Homework 1: Discrete-Event Simulation

Hints, Partial solution, Suggested format of Log File, Directions for mailing homework to Lab Instructor,

Write in C a program to do the discrete-event simulation of a system representing a dentist's office. We assume that there is a dentist and a nurse and that we have five types of patients:

1. Patients that see first the nurse then the dentist
2. Patients that only see the nurse
3. Patients that only see the dentist
4. Patients that see first the dentist then the nurse
5. Patients that see first the nurse, then the dentist, then again the nurse.

Thus the situation is as described in the following queueing network:

                 +<----------------------------------------------+
                 |                                               ^
         SWITCH1 v  ----+       +--+  SWITCH2                    |
 INPUT ---->x----+-->||||------>|  |---->x------------------------>+---> OUTPUT
            |       ----+       +--+     |                       | ^
            |       QUEUE1     SERVER1   |                       | |
            |                            |               SWITHC3 +-+
            v                  (nurse)   v    ----+     +--+     |
            x--------------------------->x---->||||---->|  |---->x
                                              ----+     +--+
                                             QUEUE2     SERVER2
                                                       (dentist)

• The dentist's office is open from INITTIME to FINTIME.
• Patients(Jobs) arrive to the INPUT randomly, say, with interarrival time uniformly distributed in the time interval AMIN..AMAX.
• Arriving patient can be of any type with equal probability.
• At SWITCH1 the patients goes to the nurse or to the dentist in accordance to their types (types 1, 2, and 5 go to the nurse; types 3 and 4 go to the dentist)
• At SWITCH2 the patients that are done with the nurse will go to the OUTPUT if of type 2, 4, or 5 and it is the second visit to the nurse. They will go to the dentist if of type 1, or 5 and it is the first visit to the nurse.
• At SWITCH3 the patients will go back to the nurse if of type 4 or 5, otherwise they will go to the output.
• When patients get to a server (nurse or dentist), if the server is free the patients move directly to the server; otherwise they wait in the queue of that server until their turn to move to the server arrives.
• Patients are serviced at a server for a random amount of time, uniformly distributed, say, in the interval S1MIN..S1MAX and S2MIN..S2MAX respectively at the nurse and the dentist.
• At FINTIME the simulation terminates, that is, we do not consider further the patients that are being serviced or that are waiting in the queues.

The user is asked at run time to specify:

• a seed for the random number generator
• the values of INITTIME, FINTIME, AMIN, AMAX, S1MIN, S1MAX, S2MIN, S2MAX
• the name of a log file where will be logged each significant event [the significant events are the arrival of a new patient into the system, the start of a job at a server (nurse or dentist), and the completion of a job at a server] [Here is the suggested format for the log file.]

Determine:

• The average and the maximum size of each Queue.
• The Utilization of each Server [= time-the-server-is-busy / total-time; where total-time = time of termination of the simulation( = FINTIME) - start time of simulation(= INITTIME)].
• The Average and Maximum Response Time at each Server [or Turn-Around Time = Time a job completes service at the Server - Time that job arrived at that server and queue].
• For each kind of user, compute the Average and Maximum Response Time for the whole system

Run the program a number of times with different values for the parameters and random seed. Examine in correspondence the utilizations and queue sizes. If for a given choice of parameters by changing the random seed we obtain utilization and size values that are stable [i.e. do not change much, say, change at most 10%], then we have a good simulation.

Mail the homework to the TA. Be sure to receive within 24 a message back from the TA confirming that he has received the homework. If you do not receive the reply from the TA contact both the instructor and the TA.