Table Of ContentIF AC 1977 and 1978 Conference Proceedings
ATHERTON: Multivariate Technological Systems
BANKS & PRITCHARD: Control of Distributed Parameter Systems
CICHOCKI & STRASZAK: Systems Analysis Applications to Complex Programs
DUBUISSON: Information and Systems
GHONAIMY: Systems Approach for Development
HASEGAWA & INOUE: Urban, Regional and National Planning - Environmental Aspects
LEONHARD: Control in Power Electronics and Electrical Drives
NIEMI: A Link Between Science and Applications of Automatic Control
OSHIMA: Information Control Problems in Manufacturing Technology
RIJNSDORP: Case Studies in Automation Related to Humanization of Work
SAWARAGI & AKASHI: Environmental Systems Planning, Design and Control
SINGH & TITLI: Control and Management of Integrated Industrial Complexes
SMEDEMA: Real Time Programming 1977
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SOFTWARE FOR
COMPUTER CONTROL
Proceedings of the Second IFAC/IFIP
Symposium on Software for Computer Control,
Prague, Czechoslovakia, 11-15fune 1979
Edited by
M. NOVAK
General Computing Centre, Prague, Czechoslovakia
Published for the
INTERNATIONAL FEDERATION OF AUTOMATIC CONTROL
by
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Copyright© IFAC 1979
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First edition 1979
British Library Cataloguing in Publication Data
IFAC/IFIP Symposium on Software for Computer
Control, 2nd, Prague, 1979
Software for computer control.
1. Automatic control • Data processing - Congresses
I. Title II. Novak, M III. International
Federation of Automatic Control IV. International
Federation for Information Processing
V. Ceskoslovenská Akademie Ved. General Computing
Centre
629.895 TJ213 79-42865
ISBN 0-08-024448-3
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Therefore, the readers are asked to excuse any deficiencies
of this publication which may be due to the above men
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The Editor
Printed in Great Britain by A. Wheaton & Co., Ltd.^Exeter
SECOND IFAC/IFIP SYMPOSIUM ON SOFTWARE
FOR COMPUTER CONTROL
Organized by
The General Computing Centre of the Czechoslovak Academy of Sciences, Prague
Sponsored by
The International Federation of Automatic Control Technical
Committee on Computers
The International Federation of Automatic Control Technical
Committee on Applications
The International Federation of Automatic Control Technical
Committee on Education
Co-sponsored by
The International Federation for Information Processing Technical Committee on
Computer Applications in Technology
International Program Committee National Organizing Committee
V. Amoia, Italy M. Novak (Chairman)
K. J. Astrorn, Sweden J. Mayerová
A. Csurgay, Hungary Z. Fabián
J. Gertler, Hungary M. Chábová
A.J. Gobzemis, U.S.S.R. Z. Kominek
M. Günther, D.D.R. L. LukSan
R. Isermann, F.R.G. J. Poupé
J. Kulikowski, Poland M. Pravdovã
N. E. Malagardis, France S. Rízek
H. W. Meier, D.D.R. V. Sebesta
G. Musstopf, F.R.G. Z. Zázvorková
V.V. Petrov, U.S.S.R.
I. Plander, Czechoslovakia
J. Puzman, Czechoslovakia
J. Sedlak, Czechoslovakia
C. H. Smedema, Netherlands
V. Strejc, Czechoslovakia
B. Tamm, U.S.S.R.
T.J. Williams, U.S.A.
FOREWORD
During the last fifteen, years, digital computers (maxi, mini, micro) have been increasingly
used to control various industrial processes and experiments. Design and programming of
process control systems is still a very difficult job, as heterogeneous hardware is used for
this purpose and the process control algorithms are not yet fully clear. In order to find the
best way of programming these process control systems, a triannual international symposium,
IFAC/IFIP Software for Computer Control (SOCOCO), was organised. Following the first success
ful SOCOCO f76 at Tallin, the
2nd IFAC/IFIP Symposium SOCOCO f79
was held in Prague from 11 to 15 June, 1979. The interest in this new sector of computer
sciences (informatics) can be illustrated by the fact that the number of papers submitted to
the IPC for SOCOCO ! 79 was three times that for SOCOCO f76. The three years following SOCOCO
f76 brought about a rapid development of process control software especially in microcomputers,
new access to standardization of computer control software and to exploitation of distributed
computer systems, which have a multilevel hierarchical structure.
