Journey Of Line Encoding Methods

                 

 

                     

There is data everywhere. We live in a period where data is transferred in milliseconds, and the world is becoming more and more computerized. Data is kept and transported in digital form, where every institution and organization has invested, and even a tiny transmission fault could result in a huge loss.

Line encoding techniques can be used in this situation, though. Prepare to buckle up as this blog will take you on a tour through the history of line encoding methods and how they have been used to the telecommunications industry.

What is Line Encoding?

A line code is a configuration of voltage, current, or photons used in communications to represent digital data conveyed down a transmission line.

A transmitter will utilise line coding to convert binary digits of data into a baseband digital signal that will serve as the data's representation on a transmission line. It is also known as digital baseband modulation or digital baseband transmission. The transmission line in question might connect two computer network nodes or it might be a component of a much broader communications network. The line-coded signal must be translated back into binary data by the receiver.

Why Line Encoding?

Utilizing line coding has several benefits. Each of the line codes you'll be looking at has one or more of the benefits listed below:

• Relocation and Spectrum Shaping without Modulation or Filtering. This is essential for telephone line applications, for example, where the transfer characteristic is greatly reduced below 300 Hz.

• Recovery of the bit clock can be streamlined.

• By removing the DC component, it is possible to couple AC between stages (using a capacitor or transformer, like in telephone lines). can prevent baseline wander, which causes a significant degradation of the noise margin by shifting the location of the signal waveform in relation to the detector threshold.

• Capabilities for error detection.

• Bandwidth utilization; the potential to transport data more quickly than with other methods using the same bandwidth.

Line Encoding techniques :

Line coding techniques come in a wide variety, with levels of complexity ranging from the most simple unipolar schemes, in which the presence or absence of a voltage is used to represent a binary one or a binary zero, to the most complex multilevel schemes, in which various signal amplitudes are used to represent various groups of binary digits.

Only very slow-moving asynchronous transmission, in which data is sent in discrete, independent blocks, makes use of the most basic line coding techniques, allowing the receiver to resynchronize with the transmitter at the beginning of each incoming block. Timing becomes much more important for high-speed synchronous applications, as substantially larger blocks of information are transferred, together with other considerations like noise and the possibility of a signal.

Now let's Start with the First Encoding Technique : 

Unipolar NRZ :

The First Encoding Technique was developed to produce unipolar signalling, in which all non-zero signalling elements have the same polarity—either they are all positive or they are all negative. It was an attempt to emulate a basic on-off keying method where a voltage pulse denotes a binary one and a pulse denotes a binary zero. The phrase "Non-return-to-zero" denotes the signal's absence from zero in the midst of the bit time. Binary one is represented by a positive voltage, and binary zero by a zero voltage.

Example of Unipolar NRZ Encoding

This encoding algorithm was useful since it was simple to use and used little bandwidth, but it also had some problems because it didn't fulfil all the requirements.

Polar Encoding (NRZ) :

A method for Polar Encoding, which use both poles to represent a given data, was developed with the advent of Unipolar Encoding.

NRZ Encoding Techniques come in two different varieties.

• NRZ-I(Inversion)
• NRZ-L(Level)

Example of NRZ-Level Encoding

Example of NRZ-Inversion Encoding

The first one is NRZ-Level (NRZ-L), and it will be examined first. The value of a bit in this case is determined by the voltage level. Typically, a positive voltage is used to represent a logic low (binary zero), while a negative voltage is used to represent a logic high (binary 1).

NRZ-Invert (NRZ-I) is the name of the second polar NRZ line coding scheme that we will examine. The value of a bit is here determined by the presence or absence of a transition from a positive voltage to a negative voltage, or vice versa. When there is a transition, the logic level is high (binary 1), but when there isn't one, the logic level is low (binary 0).

Despite the fact that the method has some success, neither polar-NRZ-L nor polar NRZ-I are DC balanced. Long zero sequences still leave the Baseline Wandering problem unresolved.

Polar RZ ( Return-to-Zero) :

Here, the employment of three signaling levels helps to reduce some of the issues with polar NRZ line coding systems. Although the signal level in both cases returns to zero halfway through the bit time and remains there until the next bit is transmitted, logic low and logic high are still (typically) represented by positive voltages. It should be noted that, somewhat confusingly, some sources refer to this line coding scheme as bipolar RZ or BPRZ.

Example of Polar RZ Encoding

Polar RZ turned out to be advantageous since it consumes less power than polar NRZ-L or polar NRZ-I and because the receiver could resynchronize itself during the transitions in the midst of each bit time. However, even though the DC component will be insignificant if the ratio of ones to zeros is roughly equal, the technique is not DC balanced. Compared to either polar NRZ-L or polar NRZ-I, it needs twice as much bandwidth.

Manchester Encoding(Bi-phase and Differential) :

A popular line coding technique called Manchester encoding incorporates temporal information into the transmitted signal. This is accomplished by making sure that each bit time has a transition (high-to-low or low-to-high), which makes it simple for the receiver to pull a clock signal from the incoming bitstream and keep synchronisation with the outgoing signal.

Example of Bi-Phase Manchester Encoding

In bi-phase encoding, a positive pulse with a period of half a bit time, followed by a negative pulse with the same duration, represents logic high (binary one).  Similar to a logic high, a logic low (binary zero) is composed of two pulses, each lasting half a second: a negative pulse and a positive pulse.

