8/23/09

TRANSMISSION IMPAIRMENT

TRANSMISSION IMPAIRMENT


Signals travel through transmission media, which are not perfect.
The imperfection causes signal impairment.
This means that the signal at the beginning of the medium is not the same as the signal at the end of the medium.
What is sent is not what is received.
Three causes of impairment are attenuation, distortion, and noise.

Attenuation


Suppose a signal travels through a transmission medium and its power is reduced to one-half.
This means that P2 is (1/2)P1.
In this case, the attenuation (loss of power) can be calculated as

A loss of 3 dB (–3 dB) is equivalent to losing one-half the power.


Distortion

Noise

DATA RATE LIMITS
A very important consideration in data communications is how fast we can send data, in bits per second, over a channel.
Data rate depends on three factors:
1. The bandwidth available
2. The level of the signals we use
3. The quality of the channel (the level of noise)
Increasing the levels of a signal may reduce the reliability of the system.

Nyquist Theorem

In baseband transmission, we said the bit rate is 2 times the bandwidth if we use only the first harmonic in the worst case.
However, the Nyquist formula is more general than what we derived intuitively; it can be applied to baseband transmission and modulation.
Also, it can be applied when we have two or more levels of signals.

Shannon Capacity

In reality, we can not have a noisless channel

For noisy channel,
Capacity = Bandwith x log2(1+SNR)
The Shannon capacity gives us the upper limit;
the Nyquist formula tells us how many signal levels we need

Example

Consider an extremely noisy channel in which the value of the signal-to-noise ratio is almost zero.
In other words, the noise is so strong that the signal is faint. What is the channel capacity?


Solution
This means that the capacity of this channel is zero regardless of the bandwidth.
In other words, we cannot receive any data through this channel.


PERFORMANCE

One important issue in networking is the performance of the network—how good is it?


In networking, we use the term bandwidth in two contexts
The first, bandwidth in hertz,
refers to the range of frequencies in a composite signal or the range of frequencies that a channel can pass.
The second, bandwidth in bits per second,
refers to the speed of bit transmission in a channel or link.

Examples
The bandwidth of a subscriber line is 4 kHz for voice or data.
The bandwidth of this line for data transmission
can be up to 56,000 bps using a sophisticated modem to change the digital signal to analog.
If the telephone company improves the quality of the line and increases the bandwidth to 8 kHz,
we can send 112,000 bps.

Concept of bandwidth-delay product

We can think about the link between two points as a pipe.
The cross section of the pipe represents the bandwidth, and the length of the pipe represents the delay.
We can say the volume of the pipe defines the bandwidth-delay product.

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