Problems with a node of computer networks

69

Applications Problems with a Node of Computer Networks Vukagin P. Masnikosa Institute "'Mihajlo Pupin'; Volgina 15, POB 15, Beograd 11 060, Yugoslavia The paper provides an analysis of structural characteristics of a node of computer networks which are intended for the control of large-scale technical systems. The problems of constraints arising in the node related to the time and volume of information and processing are discovered as a result of experience gained from practical systems operation. The problems being solved are: the variable structure of information with respect to which the structure of a node computer system is invariant; the variable configuration (slsD - mmI~) of a computer node which is invariant to time constraints as well as other constraints; the development of each node into a central control node. Finally, proposals for solutions (computer system architecture) are given which include the use of a node computer system for automatic control.

Keywords: Computer networks, Large-scale technical systems control, Artificial intelligence. . . . .

Vul~in P. Masnikosa was born in 1925 in Ivo~evci, Dalmatia, Yugoslavia. He graduated on Electrical Engineering Faculty, Belgrade in 1953. He was promoted on Electrical Engineering Faculty, Zagreb in 1971 with Ph. D. degree (Pattern Recognition Using Method of a Pieces Information's Multimapping). He has worked in the Hydro-technical Institute, Belgrade on the problems of nonelectrical magnitude measurement. Since 1955 he has worked in the Institute of Nuclear Sciences, occupying himself with computer design and programming, continuing the work on the same problems in the Institute "Mihajlo Pupin", Belgrade ever since. While he was working in the domain of computer applications he was keen on the problems of pattern recognition and artificial intelligence. Since 1975 he has been occupying himself only with artificial intelligence. He was professor at the Faculty of the Technology in Novi Sad from 1972 to 1976. M o r e t h e n 100 professional and scientific papers were published or presented on various Yugoslavian and International Conferences by him, as well as a lot of other publications. North-Holland Computers in Industry 10 (1988) 69-77

1. Introduction

The evolution of systems for complex process control [e.g. 1,2] shows a concordance between the increase in requirements and the use of technology for their realisation. Today we have reached a stabilized request level (there are, at the moment, very rare cases of complete, automatic control of complex systems or their parts). The explanation for this phenomenon may be sought in a slow growth of the Control System Reliability (csR), and in the price. (A change in requirements or in equipment technology requests the exchange of the control system as well, and not only parts of it). These phenomena are the consequence of: (1) the culture of the use and maintenance of the System For Control (SFC) equipment (in Yugoslavia at least) on the one hand, and on the other of the sFc structure, which mainly does not offer a possibility for the increase of reliability; (2) it does not allow mixing of technologies and the devices of different manufacturers. The first cause is partly a consequence of the education, and partly of the second phenomenon mentioned above. In this paper a complex control computer system structure, through which a further growth of reliability of operation and requirements is possible, shall be presented. The key problem of the structure proposed is intercommunications between computers. Let us suppose that a power control system ~ is in question (Fig. 1). From Fig. 1 it may be depicted that participants in the process are (G i, TSj)= (nodes) which are at a distance from each other. It may be observed that there are four types of nodes. The structure 1P ought to be

1 The control is a process (complex activity) into which the participants are: operational part (OP) and control system (informational part = IP)

0166-3615/88/$3.50 © 1988 Elsevier Science Publishers B.V. (North-Holland)

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('omputer~ m Indmtm

invariant with respect to (1) the type of node (not dependent on the number and type of information) (In); (2) multiuser processing required ( Ps); (3) autonomy of the node control (Cn); (4) change of the IP node technology or its parts (Tn); (5) speed of subset operation IP (Wp), i.e.

S =f(Zn, Ps, Cn, Tn, Wp)= const.,

---4

Primary

(1)

I

where

S = IP Structure;

f = function (dependence).

A clear insight in the solution depends on the aims of control, which are: (1) RD and ID (ID = IP) equipment protection; (2) maintenance of the node into nominal operation mode; and (3) control costs minimization.

