139 lines
4.7 KiB
Markdown
139 lines
4.7 KiB
Markdown
# 1. Physical Layer
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This layer is responsible for the actual physical transfer of data between devices via cables or other mediums using switches, routers, etc..
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This layer also converts the data into a bit-stream of 1's and 0's.
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The signal convention must be the same on both devices. (Meaning of a 0 and a 1)
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## In depth
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Information on computers is stored in a digital format. To transmit this information we have to convert the information to signals.
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Physical layer is responsible for this conversion. The Physical layer deals with the actual physical transmission of electrical signals.
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You could say the physical layer is a bridge between real life and software.
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## Waves
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Waves, like you would see on a lake, or in electrical cables are made when a physical quantity changes in the form of a moving ridge.
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Waves by them self store no real information.
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## Signal
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A signal is a wave (carrier) that holds information. (wave + data = signal)
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The carrier is a type of wave we add our information to. In most cases a sine wave is used as a carrier wave,
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![[encoder.png]]
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A encoder converts analog or digital information in signals that can be transmitted across different [[Comminucation Mediums]]
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The process of adding data to a carrier wave is called Modulation
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## Modulation
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Adding data to carrier waves is called modulation.
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This can be done in 3 different ways.
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### Amplitude
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![[modulation-amplitude.png]]
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Amplitude or the strength (height) of a wave changes over time.
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### Frequency
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![[modulation-frequency.png]]
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Frequency is the amount of waves that pass a fixed point in a given amount of time.
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### Phase
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![[modulation-phase.png]]
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Phase specifies the location or timing of a point within a wave cycle of a repetitive waveform.
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![[modulation-phase-2.png]]
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## Analog
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**Analog Signals** can have a infinite number of possible values in a range.
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For example: 0.0 <-> 1.0
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- 0.1436367436473......
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- 0.54454764654765......
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**Analog data** is continuous and have continuous values.
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Values in analog data are always changing.
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A good analogy for analog data is like a physical (analogue) clock with hands.
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The hands on a clock like that are always moving (continuous) and do not have a specific value.
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## Digital
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Digital data is information represented as a string of discrete symbols each of which can take on one of only a finite number of values from some alphabet, such as letters or digits.
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For example:
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- a
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- b
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- 1
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- 0
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These values are specific and are not changing.
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## Transmitting Impairments
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Signals most often have to deal with problems during transmission. Problems like this are called transmission impairments.
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Important transmission impairments are:
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### Attenuation
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Attenuation is the strength or amplitude of a signal is decreasing over time and distance.
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A good analogy for this would be the loss of volume you would hear from a speaker the further you are away from it.
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![[attenuation.png]]
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This makes is more difficult to correctly capture the signal on the receiving end.
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We can solve this issue by dividing the communication line up in smaller segments and amplify the signal between segments.
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### Distortion
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![[distortion.png]]
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Distortion is the change that occurs in a signal that affects how the signal looks.
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For example talking to someone while behind a thick glass pane changes how you sound on the other side.
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We can solve this by using a lower bit rate. Bit rate is kinda like the information per second. This way the signal is less susceptible to distortion.
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### Noise
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![[noise-signal.png]]
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Noise is when data is added to a signal and therefore changing the data itself.
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Analogy for this when you are listening to someone talk and somebody else is talking at the same time trough your conversation.
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We can solve this by using a stronger signal (bit like talking louder).
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This will reduce the impact noise has on the signal.
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This is also called signal to noise ratio (SNR)
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## Transmission Media
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These signals are send through a transmission medium. A transmission medium is a physical object that can carry the information from source to destination.
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Transmission mediums are often space (air), metallic cable or fiber optic cables.
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Transmission media are divided in 2 groups.
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### Guided media
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Guided media like cables provide a conduit from one device to another.
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I like to think of it as you can steer the signal by moving the cable for example.
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Coaxial cable, twisted pair cable and fiber optic cables.
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![[type-cables.png]]
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### Unguided media
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Unguided media transport electromagnetic waves without using a physical conductor.
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This is called wireless transmission. (Technically using the air around you as a conductor)
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Signals are broadcast tough space and thus available for everyone to pick up. |