What is Data Communications?
The distance over which data moves within a computer may vary from a few thousandths
of an inch, as is the case within a single IC chip, to as much as several feet along
the backplane of the main circuit board. Over such small distances, digital data
may be transmitted as direct, two-level electrical signals over simple copper conductors.
Except for the fastest computers, circuit designers are not very concerned about
the shape of the conductor or the analog characteristics of signal transmission.
Frequently, however, data must be sent beyond the local circuitry that constitutes
a computer. In many cases, the distances involved may be enormous. Unfortunately,
as the distance between the source of a message and its destination increases, accurate
transmission becomes increasingly difficult. This results from the electrical distortion
of signals traveling through long conductors, and from noise added to the signal
as it propagates through a transmission medium. Although some precautions must be
taken for data exchange within a computer, the biggest problems occur when data
is transferred to devices outside the computer's circuitry. In this case, distortion
and noise can become so severe that information is lost.
Data Communications concerns the transmission of digital messages to devices external
to the message source. "External" devices are generally thought of as
being independently powered circuitry that exists beyond the chassis of a computer
or other digital message source. As a rule, the maximum permissible transmission
rate of a message is directly proportional to signal power, and inversely proportional
to channel noise. It is the aim of any communications system to provide the highest
possible transmission rate at the lowest possible power and with the least possible
Explain Communications Channels?
A communications channel is a pathway over which information can be conveyed. It
may be defined by a physical wire that connects communicating devices, or by a radio,
laser, or other radiated energy source that has no obvious physical presence. Information
sent through a communications channel has a source from which the information originates,
and a destination to which the information is delivered. Although information originates
from a single source, there may be more than one destination, depending upon how
many receive stations are linked to the channel and how much energy the transmitted
In a digital communications channel, the information is represented by individual
data bits, which may be encapsulated into multibit message units. A byte, which
consists of eight bits, is an example of a message unit that may be conveyed through
a digital communications channel. A collection of bytes may itself be grouped into
a frame or other higher-level message unit. Such multiple levels of encapsulation
facilitate the handling of messages in a complex data communications network.
Explain Asynchronous vs. Synchronous Transmission?
Serialized data is not generally sent at a uniform rate through a channel. Instead,
there is usually a burst of regularly spaced binary data bits followed by a pause,
after which the data flow resumes. Packets of binary data are sent in this manner,
possibly with variable-length pauses between packets, until the message has been
fully transmitted. In order for the receiving end to know the proper moment to read
individual binary bits from the channel, it must know exactly when a packet begins
and how much time elapses between bits. When this timing information is known, the
receiver is said to be synchronized with the transmitter, and accurate data transfer
becomes possible. Failure to remain synchronized throughout a transmission will
cause data to be corrupted or lost.
Two basic techniques are employed to ensure correct synchronization. In synchronous
systems, separate channels are used to transmit data and timing information. The
timing channel transmits clock pulses to the receiver. Upon receipt of a clock pulse,
the receiver reads the data channel and latches the bit value found on the channel
at that moment. The data channel is not read again until the next clock pulse arrives.
Because the transmitter originates both the data and the timing pulses, the receiver
will read the data channel only when told to do so by the transmitter (via the clock
pulse), and synchronization is guaranteed.
Explain Parity and Checksums?
Noise and momentary electrical disturbances may cause data to be changed as it passes
through a communications channel. If the receiver fails to detect this, the received
message will be incorrect, resulting in possibly serious consequences. As a first
line of defense against data errors, they must be detected. If an error can be flagged,
it might be possible to request that the faulty packet be resent, or to at least
prevent the flawed data from being taken as correct. If sufficient redundant information
is sent, one- or two-bit errors may be corrected by hardware within the receiver
before the corrupted data ever reaches its destination.
A parity bit is added to a data packet for the purpose of error detection. In the
even-parity convention, the value of the parity bit is chosen so that the total
number of '1' digits in the combined data plus parity packet is an even
number. Upon receipt of the packet, the parity needed for the data is recomputed
by local hardware and compared to the parity bit received with the data. If any
bit has changed state, the parity will not match, and an error will have been detected.
In fact, if an odd number of bits (not just one) have been altered, the parity will
not match. If an even number of bits have been reversed, the parity will match even
though an error has occurred. However, a statistical analysis of data communication
errors has shown that a single-bit error is much more probable than a multibit error
in the presence of random noise.
What is Data Compression?
If a typical message were statistically analyzed, it would be found that certain
characters are used much more frequently than others. By analyzing a message before
it is transmitted, short binary codes may be assigned to frequently used characters
and longer codes to rarely used characters. In doing so, it is possible to reduce
the total number of characters sent without altering the information in the message.
Appropriate decoding at the receiver will restore the message to its original form.
This procedure, known as data compression, may result in a 50 percent or greater
savings in the amount of data transmitted. Even though time is necessary to analyze
the message before it is transmitted, the savings may be great enough so that the
total time for compression, transmission, and decompression will still be lower
than it would be when sending an uncompressed message.
Some kinds of data will compress much more than others. Data that represents images,
for example, will usually compress significantly, perhaps by as much as 80 percent
over its original size. Data representing a computer program, on the other hand,
may be reduced only by 15 or 20 percent.
A compression method called Huffman coding is frequently used in data communications,
and particularly in fax transmission. Clearly, most of the image data for a typical
business letter represents white paper, and only about 5 percent of the surface
represents black ink. It is possible to send a single code that, for example, represents
a consecutive string of 1000 white pixels rather than a separate code for each white
pixel. Consequently, data compression will significantly reduce the total message
length for a faxed business letter. Were the letter made up of randomly distributed
black ink covering 50 percent of the white paper surface, data compression would
hold no advantages.
