Data or information can be stored in two ways, analog and digital.
For a computer to use that data is must be in discrete digital form.
Like data, signals can also be in analog and digital form. To transmit
data digitally it needs to be first converted to digital form.
Digital-to-digital conversion
This section explains how to convert digital data into digital
signals. It can be done in two ways, line coding and block coding. For
all communications, line coding is necessary whereas block coding is
optional.
Line Coding
The process for converting digital data into digital signal is said
to be Line Coding. Digital data is found in digital format, which is
binary bits. It is represented (stored) internally as series of 1s and
0s. [Image: Line Coding]
Digital signals which represents digital data, represented as
discrete signals. There are three types of line coding schemes
available: [Image: Line Coding Schemes]
Uni-Polar Encoding
Unipolar encoding schemes uses single voltage level to represent
data. In this case, to represent binary 1 high voltage is transmitted
and to represent 0 no voltage is transmitted. It is also called
Unipolar-Non-return-to-zero, because there’s no rest condition i.e. it
either represents 1 or 0. [Image: UniPolar NRZ Encoding]
Polar Encoding
Polar encoding schemes multiple voltage levels are used to represent
binary values. Polar encodings are available in four types:
Polar-NRZ (Non-return to zero)
It uses two different voltage levels to represent binary values,
generally positive voltage represents 1 and negative value represents 0.
It is also NRZ because there’s no rest condition.
NRZ scheme has two variants: NRZ-L and NRZ-I. [Image: NRZ-L and NRZ-I]
NRZ-L changes voltage level at when a different bit is encountered whereas NRZ-I changes voltage when a 1 is encountered.
RZ (Return to zero)
Problem with NRZ was the receiver cannot conclude when a bit ended
and when the next bit is started, in case when sender and receiver’s
clock are not synchronized. [Image: Return-to-Zero Encoding]
RZ uses three voltage levels, positive voltage to represent 1,
negative voltage to represent 0 and zero voltage for none. Signals
change during bits not between bits.
Manchester
This encoding scheme is a combination of RZ and NRZ-L. Bit time is
divided into two halves. It transitions at the middle of the bit and
changes phase when a different bit is encountered.
Differential Manchester
This encoding scheme is a combination of RZ and NRZ-I. It also
transitions at the middle of the bit but changes phase only when 1 is
encountered.
Bipolar Encoding
Bipolar encoding uses three voltage levels, positive, negative and
zero. Zero voltage represents binary 0 and bit 1 is represented by
altering positive and negative voltages. [Image: Bipolar Encoding]
Block Coding
To ensure accuracy of data frame received, redundant bits are used.
For example, in even parity one parity bit is added to make the count of
1s in the frame even. This way the original number of bits are
increased. It is called Block Coding.
Block coding is represented by slash notation, mB/nB, that is m-bit
block is substituted with n-bit block where n > m. Block coding
involves three steps: division, substitution and combination.
After block coding is done it is line coded for transmission.
Analog-to-digital conversion
Microphones creates analog voice and camera creates analog videos,
which here in our case is treated is analog data. To transmit this
analog data over digital signals we need an analog to digital
conversion.
Analog data is wave form continuous stream of data whereas digital
data is discrete. To convert analog wave into digital data we use Pulse
Code Modulation.
Pulse Code Modulation is one of the most commonly used method to
convert analog data into digital form. It involves three steps:
Sampling, Quantization and Encoding.
Sampling
[Image: Sampling of Analog Signal]
The analog signal is sampled every T interval. Most important factor
in sampling is the rate on which analog signal is sampled. According
to Nyquist Theorem, the sampling rate must be at least two times of the
highest frequency of the signal.
Quantization
[Image: Quantization of sampled analog signal]
Sampling yields discrete form of continuous analog signal. Every
discrete pattern shows the amplitude of the analog signal at that
instance. The quantization is done between the maximum amplitude value
and the minimum amplitude value. Quantization is approximation of the
instantaneous analog value.
Encoding
[Image: Encoding from quantization]
In encoding, each approximated value is then converted into binary format.
Transmission Modes
How data is to be transferred between to computer is decided by the
transmission mode they are using. Binary data i.e. 1s and 0s can be
sent in two different modes: Parallel and Serial.
Parallel Transmission
[Image: Parallel Transmission]
The binary bits are organized in to groups of fixed length. Both
sender and receiver are connected in parallel with the equal number of
data lines. Both computer distinguish between high order and low order
data lines. The sender sends all the bits at once on all lines.
Because data lines are equal to the number of bits in a group or data
frame, a complete group of bits (data frame) is sent in one go.
Advantage of Parallel transmission is speed and disadvantage is the cost
of wires, as it is equal to the number of bits needs to send
parallelly.
Serial Transmission
In serial transmission, bits are sent one after another in a queue
manner. Serial transmission requires only one communication channel as
oppose parallel transmission where communication lines depends upon bit
word length. [Image: Serial Transmission]
Serial transmission can be either asynchronous or synchronous.
Asynchronous Serial Transmission
It is named so because there’s no importance of timing. Data-bits
have specific pattern and helps receiver recognize when the actual data
bits start and where it ends. For example, a 0 is prefixed on every
data byte and one or more 1s added at the end.
Two continuous data-frames (bytes) may have gap between them.
Synchronous Serial Transmission
It is up to the receiver to recognize and separate bits
into bytes. The advantage of synchronous transmission is speed and it
has no overhead of extra header and footer bits as in asynchronous
transmission.
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