2
Installation and Maintenance of E1 circuits
1 A
N
I
NTRODUCTION
TO
C
OMMUNICATIONS
S
YSTEMS
One of the first communications networks known was built by Mediterranean cul-
tures more than 1,000 years ago and consisted of a series of successive towers with
a distance of about 5 to 12 km between them. A message could be coded and trans-
mitted from the first tower to the second one by using optical signals, and then be
passed on along the line until it reached its final destination.
In this primitive system we can already identify all the elements of a genuine
communications network (see Figure 2):
•
Information
consists of the messages interchanged between final users. In or-
der to be introduced into the network, information needs to be coded into sig-
nals.
•
Signals
are a physical magnitude, specific for each transmission medium, that
change with respect to time.
•
The
transmission medium
consists of the links that connect distant nodes.
•
Nodes
are those network elements that receive the signals and retransmit them
further along until reaching the final users.
Figure 1
ALBEDO AT.2048, E1, Datacom, Jitter, and Wander tester.