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TRANSCRIPT
QoS Design and Deployment for Wireless LANs BRKEWN-3003
Robert Barton, P. Eng
Systems Engineer
CCIE #6660
CCDE #2013::6
© 2014 Cisco and/or its affiliates. All rights reserved. BRKEWN-3003 Cisco Public
Presentation Source Material
End-to-End QoS Network Design, 2nd Edition
First Edition is one of better selling Cisco Press books of all time, but . . .
– It was time for a 2nd edition!
Book is organized around PINs (Places in the Network), e.g. Data Center, WAN, Wireless LANs, VPN, Campus
Content in this session primarily based on the “WLAN QoS Design” section
3
Now Available!
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• Wireless QoS Building Blocks
• IEEE 802.11e and Wireless Multimedia (WMM)
• Wireless QoS Design Considerations
• Cisco WLC 5500 QoS Design Strategies
• Developing a Unified QoS Strategy for the WLAN
• Wrap-up and Final Thoughts
QoS Design and Deployment For Wireless LANs AGENDA
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Comparing Wired and Wireless QoS
Wired environments are Full Duplex, Wireless is Half Duplex
– Half duplex environments are very susceptible to collisions
Thus, wired QoS is mostly concerned with managing packet loss due to congestion problems (queuing), however . . .
6
Wireless QoS is focused on a much bigger problem:
1. WLAN QoS mostly revolves around reducing the probability of a collision for high-priority traffic, based on it’s QoS classification
2. Managing congestion is a secondary concern
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Carrier Sense Multiple Access / Collision Avoidance
7
Wired Hubs use CSMA/CD (collision detection)
– A transmitting station will first listen to the medium to see if it is idle before sending it’s frame. When it seems idle, it sends the frame.
– After sending, it listens to see if a collision occurred (the waveform is reflected back)
802.11 networks use CSMA/CA (collision
avoidance)
– Wireless networks have no way to detect that a collision even occurred!
– CSMA/CA accepts collisions will occur, but has a system for dealing with then once they do occur.
– What do you do on a conference call when more than one person speaks at the same time?
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The Distributed Coordination Function (DCF) Algorithm for Media Access
Using CSMA/CA, DCF Does the Following:
1. Listen first to see if any other transmissions are in progress
2. After the WLAN is idle, wait for a set period before trying to transmit (called DIFS)
3. If the medium is still idle, sends the frame
4. If a collision occurs (because it hears another station transmit at the exact same time, or because you didn’t get an ACK from the station your are sending to, wait another random backoff period and retry
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Who Get the Higher Transmission Priority?
1. Access Point
2. Wireless Client
3. They all get equal access
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Other channel access models have been proposed. These models allow the AP to organizes channel access in a structured way, but these have never been widely adopted
1. Legacy Point Coordination Function (PCF)
2. 802.11e Hybrid Controlled Channel Access (HCCA)
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Backoff Timers and Contention Windows
10
Random Contention Window (CW)
Time (t)
Medium is Busy
Begin Transmission
DIFS ……….
After the medium is idle, all stations must wait the DCF Interframe space (DIFS) – This is a pre-established wait time that all stations must wait (they all wait the same
DIFS period)
Once DIFS has expired, a random countdown timer, called the Contention Window (CWmin) begins
– Once CWmin expires, the station tries to resend
– If the medium is still busy double the CW up to a value of CWmax until you can send
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The DCF Decision Process
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Once the medium is free, countdown the DIFS period
Transmit the Frame
YES
NO
Is the medium available for Transmission?
NO
YES
Wait the random backoff period between 0 and
CWmin
If the medium still busy after another attempt, wait another random backoff
period between 0 and CWmax
Is the medium free?
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DCF In Action: Consider an Example of Five Stations Trying to Unicast all at Once
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Station A
Station B
Station C
Station D
Station E
Frame
Defer
Defer
Defer
Frame
DIFS DIFS
Defer
Frame
DIFS
Frame
Backoff time
Backoff time remaining
Defer
Defer
Defer
Defer
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DCF Needed an Update
QoS is not possible with DCF:
1. No method of classification (all traffic is treated equally)
2. The DIFS and Contention Window values are the same for all traffic
IEEE 802.11e was formed and finally ratified in the 802.11e-2007 standard (which is now part of the 802.11e-2012 “rollup”)
– 802.11e set out to make tweaks to DCF that would allows priority handling of high priority traffic
– The Goal: Improve the probability that high priority traffic will always be transmitted first
By the way, what is the difference between IEEE 802.11e and WMM??
