7921G perform poorly in Abruba wireless environment

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Jul 12th, 2009
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Hi All,

Here we have a VoWLAN project troubled us long time. Could you give me some suggestions please?

The situation is that we deployed VoWLAN in our hotel customer this year. The wired network infrastructure device is Cisco switch C3560 and C6509, Call control is callmanaer 5.1, and the wireless infrastructure device is Aruba, including Aruba Wireless Controller and about 600 Aruba APs. The wireless IP Phones are Cisco CP-7921G and several old CP-7920G.

After a few weeks of use, we found that CP-7921G performed poorly in the hotel. In some corner of the room, 7921 loses voie stream during the session usually, sometimes we can't answer the call and one-way voice connection. But the RSSI and SNR on the corner is acceptable, which is about -65dbm and 26.

For the comparison, we found 7920 works better than 7921G, and can cover more bigger range. Therefore we contacted TAC for the problem, and TAC told me that 7920 has higher Tx power level(40mW) than 7921, so 7920 can work better than 7921. But by in searching the doc from CCO, I found that is not true.

So could you tell me what is real reason of the poor performance of 7921 in Aruba Wireless enviroment, even worse that 7920. And how can we resolve the problem? Any suggestions or solutions?

Thanks very much


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Overall Rating: 4 (2 ratings)
qilin zhang Tue, 07/14/2009 - 03:09
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Can we adjust the Tx power level of the 7921G manually, so that we can improve the 7921G performance.

migilles Tue, 07/14/2009 - 14:51
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The 7921 will use the max power by default, which is 17 dBm for 802.11b and 16 dBm for 802.11a and 802.11g.

However 3rd party APs (i.e. Aruba) are not supported as we do not test for 100% interoperability with those platofrms.

The Cisco APs have a feature called DTPC, which can control what tx power the client will use.

I am not sure if your Aruba controller version supports WMM, U-APSD and CAC (TSPEC) or not. Need to ensure that the QoS markings downstream to the 7921 are marked as DSCP = EF and UP = 6 (VO).

If U-APSD is not supported, then the phone will attempt to use PS-POLL which is higher overhead as the client must poll for each packet and the ap must buffer those packets.

Can you try to put the phone into active mode? You do this by changing the "On Call Power Save Mode" to "None".

If this is works, then there is an interop issue with power save here and suggest you contact Aruba for an updated release.

Otherwise, please see the 7921G Deployment Guide for more info @


qilin zhang Tue, 07/14/2009 - 20:22
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Hi migilles,

What troubled me is that 7920G works better than 7921G in the same wireless environment. But as I know 7921 has more Tx power than 7920G, so any other difference between them?



migilles Fri, 07/17/2009 - 16:11
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That is incorrect. The 7920 can tx up to 100mW as it is an 802.11b device.

The 7921/7925 can tx up to 50mW for 802.11b, but up to 40mW for 802.11a and 802.11g.

The 7920 and 7921/7925 are very different products and basically the only thing in common is they are Cisco Wireless Phones. :)

The 7921/7925 support WMM, TSPEC, TCLAS, U-APSD, etc. etc. etc.

qilin zhang Fri, 07/17/2009 - 23:15
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So is that why 7920 works better than 7921 in the 3-party wireless environment?

Yes, the 7921/7925 can support WMM, TSPEC, TCLAS, U-APSD, etc. But in the 3-party wireless environment it is useless, because all of the feature above is cisco private, right?

qilin zhang Tue, 07/14/2009 - 22:55
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Hi migilles,

I have changed the the "On Call Power Save Mode" to "None", but the problem is still exist.

How can I check the current Tx Power? I see the Tx Power (device status->WLAN->Tx power) is always 1dBm. Does that work normally?

I can confirm these Aruba AP preferences from the 7921G AP detail information menu:


Available Admission Capacity:0



Proxy ARP:NO


Admission Control:NO

Power Constraint:10dBm

migilles Fri, 07/17/2009 - 16:18
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There was a bug, where the tx power may show incorrectly when accessing the first time.

Above you showed the following, but we don't have a such item in our site survey menu.

