Data Transmission and Security

• Copper cables using electrical pulses • Radio frequency (wireless) • optical fibre (light) • infrared

Carry almost 100% of worldwide internet and phone traffic

Amount of data that can be carried at one time

Bits are sent one signal at a time Advantages over parallel: • smaller and simpler connectors • crosstalk creates interference between parallel lines and can result in corrupted bytes which would need to be retransmitted • serial links are much more reliable over greater distances than parallel

• Signals (bits) are sent simultaneously over a number of parallel wires

Crosstalk occurs when a signal from one line gets transferred to another line

Each wire in a parallel cable has slightly different properties so the signals travel down the wires at slightly different speeds. This means the transmission must be over very short distances to avoid this problem

Data travels in one direction only

Data can travel in both directions along a single cable, but not simultaneously (not in both directions at the same time)

Data can travel in both directions simultaneously using two cables

The speed of data is measured in bits per second The baud rate is the rate at which the signal changes Baseband means one signal (bit) is sent at a time down a serial connection In standard baseband, bit rate = baud rate

With higher bandwidths, more than one bit can be coded in a signal, increasing the bit rate • Bit rate of channel = baud rate x number of bits per signal

When data is transmitted, it doesn’t always arrive in the same format that it was sent • Electric interference • Power surges • Synchronisation • Wear and tear on the cable or connectors

Computers need methods to check for data transmission errors caused by interference These methods include the use of: • Parity bits, blocks • Check digits • Check sums • Automatic Repeat reQuests (ARQ)

Computers use either odd or even parity When sending a byte of data one bit is used as a parity bit This is bit is set to a 1 or a 0 to make the total number of 1s and 0s in the byte (including the parity bit) odd or even depending on the machine

When data is transmitted, the parity bit is set at the transmitting end and parity is checked at the receiving end If the wrong number of bits are ‘on’, an error has occurred The receiving computer notifies the transmitting end and the data is resent

A check digit is an additional digit at the end of a string of other numbers designated to check for mistakes in input or transmission Printed books and other products have a unique barcode with an ISBN (International Standard book number) EAN (European Article Number) The first 12 digits of the barcode are the unique item number, the 13th is the check digit This can be calculated using the Modulo 10 system

A check sum works similar to a check digit The checksum of all bytes in a data transmission is calculated using an algorithm The check sum is sent with the data The receiving computer recalculates the checksum based on the data it received and compares it with the checksum received or a known, expected value If it does not match, the data may have been altered or corrupted during transmission and the data is resent

Computers using ARQs will automatically return an acknowledgement that the data was correct The simplest stop and wait ARQ will resend the data if an acknowledgement is not received within a specific period of time period, it will assume that there was a flaw in the data or transmission error

A collection of interconnected networks, not the world wide web as that is a part of the internet

ISPs are companies supplying connectivity to the internet, email services, web hosting and VOIPs. You connect to the Internet service provider that you are contracted with, when you connect to the ISP you become a part of their network. This ISP may connect to a larger network and become a part of that. Every ISP has its own data storage facility.

Internet explorer, google chrome and firefox are all browser softwares. One way to access a particular page is by typing its address into an address bar.

Servers host software or services that you can access. Online storage: Dropbox OneDrive Google Drive

PC > Router > Modem > ISP > Internet router provides access to local area network modem connects to your internet service provider (ISP) - modem no longer used in modern technology ISP connects you to the internet