Individual computers are capable of gathering, processing, storing and distributing information, under the direction of humans. They are not only found in distinctive boxes with keyboards, mice and screens attached, or in large cabinets bristling with flashing lights and whirling tape drives, as seen in ageing science fiction films. Their basic information-handling capabilities can be harnessed for controlling other machinery, and so they are also hidden inside things like wristwatches, microwave ovens, central heating systems, factory production line equipment and nuclear power plant safety systems.
There are three main areas where benefits can be expected if one computer is able to communicate with others: it can get information that is stored by other computers; t can get other computers to do specialized work; and it can communicate with humans that use other computers. The benefits need not only be in one direction -? this computer can also export its own information, its specialized abilities and access to its human users. The outcome is a beneficial sharing of resources.
At present, it is usually the case that computers inside personal items like wristwatches, or inside domestic equipment like microwave ovens, do not communicate with others. However, this situation is on the point of changing, given that communication teen the more recognizable types of computers has proved to be very useful, and that appropriate communication technologies are becoming available. It might also seem that a conventional home computer, or a single computer tucked into a musty office, is island-like, cut off from the world community of computers.
However, this is a delusion, since such computers usually have an obliging communication mechanism: human beings transferring the latest fruits of the computing trade on floppy disk or compact disk. This research is concerned with computer communications where there are no human middlemen, so that computers can inverse directly with one another. Although human participation in the role of intermediary is being eliminated, it should not be forgotten that computers only communicate because humans have instructed them to and, moreover, that this is possible because humans have instructed the computers how to communicate.
The techniques covered arise from communications between conventional types of computer. However, they are equally applicable to the world of the future, where there will be things like intelligent houses with communication not Just between domestic appliances but also with the fabric of the building itself. The act of immunization is not always easy for humans. For example, it is not feasible for every person in the world to communicate effectively with any other person whenever desired. Differences in culture, availability and physical location cause problems.
Things get even tougher if one broadens communication to include other species of animal or plant, never mind any alien life forms that might visit or be visited. Similar problems affect communication between computers. Because of this, the solutions used are rather similar to those developed by humans over the millennia, and so are usually familiar to the neophyte from normal human experience. This is good, since it means that explanations of techniques used can be well motivated by appropriate examples from human communications.
Aside from reconciling differences between communicating parties, a further problem is how communication is physically achieved. For humans, there is a mixture of movement and media. People may move to make communication easier, or even possible at all. For example, one may climb a mountain to consult a guru, travel to work or attend a concert. Once the communicating parties are in suitable positions, communication might take place using sound through the air, visible sign language, telephones, elevation or sending items through the postal system.
For computers, there is usually no movement involved in communication, robotics still being in its relative infancy. Because of this, computers communicate using media that physically reach the computers, and that are capable of transmitting computer conversations. Of course, this does not rule out the possibility of humans moving the computers to places where they can be reached by media. The main themes of the research are concerned with these communication problems. The features of communications are classified under three main headings: 1 . Information: the type of information that is communicated; 2. Mime: when, and how quickly, a communication takes place; and 3. Space: which computers, and inter-connecting channels between computers, are involved in a communication. There are two main problems associated with communication: achieving agreement between computers on the nature of communications; and implementing the required communications using available physical communication media. The processes of achieving agreement and implementation require a lot of human decision making prior to communications being possible. This entails human communications of a rather specialist kind.
The section ends by introducing the ways in which these technical and political human communications have a major influence on what is possible and not possible in the computer world. 1. 2 USES OF COMPUTER COMMUNICATIONS Present-day uses for computer communications arise from a convergence between two different worlds. The first is a computer-centered world, where computers existed, and then it became convenient to inter-connect them. The other is a human- oriented world, where communication facilities existed, and then it became convenient to computerize these facilities.
In the latter world, there was also convergence between telecommunications facilities largely used for inter-personal communication, such as the telephone, and broadcasting facilities largely used for entertainment, such as television. One term often used to describe the fruits of this general convergence is the global village. It is interesting to note that, when this phrase was coined by Marshall Mclean in 1964, it was based on an extrapolation of existing human oriented facilities, and did not envisage the future involvement of computer communications. However, it is still equally apt to embrace computer- eased facilities.
The technology shift is captured in another, more modern, term: the information superhighway. This refers to the collection of communication technology and information technology that will be used to underpin the global village of the future. Just as roads underpin the movement of people and goods, so the information superhighway will underpin the movement of information. To understand the current uses made of computer communications, and to point ahead to future developments, it is useful to conduct a brief historical survey of how different communication systems emerged and then converged.
