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WJEC A2 Computing Unit 4.2 Cheat Sheet (DRAFT) by

This is a draft cheat sheet. It is a work in progress and is not finished yet.

Data Transm­ission

Parallel Transm­ission
This is multiple wires sending a number of bits at the same time.
This is used in buses in the CPU but can only be used for short distances due to interf­erence.
This is much faster than serial transm­ssion, but may have synchr­oni­zation problems.
Serial Transm­ission
Bits are sent serially across a single wire.
This is used for longer distances and is frequently used in computer cables.
There are three types of serial transm­ission:
1. Simplex - Transm­ission in one direction only.
2. Half-D­uplex - Transm­ission in both direct­ions, but only one at a time.
3. Full-D­uplex - Transm­ission in both directions simult­ane­ously.


Bandwidth is the amount of data that can be transm­itted at a time along a single commun­ication channel.
This is related to the range of freque­ncies available on the carrier wave.


Multip­lexing involves combining more than one signal through a single commun­ica­tions channel.
When the signal reaches its destin­ation, it has to be demult­iplexed or demuxed. This involves decoding the multip­lexed signal to extract the original data.
Inverse multip­lexing breaks one data stream into several data streams to be sent seperately over multiple chnanels, where it can be re-ass­embled at the destin­ation.

Bit Rate

The speed at which a transm­ission is taking place. This is propor­tional to bandwidth, since bandwidth can determine how many bits can be transm­itted at a time.
This is measured in bits per second or bps.

Baud Rate

One baud represents one electronic state change per second.
In a system where one bit is sent per each state change, one baud would equate to one bit per second.
However, it is possible to store multiple voltages as repres­enting combin­ations of bits.
For example, 1Hz might represent "1010", 2Hz might represent "1111", etc.
This way the number of state changes can remain constant while bits per second increases.

Types of Switching

Circuit Switching
This is when a specific route of switches is determined and data is sent only down that specific path. This path is maintained for the duration of the transm­ission.
Packet Switching
Data is split into packets. Each packet contains data, the source address, the destin­ation address, and a checksum. Each packet may take a different route around the network.
This means that if one computer fails, the message can still be sent. Also, it is more difficult to intercept.