FRA and Macro/Micro Cell Operation - Part 2

Part 1 of this blog series looked at how Cisco 2800/3800 APs running in dual-5 GHz mode can steer clients from the macro cell to the micro cell using 802.11v BSS Transition Management frames. In this installment, I will look at what methods can be used if your clients don't support 802.11v.

Before going into the details of the other method (probe suppression), here is what I have observed while testing a mix of clients:

• Both Android and iOS devices responded well to 802.11v Transition Management Requests. Sometimes the iOS device I was testing with would reject the request with a reason code 6, but most of the time it accepted the transition. 
• If there are enough clients connected to the macro cell to warrant a transition to the micro cell, a client that does support 802.11v will be moved, even if it was in the macro cell "first." 
• According to the latest RRM White Paper, if a client does not support 802.11v, but does support 802.11k, it can be transitioned, but not as gracefully. The client must request a neighbor report, and the returned neighbor list will be limited to the BSSID of the micro cell. The client will then be disassociated, after which it will hopefully connect to the micro cell. I was not able to replicate this; it was hard to find a client that supported 11k but not 11v. Turning off 802.11v on the WLAN resulted in no clients being transitioned at all, whether or not they supported 11k. 

Configuring probe suppression is shown below. Probe suppression can be configured to suppress only probe responses, or both probe responses and auth responses. 

(Cisco Controller) >config advanced client-steering probe-suppression enable probe-and-auth

(Cisco Controller) >show advanced client-steering

Client Steering Configuration Information

  Macro to micro transition threshold............ -55 dBm
  micro to Macro transition threshold............ -65 dBm
  micro-Macro transition minimum client count.... 1
  micro-Macro transition client balancing win.... 1
  Probe suppression mode......................... probe-and-auth
  Probe suppression validity window.............. 100 s
  Probe suppression aggregate window............. 200 ms
  Probe suppression transition aggressiveness.... 3
  Probe suppression hysteresis................... -6 dBm

The macro to micro transition threshold has a similar meaning with probe suppression as it did with 11v transition. If a new client is a transition candidate, probes received on the macro radio with an RSSI stronger than the macro to micro threshold will have their responses suppressed.

Probe suppression steering introduces four new parameters, only two of which are user configurable. The parameters perform the same function as those under Wireless -> Advanced -> Band Select, but have slightly different names.

The probe suppression aggregate window is an amount of time that a burst of probes from a client on a single change are considered a single probe. This is similar to the Scan Cycle Period Threshold value in Band Select. Sometimes clients will sends out probes in bursts of multiple probes. Below is a Motorola G4 probing out on 5 GHz. It sends bursts of 5 probes on the same channel, just milliseconds apart. The client-steering engine will treat these 5 probes as a single probe because they all happened within 200ms.

Probe Bursts

The probe suppression validity window is the amount of time that could elapse between probes (or bursts of probes) from a single client received on the macro radio. The default value is 100 seconds, and it acts as an age-out timer.

The validity window works with the transition aggressiveness value, which corresponds to the probe cycle count value under Band Select. The transition aggressiveness value sets a limit on the number of times probe responses from the macro radio will be suppressed. The default is 3. If a probing client was a candidate to have probe responses from the macro cell suppressed, and the client had probed out on the macro channel 3 times within 100 seconds, the fourth probe (or burst) on the macro radio would be answered. This allows clients to connect to the macro cell if they refuse to connect to the micro cell because the RSSI at the client is too low.

The probe suppression hysteresis is a user configurable value between -3 and -6 dBm, with the default being -6. When Cisco uses the word hysteresis, it refers to a dampening method to prevent clients from bouncing back and forth between radios. In the context of Client Roaming, under Wireless -> 802.11a/b, the hysteresis value tells CCX clients to move to a new AP only if the RSSI value is 3 dB better than the current AP. I stumbled across the meaning of the hysteresis in probe suppression by trying to adjust the values of the transition RSSI thresholds.

(Cisco Controller) >config advanced client-steering transition-threshold macro-to-micro -60

Value must be greater than micro to Macro RSSI - probe suppression hysteresis

(Cisco Controller) >config advanced client-steering transition-threshold micro-to-macro -60

Value must be less than Macro to micro RSSI + probe suppression hysteresis

In this case, it looks like the -6 dBm hysteresis means that probes for clients already associated to the AP would have to be 6 dB weaker/stronger to get moved to the other cell. This makes sense, as you don't wont the client bouncing back and forth between the micro and macro cells because of small differences in RSSI that could just be from different client device orientations.

