Switched Networks; Nonswitched Networks; Common Terms - IBM 5100 Maintenance Information Manual

SWITCHED NETWORKS
The communications network in the preceding example is
called a switched network. A switched network uses the
same lines and exchange equipment that telephone com-
panies use for normal voice communications. To establish
a connection between the 5100 and a remote system, you
must dial the remote location.
Switched lines may take different routes each time you
dial a certain location. Some reasons for this include sit-
uations, such as unavailable lines during peak calling periods,
and problems with equipment. It is common for data being
transmitted a few hundred miles to actually travel thousands
of miles because of busy lines during peak calling hours. As
it travels, the data can encounter many devices, such as
amplifiers, microwave equipment, and terminating and
switching equipment. Therefore, if you have transmission
problems, dialing again might establish a different route,
which could solve the problem.
NONSWITCHED NETWORKS
Unlike switched networks, nonswitched networks (leased
lines) always use the same route to the remote system and
do not require dialing to establish the data connection.
As in switched networks, nonswitched lines may also be
routed through many different network devices, such as
amplifiers, exchange equipment, and microwave equip-
ment. When you have problems with nonswitched lines,
you cannot establish a different route by dialing. You will
usually find it necessary to work with a representative from
a telephone company.
COMMON TERMS
When you are working with a representative of a telephone
company, it will be helpful if you are familiar with the
terms that are used to describe problems and components
of a communications network. The following are some of
the terms that apply to the 5100 communications and
serial I/O adapter features:
4-2
Asynchronous is a type of transmission in which each
character is synchronized individually with a start and
stop bit. (See Data Format in Theory of Operation.)
Baud is a term that represents the data rate. One baud
is equal to 1 bit of data per second (1 bps). For example,
300 baud is equal to 300 bits per second.
Carrier is a high-frequency signal that is modulated by
voice or data signals (the frequency depends on the data
set design). When you dial the remote system, the tone
you hear in the receiver is the carrier. If the tone is
steady, it is not modulated. If the tone sounds like
many tones combined (varying tone), it is modulated.
Data phone l is a telephone designed for data
transmission.
Data set (modem) is a device that supplies the carrier
during data communications, modulates the carrier with
the data signals, and transmits it over the communica-
tions lines. The data set also receives the modulated
carrier from the communications lines, demodulates
it (removes the data from the carrier) and sends it to
the terminal or computer.
Demodulation is the process of removing the data from
the carrier.
Duplex indicates that data can be transmitted in both
directions.
Full duplex indicates that data can be transmitted in
both directions simultaneously.
Half duplex indicates that data can be transmitted in
both directions, but not simultaneously.
Line adapter is an impedance matching device used to
connect other devices to communications lines. Line
adapter is also used, at times, as another term for data
set or modem.
Line hit is electrical interference that causes spurious
signals on a data circuit.
Mark indicates the presence of a bit on a data circuit.
(See Data Format in Theory of Operation.)
Modem. (See data set under Common Terms.)
Off hook indicates that the line is connected or active.
On hook indicates that the line is disconnected or
inactive.
1 A trademark of the Bell system.
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