This chapter describes quality of service (QoS) in Real-Time Traffic WLAN implementations. It describes WLAN QoS in general and does not discuss security and segmentation in depth, though QoS is a part of these components. It also provides information about Cisco Centralized WLAN architecture features.
QoS refers to the capability of a network to provide differentiated service to selected network traffic over various network technologies. QoS technology provides the following benefits:
Provides building blocks for business multimedia and voice applications that are used in campus, WAN, and service provider networks.
Allows network managers to establish service-level agreements (SLAs) with network users.
Enables network resources to be shared more efficiently and expedites the handling of mission-critical applications.
Manages time-sensitive multimedia and voice application traffic to ensure that this traffic receives higher priority, greater bandwidth, and less delay than best-effort data traffic.
Application visibility and control for WLAN
With QoS, bandwidth can be managed more efficiently across LANs, including WLANs and WANs. QoS provides enhanced and reliable network service by:
Supporting dedicated bandwidth for critical users and applications
Controlling jitter and latency (required by real-time traffic)
Managing and minimizing network congestion
Shaping network traffic to smooth the traffic flow
Setting network traffic priorities
QoS importance to
Real-Time Traffic over WLAN
The WLAN used for
both packet transmission and reception is unlicensed, unprotected, and
unshielded. Multiple specifications, protocols, and devices take advantage of
unlicensed and no-cost media (radio frequencies) by a WLAN. Consider the
A tablet user in
a business office is using Bluetooth to print a document. Another laptop user
in the same office is using 2.4 GHz frequency Wi-Fi for a video conference and
presentation. A new guest user, in the lobby, is using a smartphone to check
email through the guest VLAN on the Wi-Fi network. The Wi-Fi network must
prioritize the 2.4 GHz radio frequency shared by the three devices, to give
real-time video conference application priority over the guest smartphone user
and tablet user. In addition, the Wi-Fi network must also address the tablet
Bluetooth transmission interference.
WLAN queue and
802.11 WLAN has
its own queue and schedule mechanism, which is divided into four access
categories (ACs). These four Wi-Fi AC queues provide differentiated access to
the Wi-Fi channel. The four Wi-Fi QoS categories also align to the 802.1P
access categories. Because Wi-Fi is designed to carry multiple Layer 3
protocols, specific implementations typically map the Differentiated Services
Code Point (DSCP) values in the IP header of the packet to be sent by the Wi-Fi
radio into one of the ACs.
The voice packets
are placed in queue with the highest priority for WLAN depending on the DSCP
value or IP QoS value of voice packets in the phone call, known as voice access
category. In Cisco wireless controllers, voice packets also map to the Platinum
QoS profile. Voice and video packets from a voice or video call have quicker
and more frequent access to the Wi-Fi channel than data packets. There will be
packet collisions between the phone call and data application, because Wi-Fi is
a shared medium. Wi-Fi QoS prioritizes the backoff and packet retry logic for
both real-time voice and video traffic and data traffic, based on configuration
values in the WLAN enhanced distributed coordination function (EDCF).
The Bluetooth (BT)
radio, as well as the Wi-Fi radios in the laptop, smartphone, tablet, and
access point are all half-duplex. Therefore, when each of these four radios
transmit a packet, they all change to a receive-packet state, waiting for
packet that acknowledges that the packet they sent was received correctly. This
is where the 802.11e specification of 2005 for Wi-Fi plays an important role
with QoS channel prioritization. 802.11 WLAN protocols have at their basic
media access logic a process called carrier sense multiple access with
collision avoidance (CSMA/CA). The Wi-Fi units wait in receive packet mode for
the absence of a carrier (radio frequency) before they transmit a packet.
Therefore, all Wi-Fi devices in the vicinity of a BT radio wait for the BT
radio to quit transmitting before they can transmit. The dominant BT data rate
used in smartphones to earbuds is 2 Mbps. Therefore, a G.711 voice packet of
256 bytes sent from a BT radio is going to delay the Wi-Fi devices on the same
2.4 GHz frequency for over 1100 microseconds, while the same G.711 voice packet
on 802.11n Wi-Fi takes about 50 microseconds to send.
CleanAir technology defines and locates the general area where BT and other
interferers exist and helps avoid them. But, BT uses the entire 2.4 GHz
frequency allocation that Wi-Fi uses. Therefore, it is a protocol that cannot
be avoided by all the Wi-Fi channels thus making the QoS mechanism the best
solution for mitigating BT interference.
The Wi-Fi QoS
protocol is known as Wi-Fi Multimedia (WMM). WMM is a subset of the 802.11e
specification. The 802.11e specification was approved in 2005; however, it was
extensively used by Wi-Fi Alliance and Microsoft even before 2005. With the
802.11e specification, devices required new drivers to become Wi-Fi QoS capable
with no hardware changes. The legacy devices without QoS have specialized
hardware designs with limited firmware memory.
