kickers.

5.4. CONTENTION GENERATOR

The contention generator is a specialized transmitter capable of

transmitting:

1. pseudo silence on the uplink to the headend,

2. a frame fragment on the uplink,

3. pad- idle then continuous {4} symbols,

4. continuous {4} symbols followed by silence,

5. test frame A with an invalid end delimiter, an invalid

FCS , or with octet misalignment.

5.5. ESTABLISH A LOGICAL RING

Establishing a logical ring requires a Media Access Control (MAC)

capability not strictly required for physical layer testing.

However, in developing this interoperability methodology, all

participants felt strongly that establishing a ring, passing the

token and sending data frames is crucial to interoperability

9
testing. The MAC implementation requires initialization of 13

MAC parameters. Figure 8 identifies the MAC options chosen for

these parameters. (reference: section 6.4 page 79 ANSI/IEEE

802.4-1985)

(1) TS (station address) individually assigned

( 2 ) address_length 6 octets

(3) slot time (octet times) 300 and 2000

( 4 ) hi pri token hold time > 25 ms

( 5 ) mac ac 4 rotation time > 250 ms

(6) max ac 2 rotation time > 250 ms

( 7 ) max ax 0 rotation time > 250 ms

( 8 ) max ring maintenance rotation time off, >250 ms

(9) ring maintenance timer initial value solicit immediately

(10) max inter solicit count 255

( 11 ) min_post_silence_preamble_length 4 bytes

(12) in ring desired true

( 13 ) max__retry_limit not applicable

Figure 8. MAC PARAMETERS

6. TEST DEFINITIONS

Five test definitions exercise the IEEE 802.4 broadband physical

layer. Each definition is described below. Three vendor headend

remodulator and four vendor modem equipments were used during the

testing.

6.1. BASELINE

The BASELINE test provides each vendor with assurance that the

cable plant facility is operational and capable of use. The

vendor performs a visual inspection, conducts an independent

’’proof -of -performance" test, conducts a self test or any

initialization required and verifies that the headend remodulator

connected to the cable plant facility transmits pseudo silence.

The cable plant system schematic is shown in figure 9a. Figure

9b shows the headend rack, patch panel arrangement, switchable

attenuators, splitters and diplex filter. Figure 10a is a sweep

of the cable plant system from 10 MHz to 500 MHz at 50 MHz per

division. Attenuation is 10 dB per division. The sweep was

obtained by injecting the low frequencies into the reverse

channel attenuator; then the high frequencies into the forward

channel. The spectrum analyzer, connected to the output of one

four way splitter was set to display the max value. This

provides the "notch" display of the diplex filter as well as the

slope of the cable plant. It is down 10 dB at 500 MHz. The

10
reference line at 0 dB is the direct output of the sweep

generator

Figure 10b shows a sweep of the forward channel. The center

frequency is 258 MHz; the span is 18 MHz. The straight line is

the reference output from the sweep generator. Vertical gain is

1 dB per division. Figure 10c shows a sweep of the reverse

channel. The center frequency is 65.75 MHz; the span is 18 MHz.

The positively sloped line is the reference output from the sweep

generator. Vertical gain is 1 dB per division. The cable plant

system falls within the 802.4 specifications.

Figures 11a, lib, and 11c show the RF output at a four way

splitter from each headend remodulator transmitting pseudo

silence. Slight differences in the levels were noted.

11
Fieri ’'•'n 9a SCHEMATIC DIAGRAM OF CABLE PLANT

Figure 9b HEADEND CONFIGURATION

12
~~T~

Jr"

50 MHZ per Division

ATTN 10 dB per Division

Start Frequency 10 MHz

Top trace is reference

Figure 10a CABLE PLANT SWEEP

Center Frequency 258 MHz

Span 18 MHz

1 dB per division

Straight line is reference

Figure 10b FORWARD CHANNEL

Center Frequency 65.75 MHz

Span 18 MHz

1 dB per division

Negative slope is reference

Figure 10c REVERSE CHANNEL

13
11a lib

11c

Figure 11. PSEUDO SILENCE

14
6.2. SHORT FRAME TEST

Test frames A and A’ (prime) are transmitted during the short

frame tests. Each test run consisted of a modem transmitting a

large number of the test frames through a remodulator to the

receiving modems. Each receiving modem (controller) reports the

number of frames received correctly and, when possible, the

number received in error. The number sent, the number received

correctly by each modem, and, when possible, the number received

incorrectly by each modem is recorded. After each modem has

transmitted the test frame through the remodulator, another

remodulator is selected and the process repeated.

