1. Distributed Simulation Modeling of Warehousing Operations
M. Zhou, Y. J. Son, C. Chen, and Q. Zhang
Center for System Modeling and Simulation
Indiana State University
Terre Haute, Indiana 47809

2. Intro to distributed simulation
What is a “distributed simulation”? A super-simulation process that contains multiple sub-simulation programs that are independently executing and interacting with each other, and coordinated through the super-simulation process
Why distributed simulation?
Composite and complex nature of real world systems
Needs for synchronizing multiple asynchronized software processes/systems
Needs for simulating geographically distanced systems (e.g. military applications)
Applications

3. Intro to Warehousing/DC operations
Key operational processes in warehouse-based Distribution Centers (DCs):
Truck-docking process: activities to prepare trucks for loading or offloading at a DC
In-bound process: receiving operations, e.g. offloading, inspecting, sorting and storing.
Out-bound process: shipping operations, e.g. picking, staging, sorting, packing, and loading
Cross-docking: breaking bulk, mixing and directly shipping without storage

4. Intro to warehousing processes
In real world these processes are considered running separately or “independently”
The activities/operations in the processes are inter-related in terms of resource (e.g. sharing the same crew of labor and equipment) and flow logic (e.g. truck docking affects both inbound and outbound operation, inventory management also affects inbound (replenishment) and outbound flows)

5. Exemplary interactions
Inbound process assigns resource (e.g. an offloading crew that involves labor and equipment) and starts sorting and put-away storage operation when truck-docking process seizes and finishes connecting to a dock-door
When Inbound process finishes off-loading activity, the truck will release the door and leave the yard (a waiting or staging space)
Outbound process initializes picking-sorting-packing jobs as a result of order-processing (an activity of inventory management)
Interactions are triggered by the events that are the consequences of the activities belonging to different processes

6. Simulation of subsystems and overall system
To reduce modeling complexity, we usually build simulation models separately for each subsystem to evaluate its performance with assumptions (and constraints) about the interfaces between the subsystems
Subsystems usually assume different “forms” of entities defined to satisfy the different objectives of analysis__ the objectives for analyzing that subsystem

7. An example …
In a CD/DVD distribution process, three subsystems, a location-based picking process, an automatic sorting process, and a wrapping/packing/shipping process are running separately but coordinated through the control of a sequence of events
Location-based
Picking process
Automatic
sorting process
Wrapping and
shipping process
Solid arrows: material flow; dashed arrows: information flow

8. Distributed simulation
In order to effectively and efficiently simulate the overall performance of a DC, we need to integrate the individual simulation models developed for the subsystems (legacy systems), i.e. coordinate their executions in a distributed environment to accomplish the analysis
Distributed simulation provides a technology that enables us to integrate and coordinate the simulation models developed for each subsystems, namely, truck-docking, inbound, and outbound process so that the performance of the overall system (a distribution center) can be more realistically and accurately described and evaluated through simulation

9. A distributed simulation environment
Architectural components:
Legacy models/systems (HLA based federates)
Coordination/manager model (HLA based Federation manager)
Data repository
Interfaces/adapters (transmission, receipt and internal updates to all FOM objects used by a federate)
Tools (e.g. output analysis)
Integration infrastructure/platform (e.g. RTI based on the HLA standard)

10. A distributed simulation environment
Data integration
- Communications between subsystems are realized via messages that trigger the events and transfer common data structures
MessageName {Sender; Receiver; List_FOM Object Attributes};
For example:
Request_IB {TD; IB; (T.LoadType, T.SkuList, T.SkuQty; T.RequestedTaskType, …)}

11. A distributed simulation environment
Time coordination
When Tsub(i) < Tsys and need to advance
if (Tsys – Tsub(i) ) > 
then Tsub(i) = Tsub(i) + 
else Tsub(i) = Tsys
When Tsub(i) = Tsys and system advance not completed
Do nothing (i.e. halt the local simulation until the system finishes all the advancement requests of subsystems)

12. A distributed simulation environment
Legacy
Model1
Adapter
Coordination model
(Federation
Manager)
Data
Repository
Run Time Infrastructure and Communication network

13. A distributed simulation environment
High Level Architecture (HLA): an architectural standard for integrating distributed simulation or multiple software processes, by the DoD of USA.
Run Time Infrastructure (RTI): a software implementation of HLA specifications
Federation Object Model (FOM): a common data definition created for the domain data shared across the federation; each federate has a FOM object that defines the elements of the FOM that it implements
Developing implementation of HLA RTI and the interfaces between federates and federation manager is NOT a trivial task!

