# A-level Computing/WJEC (Eduqas)/Component 2/Data transmission

Data is sent via packets. Packets are small pieces of data that can be combined later on to make a full file. Depending upon the protocol, the max size of a packet will vary. Therefore, large files are sent by splitting them up into many different packets, such as a large sized video file. To ensure that each packet is sent to the receiver, each packet contains essential data about the origin and the destination.

## Contents of a Packet

A packet contains all the details needed to ensure its successful transfer - most importantly where it began (its origin) and to where it needs to go (its destination). This is added to the packet in the form of a Source IP address and a Desintation IP address, both types of 'control information', which ensures that a packet can indeed travel across the network. As well, the control information includes the order number of the packet, any error-checking checksums/parity bits and control signals.

## Methods of Transmission

There are two ways that connections can send information: serial and parallel. In a serial connection, only one bit is sent after another in order, whereas in parallel multiple bits are sent at a time.

### Simplex

In a simplex connection, data can only travel in one direction, meaning that no data can be returned back to the sender, for example satellite TV can only receive data, but not send any back.

### Half-Duplex

In a half-duplex connection, data can travel in both directions, however can only travel one way at a time e.g. a Walkie-talkie. As the packets can travel in either direction, there can be collisions between them.

### Full-Duplex

Full-duplex connections make use of multiple cables so data can travel both ways concurrently, for example a phone call. Ethernet cables also make use of this method with pairs of wires to send and receive data from the network.

### Multiplexing

Multiplexing increases the speed in which data is transmitted, this works by combining several independent data sources and sending them together to one end destination, via a single wire.

## Switching

Switching is the process of changing the route a packet will follow. There are many devices each connected to a network, which are called nodes, meaning packets can travel in a large variety of ways through different nodes. Some ways may be very efficient however some ways may take a much longer time to reach their end destination.

### Circuit Switching

In circuit switching, routes must be setup before they can be utilised. All packets are sent down this route and they will all arrive one-by-one at the endpoint. This is vulnerable to any faults that may occur, for example if the circuit fails no packets can be sent down the circuit. Switchboards made use of circuit switching. This is where circuits would be physically unplugged and you would be directed to the person you wanted to call. No one else can use the line while you're on the line (this is why you get an 'engaged' tone when you call someone already on the phone today) and after the call was finished someone would need to unplug the circuit. This still works the same way today, but the switching is done automatically rather than manually.

### Packet Switching

Packet switching is where the best route is chosen, with the shortest round trip time. This is a dynamic process, where packets can handle any broken links and act accordingly. Since they are not on a single circuit, the destination must read the order number of the packet and wait until all the packets have arrived.

## Data Collisions

A data collision is when two packets are sent at exactly the same time. When a data collision happens, the data that is sent is completely lost and must be re-transmitted. To avoid another collision, each packet waits a short, random amount of time before it is re-sent across the network. This ensures that further collisions do not happen.