With the approved
Wi-Fi QoS protocol in 2005, the need for Wi-Fi channel bandwidth has grown
immensely along with continuous improvement in quality, range, and speed. Thus,
it is now possible for a site that was dependent on the legacy data rates of 1,
2, 5.5 and 11 Mbps to disable those rates completely with valid reasons.
channel bandwidth is a managed media resource. The Wi-Fi channel is the first
hop upstream and the last hop downstream. Because this medium is open to radio
interference and non-Wi-Fi protocols, it is the media hop that influences the
performance of the applications running on the Wi-Fi devices. When considering
voice and video applications like Cisco Jabber, the Wi-Fi channel is the medium
most likely to have a negative impact on the mean opinion score (MOS) value of
a call. Therefore, bandwidth must be managed to insure that applications can
perform to meet the expectations of the users.
data rates can double bandwidth in a 2.4 GHz Wi-Fi channel. The sites that
require 802.11b and have performance issues must consider whether to disable
the data rates of 1, 2 and 5.5. A required data rate of 11 Mbps provides all
the support required for legacy and special application devices. Cisco APs with
beam forming technology use the orthogonal frequency-division multiplexing
(OFDM) modulation to further enhance client performance and Wi-Fi channel
bandwidth. The OFDM radio modulation was introduced to Wi-Fi in 1999 in the 5
GHz bands with the 802.11a specification. OFDM came into practice for 2.4 GHz
with the 802.11g specification that was approved in 2003. With over 10 years of
use and technology development, 2.4 GHz OFDM modulation eliminates the high
cost of maintaining backwards compatibility for 802.11b technology.
four WMM QoS priority options, also known as QoS profiles, are used for
WLAN/SSID configuration with Cisco Wireless LAN Controllers:
These WMM options set the priority limit of traffic between the AP,
the wireless LAN controller upstream, and the priority limit from the wireless
controller to the Wi-Fi endpoint downstream.
For example, a
voice packet with a DSCP voice priority value in a WLAN/SSID configuration of
Silver with best effort has an IP Control and Provisioning of Wireless Access
Points (CAPWAP) wrapper header DSCP value of best effort. A data packet with a
DSCP value of zero in a WLAN/SSID with a Platinum voice configuration maintains
a best effort priority, while a voice packet in the same WLAN/SSID maintains
its voice priority.
The Cisco WLAN
Controller (WLC) provides several methods to upgrade video or voice packets
that originate with a DSCP value less than a video or voice value to be
upgraded to the highest priority that is configured for the WLAN/SSID, for
example, a desktop client like Cisco Jabber on a Windows computer. While the
call control server can be configured to have the software application mark
audio and video packets with appropriate DSCP values, the Windows operating
system may be in a default configuration that does not allow QoS marking.
Therefore, the audio and video is sent to the AP with best effort Wi-Fi media
access. But upstream from the AP, those packets can be inspected using deep
packet snooping, resulting in subsequent packet marking upgrades.
The Cisco Jabber
application with Cisco Enterprise Medianet has the advantage of the peer
relationship with Medianet switches. A Medianet-aware client application has
built-in intelligence to address the unique challenges of video and rich media
performance and end-user quality of experience (QoE) over the network
installation and management of video endpoints
troubleshooting for voice, data, and video applications
assess effect of video, voice, and data in the network
The first hop to
the network from a Wi-Fi client is the shared Wi-Fi channel. The client device
access to the Wi-Fi channel is the most influential aspect in the overall user
acceptance of performance.
configuration holds no control on the marking of the packets from the endpoint
Wi-Fi client to the AP or vice-versa. The application DSCP markings, operating
system, and the WMM driver control the marking values and the AC queues.
Therefore, is it most important that you manage these three aspects of the
source client. The delay incurred on the first hop because of the lack of QoS
on the endpoint Wi-Fi client cannot be made up by deep packet inspection and
remarking logic in the AP or upstream from the AP.
Wireless products support WMM, a QoS system based on IEEE 802.11e published by
the Wi-Fi Alliance, WMM Power Save, and Admission Control.
figure shows an example of deployment of wireless QoS based on the Cisco
Unified Wireless technology features.
Figure 1. QoS
QoS is the
measure of performance for a transmission system that reflects its transmission
quality and service availability. Service availability is a crucial element of
QoS. Before you implement QoS successfully, the network infrastructure must be
highly available. The network transmission quality is determined by latency,
jitter, and loss, as described in the following table.
Table 1 QoS
(or delay) is the amount of time it takes for a packet to reach the receiving
endpoint after being transmitted from the transmitting endpoint. This time
period is called the end-to-end delay and can be divided into two areas:
Fixed network delay:
includes encoding and decoding time (for voice and video), and the finite
amount of time that is required for the electrical or optical pulses to
traverse the media en route to their destination.
delay: refers to network conditions, such as queuing and congestion, that
can affect the overall time that is required for transit.
(or delay-variance) is the difference in the end-to-end latency between
packets. For example, if one packet requires 100 ms to traverse the network
from the source endpoint to the destination endpoint, and the next packet
requires 125 ms to make the same trip, the jitter is calculated as 25 ms.
packet loss) is a comparative measure of packets that are successfully
transmitted and received to the total number of packets that were transmitted.
Loss is expressed as the percentage of packets that were dropped.
figure defines radio upstream and downstream and includes the following:
Radio downstream QoS:
Traffic leaving the AP and traveling to the WLAN clients. Radio downstream QoS
is the most common deployment. The radio client upstream QoS depends on the
Radio upstream QoS:
Traffic leaving the WLAN clients and traveling to the AP. WMM provides upstream
QoS for WLAN clients that support WMM.
Traffic leaving the WLC and traveling to the AP. QoS can be applied at this
point to prioritize and rate-limit traffic to the AP. Configuration of Ethernet
downstream QoS is not covered in this chapter.
Traffic leaving the AP and traveling to the WLC. The AP classifies traffic from
the AP to the upstream network according to the traffic classification rules of
Figure 2. Upstream and
QoS/WMM and Wi-Fi
of QoS features cannot be detected on a lightly loaded network. QoS features
begin to apply on the application performance as the load on the network
increases. If you can measure latency, jitter, and loss when the medium is
lightly loaded, it indicates either a system fault, poor network design, or
that the latency, jitter, and loss requirements of the application are not a
good match for the network.