2. Node Control Analysis

The control of a node, of whatever type, may be roughly presented graphically in the form shown in Fig. 2. Analysis of control will point out the following actions:

(RD,):(E i, ( k , } , (rvc}) = (E,+,, 8E~+,),

(2)

where

8E~+l=f({sj}, {rn~ }) and

(ID,):({sy), { m , } , I ) = ( { k i } , {rvc})'

(3)

which reads as: operational part (RD) acts upon input work (Eg) and input c o m m a n d s (( kl }, ( rye }), transforming them into output work (Ei+l), and into informations about its state ((s;}, (mi}) (see Eq. (2)), with information part (ID) acting upon informations ((sj}, { m i d and upon the external informations ( I ) transforming them into control commands ({ k 1}, (rv~)).

(RO)

L

W°rking i)art

I

Input

I~

ExecuLive I

Ii T,

4 i denti fica~io I,

,II

I

l

Fig. 2. Structural representation of the control of a node.

From Fig. 1 it may be seen that the number of data from one node may be different, depending on the complexity of the node. The control of a node may be autonomous or partially autonomous, and can be realised from a remote control centre controlling a complex process. The primary equipment protection is the source of process destruction. This requires of ID to be fast enough to exclude its effect, or to reduce it to a minimum, requiring ID to operate in real time and reliably. During the process of ID structure selecting, the program support must be taken into the account. The aspiration is directed towards the possibility for ID structure to allow expansion from minimal to the most complex configuration, by add-on subsets (elements of the structure) of different technologies, and operation speed enabling in this way the possibility of building up ID in accordance with the user requirements (related to maintenance, replacement, expansion, different additional informations processing etc.). 3. Selecting the ID Structure

3 j

It may be seen from Fig. 2 that the information processing jobs in ID are real and related in the following way:

IT: ( [ ( I S , ) : ( I S u ) ] : I } : ( P T ) : ( ( s j } , mJ

Fig. 1. Complex process control by m e a n s of an information system,

(m,})

= ( { k , } , {rVc} ) (4) (to be interpreted: (: = two stops) as the action sign).

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V.P. Masnikosa I Computer Networks Node

From Equations (2) and (3) it is concluded that the RD and 11) connections and the ID and RD ones are unique, connecting rigidly RD and ID. Different RD point out to the possibility of different forms of primary signals. Starting with this fact and the above mentioned requirements, it is necessary to standardize the form of presenting data after P T and before IT. With this, further signal action on 119 structure is excluded, enabling, in each node, control to be realized at one or more parameters by direct connections between P T and IT. A more complex control will require the introduction of secondary processing, i.e. of part [(ISz) : (ISu) ]. With this part, the following is provided for: (1) a set level of autonomy of node control (autonomy of a node depends on the E, availability and permitted variations of 8E~+ 1); (2) processing and preparation of the information for other users (adjacent node and central control, and the others). The fitted function [(~tSI) : (1Su) ] must possess a equal correspondence either with P T and I T or, through PP, with the remaining nodes and users. The proposed node control organization (Fig. 2) points out unambiguously that the manner of data representation during the cross-section at their flow (marked A - A') is an invariant parameter. Starting with the assumption that each at the data is uniquely described by tree

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parameters (although there are even more), the conclusion is that information will occur in A A' in the following form:

I=f(p,

n, a),

(5)

where I = information; f = function (subordinate, dependeble); p = address of location out of which it is taken; n = class, a = value (amplitude). On the basis of the above stated facts and the characteristics of modern computers, a structure is required which will meet all requirements (1) and successfully satisfies the needs of the complex process control system, represented in Fig. 3. Easily observed from the picture is that there are a lot of sharp limitations, such as: fast primary data (signals) processing; a quick information classification and editing; a fast node state identification and selecting of the solution; a fast classification and distribution of control commands that requires a fast selection of program for selected solution realization; a fast transformation of control commands into the appropriate signals. The restrictions mentioned will lead to problems related to intercomputer communications. A proposal for solving this problem will now be presented in a practical realization of the structure shown in Fig. 3.

