February 2000 doc.: IEEE 802.11-00/028
IEEE P802.11
Wireless LANs
IEEE 802.11 Quality of Service – White Paper
February 15, 2000 Date:
Author(s): Arun Ayyagari, Yoram Bernet, Tim Moore
Microsoft Corporation
One Microsoft Way, Redmond, WA 98052-6399
Phone: (425) 936-8080
Fax: (425) 936-7329 e-Mail: aruna@microsoft.com
, yoramb@microsoft.com
, timmoore@microsoft.com
Abstract
IEEE 802.11 provides high bandwidth connectivity in a local area network (LAN) environment that is suitable for most data applications. However, it does not meet the quality of service (QoS) requirements for real-time data traffic applications such as voice and video transmissions. There have been recent proposals on enhancements to the IEEE 802.11 Medium access control (MAC) protocol to provide expedited access to the shared wireless network for real-time data transmissions. However, these proposals would require changes to the IEEE 802.11 standard that may potentially impact backward compatibility and future adoption of the IEEE 802.11 wireless
LAN. Therefore, in this white paper we propose the combined use of resource based admission and traffic flow control and expedited transmission of higher priority data over the shared wireless network as the mechanisms to provide QoS guarantee for real-time application data traffic in IEEE 802.11. Our underlying objective was to define a QoS approach without any changes to the fundamental IEEE 802.11 MAC protocol.
Submission page 1 A. Ayyagari, et. al., Microsoft Corp.

February 2000
doc.: IEEE 802.11-00/028
IEEE 802.11 is a shared wireless local area network (LAN) standard using the carrier sense multiple access (CSMA) medium access control (MAC) protocol with collision avoidance (CA)
[1]. The standard allows for both direct sequence (DS) and frequency-hopping (FH) spread
spectrum transmissions at the physical layer. The maximum data rate offered initially by the standard was 2 Mb/s. However, a higher speed version with a physical layer definition under the
IEEE 802.11b specification allows a data rate of up to 11 Mb/s using DS spread spectrum. The
IEEE standards committee has also defined physical layer specifications under the IEEE 802.11a specification based on orthogonal frequency-division multiplexing (OFDM) that will permit data transfer rates up to 54 Mb/s. While IEEE 802.11 provides high bandwidth connectivity in a LAN environment that is suitable for most data applications, it does not meet Quality of Service (QoS) requirements for real-time data traffic applications such as voice and video transmissions.
IEEE 802.11 wireless station (STA) is intended to operate in either of the following two modes:

Ad hoc network mode: STAs within a basic service set (BSS) are able to communicate directly with peer STAs within the BSS.

Infrastructure network mode: STAs communicate only with an access point (AP) in the
BSS. An AP provides connectivity to distribution system (DS) services in addition to performing as an STA. STAs within a BSS communicate with other stations within and outside the BSS via the AP. Therefore, all communication from/to STAs are directed to/from the AP in the BSS.
We envision that in most wireless environments such as corporations and public places, infrastructure network mode will be predominantly used. Usage of the infrastructure network mode is driven by the need to provide access control, accounting, and content provisioning mechanisms in wireless environment that follow the client/server interaction paradigm between the wireless station and a remote server. Ad hoc network mode would still be used in areas such as home networking environments. In this white paper, we will focus our discussion on the infrastructure network mode, however a many of the approaches and conclusions pertaining infrastructure network mode would be equally applicable to the ad hoc network mode.
The need for resource based admission and traffic flow control is of greater importance in wireless networks because bandwidth in wireless networks is more constrained compared to wired networks. Since an AP and all the STAs communicating with the particular AP are given equal access priority to the shared wireless network in IEEE 802.11 infrastructure network mode, the AP becomes a bottleneck for typical client/server interactions. While the STA’s would be able to transmit data to the AP, data destined to the STAs would be queued at the AP since it has to service all the STAs within its BSS with access priority similar to the STAs. Therefore in addition to the latency and bandwidth requirements for application level data traffic, characteristics of wireless communication and network architecture evinces the need for QoS
Submission page 2 A. Ayyagari, et. al., Microsoft Corp.

