The OSI network communications model in diagrammatic contextJim CurranDissertation submitted in partial fulfilment of the ...
AbstractI examine a popular model of computer network communications from three primary angles:history, taxonomy, and psyc...
AcknowledgementsMy thanks to these people who went out of their way to make sure I got copies of papers Icouldn’t find any...
ContentsIntroduction 1  What are diagrams? 2     Meaningful space 2     Making the invisible visible 3  The value of diagr...
Achieving successful transformation 34   Addressing the viewers’ needs 34      The viewers’ tasks 34      The viewers’ vis...
IntroductionIn this dissertation I examine the Open Systems Interconnection reference model (the ‘OSImodel’) and place it ...
Although the OSI model was originally intended to be used as a reference structure forthe development of communications st...
Tversky (2001; 2002) maintains that that spatial arrangements are ‘usually not accidentalor arbitrary’ and that some devic...
Why study diagrams?In other words, by studying them, what do we hope to achieve?   The ideal, from a practical rather than...
Background and historyIn the field of computer networking, diagrammatic explanations are frequently used. Semi-literal dra...
Figure 4. A simplification of the OSI model (adapted from X.200)   Each column represents a computer, and each rectangle i...
The power of the OSI modelWhen a preliminary OSI model was first published in the 1978, it was praised as a conceptualbrea...
The problem of incompatibility and potential solutionsIn actuality, the situation began to change in 1966, when we find Ma...
signals being sent and received are at the lowest level. For the rest, a process of ‘packing’ and‘unpacking’ the messages ...
Davies and Barber (1973) also show ambivalence in the way they represent layers. In an earlychapter, they arrange the prot...
That they redrew the diagram with stacked protocol layers and used terms such asHOST–HOST protocol, HOST–IMP control modul...
Figure 11. Hierarchy of interface levels (from Cotton and Folts 1977)          (I have ‘ghosted in’ the right half that wa...
Active work on the modelIn 1977, ISO created a new subcommittee called ‘Open Systems Interconnection’(Zimmerman 1980). The...
What the model was expected to be used forThe model was intended to be used as a reference structure for the development o...
TaxonomyDiagram research is replete with taxonomies. In fact, there are so many competingtaxonomies, with no single standa...
Table 1. Taxonomic dimensions of Blackwell and Engelhardt (1998)Dimension                                          Sub-dim...
To narrow the field to the dimensions I was most interested in, I assigned weightings tothe dimensions. They were as follo...
Table 3. Taxonomic aspects of Blackwell and Engelhardt (2002)First grouping            Second grouping      AspectRepresen...
A similar weighting analysis yielded Tversky, Doblin, Richards, and Bertin. I ruled out,perhaps injudiciously, delving int...
Because his operational taxonomy deals with interaction – the way people change diagramswhile working with them – I find i...
By structure Owen, echoing Doblin, begins by defining a continuum between sequentialand presentational graphic systems and...
Figure 19. Owen’s ‘kit of parts’, showing where the system diagram fits in on each (adapted    from Owen 1986 – note: seve...
PsychologyIn an attempt to make sense of the myriad angles from which psychological aspects ofdiagrams have been studied, ...
transformation process and needed to be encouraged to put more emphasis on the viewer,which is quite the opposite of most ...
Visual syntaxRichards (2002) holds that the viewer’s first task when approaching a diagram is to ‘work outthe visual synta...
Indeed others have (unwittingly, I would guess) put these principles to work in makingtheir own renditions of the model. F...
actual shapes are irrelevant, thus schematized’ (Tversky et al 2000). Lines depictconnections among objects as well as ord...
Figure 23. A more accurate use of arrows in the OSI model (from Montes 2001)Cognitive processingKim et al. (2000) outline ...
Background knowledgeLack of sufficient background knowledge of the system represented by a diagram has beenfound to reduce...
Visuo-spatial abilityVekiri (2002), citing Carrell, defines visuo-spatial ability as ‘the ability to mentally generateand ...
The orientational metaphors Lakoff and Johnson discuss all involve verticality. Inaddition to vertical metaphor, Tversky (...
A highly appealing visual metaphor is sometimes applied to the network communicationprocess: the postal metaphor. If the ‘...
‘The transformer’s job is to put the message in a form the reader can understand’(Macdonald-Ross and Waller 2000). The tra...
Achieving successful transformationMacdonald-Ross and Waller stress that ‘a good communication is selected for a purpose a...
The viewers’ background knowledgeInterpreting abstract technical diagrams is a cognitively demanding task (e.g., Lowe 1994...
concepts and principles’ (Winn and Holliday 1982). An example of how this can work isshown in Figure 25.    Figure 25. How...
that are perceptually sound…There is a range of possibilities between the rectangular boxand line diagram and fully render...
ReferencesANSI (American National Standards Institute) (1977). USA position on datagram service.  ACM Computer Communicati...
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  1. 1. The OSI network communications model in diagrammatic contextJim CurranDissertation submitted in partial fulfilment of the requirements for theMaster of Arts in Information DesignUniversity of Reading2004
  2. 2. AbstractI examine a popular model of computer network communications from three primary angles:history, taxonomy, and psychology. I argue that the model in question is important becauseit provides structure to an otherwise invisible, intangible system and facilitates teachingabout and understanding of its concepts. In tracing the development of the model over aperiod of about 15 years I reveal that it emerged in parallel to the challenges encountered andproblems solved when disparate, geographically dispersed computer systems were to beinter-connected. I attempt to place the model in the context of diagram taxonomy and reviewpsychological literature relevant to the diagrammatic communication process. I analyse themodel in the light of visual ‘grammars’ based on perceptual research and of studies ofmetaphor. I discuss the idea of transformation and conclude by explaining the mostimportant factors in achieving successful transformations of abstract technical material: thetransformer’s knowledge of viewers’ tasks, visuo-spatial abilities, and backgroundknowledge, the relation between the representation and the real system, and therepresentation’s adherence to perceptual conventions.
  3. 3. AcknowledgementsMy thanks to these people who went out of their way to make sure I got copies of papers Icouldn’t find anywhere else: Alison Black, David Feinstein of the School of Computer andInformation Sciences at the University of South Alabama, Lawrence Lipsitz of EducationalTechnology Magazine, and Barbara Tversky of Stanford University. Special thanks go toRichard Lowe of Curtin University of Technology, Australia, for discussing my project withme and sending me copies of several enlightening articles. I’m grateful to the library staff at Imperial College London, the University of Illinois atChicago, Illinois Institute of Technology, and Northwestern University for allowing meaccess to their collections. To my parents, Jim and Fran Curran, who helped me manage my affairs in the US while Iwas at Reading, all thanks and love and especially to Sheow Lu, my fiancée, who tolerated myabsence for a year and gave me support and encouragement throughout.
