"Computer Network Principles" Chapter 3 Data Communication Technology

3.1 Overview

3.2 Theoretical Basis of Data Communication

main content

• The mathematical representation of a signal and the constraints it imposes on it as it travels over a communication channel.
• The transmission medium uses changes in physical quantities such as voltage, current, and optical signals to transmit binary bit streams.
• Voltage, current, etc. can be expressed as a single-valued function f(t) of time
• In this way, the change of the signal can be described mathematically and analyzed mathematically
  

3.2.1 Fourier Analysis

• Fourier proves that any normal function g(t) with period T can be composed of an infinite number of sine and cosine functions
• 
  

3.2.2 Spectrum characteristics of periodic rectangular pulse signal

• The attenuation is different for different Fourier components, thus causing distortion of the output
• The more harmonics that pass through the channel, the more realistic the signal
• 
  

3.3 Data Communication System Model

3.3.1 Basic Structure of Data Communication System

Various transmission methods of data on communication channels and the technologies used

• Basic structure of data communication system
  • 
• The tasks of the data communication system
  • The data carrying the information is transmitted to the destination through the medium (channel) in the form of a physical signal
  • Information and data cannot be transferred directly on the medium
  • Solution: information (raw) -> data (storage) -> signal (transmission on medium)
    

3.3.2 Data and Signals

• data representation
  • Analog data continuous value
  • Numeric Data Discrete Values
• data transfer method
  • analog signal
  • Digital signal
• Signaling method
  • Analog Signaling (Analog Channel)
  • Digital signaling (digital channel)
• Data synchronization method
  • Synchronization means that the receiving end receives data strictly according to the repetition frequency and the start and end time of each symbol sent by the sending end, that is, the time base must be consistent.
  • According to the different objects to be synchronized, it can be divided into:
    • bit synchronization
      • 
    • character synchronization
      • There are two ways of character synchronization: asynchronous and synchronous
        • Asynchronous character synchronization
        • 
        • synchronous character synchronization
        • 
    • frame sync
• Sending of analog and digital signals
  • Analog Signaling (Analog Channel)
    • 
  • Digital signaling (digital channel)
    • 
      

3.3.3 Channel Communication Mode

In order to meet different needs, the communication lines adopt different connection methods

• point-point method
  • 
• multipoint mode
  • 

way of communication

• From the perspective of the relationship between information transmission direction and time
  • simplex communication
    • Features: Information can only be transmitted in one direction, and monitoring signals can be sent back.
    • 
  • half-duplex communication
    • Features: Information can be transmitted in two directions, but at a certain moment it can only be transmitted in one direction.
    • 
  • Full duplex communication mode
    • Features: Information can be transmitted in both directions at the same time, generally using a four-wire structure.
    • 
      

3.3.4 Data transmission method

• Baseband Transmission and Frequency Band Transmission
  • **Baseband signal: **The original electrical signal sent by the source without modulation
  • The transmission method that directly sends the baseband signal to the communication line is called baseband transmission.
    • The signal transformed by the analog signal source is called the analog baseband signal
    • The binary signal generated by the computer is called the digital baseband signal
  • The transmission mode that sends the baseband signal to the communication line after modulation is called frequency band transmission .
• Baseband transmission of digital data
  • Baseband transmission : directly use the baseband signal when transmitting
    • Baseband transmission is the most basic transmission method, generally low level 0 high level 1
    • Applicable to all situations at low and high speeds
    • Because baseband signals occupy a wide frequency range, there are certain requirements for transmission lines
  • Core content : coding method
• Analog transmission of digital data (band transmission)
  • Frequency band transmission : Refers to carrier transmission on lines within a certain frequency range . The carrier is modulated with a baseband signal to make it suitable for transmission on the line .
  • Modulation : Use the baseband pulse to control some parameters of the carrier signal so that these parameters change with the baseband pulse.
  • Demodulation : Modulated Fan Conversion
  • Modem MODEM
    • The modulator is a waveform converter that converts the waveform of the baseband digital signal into a waveform suitable for analog channel transmission. (do not change data content)
    • The demodulator is a waveform recognizer , which restores the analog signal transformed by the modulator into Yuankai's digital signal, and if the recognition is incorrect, a bit error will occur.
• broadband transmission (optical fiber)
  

3.4 Transmission medium

• Classification of transmission media
  • wired media
    • Coaxial cable, twisted pair, optical fiber, etc.
    • Features: Need wiring, good anti-interference performance.
  • wireless medium
    • Various forms of transmission through the atmosphere
    • microwave, infrared, satellite, etc.
    • Features: no wiring required, poor anti-interference
• Choice of transmission medium
  • safety
  • electromagnetic interference
  • cost
  • speed
  • Signal attenuation
    

