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Lesson on half/full duplex communications and hubs and switches

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Why do half duplex Ethernet 10baseT topologies have a slower data transmission rate than full duplex Ethernet 10baseT topologies?

And, do you know which part of the mac address will help you determine the NIC's manufacturer?

Your question is a very good one indeed.

In order to answer your question, we need to provide some background information that will help you understand a few basics about half/full duplex communications and how they are affected with hubs and switches.

All hubs provide a half duplex communication channel, which means that when two computers are communicating e.g. transferring data, and then only one at a time can talk.

On the other hand, Switches allow full duplex communications, which means that both computers can send and receive data at the same time.

All the above characteristics play a big role in how fast the actual data transfers occur, but there is also a few more key points which help make a big difference:

Hubs are simple repeaters that take any incoming data and send it out all the available ports, this generates quite a big of traffic, and if you have a hub with 4 or more computers that are trying to transfer data at the same time, you will see the collision light hard on, this effectively means that the network throughput has decreased immensely and therefore data transfer rates will also decrease, making the network very slow.

Switches on the other hand cerate circuit connections between the two ends that are sending/receiving data, without allowing the data to escape to the rest of the ports the switch might have. This means there is a dedicated channel for the two computers that is not affected in any way by another transfer that might be occurring at the same time between other computers in the network.

This as you would understand has a very positive effect on the overall network throughput and increases the transfer rates between computers.

So, summing all the above in order to conclude, hubs offer half duplex communications that flood the whole network when transfers are taking place, where as switches offer full duplex –dedicated- channels between the two computers sending/receiving data and therefore we have much greater speeds.

Coming now to your second question – MAC Addresses!

A MAC address is a 48 bit or 6 byte address that uniquely identifies a physical host on a local area network.

When looking at a MAC address, let's take for example my workstation's: 00-E0-7D-8D-C2-0A the first 24 bits –3 bytes- represent the NIC's manufacturer, while the next 24 bits are the network card's unique serial number.

Taking my network card's MAC address above, the 00-E0-7D is the manufacturer (Realtek) and the 8D-C2-0A is the unique serial number.

You should also be aware that a small portion of the MAC address changes when the Ethernet frame is a multicast, and you can read more about it on the website provided at the next paragraph.

I hope the above has helped answer your questions. If you would like to learn more on hubs, switches, MAC addresses and mutlicasts, you can visit www.Firewall.cx.

This was first published in July 2003

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