Chapter 30. Physical Layer

The OSI Model Physical Layer concerns itself with the transmission of bits through the communication medium. The order of the bits--and importance--is determined by the Protocol's packet.

Asynchronous & Synchronous Communication

In Asynchronous Communications, the OSI Physical layer concerned itself with the RS-232D standard (and the Voice Channel). The RS-232D standard stated the electrical and mechanical characteristics of the cable: these characteristics were for the transmission of the digital signal between the DTE (PC) and DCE (modem). The Voice Channel stated the electrical and mechanical characteristics of the connection between DCE to DCE (modem to modem) through the phone lines.

The order of the bits was determined by the following: ASCII characters, the parity (Odd/Even/None), the number of Stop Bits, and the Transfer Protocol. Examples of Transfer Protocols are shown below:

• Kermit

• Xmodem

• Ymodem

• Zmodem

Similarly, in Synchronous Communications, the electrical and mechanical characteristics of the cable (for the transmission of the signal) are defined by the protocol that's used between Network Interface Cards.

The electrical characteristics associated with the OSI Model's Physical layer are as follows:

• Transmission rate (bits/sec)

• Voltage levels

• Line Encoding

• Propagation delay

• Termination impedance

The mechanical characteristics associated with the OSI Model's Physical layer are shown below:

• Connector type

• Cable type & size

• Cable Length

• Topology

• Shielding

In summary, the OSI Physical Layer is concerned with the transmission of bits on the network: the order of bits, bit level error-checking, and the electrical / mechanical characteristics.

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