Cordless Telecommunications

The familiar cordless telephone, introduced in the early 1980s, has become a key factor in reshaping voice communications. Since people cannot be tied to their desks, as much as 70 percent of business calls do not reach the right person on the first attempt. This situation has seen dramatic improvement with cordless technology, which makes phones as mobile as their users. Now almost 30 percent of business calls reach the right person on the first attempt.

Cordless versus Cellular

Although cellular phones and cordless phones are both wireless, they have come to assume quite distinct and separate applications based on their areas of use and the differing technologies developed to meet user requirements. Cellular and cordless are implemented with their own standardsbased technologies.

Briefly, cellular telephones are intended for off-site use. The systems are designed for a relatively low density of users. In this environment, macrocellular technology provides wide area coverage and the ability to make calls while traveling at high speeds.

Cordless telephones, on the other hand, are designed for users whose movements are within a well-defined area, such as an office building. The cordless user makes calls from a portable handset linked by radio signals to a fixed base station. The base station is connected either directly or indirectly to the public network.

Cordless Standards

The cordless system standards are referred to as CT0, CT1, CT2, CT3, and DECT, with “CT” standing for Cordless Telecommunications. CT0 and CT1 were the technologies for first-generation analog cordless telephones. Comprising base station, charger, and handset and intended primarily for residential use, they had a range of 100 to 200 meters.

They used analog radio transmission on two separate channels, one to transmit and one to receive. The potential disadvantage of CT0 and CT1 systems is that the limited number of frequencies can result in interference between handsets, even with the relatively low density of residential subscribers. Also targeted at the residential user, CT2 represented an improved version of CT0 and CT1.

Using Frequency Division Multiple Access (FDMA), the CT2 system splits the available bandwidth into radio channels in the assigned frequency domain. In the initial call setup, the handset scans the available channels and locks onto an unoccupied channel for the duration of the call. Using Time Division Duplexing (TDD), the call is split into time blocks that alternate between transmitting and receiving.

The Digital Enhanced Cordless Telecommunications (DECT) standard started as a European standard for cordless communications, with applications that included residential telephones and wireless Private Branch Exchange (PBX) and wireless local loop (WLL) access to the public network. Primarily, DECT was designed to solve the problem of providing cordless telephones in high-density, high-traffic office and other business environments.

CT3, on the other hand, is a technology developed by Ericsson in advance of the final agreement on the DECT standard and is designed specifically for the wireless PBX application. Since DECT is essentially based on CT3 technology, the two standards are very similar. Both enable the user to make and receive calls when within the range of a base station.

Depending on the specific operating conditions, this amounts to a distance of between 164 feet (50 meters) and 820 feet (250 meters) from the base station. To provide service throughout the site, multiple base stations are set up to create a picocellular network. Signal handoff between the cells is supported by one or more radio exchange units, which are ultimately connected to the host PBX.

Both DECT and CT3 have been designed to cope with the highest-density telephone environments, such as city office districts, where user densities can reach 50,000 per block. A feature called Continuous Dynamic Channel Selection (CDCS) ensures seamless handoff between cells, which is particularly important in a picocellular environment where several handoffs may be necessary, even during a short call.

The digital radio links are encrypted to provide absolute call privacy. The two standards, DECT and CT3, are based on multicarrier Time Division Multiple Access/Time Division Duplexing (TMDA/TDD). They do not use the same operating frequencies, though, and consequently have different overall bit rates and call-carrying capacity.

It is the difference in frequencies that governs the commercial availability of DECT and CT3 around the world. Europe is committed to implementing the DECT standard within the frequency range of 1.8 to 1.9 GHz. Other countries, however, have made frequencies in the 800- to 1000- MHz band available for wireless PBXs, thereby paving the way for the introduction of CT3.

Many of the problems arising from the nonavailability of staff to a wired PBX can be avoided with cordless telephones. They are ideal for people who by the very nature of their work can be difficult to locate (e.g., maintenance engineers, warehouse staff, messengers, etc.) and for places on a company’s premises that cannot be effectively covered by a wired PBX (e.g., warehouses, factories, refineries, exhibition halls, dispatch points, etc.).

A key advantage of cordless telecommunications is that it can simply be integrated into the corporate telecom system with add-on products and without the need to replace existing equipment. Another advantage of cordless telecommunications is that the amount of telephone wiring is dramatically reduced.

Since companies typically spend between 10 and 20 percent of the original cost of their PBX on wiring the system, the use of cordless technology can have a significant impact on costs. There is also considerable benefit in terms of administration. For example, when moving offices, employees need not change extension numbers, and the PBX does not have to be reprogrammed to reflect the change.