Test frame A is shown in figures 12a, 12b, 12c and 12d. The

bottom trace shows the reverse channel; the top trace shows the

forward channel. Notice the (0}s and {0} kickers. While the

interframe time is not shown, it was 500 microseconds. Notice

that each vendor's test frame A is similar.

15
12c 12d

Figure 12. TEST FRAME A

16
The results of testing interoperability by transmitting and

receiving test frame A under a variety of test conditions is

summarized below. "ok" indicates proper operation under all

conditions tested. "E" indicates that errors were observed under

some test condition tested. A blank indicates that a full set of

tests could not be completed.

FRAME REMOD XMITTER A RCVD B RCVD C RCVD D RCVD

A A A

B

1

ok ok ok

C E ok ok

D ok ok ok

A B A

B

1

ok ok ok

C ok ok ok

D ok ok ok

A C

2 A E E E

B

1 E E E

C E ok ok

D E ok ok

1. Vendor B was unable to generate Test Frame A when receiving

low signal level at the transmitting modem.

2. Vendor C's remodulator was unable to operate when receiving

high signal and high noise levels.

Table 1. Summary of Short Frame Tests

17
Test frame A' (prime) is shown in figures 13a, 13b, 13c and 13d.

Notice the {4} kicker near the end of the frame. Also notice

that more than 16 ones are transmitted before the kicker. Figure

14 shows a detailed trace of the last few bytes followed by the

end delimiter. Notice there are four {2} symbols in a row -- the

kicker for the last data symbols, then the start of the end

delimiter

.

Short frame test results were found to be a function of signal

level and noise level at the modem and headend remodulator

receivers. Complete interoperability testing, therefore,

requires that tests be run for all signal and noise level

combinations listed in Figure 3.

18
19
Figure 14. END OF TEST FRAME A' (PRIME)

20
6.3. LONG FRAME TEST

Test frames B , B' (prime), C and C' (prime) were transmitted

during the long frame tests. The B test frames were used during

the first interoperability meeting and the C test frames were

used during the second operability meeting. The C test frames

are a simple modification of the B test frames, extending their

length to the longest permissable frame, 8191 octets. For each

test, a headend remodulator was switched into the cable plant;

then, required initialization was completed and the LONG FRAME

GENERATOR transmitted the test frame. Each modem reported the

number of frames received and the number received in error. The

tests were repeated with each headend remodulator for each test

frame

Figure 15a shows test frame B. The bottom trace is the reverse

channel which turns off after 4000 bytes. The 500 microseconds

inter frame time is easily seen. Figure 15b expands the trace

showing the {0} kicker. Figure 16a and 16b show test frame B'

(prime). Here the {4} kickers are clearly visible.

21
15a 16a

15b 16b

Figure 15. TEST FRAME B Figure 16. TEST FRAME B’ (PRIME)

22
The results of testing interoperability by transmitting and

receiving long test frames under a variety of test conditions is

summarized below. "ok" indicates proper operation under all

conditions tested. "E" indicates that errors were observed under

some test condition tested. A blank indicates that a full set of

tests could not be completed.

FRAME REMOD A RCVD B RCVD C RCVD D RCVD

C A E ok ok

B E E ok

C

1

C' A ok ok ok

B ok E ok

C

1

1. Vendor C's remodulator was unable to operate when receiving

high signal and high noise levels.

Table 2 . Summary of Long Frame Tests

Long frame test results were found to be a function of signal

level and noise level at the modem and headend remodulator

receivers. Complete interoperability testing, therefore,

requires that tests be run for all signal and noise level

combinations listed in Figure 3.

6.4. REMODULATOR RESPONSE TO SIMULATED CONTENTION

Initially, four headend response subtests to simulated contention

were defined and conducted. Analysis of the subtest results led

to augmentation and consideration of seven additional subtests.

Section 6.4 describes the four original tests and the seven

augmented tests.

The response of each remodulator was documented by recording

oscilloscope traces. Subtest 1 requires each remodulator to

respond to uplink pseudo silence followed by silence. Subtest 2

requires each remodulator to respond to a frame fragment defined

by pad idle, a start delimiter 32 {4} symbols then silence.

Subtest 3 requires each remodulator to respond to pad idle

23
followed by 32 {4} symbols without a start delimiter. Subtest 4

requires each remodulator to respond to 32 {4} symbols then

silence

.

