What does physical layer do in OSI model

Layer 1 devices view network topologies as physical (as opposed to logical). Physical topologies include a bus, ring, or star. The data transfer medium has a physical and electrical component. This includes cables, jacks, patch panels, punch blocks, hubs, and multi-station access units (MAUs). Data is represented as binary expressions on a computer network. At the physical layer, data is represented as binary (a series of ones and zeros). A binary expression is a combination of bits, where a bit is either a 1 or a 0. These bits are represented by voltage (copper wiring) or light (fiber optic cable). When voltage is present or absent on a wire, it represents either a binary 1 or binary 0.

The presence or absence of light in a fiber-optic cable also produces a binary 1 or 0. It’s called current state modulation. Physical layers include topologies, analog vs. digital/encoding, bit synchronization, baseband vs. broadband, multiplexing, and serial (5-volt logic) data transfer. Hubs, wireless access points, and network cabling are examples of devices defined by physical layer standards. This layer is measured in bits. When two networked devices communicate at the physical layer, they must agree on when one bit stops, and another bit starts. A method of synchronizing bits is needed by the devices. Bit synchronization can be asynchronous or synchronous.

 

In the Synchronous method, the internal clocks of the sender and receiver ensure that they agree on when bits begin and end. To achieve this synchronization, an external clock is used (for example, a clock provided by a service provider). Both sender and receiver use this external clock. Asynchronous is sending a start bit to the receiver, a sender informs the receiver that it is about to start transmitting. The receiver starts its own internal clock when it sees this. The sender sends a stop bit after transmitting its data. Channels are ways to divide the available bandwidth on a medium (like copper or fiber). Different communication streams can then be sent over different channels. A cable modem uses frequency-division multiplexing (FDM). Multiple communication sessions can share a single physical medium via multiplexing. With cable TV, you can receive many channels over a single physical medium (for example, a coaxial cable plugged into the back of your TV). Specifically, a cable modem which operates in broadband, transmits data using a range of frequencies carried by the cable company and transmits data to and from TV stations. The other type of transmission is baseband. During the transmission of data, baseband technologies utilize all the available frequencies on the medium. An example of a network technology that utilizes baseband is Ethernet.