Lecture 8 - Transmission Medium

Question 1

State Nyquist’s Theorem

Answer 1

The maximum data rate for a noiseless channel is
2H log2 V bits/s
where H is the bandwidth
and V is the transmission speed

Question 2

Define Bandwidth

Answer 2

Bandwidth is the frequency range in a transmission media that is available for communication. Bandwidth is measured from 0 up to a cutoff value

Question 3

The amount of noise that is present in a transmission media is called the Signal-to-Noise ratio (SnR). State Shannon’s Theorem which relates to SnR.

Answer 3

Shannon’s Theorem states that the maximum data rate for a noisy channel is H log2 (1 + S/N) bits/s
where H is the bandwidth
and S/N is the Signal-to-Noise ratio

Question 4

Define “Transmission Media”

Answer 4

A Transmission Medium is a physical path that carries signals (raw bit streams) from one location to another.

Question 5

Transmission Media can be classified as either guided or unguided. What is a guided transmission medium?

Answer 5

A guided media is one in which signals are confined to a narrow path and will behave predictably.

Question 6

Advantages of Fiber Optics [6]

Answer 6

1. Higher Bandwidth
2. Less attenuation -> less repeater -> less cost
3. not affected by power surge, EM interference, power failure and chemicals
4. Thin and Lightweight
5. Lower INSTALLATION cost
6. Resistant to noise

Question 7

Disadvantages of Fiber Optics [4]

Answer 7

1. requires specialized tools for installation
2. Easily damaged
3. Two-way communication is more complex (two fiber optics or use frequency bands)
4. Fiber interface cost more

Question 8

Explain how data travels along a Fiber Optics [3]

Answer 8

• Total Internal Reflection
• When light moves from one medium to another, it is refracted (bent)
• If the angle of incidence is greater than a certain value (critical angle) the light is refracted into the medium and does not escape

Question 9

Differentiate between single-mode fiber and multi-mode fiber

Answer 9

Single Mode Fiber: The diameter of the fiber is around a few wavelengths of the signal (in microns). Hence light can only travel in a straight line

Multimode Fiber: The core has a relatively larger diameter, thus allowing for different rays to bounce at different angles. Each ray is said to have a different mode.

Question 10

Calculate the attenuation (in dB per km) given the transmitted power is T and the received power is R

Answer 10

attenuation in dB/km = 10 log10 (T/R)

Question 11

What is the advantage and disadvantage of using fiber optic ring with active repeat?

Answer 11

+ Since the signal is regenerated at each interface, the individual computer-to- computer links can be kilometers long, with virtually no limit on the total size of the ring.

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