Chapter 2 Physics layer
2.1 Communication basics
1. Data, signals and code elements
The purpose of communication is to transmit messages (voice, text, images, etc.). Data refers to the entity that transmits information. Signal is the electrical or electromagnetic representation of data, which is the transmission of data Theform of existencein the process. Both data and signals can be qualified as "analog" or "digital":
①Continuously changing data (or signal) is calledsimulationData (or analog signal);
②Data (or signals) whose values are only allowed to be a limited number of discrete values are calledDigitaldata (or digital signal).
Data transmission methods can be divided into serial transmission and parallel transmission：
①Serial transmission: Send the 8-bit binary number representing a character in order from low to high in sequence, speed a>Slow, costlow, suitable for long distances Distance;
②Parallel transmission: Send the 8-bit binary number representing one character at the same time through 8 channels, speedFast, CostHigh, SuitableClose distance.
Code element: refers to a fixed durationSignal waveform (digital pulse) represents a k-base number, which represents the basic waveform of different discrete values. It is the measurement unit of digital signals in digital communication. The signal within this duration is called k base code unit , and this duration is called code unit width . When there are M discrete states of a symbol (M≥2), the symbol is an M-ary symbol. One symbol can carry multiple bits of information. For example, when using binary encoding, there are only two different code elements: 0 state and 1 state.
2. Information source, channel and information destination
Data communication refers to communication between digital computers or other digital terminals. A data communication system is mainly divided into three parts: source, channel and sink. The source is the source that generates and sends data, and the sink is the destination for receiving data (usually a digital computer or other digital terminal equipment). The information sent by the source at the sending end needs to be converted into a signal suitable for transmission on the channel through a converter, and the signal transmitted to the receiving end through the channel is first converted into the original information by an inverse converter and then sent to the sink.
Channels are not the same as circuits. Channels are the transmission medium for signals. A channel can be regarded as a logical component of a line, and is generally used to represent a medium that sends information in a certain direction. Therefore, a communication line often contains a sending channel and a receiving channel. Noise sources are concentrated representations of noise on the channel (i.e., interference to the signal) and noise scattered elsewhere in the communication system.
Channels can be divided into analog channels (sending analog signals) and digital a>channel. according to the transmission medium Cable channel and wireless channel (sending digital signals), which can be divided into
The signals transmitted on the channel are divided into baseband signals and broadband signals. The baseband signal directly represents the digital signals 1 and 0 with two different voltages, and then sends them to the digital channel for transmission (called baseband transmission); the broadband signal modulates the baseband signal to form a frequency division multiplexing analog signal, and then sends it to Transmission on analog channels (called broadband transmission).
From the perspective of the communication methods of information interaction between the two parties, it can be divided into three basic methods:
①Simplex communication: Communication in one direction without interaction in the opposite direction, only requires A channel. (broadcast)
②Half-duplex communication/Two-way alternating communication: Both parties to the communication can send or receive information, but neither party can send at the same time and reception requiretwo channels. (walkie-talkie)
③Full-duplex Communication/two-way simultaneous communication: Both parties to the communication can send and receive information at the same time, requiringTwo channels. (phone)
3. Rate, baud and bandwidth
The rate is also calleddata rate, which refers to the data transmission rate and represents the amount of data transmitted per unit time. It can be used means: Information transmission rate and Code element transmission rate
①Code element transmission rate. Also known as the baud rate, it represents the number of code elements transmitted by the digital communication system per unit time (also called the number of pulses or signal changes degree), the unit is Baud (Baud). 1 baud means that a digital communication system transmits 1 symbol per second.
②Information transmission rate. Also known as information rate, bit rate, etc. It represents the number of binary symbols transmitted by the digital communication system per unit time (that is, the number of bits), and the unit is bits/second (b/s).
Relationship: If one symbol carries n bits of information, the information transmission rate corresponding to the symbol transmission rate of M Baud is M×n bit/s. (Bit rate = baud rate × log2 in N base)
Bandwidth originally refers to the frequency bandwidth of a signal, and its unit is Hertz (Hz). In actual networks, since the data rate is one of the most important indicators of the channel, and there is a numerical interchange relationship between bandwidth and data rate, it is often used to represent the ability of the network's communication lines to transmit data. Therefore, bandwidth represents the "highest data rate" that can pass from one point in the network to another point in a unit of time. Obviously, the unit of bandwidth at this time is no longer Hz, but b/s.
