Midterm 1 Flashcards
Explain the difference between a continuous signal and a discrete signal
Continuous signal - have varying amplitudes and infinite possible values within a given range -
Discrete Signals - distinct and separate values (1, 0)
What is a periodic signal?
Signals which are exemplified by sinusoidal waves with a repeating pattern
What is the signal spectrum?
range of frequencies a signal has
What is the absolute bandwidth?
The approximation that a signal has practically all its power between two points (F1 and F2) -> Absolute bandwidth = (F1 - F2)
What are signal impairments?
Random changes on the physical signal that the transmitter sends out
Types: - attenuation, delay distortion, noise
What is Attenuation and how is it combatted?
when the signal weakens due to distance from the transmitter. The amplitude decreases. It is combatted with Amplifiers or repeaters which retransmit the signal
What is the difference between an amplifier and a repeater?
Amplifiers just re-transmit a received signal including the noise
Repeaters - take in the signal, decide what it was supposed to be and retransmit it without the noise
What is noise?
the noise is the random, unwanted variation or fluctuation that interferes with the signal.
Explain thermal noise
Movement of atoms due to heat causes thermal noise -> as such it increases with heat and is only not present in absolute zero conditions
Explain intermodulation noise
Noise caused by intermodulation of two frequencies due to a non-linear device that is not equipped to handle them. It produces signals that are multiples or sums or differences of the frequencies the original signal contains
Explain Crosstalk noise
Unwanted coupling between signal paths, like hearing others conversations interrupt your own
Explain Impulsive noise
irregular noise that causes high amplitude increases to the signal caused by electromagnetic disturbances
What are some of the impairment causes?
Reflection - signal bounces off a large surface
Diffraction - occurs at edge of impenetrable body
Scattering
Explain what bit rate and Baud rate are
Bit rate - number of transmitted bits per second
Baud rate - number of signal changes per second
What is data encoding?
When a device generates bits from a signal and transforms it into belts
- Unipolar signal belt where low represents 0 and high represents 1 but all elements are positive
-Bipolar - when one logic state is represented by positive voltage level and the other negative
List the three main digital encoding schemes
Non-return to zero
Multi-level binary
Biphase
Explain the NRZ encoding scheme and list its advantages and drawbacks
Maintains a constant value for the duration of a bit time
Lacks synchronization but makes efficient use of bandwidth
Explain the multilevel binary encoding scheme and its advantages and drawbacks
Uses more than 2 signal level (Bipolar AMI, Psuedoternary)
advantages - since signal alternates in voltage there is no net DC component
Drawback - loss of synchronization if a long string of 0’s occur in bipolar AMI or if a long string of 1’s happens in psuedo-ternary
Explain the Biphase encoding scheme and its advantages and drawbacks
Transition at the middle of each bit period
Advantages - synchronization, absence of expected transition provides error detection, no DC component
Disadvantages - higher modulation rate therefor requiring more bandwidth
(manchester and differential manchester)
What is Frequency-Division Multiplexing (FDM)
- Total bandwidth is divided into a series of non-overlapping frequency bands - with guard band in between channels
- A channel is allocated to a source but if a source is not fully utilizing that channel bandwidth is being wasted
What is Synchronous Time Division Multiplexing?
radio spectrum divided into time slots and only one user can transmit or receive during one time slot
What is statistical Time division multiplexing?
Time slots are allocated based on demand - input lines are scanned and data is only sent when a frame is full
Explain Spread spectrum (aka code division multiple access (CDMA))
Users share both time and frequency domains, separation is achieved by assigning different codes to each user
Explain Frequency Hopping Spread Spectrum (FHSS)
- Signal broadcast over seemingly random series of frequencies. The receiver hops frequencies in sync with the transmitter
What is synchronization and why is it important?
Synchronization is ensuring that the receiver and the transmitter have the same pulse timing
If a loss of synchronization occurs the receiver can misidentify bits and interpret the wrong signal
Explain the two types of errors
Random errors - when a bit is altered between transmission and reception. statistically uniformly spread errors
Burst errors - errors occurring in sequence - string of errors due to impulsive noise likely caused by environmental factors
How does the parity check error detection scheme work?
Append a 1 bit to the end of your string of bits, then add all of the bits together -> if there is an even number of ones and you see a zero in the 9th bit there is an error
How does the Longitudinal Redundancy Check error detection scheme work?
in a grid of bits you do parity check vertically and horizontally - it adds redundancy, if both horizontal and vertical error detected
-if only a column indicates an error and a row doesn’t we know there is two errors