February 2000 doc.: IEEE 802.11-00/028 mechanisms in IEEE 802.11. This white paper present an alternative approach to provide QoS guarantees to application level data traffic in IEEE 802.11 wireless LAN.
A QoS guarantee for real-time application data traffic is currently not available in IEEE 802.11.
There have been recent proposals from Sharewave (Whitecap protocol) [2] and Lucent (Black-
Burst protocol) [3] on enhancements to the IEEE 802.11 MAC protocol to provide expedited
access to the shared wireless network for real-time data transmissions. However, both these proposals would require changes to the IEEE 802.11 standard that may potentially impact backward compatibility and future adoption of the IEEE 802.11 wireless LAN. Therefore, we believe that it would be prudent to avoid or at the least limit the changes to the IEEE MAC
802.11 protocol by adopting QoS approaches that do not warrant major changes.
We propose the combined use of resource based admission and traffic flow control and expedited transmission of higher priority data over the shared wireless network as the mechanisms to provide QoS guarantee for real-time application data traffic in IEEE 802.11. The resource based admission and traffic flow control mechanism manages the number and traffic flow characteristics of peer application sessions to ensure that their QoS requirements are satisfied.
The expedited transmission of higher priority frames by the STAs over the shared wireless network ensures that the latency requirements of real-time applications are satisfied. This also results in smaller latency variability for real-time applications that leads to a better utilization of the shared wireless network by accommodating a greater number of real-time peer application sessions.
Resource Based Admission and Traffic Flow Control
In order to maintain QoS guarantees, the resource based admission and traffic flow control would assist in the management of application data traffic transmissions over the shared wireless
network. In the proposed approach resource reservation protocol (RSVP) signaling [4-5] and the subnet bandwidth manager (SBM) [6] would be used to establish end-to-end, per-conversation,
QoS to peer applications between the IEEE 802.11 STA and a remote station. While RSVP is a layer-3 protocol, SBM is a layer-2 protocol that extends the functionality of RSVP to the shared wireless network (between the STAs and AP) and beyond. In addition to the admission and traffic flow control, the RSVP signaling describing the traffic characteristics is sent to the network devices for aggregate traffic handling mechanisms suitable for IEEE 802.11 shared wireless network.
Figure 1 illustrates a typical IEEE 802.11 architecture that consists of a number of BSSs that are interconnected via the DS forming the extended service set (ESS). All the STAs in the ESS are members of a single subnet. In a typical deployment, the APs within the ESS are interconnected via layer-2 switched network, for example, using a point-to-point full-duplex fast Ethernet connection between a given AP and a layer-2 switching device at the core of the network in the
DS. Resource based admission and traffic flow control within the ESS may be aggregated across
Submission page 3 A. Ayyagari, et. al., Microsoft Corp.

February 2000 doc.: IEEE 802.11-00/028 the entire DS for all BSSs or specific to each BSS resulting in SBM protocol being enabled at
various densities within the IEEE 802 network [7]. Also, each STA within the BSS is expected to
have full SBM client functionality in order to maintain control of the IEEE 802.11 shared wireless network resources.
Aggregation of resource based admission and traffic flow control across the entire DS results in the lowest density where a single designated SBM (DSBM) in the core of the layer-2 network acts as the admission control agent for the entire layer-2 subnet. In this scenario the SBM client at each STA within the BSS detects the presence of a DSBM in the DS (core network) and routes
RSVP signaling requests via the DSBM. In addition, the DSBM may also be configured to advertise a NonResvSendLimit on the managed subnet to limit the maximum rate at which STAs may transmit in the absence of approved reservation.
At the other extreme, each AP is an admission control agent (DSBM) for the STAs within a BSS.
In this case, the DSBM election protocol will result in the division of the ESS such that each
BSS becomes a managed segment managed by an AP (DSBM). The higher density of DSBMs results in better management of the shared wireless network resources within each BSS.
However, this would require that each AP should include the SBM server functionality on the port that interfaces with the STAs within its BSS. Similar to the single DSBM scenario, all STAs within the BSS detects the presence of the DSBM in the AP and route RSVP signaling requests via the DSBM. Since all the traffic from STAs would normally be routed via the AP this does not deviate from the normal data traffic path. Also, the DSBM co-located with the AP may also be configured to advertise a NonResvSendLimit on the managed subnet to limit the maximum rate at which STAs may transmit in the absence of approved reservation.
Relative to wired LANs, bandwidth is more severely constrained in IEEE 802l.11 shared wireless LAN. Therefore, in order to guarantee QoS between the STA and a remote station, resource based admission and traffic flow control between the STA and the AP is essential. Since data traffic from all STAs within a BSS has to flow via the AP, it would be natural for the AP to assume the role of an admission control agent (DSBM) for the particular BSS. One may implement a single DSBM in the DS as an initial approach. However, DSBM functionality should be co-located with the APs to avail of the optimal admission control. In general, wireless stations in infrastructure network mode communicate with an AP that provides them with connectivity to other stations on the wired and wireless networks beyond the AP. Therefore, this concept of co-locating admission control agent (DSBM) at APs is valid for wireless networks in general.
Submission page 4 A. Ayyagari, et. al., Microsoft Corp.

