Shulou(Shulou.com)06/01 Report--

Ethernet II:

The Ethernet II consists of the MAC address of the 6-byte program, the 6-byte source MAc address, and two types (used to indicate the type of data encapsulated in the Frame) which are above Frame Header, followed by 46m / m 1500-byte data, and 4-byte frame check).

The minimum length of an Ethernet II type Ethernet frame is 64 bytes (6'6'2'46'4) and the maximum length is 1518 bytes (6'6'2 '1500' 4). The first 12 bytes identify the MAC address of the source node that sends the data frame and the MAC address of the destination node that receives the data frame. (note: 1548 bytes after ISL encapsulation and 1522 bytes after 802.1Q encapsulation).

Ethernet II protocol header structure

Each field is explained in detail as follows:

6-byte destination address: the destination MAC address of this packet.

6-byte source address: the source MAC address of this packet.

2-byte protocol type: upper layer protocol, indicating the protocol used by the network layer.

Data: high-level protocols, data, and fillers, ranging from 46 to 1500 bytes.

4-byte FCS: data frame check sequence used to determine whether a packet is damaged during transmission.

PPP:

Frame format:

Flag field-address field-Control field-Protocol-Information part-FCS-- flag field

PPP uses 7EH as the start and end mark (F) of a frame, wherein the address domain (A) and control domain (C) take fixed values (A) and control domain (C). Protocol domain (two bytes) takes 0021H to represent IP packet, 8021H to represent network control data, and C021H to represent link control data. Frame check domain (FCS) is also two bytes, which is used to check the information domain. If 7EH appears in the information domain, it will be converted to (7DHp5EH) two characters. When 7DH appears in the information domain, it will be converted to (7DHPEM 5DH). When the control character of the ASCII code appears in the information flow (that is, less than 20H), a 7DH character is added before the character.

The main difference from HDLC is that PPP is character-oriented and HDLC is bit-oriented.

Features:

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 type oriented protocol.

HDLC:

Frame format:

In HDLC, data and control messages are transmitted in a standard frame format. The frame in HDLC is similar to the character block of BSC, but the data message and control message in BSC protocol are transmitted independently, and the commands in HDLC should be transmitted frame by frame in a uniform format. The complete frame of the HDLC consists of a flag field (F), an address field (A), a control field (C), an information field (I), a frame check sequence field (FCS), etc.

Different from PPP protocol:

Although the format of the two protocols is basically the same, the difference is still obvious. PPP protocol, as its name implies, is point-to-point protocol (POINT TO POINT), which can only communicate between two points and does not have the function of multipoint addressing. HDLC has the function of multipoint addressing. The PPP protocol only borrows the format of HDLC. PPP protocol is defined by IETF and is more and more widely used on INTERNET. The source of HDLC is ITU, which is mainly used in traditional telecom network equipment. For point-to-point (that is, serial port, the package must be PPP), the broadcast port encapsulates HDLC.

Features:

HDLC has the following characteristics: the protocol does not depend on any character coding set; the data message can be transmitted transparently, and the "zero bit insertion method" for transparent transmission is easy to be implemented in hardware; full-duplex communication, which can send data continuously without waiting for confirmation, has high data link transmission efficiency; all frames are checked by CRC, and the information frames are numbered, which can prevent missed reception or reduplication, and the transmission reliability is high. The transmission control function is separated from the processing function, which has greater flexibility and more perfect control function. Because of the above characteristics, HDLC is widely used as the data link control protocol in network design.

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