Digital Transmission in Physical Layer

In this chapter we are going to discuss the digital transmission in physical layer. We already know that the physical layer is responsible for sending the data and when this data is in form of digital signals, it is known as digital transmission.

As per the OSI Model, physical layer is the last layer or you can say that it is the last layer at the sender side that adds it’s functionality and it is the first layer at the receiving side as shown in the diagram below (left side is sender and right side is receiver).

As shown in the diagram. physical layer receives the data from data link layer in form of frames (consists of bits), now physical layer needs to send these bits to the receiver side through transmission media. In order to send the data through transmission media, it needs to be converted to either digital or analog signal.

In the last chapter(data and signals in physical layer), we discussed that data can be digital or analog. In this chapter, we will mainly focus on digital transmission, we will discuss various techniques that can be used to convert the data to digital signal for transmission. First we will discuss how to convert digital data to digital signal, this technique is known as digital to digital conversion and then we will discuss how to convert analog signal to digital signal which is known as analog to digital conversion.

Digital To Digital Conversion:

Here we will learn how to convert digital data to digital signal, for this conversion we will use three popular techniques:
1. Line Coding
2. Block Coding
3. Scrambling

In this article, we will mainly discuss the Line coding in detail.

Line Coding

The main purpose of this technique is to convert the digital data to digital signal.

Line Coding technique is further divided in the following categories:
a) Unipolar
b) Polar
c) Bipolar
d) Multilevel
e) Multitransition

a) Unipolar: In Unipolar scheme all the signals are on the one side of the time axis, it can be either above the axis or below the axis.

• In this Scheme the voltage is either 0 or 1, when there is a voltage fluctuation in any direction its is represented as 1 and when there is no fluctuations, it is represented as 0.
• In this Scheme “1” is considered as high voltage while the ‘0″ is considered low voltage.
• If we compare it with the polar scheme (discussed in next section), then it is very costly because the power needed for this scheme is double than the power required for polar schemes.
• Due to its heavy cost, this scheme is not used in data communications now a days.

Here in the following diagram, you can see that all the signals are above the time axis.

b) Polar: In Polar scheme signals can be on both sides of the time axis as shown in the following diagram.

The polar Schemes are categorized as three types:

• Non-Return-To-Zero (NRZ)
• Return-To-Zero (RZ)
• Biphase: Manchester and Differential Manchester

Non-Return-To-Zero (NRZ): NRZ scheme used two encoding NRZ-L and NRZ-I.

In NRZ-L, L stands for Level, as the name suggests level of voltage determines the value of the bit.

In NRZ-I, I stands for Inversion, here change (inversion) of voltage determines the value of the bit. In layman terms you can say when there is voltage fluctuation in either direction of the time axis, it is represented as “1” and if there is no fluctuation, it is represented as “0”.

Return-To-Zero (RZ): As suggested by the name itself, In this scheme signal drops to 0 between each pulse. In this scheme the “1” is represented by the pulse above the time axis and “0” is represented by the pulse below the time axis. The signal will return to zero after each pulse, if if there are consecutive 0’s and 1’s present in the signal.

c) Bipolar: In the Bipolar coding scheme, we have three voltage levels: zero, positive, and negative. Here either 0 or 1 is represented by neutral voltage on time axis and the other binary value is represented by negative and positive voltages on time axis.

• AMI (Alternate Mark Inversion) – In this type of binary scheme, the binary value 0 is represented by a neutral voltage and the binary value 1 is represented by alternating positive and negative voltages.
• Pseudoternary – This type of binary scheme is the exactly opposite of the AMI scheme, Here the binary value 1 is represented by a neutral voltage and the binary value 0 is represented by alternating positive and negative voltages.

Analog To Digital Conversion:

Here we will discuss the schemes to convert analog signals to digital signals. The analog signals are in wave forms, here we will discuss the techniques to convert these wave signals to digital signals (pulses).

There are two popular techniques to convert Analog signals to digital signals:

1. PAM (Pulse Amplitude Modulation)
2. PCM (Pulse Code Modulation)

PAM (Pulse Amplitude Modulation)

As the name suggests, it’s a amplitude modulation scheme. In this scheme, the amplitude of input analog signal is measured at equal intervals and converted into the pulses. However this technique cannot be used as a data communication because the converted form is in pulses and not in digital format. To convert these pulses to digital format, PCM (Pulse Code Modulation) technique is used.

PCM (Pulse Code Modulation)

PCM technique is used to convert the pulses generated by PAM to digital signal. To do this conversion, PAM uses process called quantization, in which it assigns the integer values to the PAM’s pulses based on the range and then the integer values are converted to the binary numbers.

The whole process involves four steps.

1. PAM converts analog signal to digital pulses.
2. PCM uses quantization process to convert these pulses into integer values.
3. Integer values are converted into binary numbers using binary encoding.
4. Digital to Digital encoding is used to convert the binary numbers to digital signal.