Unit 3 Physical Layer

OSI Model The Open Systems Interconnection (OSI) model is a reference framework for understanding data communications between any two networked systems. It divides the communications processes into seven layers. Each layer both performs specific functions to support the layers above it and offers services to the layers below it. The three lowest layers focus on passing traffic through the network to an end system. The top four layers come into play in the end system to complete the process.

OSI Model

OSI Layers

An Exchange Using the OSI Model

Physical Layer

Functions of physical Layer Physical characteristics of interfaces and media: it defines the characteristics of the interfaces between the devices and the transmission medium. It also defines the type of transmission medium. Representation of bits: this layer consists of a stream of bits (sequence of 0s and 1s) without any interpretation. To be transmitted, bits must be encoded in to signals- electrical or optical. This layer also defines the type of encoding (how 0s and 1s are changed in to signals). Data rate: the transmission rate – the number of bits sent each second-is also defined by the physical layer. Synchronization of bits: the sender and receiver must be synchronized at the bit level. In other words, the sender and the receiver clocks must be synchronized.

Line configuration: this layer is also concerned with the connection of devices to the medium. In a point-to-point configuration, two devices are connected together through a dedicated link. In a multipoint configuration, a link is shared between several devices. Physical topology: the physical topology defines how devices are connected to make a network. Device can be connected using following topologies: Mesh topology Star topology Ring topology, and Bus topology

Transmission mode: the layer also defines the direction of transmission between two devices. It can be: Simplex Half duplex, and Full duplex

Physical Layer Design Isuue Physical layer design issues The major task of the physical layer is to provide services for the data link layer. Following are the services provided or duties of the physical layer or the design issues of the physical layer Duties of physical layer Transformation from bit to signal Bit rate control Bit synchronization Multiplexing Circuit switching

Transformation from bit to signal: The data link layer consists of 0’s and 1’s in the bit form. This bit stream cannot travel as it is on the transmission medium. So the physical layer converts the bit stream into a signal which is suitable for the transmission medium. Bit rate control: The transmission medium decides the highest value of bit rate and the physical layer acts as a bit rate controller. The design of the physical layer hardware and software will determine the data rate.

Multiplexing: Physical layer can use different techniques of multiplexing, in order to improve the channel efficiency. Switching: There are three switching methods, namely circuit switching, message switching and packet switching. Out of which circuit switching is the function of physical layer.

Physical layer specifications Installation of cables or other physical mediums for the physical layer needs the awareness of all the specification that affects the process. For example one needs to know about the Ethernet standards published by IEEE 802.3 which specifies the wiring configuration, the Media Access Control (MAC) and collision detection mechanism.

They also specify the maximum length of the cable segment, the distance between workstations and number of repeaters allowed to be used. One should also know about the other standards such as ANSI/EIA/TIA 568, commercial building telecommunication cabling standards, local building codes etc.

Physical Layer Signalling Physical layer signaling: The digital data coming from a digital computer or some other source cannot be put directly on the network medium, because the format of this signal is not suitable for its direct transmission. This takes place at the physical layer and it is called as physical layer signaling. This data is converted into a suitable format or line code and then transmitted over a communication channel.

The various formats used are called as line codes. The formats or line codes used commonly in networking are: Non-return to zero (NRZ) and return to zero (RZ) unipolar formats. NRZ and RZ polar format Non-return to zero bipolar formats Manchester format Differential Manchester code Write others also

Network Devices Used At Physical Layer All types of hubs Terminators Couplers Connectors Repeaters Multiplexers Receivers Transceivers Serial Interfacing Devices RS 232 RS 449 RS 530 RS 422 & RS 423