1. CSCD 433 Network Programming
Fall 2012
Lecture 4a
Physical Layer Line Coding 1 1

2. Physical Layer Topics Physical limits of networks for data
Encoding data onto signals 2

3. Physical Layer Looked at physical media for networks
Many types of wired and wireless connections
All have different capacities and purposes with regards to network creation
Next, look at some theoretical limits of networks, encoding schemes for digital modulation and several multiplexing methods

4. Data Rate Limits 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 available bandwidth
2. The number of levels used to represent
signals
3. The quality of the channel (the level of noise)

5. Nyquist Maximum
1924, Henry Nyquist of AT&T developed an equation for a perfect channel with finite capacity
His equation expresses
Maximum data rate for a finite bandwidth noiseless channel

6. Noiseless Channel: Nyquist Bit Rate
Defines theoretical maximum bit rate for
Noiseless Channel:
Bit Rate=2 X Bandwidth X log2L
L = number of signal levels

7. Example Have a noiseless channel
Bandwidth of 3000 Hz transmitting a signal with two signal levels
The maximum bit rate can be calculated as
Bit Rate = 2  3000  log2 2 = 6000 bps

8. Example Consider the same noiseless channel
Transmitting a signal with four signal levels
For each level, we send two bits
The maximum bit rate can be calculated as:
Bit Rate = 2 x 3000 x log2 4 = 12,000 bps

9. Note
Increasing the levels of a signal may reduce the reliability of the system

10. Claude Shannon Noisy Channel
Claude Shannon developed mathematical theory in the 1940's for noisy channels
He used Entropy in his equation, which is the amount of randomness for a channel
Then, defined the amount of information that a message could carry
This allowed networks to plan for capacity of information

11. Noisy Channel: Shannon Capacity
Defines theoretical maximum bit rate for Noisy Channel:
Capacity=Bandwidth X log2(1+SNR)

12. C = B log2 (1 + SNR) = B log2 (1 + 0) = B log2 (1) = B  0 = 0
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
For this channel the capacity is calculated as
C = B log2 (1 + SNR) = B log2 (1 + 0) = B log2 (1) = B  0 = 0

13. C = B log2 (1 + SNR) = 3000 log2 (1 + 3162) = 3000 log2 (3163)
Example
We can calculate the theoretical highest bit rate of a regular telephone line
A telephone line normally has a bandwidth of 4KHz
The signal-to-noise ratio is usually 3162
For this channel the capacity is calculated as
C = B log2 (1 + SNR) = 3000 log2 ( ) = 3000 log2 (3163)
C = 3000  = 34,860 bps

14. Example
We have a channel with a 1 MHz bandwidth
The SNR for this channel is 63,
What is the appropriate bit rate and signal level?
Solution
First, we use the Shannon formula to find our upper limit
C = B log2 (1 + SNR) = 106 log2 (1 + 63) = 106 log2 (64) = 6 Mbps
Then we use the Nyquist formula to find the
number of signal levels.
6 Mbps = 2  1 MHz  log2 L  L = 8

15. Digital Modulation

16. Digital Modulation
Process of converting between bits and signals is called digital modulation
Convert voltages into bits
Mostly for wired media
Other schemes regulate the phase or frequency of a carrier signal
Mostly for wireless media

17. Line Coding Schemes

18. In unipolar encoding, we use only one voltage level, positive
Note
In unipolar encoding, we use only one voltage level, positive

19. Unipolar Encoding

20. In polar encoding, we use two voltage levels: positive & negative
Note
In polar encoding, we use two voltage levels: positive & negative

21. Polar: NRZ-L and NRZ-I Encoding

22. In NRZ-L, level of voltage determines value of the bit
Note
In NRZ-L, level of voltage determines value of the bit
In NRZ-I, inversion or lack of inversion determines value of the bit

23. Polar: RZ Encoding

24. Polar: Manchester Encoding

25. In Manchester and differential Manchester encoding, the transition
Note
In Manchester and differential Manchester encoding, the transition
at the middle of the bit is used for synchronization.

26. Note
In bipolar encoding, we use three levels: positive, zero, and negative.

27. Bipolar: AMI (Alternative Mark Inversion) Encoding

28. Summary

29. Summary
Many types of encoding for sending data over analog types of lines
Multiplexing allows sharing
More on this later ….
There are actually limits to how much data can be sent within a network

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