Wireless Access

Reply
Occasional Contributor II
Posts: 81
Registered: ‎01-16-2016

overlap

Hi,

 

The below  text is copied from the  link  https://support.apple.com/en-us/HT203068

 

Trigger threshold

The trigger threshold is the minimum signal level a client requires to maintain the current connection.
iOS clients monitor and maintain the current BSSID’s connection until the RSSI crosses the -70 dBm threshold. Once crossed, iOS initiates a scan to find roam candidate BSSIDs for the current ESSID.
This information is important to consider when designing wireless cells and their expected signal overlap. For example, if 5 GHz cells are designed with a -67 dBm overlap:
iOS uses -70 dBm as the trigger and will therefore remain connected to the current BSSID longer than you expect.
Review how the cell overlap was measured. The antennas on a portable computer are much larger and more powerful than a smartphone or tablet, so iOS devices see different cell boundaries than expected. It is always best to measure using the target device

 


What does it mean here -67dBm overlap . Does it mean station's rssi from AP 1 and AP 2 is -67 dBm

 

Thanks 

MVP
Posts: 514
Registered: ‎11-04-2011

Re: overlap

It may be a bit guessing, as to my knowledge the cell overlap is not fully defined. After reading the Apple article, they mention that as long as the signal does not drop below -70 dBm, the client will not initiate a roaming event.

In wireless planning, you typically set a minimum signal strength value (let's assume -67 for the example), which you want to have at every location in your coverage space. Such a requirement translates in the fact that at any location there need to be at least 1 AP at a signal strength of -67 or more; and you will be planning that where the AP coverage patterns overlap (for roaming), exactly where the original AP goes down to -67 if you walk away from it; another with the signal strength is nearby:

2016-08-16 14_51_29-Untitled.esx_ - Ekahau Site Survey.png

In this drawing, the AP's just touch a -67 dBm level (note that your IOS device, or other client, may see a different signal level depending on antennas, chipset, radio, driver version, product quality).

 

If you put that 'overlap' level on a very strong signal, the clients will assume the original AP is still strong enough and not attempt to roam.

 

What Apple tries to tell here (or how to resolve it..) is not fully clear to me, but the result may be that Apple devices may not roam to the strongest AP and stick to a more distant AP (but still at -70 dBm at minimum). Seems like 802.11k does not help either in this case :-( It does explain though what many are seeing that clients are sticky to distant APs instead of immediate roaming to the nearest/strongest AP.

 

In Aruba Instant or Controllers, turning on ClientMatch should improve the roaming in such cases.

 

--
If you have urgent issues, please contact your Aruba partner or Aruba TAC (click for contact details).
Occasional Contributor II
Posts: 81
Registered: ‎01-16-2016

Re: overlap

Hi

 

 

If I am using application like messenger ,WhatsApp  when roaming  , the application shows connecting and disconnecting

 

Does it mean the rssi does not meet  ?

 

 

Thanks

MVP
Posts: 514
Registered: ‎11-04-2011

Re: overlap

The Apple article basically states that if you have very strong coverage, your Apple device may stick at an AP that is strong enough (stronger than -70 dBm) even when there is a stronger AP.

 

Do you see just the application connecting/disconnecting? Or also the Wifi signal?

How does your coverage look like? Do you have big overlap, where a client can see multiple APs when roamong, or is the Wifi signal weak at the places where roaming occurs?

 

Also, what WLAN solution do you have deployed (type, version etc..)?

--
If you have urgent issues, please contact your Aruba partner or Aruba TAC (click for contact details).
Occasional Contributor II
Posts: 81
Registered: ‎01-16-2016

Re: overlap

Hi,

Iphone ( from the airport utility ) shows it can hear from AP at - 80dBm.

Maybe that is the reason it chooses 2.4 Ghz instead of  5 Ghz .

And is it true that devices  like iphone prefer  2.4Ghz for roaming

 

Thanks 

 

 

 

MVP
Posts: 514
Registered: ‎11-04-2011

Re: overlap

Most devices give preference to the strongest signal. And as the attenuation for 5GHz is slightly higher, many devices see the same AP with both radios on the same power (EIRP) a stronger signal on the 2.4GHz.

 

For that reason, it might help to lower the power on the 2.4GHz radio to about 6 dB under the 5 GHz power to improve roaming and the use of the 5 GHz band.

 

Please check the following VRD for some more information: http://community.arubanetworks.com/t5/Validated-Reference-Design/RF-and-Roaming-Optimization-for-Aruba-802-11ac-Networks/ta-p/227716

--
If you have urgent issues, please contact your Aruba partner or Aruba TAC (click for contact details).
Occasional Contributor II
Posts: 81
Registered: ‎01-16-2016

Re: overlap

Hi,

"many devices see the same AP with both radios on the same power (EIRP) a stronger signal on the 2.4GHz"

 

EIRP and  Signal different ? 

 

Thanks

MVP
Posts: 514
Registered: ‎11-04-2011

Re: overlap

Yes, that is because attenuation is higher at higher frequencies. EIRP is measured at the antenna, so no attenuation is taken into account.

 

Found Friiss transmission formula: https://en.wikipedia.org/wiki/Friis_transmission_equation and if you see the wavelength (lambda), which is squared in the last part of the formula. If you assume 5 GHz is about half the wavelength of 2.4 GHz (double frequency), the received power is 1/4th ( 1/2 to the second power). Which brings you to -6 dB, or that the 5GHz radio should have 6 dB more than 2.4 GHz radios.

--
If you have urgent issues, please contact your Aruba partner or Aruba TAC (click for contact details).
Search Airheads
Showing results for 
Search instead for 
Did you mean: