Multicast on a Cisco Wireless Network

This is not meant to be a definitive guide to multicast. There are more detailed documents and white papers from Cisco and others on this subject. This is just an attempt to give a basic understanding and encourage you to learn more about this subject.

This is a two-part blog. The 1st blog named “Settings that can improve Multicast on a Wireless Network” which discussed the settings needed to optimize a Wireless/Wired network for multicast traffic. This blog will discuss more about settings and troubleshooting on a Cisco Controller based wireless network.

I work for a wireless communications vendor that uses multicasts as an integral part of the product. I have been onsite and on conference calls with customers trying to troubleshoot multicast issues. The issue almost always turns out that multicast packets are being blocked somewhere on the network. There seems to be a lot of confusion about what happens to multicast packets and how to prevent them from being blocked on your network. This blog will try to explain the paths the multicast packets take on the wireless and wired network.

The topics I will cover in this blog are multicast packets on a controller-based network, configuring multicast on a Cisco Controller, IGMP snooping on the controller, troubleshooting multicast issues, troubleshooting using Multicast Hammer, Cisco commands and some links to blogs and articles I found helpful.

Background

The Cisco network uses IGMP (Internet Group Messaging Protocol) to manage the multicast traffic. When a multicast session is started a client or server will send an IGMP join message to a certain multicast address. Any clients that want to be part of this group will send a join message to the same multicast address. These join messages will get recorded on your local network’s router as well as the Rendezvous point (RP). If your local router sees traffic for this multicast address it will pass the traffic to your local network. If the router sees multicast for a different address it will dump these packets. When IGMP snooping is enabled on the switch, it will record which ports need multicast traffic.

Terms discussed in this blog

Multicast Group ID (MGID) is created by the controller and passed to each AP. The MGID is the ID number which maps multicast source, the multicast address and the VLAN. This helps the controller and APs to keep track of multicast groups.

Delivery Traffic Indication Map (DTIM) is an informational element in a beacon that will inform a station if there is any multicast traffic the AP has for the station.

Rendezvous Point (RP) is a router in a multicast network domain that acts as a shared multicast tree. There can be multiple RP on any given network.

 

Configuring Multicast on Cisco Controller

There are two modes you can use on your controller; Multicast-Unicast and Multicast-Multicast. If you use Multicast-Unicast, then the controllers will make a copy of every multicast packet and sends these packets out as unicast packets to every AP. This would work OK for a smaller network but in a larger network, this would create too much traffic. In larger networks Multicast-Multicast mode is much more efficient use of bandwidth.

When selecting the Multicast-Multicast option on the controller you will need to choose a multicast address. This address must be different on every controller on your network.

Screenshot courtesy of mrn-cciew

When an AP connects to a controller, the controller will tell the AP to join this multicast address you have chosen. All the APs will send a join message to this multicast address. When the controller receives a multicast packet it sends out one packet and all the APs who have joined the multicast group will receive this packet. Multicast can be best described as one to many, where unicast is described as one to one.

IGMP snooping on the Controller

When IGMP snooping is enabled on the controller, it creates a Multicast Group ID (MGID) table, which keeps track of the clients and what multicast groups they have joined. The controller sends this MGID table to all the APs that are connected to it. When multicast packets are sent to the APs, the APs will check the MGID table to see if they have any clients that have joined this multicast session. If they have a client, the AP will send the traffic. If the AP doesn’t have a client, the AP will discard the traffic.

If IGMP snooping isn’t enabled on your controller all APs that have the same VLAN where the multicast traffic had been requested (even by a client on another AP), will receive these multicast packets and will send them out. This means every AP will be sending out multicast packets even if they do not have a client for this traffic. IGMP snooping will limit the multicast packets that are sent by the APs.

 

Multicast Packets on a Controller based network

When the wireless client sends an IGMP join message to the AP, the AP sends this join message through the CAPWAP tunnel to the controller. The controller absorbs this join message and sends a new join message to the local router on the VLAN of the client using its own IP address (not the IP address of the client). The router then adds this multicast group to the interface creating a (*,G) entry.

When multicast packets are sent from the wired side of the network, the router which is acting as Rendezvous Point (RP) will send these packets to the controller. The controller receives these packets and encapsulates them in its multicast address. The controller then sends these multicast packets to all the APs on that controller. The AP strips the controller’s multicast address and sees the original multicast address. The AP checks its MGID table to see if it has a client for this multicast address. If the AP has no clients that have subscribed to the multicast, then the AP drops the packets. If the AP has clients that have subscribed to the multicast address, then the AP sets the AID of that client in the DTIM section of the beacon. The AP then sends the packets down to the client immediately after the beacon. The AP cannot hold multicast packets longer than the DTIM value, the AP sends these packets even if the client is sleeping. This is different from unicast packets where the AP will hold onto them until the client requests the packets.