Figures 17a, 17b and 17c show the remodulator response to uplink

pseudo silence. Figure 17a shows the pattern {4}{4}{4}{0}

repeated several times followed by an abort sequence. Figure 17b

shows preamble followed by "garbage” followed by zeros. Note

that the time scale was changed in figure 17b to 1.51

microseconds per centimeter. Also note that the sequence

(4}{0}{0}{0}{4> precedes a return to pseudo silence. Figure 17c

shows about 8 microseconds of preamble followed by pseudo

silence.

Figure 18a, 18b and 18c show the remodulator response to an

uplink frame fragment. Figure 18a shows a short period of

preamble; 2 start delimiters; an abort sequence; about 5 {4};

another abort sequence; then pseudo silence. Figure 18b shows

repeated {4}’s followed by {2}'s followed by the sequence

{4}{0}{0}{0}{4}. Figure 18c shows repeated start delimiter and 1

octet of { 4

}

1 s

;

pad idle; an abort sequence; pad idle; then

pseudo silence.

Figure 19a, 19b and 19c show the remodulator response to uplink

continuous wave (CW) with pad idle. Figure 19a shows a switched

to pad idle followed by "garbage" followed by an abort sequence.

Figure 19b shows a switch to pad idle followed by several

{2}{2}{0} sequences followed by a {4}{0}{0}{0}{4} sequence.

Figure 19c shows only pad idle.

Figure 20a, 20b and 20c show the remodulator response to uplink

CW then silence. Figure 20a shows pseudo silence as if nothing

was received. Figure 20b shows preamble. Figure 20c shows less

preamble than figure 20b.

24
17a 17b

17c

Figure 17. UPLINK PSEUDO SILENCE TEST

25
18b

18c

Figure 18. UPLINK FRAME FRAGMENT

26
19a 19b

19c

Figure 19. UPLINK CW WITH PAD IDLE
20a 20b

28
Analysis of the test results abov.e led to seven additional

remodulator response tests. Even though these tests were

conducted under rather severe time limitations and the results

obtained require the interpretation of oscilloscope waveforms,

the tests clearly show that the remodulators differ in their

response to these test inputs. The test definitions and

preliminary results are summarized below:

1. Contention Simulated by White Noise

The response of each remodulator to repetitive pulses

of white noise was observed on an oscilloscope. The

source was a Micronetics Noise Generator, Model PNG

5109, transmitting + 3 dBmV pulses of 20 microsecond

duration every 120 microseconds.

In each case, the oscilloscope display was unstable,

indicating that the response of the remodulators to

white noise is not repeatable. This lack of

repeatability is due to nature of the noise input; the

noise may occasionally look like the beginning of a valid frame. Remodulator response tests should be

repeatable, that is, the same response should be

observed every time the input of the remodulator is

stimulated by the simulated contention signal.

Therefore, white noise is not a suitable simulation of

contention.

2. Invalid End Delimiter - (E Bit Set)

The response of each remodulator to repetitive frames

with an invalid end delimiter was observed on an

oscilloscope. The source was the contention generator

transmitting Test Frame A at medium signal level and

low noise and with the end delimiter modified:

from {2}{2}{4}{2}{2}{4}{0}{0}

to {2}{2}{4}{2}{2}{4}{0}{4}.

Remodulator Response

A

B

C

E bit in end delimiter is cleared

E bit in end delimiter is cleared

Unstable display on oscilloscope

29
3. Invalid End Delimiter - (I Bit Set With No Frame

Following)

The response of each remodulator to repetitive frames

with an invalid end delimiter was observed on an

oscilloscope. The source was the contention generator

transmitting Test Frame A at medium signal level and

low noise and with the end delimiter modified:

from {2}{2}{4}{2}{2}{4}{0}{0}

to {2}{2}{4}{2}{2}{4}{4}{0}.

Remodulator Response

A Repeat frame, either AS or 6 {0}, AS, PS

B Repeat frame, 8 bits of preamble, PS

C Repeat frame, 22 {0}, AS, AS, PS

where AS is an abort sequence and PS is pseudosilence.

4. Invalid End Delimiter - (Back to Back Frames with I Bit

Reset

)

The response of each remodulator to repetitive frames

with an invalid end delimiter was observed on an

oscilloscope. The source was the contention generator

transmitting Test Frame A' , 3 octets of preamble, and

Test Frame A at medium signal level and low noise.

