14-WLAN Configuration Guide (Fat AP)

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07-WLAN QoS configuration
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Configuring WLAN QoS

About WLAN QoS

An 802.11 network provides contention-based wireless access. To provide applications with QoS services, IEEE developed 802.11e for 802.11-based WLANs.

WLAN QoS features include WMM and SVP.

 

 

NOTE:

The term "AP" in this document refers to MSR routers that offer WLAN services as fat APs. For more information, see "Compatibility of hardware and AP functionality."

WMM protocol

About WMM

Wi-Fi Alliance defined the Wi-Fi Multimedia (WMM) standard to allow QoS provision devices of different vendors to interoperate. WMM enables a WLAN to provide QoS services, so that audio and video applications can have better performance in WLANs.

The Distributed Coordination Function (DCF) in 802.11 requires APs and clients to use the carrier sense multiple access with collision avoidance (CSMA/CA) access mechanism. APs or clients listen to the channel before they hold the channel for data transmission. When the specified idle duration of the channel times out, APs or clients randomly select a backoff slot within the contention window to perform backoff. The device that finishes backoff first gets the channel. With 802.11, all devices have the same idle duration and contention window. Therefore, they are equal when contending for a channel.

To provide QoS services, WMM divides data traffic into four ACs that have different priorities. Traffic in an AC with a high priority has a better chance to use the channel.

Terminology

·     Enhanced distributed channel access—EDCA is a channel contention mechanism defined by WMM to preferentially transmit packets with high priority and allocate more bandwidth to such packets.

·     Access category—WMM defines the following ACs: AC-VO for voice traffic, AC-VI for video traffic, AC-BE for best effort traffic, and AC-BK for background traffic. The priorities of the four ACs are in descending order.

·     Connect Admission Control—CAC limits the number of clients that can use high-priority ACs (AC-VO and AC-VI) to make sure there is enough bandwidth for these clients.

·     Unscheduled automatic power save delivery—U-APSD is a power saving method defined by WMM to save client power.

EDCA parameters

·     Arbitration inter-frame spacing number—In 802.11-based WLAN, each client has the same idle duration (DIFS), but WMM defines an idle duration for each AC. The idle duration increases as the AIFSN increases.

·     Exponent form of CWmin/Exponent form of CWmax—ECWmin/ECWmax determines the backoff slots, which increase as the two values increase.

·     Transmission opportunity limit—TXOP limit specifies the maximum time that a client can hold the channel after a successful contention. A larger value represents a longer time. If the value is 0, a client can send only one packet each time it holds the channel.

Figure 1 EDCA parameters

 

CAC admission policies

CAC requires a client to obtain permission from an AP before it can use a high-priority AC for transmission. This guarantees bandwidth for the clients that have gained access. CAC controls real time traffic (AC-VO and AC-VI traffic) but not common data traffic (AC-BE and AC-BK traffic).

If a client wants to use a high-priority AC (AC-VO or AC-VI), it must send a request to the AP. The AP returns a positive or negative response based on either of the following admission control policies:

·     Channel usage-based admission policy—The AP calculates the total time that the existing high-priority AC queues occupy the channel per unit time, and then calculates the time that the requesting traffic will occupy the channel per unit time. If the sum of the two values is smaller than or equal to the maximum hold time of the channel, the client can use the requested AC queue. If it is not, the request is rejected.

·     Client-based admission policy—If the number of clients using high-priority AC queues is smaller than the maximum number of high-priority AC clients, the request is accepted. If it is not, the request is rejected. During calculation, a client is counted as one client if it is using both the AC-VO and AC-VI queues.

If the request is rejected because of lack of media resources, the AP assigns AC-BE to the client. Clients that already use high-priority AC queues will not be affected.

When calculating media resources, the AP takes requests before CAC is enabled into account. Whether subsequent requests for high-priority AC queues will be accepted is greatly restricted by the resource usage.

