The shared radio

Sixteen channels in a shared band

Zigbee's 2.4 GHz radio has sixteen channels, numbered 11 to 26, 5 MHz apart. Wi-Fi uses the same band, and one Wi-Fi network is about 20 to 22 MHz wide, so it sits across several Zigbee channels at once. Wi-Fi channels 1, 6 and 11 are the usual choices.

Zigbee channels 15, 20 and 25 fall between those three, so they overlap less. They are not clean: neighbours, power and the receiver's selectivity still matter. No channel is clean for everyone, so scan the site.

Two numbers that sound like one

The radio runs at 250 kbit/s, the physical layer rate. One octet takes 32 microseconds on the air, and a frame carries at most 127, which is 4.256 ms for the largest frame.

Your data is only part of that frame. Addressing, security and the cluster message take a share. With the bench's example headers about 79 octets are left, a ceiling near 148 kbit/s before waiting, acknowledgements and gaps, all of which only lower it. The header sizes are an example budget, not constants of Zigbee.

What it costs when it goes wrong

Wi-Fi traffic on a Zigbee channel does not break the network outright. It costs retries, and each retry is airtime. The network works, slowly, and every device still shows as present, so the cause is easy to miss.

For the contrast with a network that has an access point and a password, see the Wi-Fi course. The next lesson, Three roles, looks at who does what once a channel is chosen.

Common mistakes

• Reading 250 kbit/s as throughput. It is the radio's rate. Waiting, acknowledgements and headers take most of it.
• Picking channel 15, 20 or 25 and stopping. A gap between Wi-Fi channels overlaps less. It is not clean, and a scan tells you more than a rule.
• Treating the loss figure as a measurement. On the bench it is an estimate from overlap and load. A real network reports retries and failures instead.