Critica a los impuestos especiales como extrafiscales

The backbone of a multi-agent scheduling system is its agent structure and the communication and control mechanism among agents. According to the agent structures, Multi-Agent Systems can be generally classified into the two major categories as discussed previously, namely, systems with a hierarchical structure and systems with a heterarchical structure.

6.3.4.1

Hierarchical structure

In a hierarchical structure, there is only one agent at the highest level whose responsibility is to supervise the agents at the lower levels so that the overall objective is achieved. Below this agent there are one or several levels, in which various kinds of agents are responsible for their own share of the work. Each agent in the intermediate levels reports its work only to its immediate supervisor, meanwhile monitoring its immediate lower-level agents. At the lowest level the agents usually perform the basic execution, calculation, or information collection tasks. Communication among agents at the same level is possible, but it is regulated by their supervisors. The hierarchical structure is presented, for example, in papers by [Games et al.,

1 994, Lath on et al., 1 994, Maturana & Norrie 1 996, Parunak, 1 987b, Sycara et al. , 1 99 1 b] .

Maturana and Norrie used "mediators", a special kind of agent, to manage manufacturing and

scheduling operations in an agent-based manufacturing system. [Maturana & Norrie 1 996].

These mediators have a three-level architecture. The highest level is called template mediator who decomposes and integrates the tasks. The lower level includes a data-agent manager and an active mediator. Decomposed tasks are scheduled and dispatched to various resources through the coordination of these two types of agents. The lowest level relates to resource agents (controllers).

"MetaMorph I" system provides a framework for enterprise integration [Shen & Norrie, 1 998] . The mediator agents assume the role of system co-ordinators by promoting co-operation among intelligent agents and learning from the agents' behaviour. Mediator agents are able to expand their capabilities to include mediation behaviours, which may be focused on high-level policies to break the decision deadlocks (similarly to the "staff holon/agent" in [Brussel et al., 1 998]). Mediator agents can use brokering and recruiting communication mechanisms to find related agents for establishing collaborative sub-systems ('co-ordination clusters' or 'virtual clusters' ; similarly to "co-operation domains" i n HMS [Brussel et al., 1998]). In "MetaMorph II" (Shen et al., 1 998] the framework of "MetaMorph I" has been applied at the enterprise function level, using a hybrid agent-based mediator-centric architecture to integrate partners, suppliers and customers dynamically with the main enterprise through their respective mediators within a supply chain network via the Internet and Intranets. In MetaMorph II, agents can be used to represent manufacturing resources (machines, tools etc) and parts, to encapsulate existing software systems, to function as system/subsystem coordinators (mediators), and to perform one or more supply chain functions. A prototype implementation has been reported with four mediators: enterprise (enterprise administration centre), design (integrates a functional design system), resource (co-ordinates an agent-based manufacturing scheduling sub-system), and marketing (integrates customer services).

Lathon et al. adopted a two-level structure [Lathon et al., 1 994]. In the higher level is the coordination (global) agent (or so-called "globe agent") whose responsibility is to regulate the low level agents ("machine agents", or subsystem agents). Low level agents execute local scheduling.

Gomes et al. used a tree structure to represent the relationships between agents [Gomes et al., 1 994] . At the top is the strategic agent who checks the global correctness of the schedules. Below it are two types of tactical agents: j ob tactical agents (JTA) and resource tactical agents (RTA). Each RTA's work is further facilitated by its sub-level operational agents who are in charge of local optimisation. In general, the main bottleneck in this system occurs at the coordinating agent.

Sycara et al. presented a model, which is another paradigm of a hierarchical structure [Sycara et al., 1 99 1 b]. In their model all agents make decisions without supervision and intervention, but they all share a common information base, called the coordination agent. The contribution of their research is that they selected what they believe is the most valuable information (so called "texture measures") as the basis for individual decisions.

Parunak presented Description of one of the earliest agent-based manufacturing systems - YAMS (Yet Another Manufacturing System) [Parunak; 1 987b]. In this model each factory and factory component is represented as an agent. Each agent has a collection of plans, representing its capabilities. The Contact Net is used for inter-agent negotiation.

6.3.4.2

Heterarchical structures

In heterarchical structures [Baker, 1 99 1 , Duffie, 1 990, Duffie & Prabhu, 1 994, Lin & Solberg, 1 992, Saad et al.; 1 997, Shaw, 1 987a, 1 987b, Parunak et al., 1 998b, Wang & Usher, 2002] various types of agents have no control over one another. They simply exchange information and negotiate when conflicts between scheduling decisions occur. There is no "information, computing, and control centre." In this scheme the information and workload are more balanced because of the distributed computation, but the coordination is more difficult to regulate efficiently. Most market-like agent systems have such structure. For example, [Lin & Solberg, 1 994] modelled the manufacturing floor shop exactly like an economic market place. Each task agent enters the market carrying certain "currency." It bargains with each resource agent for the

Goran D. Colak Chapter 06

machines on which it can be processed. Similarly, each resource agent competes with other agents to get more "valuable" j obs. Many decentralised models use Smith's Contract-Net Protocol for communication and scheduling, as outlined in Section 6.2. 3 . 1 "The Contract-Net Protocol".

6.3.4.3

Hybrid structures

Between a hierarchical and a heterarchical structure, some authors adopted a mixed architecture. [Ram os, 1 994] proposed a structure in which task agents and resource agents are coordinated by a task manager and a resource manager respectively, but the two managers are independent of each other. [Brennan et al., 1 997] proposed a Partial Dynamic Control Hierarchy by combining both hierarchical and heterarchical architectures. The use of partial dynamic hierarchies assists reconfigurability and can provide a better system performance than either of the single control approaches. [Choi et al., 2000] described a hybrid control system with a shop floor activity methodology called Multi-Layered Task Initiation Diagram. The architecture of the control model identifies three levels; i.e. the shop floor controller, the intelligent agent controller and the equipment controller. In addition, holonic control was proposed to combine the best of hierarchical and heterarchical control, to provide a high performance under disturbances [Bongaerts et al., 1 999].