It has 2,090 words and takes about 15 minutes to read

2.1 Basic Concepts

The physical layer deals with how to transmit bitstreams of data over media that connect various computers, rather than the specific transmission media.

The main task of the physical layer is described as: determine some characteristics of the interface with the transmission media, namely:

Mechanical properties: example interface shape, size, number of leads.

Electrical characteristics: example specified voltage range (-5V~5V).

In this example, -5 v indicates 0, and +5V indicates 1.

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

2.2 Basic knowledge of data communication

2.2.1 Typical data communication model

2.2.2 Purpose of communication

The purpose of communication is to transmit messages.

1. Data — The entity that delivers the message.

Signal — the electrical or electromagnetic representation of data.

  • Analog signal: The values of parameters representing messages are continuous.
  • Digital signal: The values of the parameters representing the message are discrete (discontinuous).

In life we hear the sound is continuous, such as human voice, bird song and so on; A computer stores sound as a stream of discrete binary bits, which are sampled and then quantized.

3. Symbols — When a time-domain waveform is used to represent a digital signal, the basic waveform representing different discrete values is called a symbol.

4. Symbol length — in digital communication, the symbol of the same time interval is commonly used to represent a binary digit. The signal within such time interval is called binary symbol. This interval is called the symbol length. A bit can carry n bits of information.

2.2.3 Basic Concepts of Channels

A channel generally represents a medium that transmits information in one direction. So our normal communication line usually consists of a channel for sending information and a channel for receiving information.

Unidirectional communication (simplex communication) : there can be communication in one direction and no interaction in the other direction.

Two-way alternating communication (half duplex communication) : both communicating parties can send messages, but not simultaneously (and therefore not simultaneously).

Two-way simultaneous communication (full duplex communication) : The two communicating parties can send and receive messages simultaneously.

2.2.4 Baseband signals and band signals

(1) Baseband signal (that is, basic frequency band signal) : the signal from the source of confidence. Data signals, such as computer output representing various text or image files, are baseband signals. Baseband signals are the signals that are sent out to directly express the information to be output. For example, the sound waves we speak are baseband signals.

(2) Band signal: 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 (that is, only a segment of frequency range can pass the channel).

Because the attenuation of baseband signal is not big in the short range, so the signal content will not change, so when the transmission distance is relatively short, the computer network adopts baseband transmission mode. Such as from the computer to the monitor, printer and other peripherals is baseband transmission.

2.2.5 Common Encoding modes

Non-return to zero system: positive level represents 1, negative frequency represents 0.

Return to zero system: positive pulse represents 1, negative pulse represents 0.

Manchester code: the upward jump of the bit-period center represents 0, and the downward jump of the bit-period center represents 1. But you can also define it the other way around.

Differential Chester coding: there is always a jump at the center of each bit. The bit-start boundary with a jump represents 0, while the bit-start boundary with no jump represents 1. Stronger anti-interference ability.

2.2.6 Basic bandpass modulation methods

Amplitude modulation: The amplitude of the carrier varies with the baseband digital signal. For example, 0 or 1 correspond to no carrier or carrier output, respectively.

Frequency modulation: The frequency of the carrier varies with the baseband digital signal. For example, 0 or 1 correspond to the frequencies F1 or F2, respectively.

Phase modulation: The initial phase of the carrier varies with the baseband digital signal, for example, 0 or 1 corresponding to phase 0 degrees or 180 degrees, respectively.

2.2.7 Shannon formula

It is used to calculate the limit information transmission rate of the channel.

2.3 Transport media below the physical layer

2.3.1 Oriented transmission media

In a guided transmission medium, electromagnetic waves travel along the solid medium.

1. The twisted-pair cable

Shielded twisted pair cable: Improves the anti-electromagnetic interference capability of the twisted pair cable.

Unshielded twisted pair cable

Coaxial cable

50 ohm (unit) coaxial cable: used for digital transmission, also called baseband coaxial cable because it is mostly used for baseband transmission;

70 ohm (unit) coaxial cable: Used for analog transmission, i.e. broadband coaxial cable.

3. The optical fiber

Single-mode fiber: can only transmit a kind of electromagnetic wave mode, in the cable TV network used in the optical fiber is single-mode fiber, its transmission characteristics are good, broadband up to 10GHZ.

Multimode fiber: can transmit multiple electromagnetic wave modes.

2.3.2 Undirected transmission media

Refers to free space, in which electromagnetic wave transmission is called wireless transmission.

Wireless transmission uses a wide range of frequency bands.

Shortwave communication mainly relies on ionospheric reflection, but the communication quality of shortwave channel is poor.

Microwaves travel mainly in straight lines in space

  • Ground microwave relay communication
  • Satellite communications
2.3.3 hub

Working characteristics: It only plays the role of signal amplification and retransmission in the network, its purpose is to expand the transmission range of the network, but does not have the ability of signal directional transmission (sent signals can be received by the machine in the hub); Only one channel is allowed to communicate at a time, half duplex.

Maximum transmission distance: 100m

The hub is a large conflict domain.

2.4 Channel multiplexing technology

2.4.1 Frequency division multiplexing

After the user is assigned a certain frequency band, it occupies this frequency band from beginning to end in the communication process. All users of frequency division multiplexing occupy different bandwidth resources at the same time (the bandwidth here is the frequency bandwidth rather than the data transmission rate).

The process of modulation and demodulation of a modem:

Wavelength division multiplexing (WDM) is the frequency division multiplexing of light.

2.4.2 TDM multiplexing

TDM divides time into equal length TDM frames. Each TDM user occupies a fixed serial number of time slots in each TDM frame; The time slots occupied by each user occur periodically (the period is the time corresponding to the length of the TDM frame).

TDM signals are also called isochronal signals.

All users of TDM are occupying the same band width at different times.

Time multiplexing STDM statistics

Is a kind of improved TDM, it can obviously improve the utilization of channel, add the user’s address information in each time slot, mark the time slot belongs to which user.

2.4.3 Code division multiplexing

Code division multiplexing is another method of sharing a channel where each user can communicate using the same frequency band at the same time. Since each user uses a specially selected different code type, there is no interference between users. It has strong anti-interference ability.

CDMA can improve the voice quality and reliability of data transmission, and reduce the influence of interference on communication. Each station using CDMA is assigned a unique sequence of M bit bits. If a station sends a bit 1, it sends its own sequence of M bit slices. If bit 0 is to be sent, the binary inverse of the slice sequence is sent.

Disadvantages: it takes m bits to send 1bit. The larger the number of mobile phones is, the larger the m value will be.