Method of Identifying Faults on an AP's Receive Link

Applicable Models and Versions

The method applies to all AP models and software versions.

Background

An AP provides a low throughput due to various reasons. In most situations, the AP radio links are checked for the fault.

When the AP's transmit link is faulty, the signal strength becomes low or no signal is available, which can be easily detected by users. To determine a receive link fault, you need to use professional testing instruments, but few such instruments are provided onsite.

This section describes how to use commonly used network devices, such as laptops and wireless NICs to determine whether an AP's receive link is faulty.

Materials Preparation

Three PCs:

1, PC1: supports GE network ports and IxChariot traffic simulation.

2, PC2: supports 2x2 MIMO NIC (with a maximum association rate of 300 Mbit/s) and IxChariot traffic simulation.

3, PC3: supports packet obtainment using OmniPeek software.

One wireless NIC: supports packet obtainment using OmniPeek software and 2x2 MIMO, such as TOTO link N500UA and TOTO link N300UA

AP power supply device: DC adapter, PoE adapter, or PoE switch

AC: AC6605

Network cable and serial cable

Networking Mode

The receive links on the AP's two antenna ports fail at the same time.

    Verification:

    Simulate upstream traffic on the live network and compare throughput of a properly-working AP and the faulty AP.

    Fault symptom:

    When internal receive links on the AP's two 5 GHz (or 2.4 GHz) antenna ports fail at the same time, the following situations occur:

1, The throughput of the PC is high when it is near the AP antenna but decreases sharply when it moves far from the AP antenna.

2, The RSSI is low on the AP.

    Identification method: cross test

3, Construct a network according to the figure (PC3 can be left unconnected).

4, Configure the AP to go online on the AC.
   

5, Use IxChariot to simulate traffic from PC2 to PC1.

6, Move PC2 gradually far from the AP antenna to observe the throughput change.

    A: If the receive links on the AP's two antennas fail at the same time, the throughput drops sharply during movement. (Normally, the AP throughput is higher than 50 Mbit/s.)

    B: If the throughput drop is obvious, replace the faulty AP with a properly working AP.

    Ensure that the feeder cable between the AP and antenna is securely connected, and the software configuration is correct.

The receive link on one antenna port of the AP becomes faulty.

    Verification:

    Simulate traffic from PC2 to PC1 on the live network, perform packet header obtainment, and check the transmission rate of upstream data packets.

    Fault symptom:

    No obvious difference can be detected.

Identification method: cross test

1, Construct a network according to the figure.

2, Configure the AP to go online on the AC.
  

3, Use IxChariot to simulate traffic from PC2 to PC1. Keep a certain distance between PC2 and the AP antenna (the lab test uses an 11 dBi antenna and places PC2 about 5 m away from the antenna. Select the antenna and adjust the distance according to actual situations).

4, After the traffic is stable, run Omipeek software on PC3 to perform packet header obtainment. Place the wireless NIC near PC2, between the AP antenna and PC2.

A: Check the transmission rate of upstream data packets.

Normally, the uplink transmission rate of data packets from the AP can reach the rate of MCS 15 2x2 MIMO, that is, 270 Mbit/s or 300 Mbit/s.


If one of the receive links becomes faulty, the uplink transmission rate of data packets from the AP can only reach the rate of MCS7 SISO, that is, 130 Mbit/s or 150 Mbit/s, as shown in the following figure.


B: If the AP's data rate cannot reach the 2x2 MIMO rate, replace the faulty AP with a properly-working AP. Ensure that the feeder cable between the AP and antenna is securely connected, and the software configuration is correct.