2.1.2 Nyquist’s Theorem and Shannon’s Theorem
1. Nyquist theorem
The frequency range that a specific channel can pass is always limited. Many high-frequency components in the signal often cannot pass through the channel, otherwise they will be attenuated during transmission, causing the signal waveform received by the receiving end to lose clear boundaries between symbols. This phenomenon is called Symbol crosstalk. The Nyquist theorem is also called the Nyquist criterion. It stipulates that under ideal low-pass (noiseless, bandwidth-limited) conditions, in order to avoid code crosstalk, the ultimate symbol transmission rate is 2W Baud, where W is the bandwidth of the ideal low-pass channel. If V is used to represent the number of discrete levels of each symbol, then the ultimate data transmission rate under an ideal low-pass channel=2Wlog2V (b/s) .
For the Nye criterion, the following conclusions can be drawn:
① In any channel, the rate of code element transmission has an upper limit. If the transmission rate exceeds this upper limit, serious inter-symbol crosstalk problems will occur, making it impossible for the receiving end to completely correctly identify the symbols.
②The wider the channel frequency band (that is, the more high-frequency components of the signal that can pass through), the higher the rate can be used. Efficient transmission of code elements.
③The Nye criterion gives a limit on the code element transmission rate, but does not put a limit on the information transmission rate.
Since the transmission rate of symbols is restricted by the Nye criterion, in order to increase the data transmission rate, we must try to make each symbol carry more bits The amount of information requires the use of a multi-modulation method.
2. Shannon’s theorem
Shannon's theorem gives the ultimate data transmission rate for a channel with limited bandwidth and interference from Gaussian white noise. When transmitting at this rate, no error can be produced. Shannon's theorem is defined as the channel's limit transmission rate=Wlog2(1+ S/N) (b/s).
In the formula, W is the bandwidth of the channel, S is the average power of the signal transmitted by the channel, and N is the Gaussian noise power inside the channel. S/N is the signal-to-noise ratio, which is the ratio of the average power of the signal to the average power of the noise, Signal-to-noise ratio=10log10(S/N) (dB).
Regarding Shannon's theorem, the following conclusions can be drawn:
①The greater the bandwidth of the channel or the signal-to-noise ratio in the channel< a i=4>, then the limit transmission rate of information ishigher.
② For a certain transmission bandwidth and a certain signal-to-noise ratio, the upper limit of the information transmission rate is determined.
③As long as the information transmission rate is lower than the limit transmission rate of the channel, some way will be found to achieve error-free transmission.
④The limit information transmission rate obtained by Shannon's theorem is much lower than the transmission rate that the actual channel can achieve.
The Nye criterion only considers the relationship between bandwidth and the ultimate symbol transmission rate, while Shannon's theorem not only takes into account the bandwidth, but also the signal-to-noise ratio. This shows from another aspect that the number of binary digits corresponding to one symbol is limited.
2.1.3 Coding and modulation
Data, whether digital or analog, must be converted into signals for transmission purposes. The process of converting data into analog signals is called modulation, and the process of converting data into digital signals is called coding .
Signal is a specific representation of data. It has a certain relationship with data, but it is different from data. Digital data can be converted into digital signals for transmission through a digital transmitter, or converted into analog signals for transmission through a modulator; similarly, analog data can be converted into digital signals for transmission through a PCM encoder, or converted into analog signals for transmission through an amplifier modulator. . In this way, the following four encoding methods are formed.
1. Encoding digital data into digital signals
Digital data encoding is used in baseband transmission, that is, digital signals are directly transmitted without basically changing the frequency of the digital data signal. Specifically what kind of digital signal is used to represent 0 and what kind of digital signal is used to represent 1 is the so-called encoding. Common encodings include the following:
①Return-to-zero coding: In return-to-zero coding (RZ), high level represents 1 and low level represents 0 (or vice versa). The middle of each clock cycle jumps to low level (return to zero), and the receiver Adjust the local clock reference based on this transition, which provides a self-synchronization mechanism for both transmitting parties. Since zeroing requires a part of the bandwidth, the transmission efficiency is affected to a certain extent.
② Non-return-to-zero coding: The difference between non-return-to-zero coding (NRZ) and RZ coding is that there is no need to return to zero, and one cycle can be used to transmit data. However, NRZ encoding cannot transmit clock signals, and it is difficult for both parties to synchronize. Therefore, if you want to transmit high-speed synchronous data, you need to have a clock line on both sides.
