Basic knowledge of the physical layer of "computer network"

Chapter2: Physical layer key summary and record Physical Layer

The consideration is how to transmit the data bit stream on the transmission media connected to various computers, not the specific transmission medium.

The function of the physical layer is to shield the difference between different transmission media and communication means as much as possible.

The protocol used for the physical layer is also often referred to as the physical layer discipline

The main tasks of the physical layer:

Describes some of the features for determining the interface with the transport media :

• Mechanical properties indicate the shape and size of the connector used in the interface, number of leads and arrangement, fixation and locking devices, and so on.
• Electrical characteristics indicate the range of voltages that appear on each line of the interface cable.
• Functional characteristics indicate the meaning of the voltage at a certain level on a line.
• Process characteristics indicate the order in which various possible events for different functions are occurring.

Important Concepts

Data-the entity that transports the message. (The message is specific information)

Signal (signal)--the electrical or electromagnetic performance of the data.

Analog signal (analogous signal)-the value of the parameter representing the message is continuous. For example, when we speak, the sound size is continuously changing, so the sound wave that transports the voice message is the analog data, and the voice signal on the telephone line is the analog signal.

Digital signal-The value of the parameter representing the message is discrete. For example, the network card on the computer sends 0100110 form of data is digital data, the corresponding signal transmitted on the cable is a digital signal.

Unidirectional communication ( single-work communication)-there can be only one direction of communication without the opposite direction of interaction.

Bidirectional alternating communication ( half-duplex communication)-both parties to a communication can send information, but not both (and certainly not simultaneously).

Two-way simultaneous communication ( full-duplex communication)-both sides of the communication can send and receive information at the same time.

Baseband signal (i.e. basic band signal)

? A signal from a source.

? Baseband signal is discrete, that is, digital signal, is the digital signal 1 or 0 directly with only two different voltage waveforms to represent, and then served on the line up. The waveform contains only two or several different discrete voltage values

? Example: Computer output of various text images of the data signal are baseband signal.

? Disadvantage: Baseband signal often contains more low-frequency components, and even DC components, and many channels can not transmit this low-frequency component or DC component, the baseband signal must be modulated.

Modulation classification:

• Baseband modulation: Only the waveform of the baseband signal is transformed so that it can be adapted to the channel characteristics. The transformed signal is still a baseband signal (still a digital signal). This process is called coding (coding).
• Band-Pass modulation: The use of carrier (carrier) modulation, the frequency range of the baseband signal is moved to a higher band, and converted to analog signal , so that it can be better in the analog channel transmission (i.e. only within a certain frequency range can pass the channel). The signal after the carrier modulation is called the band Communication number .

Generally speaking, baseband modulation is called coding, and the modulation is referred to as band-pass modulation.

Transfer Media

Mainly guided and non-guided transmission media.

Guide type: Twisted pair:

It is divided into unshielded twisted pair and shielded twisted pair wire.

Features: Inexpensive, anti-jamming, can be used to transmit analog signals and can be used to transmit digital signals.

Coaxial cable:

Typically 50 ohms and 75 ohms.

Coaxial cable has good anti-jamming characteristics, is widely used to transmit higher rate of data, but also can be digital signal/analog signal.

Optical cable:

Optical fiber is the transmission medium of optical fiber communication. It is divided into multimode fiber and single-mode fiber.

The transmission bandwidth is much larger than the current bandwidth of various other transmission media.

Non-guided type:

Poor confidentiality.

Mainly shortwave and microwave communication:

HF Communication:

Mainly by the ionospheric reflection, but the short-wave channel communication quality is poor.

Microwave communication:

Radio- Microwave communication plays an important role in data communication

Microwave in space is mainly a straight line propagation.

Microwave communication is mainly base station direct-viewing and satellite communication, anti-jamming is good.

Bluetooth technology features-frequency hopping technology: security.

Re-use:

refers to allowing users to use a shared channel to communicate, reduce costs and improve utilization.

