CAC (Call Admission Control)

Is CAC fair to your clients that don’t support it?

I support a voice product that does not support CAC. Is it right for me to ask the Wireless Network Administrator to disable it because my device doesn’t support it? Is CAC fair? Why does it supersede WMM? I will attempt to answer some of these questions in this blog.

Cisco uses CAC (Call Admission Control) that enables access points to maintain controlled quality of service (QoS). CAC is also tasked with the ability to ensure there is a limited number of voice clients per AP.

 

How does CAC maintain control of QoS?

The AP will send a beacon frame out on each SSID, usually every 100ms or so. In these beacon frames, the AP will tell what features it will support for that particular SSID. Inside the beacon frames under the Vendor Tag: Microsoft: WMM parameters the AP tells the clients which WMM Access Category that CAC has been enabled on (see example 1 below). When a device associates to the AP (and the device doesn’t support CAC) the device will choose the highest level of WMM that doesn’t have CAC enabled on (see example 2 below). If CAC is enabled on AC_Voice, but not on AC_Video, AC_Best Effort or AC_Background then the client will choose AC_Video even if the client is expecting to use Voice grade WMM or even when the SSID and VLAN are set up to use Platinum QoS.

 

Why is this an issue?

WIFI is a contention-based shared media. The AP and clients need to know that no one else will be transmitting at the same time they are. If another device does transmit at the same time it will cause collisions and the packets will have to be resent. In order to avoid collisions, the clients and APs uses Physical Carrier Sense and Virtual Carrier Sense. A device or AP will use both Physical and Virtual Carrier Sense while trying to access the wireless medium.

Physical Carrier sense happens when the station listens to the wireless medium to see if there is RF energy on the medium. If there is, the device will then know that the medium is being used. This is called Clear Channel Assessment (CCA). Virtual Carrier sense is where the station reads the Duration/ID field and sets its own NAV (Network Allocation Vector) timer. While the NAV is still active the station will not transmit. When the NAV timer goes to 0 the station waits DIFS (Disturbed Coordination Function Interframe space), which is set per PHY that you are using.  When the DIFS expires the station will choose a random number from the Contention Window (CW) range and multiply it by the slot time of the PHY you are using.  After all the timers have ended the device will do another CCA and then transmit.

The Access Category gives the client a range called the Contention Window (CW). This range is called the CWmin and CWmax values (see chart below). The device will choose a random number in the CW range and will multiply this with a set number based on the PHY. The client will wait this random amount of time and then will do another Clear Channel Assessment (CCA) to make sure no one else is transmitting at that time. Each client will choose a different value in the Min/Max times. This gives the AC categories with the lower CW values a better chance to transmit, but it is only a probabilistic chance. The lower Categories will get a chance to transmit. When the lower priority clients hear a transmission in the middle of a count/hold sequence they will pause the count/hold, look into the Duration/ID field and sets its NAV timer. When the NAV timer expires and the air is clear the client will resume the hold sequence from where it left off. So eventually it will transmit even while the higher category might be counting down.

 

The CW values per AC Categories are below.

Category                    CWmin           CWmax

AC_Voice                            3                             7

AC_Video                            7                             15

AC_Background               15                           1023

AC_Best Effort                  15                           1023

 

When a client is sending voice packets the client expects to send these packets using the Platinum level of QoS to avoid latency or jitter. If the client does not support CAC and it has to choose the next WMM parameter that doesn’t have CAC support (in this case it was Video) the client will possibly get a much higher CWmin and CWmax time then it should. If the controller set up CAC on Video then the client would choose Background. This would give the client an even worse CWmin and CWmax range to work with. This not only affects upstream, but the packets are labeled as video which would further delay the packets through the wired network.

 

On the return traffic, the network may further strip the QoS level down to Best Effort. In a busy network, this can be problematic for voice clients.

 

Given all of this, is it right for me to ask the wireless guy at the hospital to change the CAC settings because my device does not support CAC? Or should I push back on my own engineering team to fix our client to support CAC? Or should I do both? I can see the wireless guys ponder this as I ask them to remove CAC. Most people in the wireless field are very accommodating especially when you show them the results. Our device is not seen as just another device needing access. This device is usually pushed by C Suite and the nurses on the floor. Wireless guys realize that our product if given the best environment to work in, will help caregivers communicate more effectively and will ultimately help patients. So, if you see me coming, be forewarned I don’t like CAC, DTIMs set to 2 or higher, FRA or RRM (with no limits set). I might ask for things others don’t, but when I do I will back it up with facts and will always be appreciative of your willingness to work with us.

