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The feature focuses on the background to DECT/GSM dual
mode telephony and in particular describes the new Onephone service introduced in
May 1999 by BT / BT Cellnet in the UK.
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DECT/GSM DUAL MODE
and the advent of the
DECT has come a long way since the acronym was first coined by ETSI in January 1988 as a pragmatic
compromise in the then battle between the UK's CT2 and Sweden's DCT proposals.
Coming from the ETSI stable, and originating at a time when GSM
standardisation was well in hand, the overlap of people and companies between DECT and GSM
is perhaps not surprising. What is perhaps surprising is that, despite this,
it has taken until 1999 for the first commercial DECT/GSM dual mode service to be
introduced anywhere in the world - the Onephone
service, launched last month, May 1999, in the UK.
In this feature we briefly survey some of the history
around DECT/GSM dual mode, we describe the new
offering and offer a few thoughts on where the future of such dual mode products and
services may lie.
DECT and Public Access
DECT radio access technology from its outset was conceived as a very versatile
but relatively short-range radio access technology. Although clearly capable of
supporting longer range usage, as evidenced by its success in (fixed) wireless local loop
applications, cordless technologies have had a chequered history in public access (mobile)
Mobile access (Telepoint) systems based on CT2 launched in the UK in the
early 90's failed to find commercial success as soon as the pricing of digital cellular
services declined to consumer levels with the advent of personal communications networks
based on GSM. The same story was repeated in France, after the initial success of the
BiBop service, and subsequently in Hong
Kong, in the mid-90s, despite early views that differing environmental factors in
these countries would result in a different outcome. More recently, despite
consumer acceptance of the FIDO service in Italy,
regulatory factors seem to have stifled local mobility based on DECT in a market which,
again, is seeing remarkable growth of its mobile market.
DECT and GSM are quite complementary radio access technologies, as
illustrated in the table below:
|Range - Cell Size
||Large cells - typically 1-30 km
||Small cells - typically 1-500m
|Traffic per Subscriber
|Low - eg 9.6 kb/s per slot
(but higher with GPRS and HSCSD coming)
|High - eg 28.8kb/s per slot
(but 500kb/s multislot, and higher with new
modulation schemes coming)
|Robustness to Interference
|Handset Complexity / Cost
||Moderate (and falling)
||Low (and falling)
|Basestation Complexity / Cost
||High (and falling)
||Low (and falling)
GSM (especially its 900 MHz variant) is generally better suited to
wide-area coverage, but has associated with it a higher cost for infrastructure and
handsets. DECT, by contrast, is better suited to high-user-density, small-area
coverage, with correspondingly lower infrastructure and handset costs. It is also
better suited to the provision of both higher-rate services and higher traffic densities,
having been originally conceived for operation in the high density office wireless PABX
Network Operator Trials of DECT
Recognising the complementarity described above, in the mid-90's several European
(and other) operators undertook trials of DECT wireless access, to assess its potential as
an extension of their cellular networks. Some of these trials involved the
interworking of their public access mobile networks with fixed wireline networks, in some
cases using dual mode telephones, in other cases uses separate phones. Here we
briefly summarise a few of these trials which have been described at DECT conferences in
recent years. (Some of these are summarised in the reference below).
Westel Radiotelefon is a mobile operator who in 1995/96 began interconnecting
islands of DECT access to their NMT450 analogue cellular network. The first trial
was in Vaszar, a small village with very poor wireline telephone supply located some 8km
away from he nearest cellular basestation. In this scenario, DECT proved to be a
very effective solution, providing telephone access at a cost lower than would have been
possible using cellular.
With the liberalisation of the German telecoms market, several new entrants were
keen to evaluate the potential of DECT during 1994/95, for a range of potential
applications including wireless local loop, local mobility, etc. In the latter half
of 1994 Mannesmann, an operator with both fixed and mobile networks, experimented with
interconnection of DECT access to their D2 GSM network. The DECT system was
directly attached to their network via a protocol converter at the GSM 'A' interface, with
the DECT handsets assigned a D2 number. Standard GSM and DECT handsets were
used, rather than the dual-mode terminals which might have been used in a mature system
(as now in the BT Onephone service). The trial succeeded in verifying both system
interworking and user acceptance.
