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Track: System Implementation and Integration for GIS
Liza Casey
City of Philadelphia
1234 Market Street
18th Floor
Philadelphia, PA 19107
Telephone: 215-686-8174
Fax: 215-686-8258
Arthur Petrella, Jim Querry
Requirements Analysis for GIS over a Wide Area Network
Defining Issue: The City of Philadelphia is building one of the largest integrated municipal GIS systems in the world. We have a total of twenty-two different coverages completed or in construction ranging from fire hydrants to orthophotography. Parts of our multiserver system, which also incorporates new and legacy databases, are up and running over CityNet, our wide area network. However, the GIS-generated Ethernet traffic we are looking at now is only a tiny fraction of what we anticipate, and we clearly recognize the need to design and build new network infrastructure. Methodology: In order to effectively predict the City's network requirements to support the GIS we had to identify and characterize GIS user types and access methods, taking into account the new means of accessing ArcInfo coverages such as ArcView Internet Map Server, and then measure local traffic in a number of scripted scenarios. We also take into consideration emerging communications technologies such as Fast Ethernet and ATM.
In addition, we had to find a way to mathematically model the impact that unleashing access to the GIS would have on CityNet. Recent statistical analysis of Ethernet traffic at Bellcore has shown that Ethernet traffic is self-similar or fractal in nature, that there is no natural length of a "burst." This startling but widely accepted result challenges the traditional assumptions of a Poisson model for the interarrival distribution of data communications traffic. This paper is a description of the methodology that Philadelphia, looking beyond the Poisson model, applied to the traffic measurements during scripted events to help the City design network capacity and speed appropriate for the additional demand that we anticipate GIS will be placing on CityNet.
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