Computer Network Technology and Application

Chapter 6 Fundamentals of Network Technology (1)
This chapter will start with communication technology, and introduce communication-related switching technology, synchronization technology, and multiplexing technology respectively, laying the foundation for the content of the following chapters.In the second half of this chapter, we will introduce physical transmission media, that is, different kinds of communication lines.

【Main content of this chapter】

The theoretical basis of data communication mainly includes the communication system model, data communication and channel technical indicators.

Data transmission technology, including signal modulation, multiplexing technology, data exchange technology and communication methods.

Transmission media, including twisted pair, coaxial cable, fiber optics, and wireless media.

2.1 Basic knowledge of data communication
How does one computer pass information to another computer?What rules or techniques are used?What equipment and lines are needed?What problem to solve?We know that the information in the telephone system is transmitted continuously with sine or cosine waveform signals; however, we also know that according to the basic principles of computers, data exists in the form of 0 and 1 in computers, and is transmitted as pulse signals. So how is the signal sent in the computer transmitted to the remote computer through the telephone line?
2.1.1 Data and Signals

1. Data
Data is a description of objective things, and it is a form of expression after normalizing and abstracting certain attributes of things. It can be recognized and described, such as decimal numbers, binary numbers, characters, etc.The data has the following properties:
(1) Data not only refers to numerical data in a narrow sense, but also describes objective things.It is the carrier and specific form of information.

(2) Data can be expressed in a variety of forms, including not only numbers and text that we are familiar with, but also graphics, images, and sounds.

(3) There are two forms of data: analog data and digital data. The analog data is continuous. For example, the data described by the sine function is analog, and the integer is discrete digital data.

2. Signal
A signal is a specific physical representation of data with a definite physical description.Such as voltage, magnetic field strength, etc.Data transfer always depends on certain physical signals.For example, when playing the flute, the sound made by the mouth is actually the information transmitted by the sound wave signal.When data is transmitted in a computer communication network, a binary value (0 or 1) voltage sequence signal is transmitted on the communication line, and if it is transmitted by optical fiber, the data is transmitted in the form of an optical signal.

There are two kinds of signals: analog signal and digital signal.Using a carrier instrument to detect, the analog signal is a continuous waveform, while the digital signal is a pulse waveform.

3. Coding classification of signals and data
Depending on the type of signal and data, there are four coding classifications:
(1) Analog data is transmitted using an analog signal.For example, voice is transmitted in the telephone network.

(2) Analog data is transmitted using digital signals.For example, voice is transmitted in a local area network.

(3) Digital data is transmitted using analog signals.For example, data sent from a computer travels over a telephone network.

(4) Digital data is transmitted using digital signals.For example, data transfer from one computer to another computer on a local area network.

4. Frequency band signal and baseband signal

(1) The signal in the computer is a pulsed digital signal. If it is not modulated, it is called a baseband signal.The frequency range of this signal is very small, and the signals in the local area network are generally baseband signals.

(2) The signal in the telephone network is an analog signal, and the frequency band range of the analog signal is widely distributed.

(3) In order to transmit signals of more communication pairs (both parties of communication) on one line, the usual practice is to modulate the baseband signals into frequency band signals of different frequency ranges, that is, occupy different "channels".Obviously, the larger the frequency range, the more pairs of communication in different areas of modulation can be accommodated.

(4) The party receiving the signal restores the signal to a baseband signal. This process is called demodulation.

5. Code element
The encoding unit of a signal on the time axis is a symbol.For example, if two different coded states are included in one time period, the symbol is 2.The number of symbols in a period directly reflects the frequency of signal changes, and also reflects the capacity of transmitted information.

For example, if there are 8 states in one cycle, then the number of code elements is 8, and a 3-bit binary (23) number is required to describe it; if there are 4 states in one cycle, the number of code elements is 4, and two binary numbers are needed to describe it. enough.Obviously, there is a difference in the number of transmission bits between the two, and the latter transmits one bit less in the same time, and the transmission speed is slower.

It can be said that the number of symbols is directly proportional to the transmission rate.The rate of symbols is called the baud rate and will be described in subsequent chapters.

2.1.2 Communication system model and data communication
1. Communication system model
Some of these concepts are explained below.

(1) Source

A source is a device that generates a raw data signal, such as a computer or telephone.

(2) Sink

The sink is a device that converts the restored original signal into corresponding data, such as a computer or a telephone.

(3) Sending device

The basic function of the sending device is to match the source and the channel, and transform the original signal (baseband signal) generated by the source into a signal suitable for transmission in the channel.For example, a modem used to connect a telephone line to the Internet via ADSL.

(4) Receiving equipment

The function of the receiving device is opposite to that of the sending device. It is used for demodulation, decoding, and decoding. It can restore the corresponding original signal from the received signal with interference.