This proceeding contains papers presented at SOCOCO f79 grouped according to the symposium
topics. The first part includes four main lectures:
J. Puzman, P. Porizek (Czechoslovakia)
Communication control in computer networks (case study)
G. Musstopf (FRG), H. Orlowski (Poland), B. Tamm (USSR)
Program generators for process control applications (survey paper)
G. Musstopf (FRG)
Microprocessor hardward and software (tutorial)
I.C. Pyle (United Kingdom)
Methods for the design of control software (survey paper)
and presentations from the following areas:
software for microprocessors, process control, experiment control, real-time languages
and operating systems.
The papers from the sessions
algorithms for computer control, computer aided design, interactive systems, application
of computer control in sciences
are included into the second part.
We hope that the publication of papers presented by specialists from 21 countries will make a
major contribution to the development of these important areas of computer sciences from the
practical as well as from the theoretical points of view.
Jan Sedlák
IPC SOCOCO f79
xii
INVITED PAPERS
COMMUNICATION CONTROL IN COMPUTER
NETWORKS
J. F. Puzman and R. Pofízek*
Federal Ministry of Technology and Investments, Prague, Czechoslovakia
*'Institute of Applied Cybernetics, Bratislava, Czechoslovakia
Abstract. The problem of controlling data exchange in computer
networks with respect to real, in particular stochastic, inter
ferences, and user's requirements, expressed by performance cri
teria, is discussed. First, the control problem is studied in
the 2-station model by means of the concept of communication
functions and protocols the latter being a tool for implementing
the relevant communication functions. In complex computer net
works a hierarchical decomposition of the communication control
into layers facilitates to implement the control hardware and
software. As a universal model a reference network architecture
is introduced enabling to describe all types of communication
and their control in each layer and between adjacent layers.
Keywords. Computer networks; multi-access systems; telecommuni
cation; computer network architecture; communication computer
applications.
INTRODUCTION processing equipment taking
place in short distances practi
The growing number of computer net cally without any disturbancies,
works makes it necessary to know more computer networks require long
about elements and structures of the distance data exchange over un
se networks, their cooperation and reliable communication means.
interrelation. Computer networks pri In computer networks there are
marily represent an advanced type of many pairs of entitites that need
multi-access systems and form the ba to communicate, i. e. to exchange
sis for distributed data bases* blocks of data (messages, letters,
packets), between each other, e. g.
The configuration of a computer net software modules or processes,
work consists of host computers and a terminal and a computer, transport
terminals interconnected mutually via stations, node communication compu
a data communication subsystem* Such ters. Regardless of the different
a subsystem may consists of access nature of enumerated communication
elements like FEPs, TIPs, IMPs, of con entities their action has the com
centrating and switching elements mon aim: to fulfil the specified
(MUXs, packet switching nodes, ex requirements even in the presence
changes, PABXs), data transmission me of outer interferences. Therefore it
dia such as telephone and telegraph seems to be possible to study such
(telex) lines with data sets, etc. a communication from a common view
The communication subsystem is either point.
created from own means (excluding the
lines that belong mostly to the PTT) Computer networks have generated new
or uses the services of public data problems not only in the hardware
networks if they are available. In all area but mainly with respect to the
cases, however, some communication complex structure, the organization
control governing the data exchange and the coordination of elements ha
between elements of computer network ving common features of both data pro
or communicating entities is necessary. cessing and data communication sys
In contradistinction to the data tems. This fact does not surprise
transfer between elements of a data because appropriate theoretical tools
1
2 J. F. Puzraan and R. Porizek
and new techniques needed to solve not absolutely reliable, data links
them are still missing.. Frank (1975) suffer from noise, drop-outs, fading,
has outlined the main problems after etc. which causes an erroneous receipt.