③ Reverse non-return to zero encoding: The difference between inverse non-return to zero encoding (NRZI) and NRZ encoding is that the inversion of the signal represents 0, and the signal remains unchanged to represent 1. The flipped signal itself can serve as a notification mechanism. This encoding integrates the advantages of the first two encodings, which can transmit clock signals without losing system bandwidth as much as possible. (USB 2.0 communication)
④Manchester encoding. Manchester Encoding divides a code element into two equal intervals. The first high and the last low represent 1, and the front low and the last high represent 0. Of course, the opposite stipulation can also be adopted. The characteristic of this encoding is that a level transition occurs in the middle of each symbol. The transition in the middle of the bit serves as both a clock signal (can be used for synchronization) and a data signal, but the frequency bandwidth it occupies is the original baseband width. twice. (Ethernet)
⑤Differential Manchester encoding: The rule of differential Manchester encoding is: if the code element is 1, the level of the first half code element is the same as the level of the second half code element of the previous code element; if the code element is 0, then the situation on the contrary. The characteristic of this encoding is that there is a level jump in the middle of each symbol, which can achieve self-synchronization and has good anti-interference performance.
⑥4B/5B encoding: Extra bits are inserted into the bit stream to break a series of 0s or 1s, and 5bits are used to encode 4bits of data before being transmitted to the receiver. Coding efficiency is 80%.
2. Digital data is modulated into analog signals
Digital data modulation technology converts digital signals into analog signals at the transmitting end, and restores analog signals to digital signals at the receiving end, corresponding to the modulation and demodulation processes of the modem respectively. The basic digital modulation methods are as follows:
①Amplitude shift keying (ASK). Digital signals 1 and 0 are represented by changing the amplitude of the carrier signal, while the frequency and phase of the carrier do not change. It is relatively easy to implement, but has poor anti-interference ability.
②Frequency shift keying (FSK). Digital signals 1 and 0 are represented by changing the frequency of the carrier signal, while the amplitude and phase of the carrier do not change. It is easy to implement, has strong anti-interference ability, and is currently widely used.
③Phase shift keying (PSK). Digital signals 1 and 0 are represented by changing the phase of the carrier signal, while the amplitude and frequency of the carrier do not change. It is further divided into absolute phase modulation and relative phase modulation.
④Quadrature amplitude modulation (QAM). On the premise that the frequencies are the same, ASK and PSK are combined to form a superimposed signal. Assume that the baud rate is B, m phases are used, and each phase has n amplitudes, then the data transmission rate R of the QAM technology isR=Blog2（mn）（b/s）
Example: The baud rate of a certain communication link is 1200Baud, and QAM modulation technology with 4 phases and 4 amplitudes per phase is used. What is the information transmission rate of the link?
4×4=16，log216=4，1200×4=4800 bit/s
3. Encoding analog data into digital signals
The computer internally processes binary data, which are all digital audio. Analog audio needs to be converted into a discrete sequence represented by a limited number of numbers through sampling and quantization (that is, audio digitization). PCMPulse code modulation is a waveform coding method for encoding audio signals, which mainly includes three steps: . Sampling, quantization, encoding
Sampling theorem: In the field of communications, bandwidth refers to the difference between the highest frequency and the lowest frequency of the signal. Therefore, when converting an analog signal into a digital signal, assuming that the maximum frequency in the original signal is f, the sampling frequency f* must be greater than or equal to twice the maximum frequency f to ensure that the sampled digital signal retains the integrity of the original analog signal. information. In addition, the sampling theorem is also called the Nyquist theorem.
① Sampling: Periodically scan the analog signal, turning the time-continuous signal into a time-discrete signal. According to the sampling theorem, when the sampling frequency is greater than or equal to twice the frequency band bandwidth of the analog data, the resulting discrete signal can represent the sampled analog data without distortion.
②Quantization: Convert the level amplitude obtained by sampling into the corresponding digital value according to a certain hierarchical scale and take an integer, thus converting the continuous level amplitude into a discrete digital quantity. The essence of sampling and quantization is segmentation and conversion.
③Coding: Convert the quantization result into the corresponding binary code.
4. Analog data is modulated into analog signals
To achieve transmission effectiveness, higher frequencies may be required. This modulation method can also use frequency division multiplexing (FDM) technology to make full use of bandwidth resources. The signals transmitted by telephones and local exchanges use analog signals to transmit analog data; the analog voice data is loaded into the analog carrier signal for transmission.
