Working with QoS for Voice and Video

QoS settings for voice and video applications are configured when you configure firewall roles and policies.

Understanding VoIP Call Admission Control Profile

VoIP call admission control prevents any single AP from becoming congested with voice calls. You configure call admission control options in the VoIP Call Admission Control profile which you apply to an AP group or a specific AP.

You can use the WebUI or CLI to configure a VoIP Call Admission Control profile.

In the WebUI

1. Navigate to the Configuration > AP Configuration page. Select either AP Group or AP Specific.
If you select AP Group, click Edit for the AP group name for which you want to configure VoIP CAC.
If you select AP Specific, select the name of the AP for which you want to configure VoIP CAC.
2. In the Profiles list, expand the QoS menu, then select the VoIP Call Admission Control profile.
3. In the Profile Details window pane, click the VoIP Call Admission Control profile drop-down list and select the profile you want to edit.
-or-
To create a new profile, click the VoIP Call Admission Control profile drop-down list and select New. Enter a new profile name in the field to the right of the drop-down list. You cannot use spaces in VoIP profile names.
4. Configure your desired VoIP Call Admission Control profile settings. Table 1 describes the parameters you can configure in this profile.

Table 1: VoIP Call Admission Control Configuration Parameters

Parameter

Description

VoIP Call Admission Control

Select the Voip Call Admission Control checkbox to enable Wi-Fi VoIP Call Admission Control features.

VoIP Bandwidth based CAC

Select the VoIP Bandwidth based CAC checkbox to enable call admission controls based upon bandwidth. If this option is not selected, call admission controls are based on call counts.

VoIP Call Capacity

The maximum number of simultaneous calls that the AP radio can handle. The default value is 10. You can use the bandwidth calculator in the WebUI to calculate the call capacity. To access the bandwidth calculator, navigate to Configuration > Management > Bandwidth Calculator.

VoIP Bandwidth Capacity (kbps)

Enter a rate from 1 to 600000 (inclusive) to specify the maximum bandwidth rate that a radio can handle, in kbps. The default value is 2000 kbps.

VoIP Call Handoff Reservation

Specify the percentage of call capacity reserved for mobile VoIP clients on an active call. The default value is 20%.

VoIP Send SIP 100 Trying

The SIP invite call setup message is time-sensitive, as the originator retries the call as quickly as possible if it does not proceed. You can direct the controller to immediately reply to the call originator with a “SIP 100 - trying” message to indicate that the call is proceeding and to avoid a possible timeout. This is useful in conditions where the SIP invite may be redirected through a number of servers before reaching the controller.

Select the VoIP Send SIP 100 Trying checkbox to send SIP 100-trying messages to a call originator to indicate that the call is proceeding. This is a useful option when the SIP invite is directed through many servers before reaching the controller.

VoIP Disconnect Extra Call

In the VoIP Call Admission Control (CAC) profile, you can limit the number of active voice calls allowed on a radio. This feature is disabled by default. When the disconnect extra call feature is enabled, the system monitors the number of active voice calls, and if the defined threshold is reached, any new calls are disconnected. The AP denies association requests from a device that is on call.

To enable this feature, select the VoIP Disconnect Extra Call checkbox. You also need to enable call admission control in this profile.

VOIP TSPEC Enforcement

A WMM client can send a Traffic Specification (TSPEC) signaling request to the AP before sending traffic of a specific AC type, such as voice. You can configure the controller so that the TSPEC signaling request from a client is ignored if the underlying voice call is not active; this feature is disabled by default. If you enable this feature, you can also configure the time duration within which the station should start the voice call after sending the TSPEC request (the default is one second).

Select the VoIP TSPEC Enforcement checkbox to validate TSPEC requests for CAC.

VOIP TSPEC Enforcement Period

Select the maximum time, in seconds, for the station to start the call after the

TSPEC request.