  4. 4. ContentsIntroduction 1 What are diagrams? 2 Meaningful space 2 Making the invisible visible 3 The value of diagrams 3 Externalization 3 Why study diagrams? 4 How to study diagrams 4Background and history 5 What the OSI model depicts 5 Communication protocols 6 The power of the OSI model 7 Development of the OSI model 7 The situation before the introduction of the OSI model 8 The problem of incompatibility and potential solutions 8 Widespread acceptance of the layering concept 11 The influence of X.25 11 The influence of datagram services 12 Active work on the model 13 Reception of the model 13 What the model was expected to be used for 14 What the model turned out to be best for 14Taxonomy 15 What taxonomies can do 15 Meta-taxonomies for diagram research 15 Blackwell and Engelhardt (1998) 15 Blackwell and Engelhardt (2002) 17 A taxonomic analysis of the OSI model 19 Doblin’s taxonomy 19 Owen’s taxonomy 19Psychology 23 Perceptual processing 24 Visual syntax 25 Cognitive processing 28 Background knowledge 29 Mental representations 29 Visuo-spatial ability 30 Metaphor 30 The transformer and transformation 32
  5. 5. Achieving successful transformation 34 Addressing the viewers’ needs 34 The viewers’ tasks 34 The viewers’ visuo-spatial abilities 34 The viewers’ background knowledge 35 Structuring the diagram appropriately 35 Relation between the representation and the real system 35 Accompanying text 35 Adherence to perceptual conventions 36 Drawing inspiration from exemplars 37References 38
  6. 6. IntroductionIn this dissertation I examine the Open Systems Interconnection reference model (the ‘OSImodel’) and place it in the context of diagramming research and practice in general and inparticular. The OSI model provides the standard framework for explaining how computerscommunicate with one another. First published in 1984 by the International Organization forStandardization (ISO), the OSI model was developed over several years. Its roots go back tothe late 1960s and the start of the precursor to today’s Internet, the ARPANET, and thechallenges its designers met in getting disparate, geographically separated computers tocommunicate. Though the OSI model is conceptual and completely intangible, it has been expresseddiagrammatically since its origin. Diagrams that would be recognizable to any networkengineer today were hand-drawn in the very first meeting of the committee that created themodel (McKenzie 1978). Figure 1. Layers in the reference model (from ISO 1978) Compare with the published version in Figure 2 Figure 2. The OSI model (from ITU-T 1994) 1
  7. 7. Although the OSI model was originally intended to be used as a reference structure forthe development of communications standards, it largely failed in that regard (e.g., Day andZimmerman 1983; Wikipedia 13 April 2004). The OSI model remains, however, the model ofchoice for teaching, understanding, and communicating networking concepts (Testerman1999). Open any networking textbook published since the mid-1980s and you are sure to find arendition of the OSI diagram. Walk through any organization that concerns itself withnetworking and you are sure to see diagrams based on the OSI model drawn on whiteboards.Ask a network engineer what he does and he may tell you he’s a ‘layer two’ or ‘layer three’specialist. The pervasiveness and utility of the model have convinced me of its importanceand motivated me to undertake the fairly detailed examination of it that follows. ◆The first question facing me was, ‘Just how do I go about examining a diagram?’ Behind thatquestion, I found, lurked another: ‘What is a diagram, anyway?’What are diagrams?A diagram is a form of picture. Twyman (1985) defines a picture as ‘some hand-made ormachine-made image that relates, however distantly, to the structure of real or imaginedthings’. But it is a special kind of picture, one that exhibits relationships (Garland 1979;Richards 2002) using symbols and their spatial arrangement (e.g., Vekiri 2002). Kim et al.(2000) call diagrams ‘abstractions of real systems’ and Tversky (2002) adds that graphicsused in this way are a ‘modern (18th c.), Western invention’. The OSI model is an abstraction of a real system, and it exhibits relationships among itscomponents using symbols (in the form of rectangular boxes) and their spatial arrangement.The OSI model is expressed as a static diagram, and so it aligns with Engelhardt’s (2002)definition as a ‘visible artifact on a more-or-less flat surface, that was created in order toexpress information’. As well, it agrees with this observation by Albarn and Smith, quoted in Sless (1981): ‘Thediagram is evidence of an idea being structured – it is not the idea but a model of it, intendedto clarify characteristics of features of that idea’. Among the properties of diagrams, two stand out as most important in explaining thepower of the OSI model: meaningful spatial arrangement and making the invisible visible.Meaningful spaceI have borrowed the term ‘meaningful space’ from Engelhardt (2002), and am using it torefer to a key property of diagrams. In fact, Tversky (2001) calls ‘using space and elements init to convey meaning’ the key to graphics. About this there is wide agreement (e.g., Sless 1981; Winn and Holliday 1982; Richards2002): The spatial arrangement of the elements of a diagram provides information notavailable in straight text. According to Sen, ‘When we represent problems using diagrams, itusually implies that locational or adjacency properties are important, e.g., organic chemicalstructures, free body diagrams in physics, architect’s plans, data structures in computerscience’ (Sen 1992). 2
  8. 8. Tversky (2001; 2002) maintains that that spatial arrangements are ‘usually not accidentalor arbitrary’ and that some devices are ‘cognitively natural’. Indeed the spatial arrangement of the OSI model is highly meaningful and not accidentalor arbitrary. I will have more to say about this in a later section.Making the invisible visibleThe property of diagrams that is perhaps most relevant to this dissertation is their ability torender the invisible visible. Owen (1986), Richards (2000; 2002), and Tversky (2001; 2002)make much of this. According to Richards, diagrams make the invisible visible using graphicmetaphor [something regarded as representative or suggestive of something else], whileTversky attributes the effect to analogy [equivalency or likeness of relations]. In computer networking, the visible components of the systems do little to explain theunderlying processes. At the most tangible level, data transmissions are electromagneticwaveforms. These waveforms, whether conducted over copper wire as electricity or in the airas radio waves, are naturally invisible. Even the light carried over optical fibres pulses toorapidly for the eye to detect. For this reason, abstract diagrams such as the OSI model tend tobe more useful than literal ones in explaining inter-computer communications.The value of diagramsMany claims are made for the value of diagrams. They are held to be more direct thanalphabetic written language (Tversky 2001), with reduction of complexity achieved byomitting unnecessary detail (Lowe 1994; Tversky 2001), allowing inspection of related piecesof information at a glance (Winn and Holliday 1982). The value of diagrams in facilitating learning is noted by Winn and Holliday (1982) andVekiri (2002). Lowe (1993) cites evidence from Mayer that ‘diagrams can make processingmore effective, resulting in improvements in tasks such as conceptual recall and performanceon related problem-solving tasks’. Tversky (2001) reviews a number of functions of graphic displays, including the attractionof attention and interest, stimulation of memory, the recording of ideas and the ability tomake them public, and facilitation of discovery and inference.ExternalizationThe ability of diagrams to externalize thought is given special attention by Sless (1981), Sen(1992), and Ittelson (1996), who hold that the cumulative nature of scientific and technicalprogress depends upon diagramming. This is likely because once the concepts are ‘taken outof our heads’ (Ittelson 1996), they can be more easily shared with other people, who can‘inspect, reinspect, and revise them’ (Tversky 2002). This externalization of thoughtfacilitates group communication (Tversky 2001). 3
  9. 9. Why study diagrams?In other words, by studying them, what do we hope to achieve? The ideal, from a practical rather than theoretical point of view, is to increase theeffectiveness of diagrams for users and learners (e.g., Winn 1993; Vekiri 2002). This can bedone in two ways: by using the results of studies of diagram effectiveness to inform thedesigner and by using them to inform the instructor. Lowe (1994) notes that ‘the way diagrams are used in scientific instruction typically is notinformed by a deep understanding of how people process information presented in thisformat’. This understanding is necessary because diagrams ‘have the potential to be far moredifficult to process than more “realistic” pictures because of the nature of the subject matterand their high degree of abstraction’ (Lowe 1994). The trouble for the practising designer or instructor is that the findings of particularstudies are difficult to generalize from their contexts (Scaife and Rogers 1996). Still, findingson the effects of several factors on the usefulness of diagrams to learners can provideguidance to the designer or instructor. These include knowledge of the viewers’ tasks, visuo-spatial abilities, and background knowledge in the subject, the relation between the diagramand the system it depicts, and adherence to perceptual conventions. Each will be elaboratedon in later sections.How to study diagramsNow that I have discussed what diagrams are, what they are good for, and why it isworthwhile to study them, I can return to my first question, which was ‘Just how do I goabout examining a diagram?’ The path, of course, has been trod before. The most valuable suggestions, which largelyoverlap, come from Sless (1981) and Sampson (1985). Sless calls for a ‘formal analysis ofdiagrams, a psychological account of their use, an historical study of their development, anda review of their current status in our culture’ (Sless 1981). Sampson, examining thelinguistic study of writing, proposes three categories: typology, history, and psychology.Taking their lead, I examine the OSI model from three angles: background and history,taxonomy, and psychology. For background and history, I explain the concepts involved and trace the history of thedevelopment of the OSI model in some detail. Under taxonomy, I look at classificationsystems for graphics and explain the place of the OSI model within them. For psychology, Iview diagrammatic communication as a process that occurs in two separate parts: betweenthe viewer and the diagram and between the designer and the diagram. I explore factors thataffect communication on both sides of the divide. I finish by reviewing implications for thedesign of diagrams that depict abstract systems. 4