Remodulator Response

A Repeat A' , PS, No SD, Repeat A, AS

B Exact repeat of input

C Repeat A', PS, Preamble, No SD, Repeat

A, AS

where PS is pseudosilence, SD is a start delimiter and

AS is an abort sequence.

30
5.

Invalid Frame Check Sequence

The response of each remodulator to repetitive frames

with an invalid frame check sequence was observed on an

oscilloscope. The source was the contention generator

transmitting Test Frame A at medium signal level and

low noise and with the last bit of the FCS inverted.

Remodulator Response

6.

Octet Misalignment

The response of each remodulator to repetitive frames

with a non-integral number of octets was observed on an

oscilloscope. The source was the contention generator

transmitting Test Frame A at medium signal level and

low noise and the sequence {4}{0}{0} added between the'

FCS and the ED.

Remodulator Response

where ED is an end delimiter and AS is an abort

sequence

7.

Invalid End Delimiter - (ED Replaced With SD)

The response of each remodulator to repetitive frames

with an invalid end delimiter was observed on an

oscilloscope. The source was the contention generator

transmitting Test Frame A at medium signal level and

low noise and with the end delimiter replaced with a

start delimiter.

Remodulator Response

A

B

C

Exact repeat of frame

Exact repeat of frame with E bit set

Exact repeat of frame

A

B

C

Repeat frame, correct the ED, AS

Repeat frame, ignore ED, AS

Exact repeat of frame

A

B

C

Repeat frame, correct the ED, AS

Repeat frame until ED, immediate AS

Exact repeat of frame, 22 {0}, AS

where ED is an end delimiter and AS is an abort

sequence.

31
These tests demonstrate the need for agreement among the vendors

on the remodulator response to various inputs. This need has

been brought to the attention of the IEEE 802.4 Token Bus Task

Group. Appropriate remodulator response tests can be devised

when the task group has defined the appropriate response for each

stimulus

.

6.5. TOKEN BUS TEST

The token bus test consists of the formation of a logical ring

comprised of two modems from each participating vendor. This

test requires that each modem have the full functionality of the

Media Access Control (MAC) sublayer of the Logical Link Control

( LLC ) layer of the OSI protocol. Therefore, failure to

interoperate does not imply that the physical layer of the modem

is deficient. This test does, however, provide the most

stringent testing of the physical layer and successful operation

under these testing conditions adds considerable assurance of

interoperability.

The token bus tests allow each headend remodulator to host a ring

while each vendor's modem pair initializes the ring. Once the

ring is initialized, other vendors sequentially enter the ring.

With all modems in the ring, the initializing modems transmit

data. Each combination of headend remodulator and modem pair was

tested. Also, each combination of sequential entry into the ring

was tested. In addition, the addresses assigned to the modems

were chosen so that each vendor's modems would communicate with

as many other vendor's modems as possible as the token is passed

around the logical ring. Each modem was set to a different

transmit power level within the acceptable range of the

remodulator receiver to test the remodulator's response to

dynamic power level variations.

The addresses and transmit power levels assigned to the vendor's

modems during the interoperability tests at NBS are given in the

table below:

Address Vendor Signal Level

XX XX XX XX XX 2D D + 3 dBmV

XX XX XX XX XX 2C C +10 dBmV

XX XX XX XX XX 2B B

- 7 dBmV

XX XX XX XX XX ID D + 3 dBmV

XX XX XX XX XX 1C C 0 dBmV

XX XX XX XX XX IB B + 6 dBmV

XX XX XX XX XX 1A A

- 3 dBmV

XX XX XX XX XX 0A A + 13 dBmV

Table 3 . Modem Addresses and Transmit Power Levels

32
The results are summarized below. Interoperability between the

modems is clearly demonstrated, but the summaries also show those

operating conditions where reliable operation is not obtained.

"E" indicates that the token was passed around the ring

successfully, but some errors occur. "U" indicates unstable

operation where the ring must be reformed periodically. "NO"

indicates that the required operation could not be done. It is

not within the scope of these tests to identify why units fail to

interoperate; only to identify those situations where the modem

and headend remodulator failures occur.