QoS functions to
keep latency, jitter, and loss for selected traffic types within acceptable
boundaries. When you provide only radio downstream QoS from the AP, radio
upstream client traffic is treated as best-effort. A client must compete with
other clients for upstream transmission, as well as compete with best-effort
transmission from the AP. At certain load conditions, a client experiences
upstream congestion, and the performance of QoS-sensitive applications becomes
unacceptable, despite the QoS features on the AP. The upstream and downstream
QoS can be operated either by using WMM on both the AP and WLAN client, or by
using WMM and a client proprietary implementation.
client traffic from a WLAN client with WMM support are not direct. The
applications that look for benefits of WMM assign a priority classification to
their traffic, and the operating system passes this classification to the WLAN
interface. In purpose-built devices, such as wireless voice handsets, the
implementation is integrated as part of the design. However, if you implement
it on a general-purpose platform, such as a personal computer (PC), you must
first implement application traffic classification and OS support to achieve
(WMM), formerly known as Wireless Multimedia Extensions, refers to QoS over
Wi-Fi. QoS enables Wi-Fi access points to prioritize traffic and optimize the
way shared network resources are allocated among different applications.
describes the following three considerations for WMM implementation:
WMM is a Wi-Fi
Alliance certification of support for a set of features from an 802.11e draft.
This certification is for both clients and APs, and certifies the operation of
WMM. WMM is primarily the implementation of enhanced distributed coordination
function (EDCF) component of 802.11e. Additional Wi-Fi certifications are
planned to address other components of the 802.11e.
WMM uses the
802.1P classification scheme developed by the IEEE (which is now a part of the
classification scheme has eight priorities, which WMM maps to four access
categories: AC_BK, AC_BE, AC_VI, and AC_VO. These access categories map to the
four queues that are required by a WMM device, as shown in the following table.
Table 2 802.1P and
figure shows the WMM data frame format.
Figure 3. WMM Frame
Even though WMM
maps the eight 802.1P classifications to four access categories, the 802.1D
classification is sent in the frame.
The WMM and
IEEE 802.11e classifications are different from the classifications that are
recommended and used in the Cisco network, which are based on IETF
recommendations. The primary difference in classification is the change of
voice and video traffic to 5 and 4, respectively. This allows the 6
classification to be used for Layer 3 network control. To be compliant with
both standards, the Cisco Unified Wireless solution performs a conversion
between the various classification standards when the traffic crosses the
figure shows the queuing that is performed on a WMM client or AP.
Figure 4. WMM
There are four
separate queues, one for each of the access categories. Each of these queues
compete for the wireless channel, with each of the queues using different
interframe space, contention window (CW) minumum (CWmin) and contention window
maximum (CWmax) values as defined by EDCF. If more than one frame from
different access categories collide internally, the frame with the higher
priority is sent, and the lower priority frame adjusts its backoff parameters
as though it had collided with a frame external to the queuing mechanism.
figure shows the principle behind EDCF where different interframe spacing and
CWmin and CwMax values (for clarity, CwMax is not shown) are applied per
Figure 5. Access
Category (AC) Timing
types can wait different interface spaces before counting down their random
backoff, and the CW value used to generate the random backoff number also
depends on the traffic classification. For example, the CWmin for voice
traffic is 23-1,
and CWmin for best effort traffic is 25-1. High priority
traffic has a small interframe space and a small CWmin value, giving a short
random backoff, whereas best effort traffic has a longer interframe space and
large CWmin value, that, on average, gives a large random backoff number.
figure shows the WMM information in a probe response.
Figure 6. Probe
Response WMM Element Information
The elements on
the client not only contain WMM AC information, but also define which WMM
categories require admission control. For example, in the preceding figure, the
admission control for voice AC is set to
Mandatory. Therefore, the client is required to send
the request to the AP, and have the request accepted, before it can use this
automatic power-save delivery
automatic power-save delivery (U-APSD), a WMM feature of Wi-fi devices,
provides two key benefits:
voice client to synchronize the transmission and reception of voice frames with
the AP, allowing the client to transition into power-save mode between the
transmission or reception of each voice frame tuple.
client frame transmission in the access categories supporting U-APSD triggers
the AP to send any data frames that are queued for that WLAN client in that AC.
A U-APSD client remains listening to the AP until it receives a frame from the
AP with an end-of-service period (EOSP) bit set. Once the client receives a
frame with the EOSP bit set which indicates there are no other frames, the
client goes back into power-save mode. This triggering mechanism is a more
efficient use of client power than the regular listening for beacons method, at
a period controlled by the delivery traffic indication map (DTIM) interval.
This is because the latency and jitter requirements of voice and video are such
that a voice and video over IP (VVoIP) client would either not be in power-save
mode during a call, resulting in reduced talk times, or would use a short DTIM
interval, resulting in reduced standby times.
the use of long DTIM intervals to maximize standby time without sacrificing
call quality. You can apply this feature individually across access categories;
however, only voice ACs in the AP use U-APSD and other ACs still use the
standard power-save feature.
of transmission buffered data frames from the AP with the triggering data frame
from the WLAN client allows the frames from the AP to be sent without the
accompanying interframe spacing and random backoff, thereby reducing the
figure shows an example of traffic flow with U-APSD.
Figure 7. U-APSD
In this example,
the trigger for retrieving traffic is the client sending traffic to the AP.
