CA1223316A - Management help tool for communication systems operating on local area network - Google Patents

Abstract

ABSTRACT

Management help tool for communication systems operating on local area network A tool for testing the response of the conten-tion handling systems on a local area network comprises a transceiver for connection to the bus of the network.
The tool generates information frames having an adjust-able length and delivers such frames on the bus line in response to detection of the presence of data on the bus line for causing contention to occur. The tool may be arranged for sending, at will, either information in response to detection of information frames present on the bus, or randomly, or cyclically.

Description

~3~

Manaqement help tool ~or communication sYstems operatinq on local area ~etwork ~ACKG~OUND AND SUMMARY OF TEIE INVENTION

The invention relates to data communication sys-tems operating on a local area network, of the type com-prising a ~erial transmission bus and stations, consist-ing of data processing devices, distributed along the bus. The invention more particularly relates to a tool which may be used in optimizing management of such sys-tems.
The invention is suitable for use in associationw.ith numerous systems of the above-defined type in which access by the different stations is random, rather than limited to time slots each dedicated to a particular one lS of the stations. As a counterpart for their increased traffic capacity, among which collisions, that is inter-ferences between messages when two stations simultane-ou~ly attempt to transmit on the bus. A number of solu-tions have already been proposed to overcome the pro-blem. A ~irst approach consists in avoiding collisions.For that purpose, V.5 Patent 3,445,822 to Driscoll in-volves generation of a seizure signal by a station which has a message to transmi~ on the bus, prior ~o actual transmission. Then the station compare~ ~he code which is actually present on the bus with a predermined code for determining whether the bus is available. Any non-coincidence between the ~wo codes indicates to the station that another station attempts to transmit on the bus and the message should be held in abeyance until a renewed transmi~sion of the code results into a coïnci-dence. That solution decreases the rate of transfer o~
the system, since there is a delay in use of the bus.
Another solution, which is used for instance in the system designated by the trademark "ETHERNET", accepts ~he possible occurence of collisions. A~ter 'che condition of the bus has been determined, each sta_ion may immediately transmit if it ~inds tha~ the bus is not busy. Any possible collision with an other message is detected and a retransmission attempt is carried out after a collision processing algori~hm is executed.
Detection of a collision is carried out in each transmitter-receiver unit (which will now be called a transceiver~ by an interference detector which monitors the bus and compares the message which is actually present on the bus with the message that the station is ~upposed to transmit. Any difference between the bits indicates a collision. In the event of a collision, that collision is enforced by the two s~ations which send overlapping messages by maintaining their tran~mission for a predetermined time. Then all stations connected to the network receive the collision indicative signal thxough their respective transceivers and in response delay their possible data transmission. The most frequent solution consists of programming the stations for retransmission of a message to take place after a randomly-determined time delay has elapsed after a collision.
Since the handling of contention caused by col-lisions is well known in the art, it will not be neces-sary ~o describe it in the presen~ specl~ication. I~
necessary, ~eference may be had to various documents published by companies involved in the "ETHERNF.T" pro-gram, for in~tance to "The ETHERNET-local area network;
data link layer and physical layer specifications", publi~hed by DIGITAL EQUIPMENT CORPORATION, Intel Cor-poration and Xerox Corporation (September 30, 1980).
It is important to be in a position to determine whether the layers of the stations designed for avoiding collisions or modi~ying the transmi~sion procedures in the event of a collision operate properly. The currentl-used approach consists o~ creating pseudo-collisions by unmatching the bus when that bus consists of a coaxial cable. For that purpose, it is sufficient to remove a matching terminator. That solution has drawbacks in that 5 it does not provide any control on the number of pseudo-collisions which are generated. An other approach consists in increasing the number of stations which may transmit on the bus. That approach is expensive, since it requires additional hardware. The result is further-10 more unreliable and the method lacks flexibility.
It is an object of the invention to provide amanagement assistance tool for a system of the above-defined type which provides a possibility to generate simulation representative of the various traffic condi-15 tions which may occur. It is a more particular object toprovide a tool making it possible to completely verify operation of the elements embodied in ~he systems.
For that purpose, a tool for a data communic-ation sy~tem on a local area network comprises, as a 20 regular host station, a transceiver for connection to the bus and having a device for detecting collisions and a coupler. The tool further comprises means for gener-ating information frames having an adjustable length and or delivering such frames on the bus line in response 25 to detection of the presence of data on the bus line.
The generation means are designed for transmit~ing a collision frame when they detect such presence.
A tool according to the invention makes it pos-sible to verify all software layers required for hand-30 ling the protocole used on the networlc and the connectedsystems. It also malces it possible to veri~y operation of the circuits which handle collisions, particularly in the transceivers and repeater~;
The tool may be designed for operating in a 35 number of different ways, selectable at will. Then it will be possible to test operation in a number of different conditions of use. The tool rnay be designed for delivering a frame or seyuence of binary information of adjustable length onto the bus line either as soon as the tool receives the first bits of a frame delivered by 5 a regular station, or only after an adjustable time delay. That time delay may be adjustable, for instance up to a value of some milliseconds. The tool may as well be designed for transmitting a frame only after i-t has received a predetermined number n of frames and may be 10 designed for delivering frames having an adjustable length and/or after a time period has elapsed. The tool may also be designed for continuously or randomly de-livering frames having an adjus-table length, at will.
That transmission may be asynchronous and dependent of 15 the presence or absence of data on the bus. All functions may be implementPd in a tool and then means are provided for selecting the appropriate type of operation.
The same tool or a similar tool also connecteed 20 to the bus line may be used for simulating a traffic whose nature and intensity are programmable rather than collisions. Such a tool simulating an increased degree of traffic may be used for testing the conformity of the network to the standards. The tool then typically has 25 mean~ for displaying ~tatistics carried out on ~he generated traffic and on ~he response of ~he network.
