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Question APs in a Commercial Environment

RBZ5416

Distinguished Member
A question for @mickevh or anyone else with knowledge of a large scale deployment of APs.

My Mum is currently in a nursing home with a public SSID (PSR) & two private ones GSR & SRS). With the home's permission I bought her a smart TV so she could always find something to watch on the catch-up services. Unfortunately apart from the day I set it up, the signal is too poor. She also has an Internet radio that frequently fails to connect. This despite having an AP right outside her room. Here's an InSSIDer screenshot of SSIDs by signal strength.

upload_2017-7-12_9-56-53.jpeg


This firstly confirms that the public network does indeed have a lower signal strength, to the extent that a random BT Hub is fractionally better! But I'm also curious about what else this is showing.

Apparently all three SSIDs are sharing channel 6. Surely this means they are fighting each other?

2.4GHz & 5GHz channels are sharing the same SSID, so there is no way to intentionally connect to a 5GHz channel.​

I have no knowledge of the architecture/design of a multiple AP installation but this all seems a bit odd? Grateful for any input from the pros.
 

mickevh

Distinguished Member
Hi there, this isn't unusual. There's a few things to unpick here, so the order I'll address them in might not make much sense at first , but read through a couple time and hopefully any "circular" logic will resolve...

Wi-Fi RSSI (Received Signal Strength Indication) varies constantly. One could grab 100 packets from a given AP (or any other device) and expect to see differing RSSI values reported as signalling conditions change. It's "just like that."

Remember RSSI is reported "retrospectively" by things like InSSIDer - the tool only gives an illusion of it being continuous "signal" - it's not. (In the background, InSSIDer and tools like it are manipulating meta-data associated with packet captures, it's not actually connecting to the radio antenna and monitoring their inputs.) It takes a surprisingly long time (several seconds) for InSSIDer to consider an AP to have "gone out of range" because it hasn't heard from it for a while. Some Wi-FI sniffer tools report when they "last heard from" each AP/SSID - I don't recall whether InSSIDer offers this, but "Acrylic Wi-Fi" does.

SSID "Service Set Identifier" - it is essentially a "name" for a set of devices that are all participating in a "conversation" - point being an SSID is not a named AP, channel or "signal."

Enterprise AP's usually offer the facility to support multiple SSID's. This is rarer in SOHO kit (much of which only offers 1) though some offer "main" and "guest" SSID's - e.g. BT's "FON" & "BT Wi-Fi" offerings.

It is correct that in an AP offering multiple SSID's (we'll get to dual band later) the SSID's are competing for air time, just like everything else on the same channel. It's like a restaurant with an "only person at a time can speak" regime; table one may be involved in one conversation (service set) table two is having a second conversation (service set) etc. Each table is only conversing amongst itself, but they are all competing with each other for "air time." There's no free beer - if you offer 4 SSID's out of a single (say) 300mbps capable AP, there's still only 300mbps shared (competed for) between everything.

Because of the moment to moment varying in RSSI and the way the sniffer tools "channel hop" to listen in to each radio channel (meaning readings are occurring at different times,) you do sometimes see situations where the RSSI for SSID's all belonging to the same (physical) AP differ, though it's not usually by much more than a few dB. From your screengrab, the PSR you are associated with is almost certainly coming from the same AP offering SRS & GSR a few lines above on C6 - note the similarity of the MAC addresses.

In enterprise solutions we (or at least I) normally offer the same SSID(s) in each waveband, not least because most users are not sophisticated enough to want to make the choice (or even know there is a choice) and there's still plenty of "2.4GHz only" gear out there. Some enterprise AP's use a few "tricks" to try and encourage those devices that can do to use 5.GHz, but we (I) don't usually force it - if something is determined to use 2.4GHz, we let it.

If you really wanted to force 5Ghz from a given client, and the client will let you, you could disable it's 2.4GHz. A lot of laptops will let you either mandate or "prefer" a particular waveband - probably less so in consumer devices like TV's, phones, etc. in which case you're in the gift of the equipment's designer as to what it prefers (if it even makes the choice.)
 
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Kristian

Well-known Member
Something short after Mick's long post... :)

Having multiple SSIDs on one AP and one channel isn't that bad but it's advisable to minimise this where possible (because it's shared out). What's more worrying is that there are two APs in use and they're both on the same channel 6. This will be causing interference and therefore poor performance. I can only see one PSR SSID on that screen shot but plenty of GSR/SRS. Is PSR offered on the other AP, as it looks to have a stronger signal.

One of the APs needs moving to a different channel, channel 1 looking at that screenshot.
 

RBZ5416

Distinguished Member
Thanks guys.

From some further testing today it looks very much like the public SSID simply isn't available on the nearest AP. No idea why & I gues the chances of me finding out are slim. I've also got a suspicion that all the APs are on auto channel selection & endlessly chasing their own tails!

But on the bright side I managed to blag the password to one of the other SSIDs so all's good - for now at least.
 

mickevh

Distinguished Member
I've also got a suspicion that all the APs are on auto channel selection & endlessly chasing their own tails.

That can happen, though enterprise solutions often employ mechanisms whereby the AP's talk to each other and/or a management platform in order to try and figure out an optimal channel plan in order to avoid such things.

However, in my experience the results can be very variable - though I don't criticise the vendors given how difficult a thing it is to model let alone automate.

In less sophisticated systems, it could be left to some poor overworked, under payed, put upon IT tekky who has to figure out the channel plan on a piece of paper. :D
 

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