Signalling modes

Signalling modes

Transfer of data over a transmission medium occurs in one of two modes:
Baseband signalling is the transmission of the digital signal at its original frequency, without modulation
(see Figure 3a). A string of 1s and 0s will result in a DC signal so this is also known as DC signalling. This
is commonly used in local area networks.
The capacity and the inductive effect of the wire or cable, result in distortion of the signal as shown in
Figure3b. This distortion depends on the length of the transmission medium and the frequency. Baseband
signalling is suitable only for local transmission over distances typically less than one kilometre. The actual
maximum distance depends on the transmission rate for a given transmission line at given power of a
transmitter. This is the most common method of signalling in PLC systems as distances seldom exceed
these values.

Switched or leased lines from public
carriers, telephone network, are
generally not suitable for baseband
transmission. The signals on these
lines are amplified by regenerators
which do not pass DC signals. In
addition, these carrier lines are often
loaded with an inductance to reduce
the distortion of analogue signals.
It is possible to lease an unloaded
local line, which does not have any
regenerators.
Broadband transmission uses the
digital signals to modulate a carrier
signal using one of the modulation
methods discussed below. The carrier
frequency must be within the bandwidth
of the channel. This technique
must be used for networks that use
voice-grade lines which generally
have a bandwidth of 300 - 3000 Herz.
It is becoming quite common to use

the public telephone network for PLC diagnostics and programming remotely, but seldom for data
acquisition because of the slow speed, maximum generally of 2400 baud. Broadband signalling is nearly
always used in wide area networks,(WANs), but also for some LANs based on cable television technology.