Wireless shades of grey

We often think of wireless signal as black-and-white pattern: here we have “the coverage”,- and there we don’t have it and wireless media is free. However, there are shades of grey between truly black and truly white spots. It helps to know them when we design or troubleshoot wireless networks.

WirelessCoverage

Position 1

  • We have “the coverage” and connection
  • Can detect and read frames
  • Can avoid collisions

Position 2

  • “No coverage” feeling and “No connection” on your mobile device
  • Can detect frames, but cannot read them anymore
  • Can still avoid collisions

Like the wired networks, each 802.11 frame consists of Physical and MAC header. The interesting part is that they are transmitted at different speeds. MAC header passes by like a high-speed train: for example, at 54Mbps. Physical header is always transmitted at the lowest possible rate, which makes it easier to be read. At position 2, we can still read Physical header and this will allow us to detect that a frame is coming,- and try to avoid it. However MAC frame will be unreadable, therefore, we cannot read the MAC addresses of the sender andĀ recipient, nor the data content.

Position 3

  • “No coverage” feeling and “No connection” on your mobile device
  • Can detect energy, but not frames anymore
  • Higher potential for collisions, but still a chance to avoid them

No Physical or MAC headers can be read at this point, but still enough energy detected to identify the wireless transmission. The device will still try to avoid collisions purely based on detected signal energy.

Position 4

  • True no coverage
  • Can still hear noise, but will not take it into account
  • High potential for collisions


Alex Mavrin, CCIE #7846

Visit http://www.apteriks.com and use FREE ONLINE tools for network professionals.

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References:

[1] Cisco CCNP Wireless Quick Reference
[2] Hidden node problem
[3] Exposed node problem

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2 comments

  1. Hi Alex,
    interesting topic (again). The fact that I’ve just finished the wireless-part of my CCENT it is maybe more interesting to me. I have a little question though, because I could not make that up from the article: why is the MAC-frame unreadable? General interference or is there a other logical explanation for that? Thx and keep up the good work!

    1. Roland,
      The physical header is easy to read as it is transmitted at low speed (1/2/6 Mbps depending on the flavor of 802.11). Among other things it has Rate field which tells at what rate the following MAC frame will be transmitted. Right after that the transmitter changes speed and transmits MAC frame, which contains actual data, at high speed.

      If the recipient is beyond the coverage threshold, it will not be able to read MAC frame, but still able to read Physical header.

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