Introduction To Line Telecommunications


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9. Simple Telephone Devices
  9. - Simple Telephone Devices 9.4 - Modems
  9.1 - Telephone Answering Machines 9.5 - Voicemail Systems
  9.2 - Cordless Telephones 9.6 - Specialist Devices
9.3 - Facsimile Machines

9.2 - Cordless Telephones

Together with the TAM the cordless telephone is the most widely used of the new type of simple telephone.

The cordless telephone has a base unit connected to the line and communicates with its handset via radio transmissions. Thus the user can move around freely when handling calls, provided they do not go beyond the transmission range of the base unit. The handset uses rechargeable batteries which require periodic recharging , unusually on the base unit. These batteries will require replacement at yearly intervals to maintain the best performance from the handset.

As developments are made the handsets have become smaller and speech quality improved, together with a move beginning from the original analogue system to digital methods.

Both types rely upon radio waves being able to carry information between the base and handset. Therefore the correct positioning of the base itself will have an effect upon the range or coverage that is available for the handset. The best locations are those central to the area in which the handset is needed to operate, away from obstacles , the elements and sources of interference.

9.2.1 - Analogue Cordless

The analogue cordless telephone has changed little in its basic operation since they were first introduced. The electronics have become more efficient allowing longer times away from the base station for charging, improved speech quality using signal processing and are more highly featured.

The transmission characteristics are regulated and define the operating range between the handset and base. Transmission from base to handset is a 1.7Mhz signal and from handset to base is 47Mhz at a power level of 6mV.

This signalling works best when used in direct line of sight with the base station as it is easily blocked by intervening obstacles and affected by the construction of these obstacles. It can also be affected by proximity to other electrical equipment such as televisions, computers, lighting and fridges which can reduce the range and quality of the speech across the radio link.

In ideal conditions the maximum range between base and handset will not usually exceed 100m and may be reduced considerably by the presence of the factors outlined above.

9.2.2 - Digital Cordless

Digital cordless telephones provide the features of analogue cordless telephones but with the improved performance of digital technology. These are reduced interference and improved speech quality.

These are achieved by digitising the speech, representing it as a sequence of 0’s and 1’s and transmitting these between base and handset. The digital data is used to reconstruct the speech at the receiving end. This overcomes all but the most severe interference from the transmission because the coding methods used allow the signal to be reconstructed to a true copy of the source by correcting the received signal before rebuilding the speech. Severe interference will block the signals in the same way as analogue cordless.

The overall range is the same as an analogue cordless, however the performance is improved. - CT2, DECT And The CAI

CT2 (Cordless Telephone 2), DECT (Digital Enhanced Cordless Telephone) and the CAI (Common Air Interface) are all operation technologies and specifications applied to the operation of digital cordless telephones over the years.

CT2 was the second-generation cordless telephone technology. It provides interference free cordless operation, even inside buildings making it useful when connected to PBXs. Many CT1, first generation (analogue) cordless telephones, could not function well inside large buildings and were more adversely affected by the interference from the building structure.

An extension to CT2 was the CAI or Common Air Interface. This was a standardised system which allows multiple handsets from different manufacturers to operate via the same base station. This was envisaged to be the path to cheap, mobile communications at home, the office and out and about with users having a single handset which would be able to use base stations in its immediate vicinity if the appropriate access rights were available. This means that at home the owners base is used and calls are charged against their account in the usual way. At the office the same handset automatically becomes an extension of the employers PBX with all the relevant functionality and calls being charged to the employers account. When out of the office it would log onto public base stations to allow calls to be made and billed to a customer account in a similar way to cellular telephones. This mobile network was trialed in the UK but failed due to lack of subscribers because user could only make and not receive calls. Subsequent cellular network developments brought the more functional ETACS (Extended Total Access Communication System) and now the GSM cellular systems within the reach of non-business users.

DECT is a Digital Enhanced Cordless Telephone standard which is intended to provide mobile access to cellular services throughout the world if it is adopted by the appropriate authorities. It is superseding the older systems in use today and has similarities to GSM in its method of working. The handsets will share many features of CT2 CAI telephones, being usable at home and work, though early plans to allow roaming to mobile networks have not yet materialised. Though a full realisation is some years away at this time DECT equipment is currently sold for PBX systems and domestic use by several manufacturers.




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Introduction To Line Telecommunications
Copyright Panasonic Business Systems UK Ltd 2000