[Computer Network] [Royal Notes] Chapter 2 Physical Layer

2 physical layer

2.1.1 Basic concepts of the physical layer

Basic concepts of the physical layer

Solve how to transmit data bit streams on the transmission media connecting various computers, rather than referring to specific transmission media

Main tasks: define standards and determine some characteristics related to transmission media interfaces

characteristic:

1. Mechanical characteristics: define the characteristics of the physical connection, specify the specifications, interface shape, number of leads, number of pins and arrangement used in the physical connection
2. Electrical characteristics: When specifying the transmission of binary bits , the voltage range, impedance matching, transmission rate and distance limit of the signal on the line

For example:

3. Functional characteristics: refers to the meaning of a certain level appearing on a certain line , and the purpose of the signal line of the interface component

4. Procedural characteristics/process characteristics: define the working procedures and timing relationships of each physical line

2.1.2 Basic knowledge of data communication

Typical Data Communication Model

related terms

Purpose of communication: to send a message

• Data: An entity that conveys information, usually a meaningful sequence of symbols

• Signal: the electrical/electromagnetic manifestation of data, the form in which data exists during transmission

  • Digital signal: the value of the parameter representing the message is discrete

    

  • Analog signal: the value of the parameter representing the message is continuous

    

• Source: the source that generates and sends data

• Sink: the end point of receiving data

• Channel: The transmission medium of a signal. Generally used to indicate the medium that transmits information in a certain direction

  • A communication line consists of a send channel and a receive channel

  

Communication method (3 types)

Single communication

Only one channel is required for communication in one direction and no interaction in the opposite direction

half-duplex communication

Both sides of the communication can send/receive information, but neither party can send & receive at the same time. Two channels are required

• interchangeable but not at the same time

full duplex communication

Can send and receive information simultaneously requires two channels

Data transmission method (2 types)

serial transmission

• Slow speed, low cost, suitable for long distance

Parallel transfer

• Fast speed, high cost, suitable for short distance
  • Often used inside a computer

2.1.3 Symbol Baud Rate Bandwidth

symbol

Refers to the basic waveform that uses a fixed duration signal waveform (digital pulse) to represent different discrete values, and is the unit of measurement for digital signals in digital communications

• Signal in duration - k-ary symbols
• The duration - symbol width

Symbol baud bandwidth

Rate/data rate: the transmission rate of data (the amount of data transmitted per unit time)

• It can be represented by symbol transmission rate & information transmission rate

symbol transfer rate

How many symbols are transmitted in 1s

• In the figure, they are all 5 symbols/s

information transfer rate

How many bits are transmitted in 1s

bandwidth

Indicates the " highest data rate" that can pass from one point of the network to another in a unit of time

• Indicates the ability of the communication line of the network to transmit data

• Unit: b/s

Shannon's Theorem

distortion

Signals distort and change

Factors that affect the degree of distortion:

1. symbol transfer rate
2. Signal transmission distance
3. noise interference
4. Transmission Media Quality

Intersymbol Interference

The transmission is too fast, and the division between code units is not obvious

Nye's criterion

Under ideal low-pass (noise-free bandwidth limited), in order to avoid intersymbol interference, the limit symbol transmission rate is 2W Baud

​ (W - channel bandwidth (unit Hz))

1. In any channel, there is an upper limit to the rate at which symbols can be transmitted.
   • If the upper limit is exceeded, severe intersymbol interference will occur
2. The wider the frequency band (highest-lowest) of the channel, that is, the more high-frequency components of the signal that can pass through, the higher the rate at which symbols can be effectively transmitted
3. The Nyss criterion only gives a limit on the symbol transmission rate, but does not give a limit on the information transmission rate
4. multi-system modulation method
   • Increased data rates – seek to carry more bits of information per symbol

Shannon's theorem

• Signal-to-noise ratio dB: average power of signal/average power of noise
  • Often recorded as S/N
  • Unit of measurement: dB
  • Signal-to-noise ratio $dB=10{\text{log}}_{10}(S/N)$​

Shannon's theorem

In a bandwidth-limited and noisy channel, there is an upper limit to the data rate at which information can be transmitted in order not to introduce errors.

