The Physical layer
The Physical layer of the DECT standard
(EN 300 175-2) describes the physical characteristics of a DECT
system and portable. By physical charateristics is not meant the look,
size and weight of the device, but the use of the 'radio-spectrum'.
Three dimensions
The radio spectrum has three dimensions: frequency, time and space. Using
different frequencies multiple communications can take place at the same
time without interference, just like ordinairy radio. For DECT ten
frequencies are defined between 1880 and 2000 MHz. These high
frequencies have several advantages. Interference from other sources is
usually low, they have better propagation in buildings and can use
smaller RF parts, which is especially helpfull in portables.
Time Division Multiple Access
Time can be used because it is a digital communications system. The
transmissions can be bursts of data with pauses in between. In these
pauses other users may transmit using the same frequency. This is called
Time Division Multiple Access (TDMA). It is crucial that these bursts are
transmitted at exactly the right moment, otherwise interference with other
users can occur. Therefore the Medium Access Control
layer is involved in this as well.
Coverage
The third and final dimension of the radio spectrum is space. Due to
propagation loss a transmitted signal gets weaker with distance. This
means that with a certain distance the signal gets to weak to be received
with enough quality to maintain a reliable digital communications link.
This range is called a cell. The more power is transmitted the larger this
cell is. In the DECT standard a peak transmit power of 250 mW is defined.
This results in a cell radius (omnidirectional) from 50 meters indoors
to 300 meters outdoors. Specific applications, like Wireless Local Loop,
can use directional antennae to increase that range up to 5000 meters.
The fact that the signal gets weaker with distance means that the same
channel can be used again over a certain amout of distance.
The DECT standard employs Continuous Dynamic Channel Selection (CDCS).
This means that no fixed frequencies are allocated to certain cells. Any
frequency/slot combination that seams suitable to the portable may be used
to communicate with the base system. The Medium Access
Control layer handles this selection.
Written by my own two hands and an ASCII editor.
Problems? write me
Last updated January 19, 2000
This page is
