Sharing the Local Roadway: Ethernet Hubs

An old friend of mine is a native of San Francisco and still lives there. The traffic is horrible there, in part due to the need for lots of people to commute across the San Francisco Bay. There simply aren't enough bridges for the number of cars that need to cross the Bay. His solution: Fill in the Bay with dirt and pave the whole thing. Then there would be plenty of roads to allow people to cross the Bay.

If you tried to connect 10 PCs in a network, using a cross-over cable between NICs (as shown in figure 4-10), you would begin to do the equivalent of paving the San Francisco Bay. You know how two computers can use Ethernet NICs with a cross-over cable to communicate. However, to connect to other PCs, you would need more Ethernet NICs, and your PC probably does not have enough room for all the cards. Also, you would need to run cables between your PC and all the other PCs, or at least get the electrician to run the cables. If you tried to do this for 100 PCs on the same floor of the building, and every PC wanted to connect to every other, you would have 99 cables connected to 99 NICs inside each PC!

The alternative to running a cable to every other PC is to run a cable from each PC to a wiring closet and connect the cables to a networking device, called an Ethernet hub. An Ethernet hub provides several functions, but mainly it allows the electrician to cable each device to the hub using only a single NIC and single cable, eliminating the cabling problem. The hub simply listens for incoming electrical signals, and when received, the hub repeats the same electrical signal to every other device that's connected to the hub. Figure 4-11 shows the basic operation.

Figure 4-11. Ethernet Hub Repeats Everything It Hears

As seen in the figure, Larry sends data to the hub, and the hub repeats what Larry sent out the cables to Archie and Bob. It's that simple!

Figure 4-11 shows cabling, but it does not show which pairs of wires are used. Interestingly, the hub expects straight-though Ethernet cabling between itself and the PCs. Why is that? Well, the people who make hubs, knowing that Ethernet NICs in PCs send on the twisted pair that uses pins 1 and 2 and receive on twisted pair that uses pins 3 and 6, do the opposite. Therefore, a straight-through cable works between a PC NIC and a hub. Figure 4-12 illustrates the concept.

Figure 4-12. Hubs Use Straight-Through Cabling to PCs

The hub's logic is simple:

1.	Receive traffic on pins 1 and 2 on each physical interface.
2.	When received, repeat the same electrical signal out all other ports, except the one in which the data was received.
3.	When repeating out other ports, repeat the traffic out pins 3 and 6 so that the PCs will be listening.