How to determine a wifi channel number used by wifi ap/network?

Question

Ive discovered that several android wifi-apps (WiFi Manager, WiFi Analyzer) shows a channel number of WiFi network additionally to BSSID/SSID etc. But I can't find any info on how they do it. The only thing I know is I can get some wifi frequency. Maybe they determine a channel corresponding to that frequency? Is there a way to detect channel of wifi network in android at all? Of course this info is not a big deal and I can live without it :) but still i'm curious...

Answer 1

According to Radio-Electronics.com, channel number is truly related with frequency.

CHA LOWER   CENTER  UPPER
NUM FREQ    FREQ    FREQ
    MHZ     MHZ     MHZ
  1 2 401   2 412   2 423
  2 2 404   2 417   2 428
  3 2 411   2 422   2 433
  4 2 416   2 427   2 438
  5 2 421   2 432   2 443
  6 2 426   2 437   2 448
  7 2 431   2 442   2 453
  8 2 436   2 447   2 458
  9 2 441   2 452   2 463
 10 2 451   2 457   2 468
 11 2 451   2 462   2 473
 12 2 456   2 467   2 478
 13 2 461   2 472   2 483
 14 2 473   2 484   2 495

For Android, ScanResult contains the frequency of the channel.

Answer 2

@SuppressWarnings("boxing")
private final static ArrayList<Integer> channelsFrequency = new ArrayList<Integer>(
        Arrays.asList(0, 2412, 2417, 2422, 2427, 2432, 2437, 2442, 2447,
                2452, 2457, 2462, 2467, 2472, 2484));

public static Integer getFrequencyFromChannel(int channel) {
    return channelsFrequency.get(channel);
}

public static int getChannelFromFrequency(int frequency) {
    return channelsFrequency.indexOf(Integer.valueOf(frequency));
}

Answer 3

You can express the conversion using a couple of formulas, one for each band. The function returns the channel of the given frequency or -1 in case that the frequency is not a valid wifi frequency (2.4GHz and 5GHz).

public static int convertFrequencyToChannel(int freq) {
    if (freq >= 2412 && freq <= 2484) {
        return (freq - 2412) / 5 + 1;
    } else if (freq >= 5170 && freq <= 5825) {
        return (freq - 5170) / 5 + 34;
    } else {
        return -1;
    }
}

It is a compact way to do the same.

Answer 4

According to standard [802.11-2012], there is a simpler way to work out channel number from frequency. Specifically,

channel_center_frequency = channel_starting_frequency + 5 * channel_number

For 5G band, channel_number = 0, 1, ..., 200; channel_starting_frequency = 5000 MHz.

For 2.4G band, channel_number = 1, 2, ..., 13; channel_starting_frequency = 2047 MHz.

The list of all channel frequencies can be found at WiFi channels

---

Translating this into code - refer to iw source:

int ieee80211_frequency_to_channel(int freq)
{
    if (freq == 2484)
        return 14;

    if (freq < 2484)
        return (freq - 2407) / 5;

    return freq/5 - 1000;
}