The term MSDU (MAC service Data Unit) is often seen in 802.11, which actually refers to packets sent from the network layer. Then add the header of the MAC layer to form an MPDU, and add the header of the PHY layer and the preamble to form a PPDU.
The packet format of the wired network Ethernet is similar.
The current 802.3 SNAP Ethernet format packet is as long as this:
Field: Preamble SFD Destination Address Source Address Length Destination Service Access Entry Source Service Access Entry Control Vendor Identification Ethernet Type Data Frame Check Sequence
Length: 7 1 6 6 2 1 1 1 3 2 38-1492 4
meaning:
Destination Address Source Address Length Destinamtion Service Access Point Source Service Access Point SubNetwork Access Protocol Data Frame Check Sequence
The preamble is a sequence of 1, 0, and the SFD is start of frame delimiter, 10101011. Indicates that the frame is about to start. Sometimes it is also included in the preamble. The preamble is designed to be so long because I was afraid that the PHY could not handle it, and it may be lost. If it is more, it will be lost. Now, generally 14 bits can be synchronized .
The orange part is the Data link Layer Header. This is very easy to understand, because the data comes from the network layer, the upper layer of the data link layer, so the header of this layer needs to be added.
The green part is the header of the logical link layer (LLC header). What is this for? In fact, this provides an interface for communicating with the data link layer for the upper network. What is the blue part (SNAP)? After the official version of the protocol, another patch was made to be compatible with Ethernet II, and the blue part was the product of the official patch.
Because earlier Ethernet II did not have an LLC header, but the LENGTH part of the data link layer header was Ethertype. To be compatible with this, SNAP added SNAP, and the second field was Ethertype.
Novell released a version before the official launch of Ethernet II (extended ethernet) and 802.3. It does not have an LLC header, but has a manufacturer identifier starting with FFFF ( it should be 3 bytes, because then its data is 1497 Byte ), FFFF indicates that checksum is not used. This version is called Novell or 802.3 Raw version. The difference from 802.3 is that there is no LLC header.
So is the packet currently processed by 802.11 a maximum length of 1492 or 1497? Or is it 1500 length? Or does it include LLC header and/or SNAP header?