When the AP acknowledges the frame, it indicates the client that data is in
queue and must wait. The AP then sends data to the client typically as a
transmit opportunity (TXOP) burst where only the first frame has the EDCF
access delay. All subsequent frames are then sent directly after the
acknowledgment frame. In a Real-Time Traffic over WLAN implementation, only one
frame is queued at the AP, and the real-time-capable WLAN client becomes idle
after receiving that frame from the AP.
approach overcomes both the disadvantages of the previous scheme, thus making
it efficient. The timing of the polling is controlled through the client
traffic, which in the case of voice and video is symmetric. If the client is
sending a frame every 20 ms, it waits to receive a frame at each 20 ms time
interval. This introduces a maximum jitter of 20 ms, rather than n * 100 ms
figure shows an example frame exchange for the standard 802.11 power-save
Figure 8. Standard
The client in
power-save mode first detects that there is data waiting for it at the AP from
the traffic indicator map (TIM) in the AP beacon. The client must power-save
poll (PS-Poll) the AP to retrieve that data. If the data that is sent to the
client requires more than one frame to be sent, the AP indicates this in the
sent data frame. This process requires the client to continue sending
power-save polls to the AP until all the buffered data is retrieved by the
client power-save has two disadvantages.
inefficient for the PS polls and the normal data exchange to go through the
standard access delays associated with distributed coordination function (DCF).
the buffered data is dependent on the DTIM, which is an integer multiple of the
beacon interval. Standard beacon intervals are 100 ms. This introduces a level
of jitter that is unacceptable for voice and video calls, and voice and video
capable wireless endpoints handsets switch from power-save mode to full
transmit and receive operation when a call is in progress.
standard client power-save mode gives acceptable voice and video quality but
reduces battery life. The Cisco Unified Wireless IP Phones address this issue
by providing a PS-Poll feature that allows the phone to generate PS-Poll
requests without waiting for a beacon TIM. This allows the device to poll for
frames when it has sent a frame, and then go back to power-save mode. This
feature does not provide the same efficiency as U-APSD, but improves battery
life for Cisco Unified Wireless IP Phones on WLANs without U-APSD.
Specification Admission Control
Specification (TSPEC) allows an 802.11e client to signal its traffic
requirements to the AP. In the 802.11e media access control (MAC) definition,
the following two mechanisms provide prioritized access, both provided by the
transmit opportunity (TXOP):
Contention-based EDCF option
With the TSPEC
features, a client can specify its traffic characteristics, which automatically
results in the use of controlled access mechanism. The controlled access
mechanism enables the client to grant a specific TXOP to match the TSPEC
request. However, the reverse mechanism is also possible; that is, a TSPEC
request can be used to control the use of various ACs in EDCF. In a TSPEC
mechanism, a client must send the TSPEC request before it sends any
For example, a
WLAN client device that requires to use the voice AC must first make a request
for use of that AC. You can configure the use of voice and video ACs by TSPEC
requests but the use of best effort and background ACs can happen without TSPEC
The use of EDCF
ACs, rather than the 802.11e hybrid coordinated channel access (HCCA), to meet
TSPEC requests is possible because the traffic parameters are simple to allow
them to be met by allocating capacity, rather than creating a specific TXOP to
meet the application requirements.
The Add traffic
stream (ADDTS) function is how a WLAN client performs an admission request to
an AP. Signaling its TSPEC request to the AP, the admission request can be in
action frame: Used when a voice or video call is originated or terminated
by a client associated to the AP. The ADDTS contains TSPEC and might contain a
traffic stream rate set (TSRS) information element (IE) (Cisco Compatible
Extensions Version 4 clients).
message: Uses the re-association message when the re-association message
contains one or more TSPEC and one TSRS IE if an STA roams to another AP.
element in ADDTS describes the traffic request. Apart from data rates and frame
sizes, the TSPEC element also tells the AP the minimum physical rate that the
client device will use. This helps to determine the time that the station
consumes to send and receive in this TSPEC, therefore allowing the AP to
calculate whether it has the resources to meet the TSPEC. The WLAN client (VoIP
handsets) uses TSPEC admission control during a call initiation and roaming
request. While the WLAN client is roaming, the TSPEC request is appended to the
Clients page of the Cisco WLAN Controller (WLC) which
indicates what Wi-Fi protocol the client used to associate to the WLAN. In the
figure below, the client is connected to the 2.4 GHz channel because the
protocol is 802.11bn. The client can also be 802.11b, 802.11g or 802.11n. The
client is connected to the 5 GHz channel if the protocol is 802.11an. Clicking
on the MAC Address link shows the characteristics of the selected client.
Figure 9. WLAN
Detail page, which displays detailed client
information available on the WLC. This page displays three important fields and
the values about the client connection status:
Indicates the data rate, here m15.
value of -39 dBm indicates a strong signal.
tra applications, the desired receive signal strength indicator (RSSI) is a
strength of -67 dBm at the cell edge.
QoS Level: Set to
Platinum, indicates the client can send and receive at the highest WMM
Figure 10. WLAN
Controller Clients Detail Page 1 of 2
Figure 11. WLAN
Controller Clients Detail Page 2 of 2
The values in the
rate limiting column indicate this client is not part of a rate limiting
figure shows the load statistics of the AP that is associated to the client.
Figure 12. WLAN
Controller Channel Utilization
The AP radio is
802.11b/g/n and the channel utilization for the client and the AP is 46
percent. The client and the AP utilization are both 0 percent because they are
not sending a significant number of packets to each other. However, the channel
that the client and AP are using is very busy with traffic from other APs,
other clients, and interference.
The 46 percent
channel utilization is above the channel utilization wireless packetized ALOHA
standard. The ALOHA protocol defines a radio channel as full when channel
utilization reaches 33 percent. This means that the channel is busy, so the
packets must wait for an open time slot before they are transmitted. This level
of channel utilization is not uncommon with Wi-Fi 2.4 GHz channels. In this
scenario, QoS helps to manage channel bandwidth. This is also the reason for
Wi-Fi call admission control (CAC). CAC is a part of the 802.11e specification.
figure shows WLC CAC configuration page.
