3.10 Environment-Specific Thresholds and Limits


3.10 Environment-Specific Thresholds and Limits

General thresholds and limits give us some common estimates for low-and high-performance requirements for our network. They are useful when there is a lack of information about users, applications, and devices for the network, but often there is information about what the performance thresholds should be. With this information, we can develop thresholds and limits specific to that network.

Environment-specific thresholds and limits are an acknowledgment that each network is unique and that the requirements of users, applications, and devices are specific to the environment in which they operate. The environment takes into account concepts such as the area of work that they are doing or what their objectives are (e.g., what motivates them). Thus, an academic research environment differs drastically from a retail (sales) environment, which will differ from a manufacturing environment, and so on. The thresholds that distinguish between low and high performance will be unique to each environment.

Developing environment-specific thresholds recognizes that each environment is unique and that what may be considered high performance for one environment may be low performance for another. Recognizing the uniqueness of each environment (and thus network) that you work in is a good habit to develop, especially if you are involved with many different networks.

As with general thresholds, the reason for developing environment-specific thresholds is to determine which applications have high-performance requirements. We have spent a lot of time on this, but why is it so important? For each network project that we work on, we need to do the following:

  1. Determine whether there are any high-performance applications and devices, and if so, how important are they to the success of that environment? How important is it that they be supported? If there are no high-performance (multitier) applications, the network will be supporting a set of single-tier performance applications and devices.

  2. If there are high-performance (multitier) applications and devices and they are crucial to the success of that environment, it is likely that the network architecture and design will focus on supporting those applications and devices, as well as their users.

Thus, the ability to determine which users, applications, and devices are important to the success of the organization that the network will be supporting and the ability to determine whether those applications and devices are relatively high performance for that environment or are roughly the same as all other applications and devices there will affect the success of the resulting network architecture and design.

3.10.1 Comparing Application Requirements

Developing environment-specific thresholds and limits is based on comparing the various performance requirements of applications. Typically, one, two, or all of the performance characteristics (capacity, delay, and RMA) for the applications are plotted, and the plot is used to compare relative performance requirements and develop a threshold or limits for that characteristic.

For example, consider a plot of capacity requirements for applications in a network. These may be a subset of all the applications in that environment, such as the top five or six in order of performance or importance. Figure 3.24 shows the resulting plot. There is a cluster of applications in the capacity range of 50 Kb/s—around 1 Mb/s—then there are isolated applications at around 4 and 6.5 Mb/s. We could choose to pick the top one or two applications and place a threshold between them and the rest of the applications. Most likely, the top two would be grouped together and a threshold placed around 2 to 3 Mb/s.

click to expand
Figure 3.24: Plot of capacity requirements with possible thresholds.

As you can see from Figure 3.24, the capacity requirements of the applications are spread across a range of values. We could choose to develop an environment-specific performance limit for this group of applications or a threshold between low and high performance, or both. Note that, for delay, an upper limit for high performance is actually the lowest delay value, not the highest value.

In Figure 3.24, we could estimate a couple of possible capacity thresholds. The most likely would be grouping applications C and F and placing a threshold around 2 to 3 Mb/s. This value is subjective and may need to be approved by the users or management. Sometimes, the threshold or limit is obvious; at other times, it may be difficult to determine. Consider, for example, the following plot of application capacities (Figure 3.25).

click to expand
Figure 3.25: Plot of capacity requirements with no distinct groupings.

In Figure 3.25, the capacity requirements are also spread out over a range of values, but in this case there is no clear separation between low and high performance. When performance requirements are not clearly separated, you may not be able to develop a threshold.




Network Analysis, Architecture and Design
Network Analysis, Architecture and Design, Second Edition (The Morgan Kaufmann Series in Networking)
ISBN: 1558608877
EAN: 2147483647
Year: 2003
Pages: 161

flylib.com © 2008-2017.
If you may any questions please contact us: flylib@qtcs.net