February 2000
Expedited Transmission Mechanism doc.: IEEE 802.11-00/028
Expedited transmission mechanism utilizes the description of the traffic characteristics sent to the network devices via RSVP signaling to tag frames into specific priorities using the IEEE
802.1p aggregate traffic handling mechanism. The IEEE 802.1p tag carries one of eight priority values that correspond to one of eight possible service levels and its scope is limited to the subnet in which they were generated. In the case of IEEE 802.11, the IEEE 802.1p tag generated by a STA will be valid throughout its subnet that normally consists of other STAs and APs as illustrated in a typical IEEE 802.11 architecture shown in Figure 1. The network devices transmit the frames based on the non-gated head-of-line priority scheme where transmission of the frame with the highest priority in the simple first in first out (FIFO) priority queue is scheduled ahead of the rest of the queued frames. More sophisticated buffer management and transmission
BSS 1
STA 1
IEEE 802.11 MAC/PHY
STA 2
AP
ESS
DS
Portal AP
STA 3
IEEE 802.11 MAC/PHY
STA 4
802.x LAN
BSS 2
IEEE 802.11 Components
Figure 1: Complete IEEE 802.11 Architecture schemes beyond the simple FIFO priority queues and non-gated head-of-line priority scheme may be used to schedule frame transmissions, however, one would have to weigh the increased complexity of the buffer management and frame transmission scheduling algorithm with the performance benefits under various network and data traffic characteristics. In order to minimize
Submission page 5 A. Ayyagari, et. al., Microsoft Corp.

February 2000 doc.: IEEE 802.11-00/028 the complexity of the network interface cards we suggest the use of the non-gated head-of-line priority scheme using simple FIFO priority queues since it is normally adequate for most network and data traffic characteristics.
There are two approaches to providing expedited transmission for higher priority frames in IEEE
802.11 shared wireless network. The first approach is based on the fundamental access method based on CSMA/CA called distributed coordinated function (DCF) while the alternate approach is based on the point coordination function (PCF).
DCF Access Approach
IEEE 802.11 MAC protocol consists of a fundamental access method based on CSMA/CA called
DCF that is used in both ad hoc and infrastructure network modes. An STA upon determining that the shared wireless network is busy defers transmissions until the medium is determined to be idle for a period of time (DCF interframe space – DIFS or extended interframe space – EIFS) as defined by the IEEE 802.11 standard. After the idle period, the STA generates a random backoff period before initiating the backlogged frame transmission. This additional deferral is intended to minimize collisions during contention between multiple STAs. The backoff period is a function of the pseudorandom integer and aSlotTime (function of the shared wireless network round-trip propagation delay). The pseudorandom integer is drawn from a uniform distribution over the interval [0, CW], where contention window (CW) parameter ranges from a minimum value of aCWmin and a maximum value of aCWmax. The CW parameter takes an initial value of aCWmin that is incremented upon each consecutive unsuccessful transmission attempt by a
STA until it reaches the maximum value of aCWmax. CW is again reset to aCWmin after every successful transmission.
Our proposal to provide expedited transmission of higher priority backlogged frames is to modify the minimum and maximum values of the CW parameter to ensure that backlogged higher priority frames have statistically a greater probability of transmission over backlogged lower priority frames. IEEE 802.1p tag has eight priority levels ranging from 0 at the lowest priority level to 7 for the highest priority level. The proposed approach suggests that for each of the priority levels a distinct set of aCWmin and aCWmax values are defined that follow the relationship given below: aCWmin i
> aCWmin j
, where priority level i < priority level j aCWmax i
> aCWmax j
, where priority level i < priority level j
The monotonic decrease in the aCWmin and aCWmax values for increasing priority levels ensures that higher priority backlogged frames have a higher probability of transmission over backlogged lower priority frames within an IEEE 802.11 BSS. Implementation of this mechanism does not alter the IEEE 802.11 MAC protocol while providing prioritized frame transmissions for peer applications requiring greater QoS. From the network device vendor
Submission page 6 A. Ayyagari, et. al., Microsoft Corp.

February 2000 doc.: IEEE 802.11-00/028 perspective they would have to incorporate the following enhancements to the network interfaces:

Access the priority level of the frame to be transmitted [8];

Define multiple FIFO buffers, one for each priority level;

Define distinct set of aCWmin and aCWmax value for each priority level;

