WAN emulator technology primer: What to know about network emulators

This WAN emulator technology primer details what wide area network engineers and architects should know about network emulators. Learn the basics of network traffic simulation, and find out how it is used in enterprise IT.

This WAN emulator technology primer explains what wide area network engineers and architects should know about

network emulators. In the emulator primer, you will learn the basics: Learn what a WAN emulator is; why WAN engineers need to know about them; and how they are used in enterprise IT.

VoIP, remote data replication, virtual desktop infrastructure (VDI) -- these are just a few of the applications that have been layered on to headquarter-to-branch office links in recent years. And, while bandwidth is more plentiful and less expensive than it was 10 years ago, traditional file transfer and next-generation apps, like those mentioned above, can have voracious appetites. To determine which applications will work on WAN links and which won’t, you’ll need a network emulator.

What is a WAN emulator?

“Network emulation” is a general term given to any system that provides control of bandwidth and related parameters so as to emulate or simulate an actual network in a lab environment. While, technically, emulators could be used to emulate local area networks (LANs), there is little use for that function for these two reasons:

  • LANs are easy enough to provision and test without a network simulator.
  • Given the dramatically faster speeds common in the LAN, bottlenecks are much less likely to occur.

Thus, a reference to network emulation almost always implies wide area network (WAN) emulation. WAN emulators allow test engineers and network architects to build a WAN environment in a lab environment. Once built, emulators allow network engineers to change various parameters and characteristics of the WAN environment -- the most important of which is bandwidth.

WAN emulators are implemented as a box -- either a computer or appliance -- that represents the “cloud” or network from which there are two LAN interfaces each of which represents one side of the network. While traffic enters these interfaces at LAN speeds, software inside the emulator will buffer the traffic and slow it down to a speed that roughly approximates the WAN speed setting represented by the second LAN interface.

Why WAN engineers need to know about WAN emulators

The WAN emulator is an essential tool for modeling behavior of a complex mix of applications across a WAN and accurately predicting user experience. In situations where "oversubscription" (an excessive demand for bandwidth) is expected or a given, network architects will likely implement bandwidth management solutions. In such situations, a WAN emulator allows accurate setup and testing of such solutions.

The fundamental setting of bandwidth will allow network architects to estimate application throughput and user experience with various link speeds in situations where tiered speeds/pricing are available (such as broadband ADSL and cable).

Unlike traditional dedicated T1 circuits that are symmetrical, i.e., 1.544 Mbps in each direction, broadband ADSL and cable connections are asymmetrical (hence the “a” in ADSL). This means that the link speed is different in each direction. Usually, the link speed is significantly faster in the download direction. WAN emulation can allow you to see how various upload/download speeds affect the application performance.

In addition to bandwidth settings, most WAN emulators allow users to select and configure other parameters such as latency (delay) and packet loss. Latency exists in every WAN connection -- most prominently in satellite connections -- and can have a big impact on real-time communications such as VoIP. Packet loss varies by provider and region but it is important to test applications with non-zero packet loss and often advisable to increase packet loss settings in the test environment to find out at what point the application response becomes unacceptable. This type of flexibility is impossible in the real world as a real network has bandwidth determined by the provider and latency that is determined by the distance between network endpoints and the type of communication medium. Furthermore, while the network might, at times, experience some level of packet loss, it is impossible for the user to create such a scenario for the purposes of evaluating the behavior of an application in such circumstances.

What WAN engineers need to know about WAN emulators

First and foremost, always remember that WAN emulators can only approximate real network conditions via software. Packets can be timed to arrive in such a way as to approximate the timing of a real T1, for example, but the actual lower layer protocols used, say, in T1 are not present.

Because the WAN emulators effectively replace the actual routers used in the network, WAN emulation tells you nothing about the performance of your WAN router. You will need to test WAN routers separately.

The WAN emulators available range from open source to commercial. NIST Net (from NIST) and WANem (from Tata Consultancy Services) are both available at no cost. Anue System’s GEM and Shunra’s VE Cloud are two prominent commercial products.

The level of features, ease-of-use and support will, certainly, vary among the products. Open source can provide a good starting point to get more hands-on experience with WAN emulation but commercial-grade products are likely to prove a better option for mission-critical work and if you require more advanced functionality.

Either way, if you are deploying applications across a WAN and you need to test and predict application performance, WAN emulation is mandatory. 

⇒ Learn how to set up WAN simulators to test branch application performance, in this companion tip.

This was first published in October 2010

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