Network Design Guidelines

Network design standards were used for many years in configuring network communication environments to support peak workflow traffic loads. These standards were application specific and based on typical user environment needs. The Capacity Planning Tool identifies client traffic generated for each user workflow, generating a much more accurate estimate of network suitability and projected network needs. In any case, network design standards and workflow traffic estimates provide guidelines for initial network configuration. Network administrators need to manage network infrastructure bandwidth to accommodate enterprise traffic needs.

Communication environments are statistical in nature, since only a percentage of user processing time requires transmission over the network. Network data transfer time is a small fraction of the GIS users' total response time (on properly configured networks). Network data transfer time can dominate response time when bandwidth is too small or when too many clients are on the same shared network segment.

The network must be designed to support peak traffic demands. The amount of traffic varies based on the different types of applications and user work patterns. Standard guidelines provide a place to start configuring a network environment. Once the network is operational, network traffic demands should be monitored and necessary adjustments made to support peak user requirements. Network bandwidth should be roughly twice the expected peak communication loads to avoid traffic contention and delays.

Network Design Standards
Figure 6a-1 provides recommended design guidelines for local network environments. These guidelines provide a baseline for configuring distributed LAN environments. Four separate client/server environments are included for each network bandwidth.

The number of recommended clients is based on experience with actual system implementations and does not represent worst-case environments. Networks should be configured with the flexibility to provide special support to power users whose data transfer needs exceed typical GIS user bandwidth requirements.



Web Services Configuration Guidelines
Implementation of Web mapping services places additional demands on the network infrastructure. The amount of system impact is related to the complexity of the published mapping services and the selected display output format. Map services with small images (less than 10 KB) or a limited number of complex features can have little impact on network traffic. Large images (greater than 100 KB) can have significant impact on network performance.

Figure 6a-2 provides an overview of network performance characteristics that should be considered when deploying a Web mapping solution. The top portion of the chart shows the maximum number of requests that can be supported over various WAN bandwidths based on average client display traffic. The bottom portion of the chart shows the optimum transmission time for these same map images.

Web information products should be designed to support user performance needs, which in many cases may be driven by the available client network bandwidth. Map services designed for low bandwidth clients would perform best with images from 50 KB to 100 KB in size to minimize network transport time (100 KB image requires more than 18 seconds of network transport time for 56 Kbps modem clients). Higher complexity ArcGIS Server map services can generate images from 100KB to 200KB in size. ArcGIS Desktop users may request image services from 200 KB to 400 KB in size (image size varies with user display size and resolution). Users generally demand reasonable performance, or they will not be satisfied.

Data center Internet service provider connections must have sufficient bandwidth capacity to handle expected peak traffic loads. Map services that deliver and average of 100 KB per display transaction over a single T-1 Internet Service Provider connection would reach full capacity at a maximum of 5,544 map requests per hour, at which time user display performance would slow to a crawl. This is simply the maximum number of 100 KB maps that would fit over a 1.5 Mbps line - for performance reasons you should plan for half this traffic. Proper infrastructure bandwidth and careful map information product design are required to support high-performance Web solutions.

ArcGIS Server can support high performance complex services using a pre-processed data cache. The more intelligent clients (ArcGIS Desktop, ArcGIS explorer, Web applications with adobe flash clients, etc) are able to overlay Web based vector and image services on top of a high performance local cache layer. The local cache data can be sent from the server once and used by the client many time, since the images are stored on the local client machine. Data cache can reduce network transport requirements and increase display performance when configured and used properly.

Web services can include functions that support data download to Web clients over the Internet. The data delivery service extracts data layers from the server data sources based on the identified extent, zips the data into a compressed file, and downloads the data to the client. Figure 6a-3 identifies minimum download times based on available bandwidth and the size of compressed data packages. Data downloads should be managed to protect Web service bandwidth capacity needs. Data downloads can very easily dominate available bandwidth and impact performance of other Web mapping clients.

Many network administrators establish and maintain metrics on network utilization, which help them estimate increased network demands when planning for future user deployments. Both client and data center bandwidth connections are larger as more and more services depend on Internet connectivity. As a general practice, network bandwidth must be considered when establishing design requirements. In many cases, available network bandwidth is a primary consideration in selecting the right design solution.

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