Product manual
System description, DECT air interface
Technical Product Manual - DCT1800-GAP
TD 92093 (1/LZBNB 103 108 R4D) / 2006-03-09/ Ver.C
© 2006
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6.2.3 Time Division Multiple Access (TDMA)
Radio transmission requires an available frequency band in the already overcrowded frequency
spectrum. DECT uses Time Division Multiple Access (TDMA) as access technology. TDMA makes
an efficient use of the allocated frequency band by subdividing each carrier into timeslots. In this way
more calls can take place at the same time per carrier.
In DECT, TDMA divides each carrier into 24 time slots (12 time slot pairs). Any DECT base station
and cordless phone can access these time slots. A sequence of 24 time slots on one carrier is referred
to as a TDMA frame.
6.2.4 Time Division Duplex (TDD)
The first 12 time slots are used to transmit from base stations to cordless phones. Connections in this
direction are called down-links. The other 12 time slots are used to transmit from cordless phones to
base stations and are called up-links. Thus, a time slot pair on one carrier creates a duplex channel.
The sequence numbers of the two time slots always differ 12, for example, time slot 5 and 17, or 8
and 20.
In speech applications 10 carriers and 12 time slot pairs make up 120 duplex channels. In principle,
these channels can be used by any base station and any cordless phone. The default rate for speech
is 32 kbit/s. For data communications multiple time slots (up to 24 in one direction) can be used,
resulting in a transmission rate of over 500 kbit/s.
6.2.5 Dynamic Channel Selection (DCS)
In DECT, channels are selected by the cordless phones (see figure 23). Channel selection is done in
a dynamic way, not only during call set-up, but also during a call. Each cordless phone continuously
scans the DECT channels and keeps track of their states. The cordless phone is able to detect each
base station that is within its range because base stations always transmit at least on one channel.
Thus, the cordless phone can select the strongest base station and the best free channel of that base
station. The signal/noise ratio of a free channel must be at least 8 dB.
Fig. 23 Channel matrix
10
carriers
12 time slot pairs
Ongoing call
Blind
120 channels
010