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Enhanced Distributed Channel Access (EDCA)
1. Establishment of four priority queues (known as Access Categories, ACs)
2. Instead of a single DIFS timer, give timers to each class of service (AC)
3. For each class (AC), give a different range of contention window size – preferential CWmin and CWmax for each AC
4. Transmission Opportunity (TXOP)
5. Call Admission Control (CAC) with TSpec
– Other enhancements were also introduced, including: Power Save mode
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Access Categories Act like Transmit Queues
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Background Best Effort Video Voice
Application Data
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Summarizing the Four Access Categories
EDCA / WMM AC Name Description Cisco WLC Profile Name
Voice Highest Priority Platinum
Video Video, Medium Priority Gold
Best Effort Most Other Traffic Silver
Background Low Priority Traffic Bronze
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When 802.11 frames are transmitted, a QoS value known as the User Priority (UP) is written into the L2 frame
– This is used to distinguish traffic types and place them in the correct AC
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Understanding WLAN QoS Markings Surprising Differences Emerge . . .
Traffic Type DSCP 802.11e UP 802.1p CoS WLC Profile
Network Control
56 (CS7) 7 7 Platinum
CAPWAP 48 (CS6) 7 6 Platinum
Voice 46 (EF) 6 5 Platinum
Interactive Video
34 (AF41) 5 4 Gold
Streaming 26 (AF31) 4 3 Gold
X-Data 18 (AF21) 3 2 Silver
Bulk Data 10 (AF11) 2 1 Bronze
Best Effort 0 (BE) 0 0 Silver
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How Backoff Timers Affect QoS
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EDCA / WMM AC AIFS CWmin CWmax
Legacy DCF DIFS > 2 15 1023
Voice 2 3 7
Video 2 7 15
Best Effort 3 15 1023
Background 7 15 1023
EDCF does the following:
– Variable Arbitration Interframe Spacing (AIFS)
– Variable CWmin and CWmax values depending on traffic type
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Understanding the Effect of EDCA Timers
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By combining these timers, the theoretical probability of higher priority frames is greatly improved, but is not guaranteed
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QoS Enhancements for Voice Applications
EDCA provides two key key optimizations for voice over wireless:
1. Transmission Opportunity (TXOP)
2. Transmission Specification (TSpec)
TXOP is a set period of time when a wireless station may send as many frames as possible without having to contend with other stations. – In DCF once a client has access to the medium, it just keeps sending as long as it wants (bad)
– TXOP gives a specified period of time per client to access the medium (good)
TSpec allows real-time applications, such as voice calls that are in progress, to be prioritized over requests for new calls.
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0 EF
Classification
Marking/Mutation
Shaping/Policing
Queueing
Trust
What Make up Wireless Qos?
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0 EF
Classification
Marking/Mutation
Shaping/Policing
Queueing
Trust
SET MUTATE
What Make up Wireless Qos?
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Classification
Marking/Mutation
Shaping/Policing
Queueing
Trust
What Make up Wireless Qos?
Line Rate
Shaped Rate
Traffic shaping limits the transmit rate to a value lower than line rate
with Traffic
Shaping
without
Traffic Shaping
Line Rate
Policed Rate
Policing discards traffic which exceeds policed rate
without Policing
with Policing
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Classification
Marking/Mutation
Shaping/Policing
Queueing
Trust
What Make up Wireless Qos?
Queue 2 (Access Category 2)
Queue 1 (Access Category 1)
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Classification
Marking/Mutation
Shaping/Policing
Queueing
Trust
What Make up Wireless Qos?
DSCP 46
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QoS Remarking: Upstream vs. Downstream
Upstream means traffic originating at the client, headed towards the WLC
Downstream means traffic destined for the WLAN clients
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CAPWAP Tunnel
WLC AP Client
Radio Upstream Network Upstream
Network Downstream Radio Downstream
Ethernet
Switch
802.1q/p 802.11e UP CAPWAP DSCP DSCP
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The Downstream QoS Marking Model
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CAPWAP Tunnels Si Si Si Si Si Si
WLAN Controller
AP
Ethernet Switch
CAPWAP Encapsulated DSCP 802.1p DSCP Payload
AP
AP
802.1q Trunk
1 The Ethernet frame is received over an 802.1q trunk by the WLC. The WLC uses the
DSCP value of the IP packet and maps it to the outer DSCP of the CAPWAP tunnel.