3.4.1 Electromagnetic Spectrum

The basic relationship between the frequency f of electromagnetic waves, the wavelength D, and the propagation speed c in vacuum:
DF=C

3.4.2 Twisted pair

• Consists of two insulated wires arranged in a helical configuration. The wire is copper wire or copper clad steel.
• Twisted pair cables can transmit both analog and digital signals .
• The specific bandwidth depends on the thickness of the copper wire, the transmission distance and the technology used
• Twisted pair can be divided into: STP shielded twisted pair and UTP unshielded twisted pair .
  • 
• More point-to-point connections are used.
• Anti-interference performance depends on proper shielding and twisting of the wire pairs, which is close to coaxial cable at low frequency transmission.
  

3.4.3 Coaxial cable

• Coaxial cable is a hollow outer conductor surrounded by an inner conductor .
• Coaxial cables are classified according to impedance:
  • 
• Usually multi-point connection
• Anti-interference and price are between twisted pair and fiber optic.
  

3.4.4 Fiber

• Consists of three concentric parts: core, cladding and jacket.
• Optical fibers can be composed of plastic, glass or ultra-high purity silica glass.
  • The loss, transmission distance and price of optical fibers made of different materials are also different
• Optical fibers represent binary 0 and 1 through the presence or absence of optical signals .
• The sender needs electro-optical conversion equipment, and the receiver needs photoelectric conversion equipment
• Optical cables are directly used in wiring, and an optical cable is composed of multiple optical fibers.
• 
• Classification of optical fiber
  • single mode fiber
  • multimode fiber
  • Mode: It is a quantity related to many parameters, which can be understood as the polarization direction. Single-mode fiber can transmit multiple wavelengths, but each wavelength can only have one mode.
  • Three commonly used wavelength windows
    • 
  • 
    

3.4.5 Wireless medium

• electromagnetic spectrum
• radio transmission
  • There is a wireless link between the fixed terminal point (base station) and the terminal
  • 
• microwave transmission
  • 
• Infrared and mmWave
• light wave transmission
• Satellite Communications
  • 
    

3.5 Data Encoding

3.5.1 Signal encoding

• encoding of digital data
  • 
  • Non-return-to-zero encoding (NRZ) low 0 high 1
    • Disadvantages: It is difficult to define the start of the data bit, and the DC component may cause damage to the connection point.
  • Manchester encoding
    • The middle jump of each bit, falling 1, rising 0 or vice versa
  • Differential Manchester encoding
    • The middle jump of each bit, there is a jump of 0, and there is no jump of 1
      

3.5.2 Modulation and coding

• Common Modulation Techniques
  • According to the three characteristics of the carrier: amplitude , frequency , and phase , three commonly used modulation techniques are generated:
    • 
    • Amplitude Shift Keying ( ASK ) AM
      • 
    • Frequency Shift Keying ( FSK ) FM
      • 
    • Phase shift keying ( PSK ) phase modulation
      • 
        

3.5.3 Digital encoding of analog data

• Solve the problem of digitization of analog signal during digital transmission of analog data
  • Also known as Pulse Code Modulation PCM
  • Also sampled according to the Nyquist principle
  • Divide the analog signal amplitude into multiple levels (2^n), each level is represented by n bits
• Commonly used PCM techniques
  • Differential Pulse Code Modulation
    • Principle: Instead of digitizing the amplitude value, it encodes according to the difference between the two sampling values ​​before and after, and outputs a binary number
  • Example of PCM conversion process
    • sampling-quantization-encoding
    • 
  • PCM conversion waveform diagram
    • 
      

3.6 Data Communication Performance Index

3.6.1 Latency

• In computer networks, latency refers to the time required for a data block (frame, packet, segment) to be transmitted from one end of a link or network to the other.
• Latency includes
  • sending delay
  • propagation delay
  • forwarding delay
    • queuing delay
    • access delay
    • processing delay
• The delay-bandwidth product is the product of propagation delay and bandwidth: delay-bandwidth product = propagation delay * bandwidth
• The delay-bandwidth product is also called the bit length , that is, the link length in bits
  