Explain Data Encryption?
Privacy is a great concern in data communications. Faxed business letters can be
intercepted at will through tapped phone lines or intercepted microwave transmissions
without the knowledge of the sender or receiver. To increase the security of this
and other data communications, including digitized telephone conversations, the
binary codes representing data may be scrambled in such a way that unauthorized
interception will produce an indecipherable sequence of characters. Authorized receive
stations will be equipped with a decoder that enables the message to be restored.
The process of scrambling, transmitting, and descrambling is known as encryption.
Custom integrated circuits have been designed to perform this task and are available
at low cost. In some cases, they will be incorporated into the main circuitry of
a data communications device and function without operator knowledge. In other cases,
an external circuit is used so that the device, and its encrypting/decrypting technique,
may be transported easily.
What is Data Storage Technology?
Normally, we think of communications science as dealing with the contemporaneous
exchange of information between distant parties. However, many of the same techniques
employed in data communications are also applied to data storage to ensure that
the retrieval of information from a storage medium is accurate. We find, for example,
that similar kinds of error-correcting codes used to protect digital telephone transmissions
from noise are also used to guarantee correct readback of digital data from compact
audio disks, CD-ROMs, and tape backup systems.
What is analog?
Although my artistic ability leaves much to be desired, this wave form is a depiction
of a simple analog signal. The key to the analog signal is that it is *continuous*.
In other words, notice how the wave slowly rises, peaks, slowly descends, bottoms
out and slowly climbs again. Taken as a simple example, imagine many forms of this
wave signal. Some of the waves are closer together than others, some may have more
height, still others may actually start their peaks and descents in entirely different
places! Encoding data can be done based on these various kinds of wave changes.
One of the important considerations in analog communications is the ability to decode
these continuous wave forms. With the introduction of noise, or other signal disturbance,
decoding a analog signal properly can be difficult. This is why we turn to the digital
What is digital?
Compared to the picture of the analog signal above, there is a major difference
in this wave form. The transition from the peak of the wave to the bottom of the
wave is *discrete*. In this case, the only way to represent data is by using the
high or low point of the wave. For example, the high point may represent a "on"
signal and the low point may represent a "off" signal. In the world of
computers, this is also known as a binary numbering system consisting of only two
digits. By using a digital signaling system in this fashion, it makes encoding and
decoding data very simple. Generally, it will be very easy to determine where the
peaks and valleys are, even with some signal loss or disturbance.
Digital methods are used as long as frequency response (bandwidth) is not a limitation.
Analog methods are used only because multiple signal levels must be exploited to
communicate a higher data rate of digital values in lieu of having adequate bandwidth.
A digital signaling system often has an analog component. Strictly speaking, this
means the a digital wave isn't as sharp cornered as the picture shows above.
The corners will likely be slightly rounded and even more so as the signal travels
over some distance. For our purposes, this definition should give you a basic idea
of how a digitally encoded system works.
Modulation is a prescribed method of encoding digital (or analog) signals onto a
waveform (the carrier signal). Once encoded, the original signal may be recovered
by an inverse process called demodulation. Modulation is performed to adapt the
signal to a different frequency range than that of the original signal. Here's
how it flows:
bits -> modulator -> audio -> phone network -> audio -> demodulator
Hence the name MODEM short for modulator/demodulator. The modem is necessary because
the phone network transmits audio, not data bits. The modem is for compatibility
with existing equipment.
What is crosstalk?
Crosstalk refers to the interference between channels. In the xDSL world, the interference
between nearby cables can have a negative impact on the performance of the affected
cable(s). Have you ever been on the phone and heard some other conversation, not
yours, in the background? If so, you have experienced the effect of crosstalk.
Near-end crosstalk (NEXT) occurs when the transmitter sends a signal and a nearby
transceiver at the same end of link, through capacitive and inductive coupling,
"hears" the signal.
Far-end crosstalk (FEXT) occurs when the transmitter sends a signal and a transceiver
at the far end of the link, through capacitive and inductive coupling, "hears"
the signal. FEXT will be of more concern in an asymmetrical system such as ADSL
than symmetrical systems like HDSL. This is because strong signals originating from
the near end, can interfere with the weaker signals originating at the far end.
What is the effect of noise?
Noise may be defined as the combination of unwanted interfering signal sources whether
it comes from crosstalk, radio frequency interference, distortion, or random signals
created by thermal energy. Noise impairs the detection of the smallest analog levels
which may be resolved within the demodulator. The noise level along with the maximum
clip level of an analog signal path set the available amplitude dynamic range.
The maximum data rate of a modem is limited by the available frequency range (bandwidth)
and signal-to-noise ratio (SNR) which is amplitude dynamic range. If more of either
is available, more bits may be transferred per second. The information carrying
limit was discussed theoretically by Claude Shannon and is known as Shannon's
limit, or information theory.
Because modems run close to Shannon's limit today, no further advances will
be made to traditional telephone line modems other than incremental improvement
of V.90. The frequency range of the audio channel is very limited at about 4 kHz.
V.34+ modems are limited to a maximum data rate of 33.6Kb/s by an SNR of about 36
dB caused mostly by network PCM quantization noise. While V.90 improves the SNR
by utilizing the network PCM levels directly, it is still subject to Shannon's
xDSL modems take advantage of the spectrum above the telephone audio channel. While
operating with somewhat less amplitude dynamic range they increase data rates by
greatly increasing the frequency range of the communication signal (from about 10
kHz to over 1.0mHz). To do this they require the installation of special equipment
at the central office and customer premise.