VoIP Drop SIP Invite and send status code (client)

Click the VoIP Drop SIP Invite and send status code (client) drop-down list and select one of the following status codes to be sent back to the client:

480: Temporary Unavailable
486: Busy Here
503: Service Unavailable
none: Don't send SIP status code

VoIP Drop SIP Invite and send status code (server)

Click the VoIP Drop SIP Invite and send status code (client)drop-down list and select one of the following status codes to be sent back to the server:

480: Temporary Unavailable
486: Busy Here
503: Ser vice Unavailable
none: Don't send SIP status code
5. Click Apply to save your settings.

In the CLI

wlan voip-cac-profile <profile>

bandwidth-cac

bandwidth-capacity <bandwidth-capacity>

call-admission-control

call-capacity

call-handoff-reservation <percent>

disconnect-extra-call

send-sip-100-trying

send-sip-status-code client|server <code>

wmm-tspec-enforcement

wmm-tspec-enforcement-period <seconds>

Understanding Wi-Fi Multimedia

Wi-Fi Multimedia (WMM), is a Wi-Fi Alliance specification based on the IEEE 802.11e wireless Quality of Service (QoS) standard. WMM works with 802.11a, b, g, and n physical layer standards.

WMM supports four access categories (ACs): voice, video, best effort, and background. Table 2 shows the mapping of the WMM access categories to 802.1p priority values. The 802.1p priority value is contained in a two-byte QoS control field in the WMM data frame.

Table 2: WMM Access Category to 802.1p Priority Mapping

Priority

802.1p Priority

WMM Access Category

Lowest

1

Background

 

Click to view a larger size.

2

0

Best effort

3

4

Video

5

6

Voice

Highest

7

In non-WMM, or hybrid environments where some clients are not WMM-capable, Aruba uses voice and best effort to prioritize traffic from these clients.

Unscheduled Automatic Power Save Delivery (U-APSD) is a component of the IEEE 802.11e standard that extends the battery life on voice over WLAN devices. When enabled, clients trigger the delivery of buffered data from the AP by sending a data frame.

For the environments in which the wireless clients support WMM, you can enable both WMM and U-APSD in the SSID profile.

Enabling WMM

You can use the WebUI or CLI to enable WMM for wireless clients.

In the WebUI

1. Navigate to the Configuration > Wireless > AP Configuration page.
2. Select either the AP Group or AP Specific tab. Click Edit for the AP group or AP name.
3. In the Profiles list, select Wireless LAN. Select Virtual AP, then select the applicable virtual AP profile. Select the SSID profile.
4. In the Profile Details, select the Advanced tab.
5. Select the Wireless Multimedia (WMM) option. Or, select the Wireless Multimedia U-APSD (WMM-UAPSD) Powersave option if you want to enable WMM in power save mode.
6. Click Apply.

In the CLI

wlan ssid-profile <profile> wmm

wlan ssid-profile <profile> wmm-uapsd

Configuring WMM AC Mapping

The IEEE 802.11e standard defines the mapping between WMM ACs and Differentiated Services Codepoint (DSCP) tags. The WMM AC mapping commands allow you to customize the mapping between WMM ACs and DSCP tags to prioritize various traffic types. You apply and configure WMM AC mappings to a WMM-enabled SSID profile.

 

Ensure that WMM is enabled for legacy APs for the mapping to take effect. For 802.11n APs, ensure that either WMM or high throughput is enabled.

DSCP classifies packets based on network policies and rules, not priority. The configured DSCP value defines per hop behaviors (PHBs). The PHB is a 6-bit value added to the 8-bit Differentiated Services (DS) field of the IP packet header. The PHB defines the policy and service applied to a packet when traversing the network. You configure these services in accordance with your network policies. Table 3 shows the default WMM AC to DSCP decimal mappings and the recommended WMM AC to DSCP mappings.

Table 3: WMM Access Category to DSCP Mappings

DSCP Decimal Value

WMM Access Category

8

Background

16

0

Best effort

24

32

Video

40

48

Voice

56

By customizing WMM AC mappings, both the controller and AP maintain a customized WMM AC mapping table for each configured SSID profile. All packets received are matched against the entries in the mapping table and prioritized accordingly. The mapping table contains information for upstream (client to AP) and downstream (AP to client) traffic.