Frequency Division Multiplexing FDM: All users consume different bandwidth resources at the same time (note that the "bandwidth" here is the frequency bandwidth rather than the sending rate of the data).

Time Division multiplexing TDM: is divided into a segment of the length of the division multiplexing frame (TDM frame). Each time-division multiplexing user occupies a fixed sequence number of slots in each TDM frame. The time slots occupied by each user occur periodically (their period is the length of the TDM frames). The TDM signal is also known as a isochronous signal. All users of time division are occupying the same band width at different times.

Time division multiplexing may result in wasted line resources.

Statistics Time Division multiplexing STDM: The STDM frame is not a fixed-allocation time slot, but a time slot that is dynamically allocated on demand. So statistical time division multiplexing can improve the utilization of the line.

Wavelength Division multiplexing is the frequency division multiplexing of light. Use an optical fiber to transmit multiple optical carrier signals simultaneously.

Code Division multiplexing CDM:Code Division Multiple Access CDMA

This system sends the signal to have the very strong anti-jamming ability, its spectrum resembles the white noise, is not easy to be discovered by the enemy.

Chip sequence (chip sequence):

Each bit time is divided into m short intervals, called chip (Chip).

Each station is assigned a unique m-bit sequence, such as sending bit 1, to send its own m-bit sequence of chips. If send bit 0, then send the code chip sequence of binary anti-code. The calculation represents 0 in-1.

The sequence of patches allocated for each station must not only be different, but must also be orthogonal to each other (orthogonal).

In a practical system, pseudo-random code sequences are used.

The data in the channel is the overlay of the data sent by each station.

Two different stations of the chip sequence orthogonal, is the vector S and t of the normalized inner product (inner product) equals 0

Any piece of code vector and the code slice vector own normalized inner product all is 1.

The normalized inner product value of any vector of a chip vector and the code-back code is –1.

The normalized inner product of any vector of a chip vector or other vector or other code-chip inverse code is 0.

Set a station to receive the number of stations sent to the code stack value is: M (m1,m2,m3,... MM), ask S Station what data to send: S. M, if 1 is sent 1, if 1 then S is sent 0, if 0 s does not send data.

Above is the analog transmission system, below:

Digital transmission System

Compared with analog communication, digital communication has obvious advantages both in transmission quality and economy.

Pulse coded modulation

Pulse code modulation is a time continuous, the value of continuous analog signal transformed into time discrete, the value of discrete digital signal after transmission in the channel.

The analog signal is sampled first, and then the magnitude of the sample value is quantified .

Sampling theorem:

During the conversion process of the analog/digital signal, when the sampling frequency is more than twice times the highest frequency in the signal, the digital signal after sampling retains the information in the original signal intact.

Quantization:

Amplitude is still continuous sampling signal to carry out amplitude dispersion, that is, specify the level of M, the sampling value with the closest specified level of the process of representation is called quantization. This finite level is called the quantization level.

relationship with sampling: sampling is a signal that transforms a time continuous signal into time discrete, while quantization is a sequence of sampled values that take a continuous sampling of values into discrete values.

Quantization Noise: quantization error generated by quantization

The old digital transmission system has many drawbacks:

• Rate standards are not uniform

• Not synchronous transfer

Asynchronous communication and Synchronous communication: Asynchronous communication:

Asynchronous communication refers to the sending and receiving of communication devices using their respective clocks to control the transmission and reception of the data. In order to reconcile the two sides, the sending and receiving device's clock is required to be as consistent as possible.

The characteristics of asynchronous communication: Do not require the two sides of the clock strict consistency, easy to achieve, the equipment cost is small, but each character to append two to three bits for the start and end position, there are gaps between the frames, so the transmission efficiency is not high. Asynchronous communication is transmitted in characters (frames), and the gap (interval) between characters and characters is arbitrary, that is, the characters are asynchronous, but everyone in the same word characters is synchronous.

