EE535 - Project 4

DQDB Bus Contention Resolution Interactive Applet

5. Interactive DQDB Examples - Java Applet

There are two different access modes used in DQDB: Queued arbitrated (QA) and prearbitrated (PA). This Java applet implements only the first of the two. The number of nodes in the network will be user selectable, between a minimum of 2 and a maximum of 5 nodes. If the network appears to be busy at that time, the node will wait. The state of each node (request and countdown counters) will be shown. This applet assumes that all hosts are transmitting information to another host located to their right. Therefore, the forward bus is always Bus A, and the reverse bus is always Bus B in this applet.

Operation of the applet: The user is presented with a menu to select the number of hosts that are to be simulated. Select the chosen number of hosts and click on "Start/Reset" to begin the simulation. A cell (or slot) begins to travel on the reverse bus with a request bit (Req) of 0. In order to indicate that a host wants to send a packet, the user must click with the mouse on the host. This will change the state of the host from "-Idle-" to "-Trying-" to indicate that it is now trying to get queued for access to the forward bus. The simulation then continues with slots passing by buses A and B and changing appropriately the state of the RQ and CD counters in each host.

At any moment in time, the user can press "Stop/Pause" on the control panel of the applet to pause the execution of the applet, for the user to examine more carefully the current state of all counters and registers. In order to resume the simulation, press the "Continue" button.

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