With any communications system, it must be recognized that the received signal will differ from the transmitted signal due to various transmission impairments. For analog signals, these impairments introduce various random modifications that degrade the signal quality. For digital signals, bit errors are introduced: A binary 1 is trans- – formed into a binary 0 and vice versa. In this section, we examine the various impairments and comment on their effect on the information-carrying capacity of a communication link. The most significant communication impairments are as shown in fig:
- Attenuation:
Attenuation means a loss of energy The strength of a signal falls off with distance over any transmission medium. For guided media, this reduction in strength, or attenuation, is generally logarithmic and is thus typically expressed as a constant number of decibels per unit distance. In fig. shows the effect of attenuation and amplification.
Fig-2: Attenuation
For unguided media, attenuation is a more complex function of distance and of the makeup of the atmosphere. Attenuation introduces three considerations for the transmission engineer. First, a received signal must have sufficient strength so that the electronic circuitry in the receiver can detect and interpret the signal. Second, the signal must maintain a level sufficiently higher than noise to be received without error. Third, attenuation is an increasing function of frequency.
- Distortion:
Distortion means that the signal changes its form or shape. Delay distortion is a phenomenon peculiar to guided transmission media. The distortion is caused by the fact that the velocity of propagation of a signal through a guided medium varies with frequency. For a bandlimited signal, the velocity tends to be highest near the center frequency and lower toward the two edges of the band.
Thus, various frequency components of a signal will arrive at the receiver at different times.
This effect is referred to as delay distortion, as the received signal is distorted due to variable delay in its components. The distortion effect as shown in fig-3.
Fig-3: Distortion
Delay distortion is particularly critical for digital data. Consider that a sequence of bits is being transmitted, using either analog or digital signals. Because of delay distortion, some of the signal components of one bit position will spill over into other bit positions, causing intersymbol interference, which is a major limitation to maximum bit rate over a transmission control. Equalizing techniques can also be used for delay distortion.
- Noise:
Noise is refers to any unwanted signal. For any data transmission event, the received signal will consist of the transmitted signal, modified by the various distortions imposed by the transmission system, plus additional unwanted signals that are inserted somewhere between transmission and reception; the latter, undesired signals are referred to as noise-a major limiting factor in communications system performance.
Fig-4: Noise
Noise may be divided into four categories:
- Thermal noise
- Intermediation noise
- Crosstalk
- Impulse noise
1)Thermal noise is due to thermal agitation of electrons in a conductor. It is present in all electronic devices and transmission media and is a function of temperature. Thermal noise is uniformly distributed across the frequency spectrum and hence is often referred to as white noise; it cannot be eliminated and therefore places an upper bound on communications system performance. The amount of thermal noise to be found in a bandwidth of 1 Hz in any device or conductor is
2) Intermediation noise is produced when there is some non linearity in the transmitter, receiver, or intervening transmission system. Normally, these components behave as linear systems; that is, the output is equal to the input, times a constant. In a nonlinear system, the output is a more complex function of the input. Such non linearity can be caused by component malfunction or the use of excessive signal strength. It is under these circumstances that the sum and difference terms occur.
3) Crosstalk has been experienced by anyone who, while using the telephone, has been able to hear another conversation; it is an unwanted coupling between signal paths. It can occur by electrical coupling between nearby twisted pair or, rarely, coax cable lines carrying multiple signals. Crosstalk can also occur when unwanted signals are picked up by microwave antennas; although highly directional, microwave energy does spread during propagation. Typically, crosstalk is of the same order of magnitude (or less) as thermal noise.
4) Impulse noise is generally only a minor annoyance for analog data. For example, voice transmission may be corrupted by short clicks and crackles with no loss of intelligibility. However, impulse noise is the primary source of error in digital data communication. For example, a sharp spike of energy of 0.01-second duration would not destroy any voice data, but would wash out about 50 bits of data being transmitted at 4800 bps. Figure 2.15 is an example of the effect on a digital signal.
- Signal-to-Noise-Ratio:
The Signal-to-Noise-Ratio is defined as average signal power is divided by the average noise power. SNR calculation as given below:
Fig-5: Large SNR
Fig-6: Small SNR
fig: Two causes of SNR-a high & low SNR
Data Flow Diagrams Process Modeling
Communication between two devices can be Simplex, half-duplex, or full-duplex
A transmission may be:
1) Simplex
2) Half-Duplex
3) Full-Duplex.
In simplex transmission, signals are transmitted in only one direction; one station is the transmitter and the other is the receiver.
Fig: Simplex
Data exchanges over a transmission line can be classified as full duplex or half duplex transmission.
With half-duplex transmission, only one of two stations on a point-to-point link may transmit at a time. This mode is also referred to as two-way alternate, suggestive of the fact that two stations must alternate in transmitting; this can be compared to a one-lane, two-way bridge. This form of transmission is often used for terminal-to-computer interaction. While a user is entering and transmitting data, the computer is prevented from sending data, which would appear on the terminal screen and cause confusion.
Fig: Half-duplex
For full duplex transmission, two stations can simultaneously send and receive data from each other. Thus, this mode is known as two-way simultaneous and may be compared to a two-lane, two-way bridge. For computer-to-computer data exchange, this form of transmission is more efficient than half-duplex transmission.
Fig: Full-duplex
With digital signaling, which requires guided transmission, full-duplex operation usually requires two separate transmission paths (e.g., two twisted pairs), while half duplex requires only one. For analog signaling, it depends on frequency; if a station transmits and receives on the same frequency, it must operate in half-duplex mode for wireless transmission, although it may operate in full-duplex mode for guided transmission using two separate transmission lines. If a station transmits on one frequency and receives on another, it may operate in full-duplex mode for wireless transmission and in full-duplex mode with a single line for guided transmission.
