A level revision

Advanced network topologies

In AS Computing you should have covered the basics of network topologies - Bus, Ring and star. You can recap what you've learnt here. At A level these network topologies will be taken a step further and are looked at in more depth which is covered on this page.

Bus topology

In a bus topology all the nodes are connected to a central line. Only one computer sends information at any time. The data is split up into packets which are addressed to the to the target node. The packets are then sent across the network as electronic signals in the cable. The signals are recieved by all the nodes in the network but only the one the packet is addressed with accept the data. At the ends of the central line are terminators, these absorb the electronic signal freeing up the cable for the next transmission.

As only one computer can send information at a time it is necessary for a system to make sure multiple signals aren't sent at once. Before a node beings to transmit data it makes sure the line is free. Once it starts to transmit the node also looks for other nodes beginning to transmit. If the messages collide with each other both nodes abort the transmission and wait to try again.

Segments

Bus networks work well with a small number of computers, but with large numbers of computers performance can take a big hit. This can be solved by seperating the network into smaller more mangable segments. A segment is simply a portion of a computer network with a number of connected nodes.

Bridges

Bridges are used to join segments together. When nodes transmit data the bridge looks at whether they are addressed to nodes on the same or another segment. The bridge only allows the the signal to be transmitted on the segments it needs to be, thus freeing up the lines of the other segments and improving the performance and effeciency of the network. For example:

In the above diagram two segments of a bus network are joined together with a bridge. If a computer on segment A (top) wanted to send some data to another computer in segment A then the bridge would prevent the signal travelling into segment B allowing nodes in that segment to also commuicate.

Ring topology

In a ring topology each node is connected to two others forming a loop, data is transmitted in only one direction. Data is send around the ring using what is known as token passing. A packet known as a token is passed around the ring from node to node. If a node wants to transmit data it adds it to the token and passes it on. Nodes will inspect the arriving token to see if it is addressed to them, if it is they will send a token back to the sender to tell them their data successfully arrived else they will pass it on to the next node. This sounds long winded but data travels around the ring incredibly fast.

Star topology

In a star network each node is connected to a central hub. Data is sent from the nodes to the hub which then transmits the signal to the other nodes on the network, of course only the node which the signal is addressed to does something with the data.

Switched ethernet

The hub can be replaced by a switch these are basically smarter hubs. Rather than simply transmitting a recieved signal to all the other nodes the switch only transmit the signals to the nodes it is addressed to. Two pairs of wires are used to connect the node to the switch, one for transmitting and one for receiving data. This is known as switched ethernet