Networks An introduction to microcomputer networking concepts

Transcription

1 Behavior Research Methods& Instrmentation 1978, Vol 10 (4), Networks An introdction to microcompter networking concepts RALPH WALLACE and RICHARD N. JOHNSON GA TX, Chicago, Illinois60648 and JAMES H. JOHNSON Eastern Virginia MedicalSchool, Norfolk, Virginia This paper presents a brief history of the se of internal compter networks, an introdction to networking concepts and topology. and sggestions regarding a possible system for se in a psychology laboratory. Considerations abot the design and se of a microcompter network are discssed. As additional behavioral scientists obtain microcompters for research, there is apt to be a growing interest in sharing hardware-software components with other investigators to redce costs and increase comptational power. This concept is sally referred to as compter networking. The initial ideas abot compter networking began with the introdction of timesharing and the development of data commnications technology in the late 1950s. Timesharing systems involve a large central compter acting as a host to several remote terminals. With increasing service demands, some single central compters were replaced with mltiple connected compters. These were the first tre compter networks. In sch sitations, the host processors performed large comptations, controlled data bases, and spervised network operations. Remote compting systems performed smaller comptations in a local mode nder an independent operating system, bt participated in network activity when processing demands reqired. As minicompter technology developed, networking became more individalized and tailored to the needs of sitations. Individal processors were assigned very specific tasks and programmed to commnicate with neighbor processors and data base files, This approach reslted in easier software development and improved tolerance for systems failres (Digital Eqipment Corporation, 1974). The topology of links (circits and channels) and nodes (endpoint or jnctions of a circit) also became increasingly specialized as a reslt of minicompter technology. Any node cold be set p as an exclsive ser or provider of resorces, and the geometric arrangement of a network varied from simple point- Please send reqests for reprints to James H. Johnson, Department of Psychiatry, Eastern Virginia Medical School, Norfolk, Virginia to-point operations to more complex mixed strctres. The advent ofmicroprocessor technology promises to lead to additional tailored network topologies. Becase of the lower cost ofmicrocompters, behavioral scientists are now likely to be able to afford more sophisticated network topologies in laboratory and other experimental settings. However, at present there is insfficient general knowledge in the field to make fll se ofthese concepts. For example, in a recent paper on developing a laboratory compter network, Brown and Deffenbacher (1978) sggest an approach whereby a psychology department wold have a single large microcompter for program development and tasks reqiring large memory, along with several independent low-cost microcompters to service instrment needs. This is a network only in the loosest sense, as the varios micros do not commnicate with each other following program development. The prpose of the present paper is to acqaint behavioral scientists with other more advanced networking concepts. It is hoped that this material will be of se to those considering the installation of a network of microcompters, bt nfamiliar with alternatives to a simple, independent networking approach. NETWORKING The low cost of microhardware makes it possible to locate a sizable amont of compting power near the ser. Each local node may inclde its own cathoderay tbe (CRT) terminal and a compter with sfficient memory to handle most needs. If a particlar ser has special reqirements to control instrments and actators, these can be added to the node. The local compter handles all special eqipment formatting, so that only basic data are commnicated to other processors in the network. It also does extensive calclations (e.g., matrix calclations p to 20 by 20 appear feasible), Large problems are completed at a node 522

