Page 4 - Introduction
Page 4
DECT - Installation Manual
DECT in Summary
38HBKD0001SAH – Issue 14 (27th October 2003)
DECT in Summary
The 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.
