9.3 Network Hardware

For networks to function, specialised hardware is needed to take care of the sending and receiving of information. For each network topology the method of sending and receiving of information is different. Therefore, the hardware for each network topology is also different. For example, hardware designed for a ring topology will not work with hardware designed for a bus topology. Just like a part for one car would not fit another type of car.

Network Interface Card (NIC)

To connect to an existing network a computer would need a physical media connection and hardware on the computer to perform the communication with the other computer. The hardware on the computer is called a Network Interface Card (NIC). The NIC is a circuit board that contains all the necessary circuits to convert the electrical signals from the computer to signals on the physical media, which could be electrical, optical, or wireless. The NIC is usually a circuit board that is connected through a slot, or sometimes integrated with the main circuit board of the computer. A NIC card also has to have a unique address on the network for it to be identified on the network. Without a unique address communication between two computers on the network cannot take place. This is like the postal system. If two houses have the exact same address then mail delivery cannot work properly. Similarly for information to be delivered, a unique address is required for each computer on the network. This unique hardware address is called the MAC Media Access Control address. For example, a computer on an Ethernet network will have a 12-digit hexadecimal number to identify it uniquely on the network, for example 00-00-A1-BF-C5-7A. No two Ethernet NIC cards can have the same MAC address. When an Ethernet card is manufactured, a unique MAC address is automatically assigned to it to avoid any possible MAC address conflicts.

NIC cards on computers will not allow data transfer between computers to take place through a direct connection. Network devices like hubs, switches, repeaters, bridges, etc. are also needed to create a network. These devices are responsible for the transfer of information to the right place.

Repeaters and Hubs

As information flows through a given medium, the quality of the signal begins to decrease with increasing length. For example, when electrical signals are sent through wires the voltage characteristics of the signal begins to change with distance. This happens because of the wire’s resistance. The longer the wire the greater the resistance. Therefore, the voltage drop will become significant at large distances. Similar problems are also present with other types of media as well. In the case of optical signals, the loss is due to absorption and scattering of light.  This decrease in signal quality is called attenuation. When signal attenuation is significant, then the data within the signal cannot be determined accurately. The diagram below shows the attenuated signal pattern.

To solve this problem, a device called a repeater is used. A repeater is a device that restores the quality of the signal back to its original standard. It simply detects the attenuated signal and recreates the same signal. For a repeater to work effectively it should be able to detect the attenuated signal, otherwise the data will be corrupted. So to solve attenuation problems within a given media, repeaters are placed at fixed distances to ensure signal quality and data integrity.

A centralized repeater is referred to as a hub. A hub has several ports on it with different devices connected to it. Information that is sent on one of the ports is repeated on all the other ports, thus functioning as a repeater with multiple outputs. The hub ensures good signal quality on all its output lines. A hub could be used in all the various types of topologies. The following is an example of an Ethernet (bus topology) hub at work.

In the case of the Ethernet hub, the bus topology is maintained within the hub itself, as shown in figure 9.20, and therefore it should not be thought of as a change in topology from a bus to a star topology. The physical appearance of the connection may be star like in nature, but the topology still remains a bus topology. Similarly a token ring hub might create a star like physical appearance but its topology still remains a ring, with the ring setup within the hub.

Bridges

Unlike a hub which is a passive device, a bridge is an active device. It processes information within it to control information flow within a network. Imagine two sections of a network with a bridge connecting both of them.

The bridge forwards information from one section to another based on the MAC address. It keeps track of all the MAC addresses on either side of it and forwards information from one side to another based on the destination MAC address. For example, if a computer on the section A side of the network is sending information to a computer on the section B side, then the bridge will allow it to pass from one side to another. The bridge is useful in controlling traffic within a network, thereby allowing the overall network to run efficiently. One potential problem with a bridge is that it can create a bottleneck if the traffic between the two sections is heavy. Unlike a hub the bridge has to process information for it to perform its function and this can create a problem with heavy information traffic.

Switches

Switches operate on the same principle as a bridge. It looks at incoming data on a given port and analyzes it to determine its destination MAC address. The switch then transmits the data on to the port that has the destination MAC address connected to it. A switch is similar to a hub in its physical connection, but different from a hub in its operation. A hub sends the incoming information on a single port to all the other ports, but a switch sends the incoming information to a specific port that has the MAC address connected to it. Again, like the bridge, the switch reduces network traffic by controlling the information flow within the network.

Routers

Routers are network device that operate at a level above switches and bridges. Router are focused on transferring information between networks. Like addressing of devices within a network, each network needs to be addressed uniquely for information to be transferred between networks. A router uses network addressing to send information to the right place within a WAN. Again the router acts as a network traffic controller by directing the information to the right place within a WAN. A router is also sometimes referred to as a gateway because information has to pass through it to go from one network to another network.

9.3 Practice Questions 1.     Why is unique addressing important on a network? 2.     What is the function of the NIC? 3.     How is a hub different from a switch? 4.     What are the advantages of using a switch vs a hub? 5.     How is a hub different from a repeater?