33
Date/Time October 10, 1986 10:48 - 11:40

Slot Time 2000 octet times

Channel 3'/4'/P/Q

Cable Length 15 Meters

REMODULATOR MODEM

Transmitter Signal Level Medium Many

Receiver

Receiver

Signal

Noise

Level

Level

Many

Low

Medium

Low

REMOD ORDER A B C LLC DATA TRANSMISSION

A ABC ok ok ok A SENDS ok

BAC ok ok ok B SENDS ok

CAB ok ok ok C SENDS ring unstable-*-

B ABC E E U A SENDS ok (C not in)

BAC ok E U B SENDS ok (C not in)

CAB NO NO NO C SENDS NO

C ABC ok ok ok A SENDS ok

BAC ok ok ok B SENDS ok

CAB ok ok ok C SENDS ring unstable 1

1. Vendor A's modems lose tokens received shortly after

receiving data frames.

Table 4. Summary of Ring Test (No Induced Noise)

34
Date/Time October 10, 1986 11:45 - 12:31

Slot Time 2000 octet_ times

Channel 3'/4'/P/Q

Cable Length 15 Meters

REMODULATOR MODEM

Transmitter Signal Level Medium Many

Receiver Signal Level Many Medium

Receiver Noise Level Low High

REMOD ORDER A B C LLC DATA TRANSMISSION

A ABC ok ok ok A SENDS ok

BAG ok ok ok B SENDS ok

CAB ok ok ok C SENDS ring unstable 1

B ABC E E U A SENDS ok (unstable)

BAC E E NO B SENDS ok (C not in)

CAB NO C SENDS NO

C ABC E U E

2 A SENDS ok (unstable)

BAC E ok CsJ w B SENDS ok (unstable)

CAB E E

2 ok C SENDS ring unstable 1

CBA E ok ok D SENDS ok

1. Vendor A's modems lose tokens received shortly after

receiving data frames.

2. The error rate slows when these modems enter the ring.

Table 5. Summary of Ring Tests (With Induced Noise)

35
7. TABLES USED TO CONDUCT THE TESTING

The methodology provides a summary testing matrix that indicates

completed tests. For the controlled laboratory testing conducted

at the National Bureau of Standards, four vendors participated.

Three vendors had headend remodulators; one vendor did not. All

four vendors had modems. The vendors with remodulators were

named A, B and C; the vendor with only a modem was named D. Table

6 below was used to record the test completion results.

USING

REMOD

BASELINE SHORT FRAME

A A'

LONG FRAME

B B' C C'

RE RESPONSE

TESTS

RING

A A

B

C

D

B A

B

C

D

C A

B

C

D

Table 6. Test Completion Matrix

7.1. SHORT FRAME TABLES

The short frame tests required all vendors to transmit test frame

A and test frame A' (prime). Table 7 was used to organize and

conduct the short frame tests.

7.2. LONG FRAME TABLES

The long frame tables are less complex because each modem is not

required to transmit a long frame. There was only one long frame

generator. Table 8 illustrates the Long Frame Test.

36
SHORT FRAME TEST

Date/Time

Frame Type

Channel

Cable Length

Remodulator Modem

Transmit Level

Receive Level

Signal/Noise Level

REMOD XMITTER # SENT A RCVD B RCVD C RCVD D RCVD

A A _____ _____

B _____

C

D _____

B A

B _____

C

D

C A _____

B

C

D

Table 7. SHORT FRAME TEST SHEET

37
LONG FRAME TEST

Date/Time

Frame Type

Channel

Cable Length

Remodulator Modem

Transmit Level

Receive Level

Signal/Noise Level

REMOD # SENT A RCVD B RCVD C RCVD D RCVD

A

B

C

Table 8. LONG FRAME TEST SHEET

38
RING TEST

Date/Time

Slot Time

Channel

Cable Length

Remodulator Modem

Transmit Level

Receive Level

Signal/Noise Level

REMOD ORDER A B C D TRANSMIT

A ABCD A SENDS

BDAC B SENDS

CADB C SENDS

DCBA D SENDS

B ABCD A SENDS

BDAC B SENDS

CADB C SENDS

DCBA D SENDS

C ABCD A SENDS

BDAC B SENDS

CADB C SENDS

DCBA D SENDS

Table 9. RING TEST SHEET

39
8. CONCLUSIONS

While the test results indicate that all participating vendor's

modems and headend remodulators interoperate at the physical

layer, there is evidence that current implementations do not

interoperate at all parameter settings. The test methodology

described in this report finds those combinations of parameter

values where implementations fail to interoperate. This

methodology includes five test definitions.

The first test definition describes the baseline test. This test

is the necessary first step to insure that the cable plant

facility is operating correctly and to allow each vendor to

satisfy himself that his equipment is operating according to his

expectations

.

