1. Concepts In Discrete-Event Simulation
System
Model
System state
Entity
A collection of entities (people and machines..) that interact together over time for one or more goals
An abstract representation of a system, usually containing structural,
logical or mathematical relationship that describe a system in term of
state, entities and their attributes , sets, processes,…
A collection of variables in any time that describe the system
Any object or component in system that require explicit representation (server, customer,...)

2. Activity (unconditional wait)
Attributes
List
Event
Event Notice
Event List
Activity (unconditional wait)
The properties of an entity
A collection of associated entities , ordered in some logical fashion
(FIFO, priority,…)
An instantaneous occurrence that changes the state of a system
A record of an event to occur at the current or future time (type and time)
FEL (future event list)
A duration of time of specified length (service time or interarrival time,… )

3. Delay (conditional wait)
Clock
A duration of time of unspecified indefinite length, which is not known until it ends (customer delay in waiting line)
A variable representing simulated time

4. Activity vs. Delay
Activity
Activity is known as unconditional wait
End of an activity is an event, for this an event notice is placed in the future event list
This event is a primary event
Delay
Delay is known as conditional wait
Delays are managed by placing the entity on another list, e.g.,
representing a waiting line
Completion of delay is a secondary event, but they are not placed in the future event list

5. Able-Baker Call center
System state
Entities
Events
Activities
Delay
LQ(t): the number of callers waiting to serve
LA(t):0 or 1 indicate Able is idle or busy
LB(t):0 or 1 indicate Baker is idle or busy
Neither Callers nor the servers need to be explicitly represented except in terms of state variables.
Arrival event, service completion by Able or Baker
Service time by Able/Baker and Inter-arrival time
A caller’s waiting time in queue until Able or Baker becomes free

6. Simulation and Modeling Approaches:
1. Event-Scheduling Approach
2. Process-Interaction Approach
3. Activity-Scanning Approach

7. Event-Scheduling Approach
A discrete-event simulation proceeds by producing a sequence of system snapshots over time
A snapshot of the system at a given time includes

9. Example 3.3 DESS of Grocery-Store

10. System state Entities Events Event Notices Activities Delay
LQ(t): the number of customers waiting to serve
LS(t):0 or 1 indicate Server is idle or busy
Neither customers nor the servers need to be explicitly represented except in terms of state variables.
Arrival event(A)
service completion/Departure event(D)
Stopping Event(E)scheduled to occur at time t
(A,t) Arrival Event to occur at future time t
(D,t) Departure Event to occur at future time t
(E,t) Stopping Event to occur at future time t
Service time
Inter-arrival time
Time customer spend in waiting line

14. Entities (Ci, t ), representing customer Ci who arrived at time t
Event notices
(A ,t ,Ci ), the arrival of customer Ci at future time t
(D, t ,Cj ), the departure of customer Cj at future time t
Set :CHECKOUTLINE, the set of all customers currently at the checkout
counter (being served or waiting to be served), ordered by time of arrival

16. Process Approach of Modeling