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


      • 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.



  • 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.










My Road to CWNE and study habits


There are two reasons I started this blog. The first being to help pass on the knowledge I have learned and the second is to help ensure I know what I am reading. We all learn by asking questions and by other people asking the questions we are afraid to ask. So please comment on my blog, other’s blogs, and Twitter. The Wireless World needs you. It is like a good friend of mine always says “no one is born knowing”.


I am not the best student. I can be lazy and it can take reading things multiple times before I understand the material.


Typically the best course of studying for me is reading, listening, applying and then explaining it to someone else. In the past, I have skipped the listening portion, but hearing always helps. Hearing it really helps cement what I have read. I do enjoy the hands-on portion of my training…knowledge seems to go through my fingers directly to my brain. Explaining the material I have studied is where I take it from theory to actual useful knowledge, which is the reason I started this blog.



Formal classes

IT classes can be worth taking but they can be hit or miss depending on the instructor. I took MCSE classes at a training center in Cary, NC back in 1998. The first few classes were good because the trainers knew the topics. The more advanced classes were taught by the same trainers but they clearly didn’t know the material as well. One of the instructors made it clear to me that I would have to study a lot more before taking the test. This is true with most Certs and classes but it makes the classroom type class a bit of a waste (unless you have a stellar instructor). These days I try to study the book myself as a first step and see how far that takes me.


In my time at Hill-Rom, I ran a few training courses. My training philosophy was always to teach to the top level of the class. This seems different to most corporate trainers who want to teach to the middle or lower level of the class. Personally, if I am paying for a course (even if work pays for the course) I want to be challenged. I want the course to stretch my knowledge. I also want there to be plenty of handouts and documentation so I can go over it afterward to reinforce what I have learned.


In 2008 I took an Aigmagnet course through CWNP in Atlanta. Rick Murphy (@rickMurphyWiTS) taught this course. It was the best IT course I have ever taken. He was incredibly knowledgeable and he knew how to reach all of us. He stretched my understanding and gave lots of handouts. The other day I went through those materials, I am still learning from them (funny enough the coursework was written by Keith Parsons). I would like to take more Wireless classes especially taught by Keith Parsons, Devin Akin, Blake Krone and Lee Badman. In July 2017 Devin Akin taught a CWNA course for Vocera that I was scheduled to attend but, when I passed my CWNA in April 2017 I decided to give up my spot up for a colleague. I heard the class was amazing and I look forward to one day attending a Devin Akin class.




Rules for Studying

Remove distractions

Have a daily goal, a weekly goal, and a monthly goal.

Use multiple sources

Make sure you are prepared to go into the tests

Build a home lab




Remove distractions

So how does this lazy student who really needs the reading, learning and then doing model start self-studying for my CWNA, CWAP, CWSP and the CWDP? And what kind of advice can I offer to those who are reading this blog? The first step is to remove distractions from your life. I am not talking about your wife, kids and family they are your life and you need them more than anything else in this world. My distraction was the TV. Yours might be watching sports, golfing or going out to the bars. This revelation came to me during Lent 2017 when I gave up TV. I have done this in previous Lents but when Easter rolled around I went right back to the boob tube. This time I realized how much time I wasted and how much studying I can do with the TV off. I have wasted too much time watching useless TV, so now, for the most part, the TV in my hotel room never goes on. When I am at home I will watch some TV with my wife and kids but I am really trying to avoid the TV as much as possible.




Have a daily goal, a weekly goal, and a monthly goal.

I am by nature, not a planner. Benjamin Franklin’s famous quote “If you fail to plan you are planning to fail”, never meant much to me, but as I have planned and mapped out my studying I can see his wisdom. I plan on reading 25 pages a day. This allows me to finish most tech books in 30 days or so. This might be a low number to some of you but I use it as a guideline. Some days I get so busy with work or home life that I realize I missed a day or two. Other days I get so stuck into what I am studying that I read 50 -75 pages a night. The chapters that totally elude my comprehension I go back, reread and take detailed notes. I spend the next month researching the topics online and reading over my notes. When I feel I am ready I will take two weeks and do the practice exams. After the exam, I start the process all over again.




Use multiple sources

When studying for the CWNP exams I always use the book they recommend. Sometimes this is the SYBEX books, sometimes it’s the CWNP books. I have found the SYBEX books have more details and seem to be laid out a bit better but this is a matter of opinion. I always use the CWNP practice tests which are very valuable for honing my knowledge. I will use online articles and blogs of my favorite wireless guys. Websites I am always on are,,, Keith Parson’s, Lee Badman’s Andrew Von Nagy’s and my good friend and colleague from across the pond Andrew McHale’s There are a ton of Wireless Blogs out there and most of them have blogrolls that list other WIFI bloggers. Connecting to the Wireless community on Twitter and Slack Channel has increased my knowledge as well. I find Twitter and Slack are the best places to follow all of your favorite WIFI guys and girls.




Make sure you are prepared going into the tests

This goes without saying but sometimes life gets in the way. I was ready to take my CWAP exam in late June but never got around to scheduling it. In July I had a 10 day job I was doing in NYC and after that I knew life was going to get busier, I scheduled my exam while in NYC. I was prepared for weeks beforehand, but that week I had not done much studying. I felt so unprepared going in. I assumed I would fail it. As it turns out my, Prep was still valid and I passed with over 80% (which was my lowest score of all the tests). Bottom line, make sure you are prepared going in to the exam. Life is a lot less stressful when you know the material.