This follows in the next three sub-sections, which cover developments on the computer front, on the telecommunications front, and on the broadcasting front, respectively. A central concern is the demands that are made by these differing types of systems if they are to be realized using computer communication facilities. To quantify these demands, time can be measured as usual, in seconds, or in fractions or multiples of seconds. Information can be measured using bits (short for ‘binary digits’). The quantity of information available in one bit is that required to distinguish between two possible aloes.
For example, one bit of information is enough to distinguish between ‘on’ and ‘off, ‘black and White’ or yes’ and ‘no’. A more precise definition of both information, and the bit as a measure of information, is given in Chapter 2. The bit is like any other unit, so it is convenient to talk of the kilobit (kibitz), which is 1000 bits, and the megabit (Ambit), which is 1 000000 bits. Note that these units are powers often, rather than the powers of two (2 10 and 2 20) sometimes used in computing circles. 1. 2. Computer-oriented communication In the earliest days of computers, the manful of boxes containing the component parts of the computer was the centre of attention. Information was supplied to a computer, and retrieved from a computer, by people who were physically present in the computer room. In the unusual event that there was any communication between computers at all, this was done by carrying paper tapes or magnetic tapes produced by one computer along to be read by another computer. Thus, people were the servants of computers as much as computers were the servants of people.
The first developments in communications were designed to make life easier for people, by eliminating the need to be physically adjacent to the computer when interacting with it. One of these was the introduction of a remote terminal -? a typewriter-like device with a keyboard for input and a teleprompter for output -? connected by a cable to the computer. This is illustrated in Figure 1 . 1 (b). In essence, terminals were Just extra peripheral devices for the computer, the only difference being that they were a longer distance away.
Because of this, and the fact that terminals were electro-mechanical devices, they operated at very slow speeds in computer terms: a few characters input per second and around 30 characters output per second. This basic capability evolved into a model of interaction with computers that survives to the present day. There are two major improvements. One is in terminal technology, with video screens replacing printing, giving a consequent large increase in output speed, and also with terminals containing computers as their controllers (or, indeed, computers Just emulating the behavior of terminals).
Beyond terminals, full WIMP (Window, Icon, Menu and Pointer) interfaces can also be used, if the computer is sophisticated enough to interact via such an interface. The other improvement is the one of interest to imputer communications. This is that the direct physical cable between terminal and computer can be replaced by any communication channel capable of passing information between a terminal and a computer. Useable communication channels include links through the conventional telephone system or through specialized computer networks.
This is illustrated Figure 1 . 1(c). The Internet has become the best-known specialized mechanism for providing links between computers located in all continents of the world. In 1997, it was estimated that around 20 000 000 computers could make use of the Internet. The overall effect presented to a human user is still of a direct physical link between terminal and computer, but this is an illusion. The increased distancing of terminal from computer presents two agreement problems that must be solved to enable communications.
The first is that a wide variety of terminals (or computers masquerading as terminals) may be used, and each terminal’s behavior must be reconciled with that expected by the computer. This issue, and its resolution, is discussed more fully on page 44 in Chapter 2. The second problem concerns the human using the terminal. This person issues commands to the computer, or inputs information to the computer, through the keyboard. Given that the terminal is behaving as though it is a real peripheral device of the computer, its user must know exactly how to interact with that particular type of computer.
This is acceptable if only one computer is ever used via the terminal, but the whole point is that communications advances now allow all sorts of computers, all over the world, to be used. No single person can be expected to know how to interact with all of the different computers that are accessible. A elution to this problem is for a particular user Just to interact with his or her own familiar computer, and then that computer interacts with any other computers of interest on the user’s behalf. This introduces some automation of the task faced by the human terminal user.
In early versions of such facilities, the user’s computer Just pretended to be a human user of the distant computers. That is, it transmitted information that appeared to come from a terminal keyboard (albeit with a rather faster typist than normal) and then received back information by pretending to be a terminal printer or screen. This sort of interaction allowed files to be sent to, or fetched from, distant computers, electronic mail to be sent or received, or processing jobs to be given to distant computers and their results to be retrieved. These operations were directed by the human user.
In time, such procedures became refined, to eliminate the unnecessary humiliation of the dialogue between computers. That is, new agreements to specify appropriate direct interactions between computers replaced the more verbose means used by humans. The resulting situation is illustrated in Figure 1 . 1(d). The new agreements covered things eke transferring files, sending electronic mail and submitting Jobs. These matters are discussed in more detail in Section 2. 2. 2. A further effect of such advances is that users gradually got more insulated from the vagaries of particular types of computer.
For example, the same user interface could be used for something like electronic mail, regardless of what type of computers were involved in the mail transmission. Further extensions of the insulation process led to distributed systems, where the existence of a collection of computers is completely hidden from a user. Thus, it appears that a service is being provided by one single uniform computer system. In distributed systems, it is fairly common for the computers involved to take on server and client roles.
