Physical Layer

The physical layer (layer 1) of the OSI reference model serialises the frame (i.e. converts it to a series of bits) and sends it across a communications circuit (i.e cable) to the destination (or an intermediate) system. There are a number of types of ciruit/cable (sometimes also called "media") that may be used. these include:

Signalling of Bits

The physical layer defines the representation of ecah bit as a voltage, current, phase, or frequency. Four basic schemes are used:

In NRZ transmission, each data bit is represented by a level. A high level may represent a logic 1, where as a low level may represent a logic 0. The term is derived from the earlier transmision technique of sending pulses to represent bits (called Return to Zero, RZ) in which a logic 1 is represented by a pulse and a logic 0 by the absence of a pulse. (AMI and HDB3 are technqiues derived from RZ). Manchester encoding uses a still different scheme where a logic 1 is represnted by a transition in a particular direction (usually a rising edge) in the centre of each bit. A transitition in the opposite direction (downward in this case) is used to represent a logic 0.

Timing of Bits

At the receiver, the remote system reassembles the series of bits to form a frame and forwards the frame for processing by the link layer. A clock (timing signal) is needed to identify the boundaries between the bits (in practice it is preferable to identify the centre of the bit - since this usually indicates the point of maximum signal power). There are two systems used to providing timing:

  1. Asynchronous Communication (independent transmit & receive clocks)
  1. Synchronous Communication (synchronised transmit & receive clocks)

Gorry Fairhurst - Date: 01/01/2001 EG3557