In contrast to the use of 20 mA
loop for short distances, for much longer distances,
when the telephone system could be most conveniently
employed, the modems are used. They transform the binary
bit streams into a signal which conforms to the telephone
voice bandwidth and convert back digital to analog.
This is depicted below.
Modem is an abbreviation of the terms modulator and
demodulator. It permits digital signals to be sent over
the telephone analog communications facilities. Modems
are frequently referred as data sets by telephone companies.
The interface between the modem and the computer/terminal
facility is internationally standardized.
The data signal spectrum cannot be directly transmitted
over an analog telephone line that has the Characterstics
shown below.The data contains significant low-frequency
components that would be attenuated by the telephone
channel. Therefore, the modem's main task is translating
the digital signal spectrum to an appropriate spectrum
for the analog system.
Modem in data links
There are two methods of coupling modem to
the communications line:
Indirect or Acoustic coupling.
In this case the data signals are converted into
audible sounds which are readily picked up by the telephone
handset microphone, or transmitter. The audible signals
are then converted back to electrical signals and transmitted
over the telephone system.
The disadvantage of this method is that because of the
extra conversion involved, it tends to introduce more
noise and distortion than direct coupling and hence
is limited to information rates of less than 1200 bits/sec.
Direct hook up.
It is used in permanent installations. When a non-standardized
modem is used, a Data Access Arrangement (DAA) device
or some other suitable protective device must be inserted
to make a connection to the standardized-switched network.
A DAA is not required onleast
lines.
Turn-Around and Message Acknowledgement. In a typical
modem, two different frequencies can be used to represent
the MARK or SPACE input data. As shown in illustration
below, the modem can operate in either the half duplex
or full-duplex mode.
Half duplex operation.
It is a bidirectional operational operation but
only in one direction at any given time. When modems
switch from transmit to receive and vice versa, some
data transmission reversal significantly reduces net
data through it since time is wasted in establishing
appropriate signal levels and synchronization. In order
to overcome this problem, a narrow reverse channel bandwidth
is often used simultaneously with the main, or data-channel,
transmission. The reverse or secondary channel does
not require a large bandwidth since return of only a
positive acknowledgement (PAK) or a
negative acknowledgement (NAK) is required.
However, such a system remains a half-duplex system.
Type of modem
operation
The illustration below
shows data channel bandwidth of a typical modem, which
employs a main data channel that denotes a MARK
with a 1200 Hz tone and a SPACE
with a 2200 Hz tone. Note that
such frequency shift keying (FSK) causes the
data channel to extend over a bandwidth. The secondary
channel is narrowly centered around 387 Hz. It can be
used to inform the transmitting station that the previous
message block contained errors and should be retransmitted.
Full-duplex
operation
It is characterized by
simultaneous bidirectional data transmission. Typical
modems used on a two-wire line divide the channel bandwidth
into two transmit and receive sub channels.The illustration
below shows the spectrum of a full-duplex modem using
two-wire line.
Digital Data
Transmission
We observe:
When the modem is in the originate mode, it transmits
on 1070/1270 Hz.
In the answer mode, it transmits on the 2025/2225
Hz frequencies.
The modem goes into the originate mode when a call
is originated through modem.
It goes into the answer mode, if it initially answers
a call.
Frequency spectrum in a full-duplex
modem.
For the higher bit rates, 4- wire line
is used in which a separate pair of wires is used for
each direction. Often the electrical characterstics
of the line for bit rates exceeding 2400 bits/sec is
equalized or flattened out in order to achieve satisfactory
performance. Since the attenuation characterstics are
propagated velocity experienced by the various signal.
These variations in frequency are compensated out by
employing line equalizers. For leased
lines, i.e., private links, the equalizers can be manually
adjusted. Automatic equalizers are
also used but their disadvantage is that they consume
up to 50 m/sec in the equalizing process, and results
in reduction of the net data through it. However, techniques
are rapidly being developed to reduce this training
time.
The above as well as the below illustration present
the power spectrum of a typical voice-band data signal.
Clearly, it is quite different from that of the human
voice. The power spectral density of most data signals
is designed to have a raised cosine spectrum centered
at about 1800 Hz.
The power spectrum shown in below illustration is that
of a 1200 baud, for phase PSK voice band data
signal .
In most systems, voice-band channels
are designed for speech signals (as in above illustration).
On account of different characterstics in spectral density
of voice band data, the performance is less than optimal
when voice-band data is transmitted over the same system.
Presently, new coding systems are being introduced in
the modems to resolve this incompatibility.
Click here to view the table which presents the
typical Characterstics of some of the modems commercially
available.
