rx/tx switching time.

Wireless & IoT
1

Hi all,

I’m working on a ZigBee protocol. I would like to know what’s the minimum rx/tx switching time among ZigBee devices. The 802.15.4 protocol allows for switching delay about 320us. But is there any ZigBee device that can achieve shorter delay, i.e., is it possible for a ZigBee device to respond within 20us after receiving a packet?

Thanks,

Hok

2

Rx/Tx switching time is a MAC and PHY topic, unrelated to ZigBee which is one of several networking protocols that can be used with IEEE 802.15.4 chips and modules. Most applications don’t use ZigBee, and ZigBee is not a synonym for 802.15.4.

The IEEE spec stipulates the switching time. As I recall, it’s a few microseconds. Sending the '15.4 MAC layer ACK is a good example. On receipt of a frame, the receiving device must validate the frame check (CRC) then send a MAC ACK within 120 microseconds, per the standard.

The time to change channels is not spec’d but is on the order of a few tens of mSec, to allow the PLL / oscillator to stabilize.

To respond with a user data packet in 20microseconds is not possible even if you turn off the MAC ACK option.

3

Thanks a lot for your reply!

I was thinking that WiFi devices can have response time as short as 10 microseconds (the 802.11 standard specifies that ACK must be sent within 10us after receiving the data packet). I was wondering if ZigBee devices can achieve the same switching time.

Thanks!

4

modulation rates for 802.15.4 are much slower than 802.11, and this is why 802.15.4 tolerates a much poorer signal-to-noise (weak signal), and why it uses 2MHz of bandwidth whereas 802.11b/g is 20MHz.

It’s not so much the switching time as the bit rate.