Source Components

The Sampler generates a transaction, and then waits a specified time interval before generating another transaction. The time between transactions, called the inter-arrival time, can be a sample of a random variable (from one of several distributions), a fixed amount, or the value of a variable read from a SAS data set. By default, the inter-arrival time is an observation of an exponential random variable with parameter 1. This means that by default the Samplers follow a Poisson arrival process.

Figure 2.2 shows the Control Panel for the Sampler. The combobox labeled "Inter-Arrival-Time Distribution" enables you to specify the type of distribution. If you press the down arrow a list of distributions is displayed. You select the inter-arrival time distribution by selecting one of these. See Chapter 6, "Random and Exogenous Variation in the Model," for information on the choices of distributions.

The control panel also has a slider for setting the capacity of the Sampler, that is, the number of transactions that will be generated before the Sampler shuts off. The "Transaction Image" button on the control panel enables you to choose a bitmap image that would flow through the network when the simulation is being animated.

The Transaction Pool (the other source component) differs from the Sampler only in that it does not create transactions unless it receives a request for transaction message. In other ways, it is identical to the Sampler.

Request for Transaction

A Sampler passes requests to arcs leading into it. A Transaction Pool generates a transaction and initiates its flow.

Are You Busy Message

The source components pass this request to arcs leading out of them and return their answers.

Query Message

Keyword	Meaning
capacity	returns the capacity.
id	the source component's unique identifier.
on	TRUE if the source component is started, else FALSE.
remaining	returns the number of transactions remaining to be sent.
size	returns the number of transactions sent.

Trigger Message

Keyword	Meaning
reset	stops the source component and resets the number of
	transactions remaining to be sent to 1.
setCapacity	sets the capacity from the transaction attribute "capacity."
setDistribution	sets the distribution from the transaction attribute "distribution."
	This attribute should be a character string whose value is one
	of the distributions: Exponential, Gamma, Erlang, Uniform,
	IUniform, and Deterministic.
setParameter1	sets the first parameter in the distribution from
	the transaction attribute "parameter1."
setParameter2	sets the second parameter in the distribution from
	the transaction attribute "parameter2."
start	starts the source component.
stop	stops the source component.

Icon	Component	Description
	Sampler	generates transactions with prescribed inter-arrival times
	Transaction Pool	a source of transactions