Physical Layer - Layer 1 of OSI model

- The physical layer is level one in the seven level OSI model. Actually, it is last layer that receives and processes the data from the sending device while it is first layer to receive the data at the destination end.
- It performs services requested by the data link layer.
- The Physical Layer defines the Mechanical, Electrical, Procedural and Functional specifications for activating, maintaining and deactivating the physical link between communication network systems.
- The basic objective of this layer is to transform the data in the form that is needed to be carried through the transmission media over the network. The transmission media either bounded or unbounded , carries the data in the form of electromagnetic waves or radio waves.
- The Physical Layer is responsible for bit-level transmission between network nodes.
The main functions of physical layer are :

- Definition of Hardware Specifications: The details of operation of cables, connectors, wireless radio transceivers, network interface cards and other hardware devices are generally a function of the physical layer. Devices used in the Physical Layer are
* Network Interface Cards (NIC)
* Transceivers
* Repeaters
* Hubs
* Multi Station Access Units (MAU’s)
- Encoding and Signaling: The physical layer is responsible for various encoding and signaling functions that transform the data from bits that reside within a computer or other device into signals that can be sent over the network.
- Data Transmission and Reception: After encoding the data appropriately, the physical layer actually transmits the data, and of course, receives it.
- Topology and Physical Network Design: The physical layer is also considered the domain of many hardware-related network design issues, such as LAN and WAN topology. There are four possible kinds of topologies:
* Bus
* Star
* Ring
* Mesh
In general, then, physical layer technologies are ones that are at the very lowest level and deal with the actual ones and zeroes that are sent over the network.

Quick Tech Lesson: Overview Of The Physical Layer

The physical layer is the basis of all the networks. Nature imposes two fundamental limits on all channels, and these determine their bandwidth. These limits are the Nyquist limit, which deals with noiseless channels, and the Shammon limit, for noisy channels.
Transmission media can be guided or unguided. The principle guided media are twisted pair, coaxial cable, and fiber optics. Unguided media include radio, microwaves, infrared, and lasers through the air.
A key element in most wide area networks is the telephone system. Its main components are the local loops, trunks and switches. Local loops are analog-twisted pair circuits, which require modems for transmitting digital data. Trunks are digital, and can be multiplexed in several ways, including FDM, TDM and WDM. The switches include crossbars, space divison switches, and time divison switches. Both circuit switching and packet switching are important.
The telephone system are now becoming digital from end to end and carrying both voice and data traffic over the same lines. Two variants of this new system, known as ISDN, are being introduced. Narrowband ISDN is a circuit switched digital system that is an incremental improvement over the current system. In contrast, broadband ISDN represents a paradigm shift, since it is based on cell switching ATM technology. Various kinds of ATM switches exist, including the knockout switch and the Batcher-banyan switch.
For mobile applications, the hard-wired telephone system is not suitable. Alternatives to the telephone system include cellular radio and communication satellites. Cellular radio is now widely used for portable telephones but will soon be common for data traffic as well. The current generation of cellular systems (e.g. AMPS) are analog, but the next generation (e.g. PCS/PCN) will be fully digital. Transmission communication satellites are geosynchronous, and there was some interest in bigger systems, but the failure of Iridium slowed down this process.