4. Interfície Ràdio
4.1 Blocs de Recursos i Subtrames
Having proposed to use a combination of norms and trust at negotiation time in section 5.3.2.2, here we test CREDIT to see whether this combination can actually enhance negotiation encounters. Specifically, we applied the following rules to the issues (based on those rules explained in section 5.3.2.2):
• Rule 1 : If T(SU, SP, c)≥0.9 and T(SU, SP, s)>0.95Then avoid negotiating qos.29
This rule means that the SU agent will avoid negotiating the quality of service if it trusts that the SP agent will not defect on the price and the size of bandwidth. This is based on a norm in SU’s group which says thatqosis normally understood to be of a given type if the price and size of bandwith are of a certain value (see table 5.4). The same norm might not apply in the SP’s group.
29We use thresholds above or equal to 0.9 to indicate a high threshold on trust. This imposes stringent
conditions on the trustworthiness of the opponent for the issues the proponent values most. Lower thresholds could be imposed to compensate for any noise in perceiving the performance of the opponent, but we do not consider this here.
• Rule 2: If T(SP, SU, tc)≥0.99 and T(SP, SU, l)>0.9 Thenavoid negotiating usage.
This rule means that the SP agent will avoid negotiating bandwidth usage if it trusts the SU agent will honour its payment in time and if it satisfies the security level (see table 5.4).
Given our expectations regarding the effect of norms and trust on the negotiation process and in particular on the number of offers exchanged in the process (i.e. the negotiation thread) which may determine the time taken to come to an agreement, we postulate the following hypothesis:
Hypothesis 1. The more issues negotiated due to trust being low, the lengthier will be the negotiation thread using standard negotiation strategies.
In order to test this hypothesis, we set aP-SU agent to negotiate with aP-SP agent given the rules set above.30 For this experiment, the two agents use off-the-shelf negotiation
strategies such as Relative and Absolute tit-for-tat.31 In order to vary the number of
issues to be considered at negotiation time, we reduced each agent’s trust in the premises of the norms accordingly and then kept the trust values fixed for each subset of these experiments. For example, one subset of the experiments would involve only Rule 1 above not firing given that trust in price or size would be high (i.e. T(SU, SP, c)>0.99). In order to keep all other variables constant, CREDIT was prevented from modifying the negotiation ranges at runtime as well (as per section 5.3.2.1) since changing negotiation ranges changes the number of possible agreements and the value of those agreements to the agents.
Rule 2
Rule 1 Fires Does not Fire
Fires 3.084 3.286
Does not Fire 3.308 3.47
Table 5.9: The effect of norms on the average length of the negotiation thread needed to reach an agreement (results from P-SU v/sP-SP with rules 1 and 2 firing alterna-
tively and together)
Table 5.9 outlines the effect of these rules on the negotiation encounters. As can be seen, our hypothesis is validated since agents can reduce the length of negotiation threads (by 11% in the best case) needed on average to reach an agreement whenever they trust 30In this experiment, we disconnect the trust evaluation (used in changing negotiation ranges) com-
ponent of CREDIT in order to specify conditions where trust is low independent of the strategy and thus simplify the analysis we wish to make here.
31Other negotiation strategies could be used but the variable we study here is not strictly dependent
on the negotiation strategy and it is only our intention to show that a difference in the number of issues considered will affect the negotiation efficiency. We therefore use those simple negotiation strategies that rely on very few heuristics for ease of analysis.
No. of issues 4 5a 5b 6 Average Utility 0.465 0.4640 0.4645 0.4647
Table 5.10: The effect of the number of issues negotiated on the average utility of agreements for a P-SU agent (results from P-SU v/s P-SP). 5a and 5b represent the five issues that are negotiated given that only one rule fires for either the SP or SU
respectively.
their counterparts.32 While this enables agents to achieve agreements faster (i.e. they
might take less time to negotiate), it is questionable whether the conclusion of the rules (i.e. qos and usage) should be negotiated at all.33 This is because, even though these
issues are given acceptable values according to the norms, it cannot be guaranteed that these values are the best that could be achieved given the preferences of both agents negotiating. In fact, we might expect a trade-off between accelerating negotiation based on norms and making better agreements (i.e. achieving higher utility). However, from the above experiments it was also found that the gain (or loss) in average utility achieved wasnotsubstantial when more issues were negotiated as can be seen in table 5.10 (in this case the agent lost utility when more issues were negotiated since the newly negotiated issues are assigned lower values than those they usually get when trust is high and the issue is not negotiated).34 This is because the issues, only negotiated due to rules not firing, do not have substantial weight in the utility function of the agent. Otherwise, we would expect these issues to form part of the initial negotiable set (e.g. price and size are important issues that need to be negotiated since they contribute significantly to the utility of the agent).
In summary, the above results tell us that CREDIT will cause fewer issues to be nego- tiated when trust is high and more issues to be negotiated when trust is low. Moreover, CREDIT has been shown not to significantly reduce the maximum achievable utility in negotiated contracts by applying norms (when the issues added do not significantly impact on the utility of agents). Therefore, we decided to keep the rules above in future negotiations in the following experiments in order to speed up negotiations.
32These results were tested for statistical significance using ANOVA (single factor) and the null hy-
pothesis (i.e. that the means of the groups are the same) was invalidated. This follows from the fact thatF = 9.07> Fcrit= 3.5>1, withp= 5.8×10−6,α= 0.01, and a sample size of 500. This result proves that each rule indeed has an effect on the outcome. To further investigate the interaction between different rules, we used ANOVA on the results for rule 1 and rule 2 alone firing. The results are as follows forα= 0.025 and a sample size of 500: F= 5.004< Fcrit= 5.006, andp= 0.0258> α. As can be seen, the means do not significantly differ in this context (i.e. since F < Fcrit). This is because these two samples actually test the negotiation length using the same number of issues where these issues have been obtained from different rules firing. The low value ofm= 3.084 can be explained by the fact that less issues have to be negotiated (i.e. four issues as compared to five or six in the other cases).
33In our experiments, when the issues are not negotiated, we choose the value lying in the middle of
the intersection of the agents’ acceptable ranges for these issues.
34These results were checked for statistical significance using ANOVA (single factor) which tries to
identify significant differences between means of different samples (of 500 elements) of utility of contracts. Thus, the means obtained were found not to be significantly different (i.e. the null hypothesis that the means are the same is validated) given F = 0.08< Fcrit = 3.6 with α= 0.12. This means that the difference in means is more due to chance than the number of issues’ influence.