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We are all familiar with the phenomenon that mobile phones induce in amplifiers and loudspeakers. The bip bip-da-bip bip-da-bip which happens just before you get a call, or if the phone is just feeling chatty.

I understand how the sound is produced, I don't understand why it has that particular pattern. Why a series of pulses?

I thought at first it might be an attempt to avoid interference, but of course that's nonsense: two x-millisecond transmissions are just as likely to become corrupted as one 2x-millisecond transmission.

So why the pule? Why not send the packet all at once? (Or send several packets in succession without a gap? Same difference -- the expected number of resends is surely the same.)

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Most digital cellphone systems use time division multiple access. They transmit their packets in short bursts so that other phones in the same cell coverage area can also use the frequency to transmit their own packets.

Also, transmitting in short bursts helps with managing the energy consumpution of the phone overall, for better battery life. The phone only has a certain amount of data to send per second, and by using the radio channel at its maximum rate for a short time saves power relative to sending at a slower rate continuously.

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  • \$\begingroup\$ More than just that, it tends to be a particular interaction with the network preceding receipt of a call which produces stronger interference than routine in-call operation does. \$\endgroup\$ – Chris Stratton Nov 20 '12 at 15:23
  • \$\begingroup\$ I believe that's caused by the phone transmitting blindly at full power, before the handshaking that allows it to reduce power to what the cell receiver needs. \$\endgroup\$ – Dave Tweed Nov 20 '12 at 18:49
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What you're hearing is specific to GSM. GSM is an example of a TDD system, where multiple handsets share the same transmit frequencies. Each handset that's active uses one or more timeslots to transmit, and another one uses different timeslots at the same frequency. This is one of the ways in which multiple handsets share the same transmit and receive frequencies in a cell sector.

http://en.wikipedia.org/wiki/GSM

is a decent starting point for this.

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