A tool according to the invention may be programmed for simulating traffic on a local area data communication network having a serial transmission line 30 along which a plurality of data processing stations are distributed. That ~raffic s.imulation tool may have a coupler and a processor unit programmed for storing a plurality of different traffic profiles and for locating such simulated traffic on t~e network bus line.
The invention will now be described with refer-ence ~o specific embodiments given by way of examples 5 ~3~
only. The descriptlon r~fers to the accompanying dra-wings.
SHORT ~ES~RIPTION OF THE DRAWINGS
- Figure 1 is a sXetch indicating how host sta-S tions and a tool are connected in a local area networkdata communication system, - Figure 2 is a representation of the format of data packets typically used on a network of the type illustrated in Figure 1, - Figure 3 is a block diagram indicating a pos-sible construction of a collision simulation tool on a network of the type illustrated in Figure 1, - Figure 4 is a functional sketch of the traffic simulation tool, lS - Figures 5 and 6 are functional 3ketches of automates which may fulfil the functions of traffic si-mulation and traffic monitoring.
DETAILED DESCRIPTION 0~ PARTICULAR EMBODIMENTS
The particular embodiment of the invention which 20 will be described is particularly adapted to a data communicat.ion sy~tem of the "ETHERNET" type and it may be useful to first provide some indications as to the architecture of such a system and the for~at of the messages, it being understood that the invention is also ~5 suitable for use with other systems.
Refexring to Figure 1, there is shown a network comprising a bus line 10 consisting of a coaxial cable connected to ter~inators 12 one of which is illustrated.
That terminator constitu~es a matching load. The cable 30 may consist of several portions associated in end to end relation by connectors. Host stations, such as 14 and 16, are distributed along the cable, at distance~ which ~hould ~e sufficient for avoi~ing formation of station-ary waves on the portion comprised between two stations.
35 Each station 14 or 16 is connected to the coaxial cable through a transmitter-receiver which may be either in 6 ~2~
derivation (transceiver 18 in Figure 1) or serial ltransceiver 20). A link, typically consisting of twist-ed pairs 22 provided with end connectors, constitutes a physical connection between the transceiver and the station. The station has a coupler, a central unit (processor or computer) and lnput-output elements.
Most networks use communication by packets or frames each of the type illustrated in Figure 2. Each packet 24 has a synchronisation preamble 26, a desti-nation address 28, a source address 30 (representative of the transmission station) and a code 32 for identi-fyin~ the high level protocole which is used. Parts 26-32 are of fixed length. The packet then comprises a data field 34 of variable length. The length may vary 15 between two limits, for instance 46 and 1500 bytes. The end portion of the packet consists of a redundancy code 36 for error detection. Two successive packets ~uch as 24 and 38 should be separated by a minimum packet spacing , which is 9.6 m~ on an ETHERNET network.
Tools according to the invention are coupled to the bus in the same way as the regular stations 14 and 16. Again referr.ing to Figure 1, there is shown a collision simulation tool 40 connected to the bus 10 by a transceiver 42 which is illustrated as serially loca-ted, but which may as well be located as a derivation.
If tool 90 is to simulate all possible types of colli-sions, it may have the construction illustrated in block form in Figure 3. It will be appreciated that a tool according to the invention makes use of a combination of data processing and communication means which are well Xnown per se and consequently it is not necessary to provide a full description of each element and such elements may have a number of different forms.
Referring to Figure 3, the tool has a branch for detecting occurence of bits inserted onto the bus line 10 by other stations. That branch comprises a circuit 44 for envelope detection and counting. That circuit may be programmable. In Figure 3, a programmation input 45 has been shown. The output of circuit 44 is connected to the input of a phase--shift circuit 46 having a control input 48. Circuit 46 is for adjustment of the time elapsed between transmission of a collision frame. Circuit 46 drives a circuit 50 for selection of frame length.
A~ain, circuit 50 is provided with an adjustement input 52. Last, a frame generator 59 generates frames consis-ting of bits and having a length determined by the out-put of frame selector 50. The binary frames delivered by generator 54 are shaped by a shap~r 56 and delivered via a gate 58 to transmitter 70 and transceiver 42 for ex-change with the bus.
That arrangem~nt may be connected by selection means, represented as a switch 62, either to a receiver 64 connected to the transceiver 42, or to à binary data cyclical generator 66, or to a generator 6a arranged to deliver bursts of binary data according to a random mode, but pos~ibly with adjustable characteristics. The collision detector 70 associated with transceiver 42 drives a collision counter 72 which disables gates 58 after a predetermined number of collisions has occured.
Again, the number of collisions after which gate 58 is ~5 di~abled may be adjustable. The number of collisions may be displayed on a display unit 74.
A power source 71 delivers an appropriate vol-tage, typically 12 V, to transceiver 42.
The tool which has just been described is adap-ted to simulate a number of differen~ types of colli--~ions.
Inte~ral _iamminq the envelope detector 44, generally consisting of a programmable counter, is con-nected to receiver 64 when that type of opera~ion is selected. Unit 58 is disconnected or disabled. The stations to be tested are conn~cted to bus 10. They transmit one or more binary data frames onto the bus. As soon as the first bits of a frame are received by recei-ver 64, the latter ~riggers transmission of a burst of binary data by ~nits 44, 50, 54, 56 and 60. That ~urst, whose length is preadjusted, is delivered to bus 10 where it interfers with data from an other station. A
collision occurs. It is detected by the collision detec-tor 70 and transferred by 72 to the display unit 74 which may be arranged to d:isplay the number of colli-sions.
~ r~nsmission with adiusta~le _time dela~: thesame components as for integral jamming are used and in addition the time delay or time shift 46. The only dif-ference with the above-described ~ype of operation is lS that a burs~ o~ binary data having an adjustable length is sent to the bus lO only after a time delay, after a first frame bi~ has been received by receiver 64.
~ pacçd transmission: One ~rame is transmitted for each group of m frames which is received (m being a predetermined integerj. For that purpose, all components are used, but components 66 and 68, since counter 44 re-mains operative. The time delay after reception of the first bits of the received frame from a station to be tested is adjustable by controlling the time delay unit 46.
CYC1iCa1 _asvnchrong~ V~ C~ Then detec-tor 44 is connected to the cyclical generator 66 and the time delay unit 46 may be used or not. There is a cyclic transmission of frames having an adjustable length, in-d~pendent of whether or not there are frames located onthe bus by other stations.
~ a~dom asYnchronou~ transmissio~: The only dif-ference as compared to the preceding ~ituations is that the envelope detector and counter 44 is connected -to the random generator 68 rather than to ~he cyclical gener-ator 66. That type of operation is somewhat similar to ~3~