(5) channel

A channel is a channel for transmitting information, and it is also often referred to as a transmission line.Channels can be wired or wireless, and can even contain certain devices connected to the channel.The former is called a physical channel and usually refers to a physical connection.In order to improve efficiency, the physical channel is shared with multiple communication pairs, that is, each communication pair shares a certain channel with other communication pairs, rather than exclusively.For each communication pair, the channel at this time is the logical channel.

(6) Noise source
A noise source is a collection of all the noise in the channel and the noise scattered elsewhere in the communication system, and is a source of data transmission errors.

2. Analog communication system
The method of using analog signals to transmit messages is called analog communication. Ordinary telephones, radio, television, etc. all belong to analog communication. The system consists of a source, a modulator, a channel, a demodulator, a sink, and a noise source.In a transmission system, modulation and demodulation equipment are equipped in pairs.In an analog communication system, the signal transmitted on the channel is an analog signal.

A modulator is a device that modulates a parameter of a carrier wave with a transmitted message.For example, continuous amplitude modulation, frequency modulation or phase modulation of the carrier signal.A demodulator is a device that realizes the reversible conversion of the above process.

3. Digital baseband transmission system
The difference between the digital baseband transmission system and the analog system is that what is transmitted on the channel is a digital signal, which is a discontinuous pulse signal.The baseband signal former converts the non-digital signal into a digital signal, and the receiving filter restores the source signal.

2.1.3 Communication and Connection Mode

1. Parallel communication method
A parallel communication transfer is the simultaneous transfer of multiple data bits between two devices.The parallel communication method is mainly used for short-distance communication, and the bus structure inside the computer is the parallel communication method.The advantage of this method is that the transmission speed is fast and the information processing is simple.

When the sending device transmits the data bit to the receiving device through the corresponding data line, it can also add a data parity bit.The receiving device can receive these data at the same time, and it can be used directly without any transformation.

2. Serial communication method
During serial data transmission, data is transmitted on the communication line in bit-by-bit mode.At the end of the sending device (network card) inside the computer, the internal parallel data is first converted into a serial mode, and sent to the device at the receiving station bit by bit through the transmission line, and then the data is reconverted from the serial mode to the parallel mode at the receiving end. for use by the recipient.

The speed of serial data transmission is much slower than parallel transmission, but it has greater practical significance for communication networks with extremely wide coverage.

2.5 Parallel Communication Mode
3. Directional structure of serial communication

There are three directional structures of serial data communication, namely simplex, half-duplex and full-duplex.

(1) Simplex data transmission only supports data transmission in one direction.

(2) Half-duplex data transmission allows data to be transmitted in two directions, but at a certain moment, data is only allowed to be transmitted in one direction.It is actually a simplex communication that switches directions, such as the communication method of a walkie-talkie.

(3) Full-duplex data communication allows data to be transmitted in both directions simultaneously.Therefore, full-duplex communication is a combination of two simplex communication modes.It requires that both the sending device and the receiving device have independent receiving and sending capabilities.This method is being widely used in transmission systems such as voice.

2.1.4 Data Synchronization Technology

The purpose of data synchronization is to ensure that the data at the receiving end and the sending end are consistent, that is, to ensure that the receiving end receives the data completely without losing data.At present, the main data synchronization technologies include three forms: bit synchronization, character synchronization and frame synchronization.

1. Bit synchronization

The purpose of bit synchronization is to keep every bit of information received by the receiver in sync with the sender.The specific method is to use the synchronous clock signal sent by the sender, and the receiver uses the synchronous signal to lock its own clock pulse frequency and receive data according to the clock.

This method is suitable for small-scale data transmission, which is currently used in local area networks.For example, the transition in the middle of each bit transmission period in the Manchester encoded signal is used as a clock signal, and the receiving end extracts the synchronization signal from it to lock its own clock pulse frequency.

2. Character synchronization
Character synchronization is a way to realize character synchronization with one character as the sending unit, also known as "start-stop" or "asynchronous".The transmission of each character requires 1 start bit, 5-8 data bits and 2 stop bits.

3. Frame synchronization method

The frame synchronization method realizes synchronization by identifying a pair of flag fields of a frame.A frame is a transmission unit in a data link (a line between two adjacent nodes), which contains data and control information.

(1) Character-oriented frame synchronization method

The character-oriented frame synchronization method uses the synchronization character (SYN, 16H) to mark the beginning of a frame, and is suitable for frames whose data is characters.

(2) Bit/bit-oriented frame synchronization method

The bit/bit-oriented frame synchronization method uses a special bit sequence (such as 7EH, that is, 01111110) to mark the beginning of a frame, and is suitable for frames of any data type.Its characteristics are: the frame inspection domain includes address, control and information domains, the size of the information domain is variable, and a pair of flag domains marks a complete frame.This approach is widely used in large or complex networks.