2.1.4 Data exchange method
1. Circuit switching
During data transmission, there is a dedicated physical connection line composed of intermediate nodes between the source node and the destination node. This line is maintained until the end of the data transmission a>. Circuit switching is divided into three phases: connection establishment (call/circuit establishment), communication (data transmission), and connection release (circuit teardown).
Characteristics of circuit switching:Exclusive resources, users always occupy the end-to-end fixed transmission bandwidth. It is suitable for remote batch information transmission or large amounts of data transmission with high real-time requirements between systems.
| Advantages and Disadvantages of Circuit Switching |
|
| advantage |
shortcoming |
| Small transmission delay |
Long time to establish connection |
| Data is transmitted sequentially without out-of-order problems |
The line is exclusive. Even if the communication line is idle, it cannot be used by other users, and the channel usage efficiency is low. |
| It has strong real-time performance. Once the physical link is established, both parties can communicate in real time. Suitable for interactive conversational communication |
Poor flexibility. If either party in the connection path fails, it must redial to establish a new connection. It is not suitable for sudden communications. |
| Full-duplex communication, no conflicts, the communicating parties have different channels, and there will be no contention for physical channels |
No data storage capability, difficult to smooth traffic |
| Suitable for digital and analog signals |
During circuit switching, data is transmitted directly, and it is difficult for terminals of different types, specifications, and speeds to communicate with each other. |
| Simple control, circuit switching equipment and control are relatively simple |
Unable to detect and correct transmission errors, making it difficult to control errors during the communication process |
2. Message exchange
The unit of data exchange ismessage. The message is the data unit exchanged and transmitted in the network, that is, the station sends it all at once. data block. The message contains the complete data information to be sent, its length is very inconsistent, the length is unlimited and variable. Messages carry information such as destination address and source address. Message exchange uses the store and forward transmission method at the switching node.
| Advantages and Disadvantages of Message Exchange |
|
| advantage |
shortcoming |
| There is no need to establish a connection and no delay in establishing a connection. Users can send messages at any time. |
It has poor real-time performance and is not suitable for transmitting real-time or interactive business data. After data enters the switching node, it undergoes a store-and-forward process, causing forwarding delays. |
| Dynamically allocate lines and dynamically select the best path for packets to pass, which can smooth traffic. |
|
| Improve line reliability. If a certain transmission path fails, another path can be re-selected for transmission. |
Applies to digital signals only. |
| To improve line utilization, both communicating parties partially occupy this physical channel at different times, and multiple messages can share the channel. |
Since there is no limit to the length of the message, and each intermediate node must completely receive the entire message, when the output line is not idle, several complete messages may need to be stored for forwarding, requiring each node in the network to have Larger buffer. In order to reduce costs and reduce the capacity of the node's buffer memory, messages waiting to be forwarded are sometimes stored on the disk, further increasing the transmission delay. |
| Provides multi-destination services, and one message can be sent to multiple destination addresses at the same time. |
|
| Code conversion and rate matching are easy to achieve in store and forward, and even the sender and receiver may not be available at the same time. This facilitates communication between computers of different types, sizes, and speeds. |
|
3. Packet switching
Like message switching, packet switching also uses the store-and-forward method, but it solves the problem of large message transmission in message switching. Packet switching limits the upper limit of the data block size for each transmission, divides large data blocks into reasonable small data blocks, and adds some necessary The control information (such as source address, destination address and number information, etc.) constitutes a packet. The network node sends the packet to the next node based on the control information. After receiving the packet, the next node temporarily saves and queues it for transmission, and then selects its next node based on the packet control information until it reaches the destination node.
The choice of data exchange method is: if the amount of data to be transmitted is large and the transmission time is much longer than a call, select circuit switching. Circuit switching transmission delay is minimal. When end-to-end communication consists of many links, it is more appropriate to use packet switching to send data. From the perspective of channel utilization, message switching and packet switching are better than circuit switching. Among them, packet switching has a smallerdelay than message switching, which is especially suitable for Burst data communication between computers.
2.1.5 Datagram and virtual circuit
Packet switching can be further divided into connection-oriented virtual circuit mode and connectionless datagram mode according to the services provided by its communication subnet to endpoint systems. Both service methods are provided by thenetwork layer. It should be noted that datagram mode and virtual circuit mode are two modes of packet switching.
