Data Communications and

NetworksNetworksChapter 1: Signal impairment

Pham Quang Thai pqthai.hcmut@gmail.com

1

Signal impairment: received signal is

different than transmitted signal

Attenuation: change in power

Distortion: change in time domain

Noise: addition in power

Example

1. A 1mW signal travels through a transmission medium and looses half of its power. Find the total attenuation in dB.

2. The loss in a cable is usually defined in decibels per kilometer (dB/km). If the signal at the beginning of a kilometer (dB/km). If the signal at the beginning of a cable with 0.3 dB/km has a power of 2 mW, what is the power of the signal at 5 km?

3. The power of a signal is 10 mW and the power of the noise is 1 W. The signal to noise ratio is called SNR. What are the values of SNR and SNR in dB ?

Bit rate limitation of transmission

medium

How fast we can send data, in bits per second, over a

channel depending on three factors:

The bandwidth available of the channel (B)

The level of the signals we use (L)

The quality of the channel (the level of noise N)

The foundation of information

theories

The maximum bit rate (Nyquist bit rate) in noiseless

channel: C = Blog2L

The maximum bit rate (Shannon capacity) in noisy

channel: C = Blog2(1+SNR)channel: C = Blog2(1+SNR)

Harry Nyquist Claude Shannon

Examples

4. A noiseless channel with a bandwidth of 3000 Hz transmitting a signal with two signal levels. Find the maximum bit rate

5. The same noiseless channel transmitting a signal with 2 bits per signal level. Find the maximum bit rate

6. We need to send 265 kbps over a noiseless channel with a 6. We need to send 265 kbps over a noiseless channel with a bandwidth of 20 kHz. How many signal levels do we need?

7. A telephone line normally has a bandwidth of 3000 Hz. The signal-to-noise ratio is usually 3162. Find the bit rate

8. A channel has 1-MHz bandwidth and SNR of 63. Find the signal level

The Shannon capacity gives us the upper limit;

the Nyquist formula tells us how many signal

levels we need.

Optical transmission link simulations

Questions for Simulation 1

9. Set the laser source power at 3 dBm. Turn off the fibers dispersion. Set fiber attenuation at 0.2 dB/km. Compare and explain the input/output signals

10. Set the laser source power at 3 dBm. Turn on the fibers dispersion. Set fiber attenuation at 0 dB/km. Compare dispersion. Set fiber attenuation at 0 dB/km. Compare and explain the input/output signals

11. Turn on the fibers dispersion. Set fiber attenuation at 0.2 dB/km. Set the laser source power at -3 dBm and 3 dBm. Compare and explain the input/output signals

12. Calculate the total attenuation and SNR. Why the total attenuation is larger than the fiber attenuation?

Optical transmission link simulations