Transfer methods

A network is built from multiple nodes connected to one another by communication lines. There are multiple methods for transferring data from a sending node to a receiving node:

• Circuit switching involves setting up a series of intermediate nodes, in order to propagate the sending node's data to the receiving node. In such a situation, the communication line can be likened to a dedicated communication pipe.
• Message switching involves transmitting the message sequentially from one node to another. Each node waits until it has received the entire message before sending it to the next node.
• Packet switching involves splitting information into data packets, transmitted separately by intermediate nodes and reassembled when they reach the final recipient.

Circuit switching

Circuit switching is a data transfer method that involves establishing a dedicated circuit within a network.

In such cases, a circuit made of communication lines between the sending node and receiving node is reserved at the time of communication, so that data can be sent over it; the circuit is freed again when transmission is complete.

In particular, it is the method used by the public switched telephone network (PSTN). By reserving a telephone line between two speakers, the network can ensure the best data transfer performance possible. For voice communication, it is essential that the line not be cut while the signal is being transmitted.

Packet switching

When sending data with packet switching, the data to be transmitted is split into data packets (this is called segmentation) and then sent separately over the network.

The network nodes are free to determine each packet's route individually, based on their routing table. The packets sent in this manner can take different routes, and are reassembled when they arrive at the recipient node.

In such cases, the packets might arrive in a different order than the one they were sent in, and may end up getting lost. For this reason, certain mechanisms are built into packets so that they can be reordered if need be, or resent if packets are lost.

This is the transfer method used over the Internet, as it has the following advantages:

• Withstanding intermediate-node crashes
• Rational and efficient use of transmission lines

Bus topology

Bus topology is the simplest way a network can be organised. In bus topology, all computers are linked to the same transmission line by using a cable, usually coaxial. The word "bus" refers to the physical line that joins all the machines on the network.

The advantages of this topology are that it is easy to implement and functions easily; on the other hand, it is highly vulnerable, since if one of the connections is defective, the whole network is affected.

Star topology

In star topology, the network computers are linked to a piece of hardware called a hub. This is a box which contains a certain number of sockets into which cables coming out of the computers can be plugged. Its role is to ensure communications between those sockets.

Unlike networks built with bus topology, networks which use star topology are much less vulnerable, as one of the connections can easily be removed by disconnecting it from the hub, without paralysing the rest of the network. However, a star topology network is bulkier than a bus network, as additional hardware is required (the hub).

Ring topology

In a ring-topology network, computers each take turns communicating, creating a loop of computers in which they each "have their turn to speak" one after another.

In reality, ring topology networks are not linked together in loops. They are actually linked to a distributor (called a MAU, Multistation Access Unit) which handles communication between the computers linked to it, by giving each of them time to "speak."

The two main logical topologies which use this physical topology are Token ring and FDDI.

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