Servers have special capabilities, for example, storing particular information, performing particular processing of information, or having particular input or output devices attached. Clients can make use of servers by issuing requests, and by getting responses back. Overall, this is similar to the ways that humans directly interact with computers. However, in a distributed system, the computer client-server relationships are hidden from human users. The World Wide Web (WWW), considered in detail in Chapter 9, is an example familiar to many, in which the illusion of a world of information is presented to a user.
In fact, the user’s computer acts as a client, and computers throughout the world that store particular WWW pages of information act as servers for this client. In summary, the evolution of computer-oriented communications has gone from a situation where a terminal was connected to a computer via a physical cable to a situation where a computer can be connected to numerous other computers via indirect channels supplied by complex communication systems. The types of information shared are more complex and the quantities are potentially huge. The speeds are related to computer rates rather than human rates.
Early terminal links required the communication of only 300 bits per second. Two high speed computers might be able to cope with the communication of 1 gigabit (1 000000000 bits) per second nowadays. When pushed to one extreme, a collection of communicating computers can be placed into one box, with high speed electronic interconnections, to form a supercomputer: a new, high speed computer consisting of many individual computing elements. This is the point at which computer communications meets the subject area of parallel computation. 1. 2. Telecommunications Human communication via a medium is long-established. In the mid-nineteenth century, the telegraph became an economical method for the electrical transmission of information represented in Morse code, or similar, over long distances. In style, Morse code is not far removed from the bit-focused approach to information followed by computers. The difference is that information is expressed in a three-valued form (dot, dash and pause) rather than a two-valued form. The ajar problem with the telegraph was the need for an expensive physical cable between the communicating parties.
The development of telegraph systems often occurred in parallel with railways, with telegraph cables being located alongside the railway tracks. The medium was used fairly wastefully, because the speed of transmission was limited to the speed at which people could press Morse keys to send information and the speed at which people could listen to the transmission to decode the information. Thus, the restriction came from human frailty, rather than any fundamental physical limitation of the cabling. Around the turn of the twentieth century, the telephone became available.
Like the telegraph, a large investment in cabling was necessary. However, unlike the telegraph, the telephone was an analogue device, in that human speech was directly converted to electrical waveforms for transmission. Experience of using the telephone until recent times confirms the main problem of analogue transmission: dubious effects on the electrical signal are reflected as audible crackles, whines, etc. On reception. Nowadays, most telephone systems employ digital transmission systems, with speech being represented using a series of bits.
This is because it is rather easier to reconstruct a series of bits from a damaged electrical signal than directly encoded speech. A complex collection of automated national and international telephone exchanges provides a near-worldwide telephone communication system, going a fair way towards a goal of allowing every person in the world to speak to any other. Transmission is not exclusively via cabling, with radio transmission becoming increasingly used by mobile telephones. The telephone system can be used as a vehicle for computer-oriented communication, for example, as a way of connecting a Armenia to a computer.
This does not involve computers speaking to one another in a human way over the telephone. Instead, electrical waveforms of a similar style to those used to encode speech are transmitted, but they in fact encode series of bits of computer-style information. The modern telephone system has evolved to be like a distributed computer system. The digital exchanges are Just special-purpose computers, and the links between them are similar to links between computers. To an extent, a fully digital service is offered to end users of the telephone system, wrought the Integrated Services Digital Network (KIDS) service, and other offerings.
However, in the main, the typical subscriber is still supplied with a traditional analogue-style service. This is implemented using the underlying digital facilities. Thus, if such a subscriber uses the service for computer communication, the information is needlessly passed through an analogue form before being transmitted digitally. One extension to the conventional telephone is to make it into a videophone, where a picture of the caller is transmitted as well as the speech of the caller. It is feasible to supply a videophone service over a normal telephone line, but the picture quality is fairly crude.
A further catch is that few people possess videophone equipment. However, the main problem is that much more information must be transmitted to encode pictures and so, to achieve better quality, a better communication facility is needed. For speech alone, it is enough to transmit about 10 kibitz of information per second. However, for video, around 60 kibitz of information per second is a lowest limit, with 1500 kibitz per second needed for decent quality. Other telecommunication services have been developed in addition the telephone, geared to transmitting textual-style information rather than direct human communications.
One of these is fax (facsimile), which can be used to transmit an image of a text page from one telephone user to another. This involves a digital representation of the page in terms of bits of information being transmitted between two fax machines, encoded as a telephone-style electrical signal. The fax machines are really Just special-purpose computers, which conduct a conversation over the analogue telephone system. Many computers can now emulate the behavior of fax canines, so that they can transmit page images to other computers or fax machines.
The number of bits of information needed to represent a faxed page varies according to the pattern of black on white on the page. However, on average around 200 kibitz are sufficient for one page. More details of fax representation are given on page 53 in Chapter 2. A further family of informationtransmission services are those based on telex. These are fairly close relatives of computer-oriented communications that involve textual information being sent between computers and terminals or between computers.
Telex itself is a long-established telegraph service that allows textual messages to be transmitted between subscribers. There are about 1. 2 million subscribers, mostly businesses, around the world. The messages can be composed of upper case letters, numerical digits and some punctuation symbols. They are transmitted in a digital form, but the transmission rate is only 50 bits of information per second, which equates to only 10 characters per second. A more modern offering is telex, which allows a much larger character set, including graphical symbols and word processor style facilities for composing pages.
It has a such faster transmission rate of 2400 bits per second, but this is still slow in modern computer communications terms. Both telex and telex are simple mechanisms for transmitting digital information between two specialized machines. Videotape is a variation that is akin to the client-server model for distributed computer systems. It is used over the normal telephone system. A user of videotape has a special video terminal attached to the telephone line, and this acts as the client.
It is possible to telephone computer databases worldwide, and then conduct searches for information. The databases are the servers. One place where videotape is often seen is in travel agents, where video terminals are used to interrogate holiday and travel booking systems over the telephone. Videotape is an example of a value-added service operated by telecommunications providers. In addition to a raw communication facility, extra services are provided on top. This type of service illustrates a convergence of telecommunications with computer-oriented communications. . 2. 3 Radio and television broadcasting Around the turn of the twentieth century, at the same time as the telephone was becoming available, a free worldwide immunization medium was being harnessed: the electro-magnetic spectrum. This allowed the use of physical phenomena such as radio waves, rather than expensive cabling, for transmission. The first application was a wireless version of the telegraph. Although it seems strange with hindsight, the obvious use of this medium -? broadcasting -? was not realized until after the First World War.
To an extent, this was due to an attitude that communication technology was only relevant for closed commercial and military applications, rather than the general public. The average errors was still expected to travel in order to communicate directly. A further reason was that the cost of transmitting and receiving equipment was high, although there were no wiring charges. Since these early days, of course, there has been a massive growth in public radio and television broadcasting services. Until the last years of the twentieth century, the sound and picture information of radio and television has been transmitted as an analogue signal.
These signals are received using aerials, and then decoded. Digital transmission, with sound and pictures encoded using series of its was only in its infancy in 1997. However, this is undoubtedly the transmission style of the future, marking a convergence of radio and television with computer- oriented communications. Compared with the transmission demands of the telephone and videophone, those of radio and television are much higher. In the case of radio, this is because the whole range of audible sounds must be transmittable, rather than Just human speech.
In addition, extras like stereo sound are also desirable. The information rate needed for high fidelity audio transmission is about 100 kibitz per second -? ten times the rate for telephony. In the case of television, pictures are larger, contain more detail and involve more motion over time. The information rate needed for television-quality video transmission is about 15 Ambit per second -? again, ten times the rate for decent-quality video telephony. Note that these rates are for digital transmission, where extensive computer processing of the information can compress it to fit these rates.
Without such processing, the required rate for raw video transmission is very much higher. Just as videotape allows the retrieval of digital information using the telephone system, so telex is a similar agility associated with television. Note the subtle naming difference between the television world’s telex and the telecommunication world’s telex. Telex has far less scope for interaction, compared with videotape. All of the available information is continuously broadcast in sequence, and then the television receiver has to filter out the information of no interest.
This is in contrast to videotape, where a user explicitly initiates searches for information of interest. However, like videotape, telex illustrates a convergence with computer-oriented communications. The rise of cable elevation illustrates a convergence between television and telecommunications. The same physical technology can be used to deliver both telephone and television services to an end user. It can also be used to deliver computer-oriented communication services, thus completing a convergence of communication facilities with rather different histories.
Although analogue transmission is still used for cable television, in time, with the advance of digital television, the raw physical cable will become the bearer of a digital information bit carrying service, which is then used to support different value-added communication services. This will represent each user’s slip road to the information superhighway. 1. 2. 4 Summary of uses of computer communications From the point of view of a user, the interface to the converged world of computer communications, telecommunications, radio and television should allow easy access to information, communication facilities and entertainment.
These will be underpinned by the techniques associated with computer communications today. Indeed, already, devices such as telephones and television sets are really Just special-purpose computers from a technical communications point of view. The blending of computers and everyday devices will continue, to a point where, from a user point of view, explicitly visible computers are not likely to be present. A user will only see ‘intelligent’ devices; these will be internally underpinned by computers.
The devices might be examples of wearable computing, embedded in clothes or spectacles (or even in the body), or of ubiquitous computing, embedded in numerous objects all around the user. One particular new type of device will be a knowledge robot -? a device that can speculatively gather information that is likely to be of interest to its user, an improvement on conventional facilities that only gather specific information when ordered to.