those of artificial traffic generators which have al-ready been proposed for evaluation of local area compu-ter networks. In that respect, reference may be had to ~Measurement Center for the MBS Local Area Computer Net-work", by Paul D. Amer in I.~.E.E. TRANSACTIONS ONCOMPUTERS, Vol. C31, No. 8, August 1982; "ETHERNET
DESIGNERS'GUIDE IN MICROP~OCESSORS AND MICROSYSTEMS", Vol. 6, No. 8l Octo~er 1982, pp. 405-412.
The tool as de~cribed above makes it possible to ~e~t proper operation of the algorithms for resuming operatLon after contention occured on the bus, as well as the components of the network and, particularly, the devices for handling collisions.
It will not be necessary to provide a full des-cription of the collision simulation tool, since it maybe implemented in a number of ways and components adap-ted to fulfi.l the functions identified above are avail-able on the marke~. However, some indications may be useful. The cyclical transmission generator 66 may con-si~t o~ a monostable flip--~lop triggered by a fixed fre-quency or adjustable frequency oscillator whose frequen-cy is compatible with the duration of an individual frame. The "set" duration of the monostable flip-flop will for instance be 50 ms on an ETHERNET network.
The random transmission generator 68 may also comprise a monostable flip-flop which is triggered by a digital noi~e, ~enerated from a ~ener diode or a looped sh.ift register.
The envelope detection circuit 44 for counting the envelopes of received frames may comprise a retrig-gerable monostable having a "~et" duration higher than the code duration of a frame element (150 ns for an ETHERNETTM network and a dffwn ~ounter which is pre-loaded with an adjustable content. That circuit delivers a signal when it cro~ses zero,after a predetermined num-ber of frames has been received.