2.1.5 Signal modulation and demodulation technology

In Section 2.1.1, it is introduced that there are two types of signals, analog and digital, and they must be modulated before signal transmission to ensure the quality of signal transmission.This conversion technology between baseband signal and frequency band signal is called signal modulation and demodulation technology.

1. Digital signal and frequency band signal

Any carrier signal has three characteristics: amplitude (A), frequency (f), and phase (P).There are three basic techniques for converting a digital signal to an analog signal: amplitude modulation (ASK), frequency modulation (FSK), and phase modulation (PSK).

2. Coding of baseband signals

The signal encoding method of the local area network is digital, and the unmodulated signal output from the computer is called the baseband signal.Baseband transmission is the electrical pulse that directly transmits digital signals in the line. This is the simplest transmission method. The local area network is short-distance communication and uses baseband transmission.

Baseband transmission needs to solve the digital signal representation of digital data and the signal synchronization between the sending and receiving ends.The encoding of the baseband signal has the following common forms.

(1) Non-return-to-zero coding
For the transmission of digital signals, the simplest method is to represent two binary numbers with different voltage levels, that is, the digital signal is composed of rectangular pulses.

According to whether the signal returns to zero or not, it can be divided into return-to-zero code and non-return-to-zero code. The signal in the middle of the return-to-zero code returns to 0 level, while the non-return-to-zero code encounters a 1-level flip and remains unchanged at 0.Non-return-to-zero codes have difficulty determining where one bit ends and another bit begins during transmission, requiring some method of timing or synchronization between the signal transmitter and receiver.

According to the digital coding method, it can be divided into unipolar code and bipolar code.

①Unipolar non-return-to-zero code, when there is no voltage (that is, no current), it means "0", and a constant positive voltage is used to mean "1".

The middle point of each symbol time is the sampling time, and the decision threshold is the half-amplitude level (0.5).If the value of the received signal is 0.5-1.0, it is judged as a "1" code, and if the value is 0-0.5, it is judged as a "0" code.

②Bipolar non-return-to-zero code, both "1" code and "0" code have current, "1" is positive current, "0" is negative current, positive and negative amplitudes are equal, and the judgment threshold is zero level.

Non-return-to-zero codes make it difficult to determine when one bit ends and another bit begins during transmission, requiring some method of timing or synchronization between the transmitter and receiver.

(2) Manchester encoding

Manchester encoding is often used for LAN transmission.Its characteristics are:
① There is a level transition in the middle of each bit cycle, and the time interval between two level transitions can be 1/2 cycle or 1 cycle. The transition in the middle of the bit is used as a clock signal, and the level transition is used to generate transceiver Synchronization signal for both sides.

②Manchester coded signal is also called "self-contained clock coded" signal, no need to send additional synchronization signal when sending Manchester coded signal.

③ The potential jumps from low to high to represent "1", and the potential to jump from high to low represents "0".

(3) Differential or differential Manchester encoding
Differential or differential Manchester encoding has the following characteristics.

①The transition between each bit is only used as a clock.

② Use whether there is a jump at the beginning of each bit to represent "0" or "1", if there is a jump, it becomes "0", and if there is no jump, it becomes "1".

The two Manchester codes include the clock and data in the data stream. While transmitting the code information, the clock synchronization signal is also transmitted to the other side. There is a jump in each code, so it has self-synchronization ability and good anti-interference performance.

3. Analog data and digital signal

The main method of digitizing analog data is pulse code modulation (Pulse Code Modulation, PCM), and the most typical application is the digitization of voice signals.

This method is based on Shannon's sampling theorem, that is, if a continuously changing analog signal is periodically sampled, as long as the sampling frequency is greater than or equal to twice the highest frequency of the effective signal or its bandwidth, the sampled value can contain the original signal. From these samples, the original signal can be reconstructed using a low-pass filter.The concepts that need to be understood for this are as follows.

(1) Sampling

Sampling refers to reading the level value corresponding to the analog signal at a fixed sampling period.

(2) Quantification

Quantization refers to positioning the signal value obtained by sampling to the closest quantized value according to a certain gradation scale.In this way, the continuous analog signal is converted into a discrete pulse signal, whose amplitude value corresponds to the value of the signal at the sampling moment.

(3) Coding

Encoding refers to processing discrete quantized values ​​into appropriate binary code groups.In the binary code, 2n different codewords can be composed of n-bit codes, which means that the quantized signal can have 2n different values.

2.1.6 Multiplexing Technology

When a physical link has rich bandwidth and high speed, multiplexing technology can effectively improve the utilization rate of communication lines, and can use one communication channel to transmit signals of more communication pairs.Multiplexing has the following common methods.

(End of this chapter)