1. Datagram
When an end system as a communication subnet user sends a message, the high-level protocol implemented in the end system first splits the message into a number of data units with serial numbers, and adds address and other control information at the network layer to form a datagram. Packet (i.e. PDU at the network layer). Intermediate nodes store packets for a short period of time and forward each packet as quickly as possible after finding the best route. Different groups can take different paths and reach the destination node in different orders.
Datagram service has the following characteristics:
① There is no need to establish a connection before sending packets. The sender can send packets at any time, and nodes in the network can receive packets at any time.
②The network tries its best to deliver, but the transmission does not guarantee reliability, so it may be lost; the network selects routes independently for each packet, and the forwarding paths may be different, so the packets may not necessarily arrive at the destination node in order.
③The sent packet must include the complete addresses of the sending end and the receiving end so that they can be transmitted independently.
④ When packets are stored and forwarded at switching nodes, they need to be queued for processing, which will cause a certain delay. When the traffic through the switching node is large or the network is congested, this delay will be greatly increased, and the switching node may also discard some packets according to the situation.
⑤The network has redundant paths. When a switching node or a link fails, the forwarding table can be updated accordingly and another path can be found to forward the packet. It has strong adaptability to failures and is suitable for burst communications but not suitable for Long message, conversational communication.
⑥The delay of store and forward is generally small, which improves the throughput of the network.
⑦ The sending and receiving parties do not exclusively occupy a certain link, and the resource utilization rate is high.
2. Virtual circuit
The virtual circuit method attempts to combine the datagram method with the circuit switching method, giving full play to the advantages of the two methods to achieve the best data exchange effect. Before the packet is sent, a logically connected virtual circuit is required to be established between the sender and the receiver. Once the connection is established, the physical path corresponding to the virtual circuit is fixed. Similar to circuit switching, the entire communication process is divided into three stages: virtual circuit establishment, data transmission and virtual circuit release.
In the virtual circuit mode, each time the end system establishes a virtual circuit, it selects an unused virtual circuit number and assigns it to the virtual circuit to distinguish it from other virtual circuits in the system. When transmitting data, each data packet must not only have control information such as packet number and checksum, but also the virtual circuit number it passes through to distinguish it from packets on other virtual circuits. Each node in the virtual circuit network maintains a virtual circuit table. Each item in the table records the information of an open virtual circuit, including the virtual circuit number on the receiving link and the sending link, and the previous one. The identifier of the node and next node. Data transmission is bidirectional, and the above information is determined during the establishment process of the virtual circuit.
Virtual circuit services have the following characteristics:
① The establishment and teardown of virtual circuit communication links requires time overhead, which is wasteful for interactive applications and small amounts of short packets, but is more efficient for long-term and frequent data exchanges.
②The routing selection of the virtual circuit is reflected in the connection establishment stage. After the connection is established, the transmission path is determined.
③Virtual circuits provide reliable communication functions and ensure that each packet arrives correctly and in order. In addition, the flow of the two data endpoints can also be controlled. When the receiver is too late to receive the data, the sender can be notified to postpone sending.
⑤Fatal weakness: When a node or a link in the network fails and completely fails, all virtual circuits passing through the node or link will be destroyed.
⑥The packet header does not contain the destination address, but contains the virtual circuit identifier. Compared with the datagram method, its overhead is small.
| The difference between packet switching methods |
Datagram |
virtual circuit |
| Establishment of connection |
unnecessary |
must have |
| Destination address |
Each packet has a complete destination address |
It is only used during the connection establishment phase. After that, each packet uses a shorter virtual circuit number. |
| Routing |
Each packet is routed and forwarded independently |
Packets belonging to the same virtual circuit are forwarded according to the same route |
| Grouping order |
Orderly arrival of packets is not guaranteed |
Ensure orderly arrival of packets |
| reliability |
Reliable communication is not guaranteed, reliability is guaranteed by the user host |
Reliability is guaranteed by the network |
| Adaptability to network failures |
The failed node loses packets, and the path selection of other packets changes and can be transmitted normally. |
All virtual circuits passing through the faulty node cannot work normally. |
| Error handling and flow control |
Flow control is performed by the user host and the possibility of datagrams is not guaranteed |
It can be done by the packet switching network or by the user host. |
2.2 Transmission media
2.2.1 Physical layer transmission medium
Transmission medium is also called transmission medium/transmission medium, which is the physical path between the sending device and the receiving device in the data transmission system . The transmission media is not the physical layer, it is below the physical layer. Because the physical layer is the first layer of the architecture, the transmission media is sometimes called layer 0. What is transmitted in the transmission medium is a signal, but the transmission medium does not know what the transmitted signal means. But the physical layer specifies theelectrical characteristics so that the transmitted bit stream can be identified.
