Lesson 3

With this lesson we continue with Topology - but, we start to look at the LOGICAL topology now. Remember that Physical was the tangible side of things, thus the Logical is the flow of data - we cannot see this! :)

• Logical Bus Topology

In the logical bus topology the signal sent is received by all the nodes at the same time, so one signal is broadcast and all the nodes receive the signal. - this is 802.3 CSMA/CD

• Logical Ring Topology

In the logical ring topology, the signal is sent around the Ring like a dog chasing it's tail - it always travels in a clockwise direction. (a token continually travels around the ring looking for information to pick up from the network card and transport it to the required destination)

• Logical Star Topology

This is a topology which is implemented less frequently than a Ring or bus. In the star, the nodes are connected to a central device and are all on the same bus cable. The central device continually polls the nodes to determine if the need to send information. This can be more complex and a MUX (multiplexer) can be used - this will split up different sections of the network and send only to the section that requires it.

• MSAU / MAU - multi-station access unit

In this situation, the logical ring topology has a MAU whereby the signal travels in the ring inside the unit and then sends the signal down the last section of cable once it has established its location.

When you have two different standards, namely 802.3 and 802.5 in a situation where you have a MAU, for the two topologies to be able to connect, you would require some sort of "gateway" which translates the one standard into the other so that they may communicate - this is also a good way to safeguard your network (security).

In Lesson 3 we had a look at what we would need to consider should be be designing the architecture of a network in a building, in the points following we will list the necessary steps or things to consider.

• How many floors is the building? Always try to put your server room in the centre of the building to reduce cable pulls.
• Where is the lift shaft? Usually in the middle of the building - this shaft provides you with a good spot to run your cables from floor to floor - cables need to be shielded in this environment.
• decide on a topology for the network per floor
• decide on a minimum network speed - then ensure that your back-bone which runs up and down the lift shaft is of maximum speed.
• a patch panel (there would more than like be one for the telephone system) will be required on each floor to distribute that cables to each topology on each floor, this patch panel will hold the switch and the switch is connected to the server via the back-bone.
• The switches on each floor could also be MAU devices.
• Server room should be off the ground level if possible - water issues
• Your back-bone through the lift shaft must be terminated on each end.
• where the back-bone connects to the server, you need to focus on the failure points here, you could have a switch or multiple switches between the back-bone and the server or servers to lessen the chances of failure due to cable breaks.
• Server mirroring will also reduce the effects of failure, redundancy is important!

• Network Categories

LAN
Local area network - this is a self-contained network which covers a defined small area such as an office. Can be connected directly via cable or via wireless technologies

WAN

Wide area network - this is a network which spans a large and multiple geographic locations (much like a national or international company) it would connect multiple LANs.

Now this gets interesting as a WAN is the basis of the Internet as when a a device has more than one connection it becomes a Router. Thus each LAN is connected via the WAN which includes TELECOMS.

A WAN will always use a MESH topology. SO you can see how the Internet is a giant WAN connecting all the computers around the globe, A Mesh has at least 4 connections per workstation.
Lastly there are some other Networks listed below:

• MAN - metropolitan area network, imagine the city's network
• CAN - Campus area network, imagine the network that UCT might have
• PAN - Personal area network, this connects 2 or 3 workstations together, imagine the network you might have at home or in a small office.
• WPAN - wireless personal area network, this is a small network much like the PAN but uses wireless technology rather the a wired/cabled network.

That's all for now - next update after lesson 4.