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219 lines
7.1 KiB
219 lines
7.1 KiB
// WLAN PROTOCOL (two stations)
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// discrete time model
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// gxn/jzs 20/02/02
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mdp
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// COLLISIONS
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const int COL; // maximum number of collisions
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// TIMING CONSTRAINTS
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// we have used the FHSS parameters
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// then scaled by the value of ASLOTTIME
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const int ASLOTTIME = 1;
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const int DIFS = 3; // due to scaling can be either 2 or 3 which is modelled by a non-deterministic choice
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const int VULN = 1; // due to scaling can be either 0 or 1 which is modelled by a non-deterministic choice
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const int TRANS_TIME_MAX; // scaling up
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const int TRANS_TIME_MIN = 4; // scaling down
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const int ACK_TO = 6;
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const int ACK = 4; // due to scaling can be either 3 or 4 which is modelled by a non-deterministic choice
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const int SIFS = 1; // due to scaling can be either 0 or 1 which is modelled by a non-deterministic choice
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// maximum constant used in timing constraints + 1
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const int TIME_MAX = max(ACK_TO,TRANS_TIME_MAX)+1;
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// CONTENTION WINDOW
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// CWMIN =15 & CWMAX =16
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// this means that MAX_BACKOFF IS 2
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const int MAX_BACKOFF = 0;
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//-----------------------------------------------------------------//
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// THE MEDIUM/CHANNEL
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// FORMULAE FOR THE CHANNEL
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// channel is busy
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formula busy = c1>0 | c2>0;
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// channel is free
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formula free = c1=0 & c2=0;
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module medium
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// number of collisions
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col : [0..COL];
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// medium status
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c1 : [0..2];
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c2 : [0..2];
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// ci corresponds to messages associated with station i
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// 0 nothing being sent
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// 1 being sent correctly
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// 2 being sent garbled
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// begin sending message and nothing else currently being sent
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[send1] c1=0 & c2=0 -> (c1'=1);
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[send2] c2=0 & c1=0 -> (c2'=1);
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// begin sending message and something is already being sent
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// in this case both messages become garbled
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[send1] c1=0 & c2>0 -> (c1'=2) & (c2'=2) & (col'=min(col+1,COL));
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[send2] c2=0 & c1>0 -> (c1'=2) & (c2'=2) & (col'=min(col+1,COL));
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// finish sending message
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[finish1] c1>0 -> (c1'=0);
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[finish2] c2>0 -> (c2'=0);
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endmodule
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//-----------------------------------------------------------------//
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// STATION 1
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module station1
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// clock for station 1
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x1 : [0..TIME_MAX];
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// local state
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s1 : [1..12];
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// 1 sense
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// 2 wait until free before setting backoff
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// 3 wait for DIFS then set slot
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// 4 set backoff
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// 5 backoff
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// 6 wait until free in backoff
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// 7 wait for DIFS then resume backoff
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// 8 vulnerable
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// 9 transmit
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// 11 wait for SIFS and then ACK
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// 10 wait for ACT_TO
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// 12 done
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// BACKOFF
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// separate into slots
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slot1 : [0..1];
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backoff1 : [0..15];
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// BACKOFF COUNTER
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bc1 : [0..1];
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// SENSE
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// let time pass
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[time] s1=1 & x1<DIFS & free -> (x1'=min(x1+1,TIME_MAX));
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// ready to transmit
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[] s1=1 & (x1=DIFS | x1=DIFS-1) -> (s1'=8) & (x1'=0);
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// found channel busy so wait until free
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[] s1=1 & busy -> (s1'=2) & (x1'=0);
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// WAIT UNTIL FREE BEFORE SETTING BACKOFF
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// let time pass (no need for the clock x1 to change)
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[time] s1=2 & busy -> (s1'=2);
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// find that channel is free so check its free for DIFS before setting backoff
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[] s1=2 & free -> (s1'=3);
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// WAIT FOR DIFS THEN SET BACKOFF
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// let time pass
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[time] s1=3 & x1<DIFS & free -> (x1'=min(x1+1,TIME_MAX));
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// found channel busy so wait until free
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[] s1=3 & busy -> (s1'=2) & (x1'=0);
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// start backoff first uniformly choose slot
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// backoff counter 0
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[] s1=3 & (x1=DIFS | x1=DIFS-1) & bc1=0 -> (s1'=4) & (x1'=0) & (slot1'=0) & (bc1'=min(bc1+1,MAX_BACKOFF));
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// SET BACKOFF (no time can pass)
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// chosen slot now set backoff
