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Mesh Backhaul Links is Unstable in AP6510DN-AGN and AP6510DN-AGN-FAT

Publication Date:  2015-09-25 Views:  142 Downloads:  0
Issue Description

 Video freeze and garbled images occurred in Mesh links used to backhaul video surveillance data in real time.

Mesh solution: Fit APs are fixed on the lighthouse and work as MPPs. Fat APs work as MPs and are installed on the tower cranes that move around in the port. The MPs set up Mesh links with MPPs and are connected to cameras. Video surveillance traffic is backhauled in real time over the Mesh links.

Alarm Information

Mesh link setup rate was much lower than the theoretical maximum rate.
[YV-AP-46-diagnose]disp interface radio 1 vap 15 list
ADDR           AID CHAN RATE RSSI IDLE TXSEQ RXSEQ SKIP   NORML  CAPS ACAPS ERP STATE    HTCAPS
d4b1-10b3-0a7f 0   149  64M  39   0    0     23232 0      0                 0   600200b  Q    
d4b1-10b3-0aff 0   149  19M  21   0    0     48    0      0                 0   200200b  Q    
d849-0b8a-7c5f 0   149  26M  22   0    0     224   0      0                 0   200200b  Q    
d4b1-10b3-0bff 0   149  51M  33   0    0     32    0      0                 0   200200b  Q    
d4b1-10b3-0b1f 0   149  48M  30   0    0     46576 0      0                 0   200200b  Q    

The channel usage of Mesh links on the AC was very high:                                                                                                                                                                                                                [AC6605-1]displa radio statistics ap 54 radio 1
  -----------------------------------------------------------------------
......
   Wireless PER of the last statistic period(%)          :1
   Wireless channel utilization(%)                       :87
   Wireless received signal strength(dBm)                :-
   ......
  -----------------------------------------------------------------------

The AP showed that the retransmission rate was up to 65%.
[FATAP-5G-RTG05-diagnose] radio-statistics sdk radio 1
.......
TXQ[1]:BE tx 223427 xretry 99 fifoerr 3 filtered 0 no buffs 0
TXQ[3]:VO tx 1 xretry 8 fifoerr 0 filtered 0 no buffs 0
      0.00 tx unaggregated excessive retry percent
      0.00 tx aggregated long retry percent
      0.00 tx aggregated excessive retry percent
     65.08 tx aggregate subframe retry percent
      0.00 tx aggregate subframe excessive retry percent
VOW STATS: ul_tx_calls: 0 223427 0 1644 ath_txq_calls: 0 223427 0 1644 drops(be/bk): 0 0
........

Handling Process

Analyzed the problem:

1. Analyze Mesh Link Backhaul Status

Multiple wireless packet transmit rates are available, depending on the modulation mode of the physical layer. Simply speaking, a higher rate indicates a lower anti-interference capability and higher requirements on the signal strength. Therefore, if an AP discovers a wireless communication failure, it lowers the transmit rate to improve the communication success rate.
The theoretical Mesh link setup rate is 130 Mbit/s, but the actual rate is about 50 Mbit/s. This indicates that there is a high failure rate of AP packet transmission (the AP's packet transmit rate decreases only when packet retransmission occurs). Common causes for the high failure rate of packet transmission include wireless interference and weak signals.
According to on-site information, the 5G radio has no interference signal source. APs have no conflict in packet transmission because they do not send packets simultaneously. The only interference between APs is their competition for channels. Based on this situation, the high failure rate is caused by weak signals. The Mesh link setup status also confirms existence of weak signals.

2.Analyze the Channel Usage and Retransmission Rate

Actually, there was not a large volume of service traffic on a wireless channel (less than 20 Mbit/s, mainly the video backhaul data traffic). Based on the preceding analysis, the high channel usage was not resulted from large service traffic. Since an AP may need to reduce the rate many times to retransmit a packet, the high channel usage may be caused by the high air interface cost. The command output on the AP showed that the retransmission rate was up.

Replicate of the Problem

1.constructed backhaul links with similar signal strength as the Mesh links on the customer network.
[MP-10-diagnose]display interface radio 1 vap 15 list
ADDR           AID CHAN RATE RSSI IDLE TXSEQ RXSEQ SKIP   NORML  CAPS ACAPS ERP STATE    HTCAPS
dcd2-fc9a-199f 0   132  41M  36   0    0     42560 0      0                 0   600200b  Q    
04f9-3896-8cff 0   132  94M  37   0    0     22528 0      0                 0   600200b  Q    

2.During traffic simulation, the network bandwidth was low and retransmission rate was high (higher than 70%).
[MP-6-diagnose] radio-statistics sdk radio 1
.......
TXQ[1]:BE tx 6344 xretry 1 fifoerr 0 filtered 0 no buffs 0
      0.00 tx unaggregated excessive retry percent
      0.00 tx aggregated long retry percent
      0.00 tx aggregated excessive retry percent
     73.83 tx aggregate subframe retry percent
      0.00 tx aggregate subframe excessive retry percent
VOW STATS: ul_tx_calls: 0 6344 0 144 ath_txq_calls: 0 6344 0 144 drops(be/bk): 0 0
........

3. The channel usage was high at low network bandwidths.
[AC6605-wlan-view]display radio statistics ap 4 r 1
  -----------------------------------------------------------------------
......
   Wireless PER of the last statistic period(%)          :1
   Wireless channel utilization(%)                       :88
   Wireless received signal strength(dBm)                :-
   ......
  -----------------------------------------------------------------------
Based on the preceding information, the problem of unstable Mesh links was reproduced in the lab. The weak signals lead to a high failure rate of AP packet transmission and a high retransmission rate, further resulting in high channel usage and low valid network bandwidth.

Root Cause
The root cause is that the weak signals lead to a high failure rate of AP packet transmission and a high retransmission rate, further resulting in high channel usage and low valid network bandwidth.
Solution
According to the preceding analysis, the signal strength of Mesh links cannot meet needs of high-rate packet transmission, leading to a high failure rate of AP packet transmission. To solve the problem, we attempted to optimize the algorithm used by the AP to select a packet transmit rate, so that the AP can quickly select the rate that matches the current signal strength. The optimization helps to improve the success rate of AP packet transmission, reduce retransmission rate, and ensure valid network bandwidth and stable links.

END