La sessione di domande e risposte di oggi ci viene fornita per gentile concessione di SuperUser, una suddivisione di Stack Exchange, un raggruppamento di domande e risposte di community drive.
Mentre l'uso del modem dial-up potrebbe scendere dalla saturazione quasi del 100% del mercato negli anni '90 a solo il 10% degli attuali utenti di Internet statunitensi, il suono di un modem dial-up che connette vive nei ricordi dei geek ovunque. Questa settimana daremo un'occhiata alla tecnologia che sta dietro al processo rumoroso e che cosa stava succedendo esattamente quando hai chiamato per la tua sessione internet.
La domanda
Il lettore SuperUser Celeritas pone la domanda sicuramente milioni di persone si sono chieste nel corso degli anni:
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?
Perché davvero? Cosa stava succedendo durante la parte rumorosa e perché il silenzio dopo?
Le risposte
Diversi collaboratori di SuperUser hanno dato una risposta a noi. Scott Chamberlain scrive:
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 si espande su questo e spiega come puoi manipolare il tuo modem per ridurlo:
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 la magia di AT M0; scoprire che il comando era come ricevere un mantello dell'invisibilità, una tarda notte in cui tutti cercavano la navigazione. Mentre Tylerl nota che il traffico ad alta velocità suona proprio come statico, il Supercat contribuente osserva che i modem a bassissimo baud erano una storia diversa:
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.
Hai qualcosa da aggiungere alla spiegazione? Sound off nei commenti. Vuoi leggere più risposte dagli altri utenti di Stack Exchange esperti di tecnologia? Controlla la discussione completa qui.
