How can I use one telephone line for multiple uses?
Hi Chris & Co
Love the podcast.
I have one telephone line, ie 2 twisted wires, into my home, as most people have around here. How is it possble for me to have:
--a telephone conversation
--a download to the main computer
--a download to another computer (WiFi)
--my son listenining to another program or watching a 'u tube' or whatever.
-- watching or listening to the bbc news
...all being at a low speed as i'm at the furthest possible distance from my local exchange.
How is this possible over 2 wires?
When you talk into a phone the vibrations in the air that make up the sound are converted into vibrations in the electrical current carried by the copper wire of the telephone line. In the same way that your voice is made up of different pitches, this electrical vibration can be a wide-range of different frequencies. But according to Mark Smith, Network Engineer and Telecommunications Consultant, a telephone call uses a narrow range of frequencies...
Mark - Basically, there's a pair of wires from the local exchange, all the way to your phone; but the telephone conversations are actually quite a low-frequency signal: they only go up to 4 kilohertz.
Hannah - This leaves a lot of other frequencies that can transfer data above the range of human hearing. Using an electronic filter, you can transfer data at frequencies above the range that you can hear. A modem or "modulator-demodulator" converts digital data into these vibrations. As this is done at vibrations of at least 25,000 hertz, you can send voice and internet data along the same wire at the same time, without interfering with your telephone conversation.
Mark - The modem uses all the frequencies up to about 10 megahertz and it divides the frequency into bands, a bit like the spectrum of rainbow; and based upon how good your line is, it allocates different frequencies to different parts of a band. The things like YouTube, file transfer and watching your television, they're all data that needs to go as noughts and ones over the line. All that data's split into packets of data and it's sent over a line using a very special modem that decides the maximum amount of data it can get over that and works out how to do that.
With things like video for example, if the line is good enough, you'll always get that, but if the line isn't good enough, then it will start to break up. If you're transferring things like files, if the line is really good, it would be really quick, and if the line is not good, it will be very slow.
Hannah - On the forum, Evanau adds that these packets of data each have an address on the front, and a return address, like letters in the postal service. The network equipment uses this address to send each packet to the right destination. This means that, even though the packets are sent mixed up and out of sequence, the data doesn't get scrambled...