Digital Transmission and Networks: Line Coding Schemes and Analog to Digital Conversion, Slides of Data Communication Systems and Computer Networks

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Assistant Professor^ Ehtesham^ Raza 3. Govt. Murray College Sialkot

DATA COMMUNICATION

AND NETWORKS

(IT-212)

Chapter 4

Digital Transmission

DIGITAL TO DIGITAL CONVERSION

• Data can be either digital or analog.
• Signals that represent data can also be digital or

analog.

• We can represent digital data by using digital

signals.

The conversion involves three techniques:

1. Line Coding,

2. Block Coding,

3. Scrambling.

Line coding is always needed; block coding and

scrambling may or may not be needed.

Line coding schemes are divided into five broad

categories.

Line Coding Schemes

3.

NRZ (Non-Return-to-Zero): A unipolar scheme is a non-return-to-zero (NRZ) scheme in which the positive voltage defines bit 1 and the zero voltage defines bit 0. The signal does not return to zero at the middle of the bit. This scheme is very costly. As the normalized power (the power needed to send 1 bit per unit line resistance) is double that for polar NRZ. For this reason, this scheme is normally not used in data communications today.

Unipolar Scheme

In a unipolar scheme, all the signal levels are on one side of the time axis, either above or below.

Polar Schemes: NRZ-L and NRZ-I

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

HOMEWORK:

Differentiate between NRZ-L and NRZ-I.

3.

Polar Schemes: Return-to-Zero (RZ)

The main problem with NRZ encoding occurs when the sender and receiver clocks are not synchronized. The receiver does not know when one bit has ended and the next bit is starting. One solution is the return-to-zero (RZ) scheme, which uses three values: positive , negative , and zero. In RZ, the signal changes not between bits but during the bit. The signal goes to 0 in the middle of each bit. It remains there until the beginning of the next bit

Disadvantages: a) It requires two signal changes to encode a bit and therefore occupies greater bandwidth. b) A sudden change of polarity resulting in all 0s interpreted as 1s and all 1s interpreted as 0s. c) RZ uses three levels of voltage, which is more complex to create and detect.

3.1 0

Manchester and Differential Manchester

• The Manchester scheme overcomes several problems associated with NRZ-L, and differential Manchester overcomes several problems associated with NRZ-I.
• There is no baseline wandering.
• There is no DC component because each bit has a positive and negative voltage contribution.

Advantages

• The signal rate for Manchester and differential Manchester is double that for NRZ. The reason is that there is always one transition at the middle of the bit and maybe one transition at the end of each bit.

Disadvantages

Bipolar Schemes:

• There are three voltage levels: positive, negative, and zero.
• The voltage level for one data element is at zero, while the voltage level for the other element alternates between positive and negative.
• The bipolar scheme has the same signal rate as NRZ, but there is no DC component.

Alternate Mark Inversion

• A neutral zero voltage represents binary 0.
• Binary 1s are represented by alternating positive and negative voltages.
• AMI is commonly used for long-distance communication, but it has a synchronization problem when a long sequence of 0s is present in the data.

Pseudo- ternary

• The 1 bit is encoded as a zero voltage
• The 0 bit is encoded as alternating positive and negative voltages.

Summary of Line Coding Schemes

• What is Scrambling?

Write the working of

TWO common

Scrambling Techniques.

Pulse Code Modulation

• The analog signal is sampled every Ts Sec, where Ts is

the sample interval or period.

• The inverse of the sampling interval is called the

sampling rate or sampling frequency and denoted by fs ,

where fs = 1/Ts.

• There are three sampling methods:

1. ideal 2. natural 3. flat-top

1. Sampling

For example of the Nyquist theorem, let us sample a simple sine wave at three sampling rates: fs = 4f (2 times the Nyquist rate), fs = 2f (Nyquist rate), and fs = f (one-half the Nyquist rate).

1. Sampling: Sampling Rate Example

The result of sampling is a series of pulses with amplitude values between the maximum and minimum amplitudes of the signal. The set of amplitudes can be infinite with nonintegral values between the two limits. These values cannot be used in the encoding process. The following are the steps in quantization:

1. We assume that the original analog signal has instantaneous amplitudes between Vmin and Vmax.
2. We divide the range into L zones, each of height Δ (delta).
3. We assign quantized values of 0 to L− 1 to the midpoint of each zone.
4. We approximate the value of the sample amplitude to the quantized values.

2. Quantization