Principles and technologies of local area networks - Intelligent Buildings and Building Automation - page 59

is called the frequency-shift-keying (FSK) method. In this case, a logical value
of '1' is represented by a carrier signal of frequency f 1 and logical value of '0'
is represented by a carrier signal of frequency f 0 .
The encoding of digital data into a digital signal is straightforward. Usually,
the logical value '1' is represented by a positive voltage and logical value '0'
is represented by a lower voltage. As a result, the signal level is always at one
of the two voltage levels. However, no matter whether analogue or digital
signalling is used, if two consecutive bits are identical in value (e.g. two '0s' or
two '1s'), it is difficult to tell when the first one ends and the second begins,
or how many '0s' or '1s' there are when a piece of signal is received.
To solve this problem, the signal transmitter (source node) and receiver
(destination node) have to be synchronized; that is, they have to have the
same perception of time. For example, a transmitter may provide a clock
signal for the receiver in addition to a data signal. The use of an additional
clock signal line is very efficient in solving the synchronization problem, but
it requires an additional line connecting the transmitter and receiver. To avoid
the use of a clock line while keeping the transmitter and receiver synchro-
nized, two methods of data transmission might be used, asynchronous and
self-clocked . Asynchronous transmission is used for lower data rates, whereas
high-data-rate transmission is more efficient using the self-clocked method.
Asynchronous transmission uses start and stop bits to specify the beginning
and end of each character of transmitted data. These additional bits provide
the timing or synchronization for the connection by indicating when a com-
plete character has been sent or received. Thus, timing for each character
begins with the start bit and ends with the stop bit.
Manchester encoding of digital signals is a typical method used in self-
clocked transmission, a type of method in the synchronous transmission
category, as illustrated Figure 4.14. In this case, the receiver's clock is resyn-
chronized on every bit transmission and bit streams of any length can be

1 0 0 1

Transmitted
data bits

H

Transmitted
data

L

H

Transmitted
clock

L

H

Manchester-
encoded data signal

L

Figure 4.14 Manchester encoding (self-clocked transmission).