Question of the Week

How can I use one telephone line for multiple uses?

Sat, 1st Sep 2012

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Dennis Soley asked:

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?




Hannah -   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.

Rotary dial telephoneMark -   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 on how good your line is, it allocates different frequencies to different parts of a band.  The things like YouTube and just file transfer and you're 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 at 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, Evan A-U 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.  We next move on to reflect on a question just in.



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Disclaimer: I've worked as a telecommunications engineer

The ability to use one line for several things is called 'multiplexing'. The one telephone line is divided up into different frequency bands, it's similar to the way radio works. The landline telephone voice stuff works on the lowest frequencies, below 20kHz- when you pick up the phone your voice pretty much just goes straight onto the line. If you whistle at 2kHz there's a 2kHz electrical signal on the wire to the exchange.

The computer's telephone ("ADSL") modem uses higher frequencies, up to a 1MHz or so. These frequencies are separated off as they enter the house, but are too high frequency for the human being to hear  anyway.

These frequencies, unlike radio, travel down the telephone wire; but like radio different frequencies don't interfere with each other, any more than radio stations on different frequencies do. So your voice doesn't interfere with the "ADSL" computer stuff. The use of frequencies like this is called 'wavelength division multiplexing'.

However, the ability to do several things at once on computers over this wavelength link is a bit different.

What happens is that the computer divides up the information it sends and receives into lots of short sequences of numbers called 'packets'. The modem at the exchange interleaves and mixes up the packets and sends them down the telephone line one packet at a time. So you might receive a few youtube packets, then some download packets, then another youtube, then some BBC news etc. etc. This is called 'time division multiplexing'. The packets have little addresses on them, so the modem knows which machine and which program needs to receive them. wolfekeeper, Sun, 12th Aug 2012

To add a little to Wolfekeeper's response:
There are many ways of getting broadband internet into your home, including Cable TV cable, wireless and various flavours of ADSL, which operate over the telephone line.
Since you mentioned this in conjunction with the telephone line, I assume you are using some version of ADSL (

The earliest version of POTS (Plain Old Telephone Service) and ADSL divided up the electrical signals on the telephone as follows:

DC: POTS Phone off-hook or on-hook

5-20Hz: POTS Pulse dialing (rarely used today)

20-30Hz: POTS Phone is ringing

50-60Hz (and sometimes 100-180Hz): Interference from power lines

300Hz-3.3kHz: POTS Voice (can also be used by diallup modems, up to 56kbps)

25kHz-125kHz: ADSL Sending information from your home to the network (up to 800kbps)

125kHz-1.1MHz: ADSL Sending information to your home from the network (up to 8Mbps)

Later versions like ADSL2+ used frequencies up to 2.2MHz to increase this to around 24Mbps from the network, and 1.5Mbps to the network.
Very recent versions like VDSL2 use frequencies up to 30MHz to achieve bit rates up to 100Mbps.

How ADSL works: If you could imagine your 56kbps diallup modem using 3kHz of spectrum - then just strap 250 of these side-by-side on a single telephone line, each using a separate 3kHz frequency band. The later standards take advantages of advancing chip technology to just strap on thousands of these modems at higher and higher frequencies.

However, this is a case of reducing returns, since the telephone line acts like a capacitor above voice frequencies, and this "shorts out" the increasing frequencies used by these new standards. It's as if each extra diallup modem can transmit at a slower speed, for a shorter distance. This means that to make use of VDSL2, the electronics must almost be installed in your street, rather than in the telephone exchange. evan_au, Mon, 13th Aug 2012

The packets of data over the internet 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.

Just like the mail system supports separate streams of communication to and from your house from the power company, the water company and the bank, so the internet supports simultaneous streams of data to and from Youtube, Google, iTunes and the Naked Scientists, using these addresses.

The current version of Internet Protocol, "IPv4" supports almost 4 billion separate addresses (32 bits). But so many people around the world now have internet-enabled devices, that the "spare" addresses are effectively exhausted, with only 1 address per home (often shared with other homes).

The next version, "IPv6" supports 128-bit addresses, which is around 1 with 40 zeroes - you could easily have a billion IP addresses in your home!
Meanwhile, researchers looking at current and future applications of the internet are eyeing the use of addresses of 250 bits or more. evan_au, Mon, 13th Aug 2012

You forgot to include that you may also be supplying WiFi to any old Tom Dick or Pervy who are nearby if you’re using a British Telecom “hub” ... RD, Tue, 14th Aug 2012

BTFON doesn't actually cost you anything; BTFON is essentially a different, lower priority network that shares the same wire and wavelengths (when you're not using it), and if there are pervs on your BTFON network, they're pretty damn stupid, because all their packets are labelled with their (not your) account!

And the positive point about it is you get something in return; you get free use of BTFON wherever you roam. wolfekeeper, Tue, 14th Aug 2012

The bit in brackets is not true: uninvited “guests” can logon while you’re online and take up to 512Kbps of your bandwidth, so more buffering for you, ( i.e. it costs you time ).

