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Protocol analysis

The data link layer belongs to the lower layer of the computer network, and it mainly uses two channels: point-to-point channel and broadcast channel. Here we focus on the characteristics of point-to-point channel and the functions and characteristics of HDLC protocol and PPP protocol used in this channel. No matter which protocol is used in the data link layer, there are three basic common problems: encapsulation into frames, transparent transmission and error detection.

1century HDLC protocol

In the era of poor quality of communication lines, data link transmission uses a reliable transmission protocol-HDLC protocol. HDLC protocol is not only a high-level data link control that can achieve reliable transmission, but also a typical representative of bit-oriented data link control, which does not depend on any kind of character coding set; data packets can be transmitted transparently, and the "zero-bit insertion method" for transparent transmission is easy to be implemented in hardware; full-duplex communication has high data link transmission efficiency. All frames are checked by CRC, and the information frames are sequentially numbered, which can prevent missed reception or duplicates, and has high transmission reliability; the transmission control function is separated from the processing function and has greater flexibility.

A frame message of the HDLC protocol includes the following three fields, all of which are transmitted from the least significant bit.

1.SDLC/HLC flag character

The SDLC/HDLC protocol stipulates that all information transmission must begin with a flag character and end with the same character. The flag character is 01111110, called the flag field (F). A complete unit of information is formed from the start flag to the end flag, which is called a frame. All information is transmitted in the form of frames, and flag characters provide the boundary of each frame. The receiver can establish frame synchronization by searching for "01111110" to find out the beginning and end of the frame.

two。 Address field and control field

After the flag field, you can have an address field An and a control field C. The address field is used to specify the address of the secondary station with which it communicates. The control field can specify several commands. SDLC specifies that the widths of fields An and C are 8 bits. HDLC allows the A field to be of any length and the C field to be 8 or 16 bits. The receiver must check the first bit of each address byte, which is followed by another address byte if it is "0" and the last address byte if it is "1". Similarly, if the first bit of the first byte of the control field is "0", there is a second control field byte, otherwise there is only one byte.

3 information field

The control field is followed by the information field. The information field contains the data to be transmitted and becomes the data field. There does not have to be an information field for every frame. That is, the information field can be 0, and when it is 0, this frame is mainly a control command.

4. Frame check.

After the information field is a two-byte frame check field, which is called the FC field, and the check sequence FCS. When receiving, the decoder can correct the error code within the error correction range, and check the error code within the error correction range, but can not correct it. Multi-bit errors beyond the scope of correction and error correction will not be detected by calibration.

2.PPP protocol

PPP is a multi-protocol framing mechanism that is suitable for use on modems, HDLC bit sequence lines, SONET and other physical layers. It supports error detection, option negotiation, header compression, and reliable transmission using the HDLC type frame format (optional). PPP protocol is the most widely used protocol in modern data link layer.

1. Function

Compared with other protocols, it has the following functions:

(1) PPP has the ability to assign ip addresses dynamically, allowing IP addresses to be negotiated at the time of connection.

(2) PPP supports a variety of network protocols, such as TCP/IP, NetBEUI, NWLINK, etc.

(3) PPP has the ability of error detection and correction, and supports data compression.

(4) PPP has the function of authentication.

(5) PPP can be used on many types of physical media, including serial lines, telephone lines, mobile phones and optical fibers (such as SDH). PPP is also used for Internet access.

two。 Its frame format is as follows. Characteristics

PPP protocol is a point-to-point serial communication protocol. PPP has functions such as handling error detection, supporting multiple protocols, allowing IP addresses to be negotiated at connection time, allowing authentication, and so on. PPP provides three types of functions: framing, link control protocol, LCP; network control protocol, NCP. PPP is a character-oriented protocol, as opposed to HDLC's bit-oriented protocol.

The transmission of the data link layer relies on these two protocols to achieve reliable transmission, and PPP is developed based on HDLC. It has some features and functions of HDLC and adds the functions of other protocols, which makes it the most widely used protocol.

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