24-02-2011, 02:39 PM
lucid_7.ppt (Size: 123 KB / Downloads: 192)
How WiFi Works ?
Outline for Today
Last we learned how to setup a WiFi network.
This time we will learn about the protocols that enable these networks.
Towards the end, we will discuss WiMax, an emerging fixed wireless technology which may impact Susquehanna
802.11 Operating Frequency
The 802.11 suite has been developed to enable wireless local area networking in either the 2.4 GHz or 5.2 GHz frequency bands.
Specifically, the frequencies used by 802.11 fall in the unlicensed bands, these are frequency bands which anyone can use for radio communication (without a license) as long as their radio waves do not radiate too much power.
The exact frequencies used (and how they are used) depends on whether the system follows 802.11b, 802.11a, or 802.11g.
802.11b
The 802.11b standard defines a total of 14 frequency channels.
FCC allows channels 1 through 11 within the U.S. Most of Europe can use channels 1 through 13. In Japan, only 1 choice: channel 14.
Channel represents a center frequency. Only 5 MHz separation between center frequencies of channels.
Any 802.11b signal occupies approximately 30 MHz.
Thus, 802.11b signal overlaps with several adjacent channel frequencies.
Only three channels (channels 1, 6, and 11 for the U.S.) that can be used without causing interference between access points.
Any given area can therefore support at most 3 access points (operating on different channels) at once. Equivalently, it can at most support three local ad-hoc connections.
Neighboring AP’s use different channels to reduce interference. “Reuse cluster” size is equal to 3.
Ideally, 802.11b supports wireless connections between an access point and a wireless device at four possible data rates: 1 Mbps, 2 Mbps, 5.5 Mbps, and 11 Mbps.
Specifically, as terminal travels farther from its AP, the connection will remain intact but connection speed decreases (falls back).
802.11b Spread Spectrum
When a 802.11b radio is operating at 1 Mbps and wishes to transmit a bit 1, it has to do so in 0.000001 seconds.
The way 802.11b does is this by actually transmitting a fixed sequence of 11 shorter bits (01001000111) to represent a single bit “1”. These 11 “shorter bits” (which represent one information bit) are sent in 1/11 the time, i.e., 0.0000000909 seconds.
These shorter bits are called “chips.”
When the radio wishes to transmit a 0 information bit, it uses the 0.000001 seconds to transmit a different fixed sequence of chips, 01001000111.
The chip sequence used for “1” is the complement of the chip sequence used for sending a “0”.
Why is this done?
Assume the original signal (the information stream of 1’s and 0’s) occupies a frequency bandwidth of W Hz.
When we use N chips to transmit 1 bit, the bandwidth of the resulting signal now occupies N×W Hz.
The new signal has a larger spectrum, i.e., the information signal of bandwidth W has been spread to a bandwidth of N×W. For this reason, this process is called spread spectrum.