3.6.2 Transfer rate

Information transfer rate and symbol transfer rate

• The information transmission rate refers to the binary digits of the digital data before encoding transmitted per second , and the unit is bit/second , that is, b/s, bps.
  • Information transfer rate is also called bit rate
  • In computer networks, another term that has the same meaning as the rate of information transfer is called bandwidth
• The transmission rate of the transmission signal on the channel after digital data is line-encoded is called the symbol transmission rate , which refers to the number of symbols transmitted per second, that is, the number of times the transmission signal changes per second, and the unit is baud/second ( baud/s)

Baud Rate and Bit Rate

• Baud rate RB
  • The number of times the signal changes per second, also known as the modulation rate
• bit rate Rb
  • number of bits transferred per second
• A new network can often carry multiple binary bits, so at a fixed information transmission rate, the bit rate is often greater than the baud rate. Multiple bits can be transmitted in one symbol.
• Rb = RB log2 V (V is the number of levels)
• Coding efficiency = Rb/RB
  

3.6.3 Reliability

• BER
  • The bit error rate refers to the probability that the transmitted data is transmitted incorrectly
• Bit error rate = number of wrong bits transmitted/total number of bits transmitted
  • Frame Error Rate, Packet Error Rate
    

3.6.4 Channel limit capacity

• As early as 1924, Nyquist recognized this fundamental limitation and derived an expression for the maximum data transfer rate for a noise-free channel of limited bandwidth ;

• Nyquist proved that if an arbitrary signal passes through a low-pass filter with a bandwidth of H , then 2H samples per second can completely reproduce the signal passing through this filter.

• In 1948, Shannon further extended Nyquist's results to channels affected by random (dynamic) noise .

• Nye's Criterion: For Ideal Low Pass Channels

  • 
  • The Nyquist formula provides a basis for estimating the highest rate of a noise-free channel of known bandwidth.

• Shannon's theorem: Gaussian noise interferes with the channel

  • 

• Comparison of Nye's Criterion and Shannon's Theorem

  • C = 2H log2V This formula shows that the data transmission rate C increases as the number of signal encoding stages increases.
  • C = H log2(1+S/N) No matter how high the sampling frequency is and how many levels the signal encoding is divided into, this formula gives the highest transmission rate that the channel can achieve. That is, the existence of noise will make it impossible to increase the number of encoding stages infinitely .
    

3.7 Channel multiplexing technology

Since the capacity of a transmission line far exceeds the capacity required to transmit a user signal, in order to improve line utilization, multiple signals are often allowed to share a physical line at the same time .
Common methods:

• TDM
• Frequency Division Multiplexing FDM
• WDM
• code division multiplexing CDM
  

3.7.1 Frequency division and time division multiplexing

• Frequency Division Multiplexing FDM
  • When the bandwidth of the transmission medium is greater than the requirements of a single signal, in order to effectively utilize the transmission system, the technology of transmitting multiple signals on one transmission line at the same time is frequency division multiplexing .
  • Realization of FDM
    • Modulate the frequency of different signals to different frequency ranges by modulation
    • Synthesize multiple signals into one signal with a larger frequency range for transmission
    • On the receiving side , the signal is restored to multiple signals through demodulation
    • 
• Time Division Multiplexing TDM
  • When the bit transmission rate of the transmission medium is greater than the requirement of a single signal, in order to effectively utilize the transmission system, the technology of transmitting multiple signals on the same line at the same time is called time division multiplexing .
  • 
  • Method to realize:
    • Divide time into equal time slices during transmission
    • The time slices are allocated to the specified signals in sequence through the time slice rotation method;
    • The receiving side also receives the specified signals sequentially in the specified time slots by means of time slice rotation .
      

3.7.2 Statistical time division multiplexing

• synchronous time division multiplexing
  • 
• Asynchronous (statistical) time division multiplexing
  • 
    

3.7.3 Wavelength Division Multiplexing

Wavelength Division Multiplexing WDM

• The entire wavelength band is divided into several wavelength ranges, and each user occupies one wavelength range for transmission.
• 
  

3.7.4 Code Division Multiplexing

Code Division Multiplexing CDM

• Connotation is equivalent to CDMA
• Allow multiple users to communicate using the same frequency band at the same time;
• Each user uses a different code pattern that has been specially selected
• Strong anti-interference ability
• Effectively increase the communication capacity of the system
• It was originally used for military communications, and has been widely used in civilian mobile communications, especially in wireless local area networks , as the price and volume of CDMA equipment have dropped significantly .
• How CDMA works
  • The bit time is further divided into m (or 64 or 128) short time segments called chips
  • Each station is assigned a unique m-bit chip sequence
    • Send bit 1, then send m-bit chip sequence
    • Send bit 0, then send the one's complement of the chip sequence
  • Any two chip sequences (S, T) must satisfy the orthogonal relationship
    • 
  • 
    

