Configuring Quality of Service (QoS)
Quality of Service (QoS) provides you with the ability to specify parameters on multiple queues for increased throughput and better performance of differentiated wireless traffic like Voice-over-IP (VoIP), other types of audio, video, and streaming media as well as traditional IP data over the Devicescape Enterprise-Managed AP.
The following sections describe how to configure Quality of Service queues on the Devicescape Enterprise-Managed AP:
Understanding QoS
A primary factor that affects QoS is network congestion due to an increased number of clients attempting to access the air waves and higher traffic volume competing for bandwidth during a busy time of day. The most noticeable degradation in service on a busy, overloaded network will be evident in time-sensitive applications like Video, Voice-over-IP (VoIP), and streaming media.
Unlike typical data files which are less affected by variability in QoS, Video, VoIP and streaming media must be sent in a specific order at a consistent rate and with minimum delay between Packet transmission. If the quality of service is compromised, the audio or video will be distorted.
QoS and Load Balancing
By using a combination of load balancing (see Load Balancing) and QoS techniques, you can provide a high quality of service for time-sensitive applications even on a busy network. Load balancing is a way of better distributing the traffic volume across access points. QoS is a means of allocating bandwidth and network access based on transmission priorities for different types of wireless traffic within a single access point.
802.11e and WMM Standards Support
QoS describes a range of technologies for controlling data streams on shared network connections. The IEEE 802.11e task group is in the process of defining a QoS standard for transmission quality and availability of service on wireless networks. QoS is designed to provide better network service by minimizing network congestion; limiting Jitter, Latency, and Packet Loss; supporting dedicated bandwidth for time-sensitive or mission critical applications, and prioritizing wireless traffic for channel access.
As with all IEEE 802.11 working group standards, the goal is to provide a standard way of implementing QoS features so that components from different companies are interoperable.
The Devicescape Enterprise-Managed AP provides QoS based on the Wireless Multimedia (WMM) specification and Wireless Multimedia (WMM) standards, which are implementations of a subset of 802.11e features.
Both access points and wireless clients (laptops, consumer electronics products) can be WMM-enabled.
QoS Queues and Parameters to Coordinate Traffic Flow
Configuring QoS options on the Devicescape Enterprise-Managed AP consists of setting parameters on existing queues for different types of wireless traffic. You can configure different minimum and maximum wait times for the transmission of packets in each queue based on the requirements of the media being sent. Queues automatically provide minimum transmission delay for Voice, Video, multimedia, and mission critical applications, and rely on best-effort parameters for traditional IP data.
For example, time-sensitive Voice, Video, and multimedia are given effectively higher priority for transmission (lower wait times for channel access), while other applications and traditional IP data which are less time-sensitive but often more data-intensive are expected to tolerate longer wait times.
The Devicescape Enterprise-Managed AP implements QoS based on the IEEE Wireless Multimedia (WMM) standard. A Linux-based queuing class is used to tag packets and establish multiple queues. The queues provided offer built-in prioritization and routing based on the type of data being transmitted.
The Administration UI provides a way for you to configure parameters on the queues.
QoS Queues and Type of Service (ToS) on Packets
QoS on the Devicescape Enterprise-Managed AP leverages WMM information in the IP packet header related to Type of Service (ToS). Every IP packet sent over the network includes a ToS field in the header that indicates how the data should be prioritized and transmitted over the network. The ToS field consists of a 3 to 7 bit value with each bit representing a different aspect or degree of priority for this data as well as other meta-information (low delay, high throughput, high reliability, low cost, and so on).
For example, the ToS for FTP data packets is likely to be set for maximum throughput since the critical consideration for FTP is the ability to transmit relatively large amounts of data in one go. Interactive feedback is a nice-to-have in this situation but certainly less critical. VoIP data packets are set for minimum delay because that is a critical factor in quality and performance for that type of data.
The access point examines the ToS field in the headers of all packets that pass through the AP. Based on the value in a packet's ToS field, the AP prioritizes the packet for transmission by assigning it to one of the queues. This process occurs automatically, regardless of whether you deliberately configure QoS or not.
