Data Communications FAQs

What is analog?

A good starting point in order to understand analog communications is to first take in the picture below.

+     .'^'.
+    /     \
+   /       \
+  /         \
++++++++++++++\++++++++++++
+              \         /
+               \       /
+                \     /
+                 `._.'

Although my artistic ability leaves much to be desired, this wave form is a depiction of a simple analog signal. The key to the analog signal is that it is *continuous*. In other words, notice how the wave slowly rises, peaks, slowly descends, bottoms out and slowly climbs again. Taken as a simple example, imagine many forms of this wave signal. Some of the waves are closer together than others, some may have more height, still others may actually start their peaks and descents in entirely different places! Encoding data can be done based on these various kinds of wave changes.

One of the important considerations in analog communications is the ability to decode these continuous wave forms. With the introduction of noise, or other signal disturbance, decoding a analog signal properly can be difficult. This is why we turn to the digital communications system

What is digital?

Again, with a picture let us look a simplistic view of a digital signal.

|   .--------.        .--
|   |        |        |
|   |        |        | 
|   |        |        |
|---|--------|--------|---
|   |        |        |
|   |        |        |
|   |        |        |
| --'        `--------'

Compared to the picture of the analog signal above, there is a major difference in this wave form. The transition from the peak of the wave to the bottom of the wave is *discrete*. In this case, the only way to represent data is by using the high or low point of the wave. For example, the high point may represent a "on" signal and the low point may represent a "off" signal. In the world of computers, this is also known as a binary numbering system consisting of only two digits. By using a digital signaling system in this fashion, it makes encoding and decoding data very simple. Generally, it will be very easy to determine where the peaks and valleys are, even with some signal loss or disturbance.

Digital methods are used as long as frequency response (bandwidth) is not a limitation. Analog methods are used only because multiple signal levels must be exploited to communicate a higher data rate of digital values in lieu of having adequate bandwidth.

A digital signaling system often has an analog component. Strictly speaking, this means the a digital wave isn't as sharp cornered as the picture shows above. The corners will likely be slightly rounded and even more so as the signal travels over some distance. For our purposes, this definition should give you a basic idea of how a digitally encoded system works.

What is modulation?

Modulation is a prescribed method of encoding digital (or analog) signals onto a waveform (the carrier signal). Once encoded, the original signal may be recovered by an inverse process called demodulation. Modulation is performed to adapt the signal to a different frequency range than that of the original signal. Here's how it flows:

bits -> modulator -> audio -> phone network -> audio -> demodulator -> bits

Hence the name MODEM short for modulator/demodulator. The modem is necessary because the phone network transmits audio, not data bits. The modem is for compatibility with existing equipment.

What is attenuation?

Attenuation is signal loss due to the diminishing availability of signal energy, or signal power. As a analog or digital signal traverses across a medium, it fades. High attenuation may lead to the inability to recover the signal on the far end. Signal repeaters may be used on the transmission path to periodically boost the signal strength. Baseband transmission is extremely limited to attenuation. Broadband much less so. In addition, wireless communications is much less susceptible to attenuation that is wireline communications such as xDSL or cable modems.

What is crosstalk?

Crosstalk refers to the interference between channels. In the xDSL world, the interference between nearby cables can have a negative impact on the performance of the affected cable(s). Have you ever been on the phone and heard some other conversation, not yours, in the background? If so, you have experienced the effect of crosstalk.

Near-end crosstalk (NEXT) occurs when the transmitter sends a signal and a nearby transceiver at the same end of link, through capacitive and inductive coupling, "hears" the signal.

Far-end crosstalk (FEXT) occurs when the transmitter sends a signal and a transceiver at the far end of the link, through capacitive and inductive coupling, "hears" the signal. FEXT will be of more concern in an asymmetrical system such as ADSL than symmetrical systems like HDSL. This is because strong signals originating from the near end, can interfere with the weaker signals originating at the far end.

What is the effect of noise?

Noise may be defined as the combination of unwanted interfering signal sources whether it comes from crosstalk, radio frequency interference, distortion, or random signals created by thermal energy. Noise impairs the detection of the smallest analog levels which may be resolved within the demodulator. The noise level along with the maximum clip level of an analog signal path set the available amplitude dynamic range.

The maximum data rate of a modem is limited by the available frequency range (bandwidth) and signal-to-noise ratio (SNR) which is amplitude dynamic range. If more of either is available, more bits may be transferred per second. The information carrying limit was discussed theoretically by Claude Shannon and is known as Shannon's limit, or information theory.

Because modems run close to Shannon's limit today, no further advances will be made to traditional telephone line modems other than incremental improvement of V.90. The frequency range of the audio channel is very limited at about 4 kHz. V.34+ modems are limited to a maximum data rate of 33.6Kb/s by an SNR of about 36 dB caused mostly by network PCM quantization noise. While V.90 improves the SNR by utilizing the network PCM levels directly, it is still subject to Shannon's limit.

xDSL modems take advantage of the spectrum above the telephone audio channel. While operating with somewhat less amplitude dynamic range they increase data rates by greatly increasing the frequency range of the communication signal (from about 10 kHz to over 1.0mHz). To do this they require the installation of special equipment at the central office and customer premise.