U-APSD power-save mechanism

U-APSD enables clients in sleep mode to wake up and receive the specified number of packets only after receiving a trigger packet. U-APSD improves the 802.11 APSD power saving mechanism.

U-APSD is automatically enabled after you enable WMM.

ACK policy

WMM defines the following ACK policies:

·     Normal ACK—The recipient acknowledges each received unicast packet.

·     No ACK—The recipient does not acknowledge received packets during wireless packet exchange. This policy improves the transmission efficiency in an environment where communication quality is strong and interference is weak. If communication quality deteriorates, this policy might increase the packet loss rate. For A-MPDU packets sent by 802.11n clients, the No ACK policy does not take effect.

SVP

SpectraLink Voice Priority (SVP) is developed by SpectraLink to provide QoS services for voice traffic.

Protocols and standards

·     802.11e-2005, Amendment 8: Medium Access Control (MAC) Quality of Service Enhancements, IEEE Computer Society, 2005

·     Wi-Fi, WMM Specification version 1.1, Wi-Fi Alliance, 2005

Configuring WMM

WMM tasks at a glance

To configure WMM, perform the following tasks:

1.     Enabling WMM

2.     (Optional.) Setting EDCA parameters

3.     (Optional.) Setting EDCA parameters of AC-BE or AC-BK queues for clients

4.     (Optional.) Setting EDCA parameters of AC-VI or AC-VO queues for clients

5.     (Optional.) Configuring a port to trust packet priority for priority mapping

Enabling WMM

About WMM

All 802.11n and 802.11ac clients must support WMM. For 802.11n or 802.11ac clients to communicate with the associated AP, enable WMM when the radio operates in 802.11an, 802.11gn, or 802.11ac mode.

Procedure

1.     Enter system view.

system-view

2.     Enter radio interface view.

interface wlan-radio interface-number

3.     Enable WMM.

wmm enable

By default, WMM is enabled.

Setting EDCA parameters

1.     Enter system view.

system-view

2.     Enter radio interface view.

interface wlan-radio interface-number

3.     Set EDCA parameters.

edca radio { ac-be | ac-bk | ac-vi | ac-vo } { ack-policy { noack | normalack } | aifsn aifsn-value | ecw ecwmin ecwmin-value ecwmax ecwmax-value | txoplimit txoplimit-value } *

The default values for EDCA parameters are shown in Table 1.

Table 1 Default EDCA parameter values

AC

AIFSN

ECWmin

ECWmax

TXOP Limit

AC-BK

7

4

10

0

AC-BE

3

4

6

0

AC-VI

1

3

4

94

AC-VO

1

2

3

47

 

Setting EDCA parameters of AC-BE or AC-BK queues for clients

1.     Enter system view.

system-view

2.     Enter radio interface view.

interface wlan-radio interface-number

3.     Set EDCA parameters of AC-BE or AC-BK queues for clients.

edca client { ac-be | ac-bk } { aifsn aifsn-value | ecw ecwmin ecwmin-value ecwmax ecwmax-value | txoplimit txoplimit-value } *

The default values are shown in Table 2.

Table 2 Default EDCA parameter values of AC-BE or AC-BK queues for clients

AC

AIFSN

ECWmin

ECWmax

TXOP Limit

AC-BK

7

4

10

0

AC-BE

3

4

10

0

 

Setting EDCA parameters of AC-VI or AC-VO queues for clients

1.     Enter system view.

system-view

2.     Enter radio interface view.

interface wlan-radio interface-number

3.     Set EDCA parameters of AC-VI or AC-VO queues for clients.

edca client { ac-vi | ac-vo } { aifsn aifsn-value | cac { disable | enable } | ecw ecwmin ecwmin-value ecwmax ecwmax-value | txoplimit txoplimit-value } *

The default values are shown in Table 3.

