Physical Layer

The physical layer (layer 1) serialises the bytes (i.e. converts it to a series of bits/bauds) and sends it across a communications circuit (i.e. cable) on a link towards the destination (or an intermediate) system. There are a number of types of cable (sometimes called "media") that may be used.

These include:

• Open wire circuits
• Unshielded Twisted Pair (UTP) cables
• Shielded Twisted Pair (STP) 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. Various encoding schemes are used:

• NRZ, Non Return to Zero transmission (Level signalling - as in fibre communications)
• Manchester encoding (Edge/phase signalling)
• 4b5b encoding with MLT-3 (as in Fast Ethernet)
• 8b10b encoding with PAM-5 (as in GBE)

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 often < 19.2 kbps, but can be higher)
• Used for connecting: printer, terminal, modem
• No clock sent (Tx & Rx have own independent clocks at same nominal frequency)
• Requires start/stop baud which provides byte timing and increases overhead
• Parity often adds another baud and is 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 and Internet links, telephony multiplexes, etc
• Clock is sent encoded with the data (no clock wire)
• Cyclic Redundancy Check (CRC) used to validate correct reception of a frame