, The frame generator 54 and 'che shaper ~
typically be provided for delivered a frame having an adjustable length with Manchester encoding. For that purp~se, the frame generator may comprise a monostable 5 flip-flop and JK ~lip-flops or a pre-programmed store and a Manchester encoder.
Last, counter 72 and display device 74 for indi-cating the time delay, the length and the number of frames consi~ts of conventional components such as coun-lO ter, monostable flip-flop, encoding wheels and LED
diodes.
Testing of a collision detector implemented in a transceiver 18 or 20 may be carried out by associating that transceiver to a traffic simulator, for removing the need for a network in operation. Referring to Figure 1, there i~ shown such a transceiver 76 associated with a txaffic simulator 78. The traffic simulating tool may have some similari~y with those described in the herein-before~mentioned references. It may also have the cons-~0 truction illustrated in Figure 4. Referring to Figure 4,the traffic simulator 78 includes a coupler 80 and a processing unit having a central processing unit 82 having associated stores, an output unit (printer or CRT
display unit 84) and an input device ~uch as an input 25 keyboard 86. Operation of the traffic simulating tool will involve a first step consis~ing in the description of the type of traffic to be simulated. That description is stored in the central memory. The next step consists of actual operation, possibly in cooperation with the 30 collision simulating tool. Then the traffic will be simulated by sending fxames at a predetermined rate and receiving the frames from other origins~ The transmis-sion and reception frames may be moni~ored and statis-tics may be gathered, displayed or recorded. Such a 35 recordal has already been achieved in local area net-works for diagnostic and reference may be made in that respect to "A high functionality VLSI LAN controller for CSMA/CD network" by M. STARK et al, COMPCON 83, February 28-March 3, 1983, San Francisco. Traffic simulation may -take into account the parameters of -the actual network on which the transceiver will be used, for instance dedication of the stations (master or slave), number of active slave stations, active address of such stations, mode o~ transaction, average time between frames, number of frames, data length, etc. The internal construction of the traffic simulator may be designed for it to have three po sible operatiye conditions, which may be defi~ed as states O, 1 and 2 of an automate or controller (Fig.5).
State _zero: The tool is in state O when it is turned on. The tool will be arranged for providing to 15 the operator at least an initialization control, a control for help to opexation and a display control. The software of ~he tool may be arranged for displaying or printing all allowed commands as 500n as the tool is turned on and ~o do that cyclically, for instance as:
<STATE O> Allowed commands /HELP/ /STAT/ /INIT/
The command H~LP will cause display or printing of a guide indicating to the operator the commands available in each state of operation and the operations carr.ied out in response to such commands, for instance 25 as follows:
ODel~ator' 5 tL~

