Chapter 7 Flashcards
Type of Paths
unguided and guided transmission
Unguided transmission
(wireless): no specific path
e.g., radio transmissions travels in all directions through the air
Guided transmission
(wired): specific path for transmission, e.g., copper wires or optic fibers
Form of energies
Electrical energy (guided, wires), radio transmission (unguided, wireless), light (both guided and unguided)
Electrical Energy Types
Twisted Pair
Coaxial Cable
Radio Transmission Energy Types
Terrestrial Radio
Satellite
Light Energy Types
Optical Fiber
InfraRed
Laser
Noise
random electromagnetic radiation that permeates the environment
Noise comes from:
- Cosmic radiation
- Side effect of normal operation off comm systems or other electric devices
Noise interferes with:
communication using radio or copper wires
(optical wires are immune to such electrical noise)
Optical Fiber
consists of a thin strand of glass or transparent plastic encased in a plastic cover (one strand for one direction)
- One end connects to a laser to transmit light
- Other end connects to photosensitive device to detect light
- Two fibers are used for two way communication
Cons of Optical Fibers
Reflection absorbs small amounts of energy
attenuation and dispersion cause serious problems for long optical fibers
attenuation
Reflection absorbs small amounts of energy, resulting in a pulse with less energy at the receiver
dispersion
signal traveling along a long fiber gets distorted over time
Optical Fiber vs Copper Wiring
OF:
- immune to electrical noise
- higher bandwidth
- light traveling across fiber doesn’t attenuate (degrade) as much as electrical signals traveling across copper
CW:
- less expensive
- easier to install; doesn’t require as much special equipment to expertise as optical fiber
- less likely to break if accidentally bent of pulled
Tradeoffs Among Media Types
Cost: materials, installation, operation, and maintenance
Data Rate: number of bits per second that can be sent
Delay: time required for signal propagation or processing
Affect on signal: attenuation and distortion
Environment: susceptibility to interference and electrical noise
Security: susceptibility to eavesdropping
Two most important measures to evaluate performance of a transmission system:
Propagation delay and channel capacity
Propagation Delay
time required for a signal to traverse the medium over distance. Delay can lead to timing errors, data skews, and clock and data mismatches
Channel capacity
maximum data rate that the medium can support
Nyquist’s Theorem
Measuring performance; Gives fundamental relationship between the analog bandwidth of a transmission system (its max frequency) and its capacity to transfer data
provides absolute max data rate that cannot be achieved in practice
Shannon’s Theorem
Extended work of Nyquist, specify max data rate that can be achieved over a transmission system that experiences noise
Nyquist’s Theorem vs Shannon’s Theorem
NT: encourages engineers to explore ways to encode bits on a signal (make K as large as possible) because clever encoding allows more bits to be transmitted per time unit
ST: is more fundamental because it represents absolute limit derived from laws of physics.
- No amount of clever encoding can overcome laws of physics
- It places fundamental limit on number of bits per second that can be transmitted in real communication system
Most common form of unguided communication uses:
electromagnetic energy in the Radio Frequency (RF) range
- RF energy can traverse long distances and penetrate objects such as the walls of a building
- exact properties of electromagnetic energy depends on the frequency (bluetooth only works a small distance, remotes use infrared waves and a line of sight)
