I don't even know if that's a title, but I have a quick technical question about the radio interference chirps that are sent by phones right before phone calls that can be heard in speakers or seen in CRT monitors. If you know what those are, could you email me at editor@gizmodo.com? I'd appreciate it.

And since I'm already wasting space, does anyone have a Verizon EVDO phone like the Samsung A890? Do you have problems with the voice quality?

Update: Wow, so many good answers already. Thanks, guys. I'll post one of the responses after the jump, along with a link to a more technical answer.


Nathan Schmidt writes:

In a GSM system in particular, the pre-ring chirps you see are the phone negotiating with the serving tower for a traffic [voice] channel on a particular frequency band and timeslice (versus the pre-allocated and shared control channel) to carry your conversation. If it can't get the serving tower to allocate the channel, it won't ring. Similarly, if the phone doesn't acknowledge the call (you're temporarily in a basement or something) the tower diverts to voicemail.

The control channel is a fixed frequency and only has enough capacity to announce calls and do some housekeeping. Traffic channels are allocated on the fly by the tower and the allocation exists only as long as a particular conversation.

The pre-ring shows up more dramatically on your CRT because, as you might imagine, it's imperative that the initial acknowledgment stages get through to the tower, so the phone is in effect shouting. Once the voice portion of the call begins the phone and tower lock into a tightly controlled cycle of output power control, transmitting with only enough power to be distinguished from the background noise. This saves battery power and keeps your head from getting too warm.

Consider this process happening over the course of a second or two:

Tower: Phone:

Call from 'xyz'
Ok, I heard that. Where's the call?
Channel 5.3
[Ringtone begins]


Question about cell phone interference [WiFi-Forum]