NDP off-channel from a station's perspective

[vodovozovilya]

Hello colleagues,

I understood Cisco NDP (Neighbor discovery protocol) for Wi-Fi basic concept, but I cannot realise - how an access point goes off-channel from a station's point of view. Does an AP notify somehow its clients, that they should hold thier data and do not transmit anything while the AP is going to find its Cisco neighbors, or how it works..

Thank you in advance.

[Flavio Miranda]

Hi

https://www.wi-fi.org/knowledge-center/faq/what-is-the-impact-on-clients-when-aps-perform-off-channel-scanning

What is the impact on clients when APs perform off channel scanning?

When an AP is performing an off channel scan, the client devices that are connected to it will not be able to send traffic to the network. This can be disruptive to real time streaming devices that rely on a persistent connection. Care should be taken in the configuration of off-channel scanning."

 So, there will be some trouble for clients if AP performs off channel scan. 

But the whole thing is extremelly complex. Take a look here:

"TargetWake Time
The existing Wi-Fi client power-saving mechanisms have been in use since 802.11b, where the client devices
sleep between AP beacons or multiple beacons, waking up only when they have data to transmit (they can
transmit at any time, as AP does not sleep), and beacons containing the Delivery Traffic Indication Map
(DTIM), a bit-map, indicates that the AP has downlink traffic buffered for transmission to particular clients.
If a client has a DTIM bit set, it can retrieve data from the AP by sending a Power-Save Poll (PS-Poll) frame
to the AP. This power-save scheme is effective but only allows clients to doze for a small beacon interval.
Clients still need to wake up several times per second to read DTIM from the beacon frame of the AP.
With 802.11e, the new power-saving mechanism was introduced that helps voice-capable Wi-Fi devices, as
voice packets are transmitted at short time intervals, typically 20 ms/sec. Unscheduled automatic power-save
delivery (U-APSD) allows a power-save client to sleep at intervals within a beacon period. AP buffers the
downlink traffic until the client wakes up and requests its delivery."

[vodovozovilya]

Hello Flavio, Thank you for your response.

I found that:

"the client station sends a trigger frame related to a WMM access category to inform the AP that the client is awake and ready to download any frames that the AP may have buffered for that access category. The trigger frame can also be an 802.11 data frame, thus eliminating the need for a separate PS-Poll frame. The AP will then send an ACK to the client and proceed to send a frame burst of buffered application traffic during a transmit opportunity (TXOP)."

This explains how a client tells an AP that it is ready to receive buffered DOWNLINK data (from AP to the STA). So I am still not sure, does a station store its UPLINK traffic (from STA to AP) while an AP is performing other channels scanning and not listening to the clients?

[vodovozovilya]

Hello,

I found a mentioning of the problem in CCIE Wireless v3 Study Guide, chapter 7 WLAN Media and Application Services, QoS Survival Concepts:

oblivious to the presence of voice traffic (as you use best effort), the AP suddenly jumps to another channel to scan (RRM). The AP is likely to be gone for about 80 ms, during which your phone will fail to receive an ACK for the last voice packet, will retry many times while rate shifting down, then drop
the packet and attempt to send the next one, and so on. When the AP returns to the channel, you
will have lost three consecutive packets, which is enough for the user to detect a short drop in the
audio flow and complain about Wi-Fi. Yet, troubleshooting the issue will show no oversubscription
of the link. The issue is not bandwidth. The issue is that proper priority was not given to this realtime
traffic over other tasks. Typically, 30 ms in the cell is the maximum time interval allowed for a
Wi-Fi driver to send a real-time voice frame (UP 6). After that delay, this type of frame is dropped
from the queue.

But still, in further sections it is not explained, how a client deals with the fact that AP is not listening to its frames while scanning other channels.

Will highly appreciate any help, maybe I am missing something or just got something wrong.

Thanks in advance.

[vodovozovilya · Accepted solution]

Here is the exat answer I was looking for:

 High Channel Utilization and poor SNR in a busier network will lower the opportunities NDP has to transmit. A Voice SSID will defer off channel activity any time there is has been traffic in the voice QOS que within the last 100 ms. Retaining the historical information over a period of time compensates for short duration inconsistencies.

https://www.cisco.com/c/en/us/td/docs/wireless/controller/technotes/8-3/b_RRM_White_Paper/rf-grouping.html

Thanks everyone!