【正文】 vation (P=4, R=6) to overwrite the old reservation. 38 Ring Networks (b) Station A received the frame. (1) A issues a free token with a priority of 6 (P=6, R=0) and keeps in mind that it upgrades the priority from 4 to 6 by pushing Sr = 4, Sx = 6. (2) B makes a second reservation (P=4, R=5) on this token (3) D gets this token (Pm = P = 6) (4) D transmits a frame with a priority of 6 and a reservation of 5. Token Ring Priority Scheme Example 39 Ring Networks Token Ring Priority Scheme Example (c) Station C has a frame with a priority of 7. (1) The frame transmitted by D passing B (P=6, R=5) (2) B bypasses the frame. C found that this is a data frame and it has a higher priority (Pm R). (3) C makes a reservation (P=6, R=7) to overwrite the old reservation. D begins to remove the transmitted 40 Ring Networks (d) Station D received the frame. (1) D issues a free token with a priority of 7 (P=7, R=0), and keeps in mind that it upgrades the priority from 6 to 7 by pushing Sr = 6, Sx = 7. (2) A bypasses this frame (3) B makes a reservation again (P=7, R=5) on this token. (4) C gets this token and transmits a frame with (P=7, R=5). Token Ring Priority Scheme Example 41 Ring Networks Token Ring Priority Scheme Example (e) C transmitted the frame. (1) The frame transmitted by C passing A (P=7, R=5) (2) B bypasses the frame. C begins to remove this frame. (3) C issues a free token with (P=7, R=5). (4) D found that the priority level in the ining token (7) is upgraded by itself and downgrades the priority level from 7 to 6 by popping the Sx and Sr. 42 Ring Networks (f) Station A received the downgraded token. (1) A found that the priority level in the ining token (6) is upgraded by itself and tries to downgrade the priority level from 6 to 4. Nevertheless, it also found that the reserved priority level in the token is 5. As a result, the priority level is upgraded to 5 (Sx = 5, Sr=4). (2) A issues a token with (P=5, R=0). B finally found a token for itself. (3)B uses this token to transmit a frame to C. Token Ring Priority Scheme Example 43 Ring Networks Token Ring Priority Scheme Example A B D C 5 4 6 4 5 6 0 4 1 2 4 A B D C 0 6 5 6 5 6 5 5 6 Sx = 6 Sr = 4 1 2 3 4 5 4 3 (a) (b) 44 Ring Networks Token Ring Priority Scheme Example A B D C 0 7 Sx = 6 Sr = 4 5 7 Sx = 7 Sr = 6 0 7 5 7 5 1 2 3 4 A B D C 5 Sx = 6 Sr = 4 5 6 5 6 7 6 7 1 2 3 (c) (d) 45 Ring Networks Token Ring Priority Scheme Example A B D C Sx = 6 Sr = 4 5 5 7 5 7 5 7 5 6 1 2 3 4 A B D C Sx = 6 5 Sr = 4 0 5 5 6 0 5 1 2 3 (e) (f)