When multicast packets are sent from a wireless client, the wireless client sends the packets to the AP as unicast packets to the multicast address. The AP then sends these unicast packets through the CAPWAP tunnel to the controller. The controller then makes two copies. One copy is sent out to the locally connected LAN where the router will forward these packets to the RP. The second copy the controller encapsulates in its multicast address and sends these multicast packets to all the APs on that controller. The AP strips the controller multicast address and sees the original multicast address. The AP checks its MGID table to see if it has a client for this multicast address If the AP has no clients that have subscribed to the multicast then the AP drops the packets. If the AP has clients that have subscribed to the Multicast address, then the AP sets the AID of that client in the DTIM section of the beacon. The AP then sends the packets down to the client immediately after the beacon.

Troubleshooting Multicast issues

When troubleshooting multicast issues there are many settings you should look for in the Wireless controller and on your switches and routers. These settings are discussed in greater detail in my blog “Settings that can improve Multicast on Wireless Networks” which can be found here.

Most multicast issues are caused by not having PIM set up correctly on your network. Two ways you can troubleshoot this are; 1) check all the VLANs where the multicast traffic will traverse or 2.) check to see if the join messages are being recorded on your router and switches (if you have snooping enabled).

Protocol-Independent Multicast (PIM) needs to be enabled or set on all your VLANs where multicast traffic will traverse. The VLANs you need to enable it on are the management VLAN, AP management VLAN (if different from the management VLAN), AP VLAN, the VLAN of the sending device and the VLAN of the receiving device. The management VLANs are very important since the controller sends multicast packets to the APs using either the management VLAN, AP management VLAN or the AP VLAN. These VLANs are often overlooked. Checking all the interfaces and VLANs where traffic will traverse always seems to cause Network Admin the most trouble. They usually ask me to list out all the interfaces where PIM needs to be enabled. This is difficult for them but impossible for an outsider with no direct knowledge or access to their network. We always recommend opening a case with Cisco (or your wireless vendor) to help find all the VLANs and interfaces where the multicast traffic may traverse.

If you need to verify that join messages are making it through your network, you will need to SSH to all routers and switches in the path and make sure you see the join messages from the clients. If you get to a router or switch where you no longer see join messages, you probably have found the issue.

Another common issue is multicast packets getting stuck at the core. If the multicast packets are flowing over the core, make sure the VLAN the EtherChannel/Port Channel have PIM enabled on them. I have seen issues where one EtherChannel had PIM enabled the other EtherChannel did not. This caused one multicast session to work and the next multicast session to fail.

After checking the VLANS for PIM and checking to see if the messages are traversing your network you should check other settings including DTIM, TKIP, RP, IGMP snooping, multicast buffering and roaming.

DTIM should be set to 1 especially if you are using multicast for voice. You can check this either on the GUI of the controller, in the controller config file or a wireless capture. The DTIM will be in the beacons of the wireless capture under the Traffic Indication Map. If DTIM count is set to 0 then you are seeing a DTIM beacon if the DTIM period is set to 1 then the next beacon will also be a DTIM beacon (see below).

 

 

We all know TKIP has been deprecated but you may still have it lingering on your network, if you have TKIP and AES on the same SSID this will cause issues with multicast. You can check this either on the GUI of the controller, the controller config file or you can look at the beacons in a wireless capture. The capture below shows AES in the Pairwise Cipher suite list. If TKIP was enabled, you would see it in the same section.

 

Multiple Rendezvous Points (RP) can cause issues with multicast if you have devices on multiple VLANs and each VLAN is directing you to different RPs. This will cause your devices on one VLAN to get the multicast traffic and the other devices on a different VLAN will never get the multicast traffic. If you have two RPs, they must communicate and update each other or better yet have one RP for the entire network.

If you are having issue with delivery of multicast packets you should verify you have IGMP snooping set on your switches. To verify if IGMP snooping is enabled on the switch run the command show ip igmp snooping

If it is not enabled on the switch you should set it globally, then you can set it per VLAN. Enabling it globally, run the command ip igmp snooping enable to enable it on a VLAN 2 you would use the command ip igmp snooping vlan 2 enable

If you have higher DTIM values (which causes the AP to hold on to multicast traffic for longer periods of time) and your network experiences a high volume of multicast traffic, you may experience issues where some multicast packets are being dropped. This may be caused by your multicast buffers being over-run. Each radio can handle 50 Multicast packets at any given time. These are shared equally across all SSIDs. You can verify the Multicast buffers by running the command show controller dot11radio0 | begin –\ In-Prog . You can enable a WLAN to use more than 50 using the command config wlan multicast buffer enable (30-60) <wlan-id> Increasing the Multicast buffer size will allow the AP to hold on to more multicast packets.