Figure 13. WLC Call
Admission Control Settings
mandatory (ACM) Load Based CAC for wireless phones and other devices is
effective to maintain good quality calls and preserve bandwidth. Load-based CAC
measures the load of the Wi-Fi which is best in high density deployments where
there is a high level of channel reuse across several APs. Cisco also supports
SIP CAC. For SIP CAC, the WLAN must have media session snooping enabled. If the
CAC method is load-based, then SIP CAC also uses channel load. Most softphones
and smartphones use SIP as the call connection protocol, so SIP CAC is
important. When you enable SIP CAC and deploy TCP-based SIP clients, in
scenarios where there is not enough bandwidth for a new voice or video call to
go through, the WLAN network stops forwarding SIP frames upstream and
downstream. Based on client code behavior, this may cause loss of call control
registration. In the case of SIP CAC with UDP-based SIP clients, the WLAN
network will send a 486 Network Busy message. Based on the client code
behavior, client may roam to another AP or terminate call setup. In addition to
CAC configuration for voice traffic, there are tabs for video and media
traffic. These provide configuration options to extend CAC to video and media.
With the help of these tabs, you can configure how the bandwidth of a Wi-Fi
channel is divided between real-time voice and video applications and media
applications, which in turn determines how much bandwidth remains for data
Cisco WLAN endpoint and mobile client deployments, Cisco recommends not to
enable SIP CAC support because they utilize TCP-based SIP versus UDP-based SIP.
Centralized WLAN architecture has multiple QoS features, in addition to WMM
support. Cisco WLAN Controller QoS profiles are the primary mechanism for
implementing advanced QoS feature. The following four QoS profiles and
corresponding traffic types are supported:
voice application traffic
figure shows the four available QoS profiles on the Cisco WLAN Controller.
Figure 14. WLAN
Controller QoS Profiles
For each profile,
you can configure the bandwidth contracts, RF usage control, and the maximum
IEEE 802.1P classification that is allowed.
Figure 15. Editing
WLAN Controller QoS Profiles
that you use the default values for
Bandwidth Contracts settings and use IEEE 802.11 WMM features to provide
For WLANs that
use a given profile, the IEEE 802.1P classification in that profile controls
two important behaviors:
what class of service (CoS) value is used for packets that are initiated from
The CoS value
that is set in the profile is used to mark the CoS of all CAPWAP packets for
WLAN using that profile. So, for a WLAN with platinum QoS profile, and the IEEE
802.1P mark of 6, will have its CAPWAP packets from the AP Manager interface of
the controller marked with CoS of 5. The controller adjusts the CoS to be
compliant with Cisco QoS baseline recommendations. If the network is set to
trust CoS rather than a DSCP at the network connection to the WLC, the CoS
value determines the DSCP of the CAPWAP packets that are received by the AP,
and eventually the WMM classification and queuing for WLAN traffic, because the
WLAN WMM classification of a frame is derived from the DSCP value of the CAPWAP
packet carrying that frame.
the maximum CoS value that can be used by clients that are connected to that
802.1P classification sets the maximum CoS value that is admitted on a WLAN
with that profile.
WMM voice traffic
arrives with a CoS of 6 at the AP, and the AP automatically performs a
CoS-to-DSCP mapping for this traffic based on a CoS of 6. If the CoS value in
the WLC configuration is set to a value less than 6, the WLAN QoS profile at
the AP uses this changed value to set the maximum CoS marking used and which
WMM AC to use.
The key point in
Unified Wireless Network is that you must always consider IEEE 802.11e
classifications, and allow the Unified Wireless Network Solution to take
responsibility to convert between IEEE classification and the Cisco QoS
information about Per-User Bandwidth Contracts, Per-SSID Bandwidth Contracts,
and WLAN QoS Parameters, see the WLC configuration guides that match the WLC
code release and model.
You can configure
WLAN with various default QoS profiles as shown in the following figure.
Each of the
profiles (platinum, gold, silver, or bronze) is annotated with its typical use.
In addition, a client can be assigned a QoS profile based on its identity,
through authentication, authorization, and accounting (AAA). For a typical
enterprise, the WLAN deployment parameters, such as per-user bandwidth
contracts and over-the-air QoS, should use the default values, and standard QoS
tools, such as WMM and wired QoS, must be used to provide optimum QoS to
In addition to
the QoS profiles, you can also control the WMM policy per WLAN as shown in the
preceding figure with the following options:
Disabled: WLAN does not advertise WMM capabilities or allow
WLAN allows WMM and non-WMM clients
Required: Only WMM-enabled clients are associated with this
figure shows the QoS basic service set (QBSS) information element (IE) that a
Cisco AP recommends. The
indicates the portion of available bandwidth that is currently used to
transport data on that AP.
Figure 17. QBSS
The QBSS in use
depends on the WMM and clients settings on the WLAN. Based on the requirements,
the following three types of QBSS IEs must be support:
(Draft 6 [pre-standard])
(Draft 13 IEEE 802.11e [standard])
distributed CAC load IE (a Cisco IE)
Figure 3 shows 7920 AP and Client CAC,
components of WLAN Controller (WLC) WLAN configuration that enables the AP to
include appropriate QBSS elements in its beacons. WLAN clients with QoS
requirements, such as the Cisco Unified Wireless IP Phones, use these
recommended QoS parameters to determine the best AP with which to associate.