dividing them into two categories:
difficult but tractable ones, and On the other hand the user has seve
exceptionally complex ones. Among the ral requirements that he wants to ful
latter a network control theory that fil in the communication system. He
is joined with protocols, measure may determine the readiness measured
ments and performance evaluation, is e. g., by the response, delivery or
reckoned. Therefore, protocols, net access time for certain lengths of
work architectures, communication data messages. Also he may require
functions and services belong to the some accuracy given by the residual
most discussed topics. error rate, the reliability expressed
e. g., by the mean time between failu
The purpose of this paper is to con res or by an availability, and the
tribute a little to the data communi cost that, being chosen appropriately,
cation control. The aim is to outli involves the degree of utilization
ne difficulties connected with the and efficiency (Puzman, 1977).
control of communication and to de
fine the relation between computer As the communication system is stochas
network performance and its communi tic the required values cannot be
cation control. The concept of com stated accurately but with some ranges
munication function is reintroduced that must be kept by the system (e.g.,
as a foundation-stone of communica the maximum response time, the minimum
tion control protocols that are fur probability of error). All require
ther analysed and precisely descri ments form components of the communi
bed. Finally, as the control is clea cation system performance.
red up on the simplest 2-station com
munication model, the last section It is clear that the performance of
returns to the primary system - the real communication system varies ne
computer network and is devoted to cessarily due to varying disturban-
its architecture comprising a commu cies, so that to keep the performan
nication control structure. ce values in prescribed limits the com
munication control must be introduced.
THE CONTROL OF COMMUNICATION The communication control is a system
of actions that serve to maintain
In order to simplify our considera the communication according to regui-
tions let us first deal with the fol red performance values.
lowing model. We shall call the enti
ties - the stations, and the communi We shall give here neither a formal
cation medium that connects them - definition of communication control
- the data link. The data link can nor a formulation of control problem
be either physical or virtual. The (called also a satisfaction problem)
elementary communication system thus because they can be found e. g., in
consists of one pair of stations and (Mesarovic and Takahara, 1975). But
of a data link or parallel data links we must mention the principal featu
(Fig. 1). res concerning the control of commu
nication.
While the classical control problem is
solved "out of a controlled process"
that means the control observes a pro
cess and governs it directly by its
intervention, the long distance bet
ween stations in a communication sys
tem does not enable the direct influen
Fig. 1. 2-station communication model. ce and must utilize the data link it
self. But the data link partly carries
The communication system suffers from data, partly suffers from unreliabili
many disturbances, in particular of ty so that control signals must be
a stochastic nature that influence squeezed into data and protected against
more or less all its parts. Data mes noise, failures, and perturbations.
sages having to be exchanged differ
in their volumes (number of bytes, Another problem arises when the per
bits). The occurrence of data messa formance is multivalued, i. e. it is
ges in stations is random so that the expressed by several components
magnitude of data flow varies from which form in general (in terms of
case to case and from time to time. (Mesarovic and Takahara, 1975)) a cohe
Equipment of the system structure is sive set (the response time increases
Communication control in computer networks 3
with decreasing residual error rate). control itself but also to the whole
The dependency between performance system. The cheaper the communication
and disturbance components is very system is, the more sophisticated its
complex and not yet satisfactorily design must be. Each computer network
expressed. Moreover, it is impossible is a very expensive system and the
to achieve an arbitrary set of values predominant cost being expended on it
of performance criteria for some dis concerns the communication, in parti
turbance value so that the communica cular data links and link buffers.
tion system need not be completely The effort in economizing cost must
controllable. be concentrated in this field. To sha
re data link and buffer capacities is
The role of communication control is one way, another way is to reduce
now clear: if the problem formula transmitted data and to distribute
tion corresponds to some controllable the intelligence far from CPUs. The
conditions then the communication communication control must embrace
control must govern the real communi many aspects that are appearing in
cation system in order to follow the present computer networks and will ap
user's required performance. The com pear in the future.
plexity of such a problem even for a
2-station system consists in the fol
lowing. COMMUNICATION FUNCTIONS
Suppose the value of one disturbance The notion of a communication function
component begins to change worsening is not new. First it appeared in 1972
performance so that control interven (Elie, 1972) although intuitively it
tion is needed. But the influence of has been dealt with much earlier,
control on the communication system practically hand in hand with the da
cannot react in such a way that only ta transmission development. Never
one performance criterion will be theless, the precise definition of
affected and other criteria values communication function has not been
remain unchanged. The interdependence stated, perhaps for its clear intuiti
of performance criteria causes the ve meaning.
