Local Loop

This telephony term describes the connection between the subscriber and the exchange. For most of the past 100 years, the connection has been copper twisted pair buried in the pavement or distributed overhead on poles. The investment in enormous. Now, however, convergence between voice, computer and television applications means the demarcations are being eroded and there is increasing competition for the delivery of services.

The alternative systems, their applications and limitations are listed below:

Copper

For most of the history of fixed line telephony, the bandwidth that copper provided was some 3kHz, limited by analogue techniques and designed to be the cheapest solution that the telecomms operator could get away with. However, the twisted pair is inherently capable of much higher bandwidths and over short distances can carry video or broadband data. The existing infrastructure is now being used to carry data at 56kb/sec and, using ISDN (Integrated Services Digital Network) can be extended to 128kb/sec. ISDN is now seen as an interim technology and there is massive development being devoted to Digital Subscriber Line (DSL) technology, which holds the promise of increasing the bandwidth of the copper local loop to several megabits/sec., bringing the prospect of delivering video on demand (VOD) services.

Over the years, a number of trials have been carried out, in Ipswich, Cambridge and, most recently, North London but it is only now that DSL technology is becoming viable. BT are frustrating the industry by their unwillingness to set a timetable for the rollout of DSL technology but it stems from the simple fact that the longer they wait, the cheaper it will be to install.

DSL is not a solution without problems. It comes in a number of variants collectively known of as xDSL. ADSL (Asynchronous DSL) is the most likely to see largescale implementation, giving greater bandwidth to the forward path to the home and less to the return path. The bandwidth of the forward path is limited by crosstalk at longer distances and one of the most difficult compromises for BT is the chosen datarate and the corresponding percentage of existing customers who will be served without the need for curbside repeaters. With a data bandwidth of 4Mb/sec., maybe one third of existing voice subscribers will not be able to receive without expensive improvements to the local loop.

Cable

Over the last ten years, new cable companies have invested massively in alternative connections to the home. There are some different technologies in use but the majority of these have fibre optic cable to the curbside cabinet and coax from there to the home. These are clearly capable of much higher bandwidths but also have limitations. In most cases the network was installed to deliver television to the home and was designed on the basis of broadcast TV services. Although the fibre to the cabinet is broadband, it may have limitations on the number of VOD services it can carry simultaneously, which may require the number of homes served by each cabinet to be reduced in the future if VOD becomes prevalent. ADSL, being grown on a telephony foundation, does not have the same problem.

Wireless Technologies

It is worth bearing in mind that it is possible that other technologies may become competitive in some circumstances. MMDS (Multipoint Microwave Distribution System) could provide a most effective local loop, particularly in suburban and less densely populated areas. The technology is maturing and is being successfully trialed but implementation awaits, amongst other things, Radio Agency licencing of the appropriate spectrum at 40GHz. GSM mobile phone systems could provide an effective slow speed (9.6kb/s) return path and the next generation, UMTS (Universal Mobile Telecommunications System) offers the promise of data transmission at speeds of up to 2Mb/sec. In practice, however, its use is likely to be limited by capacity.

Whilst the use of GSM for return path applications has the significant advantage of 'piggy-backing' on an extremely successful system, it must be remembered that there are specific FWA (fixed wireless access) systems which may have application in some situations. In developing countries, areas without an existing copper infrastructure may prove suitable for FWA installations in cells of 5km radius.

For television return path applications, it is also possible that a return path to the satellite or terrestrial transmitter could be established via the receive antenna. The Irish DTT rollout is planned on the basis of implementing interactive applications using an INTERACT system.