| Transmission medium |
guided transmission media |
Electromagnetic waves are directed along a solid medium (copper wire/fiber optic) |
| non-guided transmission media |
Free space (air, water, etc.) |
1. Guided transmission medium
①Twisted pair
Twisted pair is the oldest and most commonly used transmission medium. It consists oftwo wires arranged side by side according to certain rulesTwisted (Twisted can reduce electromagnetic interference to adjacent wires), composed of mutually insulated copper wires. In order to further improve the ability to resist electromagnetic interference, a shielding layer made of metal wire can be added to the outside of the twisted pair. This is a> a> (UTP). unshielded twisted pair (STP), unshielded twisted pair is calledShielded twisted pair
Features: Twisted pair is cheapcheap. It is one of the most commonly used transmission media and is commonly used in local area networks and traditional telephone networks. Both analog transmission and digital transmission can use twisted pairs, and the communication distance is generally several kilometers to tens of kilometers. When the distance is too far, for analog transmission, use amplifier to amplify and attenuate signal; fordigital transmission, userepeaters to distort signal shaping.
②Coaxial cable
Coaxial cable is composed of conductor copper core, insulation layer, mesh braided shielding layer and plastic outer layer. According to different characteristic impedance values, coaxial cables are usually divided into two categories: 50Ω coaxial cables and 75Ω coaxial cable. Among them, 50Ω coaxial cable is mainly used to transmitbaseband digital signals, also called baseband coaxial cable. It is used inWidely used in local area networks; 75Ω coaxial cable is mainly used to transmitbroadband signals, also known as broadband coaxial cable axial cable, which is mainly used incable TV systems. Due to the shielding layer of the outer conductor, coaxial cablesanti-interference properties are better than twisted pairs and are widely used to transmit higher-speed data. The transmission distance is longer, but the price is more expensive than twisted pair.
③Optical fiber
Optical fiber communication uses optical fibers (referred to as optical fibers) to transmit light pulses for communication. The presence of a light pulse represents 1, and the absence of light pulse represents 0. The frequency of visible light is approximately 108MHz, so the bandwidth of the optical fiber communication system is much greater than the bandwidth of other current transmission media. The optical fiber has a light source at the transmitting end, which can use light-emitting diodes or semiconductor lasers. They can generate light pulses under the action of electrical pulses. At the receiving end, a photoelectrode tube is used as a photodetector, which can restore the electrical pulse when the light pulse is detected. . Optical fiber is mainly composed of coreand cladding. Light waves are transmitted through the core. The cladding has a lower refractive index than the core. When light rays pass from a medium with a high refractive index to a medium with a low refractive index, the angle of refraction will be greater than the angle of incidence. Therefore, if the incident angle is large enough, total reflection will occur, that is, when the light hits the cladding, it will be refracted back to the core, and this process will be repeated. , the light is transmitted along the optical fiber.
| optical fiber |
definition |
light source |
Features |
Exterior |
| single mode fiber |
An optical fiber that directly transmits optical signals intransverse mode |
Directedness Enthusiastic Gekiko Two-way tube |
Weakness is small，matchingfardistance 传输 |
diametersmallto the point where only one light ray passes |
| multimode optical fiber |
There arevarious optical fibers that transmit light signal modes |
led |
Easily lost, FitNearDistance 传输 |
The diameter is very large, which can accommodate multiple light rays |
Characteristics of optical fiber:
1. ，对远迻传输特别经济.
②resistantlightning electric power magneticdryinggood performance.
③No crosstalk interference,Good confidentiality, not easy to be eavesdropped or intercepted.
④Small size and light weight.
2. Non-guided transmission media
①Radio waves
Radio wave signals propagate inall directions. They have strong penetrating ability and can be transmitted over long distances. They are widely used in the field of communications. (such as mobile phone communication).
