How Fast? Bit Rate vs. Throughput

There's a lot of confusion inherent in the common question: 'How fast is Wi-Fi?' Well, that depends on what you mean by 'fast.' There is the rate at which bits move through the air, from one 802.11b radio subsystem to another. This is commonly called the bit rate. The top speed of an 802.11b link is 11 Mbps, which means eleven million bits, or megabits, per second. When conditions get noisy or the two stations move too far apart to support top speed, there are slower speeds that the link will automatically fall back to: 5.5 Mbps, 2 Mbps, and at the bottom 1 Mbps.

Now, where you get in trouble is when you assume that if your bit rate is 11 megabits (million bits) per second, you can send an eleven-megabit file (which is about 1.37 megabytes) in one second.

Not so. In fact, not even close.

The problem is that your file's bits aren't the only bits moving over the radio link. To move across the link, the file is broken up into packets, each of which is a kind of data container that is itself made out of bits that form headers and footers containing addresses and routing information. Without getting too technical right here, I'll simply say that there are multiple layers of containers, all of them made from bits. When your file is properly packetized and sent over the link, it's a lot bigger than it was when it was just a file on your hard drive. Think of packing something fragile for shipping cross-country. The final box, with all the bubble pack and foam peanuts inside, is much bigger than the glass brandy snifter it contains.

So even though your bit rate might be 11 Mbps, your throughput (that is, how many actual bits worth of data file you can transfer in a given time) is a lot less. Your actual maximum possible Wi-Fi throughput, flat-out with no one else sharing the connection, will be under 7 megabits per second. How much under depends heavily

on the manufacturer of your Wi-Fi access point and client adapter, and whether or not you have WEP enabled. On a link with an 11 Mbps bit rate, most of my throughput measurements have come in at about 5 Mbps with WEP disabled, and 4 Mbps with WEP enabled. That's still pretty fast … but be careful when doing bandwidth math in your head. Bit rate is not the same as throughput!

I'll explain how to test what your throughput actually is (using free software you can download from the Web) in Chapter 9, when we actually put some Wi-Fi networks together.