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Speaker 1: Audible dot com is the leading provider of digital audio

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Speaker 1: books and spoken word, with over one tho titles to

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Speaker 1: choose from. Audible lets you listen to your favorite books anywhere, anytime.

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Speaker 1: Go to audible podcast dot com slash stuff brain to

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Speaker 1: get a free audio book download of your choice when

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Speaker 1: you sign up today. Welcome to brain Stuff from house

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Speaker 1: stuff works dot com where smart Happens. Hi Am Marshall

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Speaker 1: Brain with today's question, how does collar i D work?

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Speaker 1: If you have a color i D box attached to

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Speaker 1: your phone, or if the color i D box is

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Speaker 1: built into your phone, or if you have a cell phone,

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Speaker 1: then an amazing thing happens. Every time your phone rings,

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Speaker 1: the number, and sometimes even the name of the calling

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Speaker 1: party appears on the display right after the first ring.

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Speaker 1: It turns out that the process of making the color

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Speaker 1: i D display possible is remarkably simple at your end

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Speaker 1: of the line. If you've read the house stuff Work

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Speaker 1: article entitled how modems work, then you know that the

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Speaker 1: early modems used a technique called frequency shift keying to

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Speaker 1: transmit bits over the phone line. Frequency shift keying is

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Speaker 1: extremely simple. One tone, for example, like twelve hundred hurts

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Speaker 1: or beep represents a one, and another tone LIKEEP represents

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Speaker 1: a binary zero. A modem changes frequencies depending on whether

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Speaker 1: it wants to send a one or a zero. How

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Speaker 1: quickly it changes frequencies determines the speed or the bod

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Speaker 1: rate of the modem. To send caller i D information

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Speaker 1: to your home, the phone company uses a frequency shift

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Speaker 1: keying technique identical to a two modem from a million

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Speaker 1: years ago, and it sends asky character data to the phone.

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Speaker 1: The modem message is sent between the first and the

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Speaker 1: second ring, so the phone rings once and if you

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Speaker 1: could listen to the phone line just after that ring,

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Speaker 1: you would hear bleep sound about half a second long,

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Speaker 1: and if you decoded the bleep, you would find that

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Speaker 1: it contains first a series of alternating ones and zeros

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Speaker 1: to help the caller i D box get the timing down,

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Speaker 1: then a series of a hundred and eighty ones, and

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Speaker 1: then a bite representing the type of message that's about

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Speaker 1: to be sent, and then a bite representing the length

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Speaker 1: of the message, then a pair of bites representing the month, day, hour,

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Speaker 1: and minute that the message was sent, and then the

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Speaker 1: tendiget phone number in ten bytes, followed by a checksum

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Speaker 1: bite so the phone can make sure the message came

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Speaker 1: through okay. There's also a more advanced system that contains

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Speaker 1: the caller's name, but its technique is identical. Each character

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Speaker 1: is sent with a standard eight bit asky character, preceded

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Speaker 1: by a zero start bit and followed by a one

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Speaker 1: stop bit. The phone contained the twod modem to decode

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Speaker 1: the bits, a little circuit to detect the ringing signal,

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Speaker 1: and a simple processor to drive the display. That's really

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Speaker 1: all the into the whole thing. For more on this

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Speaker 1: and thousands of other topics, visit how staff works dot com.

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Speaker 1: Audible dot com is the leading provider of digital audio

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Speaker 1: books and spoken word, with over one thousand titles to

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Speaker 1: choose from. Audible lets you listen to your favorite books anywhere, anytime.

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00:03:20,639 --> 00:03:23,679
Speaker 1: Go to audible podcast dot com slash stuff brain to

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Speaker 1: get a free audio book download of your choice when

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Speaker 1: you sign up today.