Transcript communicatingSoftware

The Principle of Electronic Data
Serial and Parallel Data
Communication
Transmission Rate
Bandwidth
Bit Rate
Parity bits
The Principle of Electronic Data

Involves sending and receiving information data
from one computer or data processing device to
another.
E-mail,
Supermarket EPOS (Electronic Point of
Sale) Terminals,
Cash Dispensers,
Examples
Facsimile
Video Conferencing.
Serial Data Communication.

Serial Data Transmission: bits are sent
via an interface one bit at a time over a
single wire, from the source to the
destination.
0
1
0
1
1
0
Serial Data Communication.

(An example of a very high data transfer
rate is through the use of fibre-optic cable,
which has a data transfer rate of 1000Mbs
per second.)
Parallel Data Communication.

Parallel Data transmission is used inside
the computer (using various computer
buses) and for short distances (a few
metres), so to prevent data being
misread.
0
1
0
1
1
0
Parallel Data Communication.
A parallel port can send 8, 16, or 32
bits simultaneously down separate
lines.
 A printer uses the parallel port.

Parallel Data Communication.
Limited to short distances of a few metres,
e.g. between a computer and printer.
 Limitation because it is very difficult to
produce copper wire with the exact same
resistance.
 Simultaneous transmission of data over
distances can result in skewing of data
(bits not arriving in parallel) due to varying
resistance of wires.

Transmission Rate
The Baud Rate is named after Baudot who
did a lot of work with electronic signals.
 The Baud Rate is the rate that the voltage
changes and the speed at which the data
is transmitted serially.

0
1
0
1
1
0
Transmission Rate
The sender will generate one voltage to
represent 1 and a different voltage to
represent 0.
 The sender has a clock which transmits
bits at regular intervals.
 At any point in time there will be a
number of bits being carried along the
wire.
 Generally one baud is one bit per second.

Baud Rate
A baud rate 56K does not mean
that 7000 characters per second
will be transmitted: the figure will
be lower.
Why?

Framing bits have to be transmitted with
each character to provide
– start and stop bits,
– error detection
– and communication controls.
Bandwidth

Bandwidth measures signal changes per
second, and is measured in Baud.
When a signal is transmitted along a wire (or
any other medium) it is reluctant to travel as a
square since waves are naturally in the form
of sine waves (analogue signals).
Bandwidth
•All media are capable of transmitting a set of waves
that have a range of frequencies.
•The frequency of the wave is the rate at which the wave
repeats itself.
The higher
the
bandwidth
the greater
the amount
of data that
can be
transmitted.
High Frequency
Low Frequency
Bandwidth

The range of the frequencies that a
medium can transmit is the bandwidth.

If the bandwidth is wide then more data
can be transmitted.
– A normal telephone wire has low bandwidth so it is
not possible to transmit large amounts of data.
– An optical fibre has a high bandwidth and is able to
transmit large amounts of data.
Bit Rate

Is the term given to the rate that the
bits are transmitted.
Remember the baud is the rate the
voltage changes.

If the voltage changes 10 times every
second the baud is said to be ten.
Bit Rate

If four voltages were generated instead of
two, each change in signal could be used
to represent two bits.

Therefore, if the signals are transmitted
10 times a second, the baud rate is still 10
but 20 bits per second are being
transmitted.
Remember

Baud Rate refers to the rate that the
signals are transmitted along a wire.

Bit Rate refers to the rate that bits are
transmitted.
Parity bits
 A parity
bit is an extra bit added to the
character code
 Set to 0 or 1
 so that total number of 1’s in code is
even, for even parity
 or total number of 1’s in code is odd,
for odd parity.
Parity bits
Parity checks are used to determine
whether the parity of a data item is
correct when it is being sent to or from
an input, output or storage device, or
being sent over a communications line.
 Odd parity will detect the failure of a
transmission line, because all the bits
will then be set to zero.
 Note that the character code, including
the parity bit, is an example of a self
checking code.

Parity bits

Add a parity bit to the end (LHS) of each of the
following codes to maintain odd parity:
110101
001101

Remember:
– Set to 0 or 1
– so that total number of 1’s in code is even, for even parity
– or total number of 1’s in code is odd, for odd parity.
Parity bits

The ASCII codes for
 P is 1010000
 Q is 1010001
 In an even parity transmission system what will
be the value and parity bit for P and Q?
Synchronous and Asynchronous
transmission
Two types of serial mode for sending data
Asynchronous transmission
 one character at a time, with a start and stop bit and
parity bit sent with each character.
 For each character sent, 10 bits are transmitted
 Start bit alerts the receiving device and synchronises the
clock inside the receiver ready to receive the character.
 Baud rate at receiving end has to be set up be same as
senders.
 Usually used by PC’s, fast and economical for relatively
small amounts of data.
Synchronous transmission
Enables whole blocks of data to be sent in timed
sequences
 Timing signals (usually computer’s internal clock)
control rate of transmission
 There is no need for start and stop bits to
accompany each character.
 System is less error prone than asynchronous
 Much faster – usually used on mainframe
computers

Handshaking

The exchange of signals between devices to establish
their readiness to send or receive data.
– E.g. between a computer and printer
 One method of ensuring both sender and receiver are
ready before transmission begins
 The conversation between two devices along lines of:– Device 1: “Are you ready to receive some data?”
– Device 2: “Yes, go ahead.”
– Device 1: (sends data)
– Device 2: “Message received, thanks!”