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:

• Open wire cicuits
• Twisted pair cables
• Coaxial cables
• Fibre optic cables

Signalling of Bits

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

• RZ, Return to Zero (Pulse signalling)
• NRZ, Non Return to Zero transmission (Level signalling)
• Manchester encoding (Edge/phase signalling)

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)

• Simple interface (limited data rate < 19.2 kbps)
• Used for connecting: Printer, Terminal, Modem
• No clock sent (Tx & Rx have own clocks)
• Requires start/stop which provides byte timing and increases overhead
• Parity often used to validate correct reception

1. Synchronous Communication (synchronised transmit & receive clocks)

• More complex interface (high data rates supported upto ~ 1 Gbps)
• Used for: LANs, MANs, WANs, Telephony
• Clock sent with data (more configuration options)
• Bit timing from transmission or by link protocol
• Cyclic Redundancy Check (CRC) used to validate correct reception