  10. 10. Background and historyIn the field of computer networking, diagrammatic explanations are frequently used. Semi-literal drawings such as the one in Figure 1 may be useful for hardware installation but areineffective for describing the mechanisms of inter-computer communications. Figure 3. A semi-literal drawing of inter-computer connectivity. This is because the visible components of networking systems do little to explain theunderlying processes. At the most tangible level, data are electromagnetic waveforms. Thesewaveforms, whether conducted over copper wire as electricity or in the air as radio waves,are naturally invisible. Even in the case of optical transmission, the light carried over thefibres pulses too quickly for the eye to detect. The signals are further abstracted by thesoftware on each computer that controls communications, making the processes ‘even moreinvisible’. Abstract diagrams tend to be more useful than literal ones in explaining inter-computercommunications. Green explained the importance of examining the functions a systemperforms when characterising networks: ‘There are other ways of characterising networks(by application, by geography, by ownership, by topology), but ‘None of these fourapproaches really reveals what the network is actually doing. A much better scheme is toexamine the total repertoire of functions that the network must provide in making up aneffective access path between two end users’ (Green 1980a). The best framework we have for explaining how networks work is the OSI model,developed by ISO in the late 1970s–early 1980s. The OSI model introduced to a wideaudience a logical structure that can be presented in graphic form and which provides aframework for people to ‘hang concepts on’.What the OSI model depictsThe OSI model is a conceptual device that abstracts the complex functions and relationshipsinvolved in inter-computer communications. Diagrammatically, it can be described as twoidentical columns of seven rectangles each placed atop a long rectangle. 5
  11. 11. Figure 4. A simplification of the OSI model (adapted from X.200) Each column represents a computer, and each rectangle in the column represents acollection of related functions performed by software components of the computer. The longrectangle at the bottom represents the physical medium (for example, a copper wire, anoptical fibre, or air in the case of wireless transmission) through which signals exchangedbetween the two systems are propagated. The columns are hierarchically arranged. The lowest layer, closest to the physicalmedium, concerns itself with transmitting and receiving electromagnetic signals through themedium. As one progresses up through the column, the functions become more abstract.They range, for example, from error-checking and retransmission mechanisms at the lowerlayers through to message routing in the middle to setting up a file transfer near the top.Each layer has a name and number. The layers are numbered bottom-to-top from one(Physical) to seven (Application). There is a lateral dimension to the model as well. Each layer must be matched by its peerin the computer opposite (or relayed by another device) for intelligible communication totake place.Communication protocolsThe model cannot be explained without delving into communication protocols. ‘For onecomputer to send a message to another computer across a network, more has to be done thansimply pump the bit-train [a series of electromagnetic ‘on–off’ signals] down an appropriatewire’. Protocols – ‘a system of standard message formats together with a set of rules for theiruse’ (Whitby-Strevens 1976) – are necessary. Such protocols are required for intelligible communication between peer layers. ‘To caterfor the various kinds of communication between processes possible in a network, it isessential to have sets of rules governing interactions to ensure they proceed in an orderlyfashion’ (Davies and Barber 1973). Protocols can be an intimidating concept, but they are not unique to computers, as Black(1991) points out: ‘One of the most interesting aspects about computers is how they exchangeinformation with each other. Remarkably, their communications are similar to thecommunications between humans, because, like humans, computers communicate with eachother through symbols and agreed-upon conventions’. 6
  12. 12. The power of the OSI modelWhen a preliminary OSI model was first published in the 1978, it was praised as a conceptualbreakthrough. Green (1980b) called it a ‘particularly clear way of visualizing all of the layersof a network architecture and their component protocols’. By the late 1980s, the model became ‘pervasive’ (Black 1991). Today, virtually every texton computer networking presents it or takes it for granted. Its usefulness as a teaching tool isfrequently mentioned (e.g., Testerman 1999; Wikipedia 13 April 2004). But the OSI modelwas not ‘invented’ by the ISO study group that developed it in the late 1970s and early 1980s.Its roots go back farther than that.Development of the OSI modelBlack (1991) lists two developments that provided the impetus for the development of theOSI model: ‘(a) the emergence of layering and structured techniques in the design ofcomplex networks and (b) the recognition of the need for compatible communicationsarchitectures between different manufacturers’ protocols’. The ARPANET, which we know today as the Internet, evolved a layered approach. Theambitious goal of its founders was to interconnect several computer systems made bydifferent manufacturers. Green (1980a) pointed out that a layered concept naturally emergeswhen an ordered list is made of the functions involved in interconnecting heterogeneoussystems. Of course, Green says this in hindsight. ISO makes it sound similarly effortless: ‘Amodel is an abstraction or simplification that makes a concept more understandable. Inorder to comprehend models of complex systems, it is important to partition the structuresinto easily comprehended parts. Communications systems are often envisioned in terms of“layers” of functions’ (ISO 1978). The model did not spring forth from nowhere. It evolved over the course of more than adecade. I will trace its origins and development in the next sections.The situation before the introduction of the OSI modelBefore the model was created, networking was a haphazard business. ‘Everybody is buildingnetworks, but as yet nobody really knows how – we lack any formal, or “high level”,framework in which to assess networking issues’ (Whitby-Strevens 1976). Writing soon after the development of the model, Green explains that ‘For a long time ithas not been entirely clear just how one should think about the bits and pieces that make upa computer network and how they should fit together. This confusion has been felt at alllevels by researchers, architects, implementers, and researchers.’ And ‘it used to be the casethat each software implementation was neither modularly organized nor generic, but insteadwas put together ad hoc to do a particular job; when the job changed or the means ofcarrying out a single function changed, everything had to be rewritten’ (Green, 1980b). Black concurs: ‘The early computers that provided communications services wererelatively simple.…These early systems used conventions based on the telegraph and telexapplications, and transmitted messages with special codes…These codes were often used andinterpreted differently by the manufacturers of communications products.…Moreover, theearlier networks…often used several different proprietary protocols that had been added in asomewhat evolutionary and unplanned manner….The protocols in the networks were oftenpoorly and ambiguously defined’ (Black 1991). 7
  13. 13. The problem of incompatibility and potential solutionsIn actuality, the situation began to change in 1966, when we find Marill and Roberts gropingwith the problem of computer incompatibility. ‘Incompatible machines represent an oldproblem in the computer field’ (Marill and Roberts 1966). They examined the two ‘time-honored remedies’ to the problem: using identical computers and writing the programs in ahigh-level language that could be compiled on different machines. They judge that ‘theseremedies have worked quite badly in the past and will probably work as badly in future time-sharing environments’ (Marill and Roberts 1966). They explain a possible solution – ‘theestablishment of a message protocol, by which [they meant] a uniform agreed-upon mannerof exchanging messages between two computers in the network’ (Marill and Roberts 1966). In June 1967, Roberts reported that an experiment connecting one type of computer inCambridge, Massachusetts to another in Santa Monica, California using the message-protocol method had been a success. Also, a generalized ‘communication protocol’ was indevelopment and researchers across the country had ‘agreed to accept [the] single networkprotocol so that they may all participate in an experimental network’ (Roberts 1967) – theARPANET. Carr et al. (1970) report their progress in getting different ‘host’ computerscommunicating with each other. They had to use a network specified by a contracting firm.In their words, ‘The format of the messages and the operation of the network was specifiedby the network contractor (BB&N)’, and so ‘it became the responsibility of representatives ofthe various computer sites to impose such additional constraints and provide such protocolas necessary for users at one site to use resources at foreign sites’ (Carr et al. 1970). Thisimplies a clean division of functions between the host computers and the network itself. The first precursor to the OSI model that I am aware of appeared in Crocker et al. (1972),who were reporting their work on ARPANET protocols. It is reproduced in Figure 5. Theyexplain the big picture this way: ‘A user at his terminal, connected to a local HOST, controls aprocess in remote HOST as if he were a local user of the remote HOST’ (Crocker et al 1972). Figure 5. The layers of protocol (from Crocker et al. 1972) Crocker et al. (1972) make an interesting distinction between communication at thelowest layer and that at the layers above: ‘actual’ versus ‘virtual’. This is because the only 8
  14. 14. signals being sent and received are at the lowest level. For the rest, a process of ‘packing’ and‘unpacking’ the messages occurs on each host computer. (This process is explain further inthe section on metaphor on page 32.) The layered concept apparently took some time to take hold. Analysis of Mills (1972)provides evidence that the concept had not yet propagated outside the ARPANETcommunity. Mills provides what he describes as ‘a greatly simplified block diagram of atypical teleprocessing system. In this diagram the communication subsystem is shown as acollection of functional components’. A collection implies a random ordering, and indeed thediagrams in the paper reflect this, with one showing the communication network at the topand one at the bottom. Figure 6. Typical teleprocessing system (from Mills 1972) (Note that the communication network is at the top and the application programs are at the bottom, the reverse of the OSI model.) Figure 7. Typical front-end processor (from Mills 1972) (In this case, the communication network is at the bottom.) 9
