Physical Layer Introduction

What is Physical Layer in OSI Model?

Physical layer in the OSI model plays the role of interacting with actual hardware and signaling mechanism. Physical layer is the only layer of OSI network model which actually deals with the physical connectivity of two different stations. This layer defines the hardware equipment, cabling, wiring, frequencies, pulses used to represent binary signals etc.

Physical layer provides its services to Data-link layer. Data-link layer hands over frames to physical layer. Physical layer converts them to electrical pulses, which represent binary data.The binary data is then sent over the wired or wireless media.

Signals

When data is sent over physical medium, it needs to be first converted into electromagnetic signals. Data itself can be analog such as human voice, or digital such as file on the disk.Both analog and digital data can be represented in digital or analog signals.

• Digital Signals

  Digital signals are discrete in nature and represent sequence of voltage pulses. Digital signals are used within the circuitry of a computer system.

• Analog Signals

  Analog signals are in continuous wave form in nature and represented by continuous electromagnetic waves.

Transmission Impairment

When signals travel through the medium they tend to deteriorate. This may have many reasons as given:

• Attenuation

  For the receiver to interpret the data accurately, the signal must be sufficiently strong.When the signal passes through the medium, it tends to get weaker.As it covers distance, it loses strength.

• Dispersion

  As signal travels through the media, it tends to spread and overlaps. The amount of dispersion depends upon the frequency used.

• Delay distortion

  Signals are sent over media with pre-defined speed and frequency. If the signal speed and frequency do not match, there are possibilities that signal reaches destination in arbitrary fashion. In digital media, this is very critical that some bits reach earlier than the previously sent ones.

• Noise

  Random disturbance or fluctuation in analog or digital signal is said to be Noise in signal, which may distort the actual information being carried. Noise can be characterized in one of the following class:

  • Thermal Noise

    Heat agitates the electronic conductors of a medium which may introduce noise in the media. Up to a certain level, thermal noise is unavoidable.

  • Intermodulation

    When multiple frequencies share a medium, their interference can cause noise in the medium. Intermodulation noise occurs if two different frequencies are sharing a medium and one of them has excessive strength or the component itself is not functioning properly, then the resultant frequency may not be delivered as expected.

  • Crosstalk

    This sort of noise happens when a foreign signal enters into the media. This is because signal in one medium affects the signal of second medium.

  • Impulse

    This noise is introduced because of irregular disturbances such as lightening, electricity, short-circuit, or faulty components. Digital data is mostly affected by this sort of noise.

Transmission Media

The media over which the information between two computer systems is sent, called transmission media. Transmission media comes in two forms.

• Guided Media

  All communication wires/cables are guided media, such as UTP, coaxial cables, and fiber Optics. In this media, the sender and receiver are directly connected and the information is send (guided) through it.

• Unguided Media

  Wireless or open air space is said to be unguided media, because there is no connectivity between the sender and receiver. Information is spread over the air, and anyone including the actual recipient may collect the information.

Channel Capacity

The speed of transmission of information is said to be the channel capacity. We count it as data rate in digital world. It depends on numerous factors such as:

• Bandwidth: The physical limitation of underlying media.
• Error-rate: Incorrect reception of information because of noise.
• Encoding: The number of levels used for signaling.

Multiplexing

Multiplexing is a technique to mix and send multiple data streams over a single medium. This technique requires system hardware called multiplexer (MUX) for multiplexing the streams and sending them on a medium, and de-multiplexer (DMUX) which takes information from the medium and distributes to different destinations.

Switching

Switching is a mechanism by which data/information sent from source towards destination which are not directly connected. Networks have interconnecting devices, which receives data from directly connected sources, stores data, analyze it and then forwards to the next interconnecting device closest to the destination.

Switching can be categorized as:

Functions of Physical Layer

• It defines the physical characteristics and functions of the physical devices and interfaces so that transmission can occur. It states the number of pins in each of the devices as well as the purpose of each pin.
• It lays out the transmission medium and type of signal for transmitting the bits, i.e., electrical pulses, optical pulses or radio signals.
• It defines the procedure of encoding of the bits, for example, how many volts should represent a 0 bit and 1 bit in case of electrical signals.
• It states the data transmission rate, i.e., number of bits transmitted per second; and the duration of a bit, i.e., how long a bit stays.
• It defines the topology, i.e., physical layout, of the network devices.
• It also states the direction of transmission, i.e., whether the transmission is in simplex mode, half-duplex mode or full-duplex mode.