2 COMPUTER NETWORKING 523 with extra memory and floating-point hardware, bt reqests to this calclator node are minimized becase many sch reqests can be handled at the local nodes. When resorces are not available at the local node, a message can be sent to nodes that have the needed capabilities. For example, if a local node reqires a data printot, a reqest is made to a node with an attached printer. For other special resorce reqirements, sch as remote commnications, graphics display, large calclation, me handling, and special instrments, the local node sends a message to the appropriate node reqesting the se of the resorce. Sch messages are roted throgh a network control node that determines if varios resorces can be allocated to the reqesting ser. A local node can alert the network control node of nsal events. For example, if a local node is monitoring an experiment and a malfnction occrs, the network control node might se a commnications node to atomatically telephone the appropriate investigator. Similar monitoring can be sed for prposes of secrity. The program action to be taken in response to a malfnction can be located within either the local or the me node. Therefore, several different corses of action are possible depending pon the task of the local node, time of day, or severity of the malfnction. The network control node might call different phone nmbers, depending on the type of malfnction and the natre of the experiment nder control. A local node may have special instrmentation, sch as a gas cromatograph with atomatic sample finding and recording, attached. To insre that the nodes are interchangeable, a standard interface mst be sed on all of the nodes. For interfacing instrments, standards crrently exist that are sefl. The IEE488 bs standard is sitable for parallel interfaces. It has sfficient control and data lines to handle most reqirements for laboratory instrmentation. For serial interface, the EIA RS2322 can be sed on most reqired eqipment. These standard interfaces mst look the same to each program that is expected to control them. This allows one instrment to be attached to two or more local nodes for reliability or to be switched from one location to another. Also, it allows several sch instrments to be attached to different nodes sing the same programs for data collection and control. Contemporary networking technology reqires many special fnctions to be available to all local sers. The special nodes perform fnctions for the other nodes in the network and have niqe programs that reflect their specialty capabilities. Examples of special-prpose nodes inclde file handling, network control, large calclation, remote commnications, and printing. A file node handles all the storage and retrieval fnctions, inclding bffering for the network. Usally, the files are stored locally on disks or tape. File data can be dplicated locally, or in some cases, on another system throgh a remote commnications node. The me node keeps a directory of all the active flles available. In a properly designed system, files that are inactive for a fixed time are atomatically archived by the me node. This insres available space on the file system for crrent se. Programs for se by the varios nodes are stored sing the me node. A reqest for a particlar program from a local node is first directed to the network control node for prposes of athorization. Once athorization is granted, it is forwarded to the file node for retrieval and transmission to the local ser. At the local ser node, a commnication program formats the file for exection, loads it into memory, and, after the last segment of the file is received, starts exection of the program. This process can also be initiated by the network control node. The ability to download programs to the ser node nder command of the network control node is extremely sefl. For example, it allows an experimenter to se different control programs for different phases of an experiment. The network control node directs the processing throghot the network. Downloading capabilities and program-access rights from other nodes are controlled by it. Local nodes refer to it whenever otside help is needed. The network control node can take appropriate action sch as transmitting a message to a CRT, downloading a new program to the node, or reqesting an operator intervention. It starts the network when the power is applied to the system. Finally, the network control node can serve as a message switching center. A special node for calclating is sally inclded in a network. Typically, this node is qite expensive, since it reqires a large amont of memory and floatingpoint arithmetic hardware. The calclation node can accept inpt sch as a program name to be retrieved from the file node, inpt data file names, and the names of files for the storage of reslts. When a calclation program is finished, reslts are released to the reqesting node by freeing the otpt files. This allows the local node to begin accessing the otpt me to review reslts. A special node is reqired for commnications with other networks. The commnications node interprets the network protocol for commnications with remote systems, and reqests programs from the me node that are appropriate to a particlar protocol. Inpt-otpt (I/O) nodes control the physical I/O to electromechanical devices sch as a printer, card reader, and plotter. The I/O node contains bffers so that data can be transferred directly to or from a local node. However, in cases where significant activity is expected, I/O data may be roted throgh the file node. This node is responsible for qeing procedres and all standard data formatting programs.

