Šodienas jautājumu un atbilžu sesija mums priecājas par SuperUser - Stack Exchange dalību, Q & A tīmekļa vietņu kopienas diskusiju grupu.
Lai gan iezvanpieejas modems varētu izmantot mazāk nekā gandrīz 100% tirgus piesātinājums 90. gados līdz tikai 10% no pašreizējiem ASV interneta lietotājiem, iezvanes modema skaņa, kas savieno dzīvi, tiek izmantota visur no atmiņas par geeks. Šonedēļ mēs skatāmies uz tehnoloģiju, kas atrodas trokšņainā procesā, un to, kas īsti noritēja, kad uzaicinājāt interneta sesiju.
Jautājums
SuperUser lasītājs Celeritas rada jautājumu, protams, miljoniem cilvēku ir uzdoti sev gadu gaitā:
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?
Kāpēc patiešām? Kas norisinās trokšņainā daļā un kāpēc pēc tam klusēšana?
Atbildes
Vairāki SuperUser dalībnieki sniedza mums atbildi. Scott Chamberlains raksta:
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 izvēršas par to un izskaidro, kā jūs varētu manipulēt ar modemu, lai piespiestu to:
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.
Ak burvju AT M0; atklājot, ka šī komanda bija tāpat kā to, ka viņam tika dota neredzama mētelis - slepena vēlu vakara pārlūkošana visiem. Kaut arī Tylerl atzīmē, ka liela bodu satiksme skaņa tāpat kā statisks, līdzdalībnieks Supercat atzīmē, ka ļoti zemu bodu modemi bija atšķirīgs stāsts:
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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