BT sells anyone access to this network made from domestic WiFi “hubs” in the form of “Fon Minutes", (£5 per day IIRC), so Pervy / Troll isn’t necessarily an identifiable roaming BT customer.

Just because any criminality is via the “guest” wifi channel it doesn’t prove the hub owner isn’t involved, as they could log in to their own network as a guest. So The Old Bill would be remiss not to remove all the computers from the hub-owner's house for a once-over, which would be an inconvenience and stigmatizing. RD, Tue, 14th Aug 2012

The bit in brackets is not true: uninvited “guests” can logon while you’re online and take up to 512Kbps of your bandwidth, so more buffering for you, ( i.e. it costs you time ).

No, my understanding is that that is false.

If you're using 100% of your broadband speed, then you get to use 100% of your broadband speed, and the BTFON gets completely excluded, and you don't get any slow down.

The 512K limit is the maximum BTFON can ever get, not a guarantee of any kind that that's what they get. BT don't just steal 512K off your bandwidth, nobody would ever agree to that.


You need a credit or debit account to buy the time; the police would doubtless trace them that way.

There's around 2 million BTFON servers in the UK, did this ever happen to a BTFON base station? wolfekeeper, Tue, 14th Aug 2012

If I was browsing a low-bandwidth text-based website like TNS, an uninvited “guest” could use the spare capacity on my BT hub. If I then decide to stream video which will require all my bandwidth is BT going to kick the “guest” off who is paying them way more* than I am to use the internet ?  (* £5 per day for 512Kbps ).

They didn’t ask BT customers to agree : BTFon is contract-out, since circa 2010 it’s on by default.
If you don’t want your BT wifi hub to be a public convenience you have to do something about it.

I believe “Fon minutes” can be bought via mobile phone, which could be an untraceable pay-as-you go type “topped-up” with untraceable cash ... RD, Wed, 15th Aug 2012

If I was browsing a low-bandwidth text-based website like TNS, an uninvited “guest” could use the spare capacity on my BT hub. If I then decide to stream video which will require all my bandwidth is BT going to kick the “guest” off who is paying them way more* than I am to use the internet ?  (* £5 per day for 512Kbps ).

The evidence I have is that yes, they do get kicked off, the person can always use a different base station elsewhere. Don't forget that the whole thing is at the home owner's courtesy, you can switch your modem off entirely for example, at any time. The whole principle of this is to use spare capacity, not steal your bandwidth!!!

The amount of spare bandwidth out there is simply enormous, most users only use 1/50 or less of their line's capacity, and that's when they're online.

Well, unsubscribe then. The whole point of it is that the service gives you a facility when you're out and about.

I don't mind if you have justified objections to verified facts, but you seem to have made up a non existent feature of the system and objected to it, on that false basis.

I believe “Fon minutes” can be bought via mobile phone, which could be an untraceable pay-as-you go type “topped-up” with untraceable cash ...

Well then they have the phone number, and cell phone masts physically track mobile phones, even a pay-as-you go type. If the paedo is only using your WiFi then it's pretty justifiable to get a court order to search your house. If they're moving around then the police would be unlikely to do that.

Honestly, it's so unlikely that it amounts to simple paranoia. wolfekeeper, Wed, 15th Aug 2012

The reason you can get so much information from a single twisted pair is because of superheterodyning (or modulation), putting information on a carrier wave(s),  then subtracting the carrier wave frequency to extract the information.  Its the same reason why you can have one cable coming into your home yet watch different channels of TV.  In its simplest form, superheterodyning circuits created the first carrier wave radio transmission.  Your local AM radio station transmits a carrier wave, say 950Mhz,  and modulates the audio frequency information-say 10-10Khz, say the traffic report, onto the carrier wave via a mixer.  The mixer has two inputs, the desired information and carrier frequency from a local oscillator which are added together.  Your phone line has multiple carrier frequencies being transmitted on the same wire, and your individual devices are tuned to receive only specific carrier frequencies that make it so you can have multiple channels of information on one wire. BillionsNbillions, Sun, 9th Sep 2012

Thank you, Billionsnbillions. chris, Mon, 10th Sep 2012

Amplitude and Phase modulation of a Carrier Frequency works well in hardware, when there is a small amount of information in a fairly narrow frequency band (compared to the carrier frequency).

The ADSL transmission used on most telephone lines spans quite a wide band - from 25kHz to 125kHz, and so modulating a single carrier would not work very well. This wide range is broken up into sub-bands.

Modern Digital Signal Processors use a mathematical technique called the Fast Fourier Transform (FFT) to generate signals in all of the sub-bands simultaneously. The inverse FFT is used to decode them off the telephone line, while accounting for the variable frequency and phase response over this wide frequency band, and cancelling the interference from the adjacent sub-bands.

Like an increasing number of things today, this is a software-based technique, rather than a hardware-based technique. Most of the hardware effort goes into making sure the software can run as fast as possible, while consuming as little power as possible. evan_au, Mon, 10th Sep 2012

What is the connection with data and frequently imean signals are coming in frequents but we count those thing with giga bytes ARJ, Fri, 31st Oct 2014

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