Dnešná relácia otázok a odpovedí sa k nám pridelí zdvorilosťou SuperUser - podskupiny Stack Exchange, zoskupenia webových stránok Otázky a odpovede.
Zatiaľ čo používanie dial-up modemu môže byť v 90-tych rokoch skoro 100% nasycenosťou na trhu iba na 10% súčasných užívateľov internetu v USA, zvuk dial-up modemu, ktorý spája v pamätiach geekov všade. Tento týždeň sa pozrieme na technológiu za hlučným procesom a na to, čo sa práve deje pri príchode na vaše internetové stretnutie.
Otázka
Čítačka SuperUser Celeritas predstavuje otázku, ktorú si v priebehu rokov určite milióny ľudí spýtali:
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?
Prečo skutočne? Čo sa deje počas hlučnej časti a prečo ticho neskôr?
Odpovede
Viacerí prispievatelia služby SuperUser nám poskytli odpoveď. Scott Chamberlain píše:
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 sa rozširuje a vysvetľuje, ako by ste mohli manipulovať s modemom,
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.
Ach kúzlo AT M0; objavovanie tohto príkazu bolo podobné tomu, že dostal neviditeľný plášť - tajný neskoro večer prehliadania pre každého. Zatiaľ čo Tylerl poznamenáva, že vysoko prenosný prenos zvuku je rovnako statický, prispievateľ Supercat poznamenáva, že modemy s veľmi nízkymi prenosovými rýchlosťami boli iný príbeh:
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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