Data link layer (Layer 2) data frame structure

The data link layer combines bits into bytes and combines bytes into frames. Frames are used at the data link layer. packets transmitted from the network layer are encapsulated into frames for transmission based on the type of media access.

An Ethernet site uses a set of digits called Mac frames to transmit data frames between sites.

In the frame format, CRC is used for error detection. But remember, this is error detection, not error correction.

802.3 Frame Structure
Leading 8 bytes	Da 6 bytes	SA 6 bytes	Length 2 bytes	Data	FCs

 

Etllernet Frame Structure
Leading 8 bytes	Da 6 bytes	SA 6 bytes	Type 2 bytes	Data	FCs

Note: It is called the tunneling technology to encapsulate a frame into different types.
The following is a detailed description of the fields of frames 802.3 and etllernet.

Preambie)It uses an alternating mode of 1 and O to provide a 5 MHz clock signal at the beginning of each packet to allow the bitstream to be locked into by the device.

Start fra1ne deiimiter, SFD/synch)The leading value is 7 bytes, And the SFD value is 1 byte (synch ). SFD is 10101011. The last one here allows the receiver to enter the alternate 0 and 1 modes somewhere in the middle, but the receiver can still synchronize and detect the beginning of the data.

Destination Address (DA)It first uses the lowest valid bit (IEB) to transmit the 48-bit value. The receiving site uses da to determine whether a incoming data packet is sent to a specific node. The destination address can be a separate address or a broadcast or multicast MAC address. Remember, the broadcast address is full 1 (in hexadecimal format, full F) and sent to all devices, but the multicast address is only sent to a subset of the same type of nodes in the network.

Source Address (SA)Sa is a 48-bit MAC address used to identify sending failures. It uses LSB first. In the SA field, the broadcast and multicast address formats are invalid.

Length or Type)802.3 uses the length field, but the Ethernet frame uses the type field to identify the network layer protocol. 802.3 the upper-Layer Protocol cannot be recognized and must be used with a dedicated LAN (such as IPX.

Data)This is a packet sent from the network layer to the data link layer. It can be in the size of 46 ~ Changes Between 1500 bytes.

Frame verification sequence (fraiue check sequence, FCS)A field located at the end of a frame. It is used to store cyclic redundancy check (CRC ).