Max no. of simultaneous APSD calls?
Last Post: June 26, 2009:
I am searching for some typical values for the maximum number of simultaneous VoWLAN calls which can be practically achieved with U-APSD and WMM voice prioritization enabled. I know this will depend on the 802.11 chipset and the environment - I am just searching for typical values to get a rough idea. I require this information to carry out some planning. I have done some googling on this, but couldn't come across anything solid.
Reason for this concern:
U-APSD, though good for battery power savings, is known to increase contention. All the VoWLAN stations sleep for a fixed amount of time. Then, they wake up almost together and try to access the medium all at once. This may decrease the number of simultaneous calls of reasonable quality.
A VoIP phone won't be sleeping during a call so nothing to worry about.
As far as how many simultaneous calls, yeah that is a tough one. It is hard to give even a generic number without knowing what data rate or MCS it would be using.
I suggest you either test it in a live environment, or call up a VoIP manufacturer. Whatever number they give you, cut it in half and you should be safe. Ok, joking... sort of. :)
I agree that finding a concrete number would be difficult because it varies greatly between infrastructure vendors.
The good news is that 802.11n chipsets will support an enhancement to U-APSD called U-PSMP (Unscheduled Power Save Multi-Poll) where stations awake on a schedule. Since they're not all scheduled to awake at the same time, there is no contention between U-PSMP stations and regular contention with any other stations present.
Right now, the industry's 'unspoken' standard is about 7 calls per AP, though some vendors can go much higher (to as much as 25) depending on their system architecture. It also depends on how many data clients are on each AP, how much data they're moving (at what MCS rate), and how much non-modulated interference is in the band. Many variables.
Hi GTHill and devinator,
Thanks a lot for your replies!
From the below article written by Bill McFarland, CTO, Atheros, it looks like 15 U-APSD enabled STAs is a typical value:
In one section of the article, the author contrasts U-APSD with 802.11n PSMP Power Save. He says:
"All of these factors cause PSMP to have a significant amount of overhead, so this mode only pays off if the number of VoIP nodes associated with an AP exceeds approximately 15. Under APSD, the devices wake up according to their own timing and are not coordinated. With that many devices contending for the airwaves, they start colliding or waiting for each other to finish their transmissions.
As a rule of thumb, with a modest number of VoIP devices, APSD is the best approach. A network with a large number of VoIP calls (>15) going simultaneously to the same access point might benefit from PSMP. "
Number of voice calls on a WLAN is indeed dependent on a number of factors. The old "7" call rule was for 802.11b (because that's what most of the vendors had for the longest time). Now, the world is different. Being that 802.11n phones are largely not-present, let's consider 802.11g and 802.11a.
Most infrastructure gear comes right out of the box w/ 1, 2 and 5.5Mbps as basic rates, which will kill channel capacity almost immediately where you have multiple SSIDs and a high density of APs (mainly the 1 and 2Mbps rates). If you're nominal RF utilization without running any data traffic is ~5% I've been able to exceed 20 calls on a single cell w/ 802.11g.
I have this discussion a lot with my colleagues when it comes to planning for max simultaneous calls per AP because there is often so much focus around it. Think about a scenario of 20 people sitting on a quiet bus all on their cell phones. People would start smacking each other! i.e. people will tend to scatter about to get to a more quiet space and start roaming to adjacent APs.
The moral of the story is that if you're designing your cells right (not insanely low on power and not too hot on the downlink) and you've followed the VoWiFi manufacturer's deployment guide I would expect you to get ~20 and good quality if you use QoS.
Btw, if you have 802.11b phones, throw them away if you're concerned about calls/AP.