Communicating computers

An electronic information service requires a fairly standard set of facilities. The user typically watches information of two kinds presented on a computer screen—the actual content information requested by the user and the record of a dialogue between the information service and the user through which the user specifies her requests. Both kinds of information are codified and presented in various ways, with the user’s ‘input’ to the dialogue entered using a keyboard and mouse. That part of the system is usually provided by a personal computer or its more powerful cousin, the ‘workstation’, and is known as the ‘front end’. The front end machine runs what it has recently become fashionable to call ‘client’ software which displays both kinds of information to the user, acts appropriately on the user’s input, and conducts a corresponding communication of digitally encoded information with the information service ‘back end’ across an ordinary telephone line or a dedicated data communication line.

The typical ‘back end’ is provided by a more powerful computer which deals with simultaneous communications with many front ends, responding appropriately to messages from the client software by retrieving requested information from storage and transmitting it back down the line for display to the user.

Technical obstacles to transmission of digital computer data over ordinary telephone lines3 have been a major limiting factor to the growth of information service use. The conversion from digital computer data to and from an analogue telephone signal is achieved using a specialised electronic device, a ‘modulator-demodulator’, which is nowadays better known by its shortened form ‘modem’. Modems must support the standard ‘protocols’ determined by telephone system operators and their regulating authorities to ensure that the data signal can get from one end of a line to the other, through any number of ‘switches’ which have been designed primarily to ensure that acceptable voice quality can get from one end of the line to the other. The speed of modems is measured in ‘baud’ which is the number of bits4 per second, with around ten bits being required to represent a single character of text (or numeral or punctuation mark). Early 300 baud modems allowed the communication of less than half a line of type per second, but recent years have seen the growth of transmission speeds almost parallel the growth or other areas of computing capacities, to the point where 28.8 Kb (Kilobaud) modems are becoming the standard choice, but this speed is also approaching theoretical limits for transmission over a voice line.

At the other end of the scale, the need to use the vastly higher capacity of satellite and optical fibre transmission has forced the telecommunications industry to actually send voice signals over long distances by turning them into digital approximations, using the miracles of modern electronics to squeeze hundreds together down the same link or fibre, then unpacking and converting them back to analogue voice signals at a quality sufficient to satisfy the listener. In high speed communications, the preferred language has moved back from ‘baud’ to ‘bit’, and the capacity of optical fibres is now measured in gigabits per second. This means that for a home personal computer to communicate with an information service, messages are converted from digital at the computer to analogue for the ‘local loop’ between the computer and the telephone exchange, then back to a very different digital representation for transmission between exchanges, and all the way back again. It eventually became apparent that there had to be a better way.

The result was the development of Integrated Services Digital Network (ISDN) which uses the local loop to deliver 144Kb per line which is nominally shared between two 64Kb data lines and one 16Kb control signal line, with either of the data lines having more than enough capacity to carry digitised voice. ISDN has long been regarded as ‘a product in search of a market’ and subject to many derogatory reexpansions of the acronym, but during 1994 demand for non-specialised ISDN connections really started to pick up.5 Of particular interest to a latter part of this story, a number of 144Kb ISDN lines can be digitally combined into a 1.5Mb (Megabit) signal, called a ‘T1’ line, which the carriers use for their lowest level of data trunk line. The much celebrated prospect of ‘broadband’ services requires T1 or better connections into consumer premises.

When computers are close enough together, their communication has long bypassed any need for modems and telephone quality service, with local area network (LAN) technology delivering of order 10Mb communication capacity which is sharable by a number of computers, any of which can exchange messages with any other within the local subnet. The dominant LAN technology, Ethernet, was an early product of Xerox Corp’s Palo Alto Research Centre (PARC) which has been the source of several other important actors in this story. The connection of subnets and connection between LAN and ISDN communications requires specialised electronic equipment such as ‘bridges’ and ‘routers’.

The degree of integration of at least local communications is the most persistent difference between workstations and personal computers, and that difference is reflected in the operating system of choice for the respective systems. The Unix operating system has long enabled leading workstation supplier Sun Microsystems to proclaim that “the network is the computer.” With fast communications available, the question of which computer is doing the back-end processing becomes unimportant and the organisation of work more usefully divides between the client and ‘server’ processes. As we shall see, the networks have now been multiply joined so that an individual user can call on the services of servers anywhere on the network of networks. But first we will have a brief look at how an information service actually manages and presents its information so that you can use it.