complex behaviour of many or all
their values so that further interven Elie (1972) in his paper has conside
tion must be introduced in order to red a communication function to be the
eliminate undersirable changes of them transformation of information being
even when the original disturbance has generated by a source which makes it
stopped its influence. If we admit acceptable by a sink. Such an interpre
that almost all performance component tation seems to be very close to the
values cannot be achieved at the same real action of communication system
time (and this actually happens) the because the whole communication opera
control must find out some compromise ting with stations at the end of data
of solutions. link can be comprehended as a trans
formation facility which conforms the
Although many types of disturbances results of communication to given spe
affect the communication system there cifications of users (Fig. 2). Thus,
seems to exist a way out of the impas the term function is quite appropriate.
se: to decompose the control into less
complex actions and to solve each
problem separately. One approach is to
deal with communication functions that
will be the subject of the next sec
tion.
Besides the outer unpropitious dis
turbances there are other factors that
are hidden in the user's requirements
and in the communication system
performance and demand the correspon
ding control. For example, human inter Fig. 2. Functions and services.
vention into the computer network is
undesirable in contradistinction to If we admit that the communication
public networks like telephone or te function is a transformation we must
lex where in turn the human factor ask how to define it. First, the do
plays an important role. To mechani main and the range of communication
ze the whole action of computer net function must be determined. We may
work is quite a natural requirement imagine that the communication function
that is necessary in order to utilize operates in that way that it changes
efficiently up-to-date data resources. one state of communication system into
another. Thus, the domain is a set of
The cost does not apply only to the starting states while the range is
4 J. F. Puzman and R. Pofizek
a set of final ones. The states can as a control action tending to improve
be described in many ways, e. g. by the performance value after some un
values of performance criteria, by pro desirable interference then the com
perties of communication elements, by munication control is a system of com
the behaviour of communication systems, munication functions. Moreover, if
etc. After determining accurately cor communication functions act sequential
responding states the communication ly (and almost all communication
function is a set of rules transfor functions are of that nature) we may
ming the starting states into the fi conclude the communication control is
nal states. in fact an ordered system of communi
cation functions. On the other hand,
Besides the starting and final states each communication function can be de
of the whole communication it is pos fined by a system of rules of station
sible to find out more internal well- behaviour (note, the control act via
-defined starting and final states if both stations before,during and after
we carefuly follow the course of com a data transfer) so that the same holds
munication. The first control action for the communication control. We shall
is to establish a data link in a suf return to this fact in the next sect
ficiently short time period. The fi ion.
nal state of it may be done by the
existence of a physical or a virtual It remains to state the relation bet
link and the time limit within that ween outer disturbances and a communi
the link must be established. Such a cation system performance expressed by
final state is at the same time a star means of communication functions. The
ting point for synchronizing (bit, by complexity of such a task will be
te, block and/or message phasing) a re shown on a relatively simple example.
ceiver on a transmitter in both sta
tions (if necessary). Let us consider the subset of main
optional communication functions cho
These control actions or communication sen from (Pofizek and Puáman, 1978):
functions are necessary in order to rea -error control (a set of rules to de
lize the communication in general. tect transmission and/or processing
There are, however, other optional com errors and to correct them),
munication functions the use of which -flow control (a set of rules to go
depends on the specific requirements. vern the rate of transmitter accor
For example, after the synchronization ding to the receiver possibility of
process the fragmentation of data mes receipt),
sage into shorter blocks enables to put -routing control (a set of rules to
through an amount of data within a direct data blocks to outgoing data
short time period if more than one path links),
between both stations are established -capacity assignment control (a set
(then an appropriate routing must fol of rules to share the data link ca
low) . During the data exchange the er pacity among several data messages) ,
ror control takes place with the aim -fragmention/reassembly control (a set
of increasing the accuracy, etc. For of rules to split up a data message
those who are acquainted at least a lit into fragments, to envelop each
tle with data communications, the fol fragment and to form a block/ to re
lowing examples will help to understand lieve received blocks of envelopes,
the principles of the decomposition of to sequence fragments into original
communication control operation by data message),
means of communication functions. -recovery control (a set of rules to
identify exceptional states and to
The simplest data message transfer over pass over the control to higher le
a leased line may consist of synchroni vel) .
