Digi XBee modules operating in 2.4GHz band (like Digi XBee3 Zigbee modules) make use of one fixed operational channel for transmission. Users operating multiple network in common vicinity are advised to operate each network on different channel to avoid interference among them.
Digi XBee modules operating in 900MHz band make use of FSK (Frequency shift Keying) modulation technique for transmission. This results in making use of complete operational band instead of one fixed channel. Each Preamble ID values on module uses different pseudo-random sequence in which frequency shifting will take place. In other words; each "HP" value uses a different Hopping Pattern to navigate through the same frequencies.
Because the same frequencies are used for each channel there is some interference when two pairs of radios on different channels are operating in the same vicinity at the same time. Because Digi radios only transmit if their is payload to be transfered, a system with infrequent data transmissions will experience almost no interference between channels. If the channels are used at near full capacity we have found a decrease of 4% to 8% in the percent of data packets received per additional channel operating in the same vicinity.
If a base station broadcasts to two remote stations who then reply at the same time, then certainly there will be some interference. The more stations that transmit at the same time, the more interference there will be. Again, this interference can usually be overcome by enabling Reliable Delivery mode.
Aug 23, 2018