Collins Guide chapter 5
Modem (modulate-demodulate) is a device used to encode digital data onto an analogue wave so that it can be transmitted along telephone wires. The analogue signal is converted back to digital by the receiver's computer.
A modem is not required when digital lines are available. The device used with digital lines is a 'terminal adapter', not a 'digital modem'.
Modems are used to connect computers to the internet using telephone lines and a dial-up connection. A dial-up connection is only on when a connection has been made and most users would disconnect when they have finished so they don't have to go on paying for the connection. The maximum speed of a modem used in this way is 56 kbps (kilobits per second).
Broadband connections use digital data only, there is no conversion from digital to analogue and back from analogue to digital. Broadband connections are generally 'always on', the line may drop if it is not used but it quickly comes on line when you request it and you don't have to pay by the hour, there is usually a flat fee. You may, however, have to pay for data by volume, for example £10 per GigaByte of data downloaded each month.
Broadband speeds in the UK are around 1 mbps, around 20 times faster than a 56 kbps analogue line. Many internet providers advertise speeds of up to 8 mbps but factors such as the number of people using the line at the same time will keep most access speeds well below this. 8mbps is needed for the transmission of broadcast quality TV. Very few people would want to return to using 56 kbps phone lines. Some people believe the UK is falling behind other countries in its provision of broadband infrastructure and services.
ADC=Analogue Digital Converter - a device that reads the data encoded into an analogue signal. Analogue signals are waves transmitted along wires by electricity, digital data are encoded into the wave by a DAC - Digital Analogue Converter.
Computer networks can be arranged in three main ways: bus, star, ring.
A bus network is a single cable with a resistor at the end and work stations connected along it.
A star network is where there is a central device to which the work stations (computers) are connected. The central device may be a computer, a server (another computer) or a hub or switch (a box that is placed between a server and the stations, usually with sockets to connect 16, 24 or 48 computers).
A ring network is where a cable runs in a circle with work stations attached to it. Data flows very quickly round a ring and in one direction only.
The most common form of network is the star but in most networks the cables are not laid out in anything like a star pattern, that is just the theoretical design. Usually the cables are connected to a hub or switch and they run through trunking along corridors and around rooms until they reach the computers to which they are connected. To make a star network you would have to put the hub in the middle and lay the cables around it in a star pattern.
Networks consist of linked computers and peripherals. This produces a number of advantages
Advantages:
Disadvantages:
Most networks require a user to login with an ID and password. This helps to protect the integrity of the network and provides protection of personal data as other users should not be able to access it.
Passwords should be 'strong', that is they should feature numbers as well as letters and both upper and lower case letters. They should not be easy for others to guess e.g. middle name, car registration, postcode. Many networks will insist on a change of password every three months or so and will not allow re-use of old passwords. Passwords should not be written down.
One problem is that people end up with many IDs and passwords for different networks - work, bank, email, etc.
Other ICT-based communication media in common use include: