Wi-Fi test knowledge five: Wi-Fi product test PER

PER is a mandatory item for mass production of WIFI products, because the mass production test can only verify whether the DUT's RX link is normal if it passes the PER test. However, various problems will cause packet loss during PER test. The following is an analysis of common situations.

1. Packet loss caused by configuration:

1. Instrument port and DUT port; during testing, the phenomenon of "port wrong connection" is often encountered, which can be confirmed by the following simple methods:

(1) WT200 can use the "port status indicator light" of the instrument to determine whether the instrument port is correct (light on indicates that the port is working)

(2) The antenna port of the DUT needs to be checked by the customer according to the configuration and wiring of the DUT.

2. The RF cable is damaged; It is necessary to ensure that the RF cable is normal. There is a simple method for reference.

Connect both ends of the RF cable to the RF1 and RF2 ports of the WT200, perform a loopback test through WT Meter, and check the transmit and receive power to see if the cable is normal and the RF cable is attenuated.

3. Center frequency; it needs to be consistent with the DUT configuration.

4. Frame type; it needs to be consistent with the DUT configuration.

5. IFG (Interframe Gap); The standard requirement is that the SIFS is 10us (802.11ag is an example); but under the test conditions, it needs to be determined according to the processing time of the DUT. The actual DUT test is much larger than the IFG of 10us. If the IFG is too small, it will cause packet loss, but if the IFG is set too large, the test efficiency will be affected.

The general method is to set a larger IFG first. If the PER reaches the expected value, then try to slowly reduce the IFG value. The principle is that the set IFG value cannot affect the PER unreasonably.

For example, some WIFI modules transmit the test results to the PC through the serial port. Because the serial port communication speed is not fast enough, the DUT cannot transmit the results to the PC in time, which makes it impossible to continue receiving and causes packet loss.

6. Transmission power (need to consider external attenuation); too much power or too low power will not work, because the amplifier can only be applied to a certain range of power. Please determine this value according to the design of the DUT.

7. The effective length of the Waveform data; the effective length of the data (PSDU length) will also cause packet loss, because when the data packet takes a long time, the time-varying problem of the channel and the time stability of the oscillator need to be considered.

2. Waveform causes packet loss:

After the first attempt, if the DUT cannot receive data at all, then consider the MAC layer (Media Access Control Layer).

1. Some DUTs count the received data packets according to the data packet type (management frame, control frame, data frame broadcast packet, etc.). At this time, you need to pay attention to the receiving status information of the DUT.

Note: WT200 Meter software WIFI uses data broadcast packets by default.

2. Some DUT firmware needs a specific MAC to be considered valid when testing PER. At this time, baseband data needs to be generated according to requirements. Waveform is generally in the tool folder of the original chip. At this time, we need to provide these special waveform files to our company for analysis.

Eg1: When the Athros chip tests the PER, it needs a specific MAC address (default is 01.00.00.C0.FF.EE) to be considered valid. At this time, it is necessary to generate a Waveform data packet according to the requirements.

3. Special circumstances cause packet loss

Sometimes manual testing is fine, and there are errors in automation. This problem is actually a long delay between actions during manual operation, while operation time is short during automation; because some DUTs take a long time to enter the receiving state, but the automation tool does not wait for the DUT to enter the receiving state before sending data, so Cause packet loss.

Note:

SIFS is the shortest inter-frame interval, which is the shortest inter-frame interval. It is used to separate the frames belonging to a conversation. The AP should be able to switch from sending to receiving within this period of time
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