SIMTRAP Systems Architecture

The SIMTRAP demonstrator is based on a client-server architecture, taking advantage of the http (hypertext transfer) protocol. The main server provides the basic user interface and controls the user dialogue, displays information, and connects to external information resources, primarily the HPC simulation models DYNEMO and DYMOS, and optionally to monitoring data and data bases.

This communication is based on the public http protocol, and can be based on

local networks (LAN) such as 10/100 Mb Ethernet;
the Internet with alternative access mechanisms and providers;
or dedicated connections (such as ISDN phone lines)

for the physical communication layer. This protocol also forms the basis of World Wide Web browsers like Mosaic or Netscape as future implementation platforms for the user interface. The following diagram summarizes this architecture:

Client/server structure

The SIMTRAP demonstrator consists two main elements linked in this client-server system:

The main SIMTRAP Server
the SIMTRAP Model Server

SIMTRAP Server

overall integration including:
- communication with the model server(s)
- communication with the monitoring system (optional)
the interactive user interface, including:
- GIS and visualization functions
- model scenario editing and management
- DSS functionality

The SIMTRAP Server is implemented (for the purpose of the Demonstrator) on a SUN Sparc workstation under UNIX (Solaris2.5 or higher). For details, please refer to the the description of the SIMTRAP hardware and software requirements.

The MODEL Server provides:

implementation of the (parallel) simulation models DYNEMO and DYMOS
local data management for the models
direct connection to weather forecast and monitoring data (optional)

The model server can be implemented on a dedicated parallel machine (at the GMD), or on a UNIX workstation cluster or multi-processor system (see the description of hardware requirements.

The Communication Protocol

The communication between the SIMTRAP Server and the MODEL Server is based on the TCP/IP and http protocols. Using this basic communication structure (using http POST requests for the interactive communication and a cgi script at the model server side to trigger the simulation models proper), the servers use a simple protocol based on keywords and associated values and data streams.

This protocol transfers model scenario specifications or parameters compiled and edited through the SIMTRAP Serveruser interface to the MODEL Server; results from the MODEL Server are returned through the same channel with the same protocol, using again keywords and associated values and data stream to return the computation results.

The syntax of this communication is as follows:

KEYWORD:value

An example would be:

MetScenario:WorstCase
EmissionNOInd:1.05
EmissionNO2Ind:1.05
EmissionVOCInd:1.05
EmissionNOHhl:1.1
EmissionNO2Hhl:1.1
EmissionVOCHhl:1.1
BackgroundVOC:1.1
NetworkScenario:Berlin2000
Weekday:Monday
Specialday:Sommerferienbeginn
LinkClosureFrom:100125
LinkClosureTo:100126
TrafficVolume:1.2
......
......

From the MODEL server , the results are sent back using basically the same conventions. The results consists of:

a header identifying the scenario, or possibly an error message, e.g., if the server is not available;
for each output time step (every 15 minutes), lists of the variables computed by the two models.

They are transferred as lists of float values, following a keyword that identifies the parameter in question. For network related parameters, the sequence of values corresponds to the sequence of network links as defined in a basic link file (stored also at the SIMTRAP server; for air quality results, the values correspond to the cell of the computational domain (100 by 100 km) and are sent, column by column starting with the upper left cell of the domain.

The model output will be structured as follows:

MetScenario:WorstCase
EmissionNOInd:1.05
EmissionNO2Ind:1.05
EmissionVOCInd:1.05
EmissionNOHhl:1.1
EmissionNO2Hhl:1.1
EmissionVOCHhl:1.1
BackgroundVOC:1.1
NetworkScenario:Berlin2000
Weekday:Monday
Specialday:Sommerferienbeginn
LinkClosureFrom:100125
LinkClosureTo:100126
TrafficVolume:1.2
......
......
TimeStamp:199708040600
NumberOfCars:15 16 20 19 18 15 25 ..... (one value for each link) 
SpeedOfCars:40 45 48 38 40 ......
EmissionNo:100 120 110 85 110 .....
EmissionNo2:100 120 110 85 110 .....
EmissionVOC:100 120 110 85 110 .....
ConcentrationO3: 80 82 81 79 83 ....... (one value for each cell)
TimeStamp:199708040615
NumberOfCars:15 16 20 19 18 15 25 ..... 
SpeedOfCars:40 45 48 38 40 ......
EmissionNo:100 120 110 85 110 .....
EmissionNo2:100 120 110 85 110 .....
EmissionVOC:100 120 110 85 110 .....
ConcentrationO3: 80 82 81 79 83 .......

Please note that this protocol does not include any optimization potential to minimize data transfer (server side compression, transfer of deltas, etc.) is is purely designed to simplify the initial server integration and the debugging of the data transfer.

Input Parameters

Input parameters specified in the user interface are always presented as a list of options the user can chose from. Parameter values can

be derived from a list of available files, e.g., for network and meteorological scenarios;
be defined in the knowledge base of the embedded expert system (see below)
be constrained by the underlying data files such as the link file; here the selection of a link is from the map representation of the network rather than by link IDs, which are transparent for the user.

Distributed Data Bases

The data bases for the SIMTRAP system are distributed between the SIMTRAP server and the MODEL server. By default, all model input data are stored with and managed by, the MODEL server. Since the SIMTRAP servers user interface does not support access to ALL the model input, but only those that are considered:

absolutely necessary for formulate a meaningful simulation or decision scenario;
are safe for the user to modify within the ranges defined.