Section 17.1
Computers & Communication
17.1 Data Transmission
17.1.1 Comms Channels
Coaxial Cable
Coaxial cable (simliar to that used in television systems) has an outer mesh which is
'earthed' and shields the signal in the inner wire. The inner wire carries the signal.
Used in: LAN
Pros
- Inexpensive
- Accurate data transfer
Cons
- Signal degrades over distance
- Not very easy to lay
Twisted Pair Cable
This uses sets of twisted copper wire pairs.
Used in: LAN
Pros
- Easy to wire in situe
- Fast data transfer
Cons
- Susceptable to interference
Fibre Optic Cable
One of many transparent fibres can relay phases of light.
Used in: MAN and WAN (internet backbone)
Pros
- Very fast data transfer rate
- No electromagnetic interference
- Physically secure
- Less signal degradation over distance
Cons
- Cost
- Cable cannot bend around tight corners
- Difficult to interface with computer
Microwave Transmission
Data is encoded on a microwave signal.
Used in: MAN
Pros
- No cost of wiring and associated disruption
- Reasonable bandwidth
Cons
Satellite Transmission
A one or two-way link can be established by using a satellite (in geosynchronus
orbit) and a dish.
Used in: WAN
Pros
- Good bandwidth
- No wiring or boosters needed
Cons
- Expensive
- Possibly insecure
17.1.2 Transmission Directions
Simplex
Data can only be transmitted in one direction.
Half-Duplex
Data can be sent in both directions but not at the same time.
OR
Full-Duplex
Data can be sent in both directions at the same time.
17.1.3 Transmission Rate
The speed at which data is transmitted is measured in bits per second (bps).
The bandwidth is the maximum speed at which data can be sent along a
communication channel.
17.1.4 Error Detection
A parity bit can be transmitted with the code for each character.
An element of redundancy can be introduced with 2D parity. This allows
corrupted data to be recovered.
17.1.5 Modulation And Digital Transmission
Analogue Systems
MODEM stands for Modulator-Demodulator.
To send digital data, the data is imposed or modulated onto an analogue wave.
The reverse process (demodulation) retrieves the digital information.
Digital Systems
Data is sent digitally via:
- An ISDN line
- A LAN connection
- A digital radio network
- A phone line with ADSL
ISDN (Integrated Services Digital Network)
Data is sent digitally down a special ISDN line.
ADSL (Asymetric Digital Subscriber Line)
This has a maximum bandwidth of 9Mbps and runs over a standard telephone line.
17.1.6 Parallel To Serial Transmission
Parallel transmission is not pratical for long distance communication. This is
because of the cost of manufacturing the wires.
17.1.7 Multiplexing
Multiplexing allows more than one signal to be sent over a single link at one
time.
There are two main methods of doing this:
Time Division Multiplexing
The transmission time is broken up into slices and each device wishing to
transmit can take a packet in turn.
Frequency Division Multiplexing
Different frequency carrier waves are used to send many signals over the link.
A high bandwidth is required for this.
17.1.8 Data Compression
Data compression is frequently used when transmitting large amounts of data.
At the heart of the process is the method of looking for repeated patterns and
replacing them by one copy of the bit pattern plus the number of times it occurs.
