How is digital information sent over an analog line?

[SkuxDeluxe]

With a DSL connection, the digital information is converted into analog as the phone line is analog.

 

How would, for example, a byte, be sent over an analog line? Which part of the analog signal determines the data?

 

For example the byte with binary digits 10101010, would a higher amplitude signal be sent for the 1s and a lower amplitude for the 0s? And what is the time gap between the bits? I.e say there is a 1ms gap between the bits, the analog line would be in a high amplitude state for the first ms, low for the next, high for the next..... or is that the completely wrong idea?

 

Can someone please help me with this?

 

Thanks.

[Strange]

For example the byte with binary digits 10101010, would a higher amplitude signal be sent for the 1s and a lower amplitude for the 0s?

 

There are many ways of encoding data onto an analog signal. Something like the way you suggest has been used in the past. Normally it would be presence of a voltage (for 1) or absence (for 0).

 

Then there is the problem you mention, of knowing how long each bit is. Some old protocols used to also send a clock, so you knew when a new bit was on the line. However, that requires extra wires. So you want to somehow encode the data so it provides its own clocking/synchronization information. This can be done by using something like "Manchester code" so that each data bit has a change from hi to low voltage or vice versa.

http://en.wikipedia.org/wiki/Manchester_code

 

Another possibility is to encode the 1s and 0s as a change in frequency.

 

However, the bandwidth of the line means is a limit to the data rate that can be sent that way so more complicated schemes g=have been developed employed, where more bits are encoded per transmitted "symbol".

http://en.wikipedia.org/wiki/Phase-shift_keying

 

I don't know what modern ADSL modems use though ...

Edited August 13, 2013 by Strange

[Enthalpy]

From http://en.wikipedia.org/wiki/ADSL

With commonly deployed ADSL over POTS [hence not in Germany]... 138 kHz – 1104 kHz is used for downstream communication [...] further divided into 4.3125 kHz channels called bins.

 

Wiki doesn't tell it, but since ADSL transmits more than 1Mb/s, these bins must use some special modulation, probably a constellation

http://en.wikipedia.org/wiki/Quadrature_amplitude_modulation#Rectangular_QAM

[SkuxDeluxe]

 

In an analog line, what are the signals? Are they electricity? Are they sound waves? What is the difference between an analog line and a digital line?

Edited August 14, 2013 by SkuxDeluxe

[Greg H.]

The easiest difference to spot between an analog line and a digital line is mostly in how the signal is shaped. In an analog line, the signal will be a continuous sine wave (see Sine Wave). A digital signal, on the other hand, is made up of discrete square signals that indicate discrete 1's and 0's (see Digital Signal).

 

In an analog line, changes in frequency or voltage are used to indicate the digital 1's and 0's, and a modem is used to translate these analog waveforms into a digital waveform that the computer will understand. That is, in general sense, all a modem (MOdulator/DEModulator) does - converts one wave form into another.

 

To answer your initial question, what the signals are made of depends on the transmission medium. In a standard phone line or network cable, they are low voltage electrical signals. You can also use microwave transmissions, infrared (the remote control for your tv, for example), and light (fiber optic) depending on the application needed.

Edited August 14, 2013 by Greg H.

[EdEarl]

Analog signals are usually carried on sine waves which may be modulated via amplitude (AM), frequency (FM) or phase (PM). Digital data (e.g., Morse code and binary) is carried in the modulations. Whether WiFi, satellite, DSL, cellular, NASA interplanetary, amateur radio, military, or commercial EMR (radio, light, infrared, etc.) digital communications are being done, these modulation techniques are used. However, modulating a signal to carry digital signals is simple compared to the simultaneous amplitude and frequency modulations of sound used for music and voice.

 

Modulation from Wikipedia

In electronics and telecommunications, modulation is the process of varying one or more properties of a periodic waveform, called the carrier signal, with a modulating signal which typically contains information to be transmitted. This is done in a similar fashion to a musician modulating a tone (a periodic waveform) from a musical instrument by varying its volume, timing and pitch. The three key parameters of a periodic waveform are its amplitude ("volume"), its phase ("timing") and its frequency ("pitch"). Any of these properties can be modified in accordance with a low frequency signal to obtain the modulated signal. Typically a high-frequency sinusoid waveform is used as carrier signal, but a square wave pulse train may also be used.

 

There are a more modulating techniques (e.g., combinations of AM, FM and PM), which can be found in the Passband Modulation box on This page, This page is about spread spectrum communications, which are techniques that use modulation to improve communications security and reliability. However, the techniquest mentioned in the Passband Modulation box are not necessarily for security and reliability.

[SkuxDeluxe]

The easiest difference to spot between an analog line and a digital line is mostly in how the signal is shaped. In an analog line, the signal will be a continuous sine wave (see Sine Wave). A digital signal, on the other hand, is made up of discrete square signals that indicate discrete 1's and 0's (see Digital Signal).

 

In an analog line, changes in frequency or voltage are used to indicate the digital 1's and 0's, and a modem is used to translate these analog waveforms into a digital waveform that the computer will understand. That is, in general sense, all a modem (MOdulator/DEModulator) does - converts one wave form into another.

 

To answer your initial question, what the signals are made of depends on the transmission medium. In a standard phone line or network cable, they are low voltage electrical signals. You can also use microwave transmissions, infrared (the remote control for your tv, for example), and light (fiber optic) depending on the application needed.

Thanks, that helps immensely.

 

Now, how can multiple pieces of data be sent at the same time (i.e. the bandwidth of the line)? Do different voltages not interfere? Or are the amplitudes added?

[EdEarl]

Multiple signals can be sent using several methods. First, it is possible to modulate AM, and FM on the same carrier, thus sending two different signals. In addition, one can send several carrier waves, each with its own AM and FM modulation. And, third, each frequency has two sidebands that can be modulated differently; thus, one carrier with two sidebands. both modulated AM, and FM to carry four signals.