DECT stands for Digital Enhanced Cordless Telecommunication, a radio standard for short-range cordless communication, which can be adapted for many applications and can use various international frequency allocations. It is a technology suitable for voice, data and network applications with a range of up to 500 metres.
Description
The DECT system is not a mobile telephony system but a cordless telephony system, i.e. a telephone intended for home or business use that moves within a defined geographical area and/or limited by radio coverage provided by one or more antennas (e.g. a flat, a building, a villa, an airport, etc.). If the radio coverage allows it, migration from one area to another is permissible. Typically, the radio coverage could be owned by a party other than the operator offering the telephone service. For example, the radio coverage of an airport is provided by the company that owns the airport while the telephony service is offered by the fixed telephony operator of which the said company is a customer.
More generally, switching from one radio coverage to another, if they are not adjacent, is only permitted by the coverage offered by the telephone operator of which one is a customer.
This just described represents a first business scenario that was not possible with the first analogue versions of cordless telephones: a telephone (and thus a customer) moving within the first radio coverage (the airport) uses a cordless service, a telephone (and thus a customer) moving between different coverage areas also uses a mobility service.
Technical specifications
The DECT encrypted digital cordless telephony standard operates on the frequency band between 1880 and 1900 MHz in Europe and between 1920 MHz and 1930 MHz in the USA; it uses 10 dynamically allocated channels in Europe while in the USA the channels are 5; given the reduced bandwidth available, each of the channels in turn is divided into 2 x 12 time-slots. It uses GFSK modulation, has a maximum transmission speed of 384 kbps and allows interaction with other networks such as PSTN, ISDN, GSM. The audio codec is G.726, with a net transmission rate of 32 kbit/s.
The DECT physical layer uses:
Frequency Division Multiple Access (FDMA);
Time Division Multiple Access (TDMA);
Time Division Duplex (TDD)
In other words, the radio spectrum is divided both in frequency and time, similar to the GSM standard.
The 'media access control layer' of DECT is the layer that controls the physical layer and makes connection-oriented, connectionless and broadcast services available to the upper layers. It also provides encryption services with the 'DECT Standard Cipher' (DSC). The encryption is rather weak, using a 35-bit initialisation vector and encrypting the audio stream with 64-bit encryption.
The data link layer of DECT uses LAPC (Link Access Protocol Control), a special variant of the ISDN LAPD protocol, and based like the latter on an HDLC technique.
The DECT network layer always has the following protocols within it:
Call Control (CC);
Mobility Management (MM).
In addition, it may contain these other protocols:
Call Independent Supplementary Services (CISS)
Connection Oriented Message Service (COMS)
Connectionless Message Service (CLMS)
All these protocols communicate via a 'Link Control Entity' (LCE).
The call protocol is derived from the ISDN DSS1 protocol, but many DECT-specific modifications have been made. The 'mobility management protocol' contains elements similar to GSM, but also many DECT-specific elements.
DECT GAP (Generic Access Profile - documents series ETS 300 444 and ETS 300 494) defines a DECT interoperability profile. The aim is to ensure that different products from different manufacturers that follow not only the DECT standard, but also the GAP profile, are able to interoperate for simple calls. The DECT standard also includes comprehensive test procedures for GAP, and GAP products from different manufacturers are in fact interoperable for basic functions.
