Cordless Phones

Cordless Phones

Cordless telephones first appeared in the late 1970’s and have experienced spectacular growth ever since.
Roughly half of the phones in U.S. homes today are cordless. Cordless phones were originally designed
to provide a low-cost low-mobility wireless connection to the PSTN, i.e. a short wireless link to replace
the cord connecting a telephone base unit and its handset. Since cordless phones compete with wired
handsets, their voice quality must be similar: initial cordless phones had poor voice quality and were
quickly discarded by users. The first cordless systems allowed only one phone handset to connect to each
base unit, and coverage was limited to a few rooms of a house or office. This is still the main premise
behind cordless telephones in the U.S. today, although these phones now use digital technology instead
of analog. In Europe and the Far East digital cordless phone systems have evolved to provide coverage
over much wider areas, both in and away from home, and are similar in many ways to today’s cellular
telephone systems.
Digital cordless phone systems in the U.S. today consist of a wireless handset connected to a single
base unit which in turn is connected to the PSTN. These cordless phones impose no added complexity
on the telephone network, since the cordless base unit acts just like a wireline telephone for networking
purposes. The movement of these cordless handsets is extremely limited: a handset must remain within
range of its base unit. There is no coordination with other cordless phone systems, so a high density of
these systems in a small area, e.g. an apartment building, can result in significant interference between
systems. For this reason cordless phones today have multiple voice channels and scan between these
channels to find the one with minimal interference. Spread spectrum cordless phones have also been
introduced to reduce interference from other systems and narrowband interference.
In Europe and the Far East the second generation of digital cordless phones (CT-2, for cordless
telephone, second generation) have an extended range of use beyond a single residence or office. Within
a home these systems operate as conventional cordless phones. To extend the range beyond the home
base stations, also called phone-points or telepoints, are mounted in places where people congregate, like
shopping malls, busy streets, train stations, and airports. Cordless phones registered with the telepoint
provider can place calls whenever they are in range of a telepoint. Calls cannot be received from the
telepoint since the network has no routing support for mobile users, although some newer CT-2 handsets
have built-in pagers to compensate for this deficiency. These systems also do not handoff calls if a user
moves between different telepoints, so a user must remain within range of the telepoint where his call
was initiated for the duration of the call. Telepoint service was introduced twice in the United Kingdom
and failed both times, but these systems grew rapidly in Hong Kong and Singapore through the mid
1990’s. This rapid growth deteriorated quickly after the first few years, as cellular phone operators cut
prices to compete with telepoint service. The main complaint about telepoint service was the incomplete
radio coverage and lack of handoff. Since cellular systems avoid these problems, as long as prices were
competitive there was little reason for people to use telepoint services. Most of these services have now
disappeared.
Another evolution of the cordless telephone designed primarily for office buildings is the European
DECT system. The main function of DECT is to provide local mobility support for users in an in-building
private branch exchange (PBX). In DECT systems base units are mounted throughout a building, and
each base station is attached through a controller to the PBX of the building. Handsets communicate to
the nearest base station in the building, and calls are handed off as a user walks between base stations.
DECT can also ring handsets from the closest base station. The DECT standard also supports telepoint
services, although this application has not received much attention, probably due to the failure of CT-2
services. There are currently around 7 million DECT users in Europe, but the standard has not yet
spread to other countries.
The most recent advance in cordless telephone system design is the Personal Handyphone System
(PHS) in Japan. The PHS system is quite similar to a cellular system, with widespread base station
deployment supporting handoff and call routing between base stations. With these capabilities PHS does
not suffer from the main limitations of the CT-2 system. Initially PHS systems enjoyed one of the fastest
growth rates ever for a new technology. In 1997, two years after its introduction, PHS subscribers peaked
at about 7 million users, and has declined slightly since then due mainly to sharp price cutting by cellular
providers. The main difference between a PHS system and a cellular system is that PHS cannot support
call handoff at vehicle speeds. This deficiency is mainly due to the dynamic channel allocation procedure
used in PHS. Dynamic channel allocation greatly increases the number of handsets that can be serviced
by a single base station, thereby lowering the system cost, but it also complicates the handoff procedure.
It is too soon to tell if PHS systems will go the same route as CT-2. However, it is clear from the recent
history of cordless phone systems that to extend the range of these systems beyond the home requires
either the same functionality as cellular systems or a significantly reduced cost.