Transmission Impairments

Three different causes of errors
- Attenuation and Attenuation distortion
- Delay Distortion
- Noise
Attenuation
- Strength of a signal fades over distance
- This is true in both guided and unguided.
- Expressed in db/unit distance
- In guided, this is exponential.
- For unguided and depends on atmosphere.
- Three considerations
  - Sufficient strength so the signal can be detected. But not so much receiving equipment is damaged.
  - The signal must be strong enough that noise does not induce error.
  - Attenuation varies with frequency.
- 1 and 2 are dealt with using amplifiers or repeaters.
  - This is a layer 1 device
  - Hub, multi port hub, repeater, multi port repeater.
- Attenuation vs frequency.
  - The higher the frequency, the greater the attenuation.
  - But we can correct this with equalization.
  - See figure 3.15 page 88
- Attenuation is measured in db.
  - db = 10 log₁₀(P₂/P₁)
  - Example: A signal at a power of P but received at 1/3P the attenuation is 10 log₁₀((1/3P)/P) = 10 log₁₀(1/3) = -4.77
Delay Distortion
- The speed of a signal through a media varies with it's frequency.
- Or signals of different frequencies travel at slightly different speeds.
- This represents sin(2πft)+1/3sin(3*2*π*ft)+1/5*sin(5*2*π*ft) under a simulated delay distortion.
- Under extreme conditions, this can inter fear with the bits being sent.
- In this case, data from one bit is spilling over into another bit.
- This is called intersymbol interference
- Stallings says this is a major limitation to the maximum bit rate.
- Stallings tells us that this can be compensated for with equalization as well, and demonstrates this in 3.15b (p 88)
Noise
- Thermal noise
  - Random motion of electrons in a wire.
  - Distributed uniformly across all bandwidths.
  - Often called white noise
  - Places an upper bound on performance.
  - Modeled by N = kTB (watts)
    - k = Boltzmann's constant = 1.38x10^-23J/K
    - T is temperature in Kelvin (C = K - 273)
    - B is the bandwidth in Hz.
    - Example: A Network is operating at 4MHz at 20 degrees C. What is the white noise generated?
```
 N = 1.38 x10^-23*(273+20)*4x10⁶  = 1.6 x 10 ^-14
		
```
- Intermodulation noise
  - When signals of different frequencies share a wire, they generate this type of noise.
  - Theoretically it is the sum of the two frequencies.
  - But complex systems make this not quite true.
- Crosstalk
  - Phone conversations where you can hear a different connection.
  - In twisted pair, one wire acts as a transmitter.
  - The other wire acts as a receiver.
  - This is of the same order of magnitude as thermal noise.
  - I assume the different twists in wire are to reduce this.
- Impulse Noise.
  - Non uniform
  - non continuous
  - Not predictable
  - Lightening
  - A motor starting or turning off.
  - Short spikes, relatively high.
  - What would the effect of a spike .01 sec in duration be?
```
Assume 56kb/sec

56x10³bits/second x .01 sec = 560 bits
	    
```