2: The physical layer Flashcards
Digital transmission
Deals with the transmission of digital data.
Data composition
2 Binary states: 1 and 0.
Information needed for transmition
Base (or ground) information, and some other piece of information that contrasts with your base.
Example: light on vs light off.
Computer signals
Are electromagnetic waves, which can be represented using sinusoidal waves (sin and cos waves).
Wave attributes
Amplitude
Wavelength
Frequency
Phase
We are only concerned with amplitude, frequency and phase.
Amplitude
Strength of the wave
Wavelength
Length between 2 similar points
Frequency
Number of complete wavelengths within a particular time frame. Measured in Hertz (Hz).
Phase
Overall shape of the wave. We use frequency and amplitude to determine this.
Physics rules that apply to networks
- Energy cannot be created or destroyed; the maximum strength of a signal is determined at the point of transmission.
- A wave loses energy simply by moving, which means that it automatically loses energy over time.
- 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.
Attenuation
The loss of signal strength over time.
Attenuation occurrence
As electricity moves through a wire, the wire heats up and generates a magnetic field, which also generates radio waves.
Wave interference
Waves with similar phases will interact with one another, either constructively or destructively.
Electromagnetic interference (EMI)
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.
Bandwidth
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.
Bandwidth and data transmission rate
Large bandwidth implies that lots of waves can pass through uneffected, meaning more data can be recieved.
Guided transmission media
Signals sent using guided transmission are guided by physical mediums like wires.
Wireless transmission media
Any form of transmission that does not use guided media.
4 types of guided transmission media
Twisted pair
Coaxial
Single-mode fibre
Multi-mode fibre
Twisted pairs
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)
Shielding
Wrap twisted pairs in grounded conductive sheets that absorb EMI.
Tip for using twisted pairs
Never place them close to a power cable.
If they must cross, then cross them perpendicular to one another.
Coaxial
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.
Optical fibre
This makes use of light pulses. This method has 2 application variations. Multi-mode and single-mode.
Multi-mode
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.
Single-mode
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.
Modulation
The process of converting bits into signals and vice versa. There are two categories of modulation:
Baseband
Passband
Baseband
Signal used can have a base frequency of 0. In practice, only guided transmission uses baseband modulation.
Passband
Lowest frequency is greater than 0
Line codes
Schemes that convert bits into signals in baseband modulation
Non-return to Zero(NRZ)
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.
Non-return to zero invert (NRZI)
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.
Manchester
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.
Bipolar
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.
Amplitude shift keying (ASK)
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.
Frequency shift keying (FSK)
AKA frequency modulation uses distinctions in frequency to show changes from 1 to 0. Its more complex than ASK, but has higher bandwidth.
Phase shift keying
Uses variation in phases to distinguish 1s and 0s. By far the most complex but has the most efficient bandwidth use.
Multiplexing
Making use of one channel for multiple transmissions.
Frequency division multiplexing (FDM)
Each user in a channel is given a specific frequency range that they use exclusively.
Time division multiplexing (TDM)
“Everyone gets a turn”. Each user gets a time slot to use 100% of the bandwidth.
Code division multiplexing (CDM)
Allows user to access the full bandwidth at all times. It makes use of code theory to extract data sent to the receiver specifically.
maximum data rate formula
2Blog_2_(V)
B = bandwitdh
V = discrete levels
Maximum bits/sec
log_2_(1+S/N)
S = signal
N = noise
