Information Technology Reference
In-Depth Information
phones or computers attached to different centralized machines be
comes a severalstep process. In particular, when you make a long
distance call, you use your local telephone to send information to
your local switch. That switch then uses its outside connections to
send your information from switch to switch until it gets to the
switch of the person you are contacting. That switch then uses the
local connections to forward your information to the final party.
So, when you call home from school on Mother's Day, your tele
phone wires carry your information through your school's central
computer to the network in your hometown.
Suppose for the moment that your telephone is attached to
switch 1 in Figure 8.6, and your best friend has a telephone attached
to switch 3. When you dial your friend's telephone number, you
send the number to switch 1. That switch realizes that the number
does not correspond to a telephone attached to the same switch, but
information must get to switch 3 instead to complete the connec
tion. Modern telephone switching networks may use rather sophis
ticated techniques to handle your telephone call, but at a conceptual
level, information will have to be sent through either switch 2 or
switch 4. As you are talking, your voice information will go from
your telephone to switch 1, be forwarded on to switch 2 or 4, and
onward again to switch 3, where it can be sent to your friend's tele
phone. A similar process in reverse is followed as your friend speaks
to you in response. Thus, even when you call someone who is not
within your local network, your voice and the voice of your friend
will travel through telephone wires from central machine to central
machine, making it possible for you to communicate.
Although star networks can work well for small installations,
as in the phone conversation just described, they place consider
able responsibility on a central machine or switch. Star networks
for either telephones or computers rely on the central machine for
all activity. This allows relatively easy monitoring of communica
tions and maintenance of equipment, because much work is cen
tralized. This approach also has some advantages regarding relia
bility, in that the malfunctioning of any telephone or computer
(outside the central machine) is likely to affect only that one ma
chine. Further, if a wire is cut between a distant computer and the
central machine, again, only the one local telephone or computer
is affected. On the other hand, a malfunction by the central ma
chine will likely affect communications for the entire network. The
