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I've changed my AP operational mode to 802.11b+g (mixed) from 802.11b. Both the RTT and bandwidth dropped heavily. When I've switched back to 802.11b on my AP, the connection was much better.

Is that possible, or I'm missing something that misguided me? What is the exact difference between mode b and g?

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1 Answer 1

up vote 1 down vote accepted

After some research I've found the answer!

Conclusion: The 802.11b is a better choice than 802.11g when the environment is heavily crowded.

Full story:

I read a book long time ago (Computer Networks, Fourth Edition - Andrew S. Tanenbaum), so I was thinking: maybe I can't find the answer there, but I might find a trail. So did I. There, I've found a section about the Physical Layer of 802.11, which highlighted, that

  • 802.11a uses OFDM
  • 802.11b uses HR-DSSS
  • 802.11g uses OFDM

protocol. There were only a few information about what HR-DSSS and OFDM is, but Wikipedia cleared everything.

DSSS, so called direct-sequence spread spectrum is a modulation technique.

DSSS phase-modulates a sine wave pseudorandomly with a continuous string of pseudonoise (PN) code symbols called "chips", each of which has a much shorter duration than an information bit. That is, each information bit is modulated by a sequence of much faster chips. Therefore, the chip rate is much higher than the information signal bit rate.

DSSS uses a signal structure in which the sequence of chips produced by the transmitter is already known by the receiver. The receiver can then use the same PN sequence to counteract the effect of the PN sequence on the received signal in order to reconstruct the information signal.

Benefits: Resistance to intended or unintended jamming. Sharing of a single channel among multiple users. Reduced signal/background-noise level hampers interception. Determination of relative timing between transmitter and receiver.

More about DSSS on Wikipedia.

OFDM: A large number of closely spaced orthogonal sub-carrier signals are used to carry data on several parallel data streams or channels.

More about OFDM on Wikipedia.

While OFDM (802.11g) can mean a wider bandwidth, HR-DSSS (802.11b) is slower, but more reliable, since it has 7 times wider operational range.

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