802.11 DSCP Payload
1
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The Downstream QoS Marking Model
30
CAPWAP Tunnels Si Si Si Si Si Si
WLAN Controller
AP
Ethernet Switch
802.1p DSCP Payload 11e UP DSCP Payload
2 2
AP
AP
802.1q Trunk
1 The Ethernet frame is received over an 802.1q trunk by the WLC. The WLC uses the
DSCP value of the IP packet and maps it to the outer DSCP of the CAPWAP tunnel.
2 Once the Ethernet frame is received by the AP, it maps the DSCP value of the IP packet
to the 802.11e CoS value on the wireless frame. The frame is then sent to the client.
CAPWAP Encapsulated DSCP
802.11 DSCP Payload
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The Upstream QoS Marking Model
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CAPWAP Encapsulated
CAPWAP Tunnels Si Si Si Si Si Si
WLAN Controller
AP
Ethernet Switch
11e UP DSCP Payload DSCP
3 1
AP
AP
802.1q Trunk
3 1 The client 802.11e frame is received by the AP. The AP copies the 802.11e
frame header to the outer CAPWAP IP DSCP header
802.11 DSCP Payload
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The Upstream QoS Marking Model
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CAPWAP Tunnels Si Si Si Si Si Si
WLAN Controller
AP
Ethernet Switch
DSCP
3 1
802.1p DSCP Payload
AP
AP
802.1q Trunk
3 1 The client 802.11e frame is received by the AP. The AP copies the 802.11e
frame header to the outer CAPWAP IP DSCP header
At the WLC side of the CAPWAP tunnel, the 802.11e bridged to the Ethernet switch.
The original packet’s DSCP value is mapped to a CoS value on the 802.1q trunk. 2
2
11e UP DSCP Payload
CAPWAP Encapsulated
802.11 DSCP Payload
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Cisco WLC 5500 QoS Profiles
As a “best practice” Cisco recommends to have a separate WLAN (SSID) for voice.
– Not exactly practical any more
– What do you do if you are using MS Lync for Unified Communications on a laptop??
This is the most misunderstood concept in Cisco WLAN QoS
34
Cisco WLC Profile Name
Maximum DSCP Value for the Profile
Platinum EF (46)
Gold AF41 (34)
Silver DF (0)
Bronze AF11 (10)
The main feature of the profile is to set a ceiling on the DSCP value that is allowed.
This only impacts the DSCP value that is set on the CAPWAP header and the downstream UP value.
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Step 1: Configure the QoS Profile in the WLC
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These four pre-configured QoS profiles may be used and applied to each WLAN (SSID) you create.
Recommendations: 1. For enterprise class, mixed-use WLANs,
use the Platinum Profile
2. For guest SSIDs use the Silver Profile
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Step 2: Configure the QoS Profile
36
Create default bandwidth contracts for each user or each SSID
Bandwidth contracts are
bidirectional
Set the maximum priority
Set the 802.1p CoS value
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Customization Feature (AireOS 7.2): Alloy QoS
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Maximum Priority allows you to customize the upper limit QoS marking for a QoS policy
Non-WMM client packets are defaulted to the maximum priority of the WLAN. – Can introduce significant problems for multicast traffic – flooded to all clients in the
Voice queue, which degrades performance.
These fields set the default QoS marking for non-WMM clients
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Step 3: Apply the QoS Profile to Your WLAN (SSID)
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Choose the QoS profile you want to apply for this WLAN
In this example, the “Voice” profile is selected.
This sets the ceiling on all traffic to DSCP 46 and UP value to 6.
You can also set the per-user and per-SSID bandwidth contracts from this screen.
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Example 1 – Downstream QoS Marking
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CAPWAP Tunnels Si Si Si Si Si Si
WLAN Controller
AP
Ethernet Switch
CAPWAP Encapsulated DSCP 802.1p DSCP Payload
UP DSCP Payload
AP
AP
802.1q Trunk
802.11 DSCP Payload
Mixed use WLAN (tablets, phones, etc.) configured for the Platinum Profile
6 46 46
5
46
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Example 2 – Using the Gold QoS Profile
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CAPWAP Tunnels Si Si Si Si Si Si
WLAN Controller
AP
Ethernet Switch
CAPWAP Encapsulated DSCP 802.1p DSCP Payload
UP DSCP Payload
AP
AP
802.1q Trunk
802.11 DSCP Payload
Mixed use WLAN (tablets, phones, etc.) configured for the Gold Profile
DSCP 46 gets Marked down to 34, UP is 5
5 34
46 5
46
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What to do With Non-WMM Clients?
