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Chapter 12 — Going Wireless with Wi-Fi 233

To Internet To Internet

Router Router

Hub Access point

Laptop Laptop Laptop Laptop Laptop Laptop
Ethernet network Wi-Fi network
FIGURE 12-1: Wired versus wireless networks, the topology is the same

Wi-Fi modifies the algorithm a bit and is instead CSMA/CA; the CA stands for collision
avoidance. Unlike on a physical cable, with radio it’s hard to tell if someone else is transmitting
while you’re transmitting. So in CA, when a device wants to talk, it will first send a jam signal.
This is sort of like blowing a whistle or clearing your throat: it gets through when normal con-
versation tones wouldn’t. The jam signal puts all other devices in a listen state for a while as
they wait for the transmitting device.
One problem results with both CD and CA algorithm: Data transmission in both degrades
sharply once a critical mass of devices get on the shared medium (wire or radio channel). With
many devices on the same Ethernet with CD, collisions happen so frequently hardly any data is
sent. In Wi-Fi with CA, jam signals are frequent as devices start to talk; stopping all transmis-
sions and again hardly any data is sent. In both cases the solution is to reduce the number of
devices. In Ethernet that means moving devices to a different hub; in Wi-Fi that means mov-
ing them to a different access point. Hubs or access points are then connected together and talk
among themselves as if they’re each a single entity.

Roaming and Disconnects
In Ethernet, when you disconnect the cable and move a device to a different location and hub,
the hub can tell when the cable is unplugged and plugged in.These events are used to update the
internal ARP tables used by all devices to route Ethernet traffic. The result is that you’re back