Data Signals

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Analogue Signals

The public dial-up service supports analogue signals. Analogue signals are what we encounter every day of our life. Speech is an analogue signal, and varies in amplitude (volume), frequency (pitch), and phase.

The three main characteristics of analogue signals are,

1. Amplitude

   This is the strength of the signal. It can be expressed in a number of different ways (as volts, decibels). The higher the amplitude, the stronger (louder) the signal. The decibel (named in honor of Alexander Graham Bell) is a popular measure of signal strength.

   Sound level	Type of Sound
   40db	normal speech
   90db	lawn mowers
   110db	shotgun blast
   120db	jet engine taking off
   120db+	rock concerts

   
   

   Diagram below shows a single signal of various amplitudes. The base line indicates a steady state. In this example, the signal amplitude rises both above and below the steady state. The measurement of the two extremes is called the peak to peak measurement.

   

   
   

   Below diagram illustrates a speech signal, in this instance, the word "Hello". Speech is a very complex signal, and contains many thousands of different combinations of signals all mixed together. Note that it looks much more complicated than the single signal shown above.

   

   
   

2. Frequency

   This is the rate of change the signal undergoes every second, expressed in Hertz (Hz), or cycles per second. A 30Hz signal changes thirty times a second. In speech, we also refer to it as the number of vibrations per second. As we speak, the air is forced out of our mouths, being vibrated by our voice box. Men, on average, tend to vibrate the air at a lower rate than women, thus tend to have deeper voices.

   A cycle is one complete movement of the wave, from its original start position and back to the same point again. The number of cycles (or waves) within a one second time interval is called cycles-per-second, or Hertz.

   

   
   

   An example is sitting on the beach, counting the waves as they come in on the shore. If we counted the number of waves that crashes on the beach over a minute period, then divided that number by 60 (because there are 60 seconds in one minute), we would have the number of waves per second (i.e. frequency).

   Humans can hear from reasonably low frequency tones (about 100 Hz) all the way up to about 12 KHz. As we get older, our ability to hear the higher notes is lessened, due to the aging process making the bones in the ear harder and less able to vibrate.

3. Phase

   This is the rate at which the signal changes its relationship to time, expressed as degrees. One complete cycle of a wave begins at a certain point, and continues till the same point is reached again. Phase shift occurs when the cycle does not complete, and a new cycle begins before the previous one has fully completed.

   

   
   

   The human ear is insensitive to phase shift, but data signals are severely affected by it. Phase shift is caused by imperfections in cable media, such as joins and imperfect terminations.

   Analogue signals are sent via the PTSN. Digital signals cannot be sent via the PTSN without being first converted to analogue.

Digital Signals

Digital signals are the language of modern day computers. Digital signals comprise only two states. These are expressed as ON or OFF, 1 or 0 respectively. Examples of devices having TWO states in the home are,

• Light Switches: Either ON or OFF
• Doors: Either OPEN or CLOSED

Digital signals require greater bandwidth capacity than analogue signals, thus are more expensive to communicate. The diagram below shows a digital signal.

Some Networking Terms

Baud Rate

Baud rate is the reciprocal of the shortest signal element (a measure of the number of line changes which occur every second). It is the number of signaling elements that occur each second. For a binary signal of 20Hz, this is equivalent to 20 baud (there are 20 changes per second). For telephone cables, the limiting factor in speed is the number of line changes per second. A line change is defined as switching from one state to another, for instance, switching from a 1 to a 0, or from a 0 to 1 for a digital signal. If the number of line changes per second are exceeded, errors occur and the signal at the receiving end cannot be reliably reconstructed.

The term is named after J. M. E. Baudot, the inventor of the Baudot telegraph code.

At slow speeds, only one bit of information (signaling element) is encoded in each electrical/ line change. The baud, therefore, indicates the number of bits per second that are transmitted. For example, 300 baud means that 300 bits are transmitted each second (abbreviated 300 bps). Assuming asynchronous communication, which requires 10 bits per character, this translates to 30 characters per second (cps). For slow rates (below 1,200 baud), you can divide the baud by 10 to see how many characters per second are sent.

At higher speeds, it is possible to encode more than one bit in each electrical change. 4,800 baud may allow 9,600 bits to be sent each second. At high data transfer speeds, therefore, data transmission rates are usually expressed in bits per second (bps) rather than baud. For example, a 9,600 bps modem may operate at only 2,400 baud.

