Carrying Bits

Carrying Bits

Because the computer industry was “born” digital, it was the first to be confronted with the problem of “mapping” digital data onto analogue carriers, i.e. storing bits on intrinsically analogue devices. One of the first solutions – storage of bits on paper tape – was limited to small quantities of data, typically programs, but magnetic technologies were more promising because one could use tapes, drums or disks to store data with vastly improved capacity. 

Magnetic tapes for sound recording had already achieved a considerable degree of maturity, as they had been in existence for some time. The difference was that sound was analogue and already band-limited so that a suitable transducer could convert a current or a voltage directly into a magnetic field and vice versa, while binary data from computers have a theoretically infinite bandwidth. The obvious solution was to “modulate”, i.e. pre-filter, the binary data so as to minimise the interference between the binary data entering the storage device, caused by their “infinite” bandwidth. Further, to enable the identification and correct writing/reading of the data, the information had to be “formatted”, i.e. had to be given a precise structure, in very much the same way as a text is formatted in lines, paragraphs and pages. 

Not long after the computer industry had been confronted with the problem of storing digital data on analogue media, the telecommunication industry was confronted with the similar need of “sending” digital data through the analogue transmission medium called telephone cable. One example is provided by the elements of the digital transmission hierarchy where the equivalent of the magnetic disk or tape formatting is the “frame”. The primary A-law based multiplexer has a frame of 256 bits (32 time slots each of 8 bits), where TS 0 has a fixed pattern so that it can act as a “start code” for a receiving device to know where to look in order to get the start of the frame (of course there is no guarantee that this code word cannot be emulated by unconstrained telephone data because it is possible that in a frame one speech sample has exactly that value). Higher-order multiplexers are organised in a similar manner, in the case of the European hierarchy, by multiplexing 4 lower-order streams. 

The COST 211 project mentioned above did not just develop the video coding part but provided the specification of the complete transmission system for videoconference applications. In the COST 211 system TS 1 carries speech, TS 17 optionally carries a digital facsimile signal and TS 18 optionally carries other data. Several additional types of information have to be conveyed from one terminal to the other, such as information on whether TS 17 and TS 18 contain video data or facsimile and other data. 

Transmitting digital data over telephone lines is conceptually the same as storing data on a local storage device, but in general the “modulation” schemes have to be much more sophisticated because the telephone line is a bandwidth-limited transmission system with a nominal bandwidth of 4 kHz (actually the transmitted speech signal has significant energy only in the 300 to 3,400 Hz band) with unpredictable characteristics caused by extremely variable operating conditions and by the long span of time telephone cable have been deployed, while magnetic tapes and disks have better-defined characteristics thanks to well-monitored manufacturing processes and more predictable operating conditions. 

The initial modulation schemes supported a low bitrate transmission of 300 baud (“bit/s” is also called “baud”, frorm Émile Baudot, the inventor of the telegraphy code) but later “adaptive” schemes were developed that automatically adapted their performance to the characteristics of the line and higher bitrates became progressively possible. More and more sophisticated schemes were developed and the bitrate climbed to several kbit/s but always as a multiple of 300. A set of widely used ITU-T recommendations started making it possible for to a new generation of nomadic users to connect from anywhere to anywhere else in the world over distances of possibly thousands of kilometres at values as high as 56 kbit/s depending on the end-to-end link “quality”. 

An ambitious goal that the telco industry set to itself in the late 1960s was the development of ISDN. The plan was to provide telephone subscribers with two 64 kbit/s channels (so-called B-channels) and one 16 kbit/s of signalling (so-called D-channel) for a total of 144 kbit/s (so-called 2B+D). With the usual schizophrenia of the telco business, ISDN is not fully defined in all its parts. In particular the modulation scheme to be used in the local access was left to each telco. Assuming that users are static (a reasonable assumption of that time) this is not an unreasonable assumption, but it is one that has prevented the later availability of ISDN connectors in laptops, the main reason why eventually ISDN did not fly, not even in countries where significant levels of deployment had been achieved. 

At the end of the 1980s, while the ISDN standardisation project was drawing to a close, some telco R&D laboratories showed the first results of what should have been great news for companies whose assets were buried underground in the form of millions of kilometres of telephone cable. The technology was called Asymmetric Digital Subscriber Line (ADSL), which would allow downstream (central office to subscriber) transmission of “high” bitrate data, e.g. 1.5 or 2 Mbit/s, with a lower-rate upstream transmission, e.g. 64 or 128 kbit/s from the subscriber terminal to the central office. 

