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Why does the UDP Download test show poor performance?

When running the performance test cases you’ll need to consider the capabilities of your DUT and understand what is really being tested.

The Download tests (TCP and UDP) measure the amount of traffic flowing from the WAN side of a DUT to the LAN side of the DUT. This setup typically simulates traffic coming from the Internet to a client connected to the DUT’s LAN. This LAN connection can either be wired Ethernet or wireless using a DUT’s 2.4Ghz or 5Ghz radio.

These days the wired Ethernet connection speed can be a 100Mb/sec connection or more commonly 1Gb/sec (1,000Mb).

For wireless, this can be a variety of connection speeds, depending on which IEEE specs are supported (a/b/g/n/ac), and how many antennas there are, among many other factors. Typically, the speeds are less than 1Gb. This is key in understanding the poor performance you may be experiencing.

If you do not make any changes to the default configuration for performance, the CDRouter performance engine will attempt to send traffic as quickly as the interface allows. Typically, WAN connections are Gigabit Ethernet or maybe DSL/ADSL. For Gigabit connections, this means sending traffic at 1Gb/s.

When using wireless on the LAN the traffic coming in to the DUT is at a rate of 1Gb/s, so it’s not possible to send the same amount of traffic out the LAN. An enormous amount of data loss will occur in this scenario. Note that the CDRouter performance engine counts out of order data as lost, as well.

Why dosen’t the TCP Download test suffer the same issue?

TCP has a throttling mechanism (windowing and acknowledgements) built-in to the protocol. This prevents the DUT from being overwhelmed with too much traffic.

So what can I do about this?

You can adjust the testvar perfDownloadBandwidth to be a value the DUT can reasonably forward to the LAN without throttling. The default value is 0.0, which means send as fast at the underlying interface allows. A reasonable rate to set this value to, is 1/2 the published speed of the wireless interface. So, if the DUT’s wireless 5Ghz interface has a speed of 300Mb/s, set the perfDownloadBandwidth to 150.0.

Why only 1/2 the published speed?

Keep in mind that even though this is UDP, the 802.11 layer does have acknowledgements as part of the delivery of wireless traffic. Each packet must be acknowledged. It’s also a shared medium and not full duplex. These factors mean that although the speed may be 300Mb/s, the actual amount of data transfer is closer to 1/2 of the published speed.

For additional information read the Understanding the theoretical maximum for wireless interfaces in the CDRouter Performance User Guide.

The explanation in this KB can be applied any time the LAN and WAN connections have different data speeds. For example, the same would be true for a gateway switch that only has 100Mb ports on the LAN, but a single 1Gb port on its WAN.