Bandwidth

Bandwidth refers to a network's capacity to transmit data. Think of bandwidth as a highway: a two-lane road simply can't handle the traffic of an eight-lane super-highway.

Bandwidth is the amount of data that can be transmitted in a fixed amount of time. For digital devices, the bandwidth is usually expressed in bits per second (bps) or bytes per second. For analog devices, the bandwidth is expressed in cycles per second, or Hertz (Hz).

For comparison, an analog modem plugged into a typical telephone jack can accommodate as much as 56,000 bits per second while an optical fiber cable can transmit data at rates of 1,544,000 bits per second and greater, depending on the transmission protocol employed.

The right hardware can manage available bandwidth effectively.

Bits Per Second

This is an expression of the number of bits per second. Where a binary signal is being used, this is the same as the baud rate. When the signal is changed to another form, it will not be equal to the baud rate, as each line change can represent more than one bit (either two or four bits).

Digital signals sent via the PSTN need to be converted to analogue first (by using a device called a modem). Digital signals can be sent via the ISDN unmodified.

Channel

In communications, the term channel refers to a communications path between two computers or devices. It may refer to the physical medium, such as coaxial cable, or to a specific carrier frequency (sub-channel) within a larger channel or wireless medium.

Modem

Modems convert digital data from a personal computer, printer or other device featuring a computer chip into an analog signal that can be carried over the analog lines common to plain old telephone service (POTs). Computer information is stored digitally, whereas information transmitted over telephone lines is transmitted in the form of analog waves. A modem converts between these two forms.

The name stems from the process involved in converting digital data to an analog format (MOdulation) and the process by which the receiving modem reconverts data back to a digital format (DEModulation).

There is one standard interface for connecting external modems to computers called RS-232. Consequently, any external modem can be attached to any computer that has an RS-232 port, which almost all personal computers have. There are also modems that come as an expansion board that you can insert into a vacant expansion slot. These are sometimes called onboard or internal modems.

While the modem interfaces are standardized, a number of different protocols for formatting data to be transmitted over telephone lines exist. One such standard is CCITT V.34.

Most modems have built-in support for the more common protocols -- at slow data transmission speeds at least, most modems can communicate with each other. At high transmission speeds, however, the protocols are less standardized.

Aside from the transmission protocols that they support, the following characteristics distinguish one modem from another:

bps: How fast the modem can transmit and receive data. At slow rates, modems are measured in terms of baud rates. The slowest rate is 300 baud (about 25 cps). At higher speeds, modems are measured in terms of bits per second (bps). The fastest modems run at 57,600 bps, although they can achieve even higher data transfer rates by compressing the data. Obviously, the faster the transmission rate, the faster you can send and receive data. Note, however, that you cannot receive data any faster than it is being sent. If, for example, the device sending data to your computer is sending it at 2,400 bps, you must receive it at 2,400 bps. It does not always pay, therefore, to have a very fast modem. In addition, some telephone lines are unable to transmit data reliably at very high rates.

voice/ data: Many modems support a switch to change between voice and data modes. In data mode, the modem acts like a regular modem. In voice mode, the modem acts like a regular telephone. Modems that support a voice/data switch have a built-in loudspeaker and microphone for voice communication.

auto-answer: An auto-answer modem enables your computer to receive calls in your absence. This is only necessary if you are offering some type of computer service that people can call in to use.

data compression : Some modems perform data compression, which enables them to send data at faster rates. However, the modem at the receiving end must be able to decompress the data using the same compression technique.

flash memory: Some modems come with flash memory rather than conventional ROM, which means that the communications protocols can be easily updated if necessary.

Fax capability: Most modern modems are fax modems, which means that they can send and receive faxes.

Dial Up Speech Circuits provided by Telephone Communication Companies : The voice channel was designed to handle analogue voice in the range 300Hz to 3.4Khz. The nature of voice traffic is

• periodic - talk and then listen
• varying in intensity - talking softly, shouting

In addition, the voice channel was implemented using two way amplifiers, which meant special devices were used to prevent echoes or unwanted oscillations (the circuit suffering from feedback). These devices are called echo suppressors, and affect the signal by reducing the available bandwidth of the channel. The amplifiers are designed to boost low level signals and attenuate high level signals, with the intention of trying to maintain an average signal level on the channel.