The WLC provides
7920 CAC support through the client call admission control (CAC) limit, or AP
CAC limit. These features are listed below:
limit: The 7920 Client CAC maps to the old QBSS method, which is not clear
channel assessment (CCA) based, but only accounts for 802.11 traffic on that
specific AP. The client can set a fixed CAC limit, to prevent outbound calls
when that limit is reached.
limit: The 7920 AP CAC maps to the new QBSS method, which is CCA-based, and
accounts for all energy on the RF channel including 802.11 traffic for the
local AP as well as for other APs, and also energy from non-802.11 devices (for
example, microwaves and Bluetooth). The client can set a fixed CAC limit, to
prevent outbound calls when that limit is reached.
combinations of WMM, client CAC limit, and AP CAC limit result in different
QBSS IEs being sent:
If only WMM
is enabled, IE number 2 (IEEE 802.11e standard) QBSS Load IE is sent out in the
beacons and probe responses.
Client CAC limit must be supported, IE number 1 (the pre-standard QBSS IE) is
sent out in the beacons and probe responses on the bg radios.
If 7920 AP
CAC limit must be supported, the number 3 QBSS IE is sent in the beacons and
probe responses for bg radios.
QBSS IEs use the same ID, and therefore the three QBSSs are mutually exclusive.
For example, the beacons and probe responses can contain only one QBSS IE.
admission control parameters
figure shows a sample configuration screen for setting the voice parameters on
Figure 18. Voice
Cisco WLAN endpoint and mobile client deployments, Cisco recommends not to
enable SIP CAC support because they utilize TCP-based SIP versus UDP-based SIP.
control parameters consist of the maximum RF Bandwidth that a radio can use and
still accept the initiation of a voice or video over WLAN call through a normal
ADDTS request. The reserved roaming bandwidth is the capacity set aside to
respond to the ADDTS requests during association or re-association, which are
RToWLAN clients with calls in progress trying to roam to that AP.
Control (ACM) check box to enable admission control based on these
parameters. This enables admission control based upon the AP capacity, but does
not consider the possible channel loading impact of other APs in the area. To
include this channel load in capacity calculations, select Load Based from the
drop down and check the
Control (ACM) check box.
figure shows an example of voice statistics reports that are available on the
WCS, which displays the calls that are established on the radio of one AP, and
the number of calls that roamed to that AP. This report and other voice
statistics can be scheduled or ad hoc, and either graphically displayed or
posted as a data file.
Figure 19. Voice
Statistics from WCS
control is performed only for voice and video QoS profiles.
Impact of TSPEC
The purpose of
TSPEC admission control is to protect the high-priority resources. Therefore, a
client that has not used TSPEC admission control does not have its traffic
blocked but its traffic is reclassified if it tries to send, which it must not
do if the client is transmitting WMM-compliant traffic in a protected AC.
tables describe the impact on classification if admission control is enabled,
depending on whether a traffic stream has been established.
Table 3 Upstream
in behavior; the packets go into the network. UP is limited to max=WLAN QoS
in behavior; the packets go into the network. UP is limited to max=WLAN QoS
in behavior; the packets go into the network. UP is limited to max=WLAN QoS
are re-marked to BE (both CoS and DSCP) before they enter the network for WMM
clients. For non-WMM clients, packets are sent with WLAN QoS.
Table 4 Downstream
are re-marked to BE (both CoS and DSCP) before they enter the network for WMM
clients. For non-WMM clients, packets are sent with WLAN QoS.
IEEE 802.11e, IEEE
802.1P, and DSCP mapping
In a Unified
Wireless network, WLAN data is tunneled through CAPWAP (IP UDP packets). To
maintain QoS classification applied to WLAN frames, a process of mapping
classifications to and from DSCP to CoS is required.
For example, when
a WLAN client sends WMM classified traffic, it has an IEEE 802.1P
classification in its frame. The AP must translate this classification into a
DSCP value for the CAPWAP packet carrying the frame, to ensure the packet is
treated with the appropriate priority as it reaches WLC. A similar process must
occur on the WLAN Controller (WLC) for CAPWAP packets going to the AP.
A mechanism to
classify traffic from non-WMM clients is also required, to ensure the AP and
WLC give an appropriate DSCP classification to the CAPWAP packets for non-WMM
figure shows a numbered example of the traffic classification flow for a WMM
client, an AP, and a WLC.
Figure 20. WMM and
IEEE 802.1P Relationship
classification flow is described as follows:
A frame with
an 802.1P marking and a packet with an IP DSCP marking arrive at the WLC wired
interface. The IP DSCP of the packet determines the DSCP of the CAPWAP packet
leaving the WLC.
The IP DSCP
of the CAPWAP packet reaching the AP translates to an 802.11e CoS marking.
CoS marking of a frame arriving at the AP translates to an CAPWAP DSCP value,
capped at the maximum value for that QoS profile.
