Course Handout - Some Important Telecoms Concepts

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First published online 09:59 GMT 19th January 2000, Copyright Derek J. Smith (Chartered Engineer). This version [HT.1 - transfer of copyright] dated 18:00 14th January 2010


This material originally appeared in Chunking and Cognitive Efficiency (copyright Ó 1997, University of Wales Institute, Cardiff; ISBN 1900666065). It appears here significantly updated and relaid for electronic presentation.


Several important telecommunications concepts are defined in the key concept boxes below. They were developed long ago in the age of wire-based telecommunications, and sensu stricto have long since been left behind by the age of satellites and computers; nevertheless, the underlying concepts retain immense illustrative value, especially for those investigating the principles of biological processing hierarchies.


NB1. All communication is expensive, genetically so in the case of biological communication and financially so in the case of technologically aided biological communication. The financial costs may conveniently be divided into transmitting costs (because you need a transmitter of specified sophistication), carrier costs (because you need wires, or equivalent), and receiving costs (because you need a receiver of specified sophistication). And - needless to say - the more you want out of a telecommunication system, the more each of these three aspects is going to cost you. Full human conversation makes the largest number of demands, and therefore costs the most.

NB2. The need for feedback is everywhere in all communication, biological or otherwise. However, it is vitally important to distinguish between feedback from a distant station and the replies from the distant party. The former fulfils a technical requirement, whilst the latter are part of the ongoing conversation. It follows that feedback does not need to be relayed to the local party unless and until things start to go wrong with the line, whilst the latter always needs to be relayed. [Readers who are confused at this distinction need only to ask themselves how their telephones know what to do when the number they have called turns out to be engaged - you are "talking" to the distant station, sure enough, but not to the person you rang! Similarly, it is better to get a dialling tone when the remote party hangs up on you, rather than spend the next 20 seconds talking to thin air!]

NB3. Feedback works significantly better if it can come back along a different pathway to the outgoing message, meaning that an ostensibly one-way transmission will often require two physical links to be in place. This is how it works .....


Key Concept - How Many Wires?

Simplex Telecommunication: A single wire can be used to send information in either direction between two points, but only if both ends are rigged to act interchangeably as transmitter and receiver, and even then not simultaneously. Since interchangeability is expensive, single wire systems are usually reserved for one-way communication. This means that there can be no line management feedback, of course, and this means in turn that performance will be poor whenever transmission difficulties are encountered. Somewhat counter-intuitively, then, it is usually better to use two wires when sending information in one direction. The forward wire is used to carry the substantive information flow from the transmitting station, and the return wire is used to carry the counterflow of line management feedback from the receiving station. Here are some examples:

SPECIMEN ONE-WIRE ONE-WAY SIMPLEX SYSTEMS = simple key-to-buzzer telegraph; simple cordpull-to-bell chambermaid's telegraph, etc.

SPECIMEN ONE-WIRE TWO-WAY SIMPLEX SYSTEM = children's "cup and string" system.

SPECIMEN TWO-WIRE ONE-WAY SIMPLEX SYSTEM = engine-room telegraph (orders always go downwards only, but are nevertheless always explicitly acknowledged).

For the missing entry, the TWO-WIRE TWO-WAY system, see the half duplex panel below.


NB4. Due to the cost of laying and maintaining their cables, the telegraph and telephone companies were under pressure from the outset to reuse their resources to the absolute limit. One of the earliest tricks of the trade - and nowadays the mainstay of the entire global telecommunications industry - was that of making one wire behave as if it were many. Here are the key concepts .....


Key Concepts - Duplexing and Multiplexing

Duplex (1): In standard English, this simply means double or two-fold, as with poets who happen also to be critics, or lamps with two wicks (Oxford English Dictionary; earliest instance dated 1817). The word was then borrowed by early telecommunications ..... 

Duplex (2): In its technical sense, duplexing means sending two messages down a single wire (Oxford English Dictionary) by making it somehow think it is two wires. Nowadays, there are many ways to do this, but they all boil down either to time sharing or some form of selectively "filterable" encoding. The facility is expensive in terms of transmitter and receiver complexity, but it doubles the number of paying subscribers on a given cable. The practice is far from new, with Thomas Edison, for example, taking out several patents for duplexing techniques as long ago as the 1870s. And do you need to stop at only two wires? No, because duplexing is merely the simplest instance of multiplexing .....

Multiplex: ..... which is where a single wire is made to think it is many wires, thus enabling the number of subscribers for a given network investment to be similarly multiplied.

Communication Channel: The whole point about duplexing or multiplexing is that you can now no longer tell by simple inspection how many concurrent conversations a particular wire might be carrying. As soon as the practice became popular, therefore, it became safer to refer instead to communication channels in the abstract, rather than bother specifying the precise physical resources. The classic work here was Shannon and Weaver's (1948) mathematical analysis of signal processing through a noisy medium. 

