3. What are Network Components?
  1. Network Interface Card (NIC).
  2. Modem.
  3. Bridge.
  4. Router.
  5. Hub

What are Network Components?

Network Interface Card (NIC)

The Network Interface Card (NIC) is typically installed in each computing device that is directly attached to a Local Area Network (LAN). A NIC is sometimes called a network adapter or LAN adapter. It performs the hardware functions that are required to provide a computing device with physical communication capabilities.

The Network Interface Card contains the brains of a local area network, the key information required for proper addressing, reading, and transmitting of information to be carried over the LAN. It is able to determine if there is a collision on the network, and it is then able to take corrective action and retransmit its information. Just as network workstations vary considerably by vendor, network interface cards also differ considerably even thought they might share a common designation as an Ethernet card or a Token Ring card.

A good example of a typical NIC is 3Com's EtherLink Plus card designed for the PC AT bus. This is a full-sized card with its own Intel 80186 microprocessor and RAM. While this card is more expensive, it also can provide higher performance because it can respond to the faster interrupts generated by the higher end Intel 80x86-based PC's. The EtherLink card provides an 8 kilobyte buffer, while the EtherLink Plus card provides a buffer of 64 kilobytes. A workstation that rarely needs to access the network can probably get by with a small buffer while a network file server responsible for supplying information for all network users should have a large buffer. Generally. though, a large buffer does provide higher network performance.

Modem?

A Modem (or Modulator-Demodulator) is used to transmit binary information over an analogue telephone system. The modem alters (modulates) the characteristics of a carrier wave (continuous sine wave) according to the properties of the information being modulated. The sine wave can be altered using amplitude modulation (AM), frequency modulation (FM) or phase modulation (PM).

AM can be used to represent a data stream in the following way:
Binary '1' is 2xSinwt Binary '0' is Sinwt
So a binary '1' will give double the amplitude compared to the amplitude for binary '0'.

AM modulated signal
Figure 3-1 - Example AM Signal

FM can be used to represent a data stream by varying the frequency of the transmitted signal. The frequencies used over the telephone network are 3000…3400 Hz, so 1300 Hz could be used to represent binary '1' and 2100 Hz to represent binary '0'. The frequencies are kept quite far apart so that minor differences in signals at either end (due to line noise, etc.) to not cause problems when the signal is demodulated. The frequency deviation allowed on the phone line isn't allowed to go above 10 Hz.

FM modulated signal
Figure 3-2 - Sample FM Signal

When PM is used to represent data, a binary '1' is represented by no phase change: Sint, and a binary '0' is represented by a phase change of radians: Sin(t + ). PM is often used in conjunction with AM to give two levels (hence more bits can be represented in a single time frame) for high speed modems.

Currently, the fastest modems' operate at 33.6Kbps, but Rockwell have recently announced plans to release modems within the next few months that will operate at 56K. However, the increase in speed will only happen in one direction - from the exchange to the user. Data transfer rates are being held back by the fact that modems are digital. Traditional modem technologies use an audio channel of 3,600Hz, while 56K modems can operate over standard copper wiring. To reach transmissions of 56K, the chip-powered modem controls the codec at the telephone exchange across the copper cable.

Bridge

A Bridge is a device which allows two networks which use the same technology to be connected together. Let us assume there exist two LANs, one in building 'A' and one in building 'B'. The Bridge in 'A' examines all data traffic passing through it and will only allow traffic to cross that is meant for LAN 'B'. Also, by partitioning a single LAN into smaller networks, the impact of a network fault is localised to the faulty partition only, hence making the fault can easily traceable. This means that should there be a break in the network cable in one network all communication in that network stops, but all other networks continue to operate as before.

Also, a Bridge improves the performance of a network system by reducing collisions across the network. The more computers there are connected to the network, the more collisions occur and the more delays are to be suffered. By splitting a network into more smaller networks, collisions are reduced. Since a Bridge can separate a network into smaller networks, it can also be used to improve network security where sensitive data is carried between computers. Network administrators can create secure and unsecured networks by placing Bridges between areas to be separated.

The following points should be remembered when considering the use of a Bridge:

Router

A Router is in many ways very similar in operation to a Bridge. It is a device which routes data between networks of different technologies such as Ethernet and IBM Token Ring. Since the Internet consists of thousands of networks using many different technologies, Routers are an essential component of the Internet.

A Router has an address on the network, a Bridge does not. Networks frequently use a Router as an intermediate destination. A Router only examines packets of data if they are addressed to it and then sends them on to the other network. A Bridge has to examine all data in order to determine its destination. A Router can also be referred to as a 'Gateway'.

Hub

A Hub offers a common wiring point and allows the simulation of a star topology network.

The hub allocates a port to each segment connected, to form a centre of distribution for signals to and from the different parts of the network. Usually, there are twelve ports, each connected to a different segment of the network. Each segment can comprise of a maximum of 16 computers.


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