Cordless Digital Telephony (DECT)
© Mercury Communications Ltd - July 1993
2007
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If a single word is needed to profile voice and data communication in the late 1990s it would be mobility. Firstly, major changes are underway with regard to where and how workers undertake the roles allocated to them by companies. The concept of teleworking is slowly changing the face of industry by removing the need to waste time commuting to a central city location to sit in front of a terminal when they could equally well undertake the tasks from their home. Secondly, the power and capability of portable computers is increasing at a breathless pace putting more and more demand on telecommunications companies to provide mobile data services. Thirdly, the average worker in a large corporate company will be far more mobile than ever before and if a telco is to support 100% of their communications needs it could be conjectured that fixed wire access will be inadequate to support all of their needs. Understanding these changes will be key if telecommunications companies are to provide services that meet the needs of the future corporate employee. Figure 1 shows BIS Strategic Decision's classification of employee mobility and it is predicted that by the end of the century at least 20% of telephone calls will not be made with conventional desk-bound telephones.
Issue #15 of Technology Watch addressed mobile and personal digital cellular markets driven by the standards GSM and DCS 1800. This issue looks at the two main digital cordless telephone standards CT2 and DECT. DECT stands for Digital European Cordless Telecommunications.
Figure 1 - BIS Strategic Decision's Mobility Categories
History
In 1993 there are two European standards for digital telephony, CT2 and DECT. This has caused much confusion which has resulted in political infighting, delays in launching products, and non-compatible network deployments. It is a truism to say that for a technology or new product to gain acceptance in any market the ideal situation would be to have a single standard. But in this domain, as in many others, competitive standards have led to major delays in bringing digital cordless telephony to the consumer.
Throughout the 1970s and early 1980s Europe was being flooded with illegal analogue cordless telephones originating in the Pacific rim countries. Not only was the UK's Radio Communications Agency not happy with this as they all seemed to use frequencies allocated to other users and the
GSM DCS 1800
CT2 CT3 DECT
Feature (mobile) (personal)
(cordless) (cordless) (cordless)
Service Deployment 1991 1993 1989 1990 1992-1993
Frequency 935-960MHz 1.7-1.9GHz 864-868MHz 800-1000 1.88-1.9GHz
Allocation 890-915MHz (depending
on
country)
Data Structure TDMA TDMA TDMA TDMA TDMA
Channel Spacing 200kHz 200kHz 100kHz 1MHz 1.728MHz
No. of Channels 124 374 40 8 132
Speech Compression RPE-LTP RPE-LTP ADPCM ADPCM ADPCM
Algorithm 13kbit/s 13kbit/s 32kbit/s 32kbit/s 32kbit/s
Mobile Transmit 20mW to 20W 2.5mW to 1W 1mW to 10mW ? 250mW
Power
Table 1 - Comparison of Digital Wireless Technologies (Main features only)
potential for interference was high but manufacturers were unhappy as well. They could see yet another market disappearing from beneath their feet. Europe responded by creating three new cordless standards, starting with CT1.
Outside of the UK and France CT1 was adopted very quickly, the specification of which originated in the Conference of European Posts and telecommunications (CEPT) standards body. At that time the CT1 analogue specification was way ahead of the capability of semiconductor manufacturers to integrate the technology into a chip set, hence CT1 handsets turned out to be a very expensive commodity. This was because CT1 was specified to provide excellent speech quality equal to that of copper based local loop. The UK, believing the cost of handsets was too high, developed their own CT1 variant, as did France. Needless to say they were incompatible.
It was beginning to be realised that a new single digital standard was required rather urgently. However, yet again alternative solutions came to the fore, namely, CT2, CT3 and DECT.
CT2 (UK Telepoint)
The driving force for CT2 came out of the United Kingdom. CT2 was the result of development work undertaken by a consortium of companies including Ferranti, Plessey, Shaye and STC. Due to lack of co-ordination, products launched by these companies exhibited differences and, encouraged by the DTI and under the sponsorship of that organisation, they reached an agreement to work towards a standard known as Common Air Interface (CAI). There were two key standards for CT2: BS6833 which defined the signalling and speech encoding between the handset and the base station; and, MPT 1344 which defined the radio parameters to a sufficient degree to allow the various manufacturers' equipment to work together.
However, CT2 technology was actually available, while the European standard in the form of DECT was still only a paper specification. The lack of a real European standard at the time led a number of European PTTs to adopt CT2 Telepoint in trials. In the UK, Hutchison's Rabbit service is but one example. Due to regulatory limitations, CT2 allows two-way conversations within the home but only outgoing calls when out and about. This was generally seen as telepoint's Achilles heel. As with CT3 described below, CT2 was rejected for use as the basis of the European standard so CT2 remains an interim standard that must be reviewed every two years to retain its status as a standard.
CT3 (DCT 900)
DCT 900 was developed by the Swedish telecommunications manufacturer Ericsson and was based on an early DECT specification. There are many similarities to CT2 with the exception that CT3 allows full two-way access no matter whether the telephone is being used in the home, in the office, or with a Telepoint node. CT3 field trials were well underway in 1991 in the Netherlands, Sweden, Canada, USA, Australia and Andorra but it took a major blow in 1991 when it was not accepted as the European digital cordless standard.
DECT
In 1991 the Digital European Cordless Telecommunications(DECT) standard was presented for public enquiry. The standard is under development within the European Telecommunications Standards Institute (ETSI) and was planned to be the ultimate cordless standard for European cordless telephony. However, it has a higher price tag than CT2 and is more than a year behind in actual deployment.
