packets
A small part of data is transmitted over a network usually 64 KiB
What are the 3 components of a packet
Header
Payload
Trailer
Header
IP Address of sender and receiver
sequence number of packet - so that all the data can get reassembled
Size of packet In bytes
Payload
The actual data
Trailer
Method of identifying the end of
the packet
Error checking method
Cyclic Redundancy Checks
Sending computer adds all the 1 bits in the payload and sends as a hex value in the trailer.
Receiving the computer recalculates the 1 bits and compares the value to the one in the trailer.
Router
Receives a data packet and decides where to send next based on the information in the header
Packet switching
Method of data transmission where data is broken down into packets and sent independently from the start point to the endpoint.
Packets get reassembled at the destination.
General process of packet switching
Data is broken down into packets
* Each packet could take a different route
* A router controls the route a packet takes (always selects the shortest path available)
* Packets may arrive out of order
* Once the last packet has arrived, packets are
reordered
Benefits of packet switching
High transmission rate
there is no need to tie up a single communication line
it is relatively easy to expand package usage
Drawbacks of packet switchign
packets can be lost and need to be re-sent
the method is more prone to errors with real-time streaming
there is a delay at the destination whilst the packets are being re-ordered.
Node
stages in a network that receive and transmit data packets.
real-time streaming
Transmission of data for live events. It is sent as it is received or generated.
Hop number
A number in the header that helps terminates lost packets so that the network doesn’t get clogged.
Factors considered why transmitting data
» the direction of data transmission
» the method of transmission
» how will data be synchronised
Simplex
Can be transferred only in one direction.
Half-duplex
Can be transferred in both directions, but only one at a time.
Full-duplex
Can be transferred in both directions, simultaneously.
Serial
Sending data down one channel/wire one bit at a time
Serial advantage
More reliable over longer distances
Data wont get skewed
Less expensive
Parallel
Sending data down multiple channels/wires multiple bits at a time. (Usually 1 byte at a time)
Parallel advantage
Faster rate of transmission than serial
Works better in shorter distances
easier to program input/output operations when parallel used
Skewed data
Data arrives with bits that are unsynchronised.
Universal Serial Bus (USB)
Serial data transmission which has become industry standard for connecting a computer to a device via a USB port.
What happens when a device is plugged into a computer using a usb port
» the computer automatically detects that a device is present (this is due to a small change in the voltage on the data signal wires in the USB cable)
» the device is automatically recognised, and the appropriate device driver software is loaded up so that the computer and device can communicate
effectively
» if a new device is detected, the computer will look for the device driver that matches the device; if this is not available, the user is prompted to download the appropriate driver software
USB Advantage
Industry standard - is compatible almost everywhere.
Plug and play - no external drivers need to be installed in order to use it
Different transmission rates 1.5mbps - 5gbps
USB Disadvantage
Only up to 5m
V1 USB Isn’t supported on new computers
Parity
Error checking method to check whether data has been changed or corrupted. This method is based on the number of 1 bits in the byte. If odd number then odd parity, if even number then even parity
Parity bit
A bit added to the MSB of a byte to ensure that the byte follows either the even or odd parity protocol.
Parity block
Horizontal and vertical parity check on a block of data that is being transmitted.
Parity byte
Extra byte of data sent at the end of a parity block. Made of the parity bits from the vertical checks of a data block.
Checksum
Error detection method that is calculated from the block of data that has been sent. Value is sent after each block.
Process of checksum
» the checksum is calculated
from the block of data
» the calculation is done using an agreed algorithm
» the checksum is then transmitted with the block of data
» at the receiving end, the checksum is recalculated by the computer using the block of data
» the re-calculated checksum is then compared to the checksum sent with the data block
» if the two checksums are the same, then no transmission errors have occurred; otherwise a request is made to re-send the block of data.
Check digit
what errors can it detect
Calculated from other digits in the code and detect errors in data entry.
Incorrect digit entered
Transposition errors
Omitted or extra digits
Phonetic errors
ARQ - Automated Repeated Requests
Error checking method that uses a system of acknowledgements and timeouts. If the sender receives a negative acknowledgement from the receiver or it doesn’t receive any acknowledgement at all before the timeout, it will resend the data until a positive acknowledgement is received or a pre-determined number of re-transmissions has taken place. If the sender receives a positive acknowledgement then data has been transferred successfully.
Echo check
Data is sent to a receiver, and then the receiver sends that data back to the sender to get compared. If it is corrupted, the sender resends the data .
encryption
Process of making data useless without encryption keys. The correct key is required to decode the data.
Symmetric encryption
The same encryption key is used to encrypt and decrypt a message.
Asymmetric encryption
Uses public and private keys to ensure that data is secure.
Public key
Key that is known to all users
Private key
Key that is only known to a single user.
What is an eavesdropper
Person that intercepts data being transmitted on a wired/wireless network
(hacker)
What is plaintext
what is ciphertext
plaintext– the original text/message before it is put through an encryption algorithm
ciphertext – encrypted data that is the result of putting a plaintext message through an encryption algorithim
What is an encryption Algorithm
a complex piece of software that takes plaintext and generates ciphertext
Explain how asymmetric encryption works
First - Receiver uses an algorithm to generate a matching pair of keys. A public key and a private key
Second - The public key is sent to the sender.
Third - The sender uses the public key to encrypt the data (ciphertext) and sends it back to the receiver
Fourth - Receiver uses the matching private key to decrypt the ciphertext
Can you use a public key to decrypt data in asymmetric encryption?
No
Quantum computers
Very fast computers that can do calculations at a high speed. Performs calculations based on probability not 0 and 1 values. This means it can process a lot more data than a normal computer.
Causes for errors in data transmission
» interference (all types of cable can suffer from electrical interference, which can cause data to be corrupted or even lost)
» problems during packet switching
» skewing of data (parallel data transmission - cause data corruption if the bits arrive out of synchronisation).
Drawback of symmetric encryption
The security issue of keeping the key a secret since the same key is used by the sender and receiver
