Paper 1: Testimonial

Data transmission

When we want to transfer something, it is better to put it in bag rather that sending all of it in one go, thats why data lso sent in packets

Data packet

Header

Contains the IP address of the sender and the receiver
The sequence number of the packet
Size of the packet

Payload

Contains the actual data

Trailer

Includes a method of identifying the end of the packet
Error-Checking methods

Packet switching

When the data is broken into multiple packets. Packets are then sent independently from start to end and reassembled at the receiver’s computer.A router controls the route a packet takes

Adventages of packet switching

Efficient – the packets are always sent along the fastest route.
Reliable – the packets can take alternative routes if some parts of the network are down.
Secure – the packets can take different directions across a network

Disadvenatages of packet switching

packet might be lost
Delay at the receiver while the packets are being re-ordered

Types of data transmission

Serial data transmission is when data is sent one bit at a time over a single wire

Parallel data transmission is when data of several bits (1 byte) are sent down several wires at the same time

Simplex data transimssion one direction transmission

Half-duplex data transmission is in both directions but not at the same time

Full-duplex data transmission is in both directions simultaneously

Parallel,Full-duplex data transmission

Advantages

faster time of trasmission compared to serial
execelnt for short distances

Disadvantages

data can be skewed due to transmiting in multiple wires
expensive
many wires required

Serial,Simplex,Half-duplex data transmission

Advantages

Works well over a long distance using a single cable
Cheaper

Disadvantages

Slower in transmission
Data cannot be synchronised due to one cable

The Universal Serial Bus (USB) Interface

USB is an asynchronous serial data transmission method

USB consists of

Four-wire shielded cable
Two wires are used for power and earth
Two wires are used in data transmission

Error detection

Computers needs to check data that is being transmitted for errors, caused by interference, e.g. data loss, data gain and data change

Parity bit

first we need to decide on which parity we are using, odd or even If a computer system had decided to use odd parity, it means that the number of 1s and 0s must be odd.If the parity will be even, then both of 1s and 0s must be even number

Parity block

This is other method of representing parity bits system in a table or a block, with this visualised table it will be easier to spot error as we can follow rows and columns

Check digit

Check digit is additional digital added at the end of the string or number. Printed books and other products have a unique barcode with a ISBN (International Standard Book Number) or EAN (European Article Number), the first 12 digits are unique item digits and 13th is check digit

First you multiply ISBN number by weight and add them all, then you devide by 10 and remainder should be your check or last digit

Checksum

A checksum works in a similar way to a check digit, this method sums up number of bits of transmitted data and sends it with packet, the reciever computer calculates number of bits and compares it with number sent

Automatic Repeat Requests (ARQ)

Its an automatic system of error detection which uses acknowledgements and timeouts, if acknowledgement is not recieved is pre-seted time then it will resend data one more time assuming that there was error in transmission

Echo check

During this error detection method recieving computers immediatelly sends copy of data to sender, sender compares data and if there is any differences error is spotted

Topic 2 - Data transmission

In this section you will find out how computers transfer data

Encryption

Encryption is the conversion of data (plain text) into a form that is unreadable (ciphertext).We use encryption to protect data from hackers, we need encryption key to convert our ciphertext to normal text, encrypted data consists of numbers and letter and is unreadable, 128bit key is less secure than 256 bit key

Symmetric Encryption:

It uses an encryption key for the encryption process; the same key is used for encrypting and decrypting the data

Asymmetric Encryption:

Uses a public key and a private key. The public key is available to everyone, whereas the private key is only available to the user.
The receiver would have the private key, and they would send the public key to the sender. The sender can encrypt the message with the public key, and the data can be decrypted using the private key.

by: Arnur Mussabekov