  15. 15. Davies and Barber (1973) also show ambivalence in the way they represent layers. In an earlychapter, they arrange the protocol elements in a line, horizontally, as shown in Figure 7. Figure 8. Variety of protocols in a network (from Davies and Barber 1973) In a later chapter, they adopt a layered approach. This is no oversight on their part. ‘Theprotocol structure of packet switching networks was described at length in Chapter 11. It isapparently at this point that much of the conceptual difficulty arises in modern datanetworks. One of the figures of that chapter is redrawn in Figure 14.1 to show the ‘higher-lower’ relationships of these protocols’ (Davies and Barber, 1973 – italics mine). Figure 9. Examples of protocols and interfaces (from Davies and Barber 1973) 10
  16. 16. That they redrew the diagram with stacked protocol layers and used terms such asHOST–HOST protocol, HOST–IMP control module, and IMP suggests that they werefamiliar with ARPANET concepts. As Green (1980a) states: ‘The ARPANET…had a greatinfluence on all succeeding computer networks’.Widespread acceptance of the layering conceptBy 1975, the layered concept was common currency. ‘A basic principle, generally acceptednowadays, is a layered structure, made up of quasi-independent levels’ (Pouzin 1975). Pouzinincluded a diagram that takes on the familiar ‘U’ shape of the OSI model. Figure 10. Network structure (from Pouzin 1975)The influence of X.25In the mid-1970s, work proceeded on the protocol that was to be called X.25. Rybczynski(1980) contends that the development of X.25 was ‘a response to the rise of public datanetworks’, especially within countries whose communication systems were controlled bygovernment-based Post, Telephone and Telegraph administrations (PTTs). In order tointerconnect the countries’ networks, standard protocols needed to be agreed. In fact, ‘thecommercial viability of these networks hinged largely on the development and adoption ofstandard access protocols’ (Rybczynski 1980). Cotton and Folts (1977) reported that the first three levels in the ‘hierarchy of interfacelevels’ they present had been worked out for the X.25 protocol. The fourth level was simply‘higher level’ (end-to-end system and user protocols), which would later become fourindependent levels itself. While X.25 predates the OSI model, and indeed was not designed with the OSI model inmind (Cotton and Folts 1977; Marsden 1985), it could not help but to have been influencedby work on the ARPANET. We can see the resemblance clearly in Figure 11. 11
  17. 17. Figure 11. Hierarchy of interface levels (from Cotton and Folts 1977) (I have ‘ghosted in’ the right half that was implied in the original diagram.) The lower levels covered by X.25 were not where the action was, however. According toRose, ‘To be sure, OSI has introduced terminology and notation for discussing end-to-endservices in a consistent fashion. Nevertheless, in terms of technical advancement, the lower-layer infrastructure of OSI is uninteresting’ (Rose, 1990).The influence of datagram servicesI was not able to determine whether the paper Generic Requirements for Datagram Serviceswas submitted before or after ISO decided to form the committee. But this paper, submittedto ISO in February 1977 by the American National Standards Institute (ANSI), contained afairly mature diagram with six levels of protocol. It is shown in Figure 12. Figure 12. Protocols of the datagram network (from ANSI 1977) 12
  18. 18. Active work on the modelIn 1977, ISO created a new subcommittee called ‘Open Systems Interconnection’(Zimmerman 1980). The task they faced at their first meeting in February 1978 was ‘to definea model for network architecture and to consider the standardization of higher-levelprotocols’ (McKenzie 1978). The task does not seem to have presented much difficulty for the committee. During thefirst meeting, they produced a provisional model of open-systems architecture (McKenzie1978). The provisional model, shown in Figure 13, was published in July 1978. Figure 13. Layers up to network control may be chained (from ISO 1978) Several authors report the ease with which a unanimous decision was made on thediagram (e.g., ISO 1978, Zimmermann 1980). The report on the preliminary model indicatesthat there was ‘a high degree of commonality between the views expressed by all memberbodies on this subject’ and that ‘The various models which have been proposed all conformwith the principles of layered architecture’ (ISO 1978). The task was complete in less than 18 months (Zimmerman 1980), but it took a few moreyears for its approval in May 1983 (Folts 1983). The results were published in 1984 as ISOInternational Standard 7498 and CCITT Recommendation X.200. Folts concludes that ‘The architectural principles have now been firmly established, withthe definition of the seven layers of functions necessary to create an Open SystemsInterconnection environment’ (Folts 1983). The evidence presented in this section, however,suggests that most of it had already been worked out before the meeting began.Reception of the modelThe OSI model was taken as an immediate success. Green (1980b) wrote that ‘A particularlyclear way of visualizing all of the layers of a network architecture and their componentprotocols has been worked out by the International Standards Organization’. Marsdenacknowledges the value of the model in that ‘it…allows existing standards (e.g. X25) to beplaced into perspective’ (Marsden 1985). Black also seems to have thought the endeavourworthwhile: ‘The initial 2 1/2 years that SC16 spent developing the Basic Reference Modelhas more than paid off in the long run’ (Black 1991). 13
  19. 19. What the model was expected to be used forThe model was intended to be used as a reference structure for the development of openstandards for computer interconnection (ISO 1978; Zimmerman 1980; Day andZimmermann 1983). It may have been thought of this way at the time, but the future did notbear this out. Although many protocols were developed, few of them were actually wereactually implemented as they were found to be too complicated. According to the Wikipedia,‘The OSI approach was eventually eclipsed by the Internet’s TCP/IP protocol suite and itssimplified pragmatic approach to networking’ (Wikipedia 13 April 2004).What the model turned out to be best forThe true success of the model was to clarify a complex system. ‘The most significantachievement of OSI has been to provide a flexible framework for describing the diversetransmission media and protocols that combine to form end-to-end services’ (Rose 1990).Testerman (1999) acknowledges that the OSI model ‘has become the model forunderstanding and communicating telecommunications concepts’. He concludes that ‘As ateaching tool, the OSI Model is unsurpassed’ (Testerman 1999). ◆Having explained the OSI model, traced its history, and demonstrated its usefulness as anexplanatory framework, I now turn to examining the model in the context of the study ofdiagramming. 14
  20. 20. TaxonomyDiagram research is replete with taxonomies. In fact, there are so many competingtaxonomies, with no single standard (e.g., Scaife and Rogers 1996; Vekiri 2002), thatBlackwell and Engelhardt (1998; 2002) have proposed a taxonomy of diagram taxonomies(or meta-taxonomy). Before I examine the details of Blackwell and Engelhardt’s approach, let me step back anddiscuss a problem I encountered with taxonomies from the beginning of my research. Thatis, suppose that I find a place for the OSI model in a taxonomy. I can label it and see whatother kinds of diagram relate to it, but then what? What does it do for me? What is ataxonomy good for?What taxonomies can doThe purported benefits of taxonomy can be divided into those useful for practice and thoseuseful for theorizing. Taxonomies are seen as practically useful in that they provide aninventory of potential solutions to design problems (Macdonald-Ross 1989). When levels oftaxonomic variables are laid out in matrices, they can suggest possibilities for newdiagramming systems (Owen 1986). And they provide a framework for discussingapproaches to the solution of design problems – a potential means of determining whether adesign is appropriate for a given task (Engelhardt 2002; Macdonald-Ross 1989). In terms of theory, taxonomies structure domains of inquiry (Lohse et al. 1994;Engelhardt 2002) and can be used to predict future research needs (Lohse et al. 1994). Lohseet al. (1994) argue that ‘Classification lies at the heart of every scientific field’. In adeveloping field such as diagram research (e.g., Macdonald-Ross 1989; Vekiri 2002) suchrigour could certainly have its appeal.Meta-taxonomies for diagram researchBlackwell and Engelhardt (1998; 2002) have surveyed dozens of taxonomic approaches andhave produced two ‘meta-taxonomies’ that do much to make sense of them. As well, theirwork provides an excellent route into the literature. In their 1998 paper, they analysedtaxonomies in terms of six taxonomic dimensions, while in their 2002 paper, which seems tobe a refinement of the earlier one, they used nine taxonomic aspects. Since I find eachapproach useful and informative, I will review them both.Blackwell and Engelhardt (1998)In attempt to make sense of the taxonomies, Blackwell and Engelhardt (1998) propose sixdimensions: representation, message, relationship between representation and message,task and process, context and convention, and mental representation. Each of thesedimensions is divided into two categories. Each taxonomy they review can belong to one ormore of these categories depending on which aspects the taxonomy covers. 15
  21. 21. Table 1. Taxonomic dimensions of Blackwell and Engelhardt (1998)Dimension Sub-dimensionRepresentation the organization of the graphic vocabulary individual marks or graphic display components graphic structure the way the components are related to one anotherMessage the information that is information domain ontological categories represented (time, space, quantity) that constrain variation information relationships present in structure the dataRelationship the way information is pictorial from realistic to abstractbetween the mapped to the correspondencerepresentation and representation analogical structural analogythe message correspondenceTask and process interpreting and modifying information internal perception and representations processing problem solving tools interaction with the external representationContext and cultural and communicative roles of diagrams inconvention communicative context context discourse cultural conventions influence of society on diagrammatic formsMental diagrams in the head mental imagery nature of internalrepresentation representations interpersonal differences between variation people that have some constancy Figure 14. Graphic depiction of the taxonomic dimensions in Blackwell and Engelhardt (1998). They found that most of the taxonomies they reviewed covered the first few dimensions.They explain this finding this way: ‘These dimensions concern formalisable structure, andthe attributes of diagrams that are most apparent by inspection’ (Blackwell and Engelhardt1998). The later dimensions ‘concern questions of performance, interpretation, andcognition…They are less easily formalised’ (Blackwell and Engelhardt 1998). 16