Roaming can cause issues with multicast especially if your client doesn’t send a new join message after each roam. This issue is more common with Autonomous APs or even cloud based AP. Cisco controllers will prompt the client to send a join request or they will send a new join message on behalf of the client after a roam. In Autonomous APs or even cloud based APs if a new join message is not sent and you have igmp snooping enabled on your switches the multicast packets might get delivered to the wrong AP.

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Troubleshooting using Multicast Hammer

Multicast Hammer is a free tool that can help you troubleshoot multicast issues. You can download Multicast Hammer installer and setup instructions from the internet. It is a simple program to use and install that was created by Nortel (maybe the only thing left of Nortel in use these days). Multicast Hammer simulates multicast traffic on your network. It is a valuable tool that will remove questions about certain applications and show you raw multicast traffic (or lack of it) on your network. You should run MC Hammer on two machines that are on the same WLAN. Set one up as a server and one as a client. Run the test to see if data is flowing between machines. Once you see data you should move one of the machines to another network and retest. This will help you find out where multicast traffic is being blocked.

I won’t go through the install of Multicast Hammer (or as Vocera has dubbed it MC Hammer….can’t touch this!!!!) but I wanted to show you two screen shots one set up as server and one and client.

Server

The set-up is rather simple. You want to make sure you have the server radio button selected, choose your network interface, and then choose an available Multicast address (not the MC address on your controller). After you hit start on the server and client you will see data flowing. If you only see the data on the server then you know multicast traffic is being blocked somewhere on your network.

Client Side

The set-up is rather simple. You want to make sure you have the Client radio button selected, choose your network interface and then choose the same Multicast address as you chose in the server side of the application. After you hit start on the server and client you will see data flowing. If you only see the data on the server then you know multicast traffic is being blocked somewhere on your network.

 

Cisco Multicast Troubleshooting commands

show ip pim interface

show ip mroute

show ip igmp

Debug bcast * enable

Debug ip igmp

show ip igmp groups

On the controller verify multicast mode

show network

On the controller check L2 and L3 MGIDs

show network multicast mgid summary

show network multicast mgid detail <mgid>

 

Links to Multicast articles, videos and blogs

https://www.youtube.com/watch?v=Gjt2L9jAYNA From Kevin Wallace Cisco Multicast Routing for CCNA, CCNP, and CCIE candidates

https://www.cisco.com/c/en/us/td/docs/wireless/controller/technotes/7-4/vocera_config_guide/vocera_config_guide/vocera_config_guide_chapter_01011.pdf Cisco guide on how to configure multicast for Vocera

https://supportforums.cisco.com/document/56511/multicast-and-wireless-lan-controller-wlc This is from Stephen Rodriquez 7 years ago but still good info in there.

http://www.cisco.com/c/en/us/td/docs/switches/datacenter/nexus1000/sw/4_0/troubleshooting/configuration/guide/n1000v_troubleshooting/trouble_14mcast.html

https://community.cisco.com/t5/wireless-mobility-documents/understanding-multicast-in-unified-wireless-networks/ta-p/3125021 This a power point Understanding Multicast in Unified Wireless Networks by Jeff Keown Cisco Wireless TAC

http://www.labminutes.com/wl0025_wlc_multicast_videostream_1 excellent video on setting up your controller for multicast.

https://mrncciew.com/2012/11/17/configuring-multicast-on-wlc/ everything on mrc-cciew site is awesome and I learn a ton every time I visit

Thank you for reading this blog. I hope reading this blog gives you more insight into multicast on a Cisco network. Please leave comments and continue this discussion on Twitter and Slack. If you haven’t followed me on Twitter please use this link to follow me @wifi_nc

 

 

 

Settings that can improve Multicast on a Wireless Network

This is a two-part blog; this blog will go over settings that will improve multicast on your network. The next blog “Multicast on a Cisco Wireless Network” will go over more troubleshooting on a Cisco Network.

I work for a wireless communications vendor that uses multicast as an integral part of the product. There are certain settings on the wireless/wired network that when implemented will improve multicast.

The issues and settings we will discuss in the blog are; enabling multicast on the router and VLANs, Which VLANs do you enable PIM on, the purpose of DTIM and Beacon settings, basic and mandatory data rates, issues using TKIP and AES on the same SSID, Issues with Rendezvous Points (RP), IGMP snooping on switches, roaming issues with multicast and lastly issues with multicast buffers.

