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

1. Ethernet II

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.

The next two bytes identify the upper data type carried by the Ethernet frame, such as hexadecimal number 0x0800 represents IP protocol data, hexadecimal number 0x86dd represents IPv6 protocol data, hexadecimal number 0x809B represents AppleTalk protocol data, hexadecimal number 0x8138 represents Novell type protocol data, and so on.

After the variable-length data field is a 4-byte frame check sequence (Frame. Check Sequence,FCS), using 32-bit CRC cyclic redundancy check to check the data from the destination MAC address field to the data field.

The minimum transmission frame length of Ethernet is 64 bytes. According to the standard, when 10Mbps Ethernet uses repeaters, the maximum connection length is 2500 meters, passing through a maximum of 4 repeaters, so it is stipulated that the minimum transmission time of a frame for 10Mbps Ethernet is 51.2 microseconds. The data that can be transmitted during this period is 512 bits, so it is also called 512 bit hours. This time is defined as an Ethernet slot, or a collision slot. 512 bits = 64 bytes, which is why Ethernet frames are at least 64 bytes. Now, basically, switches or routers are directly connected to computers, and hub connections are rare. When switches or routers are directly connected to computers, there is an independent collision domain between each computer and the device, so there is no conflict. Therefore, it is theoretically possible to send frames less than 64 bytes in length, but in order to follow the original standards, considering that some hub devices are still in use Therefore, the minimum frame length of 64 bytes is maintained.

II. PPP

Point-to-Point Protocol (PPP) provides a standard method for transmitting multiprotocol packets over point-to-point connections. PPP was originally designed to provide an encapsulation protocol for the transport of IP traffic between two peer nodes. In the TCP-IP protocol set, it is a data link layer protocol (layer 2 in OSI mode) used to synchronously modulate connections, replacing the original non-standard layer 2 protocol, namely SLIP. In addition to IP, PPP can also carry other protocols, including DECnet and Novell's Internet Network packet switching (IPX).

PPP (Point-to-Point Protocol) is a link layer protocol designed for simple links such as transmitting packets between equivalent units. This link provides full-duplex operation and delivers packets sequentially. The main purpose of the design is to establish a point-to-point connection to send data through dial-up or dedicated line, making it a common solution for a simple connection between various hosts, bridges and routers.

Point-to-Point Protocol (PPP) provides a standard method for transmitting multiprotocol packets over point-to-point connections. PPP was originally designed to provide an encapsulation protocol for the transport of IP traffic between two peer nodes. In the TCP-IP protocol set, it is a data link layer protocol (layer 2 in OSI mode) used to synchronously modulate connections, replacing the original non-standard layer 2 protocol, namely SLIP. In addition to IP, PPP can also carry other protocols, including DECnet and Novell's Internet Network packet switching (IPX).

III. HDLC

HDLC-- bit-oriented synchronization protocol: High Level Data Link Control (High level data Link Control Protocol). HDLC is a typical representative of bit-oriented data link control protocol, 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 sequentially numbered information frames, which can prevent missed reception or retransmission and high transmission reliability. The transmission control function is separated from the processing function, so it has great flexibility.

Segmentation of frame information

One frame of SDLC/HDLC information includes the following fields (Field), all of which are transmitted from the least significant bit.

SDLC/HDLC 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, called a frame (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.

Address field and control field

After the flag field, you can have an address field A (Address) and a control field C (Contro1). 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.

Information field

The control field is followed by the information field (Information). 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.

Frame check field

Immediately after the information field is the two-byte frame check field, which is called the FC (Frame Check) field and the check sequence FCS (Frame check Sequence). SDLC/HDLC adopts 16-bit cyclic redundancy check code CRC (Cyclic Redundancy Code), which generates a polynomial of CCITT polynomial X ^ 16 + X ^ 12 + X ^ 5 + 1. Except for the flag field and the automatically inserted "0" bit, all the information participates in the CRC calculation. The encoder of CRC adds redundant supervisory code points to each code group when transmitting code groups. 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.

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