Shulou(Shulou.com)06/01 Report--

1. Basic concepts of physical layer:

Figure out how to transfer data bit streams (rather than specific transport media) over the transport media that connect various computers!

2. The main tasks of the physical layer:

Determine some of the characteristics of the interface that transmits the media, namely:

Mechanical properties: such as interface shape, size, number of leads

Electrical characteristics: such as specified voltage range (- 5v to + 5v)

Functional features: for example,-5v means 0pm 5v means 1

Process characteristics: also known as procedural characteristics, which specify the working steps of the relevant components when establishing a connection.

3. Typical data communication model (physical level):

4. the basis of data communication:

Data (data): the entity that runs the message

Signal: electrical or electromagnetic representation of data

Analog signal: continuous value

Digital signal: value discretization

Symbol: basic waveform representing different discrete values (1 symbol can carry the amount of information of N bit)

Channel: a medium that sends information in one direction (commonly referred to as a communication line: 1 channel for sending information + 1 channel for receiving information)

One-way communication (simplex): communication in only one direction and no interaction in the opposite direction

Two-way alternating communication (half-duplex communication): both sides of the communication can send and receive information, but not both sides at the same time. Such as: walkie-talkie.

Two-way simultaneous communication (full-duplex communication): both parties can send and receive information at the same time. For example, the telephone.

Baseband signal (base band): a signal from a signal source (such as data signals from various text or image files output by a computer)

Passband signal (band pass): after the baseband signal is modulated by the carrier, the frequency range of the signal is moved to a higher frequency band to facilitate transmission in the channel.

When the transmission distance is close, the computer network adopts the baseband transmission mode, such as the signal from the computer to the monitor, printer and other peripherals.

Carrier modulation method:

Amplitude modulation (AM)

Frequency modulation (FM)

Phase modulation (PM)

Digital signal coding:

Unipolar non-return-to-zero code

Bipolar nonreturn-to-zero code

Unipolar return to zero code

Bipolar return to zero code

Manchester coding

Differential Manchester coding

Nessler's criterion: when there is no signal interference, in order to avoid inter-symbol interference, the transmission rate of symbols has an upper limit (applicable to analog signals).

Xianglong Formula (shannon): C = Wlog2 (1+S/N) bUnip s (applicable to: analog signal + digital signal)

5. Transmission media:

Directed to transport media:

Twisted pair: shielded twisted pair (STP), unshielded twisted pair (UTP, daily network cable)

Coaxial cable: 50 euros (for baseband transmission), 75 euros (for analog transmission)

Optical cable

Network cable: the same kind of straight-through line, different kind of cross cable

Optical fiber: through continuous total reflection

Single-mode optical fiber (non-refraction, direct transmission, such as digital television signals)

Multimode fiber (full refraction)

Undirected transmission media:

Terrestrial microwave relay communication

Satellite communication

Physical layer devices:

Hub: only amplifies and retransmits the signal in the network, the purpose is to expand the transmission range of the network, does not have the directional transmission ability of the signal.

Features:

1) maximum transmission range: 100m

2) it is a large conflict domain, which is basically not used now.

6. Channel multiplexing technology:

Frequency Division Multiplexing (FDM): different users occupy different frequencies at the same time. Such as: telephone

Time Division Multiplexing (TDM): the same band width, the user occupies different time (inefficient)

Statistical time Division Multiplexing (STDM): tagging data frames to distinguish data sources

Wavelength Division Multiplexing (WDM): frequency division administration of light (optical modulation, optical demodulation)

7. Digital transmission system:

Pulse modulated PCM:

North American T1 standard, European E1 standard, Chinese E1 standard

8. Broadband access technology:

XDSL (asymmetric transmission): leave the low spectrum of 0~4kHz to traditional phones and give users the high-end spectrum that they did not use on the Internet (using discrete multi-tone DMT technology to modulate and demodulate, frequency division multiplexing)

Fiber coaxial hybrid network (HFC): TV, telephone and network share CATV (cable television network, using frequency division multiplexing of analog data)

FTTx (Fiber to the Home, Building, roadside): fiber Optic

Note: this series of articles is to view the notes of Mr. Han Ligang's computer network basic course.

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