Data link layer in two channels-point-to-point communication PPP protocol (1)

Data link layer in two channels-point-to-point communication PPP protocol (1)

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Blog: http://zhaoyuqiang.blog.51cto.com/6328846/1570684

Composition of PPP protocol

● The data link layer protocol can be used for asynchronous serial or synchronous serial media.

● Use LCP to establish and maintain data link connections

● NCP allows multiple network layer protocols on point-to-point connections, such:

● IP, IPX, and AppleTalk.

If the dial-up Identity Authentication succeeds, LCP is responsible for establishing a data link. After LCP establishes a link, NCP will be able to access the Internet, and NCP will assign an IP address to us.

Usage

The user connects to the ISP (China Telecom or China Unicom) through dialing, point-to-point. The ISP assigns an IP address to the user. PPP provides rules between users and ISPs: On the basis of connecting to a network cable, you must follow the rules for dialing and using IP addresses to access the Internet.

PPP can be billed to display the Internet access time and Internet traffic.

PPP protocol frame format

The PPP protocol is byte-oriented, so the length of a PPP frame is the number of full-text lines.

●Requirements for PPP Discussion:

Simple: frames do not need to be corrected, and traffic control is not required. After the receiver receives the frames, CRC is used for detection. If the frames are correctly received, errors are discarded.

Multiple network layer protocols: support the operation of multiple high-level protocols, such as IP protocols.

Multiple types of links: Supports different physical links such as light.

Error Detection: CRC can be used for error detection.

Check connection status: When the dial-up password is incorrect or the connection is incorrect, the PPP protocol will prompt an error message.

Maximum Transmission Unit: Generally, the data to be transmitted cannot exceed 1500 bytes.

Network-layer address negotiation: After successful dialing, the PPP protocol can assign a network-layer IP address to the user.

Data Compression negotiation: for example, to transmit bytes 1111111111, 16 bits. After the compression algorithm, you only need to tell the other party to upload 8 0, 8 1. After receiving the information, the receiver decompress 16 0000000011111111 1 to according to the algorithm. This can save bandwidth.

Encapsulation into frames: from the frame format of the PPP protocol, we can see that the PPP protocol is encapsulated into frames. Transmission is performed on the data link layer in frames.

Transparency: to prevent the frame demarcation sequence 7E from appearing in the information part, the network mistakenly assumes that the frame ends.