preface

This blog is xie Xiren’s reading notes for the seventh edition of Computer Network.

1 Basic Concepts

The role of the physical layer: The physical layer considers how to transmit the data bit stream on the transmission media connecting various computers, rather than the specific transmission media. It should shield the differences between the transmission media and the means of communication as much as possible, so that the data link layer above the physical layer does not feel these differences. Data is mostly transmitted in parallel inside the computer, but the transmission mode of data on the transmission media is generally serial transmission, so the physical layer has to change the transmission mode.

The main task of the physical layer is to determine some characteristics related to the interface of the transport media, namely:

  • Mechanical properties: specify the shape and size of the connector used for the interface, the number of pins, etc.
  • Electrical characteristics: indicates the range of voltages that occur on each line of the interface cable.
  • Function: Indicates the significance of the voltage at a level present on a line.
  • Process features: Indicates the order in which various possible events occur for different functions.

2 Data Communication

2.1 Data communication system model

A data communication system can be divided into three parts, namely source system (or sender, sender), transmission system (or transmission network), destination system (or receiver, receiver).

Source systems include:

  • Source point: The source point device generates the data to be transmitted.
  • Transmitter: carries the digital bit stream generated by the source pointcodingAnd then transmit in the transmission system. The typical transmitter ismodulator.

The target system includes:

  • Receiver: Receives signals from the transmission system and converts them into information that can be processed by the destination device. The typical receiver ismodem.
  • Endpoint: The endpoint device receives a stream of digital bits from the receiver and outputs the information.

Classification of signals:

  • Analog signal (continuous signal) : Indicates that the values of the parameters of the message are continuous.
  • Digital signal (discrete signal) : The values of the parameters representing the message are discrete. The basic waveform representing different discrete values is calledelement. When using binary encoding, there are only two distinct codes, 0 and 1.

2.2 channel

The difference between circuit and channel: a communication circuit usually contains a sending channel and a receiving channel.

Ways of information interaction:

  • 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 neither of them can simultaneously send or receive messages.
  • Two-way simultaneous communication (full duplex communication) : The two communicating parties can send and receive messages simultaneously.

Baseband signal: signal from the source of confidence. Modulation: Baseband signals tend to contain more low-frequency components, and even dc components, and many channels do not carry this low-frequency or DC component. Therefore, the baseband signal must be modulated.

  • Baseband modulation: only the waveform of baseband signal is transformed, and the transformed signal is still baseband signal. Is the digital signal into another form of digital signal, so also known as coding.
  • Band modulationUse:carrierTo carry out modulation, move the frequency range of the baseband signal to a higher frequency band and convert it into an analog signal. The signal after carrier modulation is calledWith communication,.

Encoding mode:

  • Non-zero system: positive level represents 1, negative level 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 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.

Bandpass modulation methods: AM, FM and PM.

2.3 Ultimate capacity of channel

Factors that limit the rate of symbol transmission: (1) the frequency range that the channel can pass: in any channel, if the rate of symbol transmission exceeds the upper limit, there will be serious inter-symbol crosstalk, so that the receiver can not judge the symbol. Signal-to-noise ratio: the ratio of the average power of the signal and the average power of the noise, often recorded as S/N, and the decibel (dB) as a unit. Signal-to-noise ratio (dB) = 10lOglO(S/N)(dB)

Shannon formula, C is the channel limit information transmission rate, W is the channel bandwidth (Hz), S is the average power of the signal, N is the average power of the noise. The higher the channel bandwidth or the signal-to-noise ratio in the channel, the higher the limit transmission rate of information. The significance of Shannon’s formula is that as long as the transmission rate of information is lower than the limit of the channel, there must be some way to achieve error-free transmission. c = Wlog2(l+S/N)(bit/s)

3 Transmission Media

Transmission media is the physical path between transmitter and receiver in a data transmission system.

3.1 Bootstrap transmission media

Twisted-pair cable: On lans, twisted-pair cables are used as the transmission media.

Coaxial cable: Coaxial cable has good anti-interference characteristics and is widely used to transmit high rate data. Coaxial cable is mainly used in residential communities of cable television networks.

Optical cable: optical fiber communication is the use of optical fiber (optical fiber) transmission of light pulse to carry on communication, its transmission bandwidth is far greater than other kinds of transmission media.

  • Multi-mode fiber: Multiple rays of different incidence angles are transmitted in an optical fiber. The optical pulse propagates in multi-mode fiber gradually, causing distortion. Therefore, it is only suitable for short distance transmission.
  • Single-mode fiber: If the diameter of the fiber is reduced to just one wavelength of light, it allows the light to travel straight forward without multiple reflections.

3.2 Unbooted transport media

Non – guided transmission media: such as wireless, infrared, atmospheric laser. Used in short wave communication, microwave communication (divided into ground microwave relay communication and satellite communication).

Channel multiplexing technology

4.1 Frequency division multiplexing and TDM

Reuse divided into:

  • Frequency division multiplexing (FDM) : Users occupy different frequency bands at the same time.
  • Time division multiplexing (TDM) : Users occupy the same frequency band at different times.

Statistical time division multiplexing (STDM) : Statistical multiplexing is also known asAsynchronous TDMAnd ordinary TDM is calledSynchronous TDM. It is an improved TDM which can obviously improve the utilization of channel.concentratorThis technique is often used.

4.2 WDM

Wavelength division multiplexing (WDM) : Frequency division multiplexing of light. Optical fiber amplifier (EDFA) : Optical signals decay after a certain distance, so the attenuated optical signal must be amplified to continue transmission. EDFA directly amplifies the optical signal without photoelectric conversion.

4.3 Code division multiplexing

Code division multiplexing (CDM) is another way to share channels. Each user can communicate at the same time using the same frequency band. Because each user uses the different code type, therefore does not cause the interference between each user. The more common term is code division multiple access (CDMA).

5 broadband access technology

Users’ broadband access to the Internet includes:

  • Asymmetric digital subscriber lineADSL: Is to use digital technology to modify the existing analog telephone subscriber line to enable it to carry broadband digital services.
  • Fiber optic coaxial hybrid networkHFC: Based on the cable network.
  • FTTx: Optical fiber to…… .

In order to utilize fiber optic resources efficiently, passive optical network (PON) is widely used between fiber optic trunk lines and users. Passive optical network does not need to be equipped with power supply, and its long-term operation cost and management cost are low. The most popular passive optical networks are Ethernet passive optical Network (EPON) and gigabit passive optical network (GPON).