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What Is The Hidden Node Problem And How Does TDMA Fix This?

With the roll out of TDMA(time devision multiple access – AKA Nstreme2) I’ve heard mention of the hidden node problem…though, no one bothers to mention what the hidden node problem is…it seems the answer to the hidden problem is hidden.

Here’s my two second rendition of the issue:

Basic hidden node configuration. One user can't hear the radio transmissions from other users due to directional antennas.

Two users can transmit at the same time…which will cause collisions. PS, that's bad 😉

802.11 uses CSMA/CA to handle collisions. Collisions occur on half duplex connections (like wireless) when multiple parties send at the same time. CSMA attempts to listen for others transmitting and wait for a clear time to send. If other users can’t hear you sending, they assume no one else is sending and it is ok for them to transmit. Thus, the other nodes or users are hidden from you and may cause collisions.

The 802.11 RTS/CTS mechanism was added to help alleviate this issue, but doesn’t completely solve the problem. It basically does a request to send and a clear to send signaling method. This does also lower overall throughput.

With TDMA, each user is given a time slice to speak in, so the fact they can't sense each other's radio transmissions doesn't matter.

TDMA creates time slices. Each user is allowed only to send in his time slice. This effectively eliminates the hidden node issue! It doesn’t matter if you can hear the other nodes or not, you only send in your designated time slot.

Comments

9 responses to “What Is The Hidden Node Problem And How Does TDMA Fix This?”

  1. Tim Avatar
    Tim

    Nice in theory, have you tried it yet? How could WDS be integrated into this, or maybe not?

  2. Greg Avatar
    Greg

    @Tim

    I’ve not used it yet, though I spoke with several who have. So far the results have been pretty positive. I have one friend who calles Nv2…and well all TDMA implementations as “a chainsaw”. The fact that it just blasts and doesn’t care about anyone else.

    I did read that as of RoS V5 RC1, WDS is working.

  3. J.J. Boyd Avatar
    J.J. Boyd

    Its only a chainsaw to non associated stations & ap’s. Whereas with standard 802.11x it uses csma which backs off when collisions are detected. Using TDMA it doesn’t backoff at all. The stations keep right on talking only when its their turn. So the effects to standard 802.11x where other TDMA gear is present on the same channel is they(802.11x) stay backed off due to csma & TDMA keeps on “sawing” thru the band. 🙂 Which is good for the TDMA guy…. Until other people in the same area start using TDMA… So on 2.4ghz & 900mhz this could get messy. just my .02 cents.

  4. J.J. Boyd Avatar
    J.J. Boyd

    OOO & BTW good read Greg.

  5. Greg Avatar
    Greg

    @JJ
    Thanks kiddo 🙂 I’m sure this was all old news to you, but I found it interesting.

    Also, thanks for your $.02, even if it is only worth $.01 hehehehehe

  6. Tom Waterloo Avatar
    Tom Waterloo

    Think of the Access Point as a gatling machine gun, with each barrel firing at only one subscriber unit. This is how Motorola Canopy works.

  7. Greg Avatar
    Greg

    @Tom

    How dare you speak that name…hehehe

  8. Tim Payne Avatar
    Tim Payne

    Doesn’t Canopy use GPS to sync its transmissions?

  9. Iam8up Avatar
    Iam8up

    @Tim

    It can. You can

    a) generate sync from the AP – this has no additional cost and is minimal for functionality
    b) sync between two APs with an rj11 terminated cable
    c) add GPS sync to every AP and do it right – this is best, helps you and your neighbors using Canopy

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