Mac层帧结构。介质访问控制层(MAC)帧被称为MAC协议数据单元(MPDU)，其长度不超过127个字节。它具有四种不同的帧形式，即信标帧、数据帧、确认帧和命令帧。-Mac layer fr a me structure. MAC layer (MAC) fr a me is called MAC Protocol Data Unit (MPDU), whose length does not exceed 127 bytes. It has four different forms of fr a mes, i.e., a beacon fr a me, data fr a me, the acknowledgment fr a me and a command fr a me.