12 ipt 0301: Characteristics of a Communication System
Communication SystemsCommunication Systems
3.1) Characteristics of a
• There are five main parts of any
• a data source
• a transmitter for encoding
• a transmission medium
• a receiver for decoding
• a data destination
• When a message is sent it goes through
three levels of being: application,
control and transmission
• Protocols are sets of rules that outline
the transfer of data between computers
• Handshaking is an agreement between
computers as to which protocols to use
• The speed of transmission is
determined by the bandwidth.
• Bandwidth is the capacity of a
• The speed of transfer can be measured
in baud rate or bits per second (bps)
• Baud rate is the number of electrical
signals that can be transmitted in a
second, while bps is how many bits can
• One signal can contain multiple bits so
bps can be greater than the baud rate
• When data arrives at its destination it
may contain errors so there needs to be
ways of detecting these errors.
• Three common error checking methods
• parity checking
• cyclic redundancy check (CRC)
• Parity checking involves adding an
extra bit to a data packet that makes
the total number of 0’s odd or even.
• In even parity checking the total
number of 0’s, including the parity bit,
must be even; in odd parity it must be
• E.g. 1000111 is sent. If we are using
even parity a 0 is added, if we are using
odd a 1 is added.
• Practice: what would be the result if the
data packet 1100110 is sent with even
• Answer: 11001100
• Practice 2: what would be the resulting
transmission if 1100010 was sent with
• Answer 2: 11000100
• This simplistic method only works for an
odd number of errors; an even number
of errors cancel each other out.
• Checksum is a method of error
checking that counts the number of bits
in a data packet.
• The count is attached to the data
packet and then the packet is counted
at the receiving end and if the two
counts match then the message is
• This method is about 90% effective.
• Cyclic redundancy check (CRC) is the
method of checking using a division
• The data is divided using a fixed
number and the remainder is attached
and sent with the data.
• The same calculation is carried out at
the receiving computer and the two
• If the two remainders do not match then
an error has occurred.
• CRC achieves 99.99% detection of
• There are several communication
settings that are agreed upon during
• These include:
• bits per second
• data bits
• stop/start bits
• flow control
• Bps is the speed of transmission.
• Data bits are the number of bits sent in
a data packet (usually 7 or 8 bits).
• Parity is whether the data contains a
parity bit for error checking.
• Stop and start bits are used in
asynchronous transmission to identify
• Flow control is the type of software
handshaking protocol used.
[Complete L.A.’s 1-4, p.80]