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DECTweb brings you occasional  feature articles on timely topics and / or from well known names in the cordless telecommunications industry.   If you would like to offer an article for consideration please e-mail us with your biographical details and an outline for the proposed feature. 

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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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) applications.   

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:

Parameter GSM DECT
Range - Cell Size Large cells - typically 1-30 km Small cells - typically 1-500m
Frequency Planning Fixed, inflexible Dynamic, adaptive
Spectrum Efficiency Moderate Very high
Traffic per Subscriber ~20 mErlang ~200 mErlang
Data Capability

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 Moderate High
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 scenario. 

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.

United Kingdom
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 information.

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 the future.

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 agreements.

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 certainty.

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 Telecommunications Worldwide'.   

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 on DECTweb
Thank you

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