The short frame tests exercise the scrambler /descrambler , the

encoder, and the kicker mechanisms in vendor's equipment under a

variety of test conditions. Headend capture, timing recovery and

response to minimum preamble are also checked. Short frame tests

are run for several combinations of receiver signal level and

receiver noise level at both the remodulator and the modems.

These tests find those combinations of vendor equipment, signal

level and noise level that exhibit an abnormally high error rate.

The long frame tests check AGC stability and phase recovery.

These test require a special frame generator to transmit the test

frame. Long frame tests are also run for several combinations of

receiver signal level and receiver noise level at both the

remodulator and the modems. These tests find those combinations

of vendor equipment, signal level and noise level that exhibit an

abnormally high error rate.

The remodulator response tests define useful test stimuli for

testing how a remodulator responds to error conditions at its

input. These tests show that different remodulators respond

differently to the specified test inputs.

The token bus tests exercise the equipment under test as it is

used in a logical ring. Correct operation under the conditions

of this test is a strong indication that equipment interoperates

at the physical layer. Correct operation, however, also requires

interoperation at the MAC level, so failure to operate correctly

does not necessarily indicate a failure in interoperability at

the physical layer. These tests also found those conditions

where the error rate increased noticeably.

The methodology will evolve as experience is gained. New

remodulator test frames and different environmental and

electrical parameters settings are easily incorporated into an

interoperability test plan that uses this methodology. A more

40
complete set of remodulator tests is required; but, the current

standard is deficient in describing responses during ring

initialization and other opportunities where modem contention

occurs. During these contention periods, the remodulator must

respond to a wide variety of reverse channel transmissions. Once

responses to these conditions are standardized, appropriate test

frames can easily be added to the test methodology and then

tested in a controlled and repeatable laboratory environment.

41
1. Manufacturing Automation Protocol Specification, Version 2.2,

July, 1986. General Motors

2. International Standard ISO 7498 -1984 (E), "Information

Processing Systems -- Open Systems Interconnection -- Basic

Reference Model, 1984. p30.

3.

"ANSI/IEEE Std 802.4-1985, ISO/DIS 8802/4," Token Passing Bus

Access Method. The Institute of Electrical and Electronics

Engineers, Inc., Distributed in cooperation with WileyInterscience , a Division of John Wiley & Sons. Inc. Approved

December 17, 1984 American National Standards Institute. p238.

4 . Implementation Agreements for Open Systems Interconnection

Protocols NBS Workshop for Implementors of Open Systems

Interconnection. NBSIR 86-3385 July 1986, pl29.

5. "ANSI/IEEE Std 802.2-1985, ISO/DIS 8802/2," Logical Link

Control

.

The Institute of Electrical and Electronics Engineers,

Inc., Distributed in cooperation with Wiley-Interscience , a

Division of John Wiley & Sons. Inc. Approved December 28, 1984

American National Standards Institute, pill.

42
NBS-114A (REV. 2-80

U.s. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET (See instructions) 4. TITLE AND SUBTITLE

. PUBLICATION OR 2 REPORT NO.

NBSIR 87-3536

. Performing Organ. Report NoJ 3. Publication Date

APRIL 1987

A Methodology for Broadband Token Passing Bus Interoperability Testing

5. AUTHOR(S)

Robert Rosenthal, Daniel P. Stokesberry

6. PERFORMING ORGANIZATION (If joint or other than N8S. see instruct! ons) 7. Contract/Grant No. national bureau of standards

DEPARTMENT OF COMMERCE 8 . Type of Report & Period Covered

WASHINGTON, D.C. 20234

9.

SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street. City . State, ZIP)

10.

SUPPLEMENTARY NOTES

, ~| Document describes a computer program; SF-185, FIPS Software Summary, is attached.

11.

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

bibliography or literature survey, mention it here) A method for testing broadband token passing bus interoperability is described. The

method was tested in a controlled laboratory environment at the National Bureau of

Standards with four modem manufacturers' and three headend remodulator manufacturers'

equipment. The tested equipment implemented specific token passing bus options found

in commercially available products.

The methodology specfies seven environmental and electrical parameters; for the first

set of tests, nominal parameter settings were selected. The methodology uses specific

test frames developed for their unique bit patterns. These frames were transmitted,

received, and counted to exercise the broadband modems and headend remodulators.

The methodology organizes the frame transmissions into five test definitions. Each

test definition is repeated for each headend remodulator.

12. KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolon s) Local Area Networks; physical layer testing; protocol testing; test methodology;

token bus

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