Basic learning of the network must be mastered: an overview of the physical layer of the network (full version)

Physical Layer Overview

The physical layer is a layer in a computer network, located at the bottom of the OSI (Open Systems Interconnection) model, responsible for transmitting the raw bitstream (bitstream) on a physical medium, such as a cable, optical fiber, or wireless channel.

The main task of the physical layer is to transmit data from one node to another, ensuring the reliable transmission of data on the transmission medium.

The following are some important concepts of the physical layer:

1. Transmission medium

The physical layer deals with transmission media, including wires (eg, twisted pair, coaxial cable), fiber optics, and wireless channels (eg, radio waves, infrared, etc.).

1.1 Twisted Pair

Twisted pair is a widely used transmission medium, divided into unshielded twisted pair (UTP) and shielded twisted pair (STP). They reduce electromagnetic interference by twisting two insulated copper wires. Twisted pair cables are used for local and wide area network connections such as Ethernet.

1.2 Coaxial Cable

A coaxial cable consists of an inner conductor, insulation, metal shield and outer jacket. It is widely used in TV cable transmission, local area network and broadband access.

1.3 Optical Fiber

Optical fiber is a transmission medium with very high transmission rate and strong anti-interference. It transmits data through optical signals, which can be divided into single-mode fiber and multi-mode fiber. Optical fibers are used in high-speed broadband networks, long-distance communications, and data center interconnects.

1.4 Wireless channel

A wireless channel is a medium for transmission through radio waves, microwaves, infrared rays, etc. Common wireless transmission technologies include Wi-Fi, Bluetooth, cellular networks (such as 4G and 5G), etc. Wireless transmission is suitable for applications such as mobile devices, mobile communications, and the Internet of Things.

1.5 Power Line Communication (PLC)

PLC uses power lines to transmit data signals, and can realize network connections in homes, offices, etc. without additional network wiring.

1.6 Infrared

Infrared is an electromagnetic wave that can be used for short-distance wireless communication, such as infrared remote control, infrared data transmission, etc.

1.7 Gekko Tsushin

Laser communication uses laser beams to transmit data, and is usually used in long-distance, high-speed communication connections, such as satellite communication and optical fiber communication.

2. Signal transmission

Signaling refers to the process of transferring data from a sender to a receiver in a communication system. During signal transmission, digital data is converted to an analog signal (modulation), then transmitted on the transmission medium, and finally the analog signal is converted back to digital data at the receiving end (demodulation).

2.1 Transmission medium

The transmission medium is the physical medium for analog signal transmission, which can be cables, optical fibers, wireless channels, etc. Different transmission media have different characteristics, such as transmission distance, bandwidth, signal attenuation and noise, etc.

2.2 Transmission and dissemination

During transmission, an analog signal propagates through the transmission medium. During transmission, you may encounter problems such as signal attenuation, distortion and interference, which may affect the signal quality.

Signal transmission is an important link in the communication system, which affects the quality and reliability of communication. Appropriate modulation technology and selection of transmission medium can ensure that data will not be lost during transmission, and the original data can be accurately restored at the receiving end. Different types of communication systems and applications may use different modulation methods and transmission media to meet specific communication needs.

3. Transmission rate

Transmission rate, also known as bit rate (Bit Rate) or data rate, refers to the number of bits (binary digits) transmitted per unit time in digital communication. It is an indicator to measure the speed of data transmission, usually expressed in the number of bits transmitted per second, and the unit is bps (bits per second). The transmission rate is used to describe the transmission speed and bandwidth utilization of data in the communication system.
The transmission rate is affected by many factors, including the signal modulation method, the characteristics of the transmission medium, the encoding technology and the design of the communication system. Here are some common transfer rate related concepts:

3.1 Physical Rate

Physical rate refers to the raw transmission rate in a channel, usually in bits per second (bps). It depends on the modulation method and channel characteristics.

3.2 Effective Rate

The effective rate takes into account some overheads in the communication process, such as error correction codes, frame synchronization, etc., and the actual number of data bits transmitted may be slightly less than the physical rate.

3.3 Symbol Rate

The symbol rate refers to the number of symbols transmitted per unit time, and a symbol can be a combination of multiple bits. In some modulation schemes, a symbol may represent multiple bits, so the symbol rate may differ from the transmission rate.

3.4 Baud Rate

The baud rate refers to the number of signal changes per unit time, which is usually used in analog modulation. In digital communications, the baud rate and symbol rate may be equal or related.

Transmission rate is one of the important parameters in communication system design. High transfer rates can support greater data transfer volumes, but also require wider bandwidth and stronger signal quality. Different applications and scenarios may require different transmission rates. Therefore, factors such as bandwidth, signal-to-noise ratio, and signal-to-interference need to be considered comprehensively when designing a communication system to achieve reliable and efficient data transmission.