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[] s1=4 -> 1/16 : (s1'=5) & (backoff1'=0 )
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+ 1/16 : (s1'=5) & (backoff1'=1 )
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+ 1/16 : (s1'=5) & (backoff1'=2 )
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+ 1/16 : (s1'=5) & (backoff1'=3 )
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+ 1/16 : (s1'=5) & (backoff1'=4 )
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+ 1/16 : (s1'=5) & (backoff1'=5 )
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+ 1/16 : (s1'=5) & (backoff1'=6 )
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+ 1/16 : (s1'=5) & (backoff1'=7 )
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+ 1/16 : (s1'=5) & (backoff1'=8 )
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+ 1/16 : (s1'=5) & (backoff1'=9 )
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+ 1/16 : (s1'=5) & (backoff1'=10)
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+ 1/16 : (s1'=5) & (backoff1'=11)
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+ 1/16 : (s1'=5) & (backoff1'=12)
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+ 1/16 : (s1'=5) & (backoff1'=13)
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+ 1/16 : (s1'=5) & (backoff1'=14)
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+ 1/16 : (s1'=5) & (backoff1'=15);
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// BACKOFF
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// let time pass
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[time] s1=5 & x1<ASLOTTIME & free -> (x1'=min(x1+1,TIME_MAX));
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// decrement backoff
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[] s1=5 & x1=ASLOTTIME & backoff1>0 -> (s1'=5) & (x1'=0) & (backoff1'=backoff1-1);
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[] s1=5 & x1=ASLOTTIME & backoff1=0 & slot1>0 -> (s1'=5) & (x1'=0) & (backoff1'=15) & (slot1'=slot1-1);
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// finish backoff
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[] s1=5 & x1=ASLOTTIME & backoff1=0 & slot1=0 -> (s1'=8) & (x1'=0);
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// found channel busy
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[] s1=5 & busy -> (s1'=6) & (x1'=0);
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// WAIT UNTIL FREE IN BACKOFF
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// let time pass (no need for the clock x1 to change)
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[time] s1=6 & busy -> (s1'=6);
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// find that channel is free
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[] s1=6 & free -> (s1'=7);
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// WAIT FOR DIFS THEN RESUME BACKOFF
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// let time pass
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[time] s1=7 & x1<DIFS & free -> (x1'=min(x1+1,TIME_MAX));
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// resume backoff (start again from previous backoff)
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[] s1=7 & (x1=DIFS | x1=DIFS-1) -> (s1'=5) & (x1'=0);
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// found channel busy
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[] s1=7 & busy -> (s1'=6) & (x1'=0);
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// VULNERABLE
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// let time pass
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[time] s1=8 & x1<VULN -> (x1'=min(x1+1,TIME_MAX));
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// move to transmit
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[send1] s1=8 & (x1=VULN | x1=VULN-1) -> (s1'=9) & (x1'=0);
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// TRANSMIT
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// let time pass
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[time] s1=9 & x1<TRANS_TIME_MAX -> (x1'=min(x1+1,TIME_MAX));
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// finish transmission successful
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[finish1] s1=9 & x1>=TRANS_TIME_MIN & c1=1 -> (s1'=10) & (x1'=0);
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// finish transmission garbled
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[finish1] s1=9 & x1>=TRANS_TIME_MIN & c1=2 -> (s1'=11) & (x1'=0);
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// WAIT FOR SIFS THEN WAIT FOR ACK
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// WAIT FOR SIFS i.e. c1=0
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// check channel and busy: go into backoff
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[] s1=10 & c1=0 & x1=0 & busy -> (s1'=2);
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// check channel and free: let time pass
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[time] s1=10 & c1=0 & x1=0 & free -> (x1'=min(x1+1,TIME_MAX));
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// let time pass
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// following guard is always false as SIFS=1
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// [time] s1=10 & c1=0 & x1>0 & x1<SIFS -> (x1'=min(x1+1,TIME_MAX));
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// ack is sent after SIFS (since SIFS-1=0 add condition that channel is free)
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[send1] s1=10 & c1=0 & (x1=SIFS | (x1=SIFS-1 & free)) -> (s1'=10) & (x1'=0);
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// WAIT FOR ACK i.e. c1=1
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// let time pass
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[time] s1=10 & c1=1 & x1<ACK -> (x1'=min(x1+1,TIME_MAX));
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// get acknowledgement so packet sent correctly and move to done
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[finish1] s1=10 & c1=1 & (x1=ACK | x1=ACK-1) -> (s1'=12) & (x1'=0) & (bc1'=0);
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// WAIT FOR ACK_TO
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// check channel and busy: go into backoff
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[] s1=11 & x1=0 & busy -> (s1'=2);
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// check channel and free: let time pass
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[time] s1=11 & x1=0 & free -> (x1'=min(x1+1,TIME_MAX));
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// let time pass
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[time] s1=11 & x1>0 & x1<ACK_TO -> (x1'=min(x1+1,TIME_MAX));
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// no acknowledgement (go to backoff waiting DIFS first)
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[] s1=11 & x1=ACK_TO -> (s1'=3) & (x1'=0);
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// DONE
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[time] s1=12 -> (s1'=12);
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endmodule
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// ---------------------------------------------------------------------------- //
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// STATION 2 (rename STATION 1)
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module
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station2=station1[x1=x2,
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s1=s2,
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s2=s1,
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c1=c2,
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c2=c1,
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slot1=slot2,
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backoff1=backoff2,
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bc1=bc2,
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send1=send2,
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finish1=finish2]
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endmodule
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// ---------------------------------------------------------------------------- //
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label "twoCollisions" = col=2;
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label "fourCollisions" = col=4;
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label "sixCollisions" = col=6;
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