In this post, we’ll explore UPR- Unsolicited Probe Response, another “In-band Discovery” method utilized in the 6GHz operation. It’s similar to the FILS Discovery method. Instead of sending short FILS discovery frames, AP sends a Probe Response frame roughly every 20ms unsolicitedly (i.e. without waiting for clients’ Probe Request frames). Because it is a Probe Response frame, it includes nearly all the information fields found in the Beacon frame. This means that the management overhead will be much higher with this method compared to FILS. It’s almost as if Beacons are sent every 20ms
In legacy bands (2.4GHz/5.0GHz), a “Probe Response” frame is sent in response to a “Probe Request” frame. However, in 6GHz-only operation, an AP can send unsolicited Probe Response frames every 20TU (approximately 20ms) to allow STAs to discover the AP passively, rather than waiting for Beacon frames, which arrive at 100ms intervals.
This time 6GHz RF profile is configured for “Broadcast Probe Response” which is sent every 20ms (default setting) once you enable it. That interval can be a value between 5-25ms in Cisco 9800 WLC.
Here is a PCAP taken once UPR is enabled in our test setup shown above (Cisco 9166 with 9800 WLC 17.12.1 firmware).
In the capture above, If you look at the size of the Probe Response frame(329B), it is almost the same size as the Beacon frame (335B). You can simply compare the contents of Probe Response and Beacon frame (Copy > All Visible items in Wireshark) to identify the exact differences.
Due to the increased management overhead, this method isn’t widely supported by many vendors. As a result, even though it’s an available option, you won’t come across it in most vendor deployments.
In the next post, we will look at PSC – Preferred Scanning Channel, another In-band Discovery method available.