Communication
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What does communication rely on?
Transmitting and receiving data in the form of electromagnetic signals
What is synchronous transmission?
Streams of bits are transferred over a communication channel at a constant rate, the transmitter and the receiver are synchronised using a common clock signal
What is asynchronous transmission?
There is no clock signal so additional data (start & stop bits) is used to control the communication. Data is transmitted when it’s available rather than at specific intervals. This means there’s periods of time when the transmission channel is idle
What pair has to be opposite in asynchronous transmission?
The start and stop bit
Why would the stop bit be longer than 1 bit?
To give the receiver time to reset
What other type of transmission is synchronous transmission used with?
Parallel
Explain serial transmission
Data bits are sent in a sequence, one after the other, over a single wire
Explain parallel transmission
Several bits are sent at the same time over their own wire
When is parallel transmission used?
Inside computers as it’s unreliable over long distances
What is crosstalk?
Electromagnetic interference between wires that are in proximity (such as parallel links) results in transmitting corrupted data that will need to be resent
What is Skew?
When the bits that are transmitted across parallel links travel at different speeds, this can result in data falling out of sync with the clock signal
What type of transmission operates better at higher bit rates?
Serial
Advantages of serial transmission
It’s not affected by skew and uses fewer wires in close proximity reducing the risk of crosstalk As it uses fewer wires it’s also cheaper and takes up little space
Define bit rate
The number of bits transmitted over a channel each second
What are the units for bit rate?
Bits per second
What does a high bit rate mean?
Data arrives faster
Define baud rate
The number of times the signal changes per second/ The symbol rate of a channel
What’s a symbol?
A set of bits that are transmitted with every signal change
How many signal levels does baseband support?
2
How does baseband transmission work?
Every signal change represents one bit ( 0 or 1) so one symbol encodes one bit and the bit rate is equal to the baud rate
How does broadband transmission work?
It supports more than 2 signal levels ( multiple frequency bands/voltage levels) so every signal change can represent more than one bit (2,4 or 8 bits) So every second the number of bits that’s transmitted equals signal changes/s multiplied by the no.bits encoded per signal change
Bit rate formula
Baud rate * number of bits per signal change
Baud rate formula
Bit rate / Number of bits per signal change
Define bandwidth
The maximum rate of data transfer of a communication channel