In building the flow specification we use various techniques, including data sources and sinks and flow models, to identify and determine individual and composite flows as well as critical flows. 206 CHAPTER 4 Flow Analysis Flow analysis is the final part of the analysis process. We began this process by gathering, deriving, managing, and tracking requirements for the network, from users, applications, devices, and networks that will be part of the planned network. In developing requirements for the network, we considered performance requirements (in terms of capacity, delay, and RMA) and the many ways to categorize requirements for users, applications, devices, and networks. This information, along with initial conditions, problem definitions, and goals, was collected in the requirements specification and mapped out in a requirements map. Performance requirements, on a perapplication basis or grouped by user, application, device, or network, are added to the directionality, hierarchy, and diversity of traffic flows to characterize them. Some tools, such as data sources and sinks, flow models, and flow aggregation points, can be used to help us determine which flows are important in a network and where flows are likely to occur. You are encouraged to develop other tools to aid in analyzing flows, or modify those presented in this book to fit your needs. While flow analysis is presented here as part of the overall analysis process, in preparation to architect and design a network, it should be noted that flow analysis can be performed on any network, regardless of what state it is in. Notice that throughout the flow analysis process, no network technologies, topologies, or underlying infrastructures were shown or mentioned. Flow analysis allows us to separate traffic movement and performance requirements from an existing network, giving us the freedom to determine what flows should look like when the network does not restrict movement or performance. If you analyze flows on an existing network (regardless of whether or not you are developing a new network or upgrading the existing network), the results of this analysis will indicate if the existing network needs to be modified to fit the traffic flows. Now that we have an idea of what to expect of the network in terms of requirements and flows, we are prepared to begin the process of network architecture. 4.11 Exercises 1. Show flows for each set of devices and applications below. Label each as either a unidirectional or bidirectional flow. a. Client–server application: Downstream (from server to client): 1.2 Mb capacity; upstream (from client to server): 15 Kb capacity. b. Streaming video (UDP) from video server to a subscriber’s PC: 300 Kb capacity, 40ms delay (oneway). Exercises 207 c. Downloading pages from the Web: Downstream: 250 Kb capacity, 5 second delay; upstream: 100 Kb capacity. d.