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Data Communications Interview Questions

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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.

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. 

Privacy is a great concern in data communications. Faxed business letters can be intercepted at will through tapped phone lines or intercepted microwave transmissions without the knowledge of the sender or receiver. To increase the security of this and other data communications, including digitized telephone conversations, the binary codes representing data may be scrambled in such a way that unauthorized interception will produce an indecipherable sequence of characters. Authorized receive stations will be equipped with a decoder that enables the message to be restored. The process of scrambling, transmitting, and descrambling is known as encryption.

Custom integrated circuits have been designed to perform this task and are available at low cost. In some cases, they will be incorporated into the main circuitry of a data communications device and function without operator knowledge. In other cases, an external circuit is used so that the device, and its encrypting/decrypting technique, may be transported easily. 

Normally, we think of communications science as dealing with the contemporaneous exchange of information between distant parties. However, many of the same techniques employed in data communications are also applied to data storage to ensure that the retrieval of information from a storage medium is accurate. We find, for example, that similar kinds of error-correcting codes used to protect digital telephone transmissions from noise are also used to guarantee correct readback of digital data from compact audio disks, CD-ROMs, and tape backup systems. 

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 

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