Page 4 - Introduction
DECT in Summary
DECT in Summary
DECT (Digital Enhanced Cordless Technology*) uses cellular radio methods to
handle multiple handsets on the same system at the same time. It is designed for
high density local area usage
The DECT system uses the frequency band of 1.88GHz to 1.9GHz. It employs
FDMA (Frequency Division Multiple Access) to divide the band into 10 separate
carrier frequencies. TDMA (Time Division Multiple Access ) further sub-divides each
carrier frequency into 24 time slots. Twelve slots are used for send and 12 for
receive, i.e. two slots are required per conversation on the carrier frequency. This is
called TDD (Time Division Duplex). The combination of a frequency and a timeslot
pair is called a channel and equates to one active call (when a handset is idle is
does not use a channel).
Fr
e
q
u
e
ncy
10
9
8
7
6
5
4
3
2
1
1 2 3 4 5 6 7 8 9 10 11 12 1 2 3 4 5 6 7 8 9 10 11 12
Basestation to handset
Handset to Basestation
Timeslots
Adjacent
channels not
used
The pair of channels used by
a call is always 12 timeslots
apart (5ms)
Alternate
timeslots not
used
The above method gives 120 call channels available to a DECT system (10 x 24
channels, 2 channels per call).
During a conversation, a handset continuously compares the speech quality
obtained in its time slot to that in the same time slot on another carrier frequency. If
the current signal falls below acceptable limits and the other band is better, the
handset will change over to that band
DECT systems use DCS/DCA (Dynamic Channel Selection/Allocation) to resolve
availability of channels between handsets and base stations. These processes allow
a high density of handsets (on the same or different DECT systems) to operate in
the same area (provided sufficient base station channels are available)
Speech within DECT is digitised using ADPCM (Adaptive Differential Pulse Code
Modulation). Traditional PCM digitises speech by sampling at time intervals and
assigning a value to the amplitude at each time interval. This sequence of amplitude
values is then sent as a digital signal. In ADPCM information is only sent when their
is a difference in amplitude. This requires more processing electronics but allows
lower signal rates to be used for the same speech quality
*Originally DECT stood for "Digital European Cordless Telephony" but this was
changed as use of the standard spread beyond the European market
Page 4
Installation Manual
38DHB0002UKCA – Issue 8 (26/03/2002)
INDeX CSLC