The goal of every WAN design should be to optimize link performance in terms of offered traffic, link utilization, and response time. To optimize link performance, the designer must balance between end-user and network manager requirements, which are usually diametrically opposed. End users usually require minimum application response times over a WAN link, whereas the network manager's goal is to maximize the link utilization; WAN resources have finite capacity.
Response time problems typically affect only users. For example, it probably does not matter to the network manager if query results are returned 120 ms sooner rather than later. Response time is a thermometer of usability for users. Users perceive the data processing experience in terms of how quickly they can get their screen to update. They view the data processing world in terms of response time and do not usually care about link utilization. The graphs in Figure 5-11 illustrate the response time and link utilization relative to the offered traffic. The response time increases with the offered traffic, until it reaches an unacceptable point for the end user. Similarly, the link utilization increases with the offered traffic to the point that the link becomes saturated. The designer's goal is to determine the maximum offered traffic that is acceptable to both the end user and the network manager.
Figure 5-11 Determining the Maximum Offered Traffic
Network Manager's View
Additional WAN Capacity Planning Additional WAN Capacity Purchasing Additional WAN Capacity Critical
However, planning for additional WAN capacity should occur much earlier than the critical point—usually at about 50% link utilization. Additional bandwidth purchasing should start at about 60% utilization; if the link utilization reaches 75%, increasing the capacity is critical.
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