Midterm 1

Question 1

Explain the difference between a continuous signal and a discrete signal

Answer 1

Continuous signal - have varying amplitudes and infinite possible values within a given range -

Discrete Signals - distinct and separate values (1, 0)

Question 2

What is a periodic signal?

Answer 2

Signals which are exemplified by sinusoidal waves with a repeating pattern

Question 3

What is the signal spectrum?

Answer 3

range of frequencies a signal has

Question 4

What is the absolute bandwidth?

Answer 4

The approximation that a signal has practically all its power between two points (F1 and F2) -> Absolute bandwidth = (F1 - F2)

Question 5

What are signal impairments?

Answer 5

Random changes on the physical signal that the transmitter sends out

Types: - attenuation, delay distortion, noise

Question 6

What is Attenuation and how is it combatted?

Answer 6

when the signal weakens due to distance from the transmitter. The amplitude decreases. It is combatted with Amplifiers or repeaters which retransmit the signal

Question 7

What is the difference between an amplifier and a repeater?

Answer 7

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

Question 8

What is noise?

Answer 8

the noise is the random, unwanted variation or fluctuation that interferes with the signal.

Question 9

Explain thermal noise

Answer 9

Movement of atoms due to heat causes thermal noise -> as such it increases with heat and is only not present in absolute zero conditions

Question 10

Explain intermodulation noise

Answer 10

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

Question 11

Explain Crosstalk noise

Answer 11

Unwanted coupling between signal paths, like hearing others conversations interrupt your own

Question 12

Explain Impulsive noise

Answer 12

irregular noise that causes high amplitude increases to the signal caused by electromagnetic disturbances

Question 13

What are some of the impairment causes?

Answer 13

Reflection - signal bounces off a large surface

Diffraction - occurs at edge of impenetrable body

Scattering

Question 14

Explain what bit rate and Baud rate are

Answer 14

Bit rate - number of transmitted bits per second

Baud rate - number of signal changes per second

Question 15

What is data encoding?

Answer 15

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

Question 16

List the three main digital encoding schemes

Answer 16

Non-return to zero

Multi-level binary

Biphase

Question 17

Explain the NRZ encoding scheme and list its advantages and drawbacks

Answer 17

Maintains a constant value for the duration of a bit time

Lacks synchronization but makes efficient use of bandwidth

Question 18

Explain the multilevel binary encoding scheme and its advantages and drawbacks

Answer 18

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

Question 19

Explain the Biphase encoding scheme and its advantages and drawbacks

Answer 19

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)

Question 20

What is Frequency-Division Multiplexing (FDM)

Answer 20

• 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

Question 21

What is Synchronous Time Division Multiplexing?

Answer 21

radio spectrum divided into time slots and only one user can transmit or receive during one time slot

Question 22

What is statistical Time division multiplexing?

Answer 22

Time slots are allocated based on demand - input lines are scanned and data is only sent when a frame is full

Question 23

Explain Spread spectrum (aka code division multiple access (CDMA))

Answer 23

Users share both time and frequency domains, separation is achieved by assigning different codes to each user

Question 24

Explain Frequency Hopping Spread Spectrum (FHSS)

Answer 24

• Signal broadcast over seemingly random series of frequencies. The receiver hops frequencies in sync with the transmitter

Question 25

What is synchronization and why is it important?

Answer 25

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

Question 26

Explain the two types of errors

Answer 26

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

Question 27

How does the parity check error detection scheme work?

Answer 27

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

Question 28

How does the Longitudinal Redundancy Check error detection scheme work?

Answer 28

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