IEEE 802.5 (Token-Ring) MAC frame

Picture 1: IEEE 802.5 (Token-Ring) MAC frame


The IEEE 802.5 standard is so called Token-Ring LAN network. The MAC frame for this standard is defined as shown on Picture 1. The first field in the frame is SD - Start Delimiter. The second field is AC - Access Control and the third field is FC - Frame Control. All three fields are one byte long. The AC field consists the bit array: PPPTMRRR. The first three bits PPP are bits for priority. Next two bits TM are T - token and M - monitoring. The last three bits RRR are for priority request. The next fields in the frame are DA - Destination Address and SA - Source Address, both of them are 2 or 6 bytes long. After them comes the INFO field which is reserved for the data information, then FCS - Frame Check Sequence field which is 4 bytes long, then ED - End Delimiter field one byte long and the last field in the frame is FS - Frame Status. As shown on the Figure 1, the three fields SD, AC and ED are actually the Token.