The DSCP of
the packet leaving the WLC will be equal to the DSCP of the packet that left
the WLAN client. The 802.1P value of the frame depends on:
protocol that is configured for that QoS profile (see
If the wired QoS protocol is configured as
no 802.1p value is set. But if the protocol is set to 802.1p, then the 802.1p
used depends on the translation table capped at a maximum value of 802.1p table
classification mechanisms and client capabilities require multiple strategies:
control frames require prioritization, and CAPWAP control frames are marked
with a DSCP classification of CS6.
clients have the classification of their frames mapped to a corresponding DSCP
classification for CAPWAP packets to the WLC. This mapping follows the standard
IEEE CoS-to-DSCP mapping, with the exception of the changes that are necessary
for QoS baseline compliance. This DSCP value is translated at the WLC to a CoS
value on IEEE 802.1Q frames leaving the WLC interfaces.
clients have the DSCP of their CAPWAP tunnel set to match the default QoS
profile for that WLAN. For example, the QoS profile for a WLAN supporting
wireless IP phones would be set to Platinum, resulting in a DSCP classification
of EF for data frames packets from that AP WLAN.
packets from the WLC have a DSCP classification that is determined by the DSCP
of the wired data packets that are sent to the WLC. The AP table converting
DSCP to WMM classification determines the IEEE 80211.e classification used when
sending frames from the AP to a WMM client.
classification that is used for traffic from the AP to the WLAN client is based
on the DSCP value of the CAPWAP packet, and not the DSCP value of the contained
IP packet. Therefore, it is critical that you have an end-to-end QoS system in
The CAPWAP AP and
WLC perform QoS baseline conversion to ensure that WMM values are mapped to the
appropriate QoS baseline DSCP values, rather than the IEEE values.
Table 5 Access Point
QoS Translation Values
Inter-network control (CAPWAP control, 802.11 management)
table shows the translations values if AP is translating CoS values, for
example, autonomous APs.
Table 6 WMM Packet
Re-marking for APs with Priority Type Configured
downstream direction, the AP takes CoS markings on the wired interface and maps
them to the UPs shown. In the upstream direction, the AP takes UPs that are
received on the dot11 interface and maps them to CoS on the wired interface.
Using this remapping results in the best match of WMM AC to CoS.
only network control traffic that must get mapped to CoS=7 is spanning-tree
traffic that is used when work group bridges are deployed or when outdoor
bridges are deployed, connecting the LANs between two or more buildings. Even
though 802.11 MAC management traffic is carried on UP=7 in autonomous APs, is
it not bridged onto the wired port of the AP.
features on CAPWAP-based APs
following when you deploy QoS on wireless APs:
CAPWAP AP interface reads or writes Layer 2 CoS (IEEE 802.1P) information. The
WLC and APs depend on Layer 3 classification (DSCP) information to communicate
WLAN client traffic classification. The intermediate routers can modify this
DSCP value and therefore the Layer 2 classification that is received by the
destination does not reflect the Layer 2 classification that is marked by the
source of the CAPWAP traffic.
The APs no
longer use NULL VLAN ID. As a result, L2 CAPWAP does not effectively support
QoS because the AP does not send the IEEE 802.1P/Q tags, and in L2 CAPWAP there
is no outer DSCP on which to fall back.
APs do not
reclassify frames; they prioritize based on CoS value or WLAN profile.
EDCF-like queuing on the radio egress port only.
first-in first-out (FIFO) queuing only on the Ethernet egress port.
WAN QoS and
For WLANs that
have data traffic forwarded to the WLC, the behavior is same as non-FlexConnect
APs (formerly hybrid remote edge access point or H-REAP) APs. For locally
switched WLANs with WMM traffic, the AP marks the 802.1P value in the 802.1Q
VLAN tag for upstream traffic. This occurs only on tagged VLANs; that is, not
traffic, FlexConnect uses the incoming 802.1Q tag from the Ethernet side and
uses this to queue and mark the WMM values on the radio of the locally switched
The WLAN QoS
profile is applied both for upstream and downstream packets. For downstream, if
you receive an IEEE 802.1P value that is higher than the default WLAN value,
the default WLAN value is used. For upstream, if the client sends a WMM value
that is higher than the default WLAN value, the default WLAN value is used. For
non-WMM traffic, there is no CoS marking on the client frames from the AP.
that you consider when you deploy QoS in a wired network apply when you deploy
QoS in a wireless network. QoS does not create additional bandwidth; it
prioritizes and optimizes the bandwidth that is allocated to different
wireless QoS requires awareness of the types of traffic and protocols
traversing the network, and understanding of the specific delay sensitivity and
bandwidth requirements of applications to properly design and configure WLAN
It is important
to consider and understand the offered traffic when you deploy IEEE 802.11 QoS.
You must consider both bit rate and frame size, because IEEE 802.11 throughput
is sensitive to the frame size of the offered traffic.
table shows how frame size affects throughput; a decrease in the packet size
decreases the throughput.
Table 7 Throughput
compared to frame size
For example, if
an application that offers traffic at a rate of 3 Mbps is deployed on an 11
Mbps IEEE 802.11b network, but uses an average frame size of 300 bytes, no QoS
setting on the AP allows the application to achieve its throughput
requirements. This is because IEEE 802.11b cannot support the required
throughput for that throughput and frame size combination. The same amount of
offered traffic, having a frame size of 1500 bytes, provides better throughput.
discusses wired switch port configuration for the wired to wireless boundary of
the following wireless infrastructure components:
configuration of the AP switch is relatively trivial because the switch must
trust the DSCP of the CAPWAP packets that are passed to it from the AP. There
is no class of service (CoS) marking on the CAPWAP frames that come from the
The use of IOS
trust dscp at the access switch enables trust of DSCP markings of the
AP as set by the WLC policy. The maximum DSCP value that is assigned to client
traffic is based on the QoS policy that is applied to the WLANs on that AP.
configuration command addresses only packet classification. Depending on local
QoS policy, you can add queuing commands and other QoS-related configuration.
classification on a WLC-connected switch is more complex than on the
AP-connected switch because you must decide to trust either the DSCP or the CoS
of traffic coming from the WLC. The following factors help to decide on the QoS
leaving the WLC can either be upstream (to the WLC or network) or downstream
(to the AP and WLAN clients). The downstream traffic is CAPWAP encapsulated,
and the upstream traffic from AP and WLAN clients is either CAPWAP encapsulated
or decapsulated WLAN client traffic, leaving the WLC.
on the WLC control the DSCP values of CAPWAP packets. The WLAN client does not
alter the DSCP values that are set on the WLAN client traffic encapsulated by
the CAPWAP tunnel header.
