PHYSICAL LAYER-WELCOMEITBOOK

PHYSICAL LAYER
It is a very Important layer in OSI Model.

The physical layer defines electrical and physical specifications for devices. In particular, it defines the relationship between a device and a transmission medium, such as a copper or fiber optical cable. This includes the layout of pins, voltages, line impedance, cablespecifications, signal timing, hubs, repeaters, network adapters, host bus adapters (HBA used in storage area networks) and more.

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. For example,contention resolution and flow control.
• 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 (such as copper and optical fiber) or over aradio link.

Parallel SCSI buses operate in this layer, although it must be remembered that the logical SCSI protocol is a transport layer protocol that runs over this bus. Various physical-layer Ethernet standards are also in this layer; Ethernet incorporates both this layer and the data link layer. The same applies to other local-area networks, such as token ring, FDDI, ITU-T G.hn and IEEE 802.11, as well as personal area networks such as Bluetooth and IEEE 802.15.4.

The physical layer consists of the basic networking hardware transmission technologies of a network. It is a fundamental layer underlying the logical data structures of the higher level functions in a network. Due to the plethora of available hardware technologies with widely varying characteristics, this is perhaps the most complex layer in the OSI architecture.

The physical layer defines the means of transmitting raw bits rather than logical data packets over a physicallink connecting network nodes. The bit stream may be grouped into code words or symbols and converted to a physical signal that is transmitted over a hardware transmission medium. The physical layer provides an electrical, mechanical, and procedural interface to the transmission medium. The shapes and properties of the electrical connectors, the frequencies to broadcast on, the modulation scheme to use and similar low-level parameters, are specified here.

Within the semantics of the OSI network architecture, the physical layer translates logical communications requests from the data link layer into hardware-specific operations to affect transmission or reception of electronic signals.