This is an ETHERNErT simulator which simula~es traffic between master and slave stations.
Allowed commands are:
/HELP/... operator's halp list /INIT/... frame formatting /EXEC/... run for frame exchange /STOP/... in-process cancelation /STAT/... statistics report /DUMP/... frames display 12 ~3~ ~
Three logi~ stat~s:
<STATE 0> : /HELP/INIT/STAT/
~STAT~ 1> : /DUMP/EX~C/HELP/INIT/STAT/
<STATE 2> : /STOP/

Referring to Figure 5, the different conditions of the automate axe indicated. The traffic to be simul-ated is defined by /INIT/ while the tool is in state 1.
Then, simulation is carried out following command /EXEC/. Any communication error brings back the tool into state O. The command /STOP/ brings back the tool into state 1, for authoxizing display of the gathered statistics as well as display of the content of the last frames which were exchanged. That display is responsive to command /STAT/.
lS A similar tool may be used for monitoring traf-fic. A traffic monitoring tool, a traffic simulation tool and a colli~ion simulation tool may be connected to a ~ame bus line for constituting an assembly for teach-ing purpose, component repair, and component qualific-ation.
The main difference between a traffic monitoring tool and a traffic simulation tool will consist in the functions to be fulfilled and the commands. The commands may be as follows, as they appear on a display screen or on a printer in response to command /H~LP/ while the tool is in state O:

13 ~ 3~;

Qpera~or's Help This is a tool for traffic and protocol~ analysis.
All~wed commands are:
/~ELP/... operator s h~lp list 5 /INIT/... reco~d's p~rameters /EXEC/... ru~ for frame hunting /STOP/... in-proc~ss cancelation /STAT/... statistics r~port /DUMP/... frame display ThreQ logic states:
<STATE 0>: /EXEC/ /INIT/ /HELP/ /srAT/
<STATE 1>: /EXEC/ /DUMP/ /HELP/ /STAT/
<STATE 2>: /STOP/

The internal construction of the tool may be as illustrated in Figure 6, where states 0, 1 and 2 of the automate are shown.
The process will a~ain involve three steps:
Step 1: Receipt of the parameter of the re-~0 cord, of the symbols used for the different fields, of the length and nature of the fields, oP the reception window (which may be selected either from the beginning of the frame or from a predetermined field in the frame).
Step ~: Record i5 carried out.
- Step 3: Display of the recorded content.
The monitoring tool may also be used indepen-dently of the other tools.on an existing network for monitoring the traffic which is exchanged between regu-lar stations. All information exchanged between stations may then be recorded and used for fix up the protocoles and software during the initial phase oP operation.
As an example, typical displays which are obtained in response to different commands when the traffic monitoring tool is put in s~rvice will be given.
In state 1 of the automate, command /STAT/ will cause display of statistics on the CRT. Those statistics which are displayed are those previously obtained from the data which have circulated through the store which constitutes a FI~O register included in the ETHERMET
controller. For instance:

**Statistics report**
number of frames r~ceived............. c R1 0010 rec~ived frames in fifo............... c R2 DD30 collision fragments re~eiv............ c R3 0 frames lost, fifo overflow............ c R~ 0 15 lost of one or more fme."L". s R5 0 frames with alignment error. c R~ 0 The abbreviations used in that table have their usual meanin~:

FIE'O: first in-first out register The fields are attributed in response to the command /INIT/. An example o~ data which may appear is as follows:
25 No. 0l234567890l234567890l234567B90l234567590l234567890l234567890l234567 NO. WORH/WORD/BYTH/BYTD/BYTA/DATA.. ~............. ~
W ORH/ Zone WORD HEXA
WORD/ Zone WORD-DECIMAL( I) BYTH/ Zone BYTE HEXA
30 BYTD/ Zone BYTE DECIMAL
BYTA/ Zone BYTE ASCII
DATA........................... O........ ~..... I Zone BYTE_ HEXA
OK ? (Y /N) Dis~lay of rec~iv~d frames?(Y/N) The operator may carry out the modifications which are felt necessary in response ~o such display.

.