Objectof control (workingpart-WP)(RO) I{k&}ITll{rVC}l

l lr A

Ii

- - - - -

I'Srl i '"Or I

l

l

l

l

J(TT)

J

Fig. 3. Node Control Structure: RD = working part, P T = primary processing, PS~ = Switching sets, MB~ = Memory blocks (distributed data bases), IS,, = State identification, IS t = Solution identification, PP = Communication set for connecting to other ID at boundary nodes, I T = Distribution and plan of identified solution realization, TT = operator terminals.

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4. Practical Realization of ID N o d e Structure

The selection of standardizing information present in the cross-section of A - A' flow under the condition to accept standardized information and to present themselves. Realization of communications between computers through an information base leads to, as a main problem, connecting of each computer with this base so that their communication is possible only about the information contents. In this way, in fact, the database is lifted to the level of an information base, and the communication of computers over the information.

Computers in h~dustt 3,

ComilOnd I block

buffer

!

I -decoderCoder~I~

Error correction

I

A .... I[ .... A'

4.1 The Structural Element PP In Fig. 3 the communication link of the ID node system is established through the element P P (there may be more than one present). The task of this part is to transform the standard telegraph signals into standardized information in a form in which they appear in the cross-section A - A'. It may also be seen in the same Figure that inside the part marked (PP), the telegraph train is transformed into a binary signal, and in the part marked (DD), the providing of binary signal transformation into a form accepted for the representation of information in cross-section A - A'. In this way the element together with the microcomputer provides for information transmission from a remote node to the appropriate information base for the transmission of a definite information to a definite remote node. Details on the solution related to the linking parts of the element marked PP are of routine character (see Fig. 4). 4.2 The Structural ElementPT The tasks of this element are manifold. The first is that of data acquisition. The acquisition of data cannot be performed separately for analog and separately for digital [3]. It is thought that the parts of the node are in such a mutual spatial relation that their separation will lead to a deterioration of the solution. The second task o f the element is to edit the information according to the user's determination of the priority of the data transmission to the data (information) base and the storing of the data itself.

11 Fig. 4. Communication flow element autonomously seperates inforrnations intended for the node and out of node to other users.

4. 3 The Structural Elements ISu and I S 1 To these structural elements very vital tasks are assigned'. The I S u element supervises definite information in the appropriate base and identifies the state and phenomena inside RD, on the basis of which it decides (makes a decision) if an intervention is to be made or not. Limiting the time of the operation of this system, and especially acting in conditions of primary protection, implies: the selection of a procedure to put in operation, which will permit parallel operation or which will provide for a direct introduction to the solution, based on the identified information. It is wellknown that the identification of the RD state may be realized through: (a) a mathematical model of RD; (b) application of some of the artificial intelligence methods (PROLOG, LIPS, Linking space); (c) identification masks. With the aspiration to accomplish a total control automatism to this element is also the task of selecting the corresponding solution assigned. Based on the solution presented element I S i identifies the procedure according to which the solution to be obtained is actually realized. The task of these elements are accomplished by means of the microcomputer.

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The access to the appropriate information base is of the highest priority. 4.4 The Structural Elements I T and T T

The task of the structural element I T is to transform the control set of information into the appropriate command signals and time sequential realization. The element T T represents the link with the users. Its task is to convey the appropriate information from the data (information) base to the users (in one of the selected forms - video-terminal and other). Realization of both elements is possible by means of the microcomputer.

V.P. Masnikosa / Computer Networks Node

I

I

~,IEIvIORYBLOCK(MBi)

Lo.

