physical layer

Definition:

 

The physical layer, the lowest layer of the OSI model, is concerned with the transmission and reception of the unstructured raw bit stream over a physical medium. It describes the electrical/optical, mechanical, and functional interfaces to the physical medium, and carries the signals for all of the higher layers. It provides:

 

Data Encoding:

 

It modifies the simple digital signal pattern (1s and 0s) used by the PC to better accommodate the characteristics of the physical medium, and to aid in bit and frame synchronization. It determines:
    • What signal state represents a binary 1?
    • How the receiving station knows when a "bit-time" starts?
How the receiving station delimits a frame?

 

Physical medium attachment, accommodating various possibilities in the medium:

 

    • Will an external transceiver (MAU) be used to connect to the medium?
    • How many pins do the connectors have and what is each pin used for?

 

Transmission technique:

 

  • It determines whether the encoded bits will be transmitted by baseband (digital) or broadband (analog) signaling.

 

Physical medium transmission:

 

  • It transmits bits as electrical or optical signals appropriate for the physical medium, and determines:

    • What physical medium options can be used?
    • How many volts/db should be used to represent a given signal state, using a given physical medium?

 

 

 

 

osi layers