Welcome to a new series of posts on Electro-ramblings concerning the wonderful but sometimes weird world of wireless comms, written by Joel Young, CTO of Digi International
Why am I able to use my 2.4GHz phone and still have a 2.4GHz 802.11 network work at the same time?
Questions like this concerning RF interference go back to a classic misunderstanding about modern day, digital radios. Especially since many people go off and buy a 5.8 GHz cordless phone because they are afraid that their WiFi won’t work very well.
Interestingly enough, these are the same people that may use a Bluetooth headset while working on their WiFi enabled laptop or better yet, may use their Nintendo DS at the same time as playing on the Wii. Unfortunately, in order to give the answer proper explanation, we need to look to how radios have evolved in the past say 40 years.
Yes it is true that all of these modern radios operate on the 2.4 GHz ISM band, spreading their transmissions across the spectrum from 2.4 to 2.5 GHz. The key lies in that all of these transmissions are (1) digital and (2) use different encoding and modulation schemes.
The technology in cordless phones, Bluetooth headsets and that Wii remote tend to use something call Frequency Hopping Spread Spectrum (FHSS) as a modulation and encoding scheme – and because of this their lower data rates tend to use narrow bunches of spectrum and hop all around.
WiFi, depending on if you are talking about 802.11b or 802.11g uses something called Direct Sequence Spread Spectrum (DSSS) or Orthogonal Frequency Division Multiplexing (OFDM), spreading their signal widely across big chucks of the spectrum.
The short of it is that these methods of encoding use complex math to encode so that if you don’t know the code, the transmission just looks like noise. This is also true about competing WiFi networks. And what do we do when we hear noise? We filter it out, tuning in on only what we want to hear – the same is true about radios in the ISM band.
Imagine you are at a dinner party and there is a dull background noise of conversation. You are still able to carry on a conversation with another person and amazingly enough, we humans are able to switch to others at will, effectively tuning out the noise. Now, as the noise gets really loud we need to compensate and sometimes we may ask someone to repeat what she just said.
Of course if someone comes up to you and screams in your ear, you will hear nothing else. The same applies to radios. When the noise level of other radios gets loud, retransmissions or clipping may occur.
Unless you put all your radios right next to each other — this is called saturating a receiver and is like someone screaming in your ear. So don’t let your radios scream at each other, but go ahead and use them in mixed company.
Joel Young, VP of Research and Development and CTO at Digi International, has more than 22 years of experience in developing and managing data and voice communications. He joined Digi International in June 2000 and in his current role he is responsible for research and development of all of Digi’s core products.
Prior to joining Digi, Joel was VP of Sales & Marketing at Transcrypt International where he was responsible for sales, marketing, and product development for all information security products. During his tenure at Transcrypt, he also served as VP of Product Development and VP of Engineering where he was responsible for engineering, research and product development for wireless communications products, cellular telephony, wireline telephony and land mobile radio, data security and specialized digital radio products.
He also served as District Manager for AT&T Business Communications Services where he was responsible for the creation and implementation of voice processing and network database strategies, including deploying new voice processing platforms into the AT&T switched network for private network and other outbound calling services.