73

,

4.5 Structural Element P S i

The structural element P S i represents the switching set through which each element of the structure is provided access to the selected information (data)base. Many solutions have been presented for this problem (access of a large number of users to one memory block). The concept of conflict evasion [4] complicates the problem considerably, easing some problems into the intercomputer communications [5]. The concept of the "bottleneck" problem [6] is avoided here by breaking up the information base according to the needs of users, so that the number of users on the same task may be enlarged as required. Connecting bases on the principle presented in [7] (minimizing Boolean functions through relations) would also require intercomputer communication, which will slow down the system. The implementation of switching connections [8] is not applicable, because the priority of access for that microcomputer processing data from a definite base is requested, and it is therefore linked to it with the highest priority, which will necessitate a separate arbitrary computer to deal with the conflict situations. Taking into consideration all of the listed properties of proposed solutions (not only the ones mentioned here but others as well) and the requirements the solution must fulfil, the structure of this set was elaborated, which to the greatest extent satisfies the requirements 1D must meet, and which is presented in Fig. 5. T h e Figure shows that priority logic will determine which of the microcomputers will access that memory block,

Fig. 5. Switching set block diagram.

because from the priority logic each user will receive a signal telling him that this memory block is at his service (i/o M B i ready). With this signal the microcomputer starts its normal communication with M B i by means of command logic which opens the appropriate switches. In case the operation speed of the microcomputer and of the memory block are not synchronized the application of a B-RA (Buffer - Rate Adapter) block is foreseen. The access conflict is resolved by means of priority logic, starting with the microcomputer requesting access, to the priority logic of the P S i itself. If as the highest priority is taken such M B i which information requiring interventions are written, then the microcomputer will have to wait his turn, prolonging in this way writing in time for so much, as for how long one of the computers will take to write only this class of information into M B r The probability for such conflict situations to arise is very small [1] and is taken as possible to occur between P T and the I S v microcomputers only. In Fig. 6, the P S i priority logic is represented. The figure shows a simple logic switching network, which directly at the output generates the signal to that microcomputer which has established connection with block, while the rest obtain

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Applications

('omputer~ m lndustrr

ual participants in ID structure, thereby, inserting a B-RA block wherever it is found necessary.

PO 5. The Main Features of the Proposed Structure

P1

Fig. 6. " O n e excludes the rest" - priority logic.

the signal telling them that the block is busy. It is to be noted that such an access to the memory block may be accomplished, if the number of users is not too large (up to 16). The solution for the control logic depends on the memory characteristics used for the memory block and the microprocessor signal used in this M B i. This block controls the operation of the switch in compliance with the control signals which define the type of access to the memory block. The block diagram showing interconnection between the control logic and switches is presented in Fig. 7. The problem of compliance between the rate of the microcomputer and M B i is solved by a FIFO buffer. The use of I D implies that the block marked as (B-RA) is used as the buffer memory, due to the fact that the quantity of data which makes up the information is finite and easily forseen, and is divisible. The diagram of this block is presented in Fig. 8. Owing to the programmable timer it is possible to programme one write rate into the FIFO buffer, and the other read rate. This provides compliance of different rates of individ-

COMAi40 LOGIC

(CL)

switch

It AUDRESS

g DATA

Fig. 7. Connection of memory block and microcomputer.

The proposed structure of the ID has several distinctions. Each one requires the appropriate attention. Standardization of information (data) presentation in the cross-section of flow A - A ' provides the possibility of combination of different technologies in realization of the ID node. This enables the expansion of the ID node without changing the already built-in equipment. This type of expansion does not require any additional works on the already implemented equipment. All works are related only to the part that is to be expanded. This enables the 1D node to be independent on the technology of its parts (Tn). Different technologies have different rates and other parameters which must be overcome in order to implement them into the system. Owing to the B-RA block this problem is easily solved. The solution of the block presented in this paper is based on the idea of a more general approach to this problem, by using the LSI technology with a whole spectre of rates which may be programmed. This enables the 1D node to be independent of the rates used in some of its parts (wp). The interface between the 1D and the object R D depends on the R D characteristics. This problem is a problem by itself, and it is solved in elements P T and IT, and it is not reflected in the ID structure by the standardization of information presented in its flow cross-section A - A ' . A similar problem occurs with the communication of the ID node with other ID nodes. In fact, the element P P presents, as do the elements PT, T T and IT, the adapter for communication with the surrounding environment. All elements of the structure can be expanded while not effecting the I D structure. This enables the ID node to be independent of the type and number of data information (In) introduced into, or extracted from the ID node. Introducing into the structure the elements for serving the user, which may also be expanded, the ID node is relieved of time constraints related to data processing. This is a consequence allowed by the structure, by using the parallel access to the