Manchester encoding improves upon polar NRZ-L and polar RZ schemes' drawbacks by ensuring that the transmitted signal contains enough embedded timing information to make it simple for the receiver to maintain synchronization (there is at least one transition per bit time), as well as by preventing Baseline wandering and the development of a DC component. Manchester encoding's ineffective utilization of bandwidth is its key drawback and the main reason it is not used for Fast Ethernet (100 Mbps) and higher speeds.

Manchester encoding has the drawback of being changed from one convention to the other if the signal is inverted (i.e., changes polarity) while in transit, resulting in polar ambiguity (the receiver will perceive ones as zeros, and vice versa). Utilising differential Manchester encoding, a type of Manchester encoding, will solve the issue. Differential Manchester is more like a combination of RZ and NRZ-I than Manchester encoding, which may be defined as combining elements of polar RZ and NRZ-L.

Example of Differential Manchester Encoding

There is still a transition in the middle of each bit time in differential encoding, but there is only a transition at the beginning of a bit time if the bit is going to be a logic high (binary one). If the bit is going to be a logic low (binary zero), there is no transition at the beginning of the bit time. What logic state a bit represents is determined by whether a transition is present or absent at the start of the bit time. In this context, it makes no difference which way the transition occurs or what voltage level is actually present on the line during the bit time.

However, differential Manchester has been utilised commercially (for instance, in IEEE 802.5 token ring local area networks), despite having the same bandwidth utilisation inefficiency as Manchester. Similar to polar NRZ-I, differential Manchester is a differential line coding scheme that uses only the presence or absence of transitions at the beginning of each bit time to convey the logic state of each bit. This is a key characteristic of differential Manchester.

Bipolar Encoding -AMI (Alternate Mark Inversion) :

Similar to polar RZ, bipolar line coding methods (also known as multi-level binary or duo-binary) use positive, negative, and zero voltage levels. The similarities end there, though, pretty much. Alternating positive and negative voltages are used in bipolar alternate mark inversion (AMI) to indicate logic high (binary one), and a zero voltage is used to represent logic low (binary 0). Although AMI is technically an NRZ line coding scheme in and of itself, it was created as an alternative to previous NRZ methods that introduced a DC component into the signal with lengthy runs of ones or zeros.

Example of AMI Encoding

Given that it consumes less power than the polar NRZ line coding system, the technology has been effective in resolving a number of challenges. There is no DC element in the signal. Additionally, it prevented problems like polar ambiguity and baseline wandering. Long zero sequences, on the other hand, can result in the receiver losing synchronisation because there are no voltage changes.

Multiline Encoding- Multi-level-Transmit(MLT-3) :

MLT-3 is a differential line coding method, much like polar NRZ-I and differential Manchester. The "3" in the name refers to the fact that MLT-3 uses three levels (positive, negative, and zero) as opposed to the two signal levels (positive and negative) that each of the aforementioned schemes use to represent binary values.

Rule for MLT-3 Encoding

MLT-3 is appropriate for transmission over twisted-pair copper wire cables because each cycle in MLT-3 requires four transitions, which lowers the maximum fundamental frequency that must be supplied by the transmission medium to only one-quarter of the baud rate. Since 100BASE-TX Ethernet operates across two wire pairs in category 5 (or above) twisted pair cables, it has been effectively used for 100BASE-TX Ethernet, the most common type of Fast Ethernet (100 Mbps).

The MLT-3 signaling diagrams above may have led us to believe that MLT-3 is not a DC-balanced line coding system. If there are lengthy runs of zeros, the receiver may experience synchronization loss. Use of block coding(multi-level) is one approach to solving this issue.

Multilevel Encoding — 8B/6T

The next multi-level line coding method we'd like to examine is called 8B6T (eight binary, six ternary), even though it only employs the three signalling levels of positive, negative, and zero. The 8B6T coding system applies three-level pulse amplitude modulation (PAM-3) to a series of six signalling elements to encode a block of eight binary digits. There are three possible values for each signal element: positive, negative, or zero.

Example of 8B/6T Encoding

Eight binary digits can be joined in just 256 different ways (2 8 = 256), whereas six ternary signal levels can be combined in 729 different ways (3 6 = 729). The remaining 473 redundant signaling choices can be employed to provide synchronization, error detection, and signal output balancing with respect to DC capability.

Encoding Table for 8B/6T

Summary :

Now that we've covered both basic and more complicated encoding methods, let's look at the summary:

• Baseband digital data transmission encounters several difficulties, such as transmission mistakes, and it is at this point that Line Encoding and Decoding enter the picture.

 

• We have discussed a variety of encoding techniques, including unipolar, polar, bi-polar, manchester, multilevel, and multiline encoding techniques, where we can see the evolution from a simple idea to something that is now so useful that we can send signals at over 1000 Mbps and even faster speeds with more sophisticated techniques.

 

• Technology is constantly changing, but the process of change and the journey are what teach us to maintain a certain perspective.

In conclusion, line encoding methods have come a long way in computer networks, from simple voltage level changes to complex symbol encoding techniques. Today, a wide range of line encoding methods is used in various digital communication systems, ranging from low-speed digital telephone systems to high-speed internet connections. As technology continues to advance, it is likely that new line encoding methods will be developed to meet the ever-increasing demand for faster and more reliable digital communication. 

References:

https://www.technologyuk.net/telecommunications/telecom-principles/line-coding-techniques.shtml

https://www.section.io/engineering-education/different-techniques-of-encoding-data-for-transmission/

https://www.codingninjas.com/codestudio/library/line-coding

Blog written by:

Srushti Hamand