Digital Cellular Technology Offers Many Advantages
Sydney Morning Herald
Monday January 27, 1992
MOBILE communications, for most telephone users, means cellular phones. The trouble with cellular telephones is that they rely on base stations, and these have only a short range - typically 30 km at best - and require a considerable investment in the network itself. No base station, no normal cellular communication.
Base stations, the transmitters and the aerials that allow mobile phones to be used, are an expensive hobby.
This explains why cellular phone calls in the US, Europe or Australia (to a much lesser extent) cost so much. Fortunately for us all, a new technology is on its way - digital cellular phones.
Digital cellular phone technology uses existing radio bandwidths for phone calls but, instead of transmitting a voice channel in its entirety, the audio signal is encoded into digital data and compressed at each end of the radio link.
Ordinary voice signals are known as analogue signals - they occupy a finite area of the radio spectrum, with transmission varying due to sound frequency and volume. Most of the time, in fact, an analogue signal consists of silence- most telephone conversations take place in one direction at a time, the other person listening to what is being said. Added to that, humans have this bad habit of breaking words up with pauses, and often pause for breath and/or effect.
All this radio silence amounts to more than two-thirds of a typical telephone call. Digital cellular allows the radio bandwidth to be used much more efficiently - three digital cellular calls can be squeezed into the bandwidth of one analogue call.
For cellular phone users in the cities, where bandwidth is scarce, this will come as good news. As an existing cellular call is passed from radio base station to radio base station, such as when a car is moving, the network has to carry out complex call handover procedures.
Added to that, radio bandwidth must be available in both transmitter areas(known as cells in cellular telephone terminology). If a call channel and/or signalling channel is not available in the new cell, then the call cannot be handed over. For the user, this means that the call is dropped - the line goes dead. (In Australia, if a cell cannot accept the call, a search is made in neighbouring cells to take it over.)
Digital cellular means that three times as many calls can be processed by a given base station. Added to that, because each call consists of digital data, with automatic error-correction, very poor signalling paths can be supported.
In the real world this means that, as a mobile phone user passes into a new cell, if there are no available radio channels, then the network can instruct an adjacent base station to begin transmitting and receiving data from the mobile phone.
Existing analogue cellular networks are capable of doing this but, due to the problems of radio signal transmissions in city areas, this facility is rarely available.
Digital cellular, meanwhile, can easily accommodate transmission distances of two or three times that available with analogue cellular - digital signals with error-correction can tolerate very noisy radio channels.
Digital cellular technology has the added advantage that, apart from changing the transmission and reception equipment and, optionally, the radio aerials, it can use the existing cellular network for call switching. Subscriber equipment needs to be replaced, but it is possible to run an analogue system alongside a digital cellular network
In Britain, where plans are well advanced for digital cellular (also true in Australia), a limited digital network - known as Groupe Speciale Mobile(GSM) - is in place and similar networks are operational in several European countries.
Since current analogue cellular phone technology dates back about five years, the mobile phone designers have redesigned the way the digital cellular phones work. Each phone on the GSM network has an ID module - about the same size as a credit card - that easily plugs in and out of a GSM phone.
Instead of the GSM phones being allocated a number, the ID module is the"holder" of the number. So, as the subscriber moves from mobile phone to mobile phone, the ID module can be slotted into the appropriate phone - that phone, while the ID module is fitted, makes and receives calls on the GSM network for that number.
GSM is so sophisticated, in fact, that the digital data transmissions can be routed over existing packet data networks to other countries' GSM networks. So, for example, a British GSM subscriber can slot an ID module into a GSM mobile phone in most other European countries.
The European GSM network has been designed so that each country's network has a record of which "foreign" GSM subscribers are logged into its network at any given time. If an incoming call is registered on the subscriber's home country network, the call can quickly be switched to another country's GSM network, where the phone is located. Because the call set-up is all-digital, the transmission delay is less than a second.
Because GSM is digital, it's relatively easy to interface a computer directly to the network - there is no difference between digital data originating from a computer and a digitally encoded voice call. Data can be fed into and out of the GSM network at up to 19,200 bits per second - the rate at which the network processes digital call data.
It is likely, then, that GSM data modems will become commonplace in portable computers in the near future.
Currently, digital cellular telephone network technology is operational only in Europe, but similar services could be operational in Australia by next year.
Because the calls are all-digital, it's likely that charges for network usage will be tailored to fit subscribers' needs. A mobile portable computer user may not need to pay a subscription, but charges based on the actual usage of the network.
This opens up the possibility of portable computers coming ready-equipped with a digital cellular transmission unit already installed, or available as a simple-to-fit optional extra. Suddenly, mobile data communications moves out of the high-tech arena and into general usage. This has to be good news for everyone.
© 1992 Sydney Morning Herald
