IEEE 80211 Wireless LANs.docx

1、IEEE 80211 Wireless LANsIEEE P802.11Wireless LANsIEEE 802.11 Quality of Service White PaperDate: February 15, 2000Author(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, yoramb, timmoor

2、e AbstractIEEE 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 h

3、ave 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 backwa

4、rd 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 g

5、uarantee 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.OverviewIEEE 802.11 is a shared wireless local area network (LAN) standard using the carrier sense multiple access (CS

6、MA) 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 vers

7、ion 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

8、 (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

9、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 p

10、oint (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 en

11、vision 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

12、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 appr

13、oaches 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 wi

14、red 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 STAs would be able to transmit data

15、 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

16、and network architecture evinces the need for QoS 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.Approach A QoS guarantee for real-time application data traffic is currently not avail

17、able 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 wo

18、uld 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

19、 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. T

20、he 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 ensur

21、es 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 Ba

22、sed 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) s

23、ignaling 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 w

24、ireless 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 il

25、lustrates 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 ne

26、twork, 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 the entire DS for all BSSs or specific to

27、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 admissi

28、on 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

29、 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

30、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 net

31、work 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 signal

32、ing 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 contro