14. The focus of our research
This research focuses on the modeling and evaluation of logistics distribution center (DC) operations via distributed simulation
One objective is to build a set of conceptual models to describe the subsystems, their interactions, and the coordination required for the evaluation of the overall system
Second objective is to construct a prototype to verify and validate the proposed distributed simulation for the target domain

15. Proposed work: conceptual modeling
Building conceptual models for the subsystems:
Truck-docking process
Inbound process
Developing legacy simulation models for the subsystems (e.g. using ARENA©)
Modeling the interactions between the subsystems via Deterministic Finite State Automata (DFSA)
DFSA graphical model (logic level)
Procedural or algorithmic model (implementation level)

16. Conceptual models for subsystems
Trucks arrive
Register at gate
Assign to doors
Connect to the door
Door available?
Offload merchandise
Wait in the yard
Assign off-loaders
Release truck No E
Yes
Truck-docking
process

17. Conceptual models for subsystems
Connect trucks
Inspection at dock
Reject and return load
Off-load goods
Accept loads?
Count, stage merchandise
Release truck 2
Sort/label products
Store or C-D?
Assign handler
Cross-dock the goods No
Update inventory E
Yes
Inbound
Process

18. DFSA model for truck docking process7 6 5 4 1 2 3
Door_available_IT
Request_IB_TI
Request_IB_ok_IT
Connect_to_door
Off-load_TI
Offload_finished_IT
Release_truck
Free_door_TI T I O
I = Incoming message
O = Outgoing message
T = Task to be performed

19. DFSA model for inbound process7 6 5 4 1 2 3
Request_IB_TI
Request_IB_ok_IT
Off-load_TI
Off-load
Free_Door_TI
Offload_finished_IT
Release_Door
Door_Available_IT T I O

20. Conceptual modeling
Define the set of messages (data structure and objects involved …, these messages trigger the events)
Define FOM objects
FOM objects are defined in a public domain, i.e. these definitions are known by a number of software processes (simulation processes)
Define these objects in a standard way, e.g. following the DMS, an implementation of HLA/RTI for manufacturing simulation

21. Conceptual modeling
Implement DFSA graphical models (Coordination Model)
Algorithm/procedures (pseudo coding)
Object-oriented design and representation
Design and development through a unified process with UML
Implement the representations with Rhapsody C++/Java (Indiana State University)

22. Conceptual modeling
Define interface between legacy models, federation manager and RTI platform:
Selection of an existing HLA/RTI implementation
Developing adaptation code between legacy systems and RTI interface
Challenges:
RTI interface is complex (about 120 methods  RTI Ambassador Interface and 40  Federate Ambassador Interface)
Legacy systems provide very limited and inadequate mechanisms for integration
Coordinated time management is required

23. Prototype: integration with DMS
Distributed manufacturing simulation (DMS) is a software developed by NIST under MISSION Project (McLean 2000)
DMS provides a distributed computing environment where non-simulation software applications are running and interacting with one or more simulation systems
Using DMS can significantly reduce the effort required in developing distributed simulation applications (i.e. developing the interface)

24. Prototype: integration with DMS
DMS Adapter
Adapter
Interface
Federate
Ambassador
HLA
Run Time
Infrastructure
RTI
Object
Model
Legacy
Simulation
Adaptation
Code

25. Architecture of a prototype
Truck-dock
model
C-Adapter
Inbound
Model
Outbound
Coordination model
(Federation
Manager)
DC Simulation
Data
Repository
DMS Adapter/Interface and Communication network

26. This research is ongoing, and …
If you are interested in collaborating with us, please contact Dr. Ming Zhou at:
OR call us at (812)