My testing with probe suppression for client steering was mostly subjective. Since the clients did not associate, I could not use "show client detail" to see the RSSI of the probe requests at the AP. I could definitely see probe suppression in action over the air. Below is a capture on channels 44 and 161. The macro cell was on channel 161, and you can see probes on 161 being ignored.

Probe Suppression of Macro Cell

The client connects to the micro cell on channel 44.

Other testing I conducted involved the transition aggressiveness factor. My Moto G4 cycles through the 5 GHz channels in about 6 seconds. With a transition aggressiveness factor of three, it should take about 24 seconds to see probe responses from the macro cell. My observations lined up with this prediction within a few seconds.

Overall, I didn't find the probe suppression method of client steering to be as predictable as the 11v method, but it did work satisfactorily. Given that most clients now support 11v I would prefer using that method over probe suppression.

FRA and Macro/Micro Cell Operation - Part 1

NOTE: This blog is not about the merits, performance, or lack thereof with dual-5 GHz radios. This is a blog about the operation or dual-cell APs, specifically how the AP transitions clients between the macro and micro cells.

Cisco 2800/3800 APs support Flexible Radio Assignment, which allows the 2.4 GHz radio to flip to either a monitor or another client-serving 5 GHz radio. When the AP is operating in this dual-5 GHz mode, the normal radio (slot 1) powers the macro cell, and the flexible radio (slot 0) powers the micro cell. The terms macro and micro are used for two reasons. When the flexible radio is put into 5 GHz mode, either automatically through the FRA algorithm or manually, the radio switches to a more directional antenna than the normal 2.4 GHz antenna. See the antenna radiation patterns of the 3800 from the AP2800/3800 Deployment Guide:

AP3800 Antenna Patterns

The second reason is the reduction in power on the flexible radio when operating in dual-5 GHz mode. The flexible radio is locked into transmitting at the lowest power supported, which is usually 2 dBm. The reduction in power, along with a mandatory separation of at least 100 MHz between the micro and macro radios, is to reduce the near-field effects of the two radios interfering with one another.

Looking at the elevation pattern (right), you can see that the macro radio has a "dead spot" directly below the AP. The micro radio (blue line) has a 15 dB advantage over the macro radio in this dead spot. Unfortunately because of the power limit on the micro radio, most clients will still perceive the macro radio as having a higher signal strength. This is even more true when not directly under the AP.

In order to take advantage of the micro cell, the AP/Controller has to have a way to nudge clients that connect to the macro cell over to the micro cell. This mechanism is called client steering, and the default method to steer clients is the 802.11v BSS Transition Request.

To see how client steering works with flexible radios the following settings must be made on the controller:

• Flexible Radio Assignment must be enabled globally under Wireless -> Advanced. 
• The flexible radio in the AP2800/3800 can either be set to Auto or client serving. When in auto mode, the FRA algorithms determines if it is better to leave the radio in 2.4 GHz client serving mode, monitor mode, or 5 GHz client serving mode. For my testing I manually configured the flexible radio as 5 GHz client serving. 
• BSS Transition Management must be enabled for the WLAN that the clients will connect to. 

Unlike normal BSS Transition Management between distinct APs, Optimized Roaming is not required for 11v frames to be used to transition clients between the macro and micro cells. Radio power and channel settings can be left to auto. The power will not be adjustable on the micro radio, even if it it set to manual. For ease of testing, I removed DFS channels from the 802.11a channel plan. This is how my AP3800 looked:

AP Name                          Channel    TxPower       Allowed Power Levels    
-------------------------------- ---------- ------------- ------------------------
FRA-AP                           44*        *8/8 ( 2 dBm) [22/19/16/13/10/7/4/2]
FRA-AP                           149*       *2/7 (16 dBm) [19/16/13/10/7/4/2/0]

It's also helpful to see the BSSID values that are assigned to the macro and micro cells, to be able to confirm values in the BSS Transition Management requests.

(Cisco Controller) >show ap wlan 802.11a FRA-AP

Site Name........................................ TestGroup
Site Description................................. 

WLAN ID          Interface          BSSID                            
-------         -----------        --------------------------       
14              xxxxxxxxxx           58:ac:78:xx:xx:3f  

(Cisco Controller) >show ap wlan 802.11-abgn FRA-AP

Site Name........................................ TestGroup
Site Description................................. 