  22. 22. To narrow the field to the dimensions I was most interested in, I assigned weightings tothe dimensions. They were as follows.Table 2. My weightings of Blackwell and Engelhardt’s (1998) dimensions andsub-dimensionsWeighting Dimension Sub-dimensionMost interested (2 points) Representation graphic structure Message information domain Message information structure Relation analogic correspondenceLess interested (1 point) Representation graphic vocabulary Relation pictorial correspondence Context and convention cultural conventionsNeutral (0 points) Context and convention communicative context Mental representation mental imagery Mental representation interpersonal variationNot interested (-1 point) Task and process information processing Task and process tools I weighted Task and process negatively because I found that most taxonomies thatcovered that dimension were concerned with logic problem solving for artificial intelligenceapplications, and I was more interested in the educational benefits of providing learners witha graphic model of a system. Adding the weights for each taxonomy gave me a good idea of which taxonomies werelikely to cover issues of relevance to the OSI model. The highest rated, in descending orderwere those of: Owen (8 points), Tversky (6 points), and Roth et al. (6 points). The work ofOwen and Tversky in particular feature in this dissertation. As I did this early in my investigation, I found later that my instincts were wrong, andthat I was more interested Context and convention: cultural conventions and Mentalrepresentation: interpersonal variation than I thought at the time. Later sections willelaborate on these topics.Blackwell and Engelhardt (2002)In their 2002 paper, Blackwell and Engelhardt enhance their meta-taxonomy, breaking itinto nine taxonomic aspects. 17
  23. 23. Table 3. Taxonomic aspects of Blackwell and Engelhardt (2002)First grouping Second grouping AspectRepresentation-related Signs Basic graphic graphic primitive vocabulary elements Types of tokens words, shapes, and pictures Pictorial abstraction continuum of pictorial abstraction Graphic structure Graphic structure principles for arranging signs Meaning Mode of correspondence relationship between a representation and its meaning The represented information information represented by the diagramContext-related Context-related Task and interaction what people do with the aspects diagram Cognitive processes mental representations, cognitive implications, and individual differences Social context cultural context and conventions of the type of medium Figure 15. Taxonomic aspects of diagram research (adapted from Blackwell and Engelhardt 2002). 18
  24. 24. A similar weighting analysis yielded Tversky, Doblin, Richards, and Bertin. I ruled out,perhaps injudiciously, delving into Bertin’s semiology of graphics, as I found it to be toocumbersome for a dissertation of this length. I do, however, discuss the work of the othersthroughout this paper.A taxonomic analysis of the OSI modelFrom the most relevant taxonomies I have chosen those of Doblin (1980) and Owen (1986)to situate the OSI model within. These two taxonomies are related to each other, and oneprovides a relatively simple introduction; the other a more elaborate analysis.Doblin’s taxonomyDoblin’s (1980) taxonomy is a good place to start because it is relatively easy to explain.Doblin divides media into presentational and sequential. A presentational medium, such as a poster, is seen all at once. It gives a total impression, then the eye tracks over it, picking up details in the order of their importance…Sequential media – books area an example – are strings of meaning units in time or space. These are perceived and matched to stored meaning units in our memories and then accumulated into a total message. (Doblin 1980) While the mechanisms Doblin explains would surely be seen as simplistic to perceptualand cognitive researchers such as Winn or Lowe, I find the division useful. Doblin proposes another dimension, that of static-dynamic. ‘The messages of static mediaare tangible, and as permanent as the material used…The messages of dynamic media aretransient, only there in real time as they are being presented’ (Doblin 1980). He proposes amatrix, which might look like the one in Table 4. Table 4. Matrixed media (adapted from Doblin 1980) presentational sequential static presentational static sequential static drawing, photography writing, printing dynamic presentational dynamic sequential dynamic movies, television speech, telephony It is clear that the OSI model fits into the static presentational category of Doblin’smodel. In the spirit of exploring the taxonomy, we can imagine what an alternativepresentation might do. For instance, a dynamic presentational version of the OSI modelmight be an animated clip of the sequence of communications between two computers, whilea static sequential version could show the sequence one step at a time – say, one step perdiagram, on pages in a book.Owen’s taxonomyOwen (1986) organizes graphics three ways: by purpose, by structure, and by operation. Histaxonomy by purpose would likely find its place in the latter half of Blackwell andEngelhardt’s meta-taxonomy, while his structural taxonomy would come up near the middle. 19
  25. 25. Because his operational taxonomy deals with interaction – the way people change diagramswhile working with them – I find it less relevant for examining the OSI model and amexcluding it.By purpose Owen plots graphic forms in a two-dimensional space with one axis as thepurpose of supplying information and the other as the purpose of creating an impression. Hefurther divides this field into four regions: identification, stimulation, enlightenment, andpersuasion.Table 5. Owen’s (1986) graphic communication purposesPurpose Used when Examplesidentification impression need not be strong and information pictograms and symbols only denotationalstimulation impression is strong and information relatively swastika, skull-and-crossbones unimportantenlightenment need for information greatly exceeds that for charts and graphs impressionpersuasion both impression and information are maximized political cartoons, business presentations Figure 16. Graphic systems ‘mapped’ according to their purpose to create impression or deliver information (adapted from Owen 1986) The OSI model would likely fall in the area occupied by organization charts when used byprogrammers and engineers, but could move up into persuasion when the goal is, forexample, to sell a customer a network system. 20
  26. 26. By structure Owen, echoing Doblin, begins by defining a continuum between sequentialand presentational graphic systems and notes that ‘it is almost possible’ to show a decreasein grammatical structure as we proceed through the continuum. Figure 17. Graphic systems ordered according to the way they are transmitted and received (adapted from Owen 1986) In addition, Owen presents what he calls a ‘kit of parts’ for graphic systems. It consists ofcontexts, entities, attributes, and operators.Table 6. Owen’s (1986) ‘kit of parts’ for graphic systems.Part Definition Optionscontexts used implicitly, may be space, time, or domain (the abstract field of the combined subjects of the diagram)entities visual elements symbolic, analogic, or iconicattributes qualities taken on by entities discrete, rank order, or continuousoperators relations among entities organizational, procedural, or spatial It is helpful to visualize the interaction of entities, attributes, and operators. Figure 18. Entities have attributes; between entities there may be relations (adapted from Owen 1986) Interestingly, he presents each part as a triangle and indicates where various systems fitin. I feel that system diagram in Figure 19 corresponds most closely to the OSI model so todraw attention to it I have shaded its circle. 21
  27. 27. Figure 19. Owen’s ‘kit of parts’, showing where the system diagram fits in on each (adapted from Owen 1986 – note: several diagramming systems have been left off each triangle) It is clear that the OSI model’s context is domain – in this case, the domain of inter-computer communication, which is not inherently spatial. Neither does the OSI model makeany effort to depict time. The OSI model’s entities are analogic – rectangles are analogous tosoftware components. They are not icons or symbols of the software components. The OSImodel’s attributes are discrete (nominal). What distinguishes each rectangle from the othersis a text label. There is a flavour of ordinality in the way the rectangles are stacked, and theyare usually numbered, but they could as easily have been numbered from top to bottom asfrom bottom to top. There is no concept of continuousness in the model. Finally, the verticalrelations between the rectangles in the OSI model are organizational, based on the layeringconcept. The horizontal relations are vaguely spatial, however, in that each stack ofrectangles represents a separate computer and vaguely procedural (for someone who knowsthe subject matter) in that communications travel ‘down’ from one computer, ‘over’, and ‘up’to the other, tracing a ‘U’-shaped path. 22
  28. 28. PsychologyIn an attempt to make sense of the myriad angles from which psychological aspects ofdiagrams have been studied, I have devised a model to structure this discussion. It isinspired by the work of Blackwell and Engelhardt (1998; 2002), but differs in that it includesthe role of the diagram’s designer (or transformer) and that its purpose is to contextualizethe psychological literature I found relevant to this dissertation rather than to analysediagram taxonomies. This model has four primary components: the diagram itself (representation), the realsystem the diagram represents, the viewer of the diagram, and the transformer. Both theviewer and the transformer approach the diagram with some goal or intent, and both rely ontheir perceptual/cognitive systems and background knowledge in arriving at a conception ofthe real system. The viewer, however, presumably does not have the same access to the realsystem and experts in its structure and function as does the transformer.Figure 20. A model for contextualizing the psychological literature relevant to diagramming One of the motivations for the devising of this model was to accommodate the stance ofSless (1981), Ittelson (1996), and Richards (2000): that the relation between the viewer andthe representation is distinct from the relation between the transformer and therepresentation. The representation might then be seen as the mid-point of a communicationprocess – the end-point for the transformer and the starting-point for the viewer. InIttelson’s (1996) words, ‘the creator of the marking starts with a set of intentions andproduces a marking: the perceiver starts with the marking and tries to reconstruct theintentions’. Another was MacEachren (1995), who, in arguing for more focus on the role of the viewerin the field of cartography, actually drew my attention to the right side of the model. Incartography, graphic depictions of the communication process ‘share a basic structure withan information source tapped by a cartographer who determines what (and how) to depict, amap as the midpoint of the process, and a map user who “reads” the map and develops someunderstanding of it by relating the map information to prior knowledge’ (MacEachren 1995).I found it fascinating that cartographic models explicitly included the transformer and 23
