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3.5 Point-to-Point Protocol PPP
Point-to-point protocol is currently the most widely used point-to-point data link layer protocol.
The PPP protocol provides a standard method for transmitting various protocol data on point-to-point links, which mainly consists of the following three parts of the process:
- Encapsulation method for various protocol datagrams (encapsulation into frames).
- Link Control Protocol LCP is used to establish, configure and test data link connections.
- A suite of Network Control Protocols (NCPs), each of which supports a different network layer protocol.
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PPP (Point-to-Point Protocol) is a point-to-point protocol, usually used to establish a serial connection between computers, such as interconnection through a modem or a serial interface. Here are some key information about PPP:
describe: PPP is a data link layer protocol used to transmit data between two nodes. It provides reliable, byte-oriented transmission and supports multiple network layer protocols, such as TCP/IP, IPX/SPX, etc.
Function: PPP provides three main functions:
Establish and terminate connections: PPP allows devices to dynamically establish and release connections.
Data Link Protocol (LCP): The LCP is responsible for negotiating and managing the configuration and parameters of the connection.
Network Control Protocol (NCP): NCP is used to configure and manage upper-layer network protocols, such as IP address allocation, compression and encryption configuration, etc.
features：
Supports two-way communication: PPP provides full-duplex communication capability, enabling nodes to send and receive data at the same time.
Provides error detection and correction: PPP uses CRC (Cyclic Redundancy Check) checksums to detect and correct errors in transmission.
support authentication: PPP supports authentication of connection identities to ensure security.
scalability: PPP supports different network layer protocols through different NCP protocols.
Application Scenario: PPP is often used in scenarios such as establishing a dial-up Internet connection, remotely accessing a LAN, and constructing a virtual private network (VPN).
3.6 Media Access Control
3.6.1 Basic Concepts of Media Access Control
One of the most important issues to be considered in the shared channel is how to coordinate the occupation of a shared transmission medium by multiple sending and receiving stations, that is: Media Access Control MAC.
With the development of technology, the maturity of switching technology and the reduction of cost, the switched LAN with higher performance using point-to-point links and link layer switches has completely replaced the shared LAN in the wired field, but due to the broadcast of wireless channels By nature, WLAN still uses shared media technology.
3.6.2 Media Access Control - Static Channel Assignment
Channel Multiplexing: Multiplexing is a major concept in communication technology. Multiplexing is to simultaneously transmit signals of multiple users through one physical line.
When the transmission capacity of the transmission medium in the network is greater than the total traffic transmitted by multiple single channels, multiplexing technology can be used to establish multiple communication channels on one physical line to fully utilize the bandwidth of the transmission medium.
Common channel multiplexing technologies: frequency division multiplexing FDM, time division multiplexing TDM, wavelength division multiplexing WDM, code division multiplexing CDM.
Code Division Multiplexing CDM is another method of sharing channels. In fact, since this technology is mainly used for multiple access, the more commonly used term is multiple access CDMA.
Different from frequency division multiplexing and time division multiplexing, each user of code division multiplexing can use the same frequency band for communication.
Since each user uses a specially selected different code pattern, there is no interference between users.
CDM was originally used for military communications, because the new numbers sent by this system have strong anti-interference ability, and its spectrum is similar to white noise, which is not easy to be detected by the enemy.
With the advancement of technology, the price and volume of CDMA equipment have dropped significantly, because it is now widely used in civil mobile communications.
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3.6.3 Random Access - CSMA/CD Protocol
CSMA/CD protocol: Carrier Sense Multiple Access/Collision Detection Protocol
Multiple Access MA: Multiple stations are connected to a bus and compete to use the bus.
Carrier Sense CS: Before each station sends a frame, it must first check whether there are other stations on the bus that are sending a frame (listen first and then talk).
- This frame is sent if the bus is detected to be idle for 96 bit times.
- If it detects that the bus is busy, it will continue to detect and wait for the bus to be idle for 96 bits, and then send this frame.
Collision detection CD: Each station that is sending frames detects collisions while sending (talking and listening)
- Once a collision occurs on the bus, stop sending immediately, back off for a random period of time and then send again (stop talking immediately once a collision occurs, and wait for the opportunity to start talking again)
3.6.4 Random Access - CSMA/CA Protocol
CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance) is a random access protocol commonly used in wireless local area networks (WLAN) to coordinate multiple devices for transmission on a shared medium. Here are some key points about the CSMA/CA protocol:
Description: The CSMA/CA protocol avoids collisions by "listening" for activity on a shared channel. It introduces the random waiting time before sending into the data transmission process to reduce the probability of collision and increase the probability of successful transmission.
Fundamental:
Listening (Carrier Sense): The device will detect whether the channel is busy before sending data. If the channel is occupied by another device, wait for the channel to be free.
Random wait time (Random Wait): When the channel is idle, the device will introduce a random wait time to prevent collisions caused by multiple devices starting to send data at the same time.
Data transmission and confirmation (Collision Avoidance): After the device sends data, the receiver will return a confirmation message. If a collision occurs, the sender will reselect a waiting time according to the backoff algorithm and try to send again.
advantage:
Collision avoidance: Due to the introduction of random waiting time, CSMA/CA reduces the probability of collision and helps to improve transmission efficiency.
Fairness: CSMA/CA allows multiple devices to share channels according to certain rules, preventing certain devices from monopolizing bandwidth resources.
shortcoming:
Delay: Due to the introduction of waiting time, CSMA/CA will introduce a certain transmission delay, especially when the channel is congested, the delay may be greater.