Antidromic: "Flowing back towards the source", and thus the opposite of orthodromic. The term can be applied to information feedback in its many senses (especially if the feedback has been duplexed back up the wire of original transmission), but has also been applied to neuronal activity, where it denotes electrical excitation applied at the distal end of an axon and propagating "up the down escalator" towards the soma. One of the classic works here was Woolsey and Chang (1947), and for a typical modern application see Lee Campbell's work at the Salk Institute.

Woolsey, C.N. & Chang, H.T. (1947). Activation of the cerebral cortex by antidromic volleys in the pyramidal tract. Research Publications of the Association for Research in Nervous and Mental Disease, 27:146-161.


NB5. We are now ready to allow conversational turn-taking into the system ..... 


Key Concept - Half-Duplex Telecommunications:

Half Duplex: This is where two wires are used to set up a channel capable of sending information in both directions, but not simultaneously. As with the simplex set-up, the backchannel is used to carry the counterflow of line management data from the receiving station. Now, however, the receiving station is free in due course to take over as transmitting station, and to reply conversationally - all that needs to happen is for the lines to be "turned around", that is to say, for both transmission directions to be reversed. The processing overheads under this sort of set up are larger than for the simplex set up, due to the need for both stations to be equipped with both transmitting and receiving functionality.

SPECIMEN SYSTEMS = radio-telephone / Citizen Band (where the right to transmit alternates between users, typically by concluding each utterance with the tag "over"); e-mail system (where the system keeps you reasonably confident that your message has arrived at the distant station, but where nothing less than an explicit conversational reply can give you 100% certainty that it has eventually been attended to).


NB6. There remains one major problem - natural human communication is full of interruptions, interjections, contradictions, and generally sloppy turn-taking. Both parties need to be able to shout simultaneously, and they cannot do this on a half duplex system. In fact, they both need half duplex systems permanently at their disposal, which gives us .....


Key Concepts - Full-Duplex Telecommunications:

Full Duplex: This is where four wires are used to set up a channel capable of sending information in both directions fully simultaneously. Unlike the half duplex set-up, there is no longer any need for the lines to be turned around before the receiving station can transmit its reply. This gives full biological functionality, that is to say, communication which is characterised by the two parties being able to interrupt each other at will (except when routed via satellite, when the occasional half second delay still plays havoc with the fluency of your conversation).

SPECIMEN SYSTEM: The telephone.

Note: For further illustration of the difference between half duplex and full duplex linking, see the recent IBM White Paper on switched ethernet LANs. For an example of four wire communication in a conventional telecommunications network, see the website of Applied Signal Technology, Inc., of Sunnyvale, California, where you will find a paper by Treichler, Larimore, and Johnson (1999/2003 online). This includes two very helpful diagrams (specifically, Figure 1 and Figure 8) to show how "four wire trunks" are used in connecting geographically separate computer CPUs. For a loosely comparable example of four-wire communication in a biological processing network, see our précis of Coltheart, Curtis, Atkins, and Haller (1993).


NB7. The required transmitting and receiver functionality is "layered", as was described in the Open Systems Interconnection (OSI) Model discussed in the main text. The operation of the lower layers is "transparent" to the end users of the system, except - as has already been noted - when technical problems occur. When that does happen, the lower layers can send what are known as interrupts up their respective processing hierarchies, ultimately and if necessary to the humans at the top of them.

NB8. It is also common telecoms practice (a) to provide for signal boosting at intermediate "repeater" stations, and (b) to route messages flexibly around the network according to where there happens to be spare capacity at the moment in question. To keep costs down, only lower layer line management functionality is provided at these repeater stations and network nodes.

NB9. What we actually have in human communication is a complex [psychology - {technology} - psychology] link, wherein the parenthesised {technology} can range in complexity from nothing (face-to-face speech) to full duplex telephony. And the thrust of 30 years of intensive psycholinguistic modeling is that it is not just the technology which has processing layers and stages, but the psychologies at either end of it as well .....


Key Concept - Biological Telecommunications

Minds are frequently described as hierarchically organised modular processors, and the literature has several good explanatory diagrams on offer. However, in their haste to get at the psychological substantives, these models frequently overlook the supporting technicalities. They show the obvious information flows - word recognition processes, perhaps - but say very little about the control logic which has to be in place for the main flows to take place. This is unfortunate, because it is here - in amongst all the biological simplex and half duplex circuitry, feedback circuits, repeater stations, and interrupts - that the true complexity of the cognitive system undoubtedly lies. Feedback, for instance, is nothing less than a theoretician's nightmare, for it may travel by a variety of routes, long and short, conscious and unconscious, direct and indirect, and possibly - using antidromic signalling - even duplexed back up the fibre tract the original message arrived down. It is precisely because mapping these ancillary information flows is so difficult, that telecommunications concepts are so important to theoretical psycholinguistics.