Physical Layer Of Osi Model

Abstract

The Physical Layer is the lowest layer of the Open Source Interconnect Model (OSI). It is the layer that deals with all the measurable, physical entities associated with the network. At this layer it is specified how much bandwidth (Baseband or Broadband) will be used in the transmission of data on the network. This layer also includes the physical topology (physical lay out) of the network such as: Bus, Star, Ring or Mesh. The Physical Layer includes these devices: Network Interface Cards (NICs), Transceivers, Hubs, Multistation Access Units (MAUs), Repeaters and Cables. It is at this layer that frames received from the Data Link layer are converted to bits for transmission over the network media to the receiving machines Physical Layer.

The Physical Layer defines all electrical and physical specifications for devices. This includes the layout of pins, voltages, and cable specifications. The major functions and services performed by the Physical Layer are: establishment and termination of a connection to a communications medium, participation in the process whereby the communication resources are effectively shared among multiple users, modulation, or conversion between the representation of digital data in user equipment and the corresponding signals transmitted over a communications channel. These are signals operating over the physical cabling copper and fiber optic. ("OSI Model", 2005)

"The Physical Layer is special compared to the other layers of the model, because it is the only one where data is physically moved across the network interface. All of the other layers perform useful functions to create messages to be sent, but they must all be transmitted down the protocol stack to the Physical Layer, where they are actually sent out over the network."(Kozierok, 2004)

Physical Layer also specifies how much of the media will be used during the data transmission referred to as Baseband or Broadband signaling.

1. Baseband Signaling: Technology in which a network uses all available signal frequencies or the entire bandwidth i.e., Most LAN technologies like Ethernet.

2. Broadband Signaling: Technology in which a network uses only one frequency or a part of the entire bandwidth i.e., multiple signals can be transmitted over a media simultaneously like TV signals, where you have various channels like CNBC, MTV, BBC, each on a different frequency and hence each occupies a part of the bandwidth. (Chandrasekaran, 2002)

The Physical Layer also deals with the way a network is laid out which is referred to as the topology of a network.