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Access Point Channel Surfing

Isaac Puckett
Intellectual 990 points
Other Parts Discussed in Thread: CC2500

Hello,

I am trying to implement a sleep state for my access point once it scans every channel that the end devices may be on. One strategy that I am thinking about is counting each channel, but I don't know how many channels they are to be counted. So something like if they're 4000 channels available then every time I scan a channel I increment the counter variable then once I reach 4000 it would go to sleep. Is that a good strategy?

-Thanks in advanced

  • 0
    Guru 227245 points

    It is a working strategy, but I don’t know whether it is a good one. As you said, the AP doesn’t know how many devices could be there, and which ones.

    In our product, we have a similar problem (even though power consumption isn’t a matter of interest).
    Our access point sends a beacon signal every few seconds. All new devices listen for some time for this signal before switching to another frequency (in your case, perhaps going to sleep for a few minutes). Once received, this beacon signal contains information about timing information: duration of a send interval and already used time slots in the interval. The device can use this information to randomly pick a free time slot, and can send a request to the access point to get a time slot assigned. From this moment on, the device know when it may send its data, without being polled. I’m sure this basic idea can be adapted to suit your needs.

  • 0 in reply to Jens-Michael Gross
    Intellectual 990 points

    I ended up using a method similar to this. I used a 'ping' system similar to one of a beeper. The end device sends a message continuously until a handshake has occurred. However, I am now looking to a wake on radio function that may be more beneficial to my experiments. Would you by chance know of any examples for the wake on radio function for the CC2500?

  • 0 in reply to Isaac Puckett
    Guru 227245 points

    Sorry, but a real wake-on-radio is a hardware function of the transceiver, and I can’t help you with this.

    It can be simulated in software by switching the receiver on in regular intervals and checking the RSSI level. If available. However, if it is done in an ongoing transmission, the start of the transmission is of course lost. So it only works if the sender requires an ACK with a short timeout, and retransmits. So the receiver can get the transmission on the next attempt.
    However, this is only useful if the wakeup interval is significantly shorter than the transmission interval. So for short transmission intervals (or multiple transmitters), it will be better to have the receiver being always on. However, usefulness shifts when the receiver is battery-driven while the sender is line-powered. Then excess sending is unimportant while not receiving all the time saves power.

    However, best is when all peers know when to send or listen. With constant re-synchronization by listening early and synchronizing when the transmission actually starts. Then everyone can sleep in the meantime and knows when to wake-up next.

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