3 524 WALLACE, JOHNSON, AND JOHNSON ( ) (1.11.1) ( ) (11.01) ( ) Figre 1. Hypercbe network showing coordinate addresses for each node. (Note the Gray Code is sed for node ordering so that adjacent nodes will differ by only 1 nit dimension.) NETWORKTOPOLOGY Network topologies vary from application to application. The particlar type of network selected is dependent pon the parameters of reliability, cost, flexibility, and expansion needs. There are no reqirements that nodes be contigos. s can be connected sing telephone lines and modem or twisted pairs of wires. Ths, network topology is not limited by the physical setting. Of the several common network configrations, the Hypercbe has the most secre commnications linkage. In a Hypercbe network (Figre 1), each node is connected to for other nodes in a symmetric manner. In order to isolate two parts of the network, a large nmber of commnications lines or node failres mst occr. In networks where reliable commnications are reqired, sch as an integrated laboratory network, this may be the best configration. However, the nmber of commnications links reqired eliminates this network topology from most applications. The Tors configration, shown in Figre 2, gives p some of the commnications reliability of the Hypercbe to redce the nmber of commnication links. For applications where concentric rings of commnications lines can be sed, this topology gives a high degree of reliability. In many process and laboratory environments, the Tors configration provides an acceptable compromise between reliability and excessive commnications needs. The Mesh network, shown in Figre 3, is sed to reliably blanket an area with ser nodes. In a psychology department it might be desirable to give every investigator some processing and laboratory eqipment fnctions. In this sitation, the Mesh network is ideal. However, once again, the nmber of commnications links reqired for a network of this type is high. The Tree topology, shown in Figre 4, minimizes the commnications links necessary in a network. Since this type of network can be separated by the failre of one commnication link or node, it shold be sed only where commnication failres can be tolerated. For example, it might be sed in sitations where the local nodes have sfficient capabilities to operate independently for some time. The Tree configration simplifies control programming and is Figre 2. Tors network showing coordinate addresses for each node. (Note the Gray Code is sed for node ordering so that adjacent nodes will differ by only 1 nit dimension.)

4 COMPUTER NETWORKING 525 relatively inexpensive. Therefore, it is apt to be the most practical system in a medim-sized environment. The Star configration, shown in Figre 5, reqires that all the commnication between nodes be roted throgh the central node. For small systems of 4-16 sers, this is practical if file-access reqirements are not great. While the Star topology is vlnerable to hardware and line failres, it is often sed becase it is relatively inexpensive. TYPICAL LABORATORY SYSTEM 0= ser o = Spec,al "" Calclations Figre 3. Mesh network. Figre 4. Tree network. Data Base U User s For a small laboratory the Star configration, modified as shown in Figre 6, wold probably be appropriate. The ser nodes actally consist of two compters linked throgh shared memory. These compters are designated as the ser processor and the commnications processor. The commnications processor has control of the node. It can stop the ser processor, load memory with a program received from the file node, and start the ser processor execting the program. The commnications processor has complete access to the memory ofthe ser processor. The ser processor signals the commnications processor for message transmission. It does this throgh a special control line attached to the commnications processor. A system of "mail boxes" in memory is sed to transfer messages. The ser processor has all the external devices for the node attached to it. This allows all commnication processors to be identical. The external devices are all attached to a similar bs strctre located at the same place in each ser node. Ths, the same program can monitor and control the same external device sing any local node. When a node fails, the network control node can download another node with the appropriate device control program, switch the device to the back-p node, and initiate the device control program. The network operating system is configred for each installation sing standard software. This software is the control program for the network control node. It User s DB Dala Base Figre S. Star network. Figre 6. Modified Star network.

5 526 WALLACE, JOHNSON, AND JOHNSON determines the fnctions of the ser nodes, file-access right, and rights to other special resorces. This software is modified as additional nodes and resorces are added to the network. DISCUSSION In this paper, we have attempted to provide the reader with an introdction to the concepts ofcompter networking, network topologies, and a potential topology for laboratory se. The paper was motivated ot of a concern that most psychologists (c.f. Brown & Deffenbacher, 1978) are nfamiliar with internal networking approaches. Networking concepts sally become important only after sfficient microcompter eqipment is installed in laboratory settings. However, initial eqipment prchases limit the ser's evental networking options. Therefore, it is imperative that psychologists be aware of the relevant concepts prior to major eqipment acqisitions in this area. In some cases, behavioral scientists will not want to make se of most of the concepts otlined above. For example, those involved in highly individalized projects or located in settings with fewer than two or three microcompter systems will probably not be able to make se of these techniqes. However, in other settings, sch as larger psychology departments, networking concepts are apt to be very helpfl as a means to providing investigators with a sefl range of eqipment at a less expensive cost. REFERENCES BROWN, E. L., & DEFFENBACHER, K. Microcompters big and little: Selecting a low cost laboratory compter network. Behavior Research Methods & Instrmentation, 1978, 10, DIGITAL EQUIPMENT CORPORATION. Introdction to minicompter networks. Maynard. Mass: (Receivedfor pblication Jne 16, 1978; revision accepted Jne 19,1978.)