②Microwave
Microwave signals propagate infixed directions. Microwave communication has a high frequency and a wide frequency range, so the data rate is very high. Microwave applications are divided into two types: 1. Ground microwave relay communication; 2. Satellite communication.
| Satellite Communications |
|
| advantage |
shortcoming |
| Large communication capacity |
Propagation time extension (250ms-270ms) |
| long distance |
Greatly affected by climate (strong winds, sunspot outbreaks, solar transit) |
| Wide coverage |
High bit error rate |
| Broadcast communications and multiple access communications |
high cost |
③Infrared ray, laser
红外线、激光的信号向固定方向传播，把要传输的信号分别转化为各自的信号格式，即红外光信号和激光信号，再在空间中传播。
2.2.2 物理层接口的特性
物理层解决如何在连接各种计算机的传输媒体上传输数据比特流，而不是指具体的传输媒体。物理层的主要任务是确定与传输媒体接口有关的一些特性（定义标准）。
物理层接口特性：
①机械特性：定义物理连接的特性，规定物理连接时所采用的规格、接口形状、引线数目、引脚数量和排列情况。
②电气特性：规定传输二进制位时，线路上信号的电压范围、阻抗匹配、传输速率和距离限制等。
③功能特性：指明某条线上出现的某一电平表示何种意义，接口部件的信号线的用途。
④规格特性（过程特性）：定义各条物理线路的工作规格和时序关系。
实现同步的数据传输方式：
①同步传输：在同步传输的模式下，数据的传送是以一个数据区块为单位的，因此同步传输又称为区块传输。在传送数据时，需要先送出1个或多个同步字符，再送出整批的数据。
②异步传输：异步传输将比特分成小组（可以是8位的1个字符或更长）进行传送，发送方可以在任何时刻发送这些比特组，而接受方不知道他们会在什么时候到达。传送数据时，加上一个字符起始位和一个字符终止位。
2.3 物理层设备
2.3.1 中继器
中继器的主要功能是将信号整形并放大再转发出去，以消除信号经过一长段电缆后而产生的失真和衰减，使信号的波形和强度达到所需要的要求，进而扩大网络传输的距离。其原理是信号再生（而非简单地将衰减的信号放大）。中继器有两个端口，数据从一个端口输入，再从另一个端口发出。端口仅作用于信号的电气部分，而不管是否有错误数据或不适于网段的数据。
中继器是用来扩大网络规模的最简单廉价的互连设备。中继器两端的网络部分是网段，而不是子网，使用中继器连接的几个网段仍然是一个局域网。中继器若出现故障，对相邻两个网段的工作都将产生影响。由于中继器工作在物理层，因此它不能连接两个具有不同速率的局域网。
从理论上讲，中继器的使用数目是无限的，网络因而也可以无限延长。但事实上这不可能，因为网络标准中对信号的延迟范围做了具体的规定，中继器只能在此规定范围内进行有效的工作，否则会引起网络故障。例如，在采用粗同轴电缆的10BASE5以太网规范中，互相串联的继器的个数不能超过4个，而且用4 个中继器串联的5段通信介质中只有3段可以挂接计算机，其余两段只能用作扩展通信范围的链路段，不能挂接计算机。这就是所谓的“5—4—3 规则”。
2.3.2 集线器
集线器（Hub）实质上是一个多端口的中继器。当Hub工作时，一个端口接收到数据信号后，由于信号在从端口到Hub的传输过程中已有衰减，所以Hub便将该信号进行整形放大，使之再生（恢复）到发送时的状态，紧接着转发到其他所有（除输入端口外）处于工作状态的端口。如果同时有两个或多个端口输入，那么输出时会发生冲突，致使这些数据都无效。从Hub的工作方式可以看出，它在网络中只起信号放大和转发作用，目的是扩大网络的传输范围，而不具备信号的定向传送能力，即信息传输的方向是固定的，是一个标准的共享式设备。
Hub主要使用双绞线组建共享网络，是从服务器连接到桌面的最经济方案。在交换式网络中，Hub直接与交换机相连，将交换机端口的数据送到桌面上。使用Hub组网灵活，它把所有节点的通信集中在以其为中心的节点上，对节点先练的工作站进行集中管理，不让出问题的工作站影响整个网络的正常运行，并且用户的加入和退出也很自由。由Hub组成的网络是共享式网络，但逻辑上仍是一个总线网。Hub的每个端口连接的网络部分是同一个网络的不同网段，同时Hub也只能在半双工状态下工作，网络的吞吐率因而受到限制。