Digital Enhanced Cordless Telecommunications
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DECT or Digital Enhanced (formerly European) Cordless Telecommunications is an ETSI standard for digital portable phones, commonly used for domestic or corporate purposes. DECT can also be used for wireless data transfers. DECT is recognised by the ITU as fullfilling the IMT-2000 requirements and thus qualifies as a 3G system, as a part of the IMT-2000 group of technologies, DECT is also referred to as IMT-FT (Frequency Time).
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[edit] Typical features in a domestic DECT implementation
Typical abilities of a domestic DECT GAP system include:
- Multiple handsets to one base station and one phoneline socket. Additional handsets usually have a battery charger station instead of a base station. No need for additional telephone sockets nor additional transceivers.
- Interference-free wireless operation to around 100 metres outdoors. Operates clearly in common congested domestic radio traffic situations. For instance, generally immune to interference from WiFi or video senders.
- Ability to transfer a call between handsets, or to have multiple handsets talking on the same call.
- Ability to make internal (intercom) calls between handsets (for example, to call from the bedroom to the kitchen).
- Caller ID, displaying the name or number of incoming calls (this is not a part of the DECT specification, but it forms part of a typical domestic DECT implementation).
- Optionally, a digital answering machine accessible from multiple handsets (again, not part of the DECT specification, but part of a typical domestic DECT implementation).
- Ability to add other DECT handsets of different brands.
Shortcomings of a typical domestic DECT system include:
- Non-DECT abilities such as Caller ID or answering machine may be brand-dependent. For this reason, a suite of DECT handsets is generally purchased together, for instance a base station with three handsets.
- It is not possible to have more than one external telephone call in operation on a typical domestic DECT system. Attempts to make an external call from one handset, whilst an external call is already being taken on another handset, generally give an error or warning, such as an engaged tone. However, this is possible with a business DECT system attached to a PABX.
[edit] Technical details
Some DECT properties:
- net bit rate: 32 kbit/s
- frequency: 1880-1900 MHz in Europe, 1920-1930MHz in the US
- carriers: 10 (1880..1900 MHz)
- timeslots: 2 x 12 (up and down stream)
- channel allocation: dynamic
- traffic density: 10000 Erlangs/km²
DECT physical layer uses:
- Frequency division multiple access (FDMA),
- Time division multiple access (TDMA) and
- Time division duplex (TDD)
This means that the radio spectrum is divided into physical channels in two dimensions: frequency and time.
The maximum allowed power for portable equipment as well as base stations is 250mW. On average, however, a portable device radiates about 10mW during a call as it is only using one of 24 timeslots to transmit.
DECT media access control layer is the layer which controls the physical layer and provides connection oriented, connectionless and broadcast services to the higher layers. It also provides encryption services with the DSC Cipher (DECT Standard Cipher).
DECT data link layer uses a variant of the ISDN data link protocol called LAPC. They are based on HDLC.
DECT network layer contains various protocol entities:
- Call Control (CC)
- Call Independent Supplementary Services (CISS)
- Connection Oriented Message Service (COMS)
- Connectionless Message Service (CLMS)
- Mobility Management (MM)
All these communicate through a Link Control Entity (LCE).
The call control protocol is derived from ISDN DSS1, which is a Q.931 derived protocol. Many DECT specific changes have been made.
There are four application areas in use in Europe:
- Domestic DECTs are connected to a base (radio fixed parts) connected to the PSTN. A base can accept more DECT handsets.
- Business DECTs connected to PBX. In such cases, many radio fixed parts. The DECT handsets dynamically connect to these (hand over feature)
- Public DECT connected to the PSTN (very rare). A high density alternative to GSM.
- Local loop (very rare). In this case, a DECT radio link replaces the normally wired connection between the final PSTN distribution point to the subscriber.
DECT GAP is an interoperability profile for DECT. The intent is that two different products from different manufacturers that both conform not only to the DECT standard, but also to the GAP profile defined within the DECT standard, are able to interoperate for basic calling. In other words, any phone that supports GAP can be registered with any base station that also supports it, and be used to make and receive calls. They will not necessarily be able to access advanced features of the base station, such as remote operation of an answering machine built into the base. Most consumer-level DECT phones and base stations support the GAP profile, even ones that do not publicise this feature, and thus can be used together.
DECT was developed by ETSI but has since been adopted by many countries all over the world. DECT is used in all countries in Europe, outside Europe it is used in most of Asia, Australia and South-America. As of 2005, the FCC changed channelization and licensing costs in a portion of 1900 MHz range spectrum which allowed DECT devices to be used without expensive changes. DECT will operate as Unlicensed Personal Communications Services (UPCS).
[edit] Safety
EMF/Wireless? Possible Health Risks asociated with such devices, as the device is held next to the head many concerns have been raised.
[edit] See also
- Generic access profile (GAP)
- CorDECT
- Wireless local loop
- Unlicensed Personal Communications Services
