A sessão de perguntas e respostas de hoje nos é oferecida por cortesia do SuperUser, uma subdivisão do Stack Exchange, um agrupamento de comunidades de perguntas e respostas da comunidade.
Embora o uso de modem discado possa estar abaixo da saturação de quase 100% do mercado nos anos 90 para apenas 10% dos atuais usuários de internet dos EUA, o som de um modem dial-up conecta-se à vida nas memórias de nerds de todos os lugares. Nesta semana, estamos analisando a tecnologia por trás do processo barulhento e o que exatamente estava acontecendo quando você discou para sua sessão de Internet.
A questão
Leitor de SuperUser Celeritas coloca a questão certamente milhões de pessoas se perguntaram ao longo dos anos:
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?
Por que de fato? O que estava acontecendo durante a parte barulhenta e por que o silêncio depois?
As respostas
Vários colaboradores do SuperUser deram uma resposta para nós. Scott Chamberlain escreve:
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 expande isso e explica como você pode manipular seu modem para canalizar:
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 a magia de AT M0; Descobrir que o comando era como receber uma capa de invisibilidade - furtiva tarde da noite, procurando por todos. Enquanto Tylerl observa que o tráfego de alta velocidade soa como estático, o colaborador Supercat observa que os modems de baud muito baixos são uma história diferente:
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.
Tem algo a acrescentar à explicação? Soe fora nos comentários. Quer ler mais respostas de outros usuários do Stack Exchange com experiência em tecnologia? Confira o tópico de discussão completo aqui.
