Page 115 - Intelligent Communication Systems

96 INTELLIGENT COMMUNICATION SYSTEMS

(CW: Call Waiting)
dailtone(A), m-cw(B), path(B, C)
dial(A, B):
ringback(A, B), cw-ringing(B, A), m-cw(B), path(B,C).
(CFV : Call Forwarding Variable):
dialtone(A), path(B, C), m-cfv(B, D), idle(D)
dial(A, B):
ringback(A, D), ringing(D, A), path(B, C), m-cfv(B, D).

A New Rule where CFV is applied
dialtone(A), path(B, C), m-cfv(B, D), m-cw(B), idle(D)
dial(A, B):
ringback(A, D), ringing(D, A), path(B, C), m-cfv(B, D), m-cw(B).
FIGURE 9.15 Conflict resolution.

In Figure 9.15, the following predicates are used.
rn-cw(B) means that a call waiting service is defined at terminal B.
cw-ringing(B, A) means that terminal A is calling terminal B and a ringing
tone is heard at terminal B because of call waiting service.
m-cfv(B, D) means that a call is transferred to terminal D when terminal B is
busy.
In this case a call forwarding variable is chosen. In general, a conflict occurs
when the initial states and operations of two or more rules are the same. In exam-
ples (1) and (2), the initial states of (1) and (2) are dialtone(A), path(B, C) and the
operations of (1) and (2) are dial(A, B). The rules that do not come into conflict are:
Rule (1): dialtone(A), idle(B) dial(A, B): ringback(A, B) ringing(B, A)
Rule (2): dialtone(A), not [idle(B)] dial(A, B): busy-dial(A, B)
The initial states of the foregoing descriptions are not the same, which means
there is no conflict. In general, rules that come into conflict are as follows:
For the event ev(A, B), the initial states A and B are the same and the next
states are defined as follows.
For terminal A, the next states are {PA1(A, B), PA2(A, B),..., PAm(A, B)}.
For terminal B, the next states are {PB 1(B, A), PB2(B, A),..., 1PB«(B, A)}.

This means that states PA1(A, B), PA2(A, B),..., PAm(A, B) may occur simultane-
ously at terminal A. In the same way, for terminal B, states PB1(B, A), PB2(B, A),...,
PBn(B, A) may occur simultaneously. Therefore conflict resolution is needed.