Impacto a nivel interno en Haití derivado de la ejecución del LEVE y LEAD:

• Problems can be diagnosed and insight can be gained in a complex system.

• Through simulation, constraints can be identified.

• An understanding can be gained on how a system really operates.

• A plan and the running of a facility can be visualised.

Banks (1998) further identifies the following disadvantages:

• The development of a model that imitates a real-world system is a complex art.

• The output of a simulation model may be random variables and may be difficult to interpret.

• Inappropriate use of simulation may occur.

Despite these disadvantages, simulation is an appropriate technique for the experiments in this study, as stated in sections 2.1 and 5.4. By using simulation, future airport concepts can be tested and the dynamic nature of airports can be modelled.

6.3 Components of a simulation model

Different components present in a simulation include the inputs, outputs and states, the entities and their attributes, activities and events, resources and sta-tistical collectors. These components will be discussed in section 6.3.1 to6.3.5.

6.3.1 Inputs, outputs and states

The actions of the environment that influence the system are the inputs to the system. In an airport traffic problem one input is the arrival rate of the aircraft.

The state of the system depends on changes in the system condition that are caused by the inputs. The state of the airport would be the number of aircraft on ground at any given time, in other words, the level of congestion at the airport.

The outputs of the system are needed to answer questions about the system and

6.3 Components of a simulation model

can be derived from the system state. In the airport problem, one of the outputs can be the average time spent by an aircraft at the airport (White & Ingalls, 2009).

6.3.2 Entities and attributes

In discrete event simulation, the arrival of entities is the input for the simulation.

The entities flow through the system and change the state of the system variables.

In the airport problem, the aircraft and the passengers are the entities. These entities have unique characteristics, called attributes. The attributes of an aircraft can be its size and its arrival time (White & Ingalls,2009).

Entities can be external and explicitly created and manipulated by the mod-eller, or internal and implicitly created and manipulated by the software. A resource provides a service to entities and often has limited capacity. Entities then have to compete or wait to be served by the resource.

When an entity has to wait for the use of the resource, it experiences a delay.

As entities move through the system, they change states. These states are the active state, ready state, time-delayed state, condition delayed state and dormant state. An entity is in the active state if it is moving. It will continue moving until it reaches a delay of some type. It will then go into another state and another entity will possibly enter the active state. In the case where two or more entities need manipulation at the same time, only one will continue moving while the others will experience a delay. The entities that are delayed are known to be in the ready state. These entities are waiting to go into the active state. Sometimes, entities are delayed for a known time period before entering the ready state. These entities are in the time-delayed state. Entities may also be delayed until some sort of condition is satisfied. These entities are in the condition-delayed state.

When entities are delayed and cannot move into another state through changes in the condition of the model, they are in the dormant state (Schriber & Brunner, 2009).

6.3 Components of a simulation model

6.3.3 Activities and events

Processes in a simulation are called activities and changes in the state of the sys-tem are called events. Entities interact with activities to create events. Activities are divided into three categories: Delays, queues and logic.

A delay is caused when the flow of an entity is put on hold for a definite time period. A delay in the flow of an aircraft would be the time spent by the aircraft at the gate/parking space. This time period can be either constant or random.

When the delay of any entity starts, an event occurs. If the time period of the delay is d time units, then the entity will be delayed until the current time plus d time units has expired, after which it will carry on with its movement (exit the parking area to depart) and create another event.

A queue occurs when an entity’s movement is put on hold for an unspecified time period. For example, if all gates are occupied, arriving aircraft will have to wait in some sort of a queue until a gate becomes available.

Logic activities are manipulation of the state variables. Thus, only allowing an entity to affect the state of the system if it is desirable. An example of a logic activity is the decision of whether or not an aircraft is allowed to land and thus affect the level of congestion at the airport (White & Ingalls, 2009).

6.3.4 Resources

Resources in a simulation are those things that have constrained capacities. In the airport problem, the resources are the gates to which aircraft must be assigned and where the aircraft park and wait for the passengers to board (White & Ingalls, 2009).

6.3.5 Statistical collectors

The parts of the simulation used to gather statistics on the values of performance measures or global variables are called statistical collectors. There are three types of statistics, namely counts, time-persistent statistics and tallies.

Counts are used to count for instance the number of aircraft that arrived at the airport in one hour. Time-persistent collectors give a time-weighted average