Types of nodes

A summary of the characteristics of the predefined node types is shown in the following list. Any node can be activated by an event that refers to it. Others (not all) may be also activated during the network scanning process. Some node types have EL and IL, others only EL, others none of them. When activation is attempted by the scanning process, it may be inhibited in certain conditions (EL empty). Otherwise the code is executed each time that the node is scanned. The user can avoid these repetitions by means of special instructions. All activation of a node starts a scanning process with the exception of type C nodes.

See Chapter 4 for a detailed account of the processing of the different node types.

• I (Input) Generates messages that enter the simulation process. It creates messages and sends them to the successor nodes. It has neither EL nor IL. It may only be activated by an event that refers to it. It is never activated again by the scanning of the network.
• L (Line) Simulates queue discipline. The messages that enter its EL are passed to its IL where they remain in the order indicated by the code. This order may be FIFO (First In First Out) LIFO (Last In First Out) or any other programmed by the user. It is activated by an event or during the scanning of the network if the EL is not empty.
• G (Gate) Stops or allows message flow. It has an EL for the retained messages. It may be activated by an event or during the scanning of the network if the EL is not empty. The instruction STATE, used to change the state of the gate, is only executed if the node is activated explicitly by an event which refers to the node.
• R (Resource) Simulates resources used by messages. A real value (called the node capacity) is associated to the Resource type node. The messages in the EL represent entities that demand a certain quantity of that capacity during a certain period of time. The code has two parts. One examines the incoming messages, the other processes the outgoing messages. The EL is examined and, for each message, it is checked if the demanded quantity is available. If it is so, the message is moved to the IL, the time of future depart is scheduled in the FEL, and the quantity of resource used for the message is subtracted from the available capacity. If there is not enough capacity, the message remains in the EL. When the depart event is executed, the message with the corresponding time of depart is extracted from the IL and it is sent to the successor node. If there are more than one successor, a copy is sent to each of them. The user can also control this process of message sending using the RELEASE instruction. Each time the node moves a message, the network is scanned. The part of the code that processes the departing message is only activated by a departing event and executed only once in the event process. The part that deals with the incoming messages in the EL may be activated during the scanning of the network, but only if the EL is not empty.
• D (Decision) Selects messages from predecessor nodes and sends them to successor nodes. It has an EL for each Š predecessor. Some messages are taking from the ELs and sent to the successor nodes according to the selection rules indicated in the code. It is activated during the scanning process and only if any of the EL has messages.
• E (Exit) Destroys messages. The message in the EL is processed. The code in the node is executed and the message is destroyed. The node is activated during the scanning of the network if the EL is not empty.
• C (Continuous) Solves systems of ordinary differential equations of first order. It accepts instructions of the form <variable>' := <expression>. The system considers a succession of such instructions as a system of differential equations. When the node is activated, the solving process starts. It may be interrupted and continued when the node is deactivated or activated by special instructions. During the execution the node schedules its own activation at each integration step. This activations does not cause scanning of the network. C type nodes have neither EL nor IL.
• A (Autonomous) Executes events at scheduled times. It is only activated by an event that refers to it. It is not activated again during the scanning of the network. It has neither EL nor IL. It can activate itself and other nodes, change variables, and send messages.
• X (General) Used to general processes programmed by the user. These nodes may be designed by any other letter different of G, L, I, D, E, R, C, A. The node is activated during scanning processes (this is the main difference with the A type). It has not EL or IL unless this is explicitly required by the user. The management of these lists must be programmed by the user with the help of GLIDER instructions and procedures.