During 1997, Airtel Movil, Spain's first private GSM operator, explored the
potential of DECT to augment their GSM offering, in a programme designed to assess the
technology as means of accessing new markets and as a response to the incumbent operator
in a highly competitive market. Scenarios explored included residential, SoHo,
WLL and corporate environments. The conclusions of their trials, which were user
centred, not simply technology evaluation, were that a dual mode service was attractive
and useful, particularly within the context of integration of GSM and the corporate DECT
PABX, but that at that time the dual mode technology was not yet sufficiently mature - in
terms of availability of GAP, dual mode terminals and integrated mobility management.
Telia in Sweden undertook an extended series of trials of DECT/GSM interworking
trials were conducted in conjunction with Ericsson, using both separate handsets and the
first dual mode prototypes - at the time described as 'Velcro' phones, referring to the
concept of simply sticking standard GSM and DECT designs together in a single case, with
little optimisation of the re-use of the designs. Detailed usage statistics were
collected in the user trial, which followed an initial technical proving activity.
These showed potential for commercial service, particularly within the corporate
environment. However, faced with changes in the commercial telecoms environment in
the country, the dual mode service was not seen as a top priority to take into commercial
service, when compared at that time with other possible new initiatives.
One of the earliest, and very effective, demonstrations of the potential of DECT/GSM
interworking was given by the Swiss PTT (today's Swisscom) at Telecom '95 in Geneva.
For this event Swisscom interconnected a GSM mobile switching centre (MSC) on
their existing GSM900/1800 network to a wireless PABX connected to over 40 DECT
basestations spread across the Telecom '95 site; some 10 or so additional DECT
basestations were also deployed at Geneva airport. Single number reachability was
implemented, regardless of whether the connection was provided via GSM or DECT.
The rationale for the trial was to provide very high density coverage within
a single number environment - Telecom '95 providing a very practical and realistic
showcase scenario for such a service. Two different methods of interworking between
the DECT and GSM infrastructures were successfully implemented, one based around the GIP -
the DECT / GSM Interworking
Profile and the other based around a corporate network - PLMN G703/R2 interface.
In the UK, BT had been trialling local area mobility and dual mode services for
business customers since 1995, alongside the parallel but more public activities described
above in Germany, Spain, Sweden and Switzerland. Unlike Switzerland, for example,
where Swisscom had full control of both mobile and fixed networks, in the UK BT are only
part-owners of Cellnet, resulting in certain regulatory constraints in seeking to
develop a dual mode service. The first technical trials in Colchester were
conducted over 18 months and involved some 70 users equipped with dual mode phones which
operated in DECT mode when within the BT offices in the town. This first trial
highlighted the difficulties of inexperienced users in coping with the full range of
features offered by such a service, but nonetheless proved its value, in that most users
were reluctant to see the trial end (a similar result to that found by Telia in Sweden).
The next stage in the BT development was its Showcase trial, designed to
explore the management and packaging of a multi-site roaming cordless / GSM service for
business customers. Some 200 DECT users and 40 DECT/GSM users were involved across 4
BT sites around the London area. Again the service was well received by the
trial users and a clear message from this work was the desirability for the phones to
support data access to laptops via the phones - something that DECT is well suited for but
which was not implemented in the equipment used for the trial.
The Onephone Service
The world's first dual mode DECT/GSM commercial service - Onephone - was launched by BT/BT Cellnet in mid-May 1999, some 18
months after the date originally planned. Limited commercial trials were
initially anticipated towards the end of 1997, but these had been apparently delayed by
the lack of availability of suitable dual mode handsets. The service launch
brings a new focus to the issue of fixed-mobile convergence and has coincided with the
rebranding of Cellnet to BT Cellnet. (The latter follows a decision from the UK
Department of Trade & Industry in January that it would not prevent BT from taking
full control of Cellnet if it could come to a suitable commercial arrangement with
Securicor, the other (40%) shareholder in Cellnet).
