What is Data Transmission?
Data transmission is the movement of data (bits) between at least two digital devices.
What is Packet Switching?
Packet Switching is the process when packets take on different routes over the network to avoid congestion and reach its final destination as soon as possible.
What do each packet contain?
Packet Header:
- IP address of the sender
- IP address of the receiver
- Sequence number
- Size of packets
Payload:
- Actual data being sent
Packet Trailer:
- Mechanism to identify the end of the packet and an error checking method
Pros and Cons of Packet Switching?
Pros:
- No single connection is fully occupying a communication line
- Busy or faulty lines will not stop data from reaching its destination
- Expansion is easy to achieve
Cons:
- Lost Packets will need to be resent
- Putting packets back into the correct sequence causes a small delay
What are the different types of transmission?
- Simplex
- Duplex
- Half Duplex
What is Simplex Transmission? and Examples
Simplex is one direction
- e.g. your keyboard to your CPU
What is Half - Duplex Transmission? and Examples
The transfer of data in both direction, one direction at a time.
- e.g. Printer cables
What is Full - Duplex Transmission? and Examples
The transfer of data in both directions at the same time.
- e.g. Telephones
What is Serial Transmission? and Examples
In serial data transmission, bits are sent sequentially (one after the other) down the same wire (channel).
- e.g. USB
What is Parallel Transmission? and Examples
In parallel data transmission, multiple bits are sent simultaneously down different wires (channels) within the same cable.
- e.g. Integrated circuits, inside computers
What is error checking?
When data is transmitted sometimes data can be corrupted during transmission.
Therfore To avoid these problems computers must check for errors.
What is parity Check? And its Method?
A method to check if errors exist using ODD or EVEN
- Before transmission, agreement made between sender and reciever
- They decide which parity wil be used ( 0 for even ) or ( 1 for odd )
- sending computeer sends data
- Recieving computer checks to make sure overall parity is agreed
- If the parity is incorrect, the recieving computer will request that the data is transmitted again.
What are the issues with Parity Check? And how to fix it?
If two bits are transposed (change places) then the computer could be fooled into thinking the data is correct and not corrupted. Therefore Parity blocks can be used.
What are Parity blocks?
They can check multiple bytes at the same time and have parity checks both vertically and horizontally using a parity byte
What is a checksum? And its method?
- A block of data sent alongside calculated checksum value
- Recieving computer calculates what it believes the checksum is
- Checksum compared to see if any errors have occured
- If checksum values dont match, recieving computer request for it to be resent.
How to calculate Checksum?
Step 1: divide value by 256
Step 1: Round down to nearest integer
Step 3: Multiply by 256
Step 4: Original value minus previous value
Transmitted value: 912
912 / 256 = 3.562
3
3256=768
912 – 768 = 144
Checksum= 144
What is Automatic Repeat Request (ARQ)?
- This form of error detection uses a system of acknowledgements and timeouts.
- An acknowledgement message is sent back to the sending computer before a timeout occurs.
- If an acknowledgment is not received the sending computer resends the message
What is Echo Check? And its drawbacks?
- receiving computer sends a copy of the data immediately back to the sending computer for comparison.
- Sending computer compares the two sets of data to check if any errors occurred during the transmission process.
- If an error has occurred, the data will be transmitted again.
Drawbacks:
- If 2 sets of data are different, there is no way of knowing whether error occured when originally sent or when it was sent back
- require lots of extra data to be transmitted
What is Encryption? And its proccess?
Data is encrypted before being sent, once it’s received at the destination computer it then gets decrypted.
- before data is encrypted its is called plain text
- an encryption algorithm is used to scramble the data this uses a special encryption key
- the scrambled data is called cipher text