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V.P. Masnikosa / Computer Networks Node

75

==

~BEAT GEiqERAL_ ~ H switch 4x4001;3 TOR4636B [ - -

--

H

switch 4x4Ol bB [

?

H

I/0 BM. reedy

FIFO

J~x4OlOSg

i'~×4O105B II

I

1

swi~cch

4x4JolB

I-

H

switch 4x401o3

I

II AODRESS

DATA

Fig. 8. Blockdiagram of the solution for memoryblock and microcomputeroperation speed coordination(B-RA).

information base related to it (write-in of information into a number of M B i blocks). This enables the I D node to be independent of requirements concerned with enlargement of different processing procedures for the user (Ps). The critical time constraint regarding the control of ID node operation can also be solved by this structure. The most difficult requirement is keeping the R D in operation [9] with primary protection being activated. Depending on the starting moment of primary protection activation (what can be detected [11]) till its final effect switching-off the device from R D - it is possible to isolate and preserve the R D node in operating mode in the part that can be used in the occurred accidental situation. In fact, this control action is possible to start and complete only in the node. Therefore, the speed of detection and memorizing of the information regarding the primary protection activity is of vital importance. According to the given structure, the flow of information from R D towards the information base is direct. Acquisition and interpretation of information taken from the base ( M B , ) is performed in the element I S v. This task can be effected, if its access time to the corresponding memory block is sufficient to enable effecting the task (access in every msec to MB~). The acquired information from the base (MBi) defines the quantity of the information from MBi necessary and sufficient for identification of the R D node status which analyzes the availability of

the related nodes. Thus, based on the identification, it enters into the appropriate solution. It is known [10] that there are a limited number of states that any object can go into. Starting from this fact, it is natural and extremely convenient to apply the self-organizing linking-space, for introduction into the solution, although it is possible to apply PROLOG, or LISP. The identification of the node status and the availability of the adjacent nodes by using the method of mathematical modeling excludes the possibility of intervention for control purposes, i.e. keeping the node in the operating mode. This does not exclude the off-line identification of the status of the R D and R D ' s node, to acquire the solutions which are available. By using this type of identification and availability it is possible to satisfy the predefined requirement even in cases when the R D node is rather complex. In fact, in such cases the node is divided into parts which enable, by detecting the activation of the primary protection, operations to continue, and to post factum put the node into its optimal operation mode. In this way, the functions I S , and IS1 can be distributed, i.e. parallel operation of the appropriate P T and I S u ( ISt ) can be achieved. The stated solution of the problem enables the structure to be independent of the level of autonomy of the R D node (Cn). Realization of the structure is possible for any type of R D node configuration. In cases when only a single datum is taken from the node, the

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P T ' s function is reduced to transforming the input signal into the standard form which is introduced into the P P element, without any influence of the microcomputer. The structure is not changed, but only necessary elements are used. Depending on number of data taken from the node RD is the number of P T elements of the structure. The structure can be realized by using only one microcomputer and one memory block (correspond to SlSO class), and it can be with several microprocessors and MB~, and the whole structure behaves as the MIMD system. In any configuration the 1D node structure is not changed, only the number of the structure elements varies. When the structure is analysed and instead of R D node parts, a data source is taken as a correspondent, it can be seen that the proposed structure of 1D can satisfy the requirements of the correspondents. Only the programmes of the elements change. Therefore, the correspondence of one P T element depends only on the transformation of the external activity into a ~tandard form of information. The only thing that has to be performed is to singulary extract the input information, to memorize them in the memory blocks, to process them in elements I S U and I S 1 and to distribute them as necessary. The structure proposed here, if interpreted in this way, can be used in all cases when processing of information is necessary, regardless of the purpose of the process. Therefore, this structure solves the problem of correspondence between computers in the computer system in general by using the information base. The described structure has two constraints, which are: a. users of the information must be self-serving (TT, PP, ISv, 1Si, I T ) b. the number of users is limited. These constraints imply that the computer network should be organized with a larger number of nodes, or with a larger number of memory units per user. The structure presented in Fig. 3 may by the ID node, but it can grow very easily into a control centre, by adding an additional element for data processing on the level of complete R D which is being monitored, without changing the basic structure.

