WLAN QoS

1. WLAN QoS Ronald Lucas

2. Introduction • With the emergence of Voice Over IP, requirements to support Voice Over IP over Wireless LAN’s without degradation of it’s quality of service has become very important. • A quality of service focused MAC Layer standard, IEEE 802.11e was developed to meet these requirements.

3. Introduction cont. • Voice over IP over Wireless LAN’s is more compelling since you can install these Wireless LAN’s in public spaces, which are backed up by the Internet. • This allows for users to have telephone service without wires anywhere that the Wireless LAN is located.

4. Introduction cont. • IEEE 802.11e enables frames from quality of service sensitive applications to be transmitted sooner than other frames, which minimizes latency. • IEEE 802.11e also has new power management features that will help with the life of the mobile device being used. • IEEE 802.11e has channel-use efficiency gains, which allow for privileged treatment to any type. • IEEE 802.11e also prioritizes time-critical data. • IEEE 802.11e will work with 802.11-compliant devices.

5. Channel Access • IEEE 802.11e provides contention-based and polled access mechanisms, these are both enhancements to the 802.11 mechanisms. • This leads to the reduction of access delay and jitter in delivering QoS-sensitive frames from the source to the destination. • WLAN served by an AP, allow stations to communicate directly with one another. • Transmit Opportunity (TXOP), allows a station to transmit a sequence of frames without having to contend for a channel, following a successful channel access attempt.

6. Legacy Contention-Based Channel Access • The legacy contention-based access mechanism, distributed coordination function, each station listens to the channel and if it is busy, postpones the transmission and enters the backoff procedure. • This is done by deferring transmissions by a random time, which avoids collision between multiple stations, which would otherwise attempt to transmit after the completion of the current transmission.

7. Contention-Based Channel Access • This extends the 802.11 standard to provide frame prioritization, frames of higher priority will access the channel sooner. • MAC protocol for 802.11e contention-based channel access is CSMA/CA, which use the parameters contention window min and contention window max, which the values assigned cause the window to be shorter or longer. • 802.11e will allow a station consecutive transmissions of frames from the same access category without the need for backoff by using spacing between consecutive frames. • 802.11e will drop frames once the expiration of the frames based on the time queued has been meet.

8. TCMA MAC Protocol • CSMA/CA protocol, a station engaged in backoff countdown must wait while the channel is idle for time equal to DIFS before decrementing its backoff immediately following a busy period, or before attempting transmission. • TCMA (Tiered-Contention Multiple Access) protocol, variable lengths of this time interval, which is called Arbitration-Time Inter-Frame Space (AIFS), lead to varying degree of accessibility to the channel. • A shorter AIFS will give a station an advantage in contending for channel access.

9. 802.11e Polled Channel Access • The improved PCF polled channel access mechanism is the 802.11e HCF controlled channel access. • It resembles the PCF, but polling is not limited to the contention-free period, the polling schedule is tailored to the time profile of the individual traffic streams which reduce overhead, delay, and jitter, and uplink transmit opportunity (TXOPs) cause frames to be transmitted sooner. • 802.11e polled access mechanism results in a polling schedule that better matches the generation of frames in a periodic traffic stream, allowing for superior delay/jitter performance and better channel use efficiency.

10. Admission Control • Admission control provides bandwidth management to ensure that QoS-sensitive applications, such as voice and video, will have satisfactory QoS. • Overloading WLAN’s with excessive number of users with high-priority access would lower the QoS, therefore requests are submitted by stations for the admission to specific traffic streams to the AP, which accepts or declines.

11. Admission Control for Contention-Based Channel Access • This is an optional feature for a station and AP, the AP decides to allow stations that employ contention-based access in the WLAN to transmit traffic using the parameters of an access category. • The AP then can track and manage bandwidth use and it is also not necessary to impose admission control on all access categories. • The AP advertises to the WLAN the access categories that are protected by the admission control. • A station’s request, submitted by and add Traffic Specification (ADDTS) frame, describes the ‘traffic stream’ to be admitted. The response to the ADDTS, if accepted, furnishes in the Medium Time field the ‘channel time’ the station is allocated for uplink transmissions.

12. Admission Control for Polled Channel Access • Admission Control is automatic when using a polled channel access, the AP will reject an add Traffic Specification (ADDTS) request if it cannot meet the requirements for a service period schedule requested by a station for a traffic stream. • The priority of a traffic stream may be considered in admission control, an admission control request from traffic stream with a higher priority may cause an admitted stream to be dropped.

13. Power Management • 802.11e standard amendment offers new mechanisms to help battery-powered devices conserve power by enabling them to power down their receivers and transmitters intermittently without loosing connectivity or data. • A station will inform the AP of being in either ‘power saving’ mode, or ‘active’. • This is done by changing the Power Management bit in the frame control field, when the frame is being sent by the station.

14. Legacy Power-Save Mechanism • Frames buffered at the AP for a power-saving station employing contention-based access are delivered when the station sends a special control frame, the power save poll. • The AP sends a single buffered frames to a station after receiving a power save poll, immediately or soon after. • More power save polls are required in order ro retrieve additional buffered frames. • Stations using legacy power-save mode rely on traffic indication map (TIM) to learn if the AP holds buffered data for it. • A power-saving station that supports legacy polled access need not send PS-polls in order to retrieve its buffered frames.

15. Automatic Power Save Delivery • This is the delivery of unicast frames from the SP to a power-saving station, this is done to reduce the signaling traffic caused by power save poll and their acknowledgements. • The AP may deliver buffered frames to their destination power-saving stations either on a previously negotiated time or in response to receiving transmissions from the station that trigger such delivery.

16. Scheduled Automatic Power Save Delivery • This mechanism is good for voice, and audio/video, because the downlink transmissions to power-saving devices will occur at a schedule that is known in advance, thus obtaining the need for special signaling between the station and AP. • The AP and the station negotiate a time in advance in which the station will be fully powered and will be able to receive any frames that are buffered for it at the AP.

17. Full Unscheduled Automatic Power Save Delivery • This was introduced for stations accessing the channel by contention, to enhance the efficiency of legacy power save. • A power-saving station may use a PS-Poll, any data, and Null frame (trigger frames) to notify the AP that it is fully powered and ready to receive transmissions. • The same as legacy power-save mechanism, a station can learn about its buffer status by listening to the beacons for its TIM.

18. Hybrid Unscheduled Automatic Power Save Delivery • This allows a station to choose between legacy power save delivery and automatic power save delivery based on access category. • Trigger frames are used to initiate the delivery of buffered frames associated with access categories that have been designated ‘delivery enabled’. • The station also designates in advance the access categories of the frames that may serve as trigger frames.

19. QoS in Wireless Mesh Networks • Wireless mesh takes on many forms, the most common is a collection of nodes that form an ad hoc network and are capable of serving as WLAN APs. • These nodes are called ‘mesh points’, can forward traffic received from 802.11 stations to other mesh points with ultimate destinations that include WLAN stations attached to other mesh points somewhere on the wired networks. • Protocols for forwarding, routing, and channel access must be specified for the mesh points, which requires an ad hoc networking standard with multi-hop capability. • A wireless mesh network can be used to enable WLAN service when wiring for APs is not readily available in an enterprise, or for a temporary network that can be easily set up and torn down. • There are challenges in routing and security, especially when mobility is contemplated. • It is not clear what choices will ultimately be made for the IEEE 802.11 mesh standard.