S1L6 - Transmission Medium (Guided) [PART 2 LEFT]

The range of frequencies transmitted without being strongly attenuated is called BANDWIDTH
.
Cutoff not really sharp so bandwidth is from 0 to the frequency at which half the power gets through
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Bandwidth is physical property of transmission medium depending on construction, thickness and length of medium

Maximum data rate for noiseless channel = 2H log2 V bits/sec
.
for noisy channel = H log2(1+S/N) bits/sec

Depending on perspective, networks classified:

  1. Based on transmission media
  2. Based on Network Size
  3. Based on Management Method
  4. Based on Topology

Transmission Media

Guided Media
Signals confined to a narrow path and will behave preictably

Twisted Pair
2 or more copper wires twisted around each other within a plastic sheath. Twisted to reduce electrical interferences

  1. STP
    Have a metal shield encasing wires that act as a ground
  1. UTP
    most commonly used because inexpensive and extremely flexible
    .
    Category:
  2. lowest quality, only good for voice. not recommended now
  3. good for voive and low data rates
  4. at least 3 twists per foot (common in phone network in residential)
  5. 16 Mbps
  6. 100 Mbps common for networks targeted for high speed communication
  7. More twists tthan cat 6, 1 Gbps

Cat 3 and cat 5 are similar but with more twists per cm which results in less crosstalk and better signal over long distances

Coaxial Cable

  1. Thinnet coaxial cable
    Similar ro the cable used bt TV companies (LAN environments)
  1. Thicknet
    Similar to thinnet except larger in diameter (loss in flexibility)

Fiber Optic Cable
Transmit data over long distances with little loss in data integrity. Data transferred as pulse of light (not subject to electromagnietic interference)

Total Internal Reflection of Light
When light passes from one medium to another it is refracted (bent) at the boundary. This depends on properties of the media (indices of refraction). For angle of incidence above a critical value, it is refracted back into the silica; thus trapped inside the fiber and can propagate for many kilometres with virtualy no loss.

Multi-mode & Single-mode Fiber

  • If fiber's diameter reduced a few wavelength, fiber acts as a waave guide and light can propagate in a straight line.
    .
  • Multi-mode has a core large diameter that allows multiple modes of light to propagate.

Attenuation of light through glass depends on wavelength of light
Attenuation = 10 log10 (transmitted power/received power)
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Light pulses sent down a a fiber spread out in length as they propagate. This spreading is known as CHROMATIC DISPERSION, the amount of which is wavelength dependent

Fiber Optic Networks

  1. Fiber Optic ring with active repeaters
    Interface at each computer passes the light pulse stream through to the next link and also serves as a T junction to allow the computer to send & accept messages

Advantage

  • Since signal regenerated at each interface, individual pc-to-pc links can be kilometres long
    .
    Disadvantage

  • If an active repeater fails, the ring is broken and network goess out of operation



    Capture1

  1. Passive Star Connection

Unguided Media

Wireless Transmission

Microwave Transmission
Waves above 100Mhz can be narrowly focused. Concentrating all energy into a small beam gives hugher signal-to-noise ratio, provided receivig antenna accuratelt aligned with each other
.
Properties
Travels in straight line, weather/frequency dependent
Even transmitter well focused at beam, some waves may be refracted which take them slightly longer. Delayed waves arrive out of phase (Multipath Fading)