Physical Layer (Layer-2) of OSI Model

The Physical Layer is the lowest layer of the OSI Model. It provides the resources to transport the bits; that make up a data link layer frame across the network media. This layer accepts a complete frame from the data link layer and encodes it as a series of signals that transmitted the local media as shown in the figure below. The encoded bits contain a frame received by either an end device or an intermediate device.

This layer also deals with the physical connection to the network and with transmission and reception of signals. This layer also defines electrical and physical details represented as 0 or a 1. It’s also decided when the data transmitted or not and how the data would be synchronized. The process that data travel from a source node to a destination node is following:

• The user data segmented by the transport layer placed into packets by the network layer and further encapsulated into frames by the data link layer. Data link layer sent these frames to the physical layer.
• It encodes the frames and creates the electrical, optical; or radio wave signals that represent the bits(0 and 1) in each frame.
• These signals then transmitted to the media.
• The destination node physical layer retrieves these signals from the media; restores them to their bit representations, and passes the bits up to the data link layer as a complete frame.

Key points of the physical layer

• Line configuration: – This layer connects devices with the medium, Point to Point configuration and Multipoint configuration.
• Topologies: – Devices can connect using the following topologies  Mesh, Star, Ring, and Bus
• Transmission Modes: –  It also defines the direction of transmission between devices ( Simplex, Half Duplex, Full Duplex).

Media

Many different types of media can be used for this layer. For example, telephone twisted pair cable, coax cable, shielded copper cable, unshielded copper cable and fiber optics are the main types used for LANs. Different transmission techniques generally categorized as baseband, or broadband transmission may apply to each of these media types. There are three basic types of media. The physical layer produces the representation and groupings of bits for each type of media as:

• Copper cable: The signals are patterns of electrical pulses.
• Fiber-optic cable: The signals are patterns of light signals.
• Wireless: The signals are patterns of microwave transmissions.

To enable physical layer interoperability, all aspects of these functions governed by standards organizations.

Standards

Upper layer Protocol: – Protocols and operation of the upper OSI layers totally performed in software designed by software engineers and computer scientists. IETF (Internet Engineering Task Force ) is an organization which defined the services and protocol for TCP/IP suite.

This layer consists of electronic circuitry, media, and connectors. Therefore, it is suitable that the standards governing this hardware are defined by the relevant electrical and communications engineering organizations.

There are many different international and national organizations, regulatory government organizations, and private companies involved in establishing and maintaining physical layer standards. For example, the physical layer hardware, media, encoding, and signalling standards defined and governed by the following:-

• International Organization for Standardization (ISO)
• International Telecommunication Union (ITU)
• American National Standards Institute (ANSI)
• Institute of Electrical and Electronics Engineers (IEEE)
• Telecommunications Industry Association/Electronic Industries Association (TIA/EIA)
• National telecommunications regulatory authorities including the Federal Communication Commission (FCC) in the USA and the European Telecommunications Standards Institute (ETSI)
• Canadian Standards Association (CSA)
• European Committee for Electrotechnical Standardization(CENELEC)

Component and Functions

Components of this layer are the electronic hardware devices; media, and other connectors such as NIC, cable, and connector that send and carry the signals. Hardware components such as NIC, interfaces and connectors, cable materials, and cable designs are all specified in standards associated with the physical layer. Physical layer deals with all the above physical components of the network. For example, the network cable, the female adapter of your NIC and the network interface card of a computer is a part of the physical layer. So, let’s go take a look into what all basic things physical layer does and what protocols are run at the physical layer.

Delivery of Packets

The physical layer takes frames from the data link layer then converts these frames into electrical, electromagnetic; optical signals through different line coding techniques. Transmit these signals through wired/wireless telecommunication links (cables/antennas) to next hop. The physical layer delivered those signals from a cable, a wifi router or an optical fiber.

Encoding

Encoding is a technique of converting a stream of data bits into a predefined “code”. Codes are groupings of bits used to give a predictable pattern that can be recognized by both the sender and the receiver. In the case of networking, encoding is a pattern of voltage or current used to represent bits; the 0s and 1s.

Signaling

The physical layer generates the electrical, optical, and also wireless signals that represent the “1” and “0” on the media. The method of representing the bits is signalling. The physical layer standards must define what type of signal represents a “1” and what type of signal represents a “0”. This can be as simple as a change in the level of an electrical signal or an optical pulse. For example, a long pulse might represent a 1 whereas a short pulse represents a 0. Modulation techniques are a common method to send data. Modulation is the process by which the character of one wave modifies another wave.