The com~and /DUMP/ dlsplays ~he frames on ~he di~play sc~een or CRT. ~ statement may be provided ~o~
causi~g with or wi~hout field indicatlon. In the first ~ituation, in a particular example where the operator selected to cause display from the first frame in res-ponse to the demand which appears on the screen:
From frame number 0001 ? (Y/N) No. WORH/WORD/BYTH/BYTD/BYTA/DATA..............................................
I ossu 5l2 4000 1 o ?.?. BBBBA5A5A5A5A5A5A5A5A5A5A5A5AsA5A

2 0950 5l2 4000 2 o ?.?. BBBBA5A5A5A5A5A5A5A5A5A5A5A5A5A5A

3 0950 512 4000 3 0 ?.?. BBBBA5A5A5A5A5A5A5A5A5A5A5A5A5A5A

4 0950 512 4000 4 0 ?.?. BBBBA5A5A5A5A5A5A5A5A5A5A5A5A5A5A
0950 5l2 4000 5 o ?.?. BBBBA5A5A5A5A5A5A5A5A5A5A5A5A5A5A
~ 0950 512 4000 6 0 ?.?. BBBBA5A5ACA5A5A5A5A5A5A5A5A5A5A5A
7 0950 sl2 4000 7 0 ?.?. BBBBA5A5A5A5A5A5A5A5A5A5A5A5A5A5A
a 0950 5l2 4000 B O ?.?. BBBBA5A5A5A5A5A5A5A5A5A5A5A5A5A5A
9 0950 5l2 4000 9 o ?.?. BBBBA5A5A5A5A5A5A5A5A5A5A5A5A5A5A
0950 512 4000 loo ?.?. BBBBA5A5A5A5A5A5A5A5A5A5A5A5A5A5A
Typa: MlMor~) *~uffer top~ ElExit) E

In the ~econd si.tuation, when the operator re-quests display ~rom frame 113, that display will be of the type:
From frame number 0001 ? IYIN) N From fr~me number oooo<N<2ose : 0113 Nb. Lg. ~rame contents ll3 74 sooso2ool4ooolooBBBBBBBBoFoFoFoFoFnFoFoFoFoFoFoFoFoFoFoFoFoF

BBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBB99EOFDO~A5 BsBBBBBBBB8BBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBF6E626A7As Type: M(More) PlPrevious) ElExit) E

All necessary information for providing a com-plete picture of the operation axe consequently avail-able.
While differen~ programs may be evolved for carrying out the necessary functions, it is of particu-lar inte.rest to use the program which is available from the assi~nee under the trade name "SPECTRE".
The possible combination of the above-defined tools provides different architectures which may provide any result of interest. For instance, as~ociating traf-fic simulation tools and collision simulating tools provides a test and repair unit which is extremely powerful. Associating all three tools which have been de~cribed on a local network provides a unit for teach-ing,repair or qualification of components.

Claims (8)

CLAIM

1. In a data communication local area network having a serial communication bus line and regular sta-tions connected to said bus line at points distributed at intervals and each including a transceiver adapted to send frames of information bits on said bus line and means for detection and handling of contention, a tool comprising a transceiver for connection to said bus line, means for detecting the presence of information on the bus line, means for generating a sequence of binary data in response to detection of a predetermined number of information frames transmitted by one of said regular stations on said bus line, and transceiver means for delivering said sequence to said bus line to cause a colli-sion.

2. A tool according to claim 1, further compri-sing means for adjusting the length of said sequence.

3. A tool according to claim 1, further compri-sing means for delaying transmission of said sequence after detection of the beginning of each said frame.

4. A tool according to claim 1, wherein said generating means are arranged to transmit one of said sequences only after it has received a plurality of said frames.

5. A tool according to claim 1, further compri-sing means for delivering sequences of binary data ran-domly and operator operable means for rendering oper-ative either one of said means for delivering sequences of binary data randomly and said means for generating a sequence of binary data upon detection of a predeter-mined number of frames.

6. A data communication system local area network comprising a serial transmission bus line; a plurality of data processing stations, each having transceiver means for connection to said bus line, an interface cable, controller means, a data processing device and means for sensing presence of a collision on said bus line and handling the resultant contention; and a collision generating tool having transceiver means connected to said bus line and controller means inclu-ding means for detecting the presence of information on the bus line, means for generating a sequence of binary data in response to detection of a predetermined number of information frames transmitted by one of said regular stations on said bus line, and transceiver means for delivering said burst to said bus line to cause a colli-sion.

7. A data communication system according to claim 6, further comprising a simulation unit connected to said bus line and arranged to simulate traffic on said bus line.

8. A data communication system according to claim 7, wherein said simulation unit has memory means for storing a plurality of different traffic profiles and means for transmitting any one of said profiles on said bus line.