DECT supports twice as many channels as CT2, incorporates encryption as standard, and supports data as well as voice. This fact has meant that DECT vendor support is split right down the middle. Half of the supporters think it's the ideal cordless telephone for the home or connected to a wireless PABX in the office, while the other half believes it's the ideal choice of technology for use in radio LANs (RLANs). DECT can actually support data rates of several hundred kbit/s while GSM and DCS 1800 can only handle up to around 19kbit/s. This fact is seen as major weakness for GSM.
It should be noted that the cordless standards provide a higher quality of speech than the mobile cellular standards and well up to that achieved with fixed networks. This is not the case for the two mobile digital technologies, GSM and DCS 1800.
Comparisons
Table 1 shows the most obvious differences between CT2 and DECT (and compares them to GSM and DCS 1800). A further clear difference is that CT2 uses 40 separate carriers to provide 40 duplex channels. Each pair of send and receive channels is multiplexed onto a particular carrier in a time division mode i.e. it uses a combination of frequency division multiple access (FDMA) and time division duplex (TDD), FDMA/TDD.
DECT on the other hand is based, yet again, on the inability of two camps to reconcile their differences. In this case those believed that frequency division multiple access (FDMA) was the right approach and those who favoured TDMA. The result is that DECT combines both approaches providing 120 duplex channels through using ten separate carriers multiplexing 12 send and 12 receive channels on each carrier. It can be argued that this is an unwieldy solution.
When the CT2 standard was upgraded to define the commmon air interface (CAI) to promote interoperability, the opportunity was taken to incorporate support for PABXs, extra signalling layers, and an applications programming interface (API) to enable the support of future additional services. However, from the very beginning DECT was developed to support a clearly defined set of advanced services needed in the 1990s. This wide remit led to a rather complex specification, that in some ways could be interpreted as supplying all things to all men.
The original DECT specification was aimed at supporting the following application domains:
- residential cordless telephones with trunk-quality voice transmission
- small business systems
- large business systems with support for wireless PABXs
- duplex Telepoint systems
- LAN access for PC data and support for 803.x Ethernet and token ring LAN standards
- DECT can support high data rates by joining channels together. For example, two slots will support 64kbit/s, while using 12 slots can provide 384kbit/s. The latter is ideal as the basis for low-data rate radio LAN applications
- evolutionary systems
- - wireless local loop
- - cordless access to cellular mobile
- This is achieved by DECT's ability to deliver services direct to the caller's premises by bypassing the copper local loop. This is achieved by fitting a DECT portable part as a fixed installation on the user's premises.
- Also:
- the specification should support both voice and data applications equally
- cost should not be penalised by data support
- specification should support ISDN functionality
Wireless PABXs
A large portion of the cordless telephony market will be wireless PABXs by the end of the decade. Although it is certainly true that many organisations that have made significant investment in conventional PABXs during the late 1980s and early 1990s will initially resist the change, the benefits to be gained in the form of flexibility, productivity, and freedom of movement will eventually prove to be overwhelming.
The ability to take one's telephone on a corporate walkabout within a single office complex or to another building in another town will be extremely attractive. Further, the abolition of hard wiring to physical desks will significantly reduce support costs. These benefits are not only pertinent to voice calls but also in the data domain as well. The portable telephone could equally well be a portable notebook. DECT (being able to support a data rate of 384kbit/s) can be used to connect a desktop personal computer a wired LAN via a wireless link.
Most of the European telecommunications manufacturers have launched DECT based wireless PABX products in recent months including, Alcatel, Ericsson, Nokia, Philips and Siemens. In fact, the first DECT product announced last year was a radio LAN product call NET3 from Olivetti which supports up to 1.5Mbit/s .
Table 2 - European Cordless Telephony Shipments
Conclusions
It is clear that we now have two standards in the cordless domain. CT2 exists, it is a relatively cheap technology and is capable of providing high quality service to mobile users. However, it is limited in its ability to support advanced services and the lack of clear definition of these services in the CT2 specification will inevitably lead to some confusion. It is also more than likely to be marginalised as newer DECT based products arrive in the market place.
In contrast, DECT is based on a well thought out and comprehensive specification that is capable of being used in several complementary roles. This initially means higher cost, but because of its universal applicability, DECT should eventually become the clear standard for cordless applications throughout Europe. And, it should not be forgotten that DECT is also one of the technologies that could be used for wireless local-loop.
Figure 2 - The Four Wireless Technology Groups
Food for Thought
It must be remembered that DECT, GSM and DCS 1800 do notaddress certain very important and high growth market areas - interactive video to the home and high-bit rate data delivery to small and medium enterprises (SMEs). The reason that GSM, DCS 1800, CT2 or DECT do not support high bit-rate data services is because all these standards started their long road to standardisation in the early 1980s when there did not seem to be any great requirement to deliver bandwidths greater than 1Mbit/s into small businesses let alone homes. In other words they are old and have been subject to the slow standard development cycles so common in telecommunications.
This limitation is leaving a gaping hole, or market opportunity, that will be filled by proprietary code division multiple access (CDMA) based wireless systems as discussed in Technology Watch #1. CDMA will be taken forward by companies that see a market opportunity not being addressed by the large service providers. Two that come to mind are Ionica and Millicom.
These markets could be addressed using alternative means including the highly interesting standards for transmitting high bit-rate data down twisted copper pairs known by the acronyms ADSL and HDSL.
