【正文】 as returned to the station. ?Frame acknowledgement can be easily implemented. ?Fairness (RoundRobin Fashion) ?Requirement of token maintenance Medium Access Control Method Token Ring 12 Ring Networks Medium Access Control Method Token Ring A B D C A B D C A B D C (a) (b) (c) 13 Ring Networks Frame Format FF = Format bits rr = Reserved bits ZZZZ = Control bits PPP = Priority bits T = Token bit M = Monitor bit RRR = Priority Reservation bits E = Error Detected Indicator A = Address Recognized Indicator C = Frame Copied Indicator 15 位元組 PCF SD DA SA VV0VV000 VV1VV10E AC00AC00 PPPTMRRR FFrrZZZZ FCS ED PCF 訊框標(biāo)頭 (Header) 資料 Ring No. Node Addr. 訊框尾標(biāo) (Trailer) 6 位元組 4 1 1 位元組 1 2 6 6 位元組 VV0VV000 PPPTMRRR VV1VV100 1 1 1 位元組 Token 14 Ring Networks TokenAccess Control ?Single token protocol: idle characters (contiguous 0s) are transmitted if the frame is too short ?The source station will remove the frame and regenerate the free token ?ARI is set by the station who finds the DA is same as its address ?FCI is set by the station who copies the frame 15 Ring Networks ?ARI is set while FCI is not set may indicate that the destination station is active but not able to copy the frame (too busy, out of buffer,...) ?Uniform access: single transmission rather than exhaustive transmission ?The FCS covers from the second PCF bytes to itself. TokenAccess Control 16 Ring Networks Token Monitor Functions ?Token Monitor: One of the active stations in the ring to monitor the token ?Token lost ?Circulating busy token (frame) 17 Ring Networks ?Token Lost ?Cause: Interference or noise ?Detection: The token monitor has a timer which is set upon the passage of either a busy token or a free token. The timer expiration indicates the loss of token. ?Action: The token monitor will regenerate the token Token Monitor Functions 18 Ring Networks ?Circulating busy token ?Cause: Transmitting station fails ?Detection: The Monitor Count flag is reset by the transmitting station and set by the token monitor. If the token monitor detects an already set flag, it is a circulating busy token. ?Action: The token monitor removes the frame and issues a free token Token Monitor Functions 19 Ring Networks Token Monitor Functions ?Multiple tokens ?Cause: Noise ?Detection: Two or more stations are in transmit state. The station will find that the removed frame has a different source address. ?Action: No free token will be issued by that station. As a result, it is in a losttoken condition. 20 Ring Networks ?Minimum Bit Length ?If all stations are idle (in listen sta