The following details of the Onephone service are summarised from the
initial marketing literature provided by BT Cellnet. It is possible that some of
these details will change in the coming months in the light of market feedback and
operational experience. Our readers may wish to check out the BT Cellnet Onephone site for current
The Onephone telephone is a DECT/GSM
dual mode phone (manufactured by Ericsson), which will operate outdoors on the BT
Cellnet GSM network as a normal digital cellular telephone. Within the home, the
phone operates in DECT-mode with a standard domestic DECT GAP basestation (manufactured by
Siemens); a separate DECT handset (from Siemens) is also supplied as part of the
package. The Onephone also allows manual override, so that if the domestic
line is in use the Onephone can be used to make a call in GSM-mode, in effect providing an
extra line when needed. The basic package comprising the Onephone, the basestation
and spare DECT phone is being launched at a price of £399. The Onephone user also
requires a subscription to the BT Cellnet network. [Editorial note - this price was
subsequently reduced to £199 in autumn 1999].
To make the most of Onephone's potential, BT is offering its
Flexinumber service, whereby the user is issued with a single number (prefixed 07041 or
07071) which allows calls to be automatically routed to his Onephone, whether he is at
home or away from home. BT charges the two types of Flexinumber at different rates,
depending upon whether the cost of incoming calls, when operating in GSM
cellular mode, is fully paid for by the calling party, or is shared between called
and calling party. The 07041 number is used for the latter case and carries a
premium to the Onephone user of £2.95 per month; the 07071 number is used for the
former case, with the Onephone user charged a premium of £4.99 per month. A
Onephone Flexinumber is also available to consumers who do not use BT as their fixed
line telephone provider - in this case they will receive their normal telephone bill from
their usual service provider and in addition a Flexinumber bill from BT. A
separate bill will also be provided by BT Cellnet for the mobile calls made by the user.
This aspect of multiple billing is clearly an undesirable feature, from the
consumer's point of view and presumably one that BT / BT Cellnet may hope to address in
In use, the Onephone does not allow intra-mode handovers - calls begun
in GSM mode must be completed in GSM mode, those started in DECT-mode must be completed in
DECT-mode. Operating in the home, the Onephone behaves as an ordinary DECT
GAP-compatible handset, which means it can be used to make intercom calls to the other
DECT handset via the basestation. As the user leaves home, the Onephone
automatically switches into GSM mode as the user goes out of range of the DECT
basestation, typically 300 metres. Like any other GSM phone, the Onephone can
be used in other countries on GSM networks with which BT Cellnet has roaming
Where To From Here ?
Whilst the BT Onephone service has been well trailed, plans for trials of similar
services in Germany and Switzerland in he coming months have been less widely
publicised. At the IBC DECT '99 conference in Barcelona in January these were
rumoured - it will be interesting to see how these develop. Certainly when players
of the stature of BT and Deutsche Telecom come into the frame one imagines that the
business case has been well considered, although how consumers will react to these new
services and their charging patterns is difficult for even these players to predict with
At CeBIT this year GSM CTS solutions were presented from Alcatel and
others - a GSM phone that operates in a cordless telephony mode when in range of a
domestic low power GSM-type cordless basestation. Will such products eclipse
the DECT/GSM dual mode solution ? Certainly it could potentially offer the same
service, with different technology - and certainly the key issue to the consumer if the
service, not the technology employed. Arguably, longer term, a GSM CTS solution
could be more cost effective.
On the other hand, the large installed base of DECT residential
basestations and cordless phones could facilitate DECT's move into data applications and
other new wireless home automation services and facilities - eg cordless fire and security
alarms that automatically call a service centre, after conducting some basic checks like
local alerting. Whilst GSM CTS could handle these low data rate applications, other
higher data rate applications will not be feasible with GSM CTS in the near future.
Whereas a year ago it looked as if DECT/GSM dual mode was finally dead,
success for the Onephone concept and the other new European trials could
represent a turning point, although such success will undoubtedly be strongly influenced
by the reaction of the consumer to the service and its pricing. At this early stage
the jury is still out.
For more information on this topic....
Further information on the potential for interworking of DECT/GSM systems and
dual mode technology may be found in the chapter 'Future Evolution of Cordless Systems'
(and other chapters) within the book 'Cordless
What do you think ?
Will DECT/GSM finally catch on and become a commercial success ?
Have you been involved in assessing DECT/GSM dual mode ?
Are you a Onephone user ? What is your experience ?
What are the plans and progress with the German and Swiss trials ?
Please e-mail us your news and views for inclusion
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