MAC Header: QoS Control

QoS Control is a 16-bit field that identifies the Quality of Service (QoS) parameter of a data frame (only in data frame type QoS-Data).

QoS Control field is comprised of five subfields:

1. [bit 0-3 ] Traffic Identifier (TID)
2. [bit 4 ]    End of Service Period (EOSP)
3. [bit 5-6 ] ACK Policy
4. [bit 7 ]   Reserved
5. [bit 8-15 ] TXOP limit, TXOP duration, AP PS buffer state, Queue Size

Below chart list which type of devices use bit 8-15 information.

Here is a QoS Data type wireless frame capture shows the above subfields in QoS control field.

WiFi uses EDCA- Enhanced Distributed Channel Access, a wireless access method that provides differentiated access for stations using 8 user priorities & 4 QoS Access categories (AC_VO, AC_VI, AC_BE, AC_BK). These UP values of a wireless frame map to QoS field (CoS/802.1D) of a 802.1q header when it translated to Ethernet frame.

WiFi alliance QoS certification called WMM-WiFi Multimedia also defined those 4 access categories. So WMM certified end client should classify its traffic on to one of those classes prior to transmit them over the air.

Below shows the relationship between WMM Access categories & 802.1D tag values: 

Here is the brief description of the QoS subfields

1st subfield (TID – Traffic Indicator)
4 bit value used to identify the user priority (UP) and traffic Access Category(AC) of a QoS data frame. 802.11 WMM clients use WMM-PS (power save) to indicate to an AP that STA is awake. Unlike in legacy PS, WMM-PS client can ask to deliver more than 1 frame.

2nd subfield (ESOP- End of Service Period)
1 bit value to indicate the end of a service period. If this bit set to 1, then client can go back to asleep.

3rd subfield (Acknowledge)
Specify the 2-bit Acknowledgement policy. There are four different options available
ACK:
No-ACK:
No Explicit ACK:
Block ACK:

4th subfield (Reserved)
Allocated for future use

5th subfield
TXOP Limit: Indicate the transmit opportunity granted by the AP
AP PS Buffer State: AP use this to indicate PS buffer state for a given client station.
TXOP Duration Requested: Client use this to tell AP how much time client station wants for its next TXOP. AP may choose to assign shorter TXOP as well.
Queue Size: Client station use that to inform AP how much buffered traffic it has to send. AP can use this information to determine duration for next TXOP to that client

Here is a two sample packet capture of QoS data frames when 7921 phone is communicating with 7960 phone in the given setup.

This is a RTP packet going from wireless client to wired phone. As you can see bit 4 set to 0 (TXOP Duration Requested in bit 8-15). Also TID is 6 indicating it is Access Class of Voice traffic.

Below show a wireless packet coming from wired phone to the wireless client. In here bit 4 is used for EOSP. Also in this case bit8-15 (5th subfield) is AP PS Buffer state to tell client whether any buffer data available for 7921 phone.

 

**** Reference: CWAP Official Study Guide and MRN-CCIEW Blog

WiFi & QoS

802.11

With the 802.11 protocol, it is not possible to prioritize between packets. Other methods must be used to successfully implement QoS. In this post you can gain some practical knowledge about how things (in overall terms) are brought together.

Distributed coordinated function (DCF)

Without going into too detailed explanation about wireless vs. wired networks, we will just relate to wireless network use CSMA/CA instead of the wired method CSMA/CD. Why is that you might ask yourself?

The wireless system is a distributed system which means that all “players” (Client and AP) share the overall media. The AP cannot control who sends a packet first. It works by the method called Distributed Coordinated Function (DCF).

Every Client starts the conversation by picking a random number and start a countdown. When the countdown reaches zero, the client will “fire” its packet (if the media is available). If the media is busy, the client doubles it counters and starts all over.

802.11e /WMM

The 802.11e protocol is designed to overcome issues regarding missing QoS on wireless network. The WMM divides the traffic into 4 categories, so instead of having only a single queue for all clients, we now have four (for WMM enabled clients). Each queue has different back-off timers and thereby different countdown windows.

