Step allows a single event to occur and the related messages to appear in the appropriate output window. As well as events which are passed to the protocol implementation, there are automatically generated events to which are used internally to display the graphical representation of the frames traversing the communications link. Labels above each output window indicate the latest time of an event in that window.
In the curses version, after Run Interactive Simulation has been selected, the simulator enters a state in which either single stepping or free running can be selected. A typical screen display is shown in Figure 3.
Figure 3: Typical Screen Display Of Interactive Simulation
The output from each protocol is displayed in separate windows. On narrow screens, such as shown in Figure 3, only one will be shown. To switch between the output window of protocol A and that of protocol B, press X key. This toggles between the two windows. The current protocol with the number of packets received is shown at the top of the window. The ``current'' time is displayed in the top right hand corner. If the terminal has a wide screen, or the simulator is running in a wide enough window on a workstation, both output windows will be displayed side by side and the X key will have no effect.
To exit the protocol immediately, press Q and to continue execution without stopping after each event press A.
At the bottom of the screen a list of events that are about to occur. These are displayed in time order and along with the time at which they will occur, the event name and the protocol to which the event is scheduled is also displayed. Only the first 5 events are shown.
The main output is displayed in the middle part of the screen. This informs the user of what event has occurred, the time at which it occurred, and the response the protocol made to the event.
This output is also redirected to an output file. The name of this file can be set by taking the Change Parameters option from the main menu. The parameters used in the particular execution of the protocol are also added to the file.
To advance to the next event for the protocol to execute, press any key (except A, X or Q).
If it is required to exit to the main menu before the simulation is complete, pressing the Q key will do so.
In both versions, if a protocol error occurs, a message box pops up, and must be acknowledged before the simulation can continue.