Build a home lab

You can read until you are blue in the face, but you really need to get hands-on experience. The best way to do this is with a home lab. When you study and have access to your own lab, it will reinforce everything you have read in the book. There is no replacement for experience and although the lab is not the real world it will teach you the tools you need to use out in the real world. Even if you have equipment at work, or even if you are lucky enough to take a lot of classes, nothing can help you learn like a home lab.


I started building mine a few years ago (before I got back into Wireless). My equipment is made up of a variety of Cisco routers, switches, a CUCM server and a few IP phones. I added an HP DL380 G7 which I use as my ESXi server. Recently I have added a vWLC and a few 1140 APs which has expanded my Voice lab into a growing wireless lab.


Don’t let the fear of high prices stop you. You can start fairly cheaply and keep adding to it each quarter. I would strongly recommend viewing Tom Carpenters CWNP video on starting your lab. It is a great video on what equipment you need and where to get that equipment. In a future blog, I will write more about my lab and what equipment you should definitely have in yours.



What Certs do I have now and what Certs do I plan on attaining?

As of this writing, I have my CWNA, CWAP, CWSP and I just passed my CWDP. I am hoping to take the Ekahau (ECSE) course sometime soon but 2018 will be all about Cisco. I am planning on taking my CCNA, CCNA Wireless and then CCNP wireless. I hope to have these by the end of the year. Years ago, I had my CCNA but unfortunately, I let it lapse. It has been hanging over my head for years and I look forward to getting that one knocked off.

Now with all the CWNP tests done, I have to focus some time on the CWNE application. I am not a big writer so getting all of the details of the application will be a challenge, but one I am looking forward to working on.


Please check back on this blog to see what happens and you can always follow me on Twitter @WIFI_NC





My Journey in WIFI

My Journey in WIFI


What is this house painter doing in this Wireless World? Even though I haven’t painted professionally in 10 years or so, painting is what I was trained to do and what I have done more than anything else in my life. I still think of myself as a painter. Painting is a job where you can see your results right away. There is no long term planning, no endless meetings, no emails, or phone calls. You just move the furniture in the center of the room, lay down your drop clothes, prep, paint and at the end of the day you have transformed a room.


In early 1999 I put my paint brush down and started working at Cisco in their OSS division. I enjoyed working there, I loved the people and the products. I enjoyed being around all of these guys and girls who knew technology. There were days when you sat in a training class or meeting and the guy running the class was one of the guys who wrote the book on the subject he was talking about. It was an amazing time. In late 2000, the market was changing and Cisco laid me off. It was a tough time. The job market dried up and I had a house, a wife, a child and another on the way. I needed a job so I went back to the one thing I really knew: house painting. I spent three long hot summers painting and dreaming about getting back into IT. My skills were getting weaker by the month and I wondered if I would ever go back.


In Nov 2004, Hill-Rom was looking for people for their Tech Support Department. They took a chance on this House Painter even though my tech skills were a bit dated. Hill-Rom was a great place to work and learn. I learned about servers, networks, Nurse Call, PBXs and integrating Nurse Call with wireless phone systems. One of the products I supported was Vocera, a wireless communication device. It was the first wireless product I had ever worked on and I was hooked. I worked closely with Chris O’Donnell CWNE #64. Chris taught me a great deal about wireless and Vocera. Every time I thought I was getting a handle on things, Chris would throw me a new topic and encourage me to learn more about it.


In 2009 Vocera decided to take over all sales, service and support of their products I was suddenly a Wireless Engineer without a Wireless product. So I spent the next 6 years at Hill-Rom doing various jobs: including installing and integrating ASCOM Wireless devices, Centrak staff and asset tracking, working trade shows and supporting Hill-Rom’s three show rooms. When I would see Chris O’Donnell (who was working for Vocera at the time) at various tradeshows and events. He would ask me if I was keeping up on Wireless especially 802.11n. I was not. Unfortunately I found I was quickly losing my wireless knowledge.


After 11 years at Hill-Rom I knew I needed to get back to the Wireless world. I began studying for my CWNA. This landed me a job at Vocera in Jan 2016 (which I had been a huge fan of since seeing it integrated with Hill-Rom Nurse Call in 2004). When I started at Vocera I spent the 1st year learning their products and processes, which didn’t leave much time for studying. In Feb 2017, I turned off the TV and started studying for my CWNA again. I passed it in April 2017 and then obtained my CWAP (July 2017), CWSP (Oct 2017), CWDP (Jan 2018). I am hoping to get my CWNE before they give out #350.


So why is this painter doing WIFI? The short answer is I really love the technology. The more I learn about it the deeper I want to dive into it. I am looking forward to taking wireless courses and getting more involved in the community. There is an amazing and supportive Wireless community on Twitter, Slack Channel and the blogs. I hope to give back to this community as much as I can. I want to thank all of those whose ’s Twitter Posts and Blogs have inspired me to get more involved, Keith Parsons (@KiethRParsons), Lee Badman (@wirednot), Andrew Von Nagy (@revolutionwifi), Devin Akin (@DevinAkin), Rick Murphy (@RickMurphyWiTS), Manon Lessard (@Mae149), Blake Krone (@blakekrone), Andrew McHale (@mac_wifi), Sam Clements (@samuel_clements) and of course Chris O’Donnell (@Vofi_CWNE).