 

 

Example 1 Beacon showing Voice is using ACM (CAC)

 

 

 

 

 

 

 

 

 

Example 2: Data packets showing Client chose QoS of Video

 

 

 

 

 

 

 

 

 

 

 

Example 3: Screenshot from the Wireless Controller Config showing the WLAN has the QoS set to Platinum

 

(Cisco Controller) >show wlan x

WLAN Identifier……………………………. x

Profile Name………………………………. xxxxxxxxx

Network Name (SSID)………………………… xxxxxxxx

Status……………………………………. Disabled

MAC Filtering……………………………… Disabled

Broadcast SSID…………………………….. Disabled

AAA Policy Override………………………… Enabled

************************Data Removed*********************

Quality of Service…………………………. Platinum

 

 

 

Example 4: Screenshot from the Wireless Controller showing CAC and ACM set on the Voice AC

 

 

Call Admission Control (CAC) configuration

Voice AC:

Voice AC – Admission control (ACM)………… Enabled

Voice Stream-Size……………………….. 84000

Voice Max-Streams……………………….. 2

Voice max RF bandwidth…………………… 75

Voice reserved roaming bandwidth………….. 6

Voice CAC Method ……………………….. Load-Based

Voice tspec inactivity timeout……………. Disabled

CAC SIP-Voice configuration

SIP based CAC ………………………….. Disabled

SIP Codec Type …………………………. CODEC_TYPE_G711

SIP call bandwidth ……………………… 64

SIP call bandwith sample-size ……………. 20

Video AC:

Video AC – Admission control (ACM)………… Disabled

Video max RF bandwidth…………………… Infinite

Video reserved roaming bandwidth………….. 0

Video load-based CAC mode………………… Disabled

Video CAC Method ……………………….. Static

CAC SIP-Video Configuration

SIP based CAC ………………………….. Disabled

Best-effort AC – Admission control (ACM)…… Disabled

Background AC – Admission control (ACM)……. Disabled

Maximum Number of Clients per AP Radio……….. 200

 

Cisco’s Flexible Radio Assignment (FRA)

 

I have heard about Cisco FRA for a while but I am only starting to see this out in the field. This technology offers great advancements over statically assigned Radios.

There are two modes of operation in FRA Macro/Macro cell and Macro/Micro cell. I will only be discussing the Macro/Micro mode in this blog. The Macro/Micro cell will have a large cell and a smaller cell inside which will increase capacity on your 5 GHz network.

The theory behind FRA is if you design a network for 5 GHz then you will more than likely have too much 2.4 GHz coverage. This is why FRA is only run against the 2.4 GHz radios.

There are only two AP models that work with FRA. They are the 2800/3800. When the AP creates a Micro cell, the power will always be set to the minimum power of the AP. In the case of the 3802, this would be 2 dBm.

 

How it Works

FRA uses the Neighbor Discovery Protocol (NDP) from RRM to figure out if there is too much coverage on the 2.4 GHz band. The output of this calculation is called Coverage Overlap Factor (COF). You can set the threshold for the COF at Low 100%, Medium 95% and High 90%. When FRA sees too much coverage based on these thresholds values, it will mark the radio as redundant. Once it is marked redundant it can be assigned another role. There are three states (roles) these radios can be in 2.4GHz/5GHz/Monitor Mode. Depending on the COF the controller will either leave it at 2.4GHz, change it to 5 GHz or put it in Monitor Mode. When the controller puts an AP in Monitor mode the only way to fix this is to reset the AP.

 

Probe Suppression

The AP can suppress Probe responses from one of the radios. When the APs receives Probe requests on both the Macro and Micro cells within a short period of time from a client who is not associated, the AP can suppress the Probe Responses on the radio which it doesn’t want the device to join. When a client is associated to either radio on the AP, the AP will suppress the Probe Response from the other radio. This should help prevent the client from roaming between radios. The Probe Suppression option is disabled by default on the controller.

 

FRA will monitor the cells and keep devices that are similar on the same radio. This will help improve throughput. FRA will use 802.11v, 802.k and Probe Suppression to keep the same type of clients on the same radio.

 

 

 

Pros and Cons of FRA

Pro

      • FRA will give you more capacity in the 5 GHz band.
      • FRA eliminates of fixes the balance between 5 GHz and 2.4 GHz radios on your wireless network.
      • The controller will limit how many devices can be on the Micro cell.

 

Con

  • If your device authenticates to the Micro Cell and moves away from the Micro cell area. This could force it to roam to the Macro cell, which would increase roaming. These additional roaming events force the device to stay awake more which will affect battery life. Cisco has safeguards against this but just like RRM, it doesn’t always work.
  • If you have 2.4 GHz clients your network, the coverage area after FRA runs could change dramatically. Depending on how often you have FRA run, this can lead to a less stable network. I know Devin Akin (@DevinAkin) would say that 2.4 GHz is dead and probably should be at this point especially for voice clients, but I just did a job last week where they insisted using 2.4 GHz for voice.