The WLC QoS
policies set the CoS values of frames leaving the WLC, regardless of whether
they are upstream, downstream, encapsulated, or decapsulated.
The use of the
trust cos IOS command enables the trust of the CoS settings of the
WLC. This allows a central location for the management of WLAN QoS, rather than
managing the WLC configuration and an additional policy at the WLC switch
connection. Customers who want a more precise degree of control can implement
QoS classification policies on the WLAN-client VLANs.
visibility and control (AVC) for wireless
Cisco wireless AVC
Improved quality of
experience for all wireless users through application-level optimization and
monitoring and end-to-end application visibility to accelerate troubleshooting
and minimize network downtime.
capacity management and planning through greater visibility of application
usage and performance.
of business-critical applications and subflows like Cisco Jabber voice or IM
AVC on the
Wireless LAN Controller has the functionality and features that are comparable
to AVC found on other Cisco products. AVC application recognition is
configurable down to the WLAN/SSID. Each WLAN can optionally enable various AVC
parameters making any WLAN unique. WLANs define the name of the SSID with which
the clients authenticate and associate. The same WLAN configuration also
defines the highest level of Wi-Fi QoS for packet transmission over the Wi-Fi
channel. A packet remarked by an AVC profile will not have a QoS priority that
is above the QoS priority that is defined by the WLAN setting. For example, if
WLAN is created for guest users with the QoS priority level of best-effort.
Voice and video packets are transmitted at a best-effort priority, even if the
AVC policy recognizes the packet as an audio packet. The guest SSID can be
configured to limit
calls to the best-effort priority. Also, the same guest SSID can be configured
to use an AVC profile that blocks
thus providing more bandwidth on other SSIDs that share the same Wi-Fi channel.
clients that are associated to a WLAN, AVC on the Wireless LAN Controller uses
application recognition through deep packet inspection to determine how packets
of a particular application should be handled per the Wireless LAN Controller
AVC configurations. The Wireless LAN Controller is the control point for
blocking packets and changing the QoS marking of packets. You can block the
FaceTime application from connecting to the servers that establishes a FaceTime
call. The blocking occurs at the Wireless LAN Controller. When the AVC profile
blocks an application, the client device remains associated to the WLAN. If AVC
profile is created to remark the FaceTime application packet, the remarking
occurs at the Wireless LAN Controller. The remarking is done in the upstream
and downstream direction. In the case of upstream traffic (from Wi-Fi endpoint
to Wireless LAN Controller through the AP), the packet remarking is from the
Wireless LAN Controller to the packets that are being forwarded to the
destination endpoint. AVC cannot control the QoS packet markings at the source
client or the markings of those packets because they are forwarded from the AP
to the Wireless LAN Controller. In the case of downstream traffic (endpoint
packets being forwarded by the Wireless LAN Controller through the AP to the
Wi-Fi endpoint), the packet remarking occurs at the Wireless LAN Controller.
The AP forwards the FaceTime traffic to the WLAN with 802.11e/WMM QoS
priorities that is representative of the DSCP values assigned in the AVC
CAPWAP is the
protocol that connects the APs and the Wireless LAN Controllers. CAPWAP packets
encapsulate IP application packets. CAPWAP QoS packet markings upstream are
based on 802.11e/WMM QoS values of the Wi-Fi header of the endpoint application
packet. CAPWAP QoS packet markings downstream are based on the WLAN
configurations. In the FaceTime example, the DSCP values on the header of the
CAPWAP packets are assigned at the Wireless LAN Controller by the DSCP values
that are configured in the AVC profile for FaceTime. You can configure AVC
profiles for each Wireless Lan Controller and assign to the WLANs.
configuration options for Wireless LAN Controller Version 7.4 and higher are
provided in the Wireless LAN Controller configuration guides by Wireless LAN
Controller release code version numbers. You can download the Wireless LAN
Controller release configuration guides from Cisco.com. A separate WLC/AVC
configuration guide is available by Wireless LAN Controller hardware type on
the same Cisco product page as the Wireless LAN Controller software page.
The Wireless LAN
Controller version of AVC runs as part of Wireless LAN Controller and does not
require a separate license. AVC on the Wireless LAN Controller became available
with Cisco Wireless LAN Controller Release 7.4.
Supported by the
Wireless LAN Controller AVC are FTP/TFTP loads of Network Based Application
Recognition (NBAR) protocol packs that are release matched to the NBAR engine
version incorporated in the Wireless LAN Controller release. For example, the
Wireless LAN Controller Release 7.5 uses NBAR engine version 13. Hence,
protocol packs that are released for Release 7.5 will have a numbering that is
similar to pp-AIR-7.5-13-4.1.1.pack.
determine the version of the protocol pack and AVC engine by executing the
following Wireless LAN Controller CLI commands:
show avc protocol-pack
show avc engine
download the AVC NBAR2 Protocol Packs by the Wireless LAN Controller type from
the same download page location as the software release versions for the
Wireless LAN Controller posted on Cisco.com.
shaping, over-the-air QoS, and WMM clients
and over-the-air QoS are useful tools in the absence of WLAN WMM features, but
they do not help to prioritize IEEE 802.11 traffic directly. For WLANs that
support WMM clients or wireless handsets, you must use the WLAN QoS mechanisms
of these clients without using traffic shaping or over-the-air QoS.