Introduction to Physical Layer

Role of Physical Layer

  1. Physical Layer
  • Signals – bit
  • Bite Rate- no. of bits sent in one seconds
  • Bit length- distance one bit occupies on the transmission medium = propagation speed x bit duration

Attenuation- Loss of energy as it travels on the medium. To compensate for loss, we may use different components such as amplifiers,

Decible- It is the unit that measure the relative strength (power or voltage) of two signals one signal at two different points.

db = 10 log10(P2/P1)       or    db = 20 log10 (V2/V1)

It should be noted that db will be +ve if signal is amplified and –ve if signal is lost/attenuated

Nyquist bit rate :For noiseless channel, the nyquist bit rate define theoretical maximum bit rate as

  • Nyquist bit rate= 2 x bandwidth x log2L


Throughput: is a measure of how fast we can send the data through a network.

Ex. 12000 frames/min, each fram carrying 10000bits, bandwidth 10Mbps

  • Throughput = (12000X 10000)/60 = 2MBPS, therefore the throughput in this case is 1/5th of the bandwidth

Latency: It is the delay and define how long it takes for an entire message to completely reach at the destination from the 1st bit that is sent out  from the source

Latency = Propagation time+transmission time + queuing time+processing delay


Operation at Physical Layer

  • Digital Transmisstion

Basic understanding

‘r’ is the ration of number of data elements carried by a signal element

Signal rate = data rate / r

S = N/ r

Line coding- Process of converting digital data to digital signals.

  1. Unipolar line coding – Non-return-to-zero(NRZ)
  2. Polar Scheme – NRZ, RZ, manchester (biphase and differential)
  3. Bipolar – AMI and Pseudoternary
  4. Multilevel- 2B/1Q, 8B/6T, 4D-PAM5
  5. Multitransition – MLT3
  6. Block Coding – it changes a block of M bits into a block of N bits referred as mB / nB such as 4B/5B, 8B/10B

Analog to Digital Conversion (A-D)

Pulse code modulation

Delta Modulation

Transmission Modes


Serial –  Asynchronous, Synchronous, Isochronous


  • Analog Transmission


Digital to Analog conversion – ASK, FSK, PSK

Analog-to-analog conversion – AM, FM, PM

  • Multiplexing


Spread Spectrum- FHSS (frequency hopping spread spectrum), DSSS(direct sequence spread spectrum)

  • Transmission Medium

Guided Medium – Twisted pair cable(TP), Coaxial cable, Fiber-optic cable

Unguided Medium – Radio Wave, Microwave, Infrared

Switching – Switching can happen at physical layer, at datalink layer, network layer or at application layer

Types – Circuit switching, message switching, packet switching

Physical Layer – Circuit Switching

Data link layer – packet switching, but we will refer them as frames or cell at DLL. Packet switching at DLL is using virtual circuit approach

Network layer – packet switching. Virtual circuit or datagram approach is used in network layer

Application layer – message switching. E-mail is a kind of message switching

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