One instance of this technique uses a large number of carriers that are placed in appropriate parts of the spectrum after an initial phase where the transmitter checks the state of the line by interacting with the receiver. This type of research work was generally ostracised within the telcos because it provided an alternative and competing solution, in terms of cost certainly not of performance, to the old telco dream of rewiring all their subscribers with optical fibres for some yet-unknown-but-soon-to-come pervasive “broadband” applications. Today ADSL provides “asymmetric” access (typically 5-10 times more bit/s downstream than upstream) to hundreds millions of subscribers around the world at increasing bitrates and is playing a major role in allowing fixed telephony service providers to survive.

If one sees the constant progress that magnetic disks are making in terms of storage capacity in one year and the snail-like progress of ADSL in the last 20 years, one could be led to think that the telco industry is simply not trying hard enough. While there is some truth in this statement ;-), one should not forget the fact that while manufacturing of hard disks happens in clean rooms, the local telephone access has to deal with a decade-old infrastructure deployed with wires of varying quality, different installation skills and unpredictable operating conditions. It is definitely not a fair comparison. 

If a comparison has to be made, it is with optical fibers used for long-distance transmission. In this case it is easy to see how the rate of increase in bitrates is even higher than the rate of increase in hard disk capacity. Again this is possible because optical fibres are a new technology and fibres are probably manufactured with as much care and using equipment as sophisticated and expensive as those used in high-capacity magnetic disk manufacturing. The problem is that, of the long distance fibres that were deployed in years of collective madness at the end of 1990s, only a few percent is actually lit, a fact that is clearly not disconnected from the slow introduction of broadband in the local loop. This underutilisation also shows the difference between the concrete advantage felt by a consumer buying a hard disk today with twice the capacity with the same or lower price compared to last year, versus financial decisions made by a telco executive based on expectations of long-distance traffic that depend on the coming of some gainful “broadband application” Messiah. 

The last physical delivery system considered in this list is the coaxial cable used for Community Antenna Television (CATV), a delivery infrastructure originally intended for distribution of analogue television signals. For this the widely chosen modulation system is Quadrature Amplitude Modulation (QAM).  The CATV industry has made great efforts to “digitise” the cable in order to be able to provide digital intercative services. The Data Over Cable Service Interface Specification (DOCSIS), now an ITU-T standard, provides high bitrates to CATV subscribers.

Another transmission medium rivaling the complexity of the local access telephone line is the VHF/UHF band used for television broadcasting on the terrestrial network. Already in the 1980s, several laboratories, especially in Europe, were carrying out studies to develop modulation methods that would enable transmission of digitised audio and television signals in the VHF/UHF bands. They came to the conclusion that such frequencies in typical conditions could carry between 1 and 4 bit/s per Hz depending on operating conditions. The lower the bitrate, the higher the terminal mobility and highest bitrates for fixed terminals. 

The modulation scheme selected in Europe and other parts of the world to digitise the VHF/UHF frequency bands is called Coded Orthogonal Frequency Division Multiplexing (COFDM). This injects a large number of carriers – up to several thousands, each carrying a small bitrate. It is a technology similar to ADSL, with the difference that in broadcasting no return channel is available to adapt the modulation scheme to the channel. In the USA a different system called 8 Vestigial Side Band (8VSB) – a single-carrier modulation system – was selected for digital terrestrial broadcasting. 

For satellite broadcasting the typical modulation scheme is Quadrature Phase Shift Keying (QPSK), a modulation system where the carrier’s phase is shifted in 90° increments. 

Digital cellular phone systems have been widely deployed in several countries. The modulation system used for Global System for Mobile (GSM) is Time Division Multiple Access (TDMA). TDMA is a multiple access technique where the access to the channel is based on time slots – like those used in digital telephony multiplexers – corresponding to digital channels, usually of a fixed bitrate. 

The 3rd generation (3G) mobile communication system is based on Code Division Multiple Access (CDMA), of which several incompatible flavours already exist (CDMA is used in some 2nd generation digital mobile telecommunication systems). This is a specialisation of a more general form of wireless communication called Spread Spectrum used in multiple-access communications, where independent users share a common channel without an external synchronisation. In this communication form the bitstream is spread throughout the available bandwidth using a periodic binary sequence, called Pseudo random Noise (PN) sequence. Because of this information scrambling, the bitstream appears as wide band noise. The receiver uses the same PN sequence as the transmitter to recover the transmitted signal and any narrow band noise is spread to a wide band signal.