Channel limit data transmission rate $=W{\text{log}}_2(1+S/N)$​(b/s)

Compared

After the calculation, take the minimum of the two

2.1.5 Coding and Modulation

Baseband Signal & Wideband Signal

baseband signal

The digital signal 1 and 0 are directly represented by two different voltages, and then sent to the digital channel for transmission (baseband transmission)

• from the source
• A signal that directly expresses the information to be transmitted
• Short distance (small attenuation)

broadband signal

The baseband signal is modulated to form a frequency division multiplexing analog signal, and then transmitted to the analog channel for transmission (broadband transmission)

• After the baseband signal is modulated by the carrier, the frequency range of the signal is moved to a higher frequency band for transmission in the channel
• big distance

Coding and Modulation

Data -> Digital Signal: Encoding

Data -> Analog Signal: Modulation

Encode digital data into a digital signal

Non-return-to-zero coded NRZ

high 1 low 0

• no error checking
• Unable to determine the start and end of a code unit
• Difficulty synchronizing the sending and receiving parties

Return to zero encoding RZ

It must be restored to 0 within one code unit

• low channel utilization

Reverse non-return-to-zero encoding NRZI

Flip 0 unchanged 1

• unchanged within a code unit

Manchester encoding

excellent!

Front low and back high 0 Front high and back low 1

• Transitions in the middle of the bit can be used as a clock signal / data signal

• The frequency bandwidth occupied is ** twice the original baseband width **

  • Symbol transmission rate = 2*bit transmission rate

Differential Manchester encoding

same 1 different 0

• Strong anti-interference and Manchester encoding

4B/5B encoding

Extra bit inserted to break consecutive 0/1

• 80% coding efficiency

Digital data modulated into an analog signal

Sending end: digital signal -> analog signal (modulation)

Receiver: analog signal -> digital signal (demodulation)

Phase modulation: a sine and a cosine

Amplitude Modulation + Phase Modulation (QAM)

Encoding of analog data into a digital signal

Convert analog audio into a discrete sequence of finite digital representations through sampling and quantization (audio digitization)

• Pulse Code Modulation PCM

  • highest fidelity level
  • Encode the audio signal

• step:

  1. Sampling: periodic scanning of analog signals

     • Time-continuous signal -> Time-discrete signal

     • Sampling theorem:
       $$2f_{\text{Signal highest}}\le f_{\text{Sampling}}$$

  2. Quantization: Convert the level amplitude obtained by sampling into the corresponding digital value according to a certain grading scale, and round it up

     • Continuous level amplitude -> discrete digital quantity

  3. Coding: Convert the quantized result to the corresponding binary code

Analog data modulated into an analog signal

increase frequency

• frequency division multiplexing
• Make the most of your bandwidth

2.2 Physical layer transmission medium

Transmission medium and classification

transmission medium/transmission medium/transmission medium: The physical path between a sending device and a receiving device in a data transmission system

not the physical layer below the physical layer

guided transmission medium

• Electromagnetic waves are directed to travel along a solid medium (copper wire/optical fiber)【Train】

twisted pair

Two copper wires that are stranded side by side and insulated from each other with certain rules

• ancient, commonly used
• delivering electrical pulses

coaxial cable

• Strong anti-interference
• Transmission distance is longer
• expensive
• delivering electrical pulses

optical fiber

• transmit light pulse
• With pulse - 1; Without pulse - 0

Composed of a core (solid) and cladding, light waves are guided through the core

1. Small transmission loss, long relay distance, long-distance economy
2. Anti-lightning/electromagnetic interference
3. No crosstalk interference, good confidentiality, not easy to eavesdrop or intercept data
4. small size and light weight

unguided transmission medium

• free space
• The medium can be air, vacuum, sea water...

radio waves

Signal travels in all directions

• Strong penetrating ability
• communication field

microwave

The signal travels in a fixed direction

• High frequency
• Wide frequency range
• high data rate

Terrestrial microwave relay communication

Satellite Communications

• advantage:

  • Large communication capacity
  • long distance
  • wide coverage
  • Broadcast Communication and Multiple Access Communication

• shortcoming:

  • 250-270ms extension when propagating
  • Highly affected by climate (strong wind, sunspot outbreak, sun transit)
  • high bit error rate
  • high cost

infrared laser

The transmission signals are converted to their respective formats

2.3 Physical Layer Devices

repeater

regenerated digital signal

• Regenerate and restore the signal to keep the same as the original data
• The network portions at both ends are network segments, not subnets

5-4-3 rule

The signal delay range is specified

Hub (multi-port repeater)

Regeneration, amplification, forwarding signal (essence: reduction & regeneration)

• It is a shared device without directional transmission capability

• Cannot split conflict domains
  • Worker hosts connected to the hub share bandwidth equally

Summarize