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Physical Layer :

It defines the electric interfaces and mechanics between the units. It indicates the following elements:

• The speed of transfer of the DATA (or speed of clock).
• The type of cable used (coaxial, twisted pair or optical fibre).
• The type of connectors (BNC, RJ45) and numbers it pins used.
• Physical topology.
• The level of the electronic or luminous signal represented by a bit (1 or 0).
• RS232/V24 and X21/V11 are of the type examples of interfaces of standard of interface. The specification has this level for environment LAN is encapsulated by the protocols IEEE 802.3, 802.4 and 802.5.

At the time of a connection of networks this level has, of the repeaters reproduces accurately the signals such as it were received. The more recent repeaters accept different type of cables on each side and are useful has to convert an electric type of signal INTO another. However, it is necessary that the two sides of the repeater use the same protocol of connection. The repeaters do not filter the data.

The types of peripherals generally met are :

• Hub
• Repeater
• Converter of media