3.8 Digital transmission system

3.8.1 PCM system

• 
• E1 = 2.048Mbit/s
• T1 = 1.544Mbit/s
• 
  

3.8.2 SONET and SDH

There are many shortcomings in the current digital transmission multiplexing rate, the most important of which are the following two aspects: the rate standard is not uniform and the transmission is not synchronous ;

• In order to solve the above problems, the United States first introduced a digital transmission standard in 1988, called **Synchronous Optical Network SONET** (Synchronous Optical Network).
  • SONET architecture
    • 
• Based on the SONET standard of the United States, ITU-T formulated the **international standard synchronous digital series SDH** (Synchronous Digital Hierarchy), that is, three recommendations including G.707~G.709 passed in 1988.
  

3.9 Data Exchange Technology

Switching : In a multi-node communication network, in order to effectively utilize communication equipment and lines , it is generally desirable to dynamically set the lines between the two parties in communication, and dynamically connect or disconnect the communication lines, which is called " switching ".
Classification of exchange methods:

• circuit switching
• storage exchange
  • message exchange
  • packet switching
  • cell switching
• mixed exchange
• 

• **电路交换                      报文交换               分组交换**
  

3.9.1 Circuit switching

Directly use the switchable physical communication line to connect the communication parties

• three phases:
  • build a circuit
  • transfer data
  • Remove the circuit
• main feature:
  • Before sending data, a point-to-point temporary , dedicated physical path must be established
  • The time to establish a physical path is longer, and the data transmission delay is shorter
    • e.g. telephone network
    • 
      

3.9.2 Message exchange

• principle
  • Information is stored and forwarded in units of messages (logically complete information segments)
  • 
• Features:
  • High line utilization
  • Requires intermediate nodes (network communication devices) to have a large buffer
  • long delay
    

3.9.3 Packet switching

• principle
  • The information is stored and forwarded in units of packets . The source node divides the message into packets, stores and forwards them at the intermediate node, and the destination node synthesizes the packets into messages.
  • Packet: A segment of information smaller than a message, usually with a maximum length limit
  • Cell: A fixed-size segment of information
• Features:
  • No pre-allocation of resources in network node devices
  • High line utilization
  • High node memory utilization
  • Easy to retransmit, high reliability
  • Easy to start new transmissions, allowing urgent messages to pass through first
  • Additional information added
• Packet switching is divided into datagrams and virtual circuits
  • Datagram
    • Each packet is routed independently
    • Suitable for transmitting a small amount of packets, eliminating the call establishment process, fast
    • Can handle congestion better
    • more reliable
    • 
  • virtual circuit
    • All packets are routed only once
    • Sending packet money requires the establishment of a virtual circuit
    • Compared with datagrams, the network is difficult and unreliable to deal with congestion
    • 
      

3.10 Physical layer procedures

3.10.1 DTE and DCE

Data Terminal Equipment DTE

• Refers to terminal devices such as data input/output equipment, terminal equipment or computers with certain data processing capabilities and transceiver capabilities .

Data Communication Equipment DCE

• Refers to the collection of automatic call answering equipment, switches and other intermediate devices , whose role is to provide signal conversion and encoding functions between DTE and transmission lines , and is responsible for establishing, maintaining and releasing data link connections .

3.10.2 Physical layer interface standard

• ISO/OSI definition of physical layer
  • The physical layer provides mechanical, electrical, functional and procedural characteristics for the purpose of initiating, maintaining and closing physical connections between data link entities for bit transfer . This connection may pass through a relay system , and the transmission within the relay system is also at the physical layer.
  • Functions of the physical layer
    • Provides transparent bitstream transfer between two network devices .
  • research content
    • The startup and shutdown of physical connections, normal data transmission, and maintenance management.
• Four characteristics of the physical layer
  • mechanical properties
    • Mainly define the boundary point of the physical connection , that is, the plug-in device. Specifies the specification, number and arrangement of pins used in the physical connection.
    • Commonly used standard interface
      • ISO 2110, 25-pin connector, EIA RS-232-C, EIA RS-366-A
      • ISO 2593, 34-core connector, V.35 broadband MODEM
      • ISO 4902, 37-pin and 9-pin connectors, EIA RS-449
      • ISO 4903, 15-core connector, X.20, X.21, X.22
  • electrical characteristics
    • When specifying the transmission of binary bits, the voltage level, impedance matching, transmission rate and distance limit of the signal on the line.
    • Earlier standards defined electrical characteristics at boundary points, such as EIA RS-232-C, V.28; more recent standards describe the electrical characteristics of transmitters and receivers and give control over the connecting cables.
    • CCITT standardized electrical characteristics standards
      • CCITT V.10/X.26: New Unbalanced Electrical Characteristics, EIA RS-423-A
      • CCITT V.11/X.27: New Balanced Electrical Characteristics, EIA RS-422-A
      • CCITT V.28: Unbalanced Electrical Characteristics, EIA RS-232-CCCITT X.21/EIA RS-449
  • Features
    • Mainly define the function of each physical line.
    • The functions of the lines are divided into four categories:
      • data
      • control
      • timing
      • land
  • procedural characteristics
    • Mainly define the working procedure and timing relationship of each physical line.
      

