Jeff Duntemanns Drive-By Wi-Fi Guide

Simple Ethernet networks built with hubs eventually fell victim to their own success. As networking hardware and software became more common, more and more computers were hooked up to networks. Still, every packet sent by any computer was received by all computers, even though in most cases that packet was addressed to only one computer. On a busy network, where most computers are sending packets most of the time, 90% of the network's computers spent most of their network time waiting for all the other computers' packets to 'play through.' The total capacity of the network to move data around, which we refer to as the network's bandwidth or throughput, was fixed, but was shared by all the network's computers. As the number of computers on the network grew, each got a smaller slice of the total bandwidth, and the network gradually slowed down until eventually it all but ground to a halt.

The solution proved to be perhaps the most significant technological advance in networking since the invention of networking itself: The network switch.

Simply put, a network switch creates a momentary, dedicated high-speed data connection between two computers. When one computer wishes to send a packet to another, the switch isolates both computers from the rest of the network just long enough for a packet to pass from one to another, and for an acknowledgement to be returned, indicating that the packet had gotten through. Because for that moment both computers are alone on a temporary, two-computer network, there is no fear of collisions, and thus no time or bandwidth must be spent on collision avoidance. Most significant of all, there is no bandwidth wasted carrying packets to computers that don't need to see them. The two computers engaging in that momentary conversation have the full bandwidth the network can offer all to themselves.

I've drawn a LAN switch schematically in Figure 2.4. The switch can very quickly create a data connection between any two computers on its network. Note that this diagram is schematic only: The actual switch mechanism is purely electronic and has no moving parts.

Figure 2.4: A Network Switch.

The connection created by the switch lasts only as long as it takes for that one packet to move between the two connected computers and acknowledgement passed back. If more than one packet needs to pass between the same two computers, that packet may have to wait its turn. The switch is careful to allow each of the connected computers to pass packets in turn so that no computer sees the network as slow or paused. Packets move so quickly through a modern Ethernet network that it almost seems like all machines can be in full connection with all other machines at once, all the time. Packets move through even slow Ethernet networks at a rate of ten million bits per second, so a typical 1,500-byte packet can move between two computers in a small fraction of a second. More modern 100-Base-T networks are ten times faster than even that.