  29. 29. transformation process and needed to be encouraged to put more emphasis on the viewer,which is quite the opposite of most work in psychology. ◆Let’s start with the viewer’s side of the model, which involves the viewer trying to make senseof the representation. The viewer draws on perceptual and cognitive processing resources,including their background knowledge, visuo-spatial abilities, and knowledge of visualconventions in constructing a mental conception of the real system depicted in the diagram.In thinking about the viewer’s task, it pays to consider the words of Ittelson: ‘The markingstands as a single, limited, and completely defined source of visual information. There is noopportunity for further exploration, although more detailed examination is usually possible,and obtaining information from other sources can be an important part of the process’(Ittelson 1996).Perceptual processingMuch is made of the strength of the match between the properties of diagrams and theprocessing capabilities of the human visual perception system (Sless 1981; Lowe 1994; Scaifeand Rogers 1996; Tversky et al. 2000). Scaife and Rogers (1996) mention object perception,search, and pattern-matching as capabilities, while Lowe (1994) cites shape, orientation, andspacing as generally applicable visuo-spatial relationships that are invoked when we look atgraphic displays. Sless (1981), discussing diagrams similar to the OSI model, acknowledges the key role ofspatial configuration and the general tendency of the Gestalt laws to organize information inspace. The Gestalt laws, established in 1912 by Westheimer, Koffka, and Kohler, describe theway we see patterns in visual displays (Ware 2000). The Gestalt laws reviewed in Ware(2000) are summarized in Table 6.Table 7. Gestalt laws as reviewed in Ware (2000)Gestalt law Definitionproximity objects that are close together tend to be perceived as grouped togethersimilarity similar objects tend to be perceived as grouped togethercontinuity we are more likely to construct visual entities out of visual elements that are smooth and continuous, rather than ones contain abrupt changes in directionsymmetry symmetrically arranged pairs of lines are perceived much more strongly as forming a visual whole than a pair of parallel lines, and bilateral symmetry produces an even stronger holistic figurerelative size smaller components of a pattern tend to be seen as objectsfigure and ground a figure is something object-like that is perceived as being in the foreground, while the ground is whatever lies behind the figure According to Winn, this perceptual structuring is immediate. ‘Perceptual structure isdetermined by the grouping of symbols by their appearance [which he calls discrimination]and by their placement and interconnection [which he calls configuration]. Note thatdiscrimination and configuration occur without any knowledge of what the symbols in thediagram mean, nor of why they are placed and connected in the way they are’ (Winn 1993). 24
  30. 30. Visual syntaxRichards (2002) holds that the viewer’s first task when approaching a diagram is to ‘work outthe visual syntax.’ Ware (2000) describes a visual syntax for what he calls node-linkdiagrams. ‘The essential characteristic of [node-link] diagrams is that they consist of nodes,representing various kinds of entities, and links, representing the relationships between theentities’. He argues that node-link diagrams have a ‘visual grammar’ in that ‘The nodes arealmost always outline boxes or circles, usually representing the entities in the system’ and‘The connecting lines generally represent different kinds of relationships, transitions, orcommunication paths between the nodes’ (Ware 2000).Table 8. The visual grammar of node-link diagram elements (after Ware 2000)Graphical code Visual instantiation Semanticsclosed contour an entity of some kind… It can be a part of a body of software, or a person in an organizationshape of enclosed entity type (an attribute)regioncolour of enclosed entity type (an attribute)regionsize of enclosed magnitude of an entity (a scalar attribute)regionpartitioning lines can delineate subparts of an entity… maywithin closed correspond to a real-world multipart objectregionattached shapes closed-contour regions may be aggregated by overlapping them. The result is readily seen as a composite entityshapes enclosed by can represent conceptual containmentcontourspatially ordered can represent conceptual ordering of some kindshapeslinking line represents some kind of relationship between entitieslinking-line quality effectively represents an attribute or type or relationshiplinking-line can be used to represent the magnitude of thethickness relationship (a scalar attribute)tab connector a contour can be shaped with tabs and sockets that can indicate which components have particular relationshipsproximity proximity of components can represent groups In the light of Ware’s grammar, the OSI model seems semantically impoverished,consisting as it does mostly of boxes. Apparently this is not unusual: ‘While generic node-linkdiagrams are very effective in conveying patterns of structural relationships among entities,they are often poor at showing the types of entities and the types of relationships’ (Ware2000). His visual grammar suggests ‘ways of extending this vocabulary that are perceptuallysound’ (Ware 2000). 25
  31. 31. Indeed others have (unwittingly, I would guess) put these principles to work in makingtheir own renditions of the model. For instance, Figure 21 uses changes in size of enclosedregion to distinguish the layers. This is a scalar attribute, and hence may not be appropriatefor this use, but it does serve to indicate that there are differences between layers. Figure 22uses changes in colour of enclosed region, which is probably more appropriate. Figure 21. Diagram showing changes in size of enclosed region (from Zacker et al. 1996) Figure 22. Diagram showing changes in colour of enclosed region (from Bitzenbytes.com 28 Aug 2003). The work of Tversky and her colleagues aligns with Ware’s. Tversky is a proponent of the‘cognitive naturalness’ of certain graphic elements and their arrangement in space (e.g.,Tversky 2001; 2002). Tversky et al. (2000) hold that certain elements are apt to ‘readily[convey] meaning’, and they call these elements ‘meaningful graphic forms’. She argues that‘The choice of visual devices for discrete, categorical concepts and for ordinal or continuousones appears to be derived from physical devices that contain or connect’ (Tversky 2001).For example, ‘Signs used for enclosure resemble physical structures that enclose actualthings, such as bowls or fences’ (Tversky 2001). The meaningful graphic forms of relevance to the OSI model are closed figures, lines, andarrows. While there is no line in the ‘official’ OSI model depicted in Figure 2, the line thatappears at the bottom of Figure 1, connecting the two columns, often appears in OSI-inspireddiagrams. Closed figures, such as boxes, ‘suggest two- or three-dimensional objects whose 26
  32. 32. actual shapes are irrelevant, thus schematized’ (Tversky et al 2000). Lines depictconnections among objects as well as order (Tversky 2002). ‘Arrows are a special kind ofline, with one end marked, inducing an asymmetry’ and ‘Arrows are frequently used to signaldirections in space. In diagrams, arrows are also commonly used to indicate direction intime’ (Tversky 2001). Spatial arrangement also communicates. Tversky holds proximity to be ‘the most basicmetaphor’, and offers that ‘In perception, things that are near by in space tend to be groupedand separated from things that are distant. To use this for conveying abstract meaningssimply requires placing things that are related in close proximity and placing things that arenot related farther away in space’ (Tversky 2001).Table 9. Meaningful graphic forms and arrangements relevant to the OSI model(summarized from Tversky et. al. 2000, Tversky 2001, and Tversky 2002)Meaningful graphic forms Meanings conveyedlines connection ordinalityarrows temporal sequence, direction in time causality, direction in causality direction in space direction in motion direction of power direction of controlclosed figures objects (whose actual shapes are irrelevant)Spatial arrangement of graphic forms Meanings conveyedinside closed figures belonging together equivalenceclustered proximity in an abstract dimension, such as time or value belonging together the sharing of a common feature or features equivalence degree of relationshipseparated differing values on the same underlying feature The closed figures in the OSI model (see Figure 2) are all rectangular. It is true that theirshapes are irrelevant, since the layers of protocol do not have any inherent shape. Theirrectangularity, however, reflects the layered concept. Ellipses, for example (see Figure 5), donot work as well in conveying the concept of a ‘layered stack’ since round objects cannot bestacked in real life. The rectangles in each stack form a cluster, indicating that they ‘belongtogether’, which is appropriate as each stack represents the software running on onecomputer. The two stacks are separated, indicating that they are different, and in fact they dorepresent the software running on two different computers. The use of arrows in Figure 2 is curious, however. The arrows that run up the left side ofthe diagram are simply callouts that link the labels to the rectangles. The labels could just aswell have been put inside the rectangles. The arrows that run horizontally between each pairof layers intend to indicate their equivalence and are actually misleading, since thecommunication flow does not go from side-to-side at each layer but ‘down, over, and up’from the top, as shown in Figure 23. 27
