Physical Layer in Computer Networks

The physical layer is the lowest layer of the OSI model and the TCP/IP model, responsible for the transmission of raw data bits over a physical medium. This article provides an in-depth exploration of the physical layer, covering transmission media, modulation techniques, encoding, and multiplexing.

2.1 Transmission Media: Wired and Wireless

Transmission media are the physical pathways that connect devices in a network. There are two main types of transmission media: wired and wireless.

Wired Transmission Media

Wired media use physical cables to transmit data between devices. Common types of wired transmission media include:

Twisted-pair cable: Consists of pairs of insulated copper wires twisted together to reduce electromagnetic interference. Twisted-pair cables come in two varieties: unshielded twisted-pair (UTP) and shielded twisted-pair (STP).
Coaxial cable: Features a central conductor surrounded by an insulating layer, a metallic shield, and an outer protective jacket. Coaxial cables provide higher bandwidth and better noise immunity than twisted-pair cables but are more expensive and less flexible.
Optical fiber: Transmits data as pulses of light through thin strands of glass or plastic. Optical fibers offer extremely high bandwidth, low signal attenuation, and immunity to electromagnetic interference but are more expensive to install and maintain than copper cables.

Wireless Transmission Media

Wireless media use electromagnetic waves to transmit data through the air or space. Common types of wireless transmission media include:

Radio waves: Radio frequency (RF) waves are used for short- and long-range communication in Wi-Fi networks, Bluetooth, and cellular networks.
Microwaves: Microwaves are high-frequency radio waves used for point-to-point communication in wireless networks, satellite communication, and radar systems.
Infrared: Infrared waves are used for short-range communication in devices like remote controls, wireless keyboards, and some wireless LANs.

2.2 Modulation Techniques and Encoding

Modulation is the process of converting digital data into analog signals for transmission over a physical medium. Various modulation techniques exist, including:

Amplitude modulation (AM): The amplitude of the carrier wave is varied according to the digital data being transmitted.
Frequency modulation (FM): The frequency of the carrier wave is varied according to the digital data being transmitted.
Phase modulation (PM): The phase of the carrier wave is varied according to the digital data being transmitted.

Encoding is the process of converting digital data into a format suitable for transmission over a specific medium. Common encoding schemes include:

Non-return-to-zero (NRZ): A binary encoding scheme in which a 1 is represented by a high signal and a 0 is represented by a low signal.
Manchester encoding: A balanced encoding scheme in which a 1 is represented by a high-to-low signal transition and a 0 is represented by a low-to-high signal transition.
Differential Manchester encoding: A variation of Manchester encoding in which a 1 is represented by the absence of a signal transition and a 0 is represented by a signal transition.
Quadrature amplitude modulation (QAM): A modulation scheme that combines both amplitude and phase modulation, allowing for higher data rates and increased spectral efficiency.

2.3 Multiplexing and Demultiplexing

Multiplexing is the process of combining multiple data streams into a single signal for transmission over a shared medium, while demultiplexing is the process of separating the individual data streams at the receiving end. Common multiplexing techniques include:

Frequency division multiplexing (FDM): Different data streams are transmitted simultaneously over different frequency bands, with each stream modulating a distinct carrier frequency.
Time division multiplexing (TDM): Data streams are transmitted sequentially in time slots, with each stream being allocated a fixed portion of the transmission time.
Code division multiplexing (CDM): Each data stream is assigned a unique code, and all streams are transmitted simultaneously over the same frequency band. The unique codes enable the receiver to separate the individual data streams.
Wavelength division multiplexing (WDM): Used in optical fiber communication, different data streams are transmitted simultaneously over different wavelengths of light.