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Network Components explained

Gateway

A gateway is a node that connects two networks. Possibly a LAN to a WAN. It will be responsible for converting data packaged according to one network's protocol to the others. For example a gateway between an Ethernet network and a packet switching network will convert the Ethernet frames to packets and vice-versa. The gateway may have to convert from one data representation to another - e.g. ASCII to an alternate character set. It will also handle any difference in transmission rate or voltage that exists, converting signals from one networks protocol to the others.

A gateway will be available to nodes on each network - it is a network resource like a printer server.

Remote Links

Remote linkage is via a public or private communications network, usually using packet switching which is more economic than circuit switching. In certain applications a dedicated line may be used to connect remote terminals to a central computer but this is an expensive option.

Digital (base band) electrical transmission is impossible over long distance because of the physical properties of conductors. Thus if electrical methods are use a modem is needed. Modulation is the process of varying some properties of a basic carrier wave to represent the data being transmitted. Since binary data is being transmitted, only two states need representing. Modulation techniques are :

  • Amplitude modulation is where the amplitude of the carrier wave is modulated to mark zero and one.
  • Frequency modulation is where the frequency modulates to represent zero and one.
  • Phase modulation where frequency or phase is changed to represent zero and one.

Amplitude modulation is not usually used because it is badly affected by interference.

The communications link is expensive and signals are often multiplexed to make more efficient use of the link. Multiplexing is when a one data link is used to carry more than one data signal. There are two approaches to multiplexing:

  • Time division multiplexing is when the different signals take turns at using the data link. The multiplexer is like a multi switch that switches the source to the data link. This approach would be used where a number of terminals were connected to a central computer, each terminal receiving a time-slice of processor time in term. During the terminal's time slice it would be connected to the central computer.
  • Frequency division multiplexing allows different signals to share the same line at the same time by assigning a range of frequencies to each of the transmissions.

Normally data will need to flow both ways along a communication line. Transmission can be full duplex which allows data to flow in both directions at the same time - this needs two communications lines or frequency division multiplexing; half duplex also provides two way communication but at any moment data is flowing in one direction only. Time division multiplexing provides a half-duplex capability. In some situations data only needs to travel one way in which case the link is called Simplex. An example of a simplex link could be a bar-code reader in a warehouse, recording deliveries of stock and passing the data to the head office computer.

Note that all the data transmission methods discussed have been serial transmission - data is transmitted as series of bits. This is slower than parallel transmission - used for communicating between a computer and a disk. Serial transmission is used over longer distances because of the costs of parallel links.

   

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