|  | 3.2 | The First Steps in WAN Design
| 3.2.3 | Analyzing requirements

You need to analyze network requirements, including the customer's business and technical goals. What new applications will be implemented? Are any applications Internet based? What new networks will be accessed? What are the success criteria? (How will you know if the new design is successful?) Availability measures the usefulness of the network. Many things affect availability, including throughput, response time, and access to resources. Every customer has a different definition of availability. You can increase availability by adding more resources. Resources drive up cost. Network design seeks to provide the greatest availability for the least cost. The objective of analyzing requirements is to determine, the average and peak data rates for each source over time. Try to characterize activity throughout a normal work day in terms of the type of traffic passed, level of traffic, response time of hosts, and the time to execute file transfers. You can also observe utilization on existing network equipment over the test period. If the tested network's characteristics are close to those of the new network, you can estimate the new network's requirements based on the projected number of users, applications, and topology. This is a best-guess approach to traffic estimation given the lack of tools to measure detailed traffic behavior. In addition to passively monitoring an existing network, you can measure activity and traffic generated by a known number of users attached to a representative test network and then calculate findings to your anticipated population. One problem with defining workloads on networks is that it is difficult to accurately pinpoint traffic load and network device performance as functions of the number of users, type of application, and geographic location. This is especially true without a real network in place. Consider the following factors that influence the dynamics of the network: The time-dependent nature of network access - Peak periods can vary; measurements must reflect a range of observations that includes peak demand. Differences associated with the type of traffic - Routed and bridged traffic place different demands on network devices and protocols; some protocols are sensitive to dropped packets; some application types require more bandwidth. The random nature of network traffic - Exact arrival time and specific effects of traffic are unpredictable. Each traffic source has its own metric, and each must be converted to bits per second. You should standardize traffic volumes to obtain per-user volumes. Finally, you should apply a factor to account for protocol overhead, packet fragmentation, traffic growth, and safety margin. By varying this factor, you can conduct what-if analyses. For example, you could run Microsoft Office from a server, and then analyze the traffic volume generated from users sharing the application on the network. This volume will help you determine the bandwidth and server requirements to install Microsoft Office on the network.
| Threaded Case Study
Washington Project: Analyzing Availability

You need to find out what availability means to your customers, in the Washington School District. These are teachers, students, administrators, and staff members. When analyzing your district's technical requirements, estimate the traffic load caused by applications and by normal protocol behavior (for example, a new node joining the network). Estimate worst-case traffic load during the busiest times for users and during regularly scheduled network services, such as file server backups. This will help you understand what availability means to your customers.

Threaded Case Study
Washington Project: Analyzing Network Traffic Load and Traffic Problems

Before you develop a district network structure and select hardware, you need to determine the network traffic load that the district WAN needs to handle. You should determine all the sources of traffic and define what source characteristics must be ascertained. At this step, it is very important to define the sources in sufficient detail that source traffic can be measured or estimated.

Additionally, you need to evaluate applications that might cause traffic problems in the Washington School District WAN. The following applications can generate large volumes of traffic and therefore can cause network problems such as congestion:

- Internet access
- Computers loading software from a remote site
- Anything that transmits images or video
- Central database access
- Department file servers

The introduction of new sources or applications into the Washington School District WAN must be projected, along with likely growth rates. Obviously, this step requires considerable consultation with district end users and application developers. Finally, district network management data is an important source that you should not overlook because it could take up more than 15% of the total traffic volume.

| Engineering Journal
Traffic Measurement

| Web Links
| Network Pre-Installation Guide
| Network Design
| Dartmouth Method for Teaching Design