What is Line Encoding?

Line Encoding is the method by which analog/digital data is converted into analog/digital electromagnetic signals for transmission via. wired/wireless transmission links. Computer Communication uses both analog and digital signal transmissions, depending on the underlying media.

All four combinations of data/signal line coding techniques are possible and are used in different forms of communication. They are described as points a) to d) in the description below.

Analog Signal Transmission

Analog signals are usually continuously varying electromagnetic signals that may be propagated over a variety of media,  depending on the frequency components of the signal. Analog signals can propagate both analog and digital data.

DIfferent types of modulation techniques are used for converting data into electromagnetic signals. In the modulation techniques, a high frequency carrier wave is modulated by data. If modulation is not used, then it is called as base-band transmission (e.g. POTS)

 a) Analog Data transmitted as Analog Signals

Here analog data is transmitted as an analog signal by altering the properties of a high frequency carrier wave. The high frequency carrier wave’s amplitude or frequency or phase is altered in accordance with the analog data. This process is known as analog modulation. Examples are  human voice transmitted over Plain Old Telephone System (POTS), AM/FM radio transmissions etc. Typically the combination of analog data getting transmitted directly as analog signals is not normally used in computer communications. Instead analog data is first digitized and converted into a binary stream and then transmitted using analog or digital transmission signals.

The figure below illustrates Amplitude modulation, where the shape of a high frequency carrier wave is altered in amplitude in accordance with the amplitude of the analog data.

Analog data modulated over a carrier signal using Amplitude Modulation (AM)

b) Digital Data transmitted as Analog Signals 

Here digital data is transmitted as analog electromagnetic signals, by digital modulation techniques like ASK, FSK, PSK etc. E.g. Email, text, digitized voice etc. carried over digital links. This combination of digital data getting transmitted as analog signals is widely used in computer communication. Considering the importance of this in computer communication, more details about the different digital modulation techniques would be given in a separate post.

Digital Signal Transmission

Digital signals are a sequence of voltage pulses that can be transmitted over a medium. They can propagate analog and digital data. They offer better noise immunity, are cheaper to implement in hardware, more secure and also allow data compression, thereby optimally utilizing the transmission link.

c) Analog Data transmitted as Digital Signals

Here analog data is first encoded into binary digital data by devices known as codecs using processes like quantization and companding (e.g. Voice over IP, Skype calls etc.). The digital data is then sent as digital signals by digital line coding techniques like Manchester encoding, AMI etc.

d) Digital Data transmitted as Digital Signals

Here digital data is first encoded into a binary stream. These binary streams are then converted into digital signals by line coding techniques like Manchester encoding, B8ZS etc.

Both types of digital encoding would be covered in detail in a subsequent post.

Electromagnetic Signals and Data Transmission

Data, Signals, Transmission and Channel

Data is the entity that is to be exchanged between different computers. Signals are nothing but electrical or electromagnetic representation of data. Data transmission is nothing but communication of data by the transmission or propagation of electromagnetic signals through either wired/wireless media (channel). Thus the basic building blocks of data communication are data, signals, data transmission and communication channels.

Electromagnetic Signals

• Electrons when propagated either in a guided media or free space creates electromagnetic waves. They are so called because they have both an electical as well as a magnetic field associated with them. 
• Signals are the actual electromagnetic voltages/waves transmitted on a physical wired/wireless medium.
• Just like data, electromagnetic signals can be either analog (continuous & can take infinite values) or digital (discrete and take only a finite set of values)
•  Also, electromagnetic signals can be expressed either as a function of time (time domain) or frequency (frequency/spectral domain), as shown in the diagrams given below

Analog Electromagnetic Signals

Time and Frequency domain representations for an analog electromagnetic signal

The above diagram shows the time and frequency domain representation of an analog electromagnetic signal (that of a typical human voice). As seen in the diagram, an analog signal typically consists of a range of continuous voltage values in both the time and frequency domains. In this example, in the frequency domain, the analog signal is made up of a weighted combination of frequency components with frequency values ranging from 300Hz to 3200Hz. Thus the signal bandwidth or the range of frequencies contained in  this signal is equal to 2900 Hz (3200 – 300). Also, this particular signal, does not have frequency components between 0 to 300 Hz and also does not have any frequency component above 3200 Hz. 

Digital Electromagnetic Signals

Time and Frequency domain representations for a Digital electromagnetic signal

The above diagram shows the time and frequency domain representation of a digital electromagnetic signal. As seen in the diagram, in the time domain, the digital signal takes only two voltage values and it alternates between them repeatedly. In the frequency domain too, the digital signal is made up of a discrete set of frequency components, with different proportions. Typically, if it is a periodic digital signal, then the frequency components would only be made up of a fundamental frequency and its harmonics (integer multiples). 

Importance of frequency perspective of an electomagnetic signal in data communication
The frequency perspective of an electromagnetic signal plays a crucial role in the communication world. Wonder how different wireless transmissions like radio channels, satellite TV channels, mobile communication signals, Wireless LAN signals all coexist together in the free space medium without interference!!. The answer lies in the different bandwidth (or range of frequencies)  of each of these signals . In order to receive a particular radio/TV channel, the receiver just extracts the signal corresponding to a particular frequency (or range). Thus it is extremelly important to understand the frequency/spectrum component  of an electromagnetic signal.

The Electromagnetic Spectrum and Data Transmission

The diagram below gives the whole range of the electromagnetic spectrum.

The Electromagnetic spectrum ( Courtesy Wikepedia)

• Though the electromagnetic signals span a wide range of frequencies, only the signals in the  radio, microwave and optical region are used for data communication purposes. Radio and micro waves are used in wireless LAN, satellite, twisted pair copper and coaxial cable medias. Optical waves are used in high speed broadband  fibre optical communication.
• Since  X-rays, UV and gamma rays are dangerous to human beings, they are not used for communication purposes, even though they are high frequency signals capable of carrying huge amounts of information.
• Even optical signals are a type of electromagnetic signals. The only difference is that the optical signals fall in the range of visible light (that can be seen by a human eye).
• In general, higher the frequency component of a signal, more the information carrying capacity of that signal. This is because frequency of a signal indicates the rate at which the signal changes its values (or number of cycles per second for a sinusoidal signal) and so more the rate of changes of signal values per second, more the number of bits that can be transmitted, assuming that each instance of signal change represents a bit of information.

Types of  Signal Transmissions used in Data Communication

• In Computer communication, though data can either be analog or digital, all data are first converted into digital form before transmission.
• Also, computer communication can use either analog signal transmission or digital signal transmission or both. It depends on the capability of the underlying media. For example, all wireless media typically support only analog signal transmission. Some wired media like Ethernet and Fibre support digital signal transmission.
• For transmission purposes, digital data are converted into analog or digital signals using different line encoding/modulation techniques and then transmitted through wired/wireless media.