Physical Layer
 

The physical layer is level one in the seven level OSI model of computer networking as well as in the five layer TCP/IP reference model. It performs services requested by the data link layer.

The physical layer is the most basic network layer, providing only the means of transmitting raw bits rather than packets over a physical data link connecting network nodes. No packet headers nor trailers are consequently added to the data by the physical layer. The bit stream may be grouped into code words or symbols, and converted to a physical signal, which is transmitted over a physical transmission medium. The physical layer provides an electrical, mechanical, and procedural interface to the transmission medium. The shapes of the electrical connectors, which frequencies to broadcast on, what modulation scheme to use and similar low-level parameters are specified here. An analogy of this layer in a physical mail network would be the roads along which the vans carrying the mail drive.

The physical layer determines the bit rate in bit/s, also known as channel capacity, digital bandwidth, maximum throughput or connection speed.

The major functions and services performed by the physical layer are:

• Bit-by-bit node-to-node delivery
• Providing a standardized interface to physical transmission media, including
  • Mechanical specification of electrical connectors and cables, for example maximum cable length
  • Electrical specification of transmission line signal level and impedance
  • Radio interface, including electromagnetic spectrum frequency allocation and specification of signal strength, analog bandwidth, etc.
  • Specifications for IR over optical fiber or a wireless IR communication link

• Modulation
• Line coding
• Bit synchronization in synchronous serial communication
• Start-stop signalling and flow control in asynchronous serial communication
• Circuit mode multiplexing
• Carrier sense and collision detection utilized by some level 2 multiple access protocols
• Equalization filtering, training sequences, pulse shaping and other signal processing of physical signals

The physical layer is also concerned with

• Point-to-point, multipoint or point-to-multipoint line configuration
• Physical network topology, for example bus, ring, mesh or star network
• Serial or parallel communication
• Simplex, half duplex or full duplex transmission mode

Physical signaling sublayer

In a local area network (LAN) or a metropolitan area network (MAN) using open systems interconnection (OSI) architecture, the physical signaling sublayer is the portion of the physical layer that:

• interfaces with the medium access control sublayer (MAC)which is a part of Data Link Layer
• performs character encoding, transmission, reception and decoding
• performs mandatory isolation functions.