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Physical Layer of Osi Model

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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. A topology is a description of any kind of locality in terms of its layout. In communication networks, a topology is a usually schematic description of the arrangement of a network, including its nodes and connecting lines. There are two ways of defining network geometry: the physical topology and the logical (or signal) topology. The physical topology of a network is the actual geometric layout of workstations and peripherals. There are several common physical topologies: Bus, Star, Ring, and Mesh. ("Topology", n.d.)

The bus topology is where every computer is connected to a common medium such as the backbone. The advantages to this is that if you have a small office and only a few computers it is cost effective because it requires less cabling to connect the machines to the network and it is easy to connect a computer or peripheral. But the disadvantages are the entire network shuts down if there is a break in the main cable, terminators are required at both ends of the backbone cable, it can be difficult to identify the problem if the entire network shuts down and it is not meant to be used as a stand-alone solution in a large building. ("Topology", n.d.)

The Star topology is designed with each client attached to a central hub. The advantages to a star topology is that it is easy to wire and setup, there are no disruptions to network during installation or removal of devices and it is easy to troubleshoot. The disadvantages of the star topology is that it is more expensive to install due to the increase in the amount of cabling required and the cost of hubs, and if a hub fails everything connected to it is disabled. (Groth and Skandier, 2005, p. 53)

The Ring topology connects a network entity directly to only two other network entities (the one preceding it and the one following it). "The vulnerability of the physical ring topology to disruption of service due to the failure of a single node makes it a poor choice in most network environments."(Groth and Skandier, 2005, p. 53)

The Mesh topology consists of every computer being directly connected to every other computer on the network. The advantage to this type of topology is that if a node fails the network is not completely disabled. The disadvantage of a Mesh topology is that it can quickly become unmanageable with just a few computers connected, add in the increase in cabling required in connecting all of the computers together.

There are several devices that operate at the Physical Layer of the OSI model they are: Network Interface Card (NIC), Transceiver, Repeater, Hub, and Multistation



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