Where does the bandwidth go?

Q: It is said that about 6M bit/sec is the true throughput on an 802.11b wireless LAN. Where does the other 5M bit/sec go? – Adam, Kansas City

A: Say your salary was $100,000 per year, but you only see $60,000 – where did the rest of it go? Just as a portion of our incomes go to taxes that help keep society running smoothly, a portion of every data packet must be reserved as tax, or “overhead,” in a wireless network.

The chart below shows the taxing structure for moving data across an 802.11 wireless network. The first thing you might notice is that some of the overhead varies in conjunction with the data rate of the wireless link, while other parts are fixed. Let’s take a closer look at the entire process.

To get data moving from your computer, first you have to pay the Internet tax man for the privilege of using the Internet. TCP/IP “usage fees” typically require about 20 bytes in overhead. Then you have to pay additional fees to use the 802.11 network. Approximately 34 bytes must be paid to the Media Access Control police, while a fixed fee of 192 usec is paid to access the 802.11b highway (20 usec is required for the 802.11a “autobahn”). If you want things to run smoothly, you must also take care of the Carrier Sense Multiple Access/ Collision Avoidance police. They require Short Inter-Frame Space, a DCF Inter-Frame Space, and a Back Off bribe, which typically run 10 usec, 50 usec, and 310 usec respectively in an 802.11b environment.

We think you can get the picture from here. The bottom line is that by the time you get done paying everyone, your 11 M bit/sec. paycheck becomes approximately 6M bit/sec. (depending on what county you live in). That’s a 45% tax rate.

For more details, check out this link (brings up a PDF file).