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【正文】 Ring Priority Scheme Operation 27 Ring Networks Token Ring Priority Scheme Operation A D B C A D B C (a) (b) Reservation for Higher Priority token Low Priority Token 28 Ring Networks Token Ring Priority Scheme Operation A D B C A D B C (c) (d) High Priority Token 29 Ring Networks Token Ring Priority Scheme Operation A D B C A D B C (e) (f) HIgh Priority Token 30 Ring Networks Token Ring Priority Scheme Operation A D B C A D B C (h) (g) Low Priority Token 31 Ring Networks Token Ring Priority Scheme ?Pm : Priority of message to be transmitted by station ?Pr: Received priority ?Rr: Received reservation ?R: Transmitted reservation ?Sr: Stack to store the old priority level 32 Ring Networks ?Sx: Stack to store the new priority level Token Ring Priority Scheme R P Rr Pr Sx 堆疊 Sr 堆疊 A Token or Frame Token or Frame Pm 33 Ring Networks Token Ring Priority Scheme ?A station wishing to transmit must wait for a token with Pr = Pm. ?While waiting, a station may reserve a future token at its priority level (Pm). ?If a data frame goes by, it sets the reservation field to its priority (R Pm) if the reservation field is less than its priority (Rr Pm). ?If a token goes by, it sets the reservation field to its priority (R Pm) if the Rr Pm and Pm Pr. ?This has the effect of preempting any lowerpriority reservation. 34 Ring Networks ?When a station seizes a token, it sets the token bit to 1, the reservation field to 0, and leaves the priority field unchanged. ?Following transmission, a station issues a new token with the priority set to the maximum of Pr, Rr, and Pm, and a reservation set to the maximum of Rr and Pm. Token Ring Priority Scheme 35 Ring Networks Token Ring Priority Scheme Issue Token and Alter Token P Pr P Max(Rr,Pm) R 0 P Max(Rr,Pm) Sx P Sr Pr R 0 P Max(Rr,Pm) Sx P PoP Sx Rr = Pr amp。 Sx Pr 傳送訊框 傳送訊標 POP Sx R 0 P Rr Sx P R Rr P Sr POP Sr Rr Sr Rr = Sr Pr = Sx 聆聽狀態(tài) 傳送訊標 Issue Token Flow Chart Alter Token Flow Chart 36 Ring Networks Token Ring Priority Scheme Ready to Transmit POP Sx R 0 P Rr Sx P R Rr P Sr POP Sr Rr Sr Rr = Sr Pm Pr Pr = Sx 欲傳送具優(yōu)先權(quán) Pm 之訊框 傳 送 訊 標 Pm = Pr 傳 送 訊 框 R Pm Rr Pm Pr 訊標到達 R Pm Rr Pm 轉(zhuǎn) 送 訊 框 訊框到達 37 Ring Networks Token Ring Priority Scheme Example (a) Station A has a frame with a priority of 4. Stations B and D have frames of priorities 5 and 6, respectively. (1) A transmitted the frame (P=4, R=0) (2) B makes a reservation (P=4, R=5) (3) D found that its priority is higher than R (65) (4) D also makes a reser
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