Power Constraint:10dBm

Ensure that TPC is not configured in the Aruba controller, where it can limit the tx power by x dB. For example if you specify 10 dB, then the max tx power would be 6 dBm as it would substract from the max 16 dBm possible power that the phone can do.

qilin zhang Fri, 07/17/2009 - 23:08
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Hi migilles,

Last week we added a Cisco WLC4402 and replaced some Aruba APs with some Cisco 1131AG APs in the same position for testing. The result is that 7921 works better than in the Aruba enviroment.

But the situation is that we have deployed large numbers of Aruba wireless devices, it is impossible to change all of the Aruba Devices.So could you give me some suggestions to resolve the problem.



dennischolmes Mon, 07/20/2009 - 06:46
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oops! Someone should have shown you the competetive analysis on Aruba hardware. Basically the Aruba APs have a lot of RF design issues surrounding their hardware. Let me know what model of AP you are using and I will explain in detail but here is one of the major issues. The antenna array uses shared antennae for both 2.4 and 5 ghz traffic. This causes MAJOR issues when using voice or latency based applications like video. There is a reason those APs were so cheap man! I could go into much greater detail but don't want you to feel too bad.

qilin zhang Mon, 07/20/2009 - 23:01
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Hi dennischolmes,

Thanks for your suggestion.

Model of AP we are using are below:

Aruba WLC: ArubaOS (MODEL: Aruba6000), Version

APs: 600*AP61,20*AP 60.



dennischolmes Tue, 07/21/2009 - 04:33
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So basically you bought the 20 dollar AP. Aruba offers a lifetime guarantee on the 60/61 line because they spend less than 20 dollars to build them. First, they use a very old chipset. Second, the shared antenna array only has one connector per antenna limiting transmissions at any given time to only one frequency. This can cause major latency and jitter issues around voice and video. Aruba of course denies this but there is excellent documentation available from some analysis studies completed by third party Novarum. Lastly, the CPU and memory of the APs is lacking. No horsepower under the hood so to speak. In a recent bakeoff performed by a customer of mine, we showed a 20% greater coverage area with Cisco 1131s vs the AP61, and a smashing 40% increased aggregate throughput over several set auditing points at set distances from the AP. The Cisco 1131s had better processing power and memory allowing for a better handling of spatial diversity vs the AP 61 allowing the 1131 to operate at higher aggregate throughputs under multipath signal load. As for the controller, I won't even go down that rabbit hole. Just know that the Cisco 4400 and 5500 series run circles around the 6000 on client scalability. The number of APs is really not as important as the amount of throughput per client and the total number of clients the controller can actually service. Lastly, their management platform is a collage of third party applications that are not Aruba developed or owned and ultimately don't work well together. In the bakeoff, Aruba themselves could not get their "stateful firewall" to work and ultimately had to take nearly a week to get up a one controller and 6 AP pilot site. Something we did in 15 minutes with a 4402 and 5 APs for the same coverage area.

migilles Fri, 07/24/2009 - 12:20
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Glad the Cisco Solution is working.

Unfortunately we don't do interoperability testing against Aruba WLAN. I know they are supposed to support WMM, U-APSD, etc.

But if having issues, you can try putting the phone into active mode by setting the on call power save mode on the 7921 to "None" in case there are issues on the AP side supporting power save clients. Would also check for any controller/AP updates from Aruba and check any open caveats with your current version.

Cisco TAC will only assist to the point of ensuring there are no issues with the 7921 handset.

jherrera2 Tue, 04/08/2014 - 07:45
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Hi all

I'm having just the same issue, a lot of problems with performance in the same scenario, but diffrent HW model /SW. OUr VoWLAN has Aruba 7210 controllers deployed with Aruba 6.3 OS and our APs are 105.135, 225 and 124, 134 and 224 with Cisco 7925g Cisco phones. OPUr Voice SSID work in 5ghz (802.11a)

I have the same issues, losses the voice stream during a session (call), sometimes call can;t be answered and one-way audio ramdonly too. Most of the phones with one or two execptions has SNR higher than 35 and the health% which is a new parameter in Aruba OS 6.3 is super high also in most of them, with not less than 50%, but btw 70 and 100% in most of them. However we see a lot of retransmission frames in both ways to/from clients, even in phones with super high numbers. The coverage in the building regarding RF  great, and phopnes with APs on top of the user's desk (without romaing) has the issues above described. Can you please


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