4. Modulation and demodulation

Modulation is the process of converting digital data into an analog signal while demodulation is the process of converting an analog signal into digital data. These two processes are especially important in analog transmission.

4.1 Modulation

Modulation is the process of converting digital data into an analog signal. Digital signals are usually discrete while analog signals are continuous. The purpose of modulation is to map a digital signal to an analog signal for transmission on a transmission medium.

4.2 Demodulation

Demodulation is the process of converting an analog signal back to digital data. The receiver uses a demodulator to detect changes in the analog signal and convert it to a digital signal.

5. Coding

The physical layer may encode data to improve the reliability and immunity of data transmission. Common encoding methods include parity check, CRC (cyclic redundancy check), etc.
The following are several common encoding methods:

5.1 Parity (Parity Encoding)

Parity is a basic method of error detection in which an additional bit (odd or even) is added to the data to ensure that the total number of bits in the data is either odd or even. The receiver can detect single-bit errors based on the parity bit.

5.2 Cyclic Redundancy Check (CRC, Cyclic Redundancy Check)

CRC is a more robust error detection method that generates redundant data by appending a polynomial to the data. The receiver can detect errors based on the received data and the attached CRC value.

5.3 Hamming Code (Hamming Code)

A Hamming code is an error-correcting code that can detect and correct multiple bit errors. It adds redundant bits to the data so that any single bit error can be detected and repaired.

5.4 Modulation Coding

In digital communications, modulation techniques generally involve mapping digital data to analog signals. Different modulation methods (such as ASK, FSK, PSK) use different signal modulation methods in data encoding and transmission.

5.5 Data Compression Coding

Data compression coding is used to reduce the storage and transmission overhead of data. It converts data into shorter codes or symbols to reduce the amount of data transmitted.

5.6 Differential Coding

Differential encoding encodes data by recording changes in the data, rather than directly encoding the data itself. It is commonly used in audio and video coding to reduce redundancy between successive frames.

6. Channel multiplexing

The physical layer involves appropriate division and sharing of the transmission medium to enable simultaneous transmission between multiple communication devices. Common channel multiplexing techniques include Frequency Division Multiplexing (FDM) and Time Division Multiplexing (TDM).

6.1 Frequency Division Multiplexing (FDM)

In frequency division multiplexing, different users or communication devices are allocated different frequency bandwidths for communication. Each user transmits data using different frequency subbands, which do not overlap in frequency spectrum. FDM is often used in wireless communication and wired communication, such as radio stations, TV signals, Ethernet, etc.

6.2 Time Division Multiplexing (TDM)

In time division multiplexing, different users or communication devices share channels according to time slices. Each user transmits data in different time periods, and the time slices are switched in turn. TDM is commonly used in digital telephone systems, sensor networks, etc.

The advantage of the channel multiplexing technology is that it can support multiple communication connections at the same time, thereby improving spectrum utilization, reducing communication costs, and improving communication efficiency.

In practical applications, frequency division multiplexing and time division multiplexing can also be combined to form a more flexible channel multiplexing technology, such as frequency time division multiplexing (Frequency Time Division Multiplexing, FTDM) or code division multiplexing (Code Division Multiplexing , CDM) etc.

7. Transmission distance and loss

The physical layer needs to consider the characteristics of the transmission medium, such as transmission distance, signal attenuation and noise, to ensure the reliability of data during transmission.

7.1 Transmission Distance

The transmission distance refers to the distance that the signal propagates in the transmission medium, usually in meters (m) or kilometers (km) as the unit. The distance of the transmission distance directly affects the strength and quality of the signal, and long-distance transmission may face signal attenuation and other challenges.

7.2 Signal Attenuation

Signal attenuation refers to the phenomenon that the signal gradually weakens during transmission. In a transmission medium, the signal is attenuated, resulting in a gradual decrease in signal strength. Signal attenuation is usually determined by the characteristics of the transmission medium and the transmission distance.

7.3 Transmission Loss and Signal-to-Noise Ratio (SNR)

Transmission loss refers to the total loss of a signal during transmission, including attenuation and other factors. The signal-to-noise ratio refers to the ratio between the signal strength and the background noise strength. A lower signal-to-noise ratio can lead to signal distortion and decoding errors.

7.4 Loss Compensation Techniques

In some cases, compensation techniques can be used to combat transmission loss, such as precoding, equalization, and forward error correction codes.

When designing and implementing a communication system, it is necessary to fully consider the influence of transmission distance and loss. Reasonable selection of transmission media, use of appropriate signal amplification technology, and reduction of signal-to-noise ratio can help overcome the challenges in the signal transmission process and ensure stable transmission of signals at different distances and environments.

8. Physical topology

The physical layer involves the physical layout and connection mode of the network, such as star topology, bus topology, ring topology, etc.

