Principles and technologies of local area networks - Intelligent Buildings and Building Automation - page 57

Signal

Ground

Ground

Output Input

Figure 4.12 Galvanic separation of network nodes.

applies to both twisted-pair and coaxial cables. Each node is equipped with
signal amplifiers, illustrated in the figure by triangles. Neither the twisted-
pair nor the coaxial cable is a lossless medium. The signal is attenuated while
it propagates through the medium. A node that transmits has to provide a
signal of sufficient energy and thus needs an output amplifier. The receiving
node has to be able to recognise the attenuated signal arriving through the
medium and therefore it needs an input amplifier also. Since all nodes in a
LAN can be transmitting or receiving, all are equipped with both output and
input amplifiers.
Because LAN nodes can be located as far as a few kilometres away, gal-
vanic separation provides better links between the nodes. This is achieved by
using radio-frequency transformers, which separate input/output amplifiers
from the medium in terms of ground potential but still allow high-frequency
signals to pass through between node I/O and medium cables. Such an inter-
connection is illustrated in Figure 4.12.

4.3.3 Signal encoding and medium interface

Data to be transferred between LAN nodes exists in parallel digital form
originally. Each message is a string of bits encoding some information. The
interconnections between nodes in LANs allow only one bit of informa-
tion to be transferred at a time, which is called serial data communication .
Thus, a transfer of a message from one node to another must be achieved
by sequentially transmitting all bits of the message from the source node to
the destination node.
The source and destination nodes of a LAN are connected by a physical
medium, such as a twisted-pair or coaxial cable. Physically, data bits are
transferred in the form of electrical signals. Two major types of signals are