zation establishment - transmission
opening - block phasing-message frag The influence of each communication
mentation - block formatting - error function on global performance crite
recovery - message reassembling - ria is shown in Table 1 where + desig
- transmission closing. nates the improving while - staids for
the worsening of performance criteria,
The packet delivery over a packet e. g. the increasing of the cost, (0
switching network would proceed from indicates that the influence can be
packet formatting through routing, neglected). For example, the error
packet phasing, flow and error control. control increases the overhead of
Using a multiaccess communication transmitted data due to check bits be
system then after the packet format ing added to information bits and due
ting an access to a slot, a buffer to retransmissions after detecting an
management and a reservation of another error so that the readiness is worse
slot is inserted. ned. But at the same time it increases
naturally the accuracy and of course
If we adopt the communication function the cost, while the flow control leaves
Communication control in computer networks 5
practically the accuracy and the re The concept of communication function
liability unchanged and decreases the is very close to another notion - the
cost owing to the shortening of buffer communication service. A transmission
capacities, etc. facility realizing some communication
functions offers a service or services
to users,being its subscribers,through
TABLE 1 The Influence of Communi an access (see Fig. 2).
cation Functions on Glo
bal Performance Criteria We shall,for our purposes, put an
equal sign between the communication
function and the communication service
although the verbal interpretation of
readi accu relia both concepts may be different. We are
cost
ness racy bility speaking about physical data link es
tablishment/disconnection as about the
communication function but from the
error subscriber's viewpoint it is expressed
- + 0 -
control by the communication service "line swit
ching" .
flow
- 0 0 +
control As the line switching and virtual call
services resemble the PTT telephone
routing service and the same holds for the
+ 0 + 0
control datagram and PTT post service, and as
each class of communication services
capacity has its specified communication fun
assign ctions, the following examples show
- 0 - +
ment con us the differences between both servi
trol ces on the basis of implemented com
munication functions.
fragmenta
tion/reas The telephone service starts with the
+ + 0 +
sembly physical or virtual link establishment,
control then it may be necessary to establish
the bit phasing and to open the trans
recovery mission. As the established link is
+ + + 0
control able to carry several independent calls
(channels) the efficiency (cost) forces
to share the link with other calls
and therefore one must control the ca
Table 1 shows the complex interplay of
pacity assignment (e. g. by means of
influences of communication functions
multiplexing, polling, multiaccess).
on performance criteria. Suppose we
The data exchange necessitates the er
must improve an accuracy. We may intro
ror and flow control accompanied by the
duce an error control and a fragmenta
message fragmentation, formatting and
tion/reassembly control (in the case
buffer management. The reassembling
of long data messages both are neces with the sequencing into the same mes
sary) . But, at the same time, the er sage as being sent is a main feature
ror control increases both the respon of a telephone service. Finally, the
se time (worsens the readiness) and closing of a transmission and the dis
the cost (the fragmentation/reassembly connection of a link are inverse
cannot counterbalance the error con functions of the transmission opening
trol) , so that the routing control is and link establishment, respectively.
necessary for improving the readiness,
and the capacity assignment control
The post service is very similar to
may decrease the cost.
the PTT service of packet delivery.
It necessitates the message fragmen
Although tables similar to Table 1 can
tation (but not correct reassembling),
be constructed for other communication
transformation of formats, headers, ad
functions and other performance crite
dresses, etc. (if necessary), the search
ria, and many measurements have been
ing of an appropriate channel direction
performed in real computer networks,
(routing). After determining the link
nevertheless the exact functional re
by routing all communication functions
lation between the methods of reali
concerning the packet (block) delivery
zing communication functions and the
over a data link with required readi
performance criteria having also a
ness, accuracy, reliability and cost
practical use is far from a solution.
have to operate.
This problem comprises the basic step
in the development of communication
For both services the exceptional sta
control theory.
te recovery is a very useful communi-