Synchronous communication:

Synchronous communication is to establish the sender clock to the receiver clock direct control, so that both sides reached full synchronization. At this point, the distance between the bits of the transmitted data is an integer multiple of the "bit interval", while the transmitted characters do not leave gaps between them, that is, the synchronization relationship is maintained, and the character synchronization is maintained. The sender's synchronization with the receiver can be achieved in two ways: out-of-sync and self-synchronizing. Left and right graphs:

Synchronous fiber network SONET and synchronous digital series SDH:

Synchronous fiber-optic network SONET (synchronous Optical Network) All levels of clocks come from a very precise master clock. n defines the line rate hierarchy of synchronous transmission for optical fiber transmission systems,

Synchronous Digital Series SDH: It is generally believed that SDH is basically the same as SONET. The SDH standard is adopted in our country.

China Digital Synchronous Network SDH (synchronous digital Hierarchy, synchronous numeral system), the main technology: multi-benchmark clock, partition level master-slave synchronization mode.

The meaning of the SONET/SDH standard:

• So that the different digital transmission system in the STM-1 level has been unified.
• For the first time, the world standard of digital transmission system was truly realized.
• has become a recognized new generation of ideal transmission network system.
• The SDH standard is also suitable for microwave and satellite transmission of the technical system.

ADSL Technology of broadband access technology

Digital technology is used to transform the existing analog telephone subscriber line so that it can carry the broadband service. ADSL technology to the 0~4 kHz low-end spectrum to the traditional telephone use, and the original unused high-end spectrum left to users to use the Internet.

DSL is the acronym for Digital Subscriber Line.

The upstream and downstream bandwidths are asymmetric. Uplink refers to from the user to the ISP, while the downlink refers to the user from the ISP.

ADSL Technology Features:

• Make full use of the installed large number of local twisted copper wire, to maintain resources.
• The use of advanced DMT line coding, anti-crosstalk and other interference is strong, and according to the length of the line, the noise situation adaptively select the transmission rate.
• The traditional pots and ADSL services can be transmitted on a single line, and they do not affect each other.
• Since each twisted pair is unique to each ADSL user, ADSL bandwidth is also exclusive to each ADSL user, not bandwidth sharing.
• Because ADSL transmission reliability is relatively low, and transmission rate is limited , it is mainly suitable for home users and small and medium-sized business users.
• ADSL can get the highest data transmission rate and the actual user line of the signal-to-noise ratio is closely related, in fact, small.

ADSL installs an ADSL modem on both ends of the user line (copper wire). The current scheme used in China is discrete multi-tone DMT (discrete multi-tone) modulation technology. Here the "multi-tone" is the meaning of "multicarrier" or "multi-sub-channel".

DMT Technology:

The DMT modulation technique uses a frequency division multiplexing method that divides the high-end spectrum of up to 1.1 MHz up to a number of sub-channels, of which 25 sub-channels are used for the upstream channel, while 249 sub-channels are used for downlink channels.

Each sub-channel occupies a 4 khz bandwidth (strictly 4.3125 khz) and is digitally modulated using different carriers (i.e. different tones). This approach is equivalent to using many small modems to transmit data in parallel on a pair of user lines.

Second generation ADSL:

Higher data rates are obtained by increasing modulation efficiency.

Fiber Coaxial Hybrid Network HFC (Hybrid Fiber Coax):

NHFC Network is a kind of resident broadband access network developed on the basis of CATV network with wide coverage.

The main line of HFC network uses optical fiber. n the coaxial cable is below the fiber junction point.

HFC Network has two-way transmission function and extends the transmission band.

FTTx Technology:

NFTTX is a scheme to realize broadband residential access network, which represents a variety of broadband optical fiber connection methods.

FTTx says Fiber to the ... (Fiber optics to ... User, community, roadside. ）

Basic knowledge of the physical layer of "computer network"