Table 3 Default EDCA parameter values of AC-VI or AC-VO queues for clients

AC

AIFSN

ECWmin

ECWmax

TXOP Limit

AC-VI

2

3

4

94

AC-VO

2

2

3

47

 

4.     (Optional.) Configure the CAC policy.

cac policy { channelutilization [ channelutilization-value ] | client [ client-number ] }

By default, the client-based admission policy is used, and the maximum number of admitted clients is 20.

Configuring a port to trust packet priority for priority mapping

About priority mapping

When the packet trust mode is disabled, an AP assigns the port priority to all packets for the service template.

Restrictions and guidelines

This feature takes effect only on uplink packets.

The port priority setting does not take effect if the trusted packet priority type is configured.

Procedure

1.     Enter system view.

system-view

2.     Enter service template view.

wlan service-template service-template-name

3.     Configure the trusted packet priority type.

qos trust { dot11e | dscp }

By default, the port priority is trusted.

4.     Set the port priority.

qos priority priority

By default, the port priority is 0.

Configuring SVP mapping

About SVP mapping

This feature assigns packets that have the protocol ID 119 in the IP header to the AC-VI or AC-VO queue to provide SVP packets with the specified priority. SVP does not require random backoff for SVP packets. Therefore, you can set both ECWmin and ECWmax to 0 when there are only SVP packets in the AC-VI or AC-VO queue.

When SVP mapping is disabled, SVP packets are assigned to the AC-BE queue.

Restrictions and guidelines

SVP mapping takes effect only on non-WMM clients.

Procedure

1.     Enter system view.

system-view

2.     Enter radio interface view.

interface wlan-radio interface-number

3.     Enable SVP mapping.

svp map-ac { ac-vi | ac-vo }

By default, SVP mapping is disabled.

To disable SVP mapping, use the svp map-ac disable command.

Display and maintenance commands for WMM

Execute display commands in any view and reset commands in user view.

 

Task

Command

Display WMM statistics for clients.

display wlan wmm client [ interface wlan-radio interface-number | mac-address mac-address ]

Display WMM statistics for radios.

display wlan wmm radio [ interface wlan-radio interface-number ]

Clear WMM statistics for clients.

reset wlan wmm client [ interface wlan-radio interface-number | mac-address mac-address ]

Clear WMM statistics for radios.

reset wlan wmm radio [ interface wlan-radio interface-number ]

 

WLAN QoS configuration examples

Example: Configuring basic WMM

Network configuration

As shown in Figure 2, enable WMM on the AP so that the AP and the client can prioritize the traffic.

Figure 2 Network diagram

 

Procedure

# Create a service template named market, set the SSID to market, and enable the service template.

<AP> system-view

[AP] wlan service-template market

[AP-wlan-st-market] ssid market

[AP-wlan-st-market] service-template enable

[AP-wlan-st-market] quit

# Bind service template market to WLAN-Radio 0/0.

[AP] interface wlan-radio 0/0

[AP-WLAN-Radio0/0] undo shutdown

[AP-WLAN-Radio0/0] service-template market

# Enable WMM.

[AP-WLAN-Radio0/0] wmm enable

[AP-WLAN-Radio0/0] quit

Verifying the configuration

# Display WMM statistics for radios.

[AP] display wlan wmm radio

Radio : 1

 Client EDCA updates : 0

 QoS mode   : WMM

 WMM status : Enabled

 Radio max AIFSN        : 15          Radio max ECWmin : 0

 Radio max TXOPLimit    : 32767       Radio max ECWmax : 0

 CAC information

 Clients accepted                  : 0

  Voice                            : 0

  Video                            : 0

 Total request medium time(μs)      : 0

  Voice(μs)                        : 0

  Video(μs)                        : 0

Calls rejected due to insufficient resources    : 0

Calls rejected due to invalid parameters        : 0

Calls rejected due to invalid medium time        : 0

Calls rejected due to invalid delay bound        : 0

Example: Configuring CAC

Network configuration

As shown in Figure 3, configure CAC to allow a maximum of 10 clients to use the AC-VO and AC-VI queues.