A different type of data is associated with each queue. The queue and associated priorities and parameters for transmission are as follows:
- Data 0 (Voice). Highest priority queue, minimum delay. Time-sensitive data such as Voice over IP (VoIP) is automatically sent to this queue.
- Data 1 (Video). High priority queue, minimum delay. Time-sensitive data such as Video and other streaming media are automatically sent to this queue.
- Data 2 (Best Effort). Medium priority queue, medium throughput and delay. Most traditional IP data is sent to this queue.
- Data 3 (Background). Lowest priority queue, high throughput. Bulk data that requires maximum throughput and is not time-sensitive is sent to this queue (FTP data, for example).
Packets in a higher priority queue will be transmitted before packets in a lower priority queue. Interactive data in the queues labeled "Data 0" and "Data 1" is always sent first, best effort data in "Data 2" is sent next, and Background (bulk) data in "Data 3" is sent last. Each lower priority queue (class of traffic) gets bandwidth that is left over after the higher classes of traffic have been sent. At an extreme end if you have enough interactive data to keep the access point busy all the time, low priority traffic would never get sent.
Using the QoS settings on the Administration UI, you can configure Enhanced Distribued Channel Access (EDCA) parameters that determine how each queue is treated when it is sent by the access point to the client or by the client to the access point.
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Note
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Wireless traffic travels:
- Downstream from the access point to the client station
- Upstream from client station to access point
- Upstream from access point to network
- Downstream from network to access point
With WMM enabled, QoS settings on the Devicescape Enterprise-Managed AP affect the first two of these; downstream traffic flowing from the access point to client station (AP EDCA parameters) and the upstream traffic flowing from the station to the access point (station EDCA parameters).
With WMM disabled, you can still set some parameters on the downstream traffic flowing from the access point to the client station (AP EDCA parameters).
The other phases of the traffic flow (to and from the network) are not under control of the QoS settings on the AP.
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EDCF Control of Data Frames and Arbitration Interframe Spaces
Data is transmitted over 802.11 wireless networks in frames. A Frame consists of a discrete portion of data along with some descriptive meta-information packaged for transmission on a wireless network.
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Note
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A Frame is similar in concept to a Packet, the difference being that a packet operates on the Network layer (layer 3 in the OSI model) whereas a frame operates on the Data-Link layer (layer 2 in the OSI model).
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Each frame includes a source and destination MAC address, a control field with protocol version, frame type, frame sequence number, frame body (with the actual information to be transmitted) and frame check sequence for error detection.
The 802.11 standard defines various frame types for management and control of the wireless infrastructure, and for data transmission. 802.11 frame types are (1) management frames, (2) control frames, and (3) data frames. Management and control frames (which manage and control the availability of the wireless infrastructure) automatically have higher priority for transmission.
802.11e uses interframe spaces to regulate which frames get access to available channels and to coordinate wait times for transmission of different types of data.
Management and control frames wait a minimum amount of time for transmission; they wait a short interframe space (SIF). These wait times are built-in to 802.11 as infrastructure support and are not configurable.
The Devicescape Enterprise-Managed AP supports the Enhanced Distribution Coordination Function (EDCF) as defined by the 802.11e standard. EDCF, which is an enhancement to the DCF standard and is based on CSMA/CA protocol, defines the interframe space (IFS) between data frames. Data frames wait for an amount of time defined as the arbitration interframe space (AIFs) before transmitting.
This parameter is configurable.
(Note that sending data frames in AIFs allows higher priority management and control frames to be sent in SIFs first.)
The AIFs ensures that multiple access points do not try sending data at the same time but instead wait until a channel is free.