 

In earlier releases, the default mappings exist for all SSIDs. After you customize a WMM AC mapping and apply it to the SSID, the controller overwrites the default mapping values and uses the configured values . If a controller is upgraded to 6.2 from an older version, the default as well as the user configured WMM-DSCP mappings in the existing SSID profiles are retained. There are no default mappings for a newly created SSID profile and for a factory default controller running 6.2 image.

When planning your mappings, make sure that any immediate switch or router does not have conflicting 802.1p or DSCP configurations/mappings. If this occurs, your traffic may not be prioritized correctly.

To view the mapping settings, use the following command:

show wlan ssid-profile <profile>

Using the WebUI to map between WMM AC and DSCP

1. Navigate to the Configuration > Wireless > AP Configuration page.
2. Select either the AP Group or AP Specific tab. Click Edit for the AP group or AP name.
3. In the Profiles list, select Wireless LAN. Select Virtual AP, then select the applicable virtual AP profile. Select the SSID profile.
4. In the Profile Details, select the Advanced tab.
5. Scroll down to the Wireless Multimedia (WMM) option. Select (check) this option.
6. Modify the DSCP mapping settings, as needed:
DSCP mapping for WMM voice AC—DSCP used to map voice traffic
DSCP mapping for WMM video AC—DSCP used to map video traffic
DSCP mapping for WMM best-effort AC—DSCP used to map best-effort traffic
DSCP mapping for WMM background AC—DSCP used to map background traffic
7. Click Apply.

The following enhancements have been made to the WMM-DSCP mapping functionality:

When a controller is upgraded to 6.2 version from an older version, the default as well as the user configured WMM-DSCP mappings in the existing SSID profiles are retained.
Default mappings are not there for a newly created SSID profile and for a factory default controller running 6.2 image.
If the mapping has no value, the original DSCP for upstream traffic is retained.
The maximum number of values that can be configured for WMM-DSCP is 8.
For the upstream traffic, if the mapping exists and incoming DSCP value matches one of the mapped values then the DSCP value is retained
For the upstream traffic, if the mapping exists and incoming DSCP value does not match any of the mapped values then the DSCP value is overwritten with the first value in the WMM- DSCP list
For Wireless to Wireless Traffic: If the AC of the incoming packet has no mapping and the incoming DSCP value is mapped to a different AC, then the DSCP value is retained and WMM priority is changed to the corresponding AC where incoming DSCP is mapped.

Using the CLI to map between WMM AC and DSCP

wlan ssid-profile <profile>

wmm-be-dscp <best-effort>

wmm-bk-dscp <background>

wmm-vi-dscp <video>

wmm-vo-dscp <voice>

Configuring DSCP Priorities

You can configure DSCP priorities for WMM packets in the following ways:

Configure the DSCP mappings in the SSID profile
Set a ToS value in the ACL
Set the ToS value as well as the 802.1p priority in the ACL

Setting a ToS value in the ACL overrides the default DSCP mappings configured in the SSID profile. Configuring a DSCP priority in both the L2 and L3 header prioritizes the WMM packets with the higher value.

For example, we can have different ToS values set for different voice traffic in a network. To prioritize all of them in the voice queue, we can set the 802.1p priority to voice.

Consider a deployment where Cisco Softphone, Lync, and Scopia are configured with the following DSCP :

Cisco Softphone - DSCP 46
Lync - DSCP 44
Scopia - DSCP 42

In the absence of doing anything, all of the DSCP above would map into the Video queue. To map all the traffic into Voice queue you can do the following ACL configuration:

wlan ssid-profile VOICE

wmm-vo-dscp 46

ip access-list session VOICE

any destination [LYNC_SERVER] [LYNC_PORTS] permit tos 44 dot1p-priority 6

any destination [SCOPiA_SERVER] [SCOPIA _PORTS] permit tos 42 dot1p-priority 6

 

You must know the ports on which each traffic is sent so that the correct traffic is identified.