The following are some common types of physical topologies:

8.1 Star topology

In a star topology, all devices are connected to a central device, usually a switch or hub. This topology helps simplify connectivity and maintenance, but if a central device fails, the entire network can be affected.

8.2 Bus topology

In a bus topology, all devices are connected to a shared main line (bus). This topology is relatively simple, but if the main line fails, the entire network may be disrupted.

8.3 Ring topology

In the ring topology, devices are connected in the form of a ring, and each device is connected to two devices before and after it. Although this topology is uncommon, it can provide redundant paths for increased reliability.

8.4 Tree topology

A tree topology is a combination of star and bus topologies, and typically consists of multiple stars connected together. This topology can provide some redundancy and scalability.

8.5 Mesh Topology

In a mesh topology, each device is directly connected to other devices, forming a complex interconnected network. This topology is highly redundant and reliable, but can be complex to maintain and manage.

8.6 Hybrid topology

A hybrid topology combines multiple topologies together. For example, a large network might use a star topology at the data center and a bus topology at the branch offices.

Choosing an appropriate physical topology depends on the size of the network, performance needs, reliability requirements, and budget constraints. In addition, with the continuous development of technology, such as wireless network, cloud computing, etc., the concept of physical topology is also constantly evolving. When designing a network, careful consideration must be given to the physical topology to meet specific needs.

9. Connectors and plugs

Connectors and plugs are physical parts used to connect different devices, cables or components. They play a vital role in electronic, electrical, communication and computer systems, enabling various devices to communicate with each other and transfer data and energy. Here are some common connector and plug types:

9.1 USB Connectors and Plugs

Universal Serial Bus (USB) connectors and plugs are used to connect computers, peripherals, mobile devices, etc. USB connectors come in several sizes, including USB-A, USB-B, Micro USB, Mini USB, and USB-C.

9.2 RJ45 connector and plug

RJ45 connectors and plugs are commonly used in Ethernet network connections, such as local area network (LAN) connections and network cables. They are commonly used to connect devices such as computers, routers, switches, etc.

RJ45 crystal head is a connector used to connect Ethernet (Ethernet) cables, usually used in local area network (LAN) and wide area network (WAN). When installing and maintaining the network, it is very important to make the RJ45 crystal head correctly to ensure a stable network connection and high-speed data transmission.

9.2.1 T568A standard

: The T568A standard is a line sequence arrangement standard for RJ45 crystal plugs. Under the T568A standard, the line sequence of the RJ45 plug is as follows:

Pin 1: 白绿
Pin 2: 绿
Pin 3: 白橙
Pin 4: 蓝
Pin 5: 白蓝
Pin 6: 橙
Pin 7: 白棕
Pin 8: 棕

9.2.2 T568B standard

The T568B standard is another commonly used RJ45 crystal connector line sequence standard. Under the T568B standard, the line sequence of the RJ45 plug is as follows:

Pin 1: 白橙
Pin 2: 橙
Pin 3: 白绿
Pin 4: 蓝
Pin 5: 白蓝
Pin 6: 绿
Pin 7: 白棕
Pin 8: 棕

Both standards are common in practical applications. In a network, in order to ensure the consistency of the connection, a standard is usually selected for the production of the RJ45 crystal head. In addition, there is a method of making a wiremap called "Crossover" or "crossover", which is used to connect two computers.

9.3 HDMI Connector and Plug

High-Definition Multimedia Interface (HDMI) connectors and plugs are used to transmit audio and video signals between high-definition televisions, monitors, projectors, and more.

9.4 Audio Connectors and Plugs

Audio connectors and plugs are used to connect audio equipment such as speakers, headphones, microphones and sound systems. Common audio connectors include 3.5mm (1/8") stereo plugs.

9.5 VGA connector and plug

Video Graphics Array (VGA) connectors and plugs are used to connect computers to monitors, especially CRT monitors, which were more common in the past.

9.6 DisplayPort Connector and Plug

DisplayPort connectors and plugs are used to connect computers, monitors, and other multimedia devices, supporting high-resolution video and audio transmission.

9.7 Power connector and plug

Power connectors and plugs are used to connect electronic equipment to a power source. They come in various sizes and forms and are used in different types of devices such as laptops, desktop computers, mobile devices, etc.

9.8 SC, LC, ST and other optical fiber connectors

These connectors are used to connect fiber optic cables to transmit high-speed data and communication signals. At present, the mainstream is the LC type fiber optic interface.

Flat Cables and Plugs

Flat cables and plugs are often used to connect internal electronic devices, such as hard drives inside computers, motherboards, etc.

Sensor Connectors and Plugs

In automation, measurement and control, various sensors often require specific types of connectors and plugs.

Summarize

The physical layer is the basic layer in a computer network, responsible for transferring data from one place to another while ensuring the "reliability" and "integrity" of the data during transmission. It involves a variety of technologies and concepts to achieve the efficient transmission of data on physical media.

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