 

Enabling Multicast on the Router and VLANs

To enable multicast routing on your routers and VLANs you will need two commands. The first command you need to issue on your router (or layer 3 switches) is ip multicast-routing. The second command is the pim command (Protocol Independent Multicast), which enables multicast routing on your VLANs. You can set this command as dense-mode, sparse-mode or sparse-dense-mode. Dense mode is good to use if you have a small network, but the PIM sparse-dense-mode will allow the router to use both sparse-mode and dense-mode. The differences between sparse-mode and dense-mode center around Rendezvous Points and how multicast traffic and multicast routes are updated in the network. Most networks I work with use sparse-dense-mode. The command pim sparse-dense mode needs to be issued on all the VLANs where you want multicast traffic to flow.

 

Which VLANs do you enable PIM on?

This is the million-dollar question that can cause a lot of confusion. The VLANs you need to enable multicast on using the pim sparse-dense mode are the management VLAN, AP management VLAN (if different from the management VLAN), AP VLAN (if different from the management VLAN) and all the VLANs of the sending and receiving devices. The management VLANs are very important since the controller sends multicast packets to the APs using either the management VLAN or the AP Management VLAN.

If the multicast packets are flowing over the core, make sure the VLAN of the EtherChannel/Port Channel have PIM enabled on them if they are different from the management VLAN. I have seen issues where one EtherChannel had PIM enabled the other EtherChannel did not. This caused one multicast session to work and the next multicast session to fail.

 

 

 

The purpose of DTIM and Beacon settings

If you are using multicast to deliver voice packets you must set the DTIM to 1 and the beacons to 100ms. These settings tell the AP how often to set either a Traffic Indication Map (TIM) information element or a Delivery Traffic Indication Map (DTIM) information element inside the beacon. There are no TIM or DTIM beacons per se. There are only Information Elements inside the beacon (but for ease of use I will use terms TIM beacons and DTIM beacons). The TIM beacon will tell the client if the AP has unicast packets buffered for that client. The DTIM beacons will tell the clients they have multicast packets about to be delivered (as well as unicast packets buffered). If you set the DTIM to a higher value to either 2 or 3 then the AP will only deliver the multicast packets to the clients every 200ms or 300ms. Most VOIP clients will have between a 90ms and 150ms buffer. If the client gets multicast packets every 200 or 300ms then, the user will hear choppy audio.

 

Some device manufacturers want you to set the DTIM to the higher value, giving the devices more time to sleep. If the devices know the DTIM will only come every 200, 300 msec or more then the client device can sleep that much longer. This saves battery life and is somewhat of a valid concern but when Voice is being delivered over multicast packets the DTIM needs to be set to 1 or the user will hear choppy audio. I said this is somewhat of a valid concern but the fact of the matter it is not mandatory that the client wakes up every DTIM. Before adjusting your DTIM check with your device manufacturer to see if the device wakes up for every DTIM, you might be presently surprised to find the devices don’t wake up every beacon. Some devices will stay asleep longer than the DTIM. You can test this by pinging the device. While the device is idle ping it. You may find the device only responds every 500msec or so (or maybe longer). You can then ping the device while on an active call and see how often it responds and then compare the two values.

 

 

Basic and Mandatory Data rates

Cisco recommends using 2 basic data rates 12Mbps and 24Mbps. When you have two basic data rates set, management traffic will go out at the lower data rate, but Cisco will send multicast traffic at the higher data rate. This can cause issues for clients that have rate shifted down to 12Mps or lower. If the AP is sending multicast traffic out at 24 Mbps and the client is only able to receive at 12 Mbps your client may miss multicast packets. This will be very difficult to troubleshoot since some devices will get the multicast packets and others will not receive them. Trying to replicate the issue would prove difficult. I would always recommend setting only one Basic Data rate to help offset this issue.

 

 

Issues with using TKIP and AES on the same SSID

I have seen issues with multicast where TKIP and AES are enabled on the same SSID. When you have both enabled on the same SSID the AP must send multicast packets out using TKIP. If your clients are using AES they will have issues decrypting the multicast packets. Hopefully, everyone is using AES instead of TKIP (especially since TKIP has been deprecated) but if you need TKIP then it is better to have only one encryption per SSID. Of course, I strongly recommend only using AES.