WLAN ID          Interface          BSSID                         
-------         -----------        --------------------------     
14              xxxxxxxxxx           58:ac:78:xx:xx:30            

Note the difference in the last octet of the BSSIDs between the macro and micro cells. I will reference this later.

The command to list the client steering parameters, their default values and explanation are shown below:

(Cisco Controller) >show advanced client-steering

Client Steering Configuration Information

  Macro to micro transition threshold............ -55 dBm
  micro to Macro transition threshold............ -65 dBm
  micro-Macro transition minimum client count.... 3
  micro-Macro transition client balancing win.... 3
  Probe suppression mode......................... disabled
  Probe suppression validity window.............. 100 s
  Probe suppression aggregate window............. 200 ms
  Probe suppression transition aggressiveness.... 3
  Probe suppression hysteresis................... -6 dBm

Macro to micro transition threshold: This is a value in RSSI above which a client can be transitioned from the macro cell to the micro cell. The default is -55 dBm. This is the RSSI at the AP. For example, if a client connects to the macro cell and its RSSI at the AP is greater than -55 dBm, it is a candidate to be transitioned to the micro cell. 

Micro to macro transition threshold: This is a value in RSSI below which a client can be transitioned from the micro cell to the macro cell. The default it -65 dBm. For example, if a client connected to the micro cell initially and had a RSSI at the AP of less than -65 dBm, it will be transitioned to the macro cell. Given the power difference between the macro and micro cells, is is rare for clients to be transitioned from the micro cell to the macro cell.

micro-Macro transition minimum client count: The minimum number of clients in either macro or micro cells that will trigger a transition for the next client connecting. The default is 3. For example, if there are 3 clients in the macro cell, the 4th client that tries to connect to the macro cell will be transitioned to the micro cell, if it meets the requirements for macro-to-micro transition threshold RSSI. 

micro-Macro transition client balancing window: This specifies the minimum difference in client count between the macro and micro cells that must exist before a client can be transitioned between cells. The default is three. Imagine a scenario where there are 5 clients in the macro cell and 3 in the micro cell. The difference in clients between the cells is 2, which is below the default balancing window value of 3. The next client that connects to the macro cell will not be transitioned to the micro cell, even it it meets the RSSI requirements. Now there are 6 clients in the macro cell and 3 in the micro cell, and the difference in client count now meets the balancing window requirement. The next client that connects to the macro cell will be transitioned to the micro cell, IF it meets the RSSI requirement. 

I only had two 802.11v capable clients to test with, so I changed both the transition minimum client count and transition client balancing window to 1.

(Cisco Controller) >show advanced client-steering

Client Steering Configuration Information

  Macro to micro transition threshold............ -55 dBm
  micro to Macro transition threshold............ -65 dBm
  micro-Macro transition minimum client count.... 1
  micro-Macro transition client balancing win.... 1

With these parameters, I could connect one client to the macro cell, and the second client to connect would (hopefully) get transitioned to the micro cell. I used 'debug client' and specified the MAC addresses for both clients.

I connected the first client to the SSID and confirmed through the CLI that it had connected to the macro cell. When using an AP setup in macro/micro, you will see extra lines in the 'debug client' output that contain XOR:

f8:95:c7:xx:xx:xx Association received from mobile on BSSID 58:ac:78:xx:xx:xx AP FRA-AP
f8:95:c7:xx:xx:xx Station:  F8:95:C7:xx:xx:xx  trying to join WLAN with RSSI 208. Checking for XOR roam conditions on AP:  58:AC:78:xx:xx:xx  Slot: 1
f8:95:c7:xx:xx:xx Station:  F8:95:C7:xx:xx:xx  is not eligible for XOR roam on AP  58:AC:78:xx:xx:xx 

The first client is not eligible for transition to the micro cell because it is the only client in the macro cell. Let's see what happens when the second client connects to the macro cell.

80:00:6e:xx:xx:xx Processing assoc-req station:80:00:6e:xx:xx:xx AP:58:ac:xx:xx:xx-01 ssid : xxxxxx thread:1a722e30
80:00:6e:xx:xx:xx Station:  80:00:6E:xx:xx:xx  trying to join WLAN with RSSI 212. Checking for XOR roam conditions on AP:  58:AC:78:xx:xx:xx  Slot: 1
80:00:6e:xx:xx:xx Station:  80:00:6E:xx:xx:xx  scheduled to transition to new BSS on AP  58:AC:78:xx:xx:xx

We see in the second line that the debug output shows a RSSI value of 212. I'm not sure how this scale of RSSI equates to a power level in dBm, but it appears to be above the threshold of -55 dBm. The third line indicates that the client will be scheduled for transition.