Configuring Dynamic WMM Queue Management

Traditional wireless networks provide all clients with equal bandwidth access. However, delays or reductions in throughput can adversely affect voice and video applications, resulting in disrupted VoIP conversations or dropped frames in a streamed video. Thus, data streams that require strict latency and throughput need to be assigned higher traffic priority than other traffic types.

The Wi-Fi Alliance defined the Wi-Fi Multimedia (WMM) standard in response to industry requirements for Quality of Service (QoS) support for multimedia applications for wireless networks. This is defined as per the IEEE 802.11e standards.

WMM requires:

The access point is Wi-Fi Certified and has WMM enabled
The client device is Wi-Fi Certified
The application supports WMM

Enhanced Distributed Channel Access

WMM provides media access prioritization through Enhanced Distributed Channel Access (EDCA). EDCA defines four access categories (ACs) to prioritize traffic: voice, video, best effort, and background. These ACs correspond to 802.1p priority tags, as shown in Table 4.

Table 4: WMM Access Categories and 802.1p Tags

WMM Access Category

Description

802.1p Tag

Voice

Highest priority

7, 6

Video

Prioritize video traffic above other data traffic

5, 4

Best Effort

Traffic from legacy devices or traffic from applications or devices that do not support QoS

0, 3

Background

Low priority traffic (file downloads, print jobs)

2, 1

While the WMM ACs designate specific types of traffic, you can determine the priority of the ACs. For example, you can choose to give video traffic the highest priority. With WMM, applications assign data packets to an AC. In the client, the data packets are then added to one of the transmit queues for voice, video, best effort, or background.

WMM is an extension to the Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) protocol’s Distributed Coordination Function (DCF). The collision resolution algorithm responsible for traffic prioritization depends on the following configurable parameters for each AC:

arbitrary inter-frame space number (AIFSN)
minimum and maximum contention window (CW) size

For each AC, the backoff time is the sum of the AIFSN and a random value between 0 and the CW value. The AC with the lowest backoff time is granted the opportunity to transmit (TXOP). Frames with the highest-priority AC are more likely to get TXOP as they tend to have the lowest backoff times (a result of having smaller AIFSN and CW parameter values). The value of the CW varies through time as the CW doubles after each collision up to the maximum CW. The CW is reset to the minimum value after successful transmission. In addition, you can configure the TXOP duration for each AC.

On the controller, you configure the AC priorities in the WLAN EDCA parameters profile. There are two sets of EDCA profiles you can configure:

AP parameters affect traffic from the AP to the client.
STA parameters affect traffic from the client to the AP.

Using the WebUI to configure EDCA parameters

Use the following procedure to define an Enhanced Distributed Channel Access (EDCA) profile for APs or for clients (stations).

1. Navigate to the Configuration > AP Configuration page. Select either the AP Group tab or AP Specific tab.
If you selected AP Group, click Edit for the AP group name for which you want to configure EDCA parameters.
If you selected AP Specific, select the name of the AP for which you want to configure EDCA parameters.
2. Under Profiles, expand the Wireless LAN menu, then select Virtual AP. In the Virtual AP list, select the appropriate virtual AP.
3. Expand the SSID profile. Select the EDCA Parameters Station or EDCA Parameters AP profile.
4. Configure your desired EDCA Profile Parameters. Table 5 describes the parameters you can configure in this profile.

Table 5: EDCA Parameters Station and EDCA Parameters AP Profile Settings

Parameter

Description

Best Effort

Set the following parameters to define the best effort queue.

aifsn: Arbitrary inter-frame space number. Possible values are 1-15.
ecw-max: The exponential (n) value of the maximum contention window size, as expressed by 2n-1. A value of 4 computes to 24-1 = 15. Possible values are 1-15.
ecw-min: The exponential (n) value of the minimum contention window size, as expressed by 2n-1. A value of 4 computes to 24-1 = 15. Possible values are 0-15.
txop: Transmission opportunity, in units of 32 microseconds. Divide the desired transmission duration by 32 to determine the value to configure. For example, for a transmission duration of 3008 microseconds, enter 94 (3008/32). Possible values are 0-2047.
acm: This parameter specifies mandatory admission control. With a value of 1, the client reserves the access category through traffic specification (TSPEC) signaling. A value of 0 disables this option.

