Elements of a Simulation Model
How can we construct a simulation model without
being concerned about implementation details?
 which are similar in every problem
 have nothing to do with the model itself
 can be dealt with automatically
Make compiler responsible
Model views

static: if time is stopped, of what elements does
world consist?

dynamic: process which moves the world from
state to state
Entities (Temporary entities)

dynamic objects that enter and leave system

represent real things or intangibles

get service from one or more resources

examples: car, patient, customer, order, …
Resources (Permanent entities)

stay in system

can be engaged with one or more temporary entities at
the same time

can represent a group of servers

examples: doctor, machine, traffic ligth, toll-booth, …
Distinction is not along lines of physical movement (or not)
doctor visits patient or vice versa
Distinction depends on situation
car in repair shop versus taxi servicing customers
Attributes (Local)

entities have attributes with specific values for
each individual entity

examples:
arrival time, due date, priority, type,
capacity needed, …
Variables (Global)

not tied to specific entity

built-in variables:
number in queue, time, …

user-defined variables:
current shift, number in system, …

statistical accumulators:
number …, highest number in …,
total time, longest time …
Queues

when an entity cannot move on
e.g. resource unavailable, …

capacity:
limited buffer space, …

policy:
FIFO, …
Events

something that happens at an instant

may change attributes, variables, …

examples
ENTER the system
LEAVE the system
TERMINATE simulation

Event Record
event type
event time
entity involved

Event Calendar
events that are supposed to happen in
the future
event records in ascending order of time
Clock

model of time: variable-length increment

close interaction with event calendar

clock update phase
take event in front of event calendar
advance clock to scheduled occurrence
time of that event
put event in current event group
WHILE (scheduled occurrence time of
next event in calendar = clock)
take event from event calendar
put event in current event group
END

scanning phase
take first event from current event group
WHILE (not end of current event group)
PROCESS EVENT
IF (blocking condition removed)
restart scanning phase
END
Process Event

remove event from current event group

schedule new event;
add event to event calendar
(or to current event group)

move entity involved as far as possible
through its life-cycle

update states and statistics
Process Event
IF (event is ARRIVAL)
update statistics
schedule next ARRIVAL
add event to event calendar
IF (server is busy)
put entity in queue
update queue length
ELSE
make server busy
schedule DEPARTURE
add event to event calendar
ELSE IF (event is DEPARTURE)
update statistics
IF (queue is empty)
make server idle
ELSE
take first entity from queue
update queue length & stats
schedule DEPARTURE
add event to event calendar
Phases of a simulation project
1. Determine purpose
2. Build model
3. Validate model
4. Design and run experiments
5. Analyze results & draw conclusions
 Sequence depends on circumstances
 Iteration possible
Identifier
Avg
Var
Min
Max
#Obs
_______________________________________________
time
: 60.0
Server Q Time
.73822
1.3780
0
3.5809
59
# in Server Q
.72592
1.7489
0
6.0000
0
Server Busy
.53786
.92694
0
1.0000
0
time
: 480.0
Server Q Time
.38699
1.7742
0
3.5809
472
# in Server Q
.38054
2.3000
0
6.0000
0
Server Busy
.47220
1.0572
0
1.0000
1
time
: 1e+006.0
Server Q Time
.49301
1.7393
0
14.418 997217
# in Server Q
.49164
2.2422
0
17.000
0
Server Busy
.49783
1.0043
0
1.0000
0
Identifier
Avg
Var
Min
Max
#Obs
____________________________________________
Replication 1 of 5 (ended at time 60.0)
Server Q Time
# in Server Q
Server Busy
.73822
.72592
.53786
1.3780
1.7489
.92694
0
0
0
3.5809
6.0000
1.0000
59
0
0
Replication 2 of 5 (ended at time 60.0)
Server Q Time
# in Server Q
Server Busy
.30073
.26064
.45774
1.7782
2.2225
1.0884
0
0
0
2.3144
3.0000
1.0000
52
0
0
Replication 3 of 5 (ended at time 60.0)
Server Q Time
# in Server Q
Server Busy
.20550
.21423
.39406
2.0470
2.7542
1.2400
0
0
0
1.8512
4.0000
1.0000
61
1
1
Replication 4 of 5 (ended at time 60.0)
Server Q Time
# in Server Q
Server Busy
.21317
.19185
.41226
1.6442
2.9451
1.1940
0
0
0
1.2277
3.0000
1.0000
54
0
0
Replication 5 of 5 (ended at time 60.0)
Server Q Time
# in Server Q
.58702
.62616
1.3541
1.8374
0
0
2.7351
6.0000
64
0
Server Busy
.56152
.88367
0
1.0000
1
Identifier
Ignore
Wait Preempt
_________________________________________
Machine Q Time
Flowtime
Machine Busy
# in Machine Q
Machine Available
Production
.25444
3.7669
.83559
.28638
.60000
2
.85275
3.7669
.71593
.40604
.60000
2
.85275
5.7669
.58260
.40604
.60000
2
Arrive
0
Arrive 1
Server
Server 1
Depart
Depart 1
Enhancing the Animation
Scrap
Rework
0
0
Part A
Prep
Inspect
Depart
Rework
Salvage
Scrap
0
Sealer
Arrive
Server
Part A Arrive
Depart
Part A Prep
Part B
Prep
Arrive
Salvaged Parts
Inspect
10
Shipping
Sealer
0
Number in
Rework Queue
Server
Part B Arrive
Part B Prep
Depart
Shipping
0
Simulate
Electronic Assembly and Test
50000
Statistics
0.000
960.000
Time
(in Minutes)Animate
Rework_R_Q
Number in Queue
Animate
Rework_R
Number Available
Small Manufacturing System
Server
Server
Cell 1
Cell 2
Depart
Arrive
Ord er R e l ea s e
E x i t S y s tem
Leave
Server
Enter
Process
Cell 3
C el l 3 M ac hi n es
Cell 4
Expressions
Sets
C e l l 1 Ti m es
Sequences
P art 1 R o ute
P art 2 R o ute
P art 3 R o ute
Variables
Tran s fer Ti m e
Fac to r
1
Simulate
S m a l l M a n ufa c turi ng S y s tem
20 00
Resource
Resource
Cell 3 New
C el l 3 Ol d
Small
Manufacturing
Resource
System
Constrained
______________________________________________
Cell
Part
Cell
Cell
Part
Cell
Part
1_R_Q Queue Time
3 Cycle Time
2_R_Q Queue Time
3 Machines_Q Queu
1 Cycle Time
4_R_Q Queue Time
2 Cycle Time
16.705
81.410
9.1815
10.795
111.64
11.626
143.21
12.362
103.69
5.5290
12.103
139.74
11.171
178.07
# in
Cell
Cell
# in
Cell
# in
# in
Cell
Cell
Cell 3 Machines_Q
2_R Busy
1_R Busy
Cell 4_R_Q
3 New Busy
Cell 2_R_Q
Cell 1_R_Q
4_R Busy
3 Old Busy
.75625
.82167
.79801
.58713
.81285
.96406
1.2173
.80659
.83263
.84723
.79957
.78298
.56975
.80512
.57226
.89013
.79621
.84121