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Physical Layer

The OSI Model's layer 1, the Physical Layer, outlines the characteristics of the physical medium between networked devices--cabling, wiring, fiber strands and the air. Transmission and receipt of data from the physical medium (copper wire, fiber, radio frequencies, barbed wire, string etc.) is managed at this layer.

The Physical Layer receives data from the data link Layer, and transmits it to the wire. The physical layer controls the electrical and mechanical functions related to the transmission and receipt of a communications signal. It also manages the encoding and decoding of data contained within the modulated signal.

Note that for two devices to communicate, they must beconnected to the same type of physical medium (wiring). 802.3 Ethernet to 802.3 Ethernet, FDDI to FDDI, serial to serial etc. Two end stations using different protocols can only communicate through a multi-protocol bridge or a router.

The physical layer is responsible for:

1. Communication with the data link layer above it.
2. Fragmentation of data into frames
3. Reassembly of frames into data link Protocol Data Units.
4. Transmission to the physical media
5. Receiving from the physical media

It should be noted that in most modern network interface adaptors, the physical and data link functions are performed by the adaptor.

Example Physical Protocols

• CSMA/CD - Carrier Sense Multiple Access / Collision Detect
• CSMA/CA - Carrier Sense Multiple Access / Collision Avoid
• FDMA - Frequency Division Multiple Access
• MSK - Minimum Shift Keying
• GFMSK - Gaussian-Fitered Minimum Shift Keying
• TDMA - Time Division Multiple Access
• CDMA - Code Division Multiple Access
• B8ZS - Binary 8 Zero Substitution
• 2B1Q - 2 Binary 1 Quaternary
• PCM - Pulse Code Modulation
• QAM - Quadrature Amplitude Modulation
• PSK - Phase Shift Keying
• SONET - Synchronous Optical NETworking

The Physical Layer | Index | The Data Link Layer >