1: About Computer Networks

1.1 What is a computer network?

A computer network is a collection of devices connected to each other, in order to allow every device to share its resources with other devices, and access other devices shared resources.

1.1.1 Computer network elements

End devices

Devices that have the resources to share with others, or devices that are used by the end users to access the shared resources.

Examples of the end devices are a ‘PC’, a ‘laptop’, a ‘printer’ and a ‘server’.

5237.jpg

Figure 1.1 a computer network

Networking devices

Devices that are used to connect between end devices.

Examples of networking devices are a ‘hub’, a switch’ and a ‘router’.

1.2 Computer network topologies

There are different network topologies that we can use to connect network devices to each other.

1.2.1 Bus network topology

Image947.PNG

Figure 1.2: bus network topology

In this network topology, we use a bus – usually a copper cable or an optical fiber cable – to connect between network devices. This bus connects to the network devices through connectors that exit from it.

1.2.2 Ring network topology

Image954.PNG 

Figure 1.3: ring network topology

In the ring topology, we connect every network device to its neighbor devices; in order to make all the network devices form a ring.

1.2.3 Mesh network topology

Full mesh topology

Image963.PNG 

Figure 1.4: full mesh network topology

In the full mesh topology, we connect every network device to all the network devices that exists in the network. The advantage of this topology is the very high redundancy that it has.

Very high redundancy means that, if a path to a certain device became unavailable, there still an alternative path that can be used to reach this device.

The disadvantage of the full mesh topology is that, it requires extensive number of connections between the devices.

Partial mesh topology

Image971.PNG

Figure 1.5: partial mesh network topology

In the partial mesh topology, every device is connected to some of the devices that exist in its network, not all devices like the full mesh topology.

The partial mesh topology requires fewer connections than the full mesh topology, which is considered an advantage. However, its redundancy is less than the full mesh topology.

1.2.4 Tree network topology

Image978.JPG 

Figure 1.6: tree network topology

In (figure 1.6), ‘device B’ is a central device, called the ‘hub device’, while ‘device A’ and ‘device C’ are called the ‘spoke devices’.

In this topology, the spoke devices talk to each other through the hub device.

The disadvantage of this topology is that, if the hub device became unavailable for any reason, the communication between spoke devices will not be possible. This is called ‘single point of failure’.

1.2.5 Star network topology

Image986.JPG Figure 1.7: star network topology

This is the most widely used network topology.

In this topology, we use a central networking device to connect between all the end devices.

1.2.6 Hybrid network topology

This topology contains more than one of the previously illustrated network topologies connected to each other.

An example of this topology is connecting a star network topology to a ring network topology.

1.3 Simplex, half duplex and full duplex communication modes

1.3.1 Simplex communication mode

Image995.JPGFigure 1.8: simplex communication mode

In the simplex communication mode, one of the devices is always the sender, while the other device is always the receiver.

As an example: the satellite (sender) and the dish (receiver).

1.3.2 Half duplex communication mode

Image1002.JPGFigure 1.9: half-duplex communication mode

In the half-duplex communication mode, every device can only send or only receive in a certain time.

It means that the data could travel from ‘device A’ to ‘device B’ or from ‘device B’ to ‘device A’ in a certain time. Data could not travel in both directions in the same time.

1.3.3 Full duplex communication mode

Image1010.JPGFigure 1.10: full duplex communication mode

In the full duplex communication mode, every device can send and receive data in the same time.

It is a two-way communication between the devices.

1.4 Carrier Sense Multiple Access / Collision Detection (CSMA / CD) protocol

Image1018.JPGFigure 1.11: CSMA/CD protocol

Carrier Sense Multiple Access / Collision Detection (CSMA/CD) protocol is used in case of using the half-duplex communication mode. It prevents the collision between the data sent from different devices.

In this protocol, the device makes sure that there is no any electrical signal – which represents a data being sent by another device – exists on the cable before sending its own data.