Background

Set the following parameters to define the background queue.

aifsn: Arbitrary inter-frame space number. Possible values are 1-15.
ecw-max: The exponential (n) value of the maximum contention window size, as expressed by 2n-1. A value of 4 computes to 24-1 = 15. Possible values are 1-15.
ecw-min: The exponential (n) value of the minimum contention window size, as expressed by 2n-1. A value of 4 computes to 24-1 = 15. Possible values are 0-15.
txop: Transmission opportunity, in units of 32 microseconds. Divide the desired transmission duration by 32 to determine the value to configure. For example, for a transmission duration of 3008 microseconds, enter 94 (3008/32). Possible values are 0-2047.
acm: This parameter specifies mandatory admission control. With a value of 1, the client reserves the access category through traffic specification (TSPEC) signaling. A value of 0 disables this option.

Video

Set the following parameters to define the background queue.

aifsn: Arbitrary inter-frame space number. Possible values are 1-15.
ecw-max: The exponential (n) value of the maximum contention window size, as expressed by 2n-1. A value of 4 computes to 24-1 = 15. Possible values are 1-15.
ecw-min: The exponential (n) value of the minimum contention window size, as expressed by 2n-1. A value of 4 computes to 24-1 = 15. Possible values are 0-15.
txop: Transmission opportunity, in units of 32 microseconds. Divide the desired transmission duration by 32 to determine the value to configure. For example, for a transmission duration of 3008 microseconds, enter 94 (3008/32). Possible values are 0-2047.
acm: This parameter specifies mandatory admission control. With a value of 1, the client reserves the access category through traffic specification (TSPEC) signaling. A value of 0 disables this option.

Voice

Set the following parameters to define the background queue.

aifsn: Arbitrary inter-frame space number. Possible values are 1-15.
ecw-max: The exponential (n) value of the maximum contention window size, as expressed by 2n-1. A value of 4 computes to 24-1 = 15. Possible values are 1-15.
ecw-min: The exponential (n) value of the minimum contention window size, as expressed by 2n-1. A value of 4 computes to 24-1 = 15. Possible values are 0-15.
txop: Transmission opportunity, in units of 32 microseconds. Divide the desired transmission duration by 32 to determine the value to configure. For example, for a transmission duration of 3008 microseconds, enter 94 (3008/32). Possible values are 0-2047.
acm: This parameter specifies mandatory admission control. With a value of 1, the client reserves the access category through traffic specification (TSPEC) signaling. A value of 0 disables this option.
5. Click Apply.

Using the CLI to configure EDCA parameters

wlan edca-parameters-profile {ap|station} <profile>

{background | best-effort | video | voice}

[acm][aifsn <number>] [ecw-max <exponent> [ecw-min <exponent>] [txop <number>]

To associate the EDCA profile instance to a SSID profile:

wlan ssid-profile <profile>

edca-parameters-profile {ap|sta} <profile>

Enabling WMM Queue Content Enforcement

WMM queue content enforcement is a firewall setting that you can enable to ensure that the voice priority is used for voice traffic. When this feature is enabled, if traffic to or from the user is inconsistent with the associated QoS policy for voice, the traffic is reclassified to best effort and data path counters incremented. If TSPEC admission were used to reserve bandwidth, then TSPEC signaling is used to inform the client that the reservation is terminated.

You can use the WebUI or CLI to enable WMM queue content enforcement.

In the WebUI

1. Navigate to the Configuration > Advanced Services > Stateful Firewall page.
2. Select Enforce WMM Voice Priority Matches Flow Content.
3. Click Apply.

In the CLI

firewall wmm-voip-content-enforcement