Before the client is transitioned, it sends a 802.11k Neighbor Report request. The debug output is interesting.

80:00:6e:xx:xx:xx Got action frame from this client.
80:00:6e:xx:xx:xx Station:  80:00:6E:xx:xx:xx  sent 802.11K neighbor request to AP  58:AC:78:xx:xx:xx 
80:00:6e:xx:xx:xx Station:  80:00:6E:xx:xx:xx  sent request with RSSI (0) to XOR roam capable AP  58:AC:78:xx:xx:xx  Slot 1
80:00:6e:xx:xx:xx Station:  80:00:6E:xx:xx:xx  limiting neighbors to sibling radios on AP  58:AC:78:xx:xx:xx 

Because the client-steering engine had already decided to transition this client to the micro cell, it limits the list of neighbors it will send back to the BSSIDs on the micro cell.

Note that the transition of the client is scheduled; it doesn't happen immediately. Perhaps this is to prevent flapping of clients transitioning between the micro and macro cells as clients join and leave the cell. It may also be delayed to allow clients in motion to roam to other macro cells, instead of being pulled back into the micro cell of a far away AP. My testing indicates that the amount of time that elapses between the association of the client that triggers the XOR roam and the transmission of the 802.11v BSS Transition Management Request varies. If I find more information I will update this blog.

Below we see the sequence of events as the second client is transitioned to the micro cell.

1 2 3 4 5 6 7

80:00:6e:xx:xx:xx apf80211vSendPacketToMs: 802.11v Action Frame sent successfully to wlc 80:00:6e:xx:xx:xx Setting Session Timeout to 4 sec - starting session timer for the mobile 80:00:6e:xx:xx:xx Setting Session Timeout to 40 sec - starting session timer for the mobile 80:00:6e:xx:xx:xx Got action frame from this client. 80:00:6e:xx:xx:xx Processing assoc-req station:80:00:6e:xx:xx:xx AP:58:ac:78:xx:xx:xx-00 ssid : xxxxx thread:18f453d8 80:00:6e:xx:xx:xx Station: 80:00:6E:xx:xx:xx trying to join WLAN with RSSI 217. Checking for XOR roam conditions on AP: 58:AC:78:xx:xx:xx Slot: 0 80:00:6e:xx:xx:xx Station: 80:00:6E:xx:xx:xx is not eligible for XOR roam on AP 58:AC:78:xx:xx:xx

Line 1 is the debug entry for sending the 11v BSS Transition Request, which is acknowledged in line 4. The client re-associates in line 5. Line 6 indicates that the client is associating to the XOR radio slot 0, which is the micro cell. In line seven we see that the client is not eligible for transition from the micro back to the macro cell: the difference in the number of clients between the cells (0) is not greater than the transition client balancing window (1).

Over the air, we see a BSS Transition Management request that includes a candidate list. The only entry in the candidate list is the BSSID of the WLAN on the micro radio.

BSS Transition Management Request

Wireshark does not completely decode the candidate list entries, but they are in the same format as an 802.11k Neighbor Report. I highlighted the important elements: the BSSID and the channel number. The BSSID matches what we expect, and so does the channel number, 44. The client responds to the request with the following action frame.

BSS Transition Management Response

The important fields here are the BSS Transition Response Status Code and the BSS Transition Target BSS. The Status Code communicates whether the client accepts or rejects the request, and a value of 0 indicates that client accepts it. The Transition Target BSS indicates the BSSID that the client intends to transition to. In this case, it matches the BSSID in the candidate list from the request frame.

Now we see that the second client has been transitioned to the micro radio on slot 0.

1 2 3 4 5 6 7 8

(Cisco Controller) >show client summary . . MAC Address AP Name Slot Status WLAN Auth Protocol Port Wired Tunnel Role ----------------- ----------------- ---- ------------- ----- ---- ---------------- ---- ----- ------- ---------------- 80:00:6e:xx:xx:xx FRA-AP 0 Associated 14 Yes 802.11n(5 GHz) 13 No No Local f8:95:c7:xx:xx:xx FRA-AP 1 Associated 14 Yes 802.11n(5 GHz) 13 No No Local

That's it for Part 1 of this series on client steering between macro/micro cells on an AP3800. Next I will look at what can be done if clients do not support 11v.