Selecting the right tool depends on who will use it and why. Discover several features to look for when you choose your next network design tool. Arnold W. Bragg Which Network Design Tool Is Right for You? e’ve all seen advertisements for flicting features, trade-offs, and thousands of rea- commercial network design tools. sonable design alternatives. Is the designer build- They usually show colorful work- ing a new network or maintaining and trouble- station and server icons suspended shooting an existing one? Will proposed changes Wlike insects in a spider web of switches and in the network handle the anticipated traffic routers, all neatly superimposed on a map of demand without violating service-level agree- Northern California. But network design is more ments? Does the design require wireless or satel- than dragging, dropping, and connecting icons to lite links? Is the tool suitable for presentations to build a graphical network infrastructure. customers, and can it provide fast answers to their Network design is challenging,requiring design- what-if questions? Is the designer testing an exper- ers to balance user performance expectations with imental protocol or a novel routing algorithm? network-resource costs, capacities, capabilities, Tools analyze (visually, mathematically, or by and use levels. Network operators want to keep simulation) what is happening in the network and resource utilization high and costs low. But then predict future behavior.Nearly every general-pur- the resulting design might produce unacceptable pose network design tool works the same way.The service levels when congested links and under- designer either uses a drag-and-drop graphical edi- sized routers cause too much delay or too many tor to create or modify a network topology or dropped packets.You need a design that will effi- imports the topology directly from a network man- ciently use your network resources and effectively agement tool like HP OpenView. Most tools have serve your users.And you want a network design extensive libraries of device and link models, so tool that can produce the design you want. building an Ethernet local area network (LAN) or Networks are a complicated mix of applications, IP-routed backbone network is straightforward. protocols, device and link technologies, traffic Next, the designer specifies (or confirms) the flows, and routing algorithms. There may be tens attributes of each device, link, or application—bit of thousands of feasible configurations, each with rate, traffic load, device throughput (in bits or different performance attributes and costs. Can packets per second), transaction rate, routing pro- network design tools help you find the combina- tocol, and so on—and simulates the network’s tion that is right for your organ- operation. Simulation runtimes vary from several ization? Yes, but it’s important minutes to several days, depending on the Inside to use the right one for the job. required level of accuracy and detail in the results. Most tools can display results graphically at the WHAT IS A NETWORK end of the simulation run, and many use anima- Other Network- DESIGN TOOL? tion or dynamic utilization bars during the course Related Tools Network design tools help IT of the simulation to highlight bottlenecks and professionals sift through con- traffic flows. Nearly every tool can identify and 1520-9202/00/$10.00 © 2000 IEEE September ❘ October 2000 IT Pro 23 Authorized licensed use limited to: University of Pittsburgh. Downloaded on January 6, 2010 at 16:31 from IEEE Xplore. Restrictions apply. NETWORK DESIGN Figure 1. Opnet Technologies’ IT DecisionGuru portrays a network of interconnected backbone IP routers. summarize network congestion, load, lost packets, failure WHO USES NETWORK DESIGN TOOLS? and error conditions, and response times. The ideal network design tool can mean different things Network design tools also let you evaluate different sce- to different people. But whether you’re a network narios.For example,suppose you need to evaluate the traf- designer, network manager or engineer, sales or market- fic load introduced by three different routing protocols on ing manager, or member of the R&D staff, you probably a portion of an existing wide area network (WAN).Figure want intuitive graphical interfaces that resemble com- 1 shows how one tool portrays the network as a set of inter- puter-aided design tools. Distances are important in LANs connected backbone IP routers, each directing traffic to and WANs, so the tool must recognize and compensate for and from one or more abstract subnetworks. In this exam- user-defined scales.You want several levels of operational ple, the border gateway protocol (BGP) generates slightly abstraction, so you can model a LAN as 37 distinct packet- less routing traffic than the open-shortest-path-first (OSPF) generating devices at one extreme, and as a cloud raining protocol,and both generate far less routing traffic than the 10,000 packets per second at the other. You also want to routing information protocol (RIP).Configuring the three test whether your proposed design or change can handle scenarios in this example may be as simple as selecting a the workload with acceptable performance. different value for the routing protocol attribute from a However, depending on your situation, concepts of the single pull-down menu. ideal network design tool can vary greatly. Quantitative network design tools produce more accu- rate and defensible results than qualitative methods or Sales and field-services staff rules of thumb. A network operator may have expected You sell network hardware or provide network services BGP and OSPF to generate less routing traffic than RIP and want to show customers reasonably accurate repre- but couldn’t predict the relative performance of OSPF and sentations of how a product, service, or technology will BGP for this specific topology. Network design tools are improve their network and support their business case. also valuable for predicting performance problems rather You want an intuitive tool that runs on a laptop computer than merely reacting to them when they arise. and that you can master in just a few days. You need fast Simulation is not the only solution. Several tools rely execution speed (tens of seconds per scenario rather than partly or solely on mathematical analysis and queuing the- tens of minutes) and extensive presentation features (such ory.Analytical tools provide solutions more quickly than as traffic animation, graphical indications of service-level simulation but don’t always achieve the required accuracy. performance, reports, diagrams, charts, and graphs). 24 IT Pro September ❘ October 2000 Authorized licensed use limited to: University of Pittsburgh. Downloaded on January 6, 2010 at 16:31 from IEEE Xplore. Restrictions apply. Network managers and engineers • confirm that service-level agreements can or cannot be You operate the networks, troubleshoot and solve per- met; formance problems, and ensure compliance with service- • predict the response times of applications before deploy- level agreements.You try to gauge how proposed changes ing them; will affect cost, performance, capacity, and availability • predict where bottlenecks and application response time before the changes go through. Changes typically include delays will appear first; introducing new routing protocols, adding new devices and • predict how many users an application can handle; and links, supporting new applications such as those for enter- • present results via animated utilization bars, online prise resource planning (ERP) or e-mail, upgrading graphical statistics and counters, diagrams, reports, servers, and changing service-level agreements. If the exist- charts, and graphs. ing network is large, you must import topology and traffic data from other tools. The set of alternatives is usually Not-so-easy problems enormous, so you need to evaluate scenarios in tens of min- General-purpose tools only partially solve some prob- utes rather than hours. lems. Here are some of the more common ones. Determine the best performance-cost ratio. Let’s say you Network designers want to decide which devices, LAN or WAN technologies, You specify and build new networks or overhaul exist- or routing protocols give the best performance at the low- ing ones.You try to reduce design time and improve design est cost. Exploring alternatives is easy, but the user must accuracy. You want your designs to meet performance specify the scenarios and evaluate the costs and results, requirements without overbuilding. And you want to be usually one by one. Some tools generate a bill of materials able to identify potential bottlenecks and overloads. for the network devices, but few general-purpose tools fac- Designers need an extensive library of link technologies, tor in communications costs. Estimating costs is challeng- devices, architectures, and protocols ing because telecommunications to build or upgrade the network, One of the most tariffs vary widely by location and and you need tools to accurately challenging decisions is type of service—for example, leased predict its performance. lines versus frame relay. determining how much Provide accurate solutions in sec- Researchers onds. You must trade off execution To reduce development costs and accuracy to sacrifice speed and accuracy. Speed comes risks, you test the effects of new or for speed. from high levels of abstraction, as modified protocols, devices, archi- when a tool models a large LAN as tectures, component designs, and a cloud generating an aggregate traffic models in the lab or on the workbench. You need traffic flow.To test a new component or protocol, you need complete control of simulated behavior at the programming to simulate every packet passing through it.To figure out language level,and you want the language to provide a rich whether you can add five workstations to that overbur- set of special-purpose modeling functions.You typically sim- dened LAN on the fourth floor, using aggregate flows ulate discrete events (packets transiting a router, protocol makes sense.There is some middle ground, though.