Figure 3 Network diagram

 

Procedure

# Create a service template named market, set the SSID to market, and enable the service template.

<AP> system-view

[AP] wlan service-template market

[AP-wlan-st-market] ssid market

[AP-wlan-st-market] service-template enable

[AP-wlan-st-market] quit

# Bind service template market to WLAN-Radio 0/0.

[AP] interface wlan-radio 0/0

[AP-WLAN-Radio0/0] undo shutdown

[AP-WLAN-Radio0/0] service-template market

# Enable WMM for AC-VO and AC-VI queues, and configure a CAC policy to limit the number of clients to 10.

[AP-WLAN-Radio0/0] wmm enable

[AP-WLAN-Radio0/0] edca client ac-vo cac enable

[AP-WLAN-Radio0/0] edca client ac-vi cac enable

[AP-WLAN-Radio0/0] cac policy client 10

Verifying the configuration

# Assume that a client requests to use a high-priority AC queue (AC-VO or AC-VI). Verify the following information:

·     If the number of clients using high-priority AC queues is smaller than the maximum number of high-priority AC clients (10 in this example), the request is accepted.

·     If the number of clients using high-priority AC queues is equal to the maximum number of high-priority AC clients (10 in this example), the request is rejected. The AP decreases the priority of packets from the client.

Example: Configuring SVP mapping

Network configuration

As shown in Figure 4, configure SVP mapping on the AP to assign SVP packets to the AC-VO queue. Set ECWmin and ECWmax to 0 for the AC-VO queue of the AP.

Figure 4 Network diagram

 

Procedure

# Create a service template named market, set the SSID to market, and enable the service template.

<AP> system-view

[AP] wlan service-template market

[AP-wlan-st-market] ssid market

[AP-wlan-st-market] service-template enable

[AP-wlan-st-market] quit

# Bind service template market to WLAN-Radio 0/0.

[AP] interface wlan-radio 0/0

[AP-WLAN-Radio0/0] undo shutdown

[AP-WLAN-Radio0/0] service-template market

# Enable WMM.

[AP-WLAN-Radio0/0] wmm enable

# Assign SVP packets to the AC-VO queue, and set EDCA parameters of AC-VO queues for clients.

[AP-WLAN-Radio0/0] svp map-ac ac-vo

[AP-WLAN-Radio0/0] edca client ac-vo ecw ecwmin 0 ecwmax 0

Verifying the configuration

# Verify that the AC assigns SVP packets to the AC-VO queue if a non-WMM client comes online and sends SVP packets to the AC.

Example: Configuring traffic differentiation

Network configuration

As shown in Figure 5, configure priority mapping on the AP to add 802.11 packets from the client to the AC-VO queue.

Figure 5 Network diagram

 

Procedure

# Create a service template named market, set the SSID to market, and enable the service template.

<AP> system-view

[AP] wlan service-template market

[AP-wlan-st-market] ssid market

[AP-wlan-st-market] service-template enable

# Set the 802.11e priority of 802.11 packets from the client to 7.

[AP-wlan-st-market] qos priority 7

[AP-wlan-st-market] quit

# Bind service template market to WLAN-Radio 0/0.

[AP] interface wlan-radio 0/0

[AP-WLAN-Radio0/0] undo shutdown

[AP-WLAN-Radio0/0] service-template market

# Enable WMM.

[AP-WLAN-Radio0/0] wmm enable

[AP-WLAN-Radio0/0] quit

Verifying the configuration

# Perform the following tasks on the AP:

·     Execute the terminal monitor command to enable monitoring of logs on the current terminal.

·     Execute the terminal debugging command to enable display of debug information on the current terminal.

·     Execute the debugging wlan wmm all command to enable all the WMM debugging features.

# Ping the client and the switch on the AP to verify that the network connections are correct.

# Verify that the priority of packets from the client to the AP is changed to 7 and the priority of packets from the AP to the client is not changed.