Unit 1 & 2

Hexadecimal is easier for humans to understand than binary, as it is a shorter representation of the binary

– the result of carrying out a calculation that produces a value that is too large for the computer’s allocated word size

The sample rate is the number of samples taken in a second * The sample resolution is the number of bits per sample

the sample rate and resolution increase

An image is a series of pixels that are converted to binary, which is processed by a computer

The resolution is the number of pixels in the image * The colour depth is the number of bits used to represent each colour

the resolution and colour depth increase

Lossless compression reduces the file size without permanent loss of data, e.g. run length encoding (RLE

Lossy compression reduces the file size by permanently removing data, e.g. reducing resolution or colour depth, reducing sample rate or resolution.

Common lossy file compression algorithms are: » MPEG-3 (MP3) and MPEG-4 (MP4) » JPEG.

The impact of this is, e.g.: – less bandwidth required – less storage space required – shorter transmission time

A packet of data in a unit of data contains a – packet header: destination address, packet number, originator’s address – payload:actual data being sent in the packet – trailer:some way of identifying the end of the packet, an error checking method

Data is broken down into packets * Each packet could take a different route * A router controls the route a packet takes * Packets may arrive out of order * Once the last packet has arrived, packets are reordered

-Simplex mode occurs when data can be sent in ONE DIRECTION ONLY -Half-duplex mode occurs when data is sent in BOTH DIRECTIONS but NOT AT THE SAME TIME -Full-duplex mode occurs when data can be sent in BOTH DIRECTIONS AT THE SAME TIME

Serial data transmission occurs when data is sent ONE BIT AT A TIME over a SINGLE WIRE/CHANNEL. Parallel data transmission occurs when SEVERAL BITS OF DATA (usually one byte) are sent down SEVERAL CHANNELS/WIRES all at the same time.

Serial data transmission works well over long distances. However, the data is transmitted at a slower rate than parallel data transmission. Because only one channel/wire is used, data will arrive at its destination fully synchronised

Parallel data transmission works well over short distances. Over longer distances, data can become skewed (that is, the data can arrive unsynchronised) and bits can arrive out of order. The longer the wire, the worse this can become. It is, however, a faster method of data transmission than serial.

When a device is plugged into a computer using one of the USB ports: » the computer automatically detects that a device is present » 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 (some systems do this automatically and the user will see a notice asking for permission to connect to the device website)

-devices plugged into the computer are automatically detected and device drivers are automatically loaded up -connections can only fit one way preventing incorrect connections being made -can support different data transmission rates -no need for external power source

-standard USB only supports a maximum cable length of 5m -even though USB is backward compatible, very early USB standards (V1) may not always be supported by the latest computers

-interference (all types of cable can suffer from electrical interference, which can cause data to be corrupted or even lost) » problems during packet switching (this can lead to data loss – or it is even possible to gain data!) » skewing of data (this occurs during parallel data transmission and can cause data corruption if the bits arrive out of synchronisation).

a copy of the data is sent back to the sender » the returned data is compared with the original data by the sender’s computer » if there are no differences, then the data was sent without error » if the two sets of data are different, then an error occurred at some stage during the data transmission.

ARQ uses positive and negative acknowledgements and timeout (this is the time interval allowed to elapse before an acknowledgement is received) » the receiving device receives an error detection code as part of the data transmission : this is used to detect whether the received data contains any transmission errors » if no error is detected, a positive acknowledgement is sent back to the sending device » however, if an error is detected, the receiving device now sends a negative acknowledgement to the sending device and requests re transmission of the data » a time-out is used by the sending device by waiting a pre determined amount of time …. » … and if no acknowledgement of any type has been received by the sending device within this time limit, it automatically re-sends the data until a positive acknowledgement is received …. » … or until a pre-determined number of re-transmissions has taken place

Encryption alters data into a form that is unreadable by anybody for whom the data is not intended.