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WMM is critical for QoS – it enables 802.11e UP
Best to use “Required” for a QoS enabled WLAN
“Allowed” will mix QoS capable and non-QoS capable devices on same WLAN
802.11n and 802.11ac enforce WMM support
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Example 3 – A WMM Client Using Lync
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CAPWAP Encapsulated
CAPWAP Tunnels Si Si Si Si Si Si
WLAN Controller
AP
Ethernet Switch
11e UP DSCP Payload DSCP 802.1p DSCP Payload
AP
AP
802.1q Trunk
802.11 DSCP Payload
The AP Recognizes
the WMM UP value
and maps it
automatically to
DSCP 34
5 34
34
34 4
34
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Example 4 – A Non-WMM Client on the Platinum Profile
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CAPWAP Encapsulated
CAPWAP Tunnels Si Si Si Si Si Si
WLAN Controller
AP
Ethernet Switch
802.11 DSCP Payload DSCP 802.1p DSCP Payload
AP
AP
802.1q Trunk
802.11 DSCP Payload
The Client is Not WMM Capable.
AP automatically maps the
CAPWAP DSCP to EF (46)
The Controller unpacks the CAPWAP
packet and only uses the inner DSCP
value from here
46
x 34 4
34
34
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Tweaking the EDCA Parameters for Voice
Choose “WMM” for networks without voice
Choose “Voice Optimized” or “Voice & Video Optimized” when using multimedia
Values that the Profile influences:
– CWmin
– CWmax
– TXOP values
– Slot Time Values
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Call Admission Control (CAC)
CAC refers to the WLC’s capability to limit the number of voice calls on a per-AP basis CAC limits the number of voice clients which can associate to the Access Point.
Configuration:
1. CAC is a subset of WMM, thus first step is to configure WMM “allowed” or “required”
2. Navigate to this page, enable and configure CAC
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Introducing Application Visibility & Control (AVC)
Introduced in AireOS 7.4, limited capabilities in IOS-based controllers
Leverages the IOS NBAR2 Engine – same list of traffic signatures as IOS & XE
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• Discover which applications are running on your
corporate and guest WLANs
• Prioritize critical wireless apps and de-prioritize
non-business apps
• Monitor voice and video performance on the
WLAN
AVC In The Wireless LAN Controller
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Key Points to Know About AVC
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CAPWAP Tunnels
WLC AP
Client
Ethernet
Switch
AVC Policy only
functions here
AVC Does Not
Function Here
AVC feature has no
concept of direction –
modifies DSCP in both
directions
AVC Modifies the inner DSCP value,
thus influencing the CAPWAP DHCP
and wireless UP values
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AVC Example: Build a Multimedia AVC Policy
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More Key Points To Know:
Applications are grouped by class (such as “voice-and-video” shown here)
From AireOS 7.6 Protocol Packs are used for signature updates
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AVC Example: Mobile Applications AVC Profile
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AVC has two basic capabilities:
1. Modify the inner packet’s DSCP to a custom value
2. Drop the packet
Select the application you want to remark (in this case MS Lync)
Decide what DSCP value you would like it remarked to.
In this case, let’s mark it to Gold (DSCP 34)
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AVC Example Cont’d:
50
The “Mobile Applications” AVC Policy is now created and ready to be applied.
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AVC Example Cont’d: Apply the AVC Policy
51
Navigate to your QoS policy for the WLAN you want to apply this to.
Enable AVC
Apply the AVC policy you created to this QoS policy.
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AVC Provides Exceptional Application Visibility
52
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QoS Features are Nice, but a QoS Design is Critical
How Does this fit with the Classic Four-Class QoS Model?
