Communication
Communication Methods
Data communication is the transmission of data form one device to another (ge
between computers), or between components along a bus within a computer such
as between RAM and the processor.
Serial transmission
• one bit at a time are transmitted between computer components. Only a
single wire is used.
• Serial transmission down a wire is sent as a change in electrical voltage which
is encoded as 0 and 1s.
• The voltages are put on a wire and sent to another computer that can read
these voltages and convert back into binary digital data.
Parallel transmission
• Parallel data transmission means that several bits of data are sent at the same
time along multiple wires between computer components.
• Often 8 bits, 16 bits, 32 bits, 64 bits are sent at the same time
• Parallel transmission is used to communicate with a networked computer.
Advantage
Disadvantages
Serial
Transmission
It can be used
over longer
distances
It is a slow method of data transfer
Parallel
Transmission
It is quicker than
serial data
transfer
Over longer distances bits can get skewed.
This results in the bits going out of
synchronisation and arriving at different
times and therefore out of order.
Interference between wires, so the data
can become corrupted
Synchronous transmission
• Synchronous transmission requires a clock to allow synchronisation between
the sender and receiver
• The sender needs to transmit the data at the same rate that the receiver reads
the data.
• The signals are sent at regular intervals in a continuous stream. Timing signals
are also sent allowing the sending and receiving clocks to be synchronised
Asynchronous transmission
• The data are sent in chunks normally as one byte or ACSII character.
• The transmission contains a start and stop bit that specify the beginning and
end of the sequence and a parity bit for error checking.
• The start bit are used in asynchronous data transmission to prepare the
receiver to start receiving data.
• The stop bit resets its state allowing it to read the next sequence.
• The values of the start bit and stop bit must be different. ie if the start bit is 0
then the stop bit is 1 and vice versa.
• Asynchronous transmission is slower than synchronous transmission.
Communication Basics
A Protocol is a set of common rules that determine how different devices
communicate
Latency is the delay in data transfer from one component to another. At the most
fundamental level the speed of data transfer is limited by the speed of light.
Latency is also caused by transmission in the medium itself, and other necessary
processing.
Baud rate is the number of changes of signal per second (frequency) and in
measure in Hertz
Bit rate is the number of bits transmitted per second.
Bit rate = baud rate x bits per signal change
The baud rate will be the same as the bit rate only when one bit is sent for each
change of signal. In this situation there are 2 voltage levels representing zero and
one. It is possible to send more that 1 bit per signal change. For instance if there
are 4 voltage levels, the voltages can be encoded as 00, 01, 10, 11.
Bandwidth
• refers to the range of frequencies at which the data can be transmitted
• relates to the volume of data that can be transmitted along the medium in a
given period of time.
• It is expressed in Hertz as the difference between the lowest and highest
frequency.
• The greater the bandwidth the greater the capacity of data transfer.
• Data are transmitted using frequency modulation (FM). Different frequencies
are used to encode different values.
• Previously when using dial up over the phone cable two frequencies were
used (2000 Hz and 1000 Hz) to transmit 0 and 1s.
• Now a greater range of frequencies can be used so a greater number of bits
per frequency can be transmitted.
• Noise limits the amount of data that can be sent.
• There is a directly proportional relationship between bandwidth and bit rate.
That is the greater the bandwidth the greater the bit rate.
Network Topologies
A network topology describes how a set of computers are arranged within a
network.
Bus topology All devices including clients, servers, printers and so on are
connected to a cable called a bus. All communication is via the shared bus. At
either ends of the bus is a terminator.
Star network topology For a star topology all devices including clients, servers,
printers and so on are connected to a central hub or switch. All communication is
via the hub.
Advantages
Disadvantages
Bus
topology
Easy and cheap to install
and does not require much
cable.
Easy to add more computers
If the main cable fails then then the
whole network fails.
Less secure as data are broadcast to
all devices on the network.
Can be slow as there are collisions
between data along the shared bus.
Will get slower as more computers
are added.
Star
topology
Greater security as data are
only sent to the intended
recipient.
If any of the connections fail
only a single node will be
affected. The rest of the
network will work as
normal.
Fewer collisions between
data packets
If the central hub fails then every
computer on the network is
affected.
Expensive as extra cable and
hardware (hubs) are needed.
Logical bus network topology
• It is possible that a network that has been physically set up with cables based
on one topology can behave as if is has been set up as another topology.
• For instance, a network that has physically been set up at a star topology can
behave as a logical bus network.
• In this situation the hub is set up so that communication received from one
computer is relayed to all the other devices on the network.