3.10.3 EIA-232

• EIA-232-E is a well-known physical layer asynchronous communication interface standard formulated by the American Electronics Industry Association EIA
• It was the earliest standard RS-232 formulated in 1962, in which RS represents a " recommended standard " of EIA, and 232 is the serial number.
  

Compare the pros and cons of analog and digital communication

• Analog communication , the technology is very mature, is to modulate the [analog signal] and [carrier] to make it [with certain carrier characteristics] without losing the uniqueness of the analog signal, and the receiving end passes through the [low-pass filter] , restore the original analog signal.
• For digital signals , first sample, [encode] the [sampling] amplitude, then [modulate], phase shift keying, etc., and restore it at the receiving end.
• The difference is that
  • Because digital communication transmits digital sampling signals, it can be restored at the receiving end, so the signal transmission rate is high and the distance is long.
  • The analog signal is the [direct modulation] of the signal, which is multiplied by the carrier. When there is interference during transmission, the impact on the system is [irreparable], so it causes [distortion].
  • Relatively speaking, digital communication is better than analog communication .

How to use the voice channel to transmit computer data?

Answer: It needs to go through three steps [PCM pulse code modulation]

• Sampling: Sampling the speech signal at a certain interval
• Quantization: round each sample to the nearest quantization level
• encoding: encode each rounded sample

The coded signal is called PCM signal and can be transmitted through the voice channel.

Try to compare the characteristics of circuit switching, message switching, virtual circuit switching and datagram switching

• Message exchange does not establish a dedicated link, and the line utilization rate is high. Due to [the difference between different messages may be quite different], the transmission [delay is large] and the transfer node [buffer management is inconvenient].
• Both datagrams and virtual circuits are packet switched, and a packet is a message [with a maximum length limit].
• Datagram switching is completely analogous to message switching.
• Virtual circuit switching, similar to circuit switching, differs from circuit switching in that a logical connection does not mean that other communications cannot use this line. It still has the advantage of [line sharing].
• The difference between virtual circuit and datagram:
  • A virtual circuit means reliable communication, which involves more technology and requires greater overhead.
  • It is not as flexible as the datagram method, and the efficiency is not as high as the datagram method.
  • Virtual circuits are suitable for interactive communication , and private datagrams are more suitable for one-way transmission of short messages. **
• Circuit switching requires the establishment of a clear physical circuit between the sender and receiver, and the circuit resources are exclusive to the current session
  

Please draw the waveform diagram of **011000101111** non-return-to-zero encoding, Manchester encoding and differential Manchester encoding

Now it is necessary to send a series of computer screen images over a fiber optic cable. The resolution of the screen is 480 640 pixels with 24 bits per pixel. There are 60 screen images per second. I would like to ask: How much bandwidth do you need? At a wavelength of 1.30 μm, how many μm wavelengths are required for this bandwidth?

Why is the PCM sampling time set to 125μs?

Compare the latency of sending a message of x bits along a path of k hops on a [circuit-switched] network versus a lightly loaded [packet-switched] network. Suppose the circuit establishment time is s seconds, the propagation delay of each hop is d seconds, the packet size is p bits, and the data transmission rate is b bps. May I ask under what conditions the delay of the packet network is relatively short?

Answer: For circuit switching, the circuit is established when t=s, the last bit of the message is sent when t=s+x/b, and
the message For packet switching, the last bit is sent at t=x/b, in order to reach the
final destination, the last packet must be forwarded k-1 times by the intermediate router, each forwarding time is p/b, so the total The total delay is x/b+(k-1)p/b+kd.
To make packet switching faster than circuit switching, the condition x/b+(k-1)p/b+kd< s+x/b+ must be satisfied kd, that is, s>(k-
1)p/b.

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