  33. 33. Figure 23. A more accurate use of arrows in the OSI model (from Montes 2001)Cognitive processingKim et al. (2000) outline the distinction between perceptual and conceptual (cognitive)processing: The perceptual process is a bottom-up activity of sensing something and knowing its meaning and value (Bolles 1991), while the conceptual process is a top-down activity of generating and refining hypotheses (Simon and Lea 1974). In other words, we search and recognize relevant information through perceptual processes and reason by inferring and deriving new information through conceptual processes….To fully exploit the potential of a diagram, it must be effectively utilized in both the perceptual and conceptual processes. (Kim et al. 2000) Others agree that interpreting diagrams requires more than just perceptual processing(e.g., Lowe 1994; Ittelson 1996; Tversky 2001). Tversky makes the fundamental point that‘interpreting a graphic depends on understanding that it can represent something other thanitself’ (Tversky 2001). Ittelson argues: ‘Markings do not exist in the real world; they exist ashuman expressive and communicative artifacts. The perception of markings must necessarilybe about that expressive and communicative content’ (Ittelson 1996). Lowe offers that for‘learning tasks – such as committing the diagram to memory, understanding its meaning orusing it as an aid to problem solving – the data it provides explicitly (typically simple linesand shapes) need to be interpreted not simply as a visuo-spatial array, but in terms of thesubject matter it depicts’ (Lowe 1994). Doing this requires background knowledge (Lowe1993; Winn 1993; Lowe 1994). 28
  34. 34. Background knowledgeLack of sufficient background knowledge of the system represented by a diagram has beenfound to reduce the ability to construct meaning from the diagram (Lowe 1994), recallunfamiliar material learned in the diagram (Winn 1993), and find information in thediagram (Winn 1993). Winn, examining how viewers search for information in diagrams, explains that viewersfirst take in the elements of a diagram that ‘enjoy perceptual precedence’ (Winn 1993).Where they look next, however, is ‘likely depend[ent] on their familiarity with the symbolsystem of diagrams and on their knowledge of the material the diagram describes’ (Winn1993). Winn writes that ‘important aspects of search in diagrams are…directed perceptuallyand do not rely on subject matter for their successful execution. Other aspects of search indiagrams are, of course, influenced by knowledge of content’ (Winn 1993). Lowe (1993) distinguishes between two types of background knowledge: domain-generaland domain-specific. Domain-general knowledge gives the ability to ‘deal with a diagram’scomponent markings on a visuo-spatial level’ (Lowe 1993). Domain-specific knowledge‘enables the viewer to go beyond the visuo-spatial level in order to represent mentally themeaning of the system depicted in the diagram’ (Lowe 1993). This is important because ‘Avisuo-spatial approach to a diagram…would be of little value in developing an understandingof the depicted subject matter’ (Lowe 1994). In Lowe’s view, background knowledge plays a key role in the development of a mentalrepresentation of a system: ‘The nature of the mental representation constructed from adisplay…can be characterized as a function of the interaction between the informationprovided in the display and the person’s background knowledge’ (Lowe 1993).Mental representationsLowe ‘assume[s] that the successful processing of a visual display (such as a diagram)involves the construction in working memory of an appropriate mental representation fromthe display’ (Lowe 1994). The nature of the viewer’s mental representations (or ‘mentalmodels’ or ‘knowledge schemata’) are held by many to be structural (Glenberg and Langston1992; Lowe 1993; Winn 1993). The structure of the mental representation must match thestructure of the real system in order to facilitate learning (Lowe 1993). When diagrams match the structure of the real system, they can help the viewer to buildan accurate mental representation of the real system (Glenberg and Langston 1992).According to Lowe (1993), diagrams that fail to ‘capture properly the aspects of the subjectmatter which have a central semantic significance’ (Lowe 1993) may cause the viewer todevelop an inaccurate mental model that does not facilitate learning. This line of reasoning is not without its detractors. Scaife and Rogers (1996) are opposedto statements such as ‘Graphic forms encourage students to create mental images that, inturn, make it easier for them to learn certain types of material (Winn 1987)’ because suchstatements do not specify a mechanism and ‘seem to rest on intuition’ (Scaife and Rogers1996). They conclude that ‘the case for an intimate relationship between graphicalrepresentation and images may not be logically compelling and is currently heavily under-specified’ (Scaife and Rogers 1996). Scaife and Rogers (1996) would prefer a focus on thekinds of internal representations people form when interacting with externalrepresentations. 29
  35. 35. Visuo-spatial abilityVekiri (2002), citing Carrell, defines visuo-spatial ability as ‘the ability to mentally generateand transform images of objects and to reason using these imagery transformations’. Itseems natural that this ability would play some part in the interpretation of diagrams. Winnand Holliday (1982) report that ‘The correct interpretation of diagrams requires variousmental skills’ and that ‘students need to have attained a certain level of “diagram literacy” inorder to extract information from them’. Twenty years later, Vekiri concurs: ‘It appears thatdiagrams may be more demanding to process, and thus less beneficial, when students do nothave high visuo-spatial ability’ (Vekiri 2002). She suggests design strategies to help viewerswith low visuo-spatial ability to process information in diagrams; these will be reviewed in alater section.MetaphorInterpreting metaphor in diagrams involves both perceptual and conceptual processing (e.g.,Richards 2000; Tversky 2001). Lakoff and Johnson (1980) examine metaphor in languagewith the goal of understanding the human conceptual system. What Lakoff and Johnson(1980) call ‘orientational’ or ‘spatialization’ metaphors are the ones most relevant toanalysing the OSI model. They argue that ‘orientational metaphors…arise from the fact thatwe have bodies of the sort we have and that they function as they do in our physicalenvironment…Such metaphorical orientations are not arbitrary. They have a basis in bothour physical and cultural experience’ (Lakoff and Johnson 1980). The orientational metaphors I found most worthy of examination are presented inTable 10.Table 10. Orientational metaphors relevant to analysis of the OSI model (from Lakoff andJohnson 1980)Direction Metaphor Language examples Physical/cultural basisup consciousness ‘Get up. Wake up. He’s We sleep lying down anddown unconsciousness under hypnosis. He sank stand up when we awaken into a coma’.up having control or force ‘I am on top of the situation. Physical size typicallydown being subject to control I have control over her. His correlates with physical or force power is on the decline. He is strength, and the victor in low man on the totem pole’. a fight is typically on topup more ‘My income rose last year. If you add more of adown less The number of errors he substance to a container or made is incredibly low. If pile, the level goes up you’re hot, turn the heat down’.up rational ‘The discussion fell to the In our culture people viewdown emotional emotional level, but I raised themselves as being in it back up to the rational control over animals, plane. He couldn’t rise above plants, and their physical his emotions’. environment, and it is their unique ability to reason that places humans above other animals and gives them control 30
  36. 36. The orientational metaphors Lakoff and Johnson discuss all involve verticality. Inaddition to vertical metaphor, Tversky (2001; 2002) researches horizontal metaphor andfinds it to be neutral (Tversky 2001). Her explanation is based on the bilateral symmetry ofthe human body and the arbitrariness of the horizontal arrangement of objects in the realworld (Tversky 2002). This is not the case for the vertical dimension: ‘What’s up defiesgravity, exhibits strength. People grow stronger as they grow taller. Larger piles, of goods ormoney, are higher’ (Tversky 2002). While ‘The vertical axis of the world has a naturalasymmetry, the ground and the sky,…the horizontal axis of the world does not’ (Tversky2001). De Sauzmarez, quoted in Sless (1981) as evidence of some designers’ over-concernwith the ‘internal dynamics’ of pictures, in this context helps to flesh out the concepts: Horizontals and verticals operating together introduce the principle of balanced oppositions of tensions. The vertical expresses a force which is of primary significance – gravitational pull, the horizontal again contributes a primary sensation – a supporting flatness; the two together produce a deeply satisfying resolved feeling, perhaps because they symbolise the human experience of absolute balance, standing erect on level ground. (de Sauzmarez 1964) What kinds of metaphor are used in the OSI model? Vertical metaphor is primary. As oneexamines a stack from bottom to top, the degree of abstraction increases and the tangibilitycorrespondingly decreases. At the lowest level, electromagnetic waveforms traverse amedium. These are physical phenomena that, while not perceptible to humans, are ‘real’ andcan be detected using instruments (such as an oscilloscope) even without any concept ofwhat they mean. Just to go one step further and convert the waveforms to ones and zeroes(binary digits) involves an abstraction that requires the computer to understand the ‘code’.The best metaphor for up relative to the OSI model is probably consciousness. At the lowestpossible level, there is no meaning ascribed to the signal that arrives. After the networkingsoftware running on the computer performs a series of transformations on the signal, itpresents an intelligible communication to an application (such as a web browser) that sitsabove the highest layer. It is interesting to trace the development of this verbal metaphor in parallel with thedevelopment of visual depictions of the layered concept that formed the basis of the OSImodel. Carr et al. (1970) use the phrase ‘to send data over a link’, which implies a separationbetween the physical connection below and the data that rides on top. But they use adifferent sort of metaphor in the same paper: ‘The network is seen as a set of data entry andexit points into which individual computers insert messages destined for another (or thesame) computer, and from which messages emerge’ (Carr et al. 1970). To me this recallsperhaps a pneumatic tube system. The paper that introduces what seems to be the earliest precursor to the OSI model (seeFigure 5), Crocker et al. (1972), is replete with metaphoric language: ‘operating just abovethe communications subnet’; ‘when we have two computers facing each other across somecommunications link’; ‘we speak of high or low level protocols’. Perhaps not surprisingly, thepaper that I use to illustrate that the layered concept took some time to take hold (Mills 1972)contains no terms that suggest spatialization. 31
