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Mens oppringt modembruk kan være nede fra nesten 100% markedsmetning på 1990-tallet til bare 10% av dagens amerikanske internettbrukere, er lyden av et oppringt modem som forbinder liv i minnene om geeks overalt. Denne uken tar vi en titt på teknologien bak den støyende prosessen, og hva som skjedde da du ringte inn for Internett-økten.
Spørsmålet
SuperUser leser Celeritas stiller spørsmålet sikkert millioner av mennesker har bedt seg selv gjennom årene:
I know that the signal was just tone pulses but why was it when (back in the 90s) when you first connected to the internet you heard a bunch of funny noises. After that if you were to use the internet, it still was using the telephone line, why no funny noises then?
Hvorfor faktisk? Hva skjedde under den støyende delen og hvorfor stillheten etterpå?
Svarene
Flere SuperUser-bidragsytere slo ut et svar for oss. Scott Chamberlain skriver:
Modems originally allowed you to send data over a network that was designed to only carry voice. Because of that, the communication method between two modems had to be in the audible hearing range (or it would not get carried on the phone line). This is no longer needed because the phone system can now carry both voice and data at the same time (DSL).
The sounds were there all the time, you just needed to pick up the phone to hear it. The reason they played it over a loudspeaker to start with is so you could hear if somthing went wrong with the connection (busy signal, wrong number, a person picked up instead of a modem on the other end, etc).
Tylerl utvides på det og forklarer hvordan du kan manipulere modemet ditt til å røre ned:
The whistles and chirps and buzzes that you hear when a modem is going through its initial handshake process is a test of the telephone line quality. A modem send precisely specified sounds and the other listens see what it actually hears on the other end. This way the modems know how clear the line is between them and what sort of frequencies they can use to communicate with each other. The more frequencies they can use and the lower the noise, the higher the speed they’ll be able to communicate at.
If a connection ever failed due to connection quality, it would generally fail during this initial handshake process. And if you were listening, you could usually tell why (e.g. you got an answering machine on the other end instead of a modem).
As such, modems were usually configured to play this handshake sequence out loud. This was configured by sending AT M1 to the modem during setup. Alternately, AT M2 means to leave the speaker on all the time, while AT M0 means don’t turn the speaker on at all. See the AT command set for more information.
The actual transmission noise that you would hear if you picked up the phone during an active session (as opposed to during this handshake procedure) just sounds like static.
Oh den magiske av AT M0; oppdager den kommandoen var som å bli gitt en usynlighet kappe-stealthy sen kveld surfing for alle. Mens Tylerl noterer seg at high-baud-trafikklyd akkurat som statisk, oppfatter Supercat at svært lave baudmodemer var en annen historie:
At 300 baud, it’s possible to audibly hear incoming data. On occasions, I’ve turned on the modem speaker if I wanted to hear when characters arrived on a generally-idle line. Higher baud rates use a “data-scrambler” circuit so that most patterns of data are no longer audibly distinguishable.
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