2: The physical layer

Question 1

Digital transmission

Answer 1

Deals with the transmission of digital data.

Question 2

Data composition

Answer 2

2 Binary states: 1 and 0.

Question 3

Information needed for transmition

Answer 3

Base (or ground) information, and some other piece of information that contrasts with your base.

Example: light on vs light off.

Question 4

Computer signals

Answer 4

Are electromagnetic waves, which can be represented using sinusoidal waves (sin and cos waves).

Question 5

Wave attributes

Answer 5

Amplitude
Wavelength
Frequency
Phase

We are only concerned with amplitude, frequency and phase.

Question 6

Amplitude

Answer 6

Strength of the wave

Question 7

Wavelength

Answer 7

Length between 2 similar points

Question 8

Frequency

Answer 8

Number of complete wavelengths within a particular time frame. Measured in Hertz (Hz).

Question 9

Phase

Answer 9

Overall shape of the wave. We use frequency and amplitude to determine this.

Question 10

Physics rules that apply to networks

Answer 10

1. Energy cannot be created or destroyed; the maximum strength of a signal is determined at the point of transmission.
2. A wave loses energy simply by moving, which means that it automatically loses energy over time.
3. As a wave passes through a medium, the medium will remove energy from the wave. This is determined by how much “work” is needed to pass through the transmission medium.

Question 11

Attenuation

Answer 11

The loss of signal strength over time.

Question 12

Attenuation occurrence

Answer 12

As electricity moves through a wire, the wire heats up and generates a magnetic field, which also generates radio waves.

Question 13

Wave interference

Answer 13

Waves with similar phases will interact with one another, either constructively or destructively.

Question 14

Electromagnetic interference (EMI)

Answer 14

When a wave is sent, and the receiver receives data that has been interfered by an outside wave, then data will be corrupted. This is electromagnetic interference.

Question 15

Bandwidth

Answer 15

The range between the lowest frequency that a wave on the channel can have, and the maximum frequency that a wave transmitted on that channel can have without being strongly attenuated.

Simply, if a wave is within certain frequency values, then attenuation will not effect it greatly.

Question 16

Bandwidth and data transmission rate

Answer 16

Large bandwidth implies that lots of waves can pass through uneffected, meaning more data can be recieved.

Question 17

Guided transmission media

Answer 17

Signals sent using guided transmission are guided by physical mediums like wires.

Question 18

Wireless transmission media

Answer 18

Any form of transmission that does not use guided media.

Question 19

4 types of guided transmission media

Answer 19

Twisted pair
Coaxial
Single-mode fibre
Multi-mode fibre

Question 20

Twisted pairs

Answer 20

1s and 0s are differentiated by the strength of the current. But current creates a magnetic field which induces current in other wires. Twisting the wires around each other reduces this interference and distortion (called cross-talk)

Question 21

Shielding

Answer 21

Wrap twisted pairs in grounded conductive sheets that absorb EMI.

Question 22

Tip for using twisted pairs

Answer 22

Never place them close to a power cable.

If they must cross, then cross them perpendicular to one another.

Question 23

Coaxial

Answer 23

Same principle as a twisted pair, but have much thicker core wires. They can thus be longer without suffering from attenuation.

It also contains a Faraday cage around the core wires, which absorbs most radio waves that reach it.

Question 24

Optical fibre

Answer 24

This makes use of light pulses. This method has 2 application variations. Multi-mode and single-mode.

Question 25

Multi-mode

Answer 25

Makes use of total internal refraction. Using correct angles, we can shoot a beam of light into the wire, and it will refract such that it remains within the wire. However, it loses a small amount of energy when it refracts. Each light frequency refracts differently. Multi-mode accommodates for a plethora of frequencies.

Question 26

Single-mode

Answer 26

Thinner than multi-mode. A light pulse can travel through without the need of refraction. It keeps more of its energy for longer. However they are difficult to work with and expensive to make.

Question 27

Modulation

Answer 27

The process of converting bits into signals and vice versa. There are two categories of modulation: Baseband Passband

Question 28

Baseband

Answer 28

Signal used can have a base frequency of 0. In practice, only guided transmission uses baseband modulation.

Question 29

Passband

Answer 29

Lowest frequency is greater than 0

Question 30

Line codes

Answer 30

Schemes that convert bits into signals in baseband modulation

Question 31

Non-return to Zero(NRZ)

Answer 31

Simplest line code High current is a 1, and no current is a 0. It keeps current high when the bit is 1, but then drops it when it should be 0. But at long distances is it effected by attenuation. NRZ has a fixed bandwidth that makes it far form optimal.

Question 32

Non-return to zero invert (NRZI)

Answer 32

One issue with NRZ is the sender and receiver desyncing. NRZI uses the changes of current to indicate data. It uses high and low values to indicate a change from 0 to 1 and vice versa. A change is 1, constant is 0.

Question 33

Manchester

Answer 33

Another attempt at solving desync. Sends a seperate clock signal, and then XORing the bits in the data with that signal. It works well in ethernet but needs a high bandwidth.

Question 34

Bipolar

Answer 34

Some technologies require both positive and negative voltage values so that the net signal is 0. You then reduce attenuation on the channel, but voltages must be used equally.

Question 35

Amplitude shift keying (ASK)

Answer 35

AKA Amplitude modulation, uses variations in amplitude to distinguish between 1s and 0s. It need not go to 0, the amplitude just needs a large enough difference change for it to work. This is the simplest for passband modulation.

Question 36

Frequency shift keying (FSK)

Answer 36

AKA frequency modulation uses distinctions in frequency to show changes from 1 to 0. Its more complex than ASK, but has higher bandwidth.

Question 37

Phase shift keying

Answer 37

Uses variation in phases to distinguish 1s and 0s. By far the most complex but has the most efficient bandwidth use.

Question 38

Multiplexing

Answer 38

Making use of one channel for multiple transmissions.

Question 39

Frequency division multiplexing (FDM)

Answer 39

Each user in a channel is given a specific frequency range that they use exclusively.

Question 40

Time division multiplexing (TDM)

Answer 40

"Everyone gets a turn". Each user gets a time slot to use 100% of the bandwidth.

Question 41

Code division multiplexing (CDM)

Answer 41

Allows user to access the full bandwidth at all times. It makes use of code theory to extract data sent to the receiver specifically.

Question 42

maximum data rate formula

Answer 42

2Blog_2_(V) B = bandwitdh V = discrete levels

Question 43

Maximum bits/sec

Answer 43

log_2_(1+S/N) S = signal N = noise