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In this article I will try to show you how both ADSL and regular voice can share a single copper phone-line ADSL technology is supplied by all major telecom infrastructure builders (like Ericsson and Alcatel - the last one being used by KPN in the Netherlands) and comes in several shapes and sizes.

Analog Telephony: PSTN or POTS

Back in the old days, copper wires where used to transport analog voice signals from location A to location B. Analog means a range between two values with infinite values in between. Discrete (or digital) is the counter part of analog: it has a limited number of values between two defined values.

Note: POTS (= Plain Old Telephony System) is the same as PSTN (= Public Switched Telephone Network).

Example:

• Analog values between 0 and 10 can be 1, 2.25, 4.333333, 9.99999999999, etc.
  
  
• Discrete (digital) values between 0 and 10 can be limited to: 1, 2, 3, 4, 5, 6, 7, 8 en 9.
  
  
• In the digital binary world we are even more limited: 0 and 1.

When comparing analog signal and digital data you will see something like this;

Red line represents analog signal and the blue stairs represent it's digital counter part. An AD (Analog-Digital) converter converts analog signals to digital data. A DA converters returns the process (however you will loose some of the information compared to the original analog source).

Voice signaling only requires a limited bandwidth, very good hearing people can only hear sounds in the range of 20 Hz (Hertz) up to 20.000 Hz (or 20 kHz). This only covers a limited amount of available bandwidth copper has to offer (currently 1.1 MHz!). Computer users had to buy a so called MODEM - MOdulator DEModulator. A modem converts the digital computer data to an analog signal and vice-versa. Because of the nature of analog signals, analog voice is sensitive to noise.

Digital Telephony: ISDN

Technology keeps improving even for telephony, soon ISDN was a fact. ISDN uses a bandwidth of up to 80.000 Hz - thus using more bandwidth on our old copper lines. Additional advantage of ISDN is that traffic is now digital meaning that computers do not need to convert their data to analog and back. Noise, as seen with analog voice, is rare. A 1 remains a 1 and a 0 remains 0. That's why the sound quality of ISDN is much better compared to PSTN (Analog). Note: for us humans to hear and understand the data, your ISDN phone still needs to convert analog speech to digital data and vice-versa. Note: ISDN = Integrated Services Digital Network.

Digital Telephony and Fast Internet: ADSL

To get even higher transfer-rates for Internet (and computer data in general), ADSL was invented. Upstream (FROM the user) uses a lower bandwidth than downstream (TO the user). In the Netherlands a system called VideoText used this asynchrone data exchange technique as well (only 75 BPS upstream and 1200 BPS downstream).

The best part of ADSL is that we can still use the good old copper lines and use both ADSL and voice simultaneous! Voice can be either PSTN (Analog) or ISDN (Digital - even 2 conversations at once!).
So why do we need voice then ?

Well, ADSL itself is not intended for voice but for data only. Of course we can use the ADSL bandwidth to transport voice data, but this requires similar solutions on both ends. For example two PC users using Yahoo! messenger, or one PC user using Yahoo! Messenger and a translation service like Net2Phone which converts Yahoo! Messenger data to standard voice.

Note: ADSL = Asymmetric Digital Line Subscriber.

How do we mix ADSL and voice?

As you might have noticed when you installed ADSL, we use a so called splitter.
Basically this is a filter splitting both signals (PSTN/ISDN and ADSL):

Incoming copper lines
Splitter
Voice (PSTN/ISDN)	ADSL

The splitter separates voice (left) and ADSL (right) so we can both surf the web and make phone calls.
This is also why connecting the phone to your ADSL connector (after the splitter) will NOT work.

For the use of ISDN we also need an NT1 box that converts it to ISDN (or rather: separates ISDN data).

Practical installation examples for both analog and digital (using MXStream) can be found on this website too.