Recently I had an experience with a group of Cisco wireless phones including both the 7921 and 7925 model in which the behaviour was quite odd. The scenario I setup was with the following infrastructure.
- CUCM 8.6 SU3
- Cisco Prime Infrastructure 2.0
- Cisco AP 3502i with LWAPP Version 7.4.110.0
- Cisco 7921 Wireless Phone
- Cisco 7925 Wireless Phone
- Cisco 7945 IP Phone
All IP Phones have been configured with a single shared line. I have both the 7921 and 7925 wireless phone connected to a single AP.
All phones are located at the same site and within the same subnet.
The behaviour monitored was that when an call came to the shared line DN, whether it be sourced from Internal or external to the CUCM Cluster all 3 phones would ring at separate times. The 7945 would always ring, so I more or less had this phone in the scenario to benchmark the two wireless phones. After a couple of seconds of the 7945 ringing, one of the wireless phones would start to ring, then soon after the second wireless phone would start to ring. This was not discriminate of the type of wireless phone, as I tested over several inbound calls, and sometimes the 7921 would ring before the 7925 and then vice versa, other times one the wireless phone would ring at the same time as the 7945 ip phone and so on. However I could not seem to get a consistent ringing behaviour for the two wireless phones.
This didn’t seem to matter if the wireless phone’s was asleep or not.
I did some digging around and found the two wireless phones had negotiated a Call Power Save Mode with the WMM policy of the AP. I logged into each of the wireless phones web page and verified that indeed the Unscheduled Automatic Power Saved Delivery (U-APSD) / PS POLL had been negotiated.
U-APSD mode is part of IEEE 80.11e standard and does provide strong benefits with lowering power consumption etc. However in this case it seemed that the AP was buffering packets until an active connection was established with the wireless phone.
In Prime Infrastructure under the WLAN configuration page, then in the QoS settings window, scroll down to you see the WMM Policy. This can be either “Required”, “Allowed” or “Disabled”. In this scenario the WMM Policy was configured to Allowed. With the wireless phones also have this setting configured to U-APSD / PS Poll allowed the wireless phone to negotiate with the AP.
On the 7921 wireless phone I configured the Power Save Mode to “None”. This basically puts the phone into an Active state and does not negotiate a call power save mode with the AP. Therefore no packets are buffered on the AP.
After I rebooted the phone (don’t have too, but I always like to flush the cache out etc when I make changes like this) the resulting behaviour is now the 7945 and the 7921 phones start the ringing at the same time, so no more delay however the 7925 phone still had a few second delay. Now once I changed the 7925 wireless phone Power Save Mode to “None”, I could ring the shared line and see all three phones ring simultaneously. Also the two wireless phones when in sleep mode still rang together with the 7945 phone. So the power save mode, although has its benefits seems to play some tricks with these phones.