Random Backoff and Minimum / Maximum Contention Windows
If an access point detects that the medium is in use (busy), it uses the DCF random backoff timer to determine the amount of time to wait before attempting to access a given channel again. Each access point waits some random period of time between retries. The wait time (initially a random value within a range specified as the Minimum Contention Window) increases exponentially up to a specified limit (Maximum Contention Window). The random delay avoids most of the collisions that would occur if multiple APs got access to the medium at the same time and tried to transmit data simultaneously. The more active users you have on a network, the more significant the performance gains of the backoff timer will be in reducing the number of collisions and retransmissions.
The random backoff used by the access point is a configurable parameter. To describe the random delay, a "Minimum Contention Window" (MinCW) and a "Maximum Contention Window" (MaxCW) is defined.
- The value specified for the Minimum Contention Window is the upper limit of a range for the initial random backoff wait time. The number used in the random backoff is initially a random number between 0 and the number defined for the Minimum Contention Window.
- If the first random backoff time ends before successful transmission of the data frame, the access point increments a retry counter, and doubles the value of the random backoff window. The value specified in the Maximum Contention Window is the upper limit for this doubling of the random backoff. This doubling continues until either the data frame is sent or the Maximum Contention Window size is reached.
Packet Bursting for Better Performance
The Devicescape Enterprise-Managed AP includes 802.11e based packet bursting technology that increases data throughput and speed of transmission over the wireless network. Packet bursting enables the transmission of multiple packets without the extra overhead of header information. The effect of this is to increase network speed and data throughput. The size of packet bursts allowed (maximum burst length) is a configurable parameter.
Transmission Opportunity (TXOP) Interval for Client Staions
The Transmission Opportunity (TXOP) is an interval of time when a Wi-Fi Multimedia (WMM) client station has the right to initiate transmissions onto the wireless medium (WM).
Navigating to QoS Settings
To set up queues for QoS, navigate to the Advanced > Quality of Service tab, and configure settings as described below.
Configuring QoS Queues
Configuring Quality of Service (QoS) on the Devicescape Enterprise-Managed AP consists of setting parameters on existing queues for different types of wireless traffic, and effectively specifying minimum and maximum wait times (via Contention Windows) for transmission. The settings described here apply to data transmission behavior on the access point only, not to that of the client stations.
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Notes
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- For the Guest interface, QoS queue settings apply to the access point load as a whole (both BSSes together).
- On a two-radio access point these settings apply to both radios but the traffic for each radio is queued independently. (The exception to this is guest traffic as noted below.)
- Internal and Guest network traffic is always queued together within each radio. This is the case on both one-radio and two-radio APs.
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Configuring Quality of Service includes:
Configuring AP EDCA Parameters
AP Enhanced Distributed Channel Access (EDCA) Parameters affect traffic flowing from the access point to the client station.
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Field
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Description
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Queue
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Queues are defined for different types of data transmitted from AP-to-station:
Data 0 (Voice)
High priority queue, minimum delay. Time-sensitive data such as VoIP and streaming media are automatically sent to this queue.
Data 1(Video)
High priority queue, minimum delay. Time-sensitive video data is automatically sent to this queue.
Data 2 (best effort)
Medium priority queue, medium throughput and delay. Most traditional IP data is sent to this queue.
Data 3 (Background)
Lowest priority queue, high throughput. Bulk data that requires maximum throughput and is not time-sensitive is sent to this queue (FTP data, for example).
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AIFs
(Inter-Frame Space)
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The Arbitration Inter-Frame Spacing (AIFs) specifies a wait time (in milliseconds) for data frames.
Valid values for AIFs are 1 through 255.
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cwMin
(Minimum Contention Window)
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This parameter is input to the algorithm that determines the initial random backoff wait time ("window") for retry of a transmission.
The value specified here in the Minimum Contention Window is the upper limit (in milliseconds) of a range from which the initial random backoff wait time is determined.
The first random number generated will be a number between 0 and the number specified here.
If the first random backoff wait time expires before the data frame is sent, a retry counter is incremented and the random backoff value (window) is doubled. Doubling will continue until the size of the random backoff value reaches the number defined in the Maximum Contention Window.
Valid values for the "cwmin" are 1, 3, 7, 15, 31, 63, 127, 255, 511, or 1024. The value for "cwmin" must be lower than the value for "cwmax".