This four queue-systems is called “Enhanced Distributed Channel Access” (EDCA)

Access Category and “Timing”

Based on the WMM functionality, the data-application will be assigned and placed in one of the four queues. In each queue, a different set of rules will decide the countdown starting value. Traffic in the Platinum queue will start with a lower counter than ex. traffic in the silver queue, and therefore reach zero first, which is a simple but effective way of prioritizing traffic in the network.

The NAV value

The basic function for this: A Counter value maintained by each station with an amount of time that must run until the medium will be free. It contains the time that the currently transmitting station will use the medium and information for how long it will require the air to transmit the frame. Remember! A station cannot transmit until the NAV = 0 (zero)

Each station will try to calculate how long it will take to transmit the frame. The calculation is based on data rate and frame length. It’s quite practical information and is included in the Duration field of the frame header. The information is used by all other stations, in the same area to set their NAV.

IEEE 802.11e, IEEE 802.1P, and DSCP Mapping

When an 802.11e (WMM) client send a frame, it will need an IEEE 802.1P classification in the frame. The AP needs to translate this classification into a DSCP value for the LWAPP/CAPWAP packet carrying the frame. This is the only way to make sure that the packet is treated with the correct priority on its way to the WLC. The same has to occur on the WLC, when it forwards packets to the AP and the connected clients.

Translation DSCP to 802.11p to 802.11e

Cisco uses the encapsulation-method “CAPWAP tunnel” to run the wireless data within.
Because the QoS value must be contained within the packet it’s somehow necessary to contain the QoS classification that has been applied to the WLAN frame. This is done by a simple mapping on classifications to and from DSCP to Class of Service, (CoS).

IEEE / IETF / Cisco

• The IEEE organization is who designed the 802.11p and the 802.11e protocol.
• The possible mapping/classification verves from 0-7, meaning eight classifications can be set.
• The IETF-organization and Cisco do not quite agree on the way to look at the IEEE QoS settings.

They both agree that 7 is to be used for emergency and there by actually never to be used on the network.

• IETF claim the most important packets (for data) are voice packets.
• Cisco claim the most important packets is the LWAPP/CAPWAP packets, cause without them we do not have any network and thereby no voice. At some point Cisco has a point here.

Information About QoS Profiles

Cisco WLAN’s by default support four levels of QoS: Platinum/Voice, Gold/Video, Silver/Best Effort (which is default), and Bronze/Background. It’s recommended to configure:

• The voice WLAN to use Platinum.
• The Video WLAN to user Gold.
• The Normal WLAN to use Silver
• Assign a lower (Bronze) QoS setting for the less important WLAN´s.

Wired to wireless and vice versa

Below an example of the traffic flow/classification for a WMM client, an AP, and a WLC connected to the network.

• 1 and 2 = Traffic from the wired side to the wireless side.
• 3, 4 and 5 = Traffic from the wireless side to the wired side.

1. A frame with 802.1p marking and a packet with an IP DSCP marking arrive on the wired interface on the Controller. The DSCP marking are used for the DSCP “setting” for the CAPWAP/LWAPP packet leaving the WLC and traveling to the wireless client.

2. The 802.1p value on the frame depends on the QoS translation table and the QoS profile for the configured WLAN. The QoS setting on the WLAN is just an expected normal setting.

3. The IP DSCP part of the packet reaching the AP and the AP will translate THE DSCP to an 802.11e CoS (Class of Service) marking that the client will understand.

4. The 802.11e CoS marking on a frame arriving at the AP is translated to an CAPWAP/LWAPP DSCP value at the maximum value for that QoS profile on the WLC.

The DSCP marking on the packet leaving the WLC on its way to the wired net, will by default have the DSCP setting equal to the packet that left the WLAN client. From the WLC to the AP it’s the DSCP marking that’s converted to 802.11e setting for CoS.

• On the trunk ports/link both the DSCP and the 802.1p marking will be transported.
• On the access port/link only the DSCP marking will be transported.

*** Reference: www.wificert.org