 

 

Issues with Rendezvous point (RP)

There are two ways you can use Rendezvous points. You can assign a router as a Rendezvous point or you can let the network assign one for you. You can program multiple RPs on your network, but this may cause issues. When a client sends a join message routers in the path will create a (*,G) entry on the interface so the router knows what interface has clients that have subscribed to this multicast address. These join messages will eventually make it to the RP. When the RP gets this join message, it will build a path back to each client/network segment.

If you have multiple RPs you may find an issue where one client may get the multicast traffic and one client will not. This happens if two devices are on different VLANs and each device sends the join message to a different RP. In this case, one device or network segment will get the multicast traffic and one will not. Having multiple RPs might be by design but if you have multiple RPs you should ensure that the RPs share information of all related VLANs.

 

 

IGMP snooping on switches

When IGMP snooping is enabled on a switch, the switch can send the multicast packets out the right interfaces. When a switch sees a normal packet, it will look up the MAC address in its CAM table, if the switch has the MAC address of the client it can forward these packets the right interface. If the MAC address is not in the CAM table, it will flood the packet out all interfaces. The same is true of the multicast packets if the switch has IGMP snooping enabled. The switch will keep track of all the join messages sent by clients/APs. The switch then records what interfaces need the multicast packets. When a multicast packet is sent to the switch it looks in its table and sends the traffic only to those interfaces that need the packets. This cuts down on multicast packets flooding the network. If the switch does not have information on a multicast address, then the switch will send the packets out on all ports.

 

Roaming issues with Multicast

When the clients roam from AP to AP the controller will request the device to send a new join message, so the controller, AP, router, and the network knows the client has moved APs. If your clients fail to send a new join message, the controller will not update its MGID table and the new AP that your client is connected to may drop multicast traffic because the AP will not know there are any clients who have subscribed to these multicast groups.

This can be a real issue in an Autonomous network or even cloud based where the client doesn’t send a join message on a roam and there is no controller to request a new join message. If your client doesn’t send a new join message you will lose Multicast session as you roam. The fix this I would contact the device manufacture to see if there is a firmware update that will fix this issue.

 

 

Issues with Multicast Buffer

The multicast buffer is shared across all BSSIDs on the AP. If there are a high number of SSIDs on your network, you may experience issues where the buffers fill up and the AP starts dumping multicasts packets. This may cause choppy audio on your multicast sessions. If your SSIDs have a higher DTIM value, the APs/Controllers will need to store packets for a longer period.  When you are experiencing issues with multicast traffic you may need to increase your multicast buffer size and then limit which WLANs can use this buffer. Multicast traffic is often crucial to voice clients and other clients/WLANs may never use multicast packets. If you limit which WLANs can use the multicast buffer there will be available space for applications that have a critical need for multicast.

 

It is important to note that the AP can only buffer multicast packets for the length of the DTIM value. When this value has expired the AP will inform the clients and immediately send the multicast packets whether the client is listening or not. This, of course, is different from the way unicast frames are delivered. If the AP has unicast frames for the client, the AP will set the clients AID in the Partial Virtual Bitmap. The AP will buffer these frames until the client wakes up and requests these frames.

 

 

Links to Multicast articles, videos, and blogs

 

https://www.youtube.com/watch?v=Gjt2L9jAYNA From Kevin Wallace   Cisco Multicast Routing for CCNA, CCNP, and CCIE candidates

 

https://www.cisco.com/c/en/us/td/docs/wireless/controller/technotes/7-4/vocera_config_guide/vocera_config_guide/vocera_config_guide_chapter_01011.pdf Cisco guide on how to configure multicast for Vocera

 

https://supportforums.cisco.com/document/56511/multicast-and-wireless-lan-controller-wlc This is from Stephen Rodriquez 7 years ago but still good info in there.

 

 

http://www.cisco.com/c/en/us/td/docs/switches/datacenter/nexus1000/sw/4_0/troubleshooting/configuration/guide/n1000v_troubleshooting/trouble_14mcast.html

 

https://community.cisco.com/t5/wireless-mobility-documents/understanding-multicast-in-unified-wireless-networks/ta-p/3125021  This a power point Understanding Multicast in Unified Wireless Networks by Jeff Keown Cisco Wireless TAC

 

http://www.labminutes.com/wl0025_wlc_multicast_videostream_1   excellent video on setting up your controller for multicast.

 

 

Thank you for reading this blog. I hope reading this blog gives you more insight into multicast and the settings needed on your network. Please leave comments and continue this discussion on Twitter and Slack. If you haven’t followed me on Twitter I am at @wifi_nc. Stay tuned for my next blog in this series called “Multicast on a Cisco Wireless Network”. That blog will go into more details and troubleshooting multicast on Cisco Networks.

 

 

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