  37. 37. A highly appealing visual metaphor is sometimes applied to the network communicationprocess: the postal metaphor. If the ‘intelligible communication’ between two computerswere to be thought of as a letter, downward traversal through the stack would be akin toputting the letter in one after another addressed envelopes and sent to the other computer,which opens each one in turn until the letter can be read. Figure 24 shows a good example ofthis metaphor. Figure 24. Illustration of the postal metaphor (from Motorola Codex 1992) ◆Now to the right (transformer’s) side of the model in Figure 20. The transformer sets aboutto explain the real system diagrammatically, drawing on his or her own conception of the realsystem, which is informed by access to expert knowledge and applied using visualconventions and, it is hoped, some idea of the task the viewer is to accomplish.The transformer and transformationMacdonald-Ross and Waller (2000) define the transformer as ‘the skilled professionalcommunicator who mediates between the expert and the reader’. They acknowledge thatOtto Neurath coined the term previously. 32
  38. 38. ‘The transformer’s job is to put the message in a form the reader can understand’(Macdonald-Ross and Waller 2000). The transformer does this by divining the ‘centralthemes or organising principles’ that unify the ‘facts, arguments, theories, problems, andprocedures’ that ‘all subjects consist of’ (Macdonald-Ross and Waller 2000). Ittelsonemphasizes the role of creativity in the transformation process: [The form] is the product of a continuous series of choices based on social practices, individual experiences, and aesthetic judgments (Willats 1990).…Many of the decisions along the way are ‘rational’. They are in principle ‘computable’ on the basis of some hypothetical algorithm. But some, perhaps most, are not. They are based on a feeling on the part of the creator of the marking that, of all possible paths, this one is the ‘right’ way to go. (Ittelson 1996) While the creative aspect of transformation is vital, the chances of achieving a successfultransformation can be greatly improved if the transformer is familiar with characteristics ofthe viewers and the task or tasks they will be expected to perform, design guidelines for thetype of artefact and medium used, and, of course, the real system itself. Even when armed with this knowledge, success is not guaranteed. As Ittelson reminds us,‘We can construct a form, but we can never fully determine how that form will be perceived.Each perceiver can, and indeed must, perceive it idiosyncratically to a greater or lesserextent’ (Ittelson 1996). But despite these difficulties, it is possible to increase the chance ofsuccess. 33
  39. 39. Achieving successful transformationMacdonald-Ross and Waller stress that ‘a good communication is selected for a purpose andhas a sound logical structure’ (Macdonald-Ross and Waller 2000). This section discussesthese and other related goals and how they might be achieved when transforming abstracttechnical material.Addressing the viewer’s needsAs Kim et al. caution, ‘Simply providing a diagram does not guarantee good performance’(Kim et al. 2000). The transformer must know something of the viewers’ backgroundknowledge, visuo-spatial abilities, and the tasks they are to perform using the diagram.The viewers’ tasksConcern for the viewers’ tasks is emphasized by Macdonald-Ross (1977; 1989) and Vekiri(2002) in particular. Vekiri puts it bluntly: ‘Displays need to address the goal of the task –displays must meet the demands of the learning tasks in order to be effective (Vekiri 2002).Macdonald-Ross stresses that ‘To choose the best format for a particular occasion one mustdecide: what kind of data is to be shown? What teaching point needs to be made? What willthe learner do with the data?… (Macdonald-Ross, 1977). He further draws a distinctionbetween the tasks of operation and conceptualization: A reader interested in operational data will be taking off precise numerical or structural information for some practical purpose. Here the graphic is used for reference in the most literal manner. On the other hand, a reader interested in conceptual relationships will be looking at trends and general structure with a view to understanding the argument presented in the text. In general, a graphic device which is optimal for one of these purposes will not be optimal for the other. (Macdonald-Ross 1989) Analysing the viewers’ tasks can lead the transformer to useful design solutions, but it isnot a formulaic technique that guarantees a satisfactory outcome: ‘It pays to remember thatgraphic communication is an art, that is, a skill which results from knowledge and practice’(Macdonald-Ross 1977).The viewers’ visuo-spatial abilitiesVekiri notes that ‘diagrams may be more demanding to process, and thus less beneficial,when students do not have high visuospatial ability’ (Vekiri 2002). Winn and Hollidaycaution that ‘diagrams are not the best way for all students to learn. The correctinterpretation of diagrams requires various mental skills that designers should not take forgranted’ (Winn and Holliday 1982). They recommend not using ‘complex and redundantdiagrams and charts with low-ability students’ (Winn and Holliday 1982). As far as Vekiri(2002) is concerned, how to design suitable materials for viewers with low visuo-spatialability is an open question. 34
  40. 40. The viewers’ background knowledgeInterpreting abstract technical diagrams is a cognitively demanding task (e.g., Lowe 1994;Vekiri 2002). Without sufficient background knowledge about the real system, viewers arelikely to interpret a diagram in terms of its visuo-spatial properties (e.g., Lowe 1994;Richards 2000). To counter this Lowe suggests that ‘instructional interventions aimed at improvingstudents’ capabilities to deal with a particular diagram should address the development ofrelevant contextual knowledge in a manner that emphasises high-level domain-specificrelation’ (Lowe 1994). Tversky (2001) and Vekiri (2002), however, argue for beginning withconcrete examples. Tversky offers that ‘research in cognition on basic level concepts and onreasoning suggests that an effective entry into a complex system might be a thoroughunderstanding of a concrete example. Once an exemplary example has been mastered,abstraction to generalities and inspection of details are anchored and supported (Tversky2001).Structuring the diagram appropriatelyAn appropriately structured diagram exhibits high fidelity with regard to the real system andadheres to perceptual and conceptual conventions.Relation between the representation and the real systemAnother way of titling this section might be ‘relation between content and graphic’, whichMacdonald-Ross calls ‘one of the most profound and important questions in graphiccommunication’. (Macdonald-Ross 1989). Winn and Holliday agree: ‘the first thing thedesigner must be conscious of is the accuracy with which the diagram or charts captures [thelogical relationships among concepts]’ (Winn and Holliday 1982). In these sorts of diagrams ‘neither the parts of the display nor their location correspondto the parts and the locations of referents’ (Vekiri 2002), but Macdonald-Ross reassures usthat ‘The mapping between a class of graphic devices and a problem domain is rarely one-to-one. A class of graphic devices can be used to represent any content that has the underlyingconceptual structure denoted by the graphic’ (Macdonald-Ross 1989). And Tversky issues areminder: ‘Diagrams…are not meant to reflect physical reality completely and veridically.Rather they are meant to be schematized renditions of actual or abstract systems.…As such,they are not meant to reflect conceptual reality. They portray an analysis of the parts of thesystem and their interrelationships, structural, causal, or power’ (Tversky 2002). The representation should be ‘selective’ (Lowe 1994) and ‘constrained’ (Scaife and Rogers1996). ‘The issue…becomes one of determining which aspects of the represented world needto be included and how they should be represented, what aspects should be omitted andwhat additional information needs to be represented that is not visible in the real world butwould facilitate learning’ (Scaife and Rogers 1996). The distances among concept labels should correspond to their positions in the realsystem (when possible) and reflect the ‘semantic distance’ between concepts (Winn andHolliday 1982). Sequences of concepts should match those in the real system, and should bepresented ‘so that they run left-to-right or top-to-bottom on the page’ (Winn and Holliday1982). For teaching concept identification, Winn and Holliday found that ‘including smalldrawings within diagrams can facilitate students’ understanding of commonly taught 35
  41. 41. concepts and principles’ (Winn and Holliday 1982). An example of how this can work isshown in Figure 25. Figure 25. How the inclusion of small drawings can be used to facilitate understanding (from Freedman 1996)Accompanying textWith abstract technical diagrams, there is often a need for accompanying text (e.g. Arnheim1969; Scaife and Rogers 1996; Richards 2000). Vekiri argues that ‘[explanations thataccompany displays] work better when they cue learners to the important graphic elementsand details necessary to extract the message(s) that graphics communicate’ (Vekiri 2002).The textual explanation should be presented near the diagram in space and time (Vekiri2002).Adherence to perceptual conventionsGenerally, diagrams should follow the visual syntax of Tversky et al. (2000), Ware (2000),and Tversky (2001; 2002) that is presented in the Perceptual processing section of thisdissertation. Ware argues that ‘it is important that a good diagram take advantage of basicperceptual mechanisms evolved to perceive structure in the environment’ (Ware 2000). Hesuggests that ‘there are ways of extending [the vocabulary of generic node-link diagrams] 36
  42. 42. that are perceptually sound…There is a range of possibilities between the rectangular boxand line diagram and fully rendered, colored, and textured 3D objects’ (Ware 2000).Drawing inspiration from exemplarsMacdonald-Ross (1989) stresses the importance of examining the work of ‘masterperformers’ to ‘stimulate and inform the creative design activities of the transformer’Macdonald-Ross (1989). Even Scaife and Rogers, who call into question the idea that we canassess adequately ‘the value of different graphical representations…from our intuitions’(Scaife and Rogers 1996) believe that ‘we should recognize the importance of the canonicalforms of diagrams’ (Scaife and Rogers 1996). I couldn’t agree more. ◆ 37
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