DECT Tutorial
- an overview or tutorial of the basics of the DECT cordless phone system,
outlining its technology, features and the way in which it operates.
DECT (Digital Enhanced Cordless Telecommunications) system is
widely used for residential, and business cordless phone communications.
Designed for short-range use as an access mechanism to the main networks, DECT
offers cordless voice, fax, data and multimedia communications, wireless local
area networks and wireless PBX.
With the flexibility offered by cordless phone
communications, DECT has become the major standard for this application and DECT
is now in use in over 100 countries worldwide.
DECT development
The standard for DECT or Digital Enhanced Telecommunications system was
developed by members of the European Telecommunications Standards Institute
(ETSI). The first release of the standard was available in 1992 after which much
of the work was focussed on inter-working protocols (DECT / GSM, DECT/ISDN,
etc).
As a result of this work, DECT / GSM inter-working has been
standardized and the basic GSM services can be provided over the DECT air
interface. This enables DECT terminals to inter-work with DECT systems which are
connected to the GSM infrastructure. All roaming scenarios based on SIM roaming
as described in GSM specifications are applicable.
Along with requirements arising from the growing use of DECT,
this work gave rise to a number of extensions to the basic DECT standard. This
led to a second release of the standard at the end of 1995. This included
facilities including : emergency call procedures, definition of the Wireless
Relay Station (WRS), and an optional direct portable to portable communication
feature.
DECT air interface operation
The most common protected spectrum allocation for DECT is 1 880 MHz to 1 900
MHz, but outside Europe spectrum is also available in 1 900 MHz to 1 920 MHz and
in 1 910 MHz to 1 930 MHz (several countries in Latin America). In addition to
these frequencies there is also a reservation in some countries in the band 2
010 MHz to 2 025 MHz.
DECT carriers have been defined for the whole spectrum range
1880 MHz to 1980 MHz and 2010 MHz to 2025 MHz in the ETSI standard. The basic
frequency plan for the 1880 to 1900 MHz DECT band provides for ten channels. The
additional frequencies beyond the this allows for expansion of the basic DECT
allocation or allows DECT services to be introduced in countries where the basic
DECT frequencies are not available. The use of extended or new frequency
allocations does not cause regulatory difficulties for roaming DECT handsets as
it is mandatory for handsets not to start transmission on carrier frequencies
others than those informed by the base station in broadcast messages.
The signal is modulated using a form of modulation called
Gaussian Frequency Shift Keying (GFSK) and has a BT of 0.5. This provides the
optimum spectral usage for the system.
The system uses dynamic channel allocation and is thereby
able to reduce the levels of interference, and ensure that links are set up on
the least interfered channels. All DECT equipment scans the frequency allocation
at least every 30 seconds as a background activity. This produces a list of free
and occupied channels along with the available timeslots to be used for the
channel selection, should this be required.
Additionally the DECT portable continuously analyses the
signals to ensure that the signals originate from the base station to which it
is connected and has access rights. The portable locks onto the h4est base
station and checks it can access the base station as detailed in the DECT
standard, and the channels with the best signal strength (RSSI - Receive Signal
Strength Indication) are used for the radio link as required. This Dynamic
Channel Selection and Allocation mechanism guarantees that radio links are
always set-up on the least interfered channel available and hence the best
performance is obtained.
DECT MC/TDMA/TDD principle
The DECT radio interface employs a number of techniques in its access
methodology. The scheme uses Multi-Carrier, Time Division Multiple Access, Time
Division Duplex (MC/TDMA/TDD).
The basic DECT system has a total of ten possible carrier
frequencies between 1880 and 1900 MHz, i.e. it is a Multi-Carrier (MC) system.
In addition to this the time dimensions for each carrier is
divided to provide timeframes repeating every 10 ms. Each frame consists of 24
timeslots, each of which is individually accessible and may be used for either
transmission or reception. For the basic DECT speech service two timeslots -
with 5 ms separation - are paired to provide bearer capacity for typically 32
kbps (ADPCM G.726 coded speech) full duplex connections.
In order to simplify the way DECT can be used when only basic
implementations are needed, the allocations of timeslots within the 10 ms
timeframe are restricted. The first 12 timeslots are used for downlink
transmissions and the remaining 12 are used for the uplink. This reduces the
level of complexity, and as this is not needed for basic implementations, it can
provide some cost savings.
DECT codecs
The basic telephony speech quality offered by DECT is very high compared to many
other wireless systems. This is the result of the use of the ITU-T
Recommendation G.726 codec that is employed. This is a 32 kbit/s ADPCM speech
codec and although it uses 32 kbps, the quality it affords is high and there is
more than sufficient bandwidth within the system to support it.
TDMA structure
The DECT TDMA structure enables up to 12 simultaneous basic voice connections
per transceiver. The system is also able to provide widely varying bandwidths by
combining multiple channels into a single bearer. For data transmission purposes
error protected net throughput rates of integral multiples of 24 kbps can be
achieved. However the DECT standard defines a maximum data rate of 552 kbps with
full security.
What is the DECT GAP profile?
All DECT systems are based on a main standard that is the Common Interface (CI),
which is often used in association with the Generic Access Profile (GAP). The
GAP profile ensures interoperability of equipment from different providers for
voice applications. The GAP defines the minimum interoperability requirements
including mobility management and security features. It has different
requirements on public and private systems. This means that the GAP is
effectively the industry standard for a basic fall-back speech service with
mobility management. This basic service is not always used, but instead it forms
the fallback that is always be available, especially when requested by a roaming
phone, etc
DECT Summary
Although DECT has been in use for a number of years now, its flexibility and
performance have meant that it is still the major technology used for cordless
phones. The standard is maintained by ETSI, and this will enable it to move
forward as new requirements appear and technology enables further facilities to
be added.
DECT Glossary
| DECT |
Digital Enhanced Cordless Telecommunications |
| DMAP |
DECT Multimedia Access Profile |
| DPRS |
DECT Packet Radio Service |
| FP |
Fixed Part - the base station |
| GAP |
Generic Access Profile |
| GSM |
Global System for Mobile telecommunications |
| IMT-2000 |
International Mobile Telecommunications 2000 |
| PP |
Portable Part - the handset |
| RES |
Radio Equipment Systems |
|
