Session Objectives#11
COULD explain 3 methods of error checking transmitted data
SHOULD describe how data is transmitted using serial, parallel, simplex and duplex
modes.
MUST identify the hardware required to connect to the Internet
Using FOR and WHILE loops to move images across the app screen
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Data Transmission Types
Data Transmission is the process of transporting data from one
place or device to another. There are different ways of doing
this: Consider 1 byte of data (8 bits):
Serial Data Transmission – sends data bit by bit (1 bit at a
time) through a single wire.
Parallel Data Transmission – sends all 8 bits at the same time
using a number of wires (one for each bit).
Can you think of any example where serial and parallel data
transmission has been used?
You would think parallel would be the preferred method as it
seems quicker BUT because serial requires less signal
processing, there are less chances for error than parallel
transmission thus the transfer rate of each individual path
may be faster.
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Data Transmission Modes
Direction of data movement:
Simplex = One way
Duplex = Both Directions at the same time
Half Duplex = Both Direction but one after the other.
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Data Transmission Speed
Bit Rate (Baud Rate)
Bit Rate – The number of bits that can be transferred in a given
space of time i.e bits per second. This is measured in units
called baud which is also known as the baud rate.
The bit rate is really determined by file use rather than file
type. For example, a film could take days to download at a slow
rate and watched later but if it were to be streamed, then it
would need a high bit rate to ensure the data is available at
the right time and not cause lag to the audio visual output.
Files that are time sensitive are files that lose their value
unless they arrive in a particular time period.
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Recap
Let’s Recap the key terms on data transmission so far:
Name 2 types of transmission.
Serial,
Parallel
Name 3 modes of transmission.
Simplex,
Duplex,
half duplex
What is transmission speed referred to and what is it
measured in?
Bit Rate and baud
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Error Checking
We have already mentioned that some data may get lost or be
transmitted in the wrong order. A bit which starts with 1 may
end with 0, for example. This causes the data or file to become
corrupt in its new location.
To reduce this chance 3 methods are used to check that data
integrity (staying in its original state) is kept when being
transmitted.
Echoing Back
Check Sums
Parity check
TASK: In small groups research one of the methods and
prepare to feedback your explanation in the form of a role
play, song or rhyme
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Error Checking
Echoing Back:
When a set of data has been sent, a copy or echo is sent back to
the sender to compare the accuracy against the original. If
there are any errors then the data will be sent again..
Check Sums: The bytes of data being sent are added together and
the answer is sent with the data ‘packet’. When the data bytes
are received they are added together and their ‘check sum’ is
calculated. If the sum totals are the same then it is assumed
that the data has not been corrupted, if they are different the
data will be sent.
Parity Check: Data is sent in fixed byte sizes (usually 8 bits).
The data being sent is in the first 7 bits and the 8th bit is
kept for use in parity. It works by organising the number of 1s
in each byte to an odd or even number, for example four 1s,
which is an even number and so the parity bit would be set to 0.
If there were an odd number then this would be 1.
i.e even parity = 01101101 and odd parity = 01101100
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