('omputers in lndustrr

6. Short Discusion and Conclusion The proposed structure of the ID node is very convenient for control in the distributed technical system. It is very simple to realize. It fulfils all the requirements. Software support (programmes) is modular, therefore, it is easy for production and implementation. Expansion does not cause a change in structure, nor in the already existing equipment of the TD node, but only the addition of the elements into the structure via the flow A - A ' and the implementation of appropriate software support. The problems that may occur are the ones concerning the form of presentation, i.e. the language which is to be used in the correspondence. This problem can be overcome if a single ID for a complex process is used. However, expansion may cause some problems. This problem can be solved by introducing an additional element into the structure, whose primary task would be to translate from one language to another. Further research should be directed to development of the information bases, which would be able to, with the appropriate information, self-initiate the users for further processing. The basis of these research programmes, which is certainly the most interesting and promising task, is the theory of the linking space [12]. A succesful solution to this problem would provide a base for the realization of some features of the artificial intelligence, thus, adding significant importance to the proposed structure.

References [1] V.P. Masnikosa, R. Rakir: Problem of Monitoring and Processing of Data in Hydroelectric Plant "Djerdap". X I I I Yug. Conference Etan, 1969, Subotica (translated title from Serbocroatian). [2] V.P. Masnikosa et al.: Design of Practical Solution for Control in Electrodistribution - Beograd. Technical Documentation of Institute "Mihailo Pupin", Beograd, 1975 (Translated title). [3] V.C. Jaswa et al.: CPAC - Concurrent Processor Architecture for Control. I E E E Trans. on Comp., C-34, No 2, Feb. 1985, pp. 163-169. [4] S.P. and S.I. Kartashev: Memory Allocations for Multiprocessor Systems that Incorporate Content-Addresable Memories. 1EEE Trans. on Comp., C-33, No 1, Jan. 1984, pp. 29-44. [5] V. Zakharov: Parallelism and Array Processing. I E E E Trans. on Comp., No. 1, Jan. 1984, pp. 45-78.

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[6] N.S. Stone: Database Application of the Fetch-and-Add Instruction, IEEE Trans. on Comp., C-33, No. 7, July 1984, pp. 652-667. [7] S.P. and S.I. Kartashev: Efficient Internode Communications in Reconfigurable Binary Trees, IEEE Trans. on Comp., C-33, No. 11, Nov. 1984, pp. 977-990. [8] C.Y. Chin, K. Hwang: Packet Switching Networks for Multiprocessors and Data-flow Computers, IEEE Trans. on Comp., C-33, No. 11, Nov. 1984, pp. 991-1003. [9] V.P. Masnikosa, S. Koprivica: Some Problems in Real-time Programming. SOCOCO 76, Tallin, IFAC/IFIP, SSSR, 1976.

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[10] V.P. Masnikosa, S. Petrovi~-Stojanovi~: Optimal Management of Production in the Hydroelectric Plant "Djerdap". SYM-OP-IS 77, Hercegnovi, 1977 (translated title from Serbocroatian). [11] V.P. Masnikosa: On Application of Telemetry and Process Computers in Electrical Power System, Automatika, Nos 3-4, 1976, Zagreb (translated title from Serbocroatian). [12] V.P. Masnikosa: Action Principle. Monography, edited by Institute "Mihailo Pupin", Belgrade, 1984 (translated title).