54
Transactional
Data
Realtime
Classes
Best Effort
Control
AF21
EF
DSCP
DF
CS3
Database Apps,
Email, FTP, Backups
CRM Apps,
Broadcast Video
Multimedia Streaming
IP Phones,
TelePresense,
WebEx,
Jabber
Application Examples
Everything Else
OAM, Routing Protocols
35% BW Guarantee
WRED
33% of BW,
Strict Priority
QoS Handling
25% BW Guarantee
WRED
7% of BW Guarantee
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A Desired Design Might Look Like This:
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Transactional Data
Realtime
4-Class Strategic
Enterprise Model
Best Effort
Control
Platinum
WMM Model +
802.11e User Priority
Silver
Gold
Bronze
UP 7
UP 5
UP 3
UP 2
UP 6
UP 4
UP 0
UP 1
AF21
EF
DSCP
DF
CS3
Packets (frames) go into the EDCA / WMM ACs based on their UP Value.
This model looks reasonable and desirable, but is it realistic?
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56
But It Doesn’t Always Work - Remember This Table?
Traffic Type DSCP 802.11e UP
Network Control
56 (CS7) 7
CAPWAP 48 (CS6) 7
Voice 46 (EF) 6
Interactive Video
34 (AF41) 5
Streaming 26 (AF31) 4
X-Data 18 (AF21) 3
Bulk Data 10 (AF11) 2
Best Effort 0 (BE) 0
Consider: In upstream direction, most wireless applications use UP 4 for signalling/control
– (Even if the inner DSCP is CS3)
Since UP 4 = AF31, this gets marked to AF31 on the CAPWAP header.
But, the network is looking to handle DSCP CS3 for signaling – meaning signaling gets put into the wrong queue across the IP transport network!
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57 57
The Impact of UP DSCP Mappings on the Network
57
CAPWAP Encapsulated
CAPWAP Tunnels Si Si Si Si Si Si
WLAN Controller
AP
Ethernet Switch
11e UP DSCP Payload DSCP 802.1p DSCP Payload
AP
AP
802.1q Trunk
802.11 DSCP Payload
AP Maps UP 3 to a
CAPWAP Header
DSCP Value of 18
24
24 3
4 24 26
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The Default DSCP UP Mappings Are Surprising
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Transactional Data
Realtime
4-Class Strategic
Enterprise Model
Best Effort
Control
Platinum
WMM Model +
802.11e User Priority
Silver
Gold
Bronze
UP 7
UP 5
UP 3
UP 2
UP 6
UP 4
UP 0
UP 1
AF21
EF
DSCP
DF
CS3
Thus, in the downstream direction, Voice Signaling ends up in the same AC as X-Data and Best Effort (ouch)
The inconsistent mapping mechanism used by 802.11e means only two queues get used
This can negatively impact wireless performance
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Solution: Use Mutation on Access Switch
Example: On Upstream, mutate the incorrect DSCP signaling value back to CS3
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C3750-X(config)# mls qos map dscp-mutation UPSTREAM-MUTATION 26 to 24
! Mutates the DSCP from AF31 back to CS3 to it is correct on the CAPWAP tunnel header
C3750-X(config)# interface GigabitEthernet1/1/10
! The interface that connects to the AP
C3750-X(config-if)# mls qos trust dscp
! Configures the port to statically trust DSCP on ingress
C3750-X(config-if)# mls qos dscp-mutation UPSTREAM-MUTATION
! Attaches the Upstream DSCP mutation map to the interface on ingress
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Through Manual Mutation, You Can Achieve This
60
Transactional Data
Realtime
4-Class Strategic
Enterprise Model
Best Effort
Control
Platinum
WMM Model +
802.11e User Priority
Silver
Gold
Bronze
UP 7
UP 5
UP 3
UP 2
UP 6
UP 4
UP 0
UP 1
AF21
EF
DSCP
DF
CS3
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Key Takeaways
Wireless QoS has matured, and is well supported by the standards bodies, particularly the IEEE 802.11e-2012 specification
The WLC 5500 Series offers a wide variety of QoS capabilities
– Per-user, per-SSID bandwidth contracts
– Policy control per WLAN
– Call Admission Control
– Application Visibility & Control
The use of Mixed-use WLANs is now the standard
AVC policies can be used to set a trust boundary and remark traffic
If the standard 4-Class QoS model us used in your network, it results in a 2-Class system over the wireless portion.
– Mutating DSCP values on the access switch can change this behavior.
62
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Call to Action…
Visit the World of Solutions:-
Cisco Campus
Walk-in Labs
Technical Solutions Clinics
Meet the Engineer
Lunch Time Table Topics, held in the main Catering Hall
Recommended Reading: For reading material and further resources for this session, please visit www.pearson-books.com/CLMilan2014
63
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