Outdoor Wireless Access Point
jmaslak at antelope.net
Sun Apr 1 18:09:12 CDT 2012
On Apr 1, 2012, at 3:44 PM, Masataka Ohta <mohta at necom830.hpcl.titech.ac.jp> wrote:
> With 802.11, you can connect to an AP and, if the AP
> fails, you may be connected to another AP, but the
> transition takes considerable amount of time not
> tolerable for voice communication, which is why it
> is not called mobility.
True under basic 802.11, at least with WPA2 + EAP, for some clients. Not all clients wait until they lose connectivity to start looking for another AP - it depends on how the client was built. However, even without needing to lose connectivity to learn what other APs are nearby, there still is a substantial associatiation delay with EAP.
That's why 802.11r + 802.11k exist. I'm sure the big name vendors support this and also support their proprietary alternatives that may or may not be better.
> If you want mobility, have different SSIDs for APs in
> the same frequency band (or, let terminals have multiple
> sets of radio interfaces) and let terminals connect
> to multiple APs simultaneously.
That's one way of doing it, provided you have a way to manage all the end devices when you add new APs. It has the disadvantage of not being a COTS solution AFAIK.
Another way to do it is Meru's "one frequency, one MAC" approach.
As for locating other access points, even without 802.11k, most solutions I have seen go into power save mode for long enough to do a quick scan every once in a while, taking into account the size of the phone's jitter buffer. That causes the AP to hold packets until the scan finishes. So one channel is not required for fast roaming.
I've seen solutions cope without 802.11r + 802.11k by using a WEP-only SSID on each AP (typically the same SSID for all APs) and throwing that into a VOIP-only VLAN. But with smartphones capable of running VoIP clients, I'd be less inclined to do it that way even if I thought WEP was secure-enough for voice calls.
The other solution that I've seen some things support is to use WDS on the VoIP device. I'm also not a fan of that personally, but others may be. WDS would require one frequency throughout the network however.
> Though you only have to modify software on terminals,
> AFAIK, there is no such commercial products.
There are plenty of commercial products that support VoIP handoff without issues. Some are proprietary, some are open standards. Many support multi-channel networks. It starts to get expensive to do this though, as most (all?) of the cheap vendors don't do what is required on the AP side. That said, I'd love to hear I'm wrong on this - I'm looking for new APs for home.
So, if I was buying an enterprise 802.11 solution and needed to support seamless VoIP roaming, I'd look at either a one-vendor solution (I'm sure Cisco phones + Cisco APs + Cisco Controller + Cisco PBX would do this just fine, for instance; you can substitute a few other big vendors for Cisco, no doubt, although not likely cheap ones; you'll be spending 10x or more per AP in many cases than if you could have used the cheap ones) or someone that complies with 802.11r + 802.11k (both for handses and APs). Obviously your network better support DSCP and/or VLAN priority marking and WMM as well.
Supporting VoIP handoff is much more complex (and, at least from what I've seen, expensive) than supporting web browsing handoff. It's also what seperates different pricing tiers of wireless equipment.
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