Define non-gated, head-of-line priority scheme to schedule frame transmissions.
This mechanism also enables continued expedited transmission of higher priority frames from ongoing sessions as the STA traverses from one BSS to another. The process ensures that QoS guarantees of ongoing sessions are not adversely impacted due to STA handoff from one AP to another within its subnet.
PCF Access Approach
IEEE 802.11 MAC protocol also includes an optional access method called PCF that is only usable on infrastructure network mode configurations. In this method, a point coordinator (PC) function at the AP of the BSS defines the schedule for STA transmissions without contention.
The PC performs the function of the polling master to provide contention-free (CF) access to
STA transmissions for a limited period of time. The PCF uses a virtual carrier sense mechanism along with access priority mechanism by using shorter idle period (interframe space – IFS) than frames transmitted via DCF. Also the PCF operates without CSMA/CA contention window randomization and backoff of the DCF. This is the alternate mechanism that may be used for expedited transmission of higher priority backlogged frames in the infrastructure network mode configuration. The PCF access procedure however has some drawbacks such as lower throughput efficiency inherent in polling mechanisms and the potential for repeated collisions if multiple, overlapping, point-coordinated BSSs are operating on the same physical layer channel.
We note that while PCF transfer procedure may be suitable for streaming transmissions such as isochronous data, its use may not be particularly advantageous to a large number of peer applications whose QoS requirements can satisfied via the expedited transmission using the DCF mechanism discussed in the DCF access approach section. Also, since PCF is an optional access method its availability under all network scenarios may not be guaranteed. Maintaining QoS guarantees for an ongoing session using PCF as the STA traverses from one BSS to another would require coordination between the PC functions within peer APs. Since the coordination of ongoing session handoff from one AP to another is not defined in addition to the coordination for operation in multiple point-coordinated BSSs on the same physical layer channel, the PCF transfer procedure would require extensive protocol development effort to standardize the coordination functions and ensure interoperability between APs.
Submission page 7 A. Ayyagari, et. al., Microsoft Corp.

February 2000
doc.: IEEE 802.11-00/028
IEEE 802.11 provides high bandwidth connectivity in a LAN environment that is suitable for most data applications, however, it does not meet QoS requirements for real-time data traffic applications such as voice and video transmissions. We propose the combined use of resource based admission and traffic flow control and expedited transmission of higher priority data over the shared wireless network as the mechanisms to provide QoS guarantee for real-time application data traffic in IEEE 802.11. We propose the use of RSVP signaling and the SBM to establish end-to-end, per-conversation, QoS to peer applications between the IEEE 802.11 wireless station and a remote station. IEEE 802.11 wireless stations in the infrastructure network mode communicate with an access point that provides them with connectivity to other stations on the wired and wireless networks beyond the access point. Therefore, we also suggest the colocation of the admission control agent (DSBM) at access points to guarantee QoS over the bandwidth constrained shared wireless network. The expedited transmission of higher priority frames by the IEEE 802.11 wireless stations over the shared wireless network ensures that the latency requirements of real-time applications are satisfied. This also results in smaller latency variability for real-time applications that leads to a better utilization of the shared wireless network by accommodating a greater number of real-time peer application sessions. There are two approaches to providing expedited transmission for higher priority frames in IEEE 802.11 shared wireless networks. The first approach is based on the fundamental access method based on CSMA/CA called distributed coordination function while the alternate approach is based on the point coordination function. Our suggested proposal to provide expedited transmission of higher priority backlogged frames over the shared wireless network is to modify the minimum and maximum values of the contention window parameter in the distributed coordination function approach to ensure that backlogged higher priority frames have statistically a greater probability of transmission over backlogged lower priority frames. We also conclude that while point coordination function approach may be suitable for streaming transmissions, such as isochronous data, its use would require extensive protocol development effort to standardize the coordination functions and ensure interoperability between access points.
We would like to thank Victoria Poncini for reviewing this white paper and providing helpful comments.
1.
Part 11: Wireless LAN Media Access Control (MAC) and Physical Layer (PHY)
Specifications , Draft International Standard ISO/IEC 8802-11, IEEE 802.11/D10, 14
January 1999.
2.
Whitecap Network Protocols , http://www.sharewave.com/technology/whitecap/whitecap.html
.
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February 2000 doc.: IEEE 802.11-00/028
3.
J. Sobrinho and A. Krishnakumar, “Quality-of-Service in Ad Hoc Carrier Sense Multiple
Access Wireless Networks,” IEEE Journal on Selected Areas in Communications, Vol.
17, No. 18, pp. 1353-1368, Aug. 1999.
4.
D. Durham and R. Yavatkar,
Inside the Internet’s Resource reSerVation Protocol
, John
Wiley & Sons, Inc., 1999.
5.
Y. Bernet, “The Complementary Roles of RSVP and Differentiated Services in the Full-
Service QoS Network,” IEEE Communications Magazine, Vol. 38, No. 2, pp. 154-162,
Feb. 2000.
6.
R. Yavatkar, D. Hoffman, Y. Bernet, F. Baker, and M. Speer, “SBM (Subnet Bandwidth
Manager): A protocol for RSVP-based Admission Control over IEEE 802-style networks, draft-ietf-issll-is802-sbm-10.txt,” IETF Internet-Draft, January 2000.
7.
Quality of Service Technical Overview , http://www.microsoft.com/windows2000/library/howitworks/communications/trafficmg mt/qos.asp
, Sept. 1999.
8.
IEEE 802.1P Packet Priority, http://www.microsoft.com/DDK/DDKdocs/Win2k/204_802p_1fxj.htm
.
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