Therefore, functions, features, and services are focused at server locations, the interfaces to client LANs, and client–server flows, as shown in Figure 5.13. The characteristics of the client–server model also apply to the hierarchical client– server architectural model. In addition to the functions, features, and services being focused at server locations and client–server flows, they are also focused at the server–server flows (see Figure 5.14). Network Clients Clients Servers, Server Farm, or Server LAN Architectural Features at Server Interfaces, Server LAN Interface FIGURE 5.13 The Client–Server Architectural Model 236 CHAPTER 5 Network Architecture Clients Clients Servers, Server Farm, or Server LAN Architectural features at server interfaces, server LAN interface, and at network between servers Clients Clients Servers, Server Farm, or Server LAN Network Network Network FIGURE 5.14 The Hierarchical Client–Server Architectural Model Computing Devices Task Manager Computing Devices Data Source Data Sink Network FIGURE 5.15 The DistributedComputing Architectural Model In the distributedcomputing architectural model (Figure 5.15) the data sources and sinks are obvious locations for architectural features. Flowbased models, like the topological models, are intuitive and can be easy to apply. Since they are associated with flows, they should map well to any flow maps you created as part of the requirements analysis process. These models are fairly general, and you may have to modify them to fit the specific requirements of your network. Architectural Models 237 5.5.3 Functional Models Functional architectural models focus on supporting particular functions in the network. In this section we present serviceprovider, intranet, single multitiered performance, and endtoend models. The serviceprovider architectural model is based on serviceprovider functions, focusing on privacy and security, service delivery to customers (users), and billing. In this model, interactions between providers (the networks) and with users are compartmentalized. While this model represents a serviceprovider architecture, many enterprise networks are evolving to this model, applying it across organizations, departments, and buildings. Figure 5.16 illustrates this model. The intranet architectural model focuses on security and privacy, including the separation of users, devices, and applications based on secure access. Note that in this model there can be several levels of hierarchy (security) (Figure 5.17). Internet Privacy Service Interop Billing Service Providers Privacy Service Interop Billing Subscribers Subscribers Subscribers FIGURE 5.16 The ServiceProvider Architectural Model External Networks Security Devices (e.g., Firewalls, Intrusion Detection) Internal Networks Internal Networks Internal Networks Security Level 1 Security Level 2 Security Level 3 Security Devices (e.g.