The physical layer is responsible for the transmission of data in the form of raw bits over a physical link between two connected nodes.

Note: The data is transmitted through a medium that can take up many different forms, as we will see later on.

Services provided by the physical layer

Now we will see some of the services that are provided by the physical layer:

Line coding

This is the process of encoding digital data in the form of bits, into a pattern of voltage, current, or photons that can be transmitted through a physical medium.

Some examples of line coding are unipolar, polar, bipolar, and Manchester code, as we can see in the following illustration.

  • In unipolar line coding, binary 1 is encoded as positive voltage and binary 0 is encoded as zero voltage.
  • In polar line coding, binary 1 is encoded as positive voltage and binary 0 is encoded as negative voltage.
  • In bipolar line coding, binary 0 is encoded as zero voltage and binary 1 is encoded alternately as a positive or negative voltage.
  • In Manchester line coding, a binary 0 is encoded as a transition from low to high voltage, while a binary 1 is encoded as a transition from high to low voltage.

Each encoding comes with its share of pros and cons. As a result, the suitability of the encoding pattern depends on other factors, such as the transmission medium.

Modulation/demodulation

This is the process of mixing a signal that corresponds to the digital data we want to transmit, known as the data signal or the message signal with a separate periodic signal, known as the carrier signal. This is done to convert the signal into a form that is more suitable for transmission.

For example, the frequency of a carrier signal depends on the data that is transmitted. Multiple data signals can be sent over a channel, if we use multiple carriers at slightly different frequencies. This also has practical benefits. Antennas need to be about one-tenth the length of the signal’s wavelength, so it’s economical to use higher frequencies.

The three basic types of modulation are:

  • Amplitude modulation
  • Frequency modulation
  • Phase modulation

Carrier sense and collision detection

This is the process that is used to determine whether or not another device is transmitting data over the medium, and if another device transmitted data concurrently in the medium, which led to a collision and potential loss/corruption of the transmitted data.

Auto-negotiation

This is the process used to make two or more sides of a network connection agree on the same set of parameters used for communication, such as speed and transmission mode for example simplex or duplex.

In practice, devices advertise what they can support, and then the highest performance transmission mode supported on both sides is selected.

Transmission medium

There are many different physical media, which can be classified into two main categories:

  • Guided media
  • Unguided media

Guided media

Guided media uses a physical path or conductor to transmit the signal in a specific direction. That’s why it is also called wired communication.

Some examples of wired media are:

Twisted pair cabling

Twisted pair cabling is a type of wiring in which two conductors of a single circuit are twisted together to improve electromagnetic compatibility through the reduction of electromagnetic radiation from the pair and external electromagnetic interference from other sources. These cables are typically made of copper wires.

Coaxial cable

Coaxial cable is a type of electrical cable, which consists of an inner conductor that is surrounded by a concentric conducting shield. The conductor and the conducting shield are separated by a dielectric (insulating material).

Fiber-optic cable

A fiber-optic cable contains one or more optical fibers, which are used to carry light. Typically, these fibers are individually coated with plastic layers and contained in a protective tube that is suitable for the environment where the cable will be deployed.

Unguided media

Unguided media broadcasts the signal through the air, and does not direct the signal. This is why it is also known as wireless communication.

Some examples of wireless media are radio waves, microwaves, and infrared light. These are all different forms of electromagnetic radiation with different frequencies.

Technologies that provide physical layer services

Many different technologies provide physical layer services, such as DSL, ISDN, Bluetooth, and USB.

Note: Ethernet and Wi-fi are the basic underpinnings of the Internet, so we will focus on them now.

Ethernet

Ethernet is part of the IEEE 802.3 standard that defines both the physical layer specifications and the associated link layer protocols, which we will examine later. It can be carried over various physical media, including coaxial cable, copper wire, and optical fiber. As a result, it comes in many different forms.

Some examples of Ethernet are 10BASE-2, 10BASE-T and 10BASE-F, where the first component indicates the speed of the standard (for example, 10Mbit/s) and the last part indicates the medium (for example, coaxial cable, twisted pair cable, or fiber-optic cable).

Wi-fi

Wi-fi is part of the IEEE 802.11 standard that defines the physical layer specifications and the associated link layer protocols used to achieve wireless communication.

Providing physical layer services to computer

Physical layer services are provided to a computer through a network interface card. This is a hardware component that connects a computer to other computers via a wired or wireless medium.

A network interface card is typically associated with an address called the MAC address, which is used to uniquely identify the device in the immediate network of devices.

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