In this article we will be discussing about digital-to-digital transmission in computer network i.e., how a digital data or information is converted into digital signal. The digital-to-digital encoding can be done by a technique called line coding.
Line Coding
The process of converting the digital data into digital signal is called as line coding which helps the receiver to get the original bits. The data that is in the form of text, numbers, audio, or video is represented internally as series of 1s and 0s. Line coding therefore transforms a set of bits into a digital signal. The sender side encrypts digital data into digital signals, while the receiving side decodes the digital signal to regenerate the digital data. The primary goal of utilizing line coding is to prevent the overlapping of pulses and distortions. The digital signal is discreet in nature. Example is sending data from computer to printer.
There are basically three different types of line coding technique:
- Unipolar
- Polar
- Bipolar
Unipolar
In Unipolar encoding, only one polarity is used i.e., bit 1 is used to represent positive voltage and bit 0 is used to represent zero voltage or idle line. It is also known as Unipolar-Non-return-to-zero. Signaling of this kind is also referred to as on-off signaling.
Problem in Unipolar
- DC Component: When we find out the average amplitude of a unipolar signal, then the value would always be non-zero because of which it creates DC component. And the signal having DC component is not able to travel through the medium which cannot handle DC component.
- Synchronization: When a signal is constant, the receiver cannot distinguish between the start and finish of each bit. For that the receiver has to rely on a timer to track the starting of each bit.
Unipolar encoding
Polar
The polar encoding is of four types. In polar encoding two levels of voltage amplitude is used. The DC component issue of unipolar encoding is minimized, and the average voltage level on the line is decreased.
- Polar Non-Return to Zero (Polar NRZ): In polar encoding, positive voltage is represented by bit 1 and negative voltage is represented by bit 0. Here two levels of voltage are used to represent binary values. If the line is idle, then there is no transition. With each inversion, the receiver is able to synchronize the timer’s start to the transmission’s real arrival. Again, Polar Non-Return to Zero (Polar NRZ) has two types: NRZ-L and NRZ-I.
NRZ-L: Changes its voltage level when different bit is encountered.
NRZ-I: Changes its voltage level when bit 1 is encountered.
Advantages of Polar NRZ
This provides synchronization as whenever a 1 bit is encountered, the signal changes.
- Return to Zero (RZ): This encoding technique uses three different voltage level to represent binary values. Bit 1 is used to represent positive voltage, bit 0 is used to represent negative voltage and zero voltage for none. During the second half of each bit, this signal enters a resting state(zero).
Problems in Return to Zero
This occupies more bandwidth as it requires two signal changes to encode one bit.
Return to Zero (RZ)
- Manchester Encoding: In Manchester encoding negative to positive transition represents binary 1 and positive to negative represents binary 0. Use the inversion at the middle of each bit interval. That means bit period is represented by two equal size intervals. Here the logic level of bit is represented by the first interval and the inverse logic level is represented by the second interval.
Manchester encoding
- Differential Manchester: In Differential Manchester, the inversion at the middle of the bit is used. Transition is represented by binary 0 and no transition is represented by binary 1.
Different Manchester
Bipolar Encoding
In Bipolar encoding, three types of different voltage level is used that is positive, negative and zero. The zero level is used to represent binary 0, positive and negative voltage represents alternatives 1’s to prevent DC component. Alternate Mark Inversion (AMI) and Pseudoternary are the types of bipolar encoding.
Bipolar Encoding
Table: Comparison of Different Digital-to-Digital Line Encoding Techniques
| Line Encoding Technique | Description | Advantages | Disadvantages |
| Unipolar | Uses only one level of voltage for bit 0 and bit 1 | Easy to implement | DC component problem, synchronization issue |
| Polar NRZ | Uses two levels of voltage for bit 0 and bit 1, no transition for idle line | Provides synchronization, DC component issue minimized | May encounter problem with long sequences of 0’s or 1’s |
| RZ | Uses three levels of voltage for bit 0, bit 1 and idle line, signal enters resting state during second half of bit | Provides synchronization | Occupies more bandwidth |
| Manchester | Uses transition from positive to negative and negative to positive to represent bit 0 and bit 1 respectively, bit period represented by two equal intervals | Provides synchronization, self-clocking | Occupies more bandwidth |
| Differential Manchester | Uses transition at the middle of the bit interval to represent bit 0, no transition for bit 1, bit period represented by two equal intervals | Provides synchronization, self-clocking | More complex encoding and decoding process |
| Bipolar | Uses three levels of voltage for bit 0, bit 1 and idle line, alternative 1’s to prevent DC component | Provides synchronization, no DC component | More complex encoding and decoding process |