The physical layer is the lowest layer of the OSI model. The physical layer consists of various networking devices such as connectors, receivers, plugs, modems, repeaters, cables, Hubs, etc. The rules and procedures that are defined for interaction between the physical layers are known as physical layer protocols. The physical connections between the devices can be point-to-point physical connections or can be point to the multi-point physical connection. The data frames that are received from the Data Link Layer are converted into data bits of 1’s and 0’s that are transmitted over the network. The quality of data is maintained by the physical layer by applying the protocols and maintaining the data bit rate.
Functions of Physical Layer in OSI Model
Below are the functions of the physical layer of the OSI model:
Establishment of Physical Connections
For transmitting data from one system to another the request travels to the data link layer from the application layer. This request is then passed by the data link layer to the physical layer and the physical connection is activated by the physical layer. When the user at one end initiates the transmission of data the receiver at another end is ready to receive the data message, hence it is a non-confirm service. Once the connection is established physical layer provides half-duplex or full-duplex transmission of data.
Signal encoding and Synchronization
Encoding of signals is provided by the physical if it is required. Synchronous synchronization and asynchronous synchronization of bits is provided by the physical layer. Performing the frame synchronization is not the responsibility of the physical layer as it is the responsibility of the data link layer.
Sequential Delivery of Data
The physical layer delivers the data bits in the same sequence as they are received by the data link layer. It provides the sequential delivery of data despite of any error control mechanism or flow control mechanism used by it. As no error control mechanism or flow control mechanism are used by the physical layer sometimes the database that are transmitted may be duplicated, lost or can be altered.
The non-confirmed services can be provided to the data link layer by physical layer. The physical layer makes the attachment of two service primitives they are request primitive and indication primitive. These primitives consists of three phases namely the activation phase, the deactivation phase and the transfer phase. The service names associated with each service primitives are given below:
1. Activation phase:
- Ph. ACTIVATE. request
- Ph. ACTIVATE. indication
2. Deactivation Phase:
- Ph. DEACTIVATE. request
- Ph. DEACTIVATE. indication
3. Data Transfer Phase:
- Ph. DATA. request
- Ph. DATA. indication
Use of Intermediate Relay
To overcome the problems such as attenuation, degradation of signal by noise the physical layer makes use of intermediate relay. In order to avoid such problems signal converting units are being used in the intermediate relay. This signal converting units are used for regeneration, amplification, modulation or equalized the media characteristics according to the needs. The activation and the deactivation of relay is being carried out by the physical layer.
Physical Service Data Unit and Data Transparency
The data link layer provides with a physical service data unit(PSDU) with one bit in a serial transmission and “n” bits in a parallel transmission to the physical layer. The physical layer can provide the transparency of data which means that the bit sequences can be transmitted over the transmission channel without any restriction.
Terminating the Physical Connection
When the user has finished the transmission of data, the connection can be disconnected. For this the physical layer disconnects the link by deactivating the physical connection once the request is received from the user. the process of deactivation of service is also non confirmed.
Frequently Asked Questions
Q.1: What is the difference between half-duplex and full-duplex communication that takes place at the physical layer?
Half duplex communication provides with the communication in both the directions but not simultaneously, where as full duplex communication mode provides with the communication facility that enables communication simultaneously in both the directions.
Q.2: Which are the protocols used at physical layer of OSI model?
The protocols used at physical layer of OSI model are Ethernet, Bluetooth, PON, OTN, DSL, IEEE.802.11, IEEE.802.3, L431 and TIA 449.
Q.3: Which devices are used at the physical layer of OSI model?
Physical layer consists of various networking devices such as connectors, receivers, plugs, modems, repeaters, cables, Hubs etc.
Q.4: What is meant by signal encoding in the physical layer?
Signal encoding in the physical layer is defined as the process where digital data is converted into analog signals for transmission. Various techniques such as line coding and modulation are used for signal encoding.
Q.5: How data is transmitted by the physical layer?
Data link layer transmits the data frames to physical layer. Physical layer converts this received frames into electrical pulses that represents the binary data. This binary data is then sent by over wired or the wireless media by the physical layer.
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