Data signals are digital in nature and are

• continuous
• constant in intensity
• require greater channel bandwidth than analogue signals

This causes several problems. The two way amplifiers tend to get overloaded, with the net result of putting too much signal level on the channel. This overflows onto other channels, affecting the signals on them also (called crosstalk).

The second problem is the signals are affected by the bandwidth of the channel, such that only some of the original signal will appear at the other end. This effect becomes more pronounced as the speed of the data signal is increased.

Problems of using Voice Channels for Digital Transmission

A digital signal is comprised of a number of signals. Specifically, the signal is represented as follows,

signal = f + f3 + f5 +f7 +f9 +f11 +f13 ....f(infinity)

This means a digital signal has a base frequency, plus another at three times the base frequency, plus another at five times the base frequency etc. f3 is called the third harmonic, f5 the fifth harmonic and so on.

The third harmonic is one third of the amplitude of the base frequency (called the fundamental frequency), the fifth harmonic is one fifth the amplitude of the fundamental and so on.

In order to send a digital signal across a voice channel, the bandwidth of the channel must allow the fundamental plus third and fifth harmonic to pass without affecting them too much.

As can be seen, this is what such a signal looks like, and is the minimum required to be correctly detected as a digital signal by the receiver.

Let us consider sending a 2400 bps binary digital signal down a voice channel rated with a bandwidth of 3.1 KHz. The base frequency of the digital signal is 1200 Hz (it is always half the bit rate), so the fundamental frequency will pass through the channel relatively unaltered. The third harmonic is 3600Hz, which will suffer attenuation and arrive severely altered (if at all). The fifth harmonic has no chance of passing the channel.

In this case it can be seen that only the base frequency will arrive at the end of the channel. This means the receiver will not be able to reconstruct the digital signal properly, as it will require f3 and f5 for proper reconstruction.

This results in errors in the detection process by the receiver.

Transmission Impairments

Transmission lines suffer from 3 major problems:

1. Attenuation

   It is the loss of energy as the signal propagates outward. On guided media (i.e. wires and optical fibers), the signal falls off logarithmically with the distance. The loss is expressed in dB/km. The amount of energy lost depends on the frequency.

   Since a signal is not transmitted as a single waveform, but as a series of Fourier components (viz. f + f3 + f5 + f7 + f9 + f11 + f13…). Each frequency component (Fourier component) is attenuated by a different amount, which results in a different Fourier spectrum at the receiver, and hence a different signal.

   Attenuation characteristics of 3 popular transmission media can be seen below

   

   If the attenuation is too much, the receiver may not be able to detect the signal at all, or the signal may fall below the noise level.

   The attenuation properties of a medium are generally known and so Amplifiers (Equalizers) can be put in to try to compensate for the frequency dependent attenuation. This approach helps but can never restore the signal exactly back to its original shape.

   
   

2. Delay Distortion

   It is caused by the fact that different Fourier components travel at different speeds. For digital data, fast components from one bit may catch up and overtake slow components from the bit ahead, mixing the two bits and increasing the probability of incorrect reception. This is delay distortion or inter-symbol interference. This can also be neutralized by using suitable equalizers.

   
   

3. Noise

   As signal is transmitted through a channel, unwanted energy from sources other than the transmitter gets mixed up with the signal, causing distortion. Noise can be of 4 types –

   1. Thermal noise

   2. Intermodulation noise

   3. Cross talk

   4. Impulse noise

   Thermal noise is caused by random motion of the electrons in a wire and is unavoidable. It is distributed across the entire spectrum and that is why it is also known as white noise.

   When more than one signal share a single transmission medium, intermodulation noise is generated. For instance, two signals f1 and f2 will generate signals of frequencies (f1+f2) and (f1-f2), which may interfere with the signals of the same frequencies sent by the transmitter.

   Cross talk is a result of bunching several conductors together in a single cable or conduit. Signal carrying wires generate electro-magnetic radiation which is induced on other conductors because of close proximity of the conductors. While using telephone, it is a common experience to hear conversation of other people in the background. This is known as cross talk.

   Impulse noise is irregular pulses or noise spikes of short duration generated by phenomena like lightning, spark due to loose contact in electrical circuits, spikes, etc. Impulse noise is a primary source of bit-errors in digital data transmission.

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