Error Emulation

As luck would have it, Apposite Technologies was kind enough to loan a Linktropy 4500 WAN Emulator (Figure 1) to help me answer this question. The 4500 is a hardware-based emulator that allows adjustment of upstream and downstream bandwidth, delay and loss. It has a web GUI that can be accessed via a dedicated Ethernet management port, or by in-band clients. If command line interfaces are more to your taste, there's also a serial console port where you can plug in and have at it.

Figure 1: Apposite Technologies Linktropy 4500 WAN Emulator

The 4500 is simply connected between any two Ethernet LAN segments via its 10/100/1000 LAN A and B ports. By default, it acts as a transparent learning bridge so will work with any protocol. It can also be switched into routing mode for IP frames only, and can't be used for multicast applications while routing.

Once connected, the desired network characteristics are entered into the management interface (Figure 2). Emulation can be turned on and off without having to change any of the settings by simply clicking on the virtual Emulation On/Off switch in the management interface screen.

As mentioned previously, you can control bandwidth, delay and loss for traffic passing in both directions through the 4500. The delay parameter is the only one that can vary with either a normal or uniform statistical distribution in addition to the static setting. I would have liked to see at least the loss and perhaps the bandwidth settings also be similarly variable, since that would have more accurately emulated a wireless connection. But I was able to get a good feel for loss effects with just the static setting.

Figure 2: Link Emulation screen (click image to enlarge)

The 4500 can also display statistics in both directions (but only when emulation is enabled) along with a running plot of transmission rate (Figure 3).

Figure 3: Link Statistics screen (click image to enlarge)

Setup consisted simply of connecting the 4500's LAN A port to one of my LAN switch's ports and its LAN B port to one of my computers. My learning curve with the 4500 was virtually nil and I was able to crap up my network performance in various ways in virtually no time. The longest thing that it took me to learn was finding the software switch that allowed me to change from using the dedicated Ethernet Management port over to accessing the web GUI via a computer connected to a LAN segment that was connected to the 4500.

Tags: Tutorial, Video streaming, WiFi,

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