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cwMax
(Maximum Contention Window)
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The value specified here in the Maximum Contention Window is the upper limit (in milliseconds) for the doubling of the random backoff value. This doubling continues until either the data frame is sent or the Maximum Contention Window size is reached.
Once the Maximum Contention Window size is reached, retries will continue until a maximum number of retries allowed is reached.
Valid values for the "cwmax" are 1, 3, 7, 15, 31, 63, 127, 255, 511, or 1024. The value for "cwmax" must be higher than the value for "cwmin".
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Max. Burst Length
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AP EDCA Parameter Only (The Max. Burst Length applies only to traffic flowing from the access point to the client station.)
This value specifies (in milliseconds) the Maximum Burst Length allowed for packet bursts on the wireless network. A packet burst is a collection of multiple frames transmitted without header information. The decreased overhead results in higher throughput and better performance.
Valid values for maximum burst length are 0.0 through 999.9.
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Enabling/Disabling Wi-Fi Multimedia
By default, Wi-Fi MultiMedia (WMM) is enabled on the access point. With WMM enabled, QoS prioritization and coordination of wireless medium access is on. With WMM enabled, QoS settings on the Devicescape Enterprise-Managed AP control downstream traffic flowing from the access point to client station (AP EDCA parameters) and the upstream traffic flowing from the station to the access point (station EDCA parameters).
Disabling WMM will deactivate QoS control of station EDCA parameters on upstream traffic flowing from the station to the access point
With WMM disabled, you can still set some parameters on the downstream traffic flowing from the access point to the client station (AP EDCA parameters).
- To disable WMM exentsions, click Disabled.
- To enable WMM extensions, click Enabled.
Configuring Station EDCA Parameters
Station Enhanced Distributed Channel Access (EDCA) Parameters affect traffic flowing from the client station to the access point.
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Field
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Description
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Queue
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Queues are defined for different types of data transmitted from station-to-AP:
Data 0 (Voice)
Highest priority queue, minimum delay. Time-sensitive data such as VoIP and streaming media are automatically sent to this queue.
Data 1(Video)
Highest priority queue, minimum delay. Time-sensitive video data is automatically sent to this queue.
Data 2 (best effort)
Medium priority queue, medium throughput and delay. Most traditional IP data is sent to this queue.
Data 3 (Background)
Lowest priority queue, high throughput. Bulk data that requires maximum throughput and is not time-sensitive is sent to this queue (FTP data, for example).
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AIFs
(Inter-Frame Space)
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The Arbitration Inter-Frame Spacing (AIFs) specifies a wait time (in milliseconds) for data frames.
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cwMin
(Minimum Contention Window)
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This parameter is input to the algorithm that determines the initial random backoff wait time ("window") for retry of a transmission.
The value specified here in the Minimum Contention Window is the upper limit (in milliseconds) of a range from which the initial random backoff wait time is determined.
The first random number generated will be a number between 0 and the number specified here.
If the first random backoff wait time expires before the data frame is sent, a retry counter is incremented and the random backoff value (window) is doubled. Doubling will continue until the size of the random backoff value reaches the number defined in the Maximum Contention Window.
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cwMax
(Maximum Contention Window)
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The value specified here in the Maximum Contention Window is the upper limit (in milliseconds) for the doubling of the random backoff value. This doubling continues until either the data frame is sent or the Maximum Contention Window size is reached.
Once the Maximum Contention Window size is reached, retries will continue until a maximum number of retries allowed is reached.
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TXOP Limit
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Station EDCA Parameter Only (The TXOP Limit applies only to traffic flowing from the client station to the access point.)
The Transmission Opportunity (TXOP) is an interval of time when a WME client station has the right to initiate transmissions onto the wireless medium (WM).
This value specifies (in milliseconds) the Transmission Opportunity (TXOP) for client stations; that is, the interval of time when a WMM client station has the right to initiate transmissions on the wireless network.
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Updating Settings
To apply your changes, click Update Settings.
