OSI MODEL-WELCOMEITBOOK

OSI MODEL
OPEN SYSTEM INTERCONNECTION MODEL
It Stands for Open System Interconnection.It has very importatn role in networking of computers.
It consists of layers.Introduction of layers is as follows

• LAYER 1:PHYSICAL LAYER
• LAYER 2:DATA LINK LAYER
• LAYER 3:NETWORK LAYER
• LAYER 4:TRANSPORT LAYER
• LAYER 5:SESSION LAYER
• LAYER 6:PRESENTATION LAYER
• LAYER 7:APPLICATION LAYER

LAYER 1:PHYSICAL LAYER

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