802.11e 学习笔记

关于802.11e (QoS)规范的重点知识点解读

802.11e简介

在IEEE 802.11e,有两种类型的QoS:

  1. prioritized QoS

    a weak requirement

  2. parameterized QoS

    a strict requirement

new field in 802.11 header

TID (traffic ID)

select a UP (user priority) for prioritized QoS or a TSPEC (traffic specification) for parameterized

0 – 7 are considered user priorities

8 – 16 refer to TSPECs.

Action

A TSPEC between an AP and an STA is negotiated by means of new management commands – ADDTS Request, ADDTS Response and DELTS.

After a TSPEC is successfully negotiated, an STA can get a TXOP by one of two ways:

  1. Using EDCA, an STA can contend for the medium, and if it acquires the medium can use the medium for TXOP time limit. The STA must observe the TXOP time limit as specified in an IE in beacons.
  2. During CP/CFP, an AP can grant an STA a TXOP using the QoS CF-Poll (called as “polled TXOP” in contrast to “EDCA TXOP”). The TXOP limit is specified in the CF-Poll frame. While the spec allows an AP to grant a “polled TXOP” to an STA during either CFP or CP, it is recommended that it not be issued during CFP, but only during CP for reasons of simplicity in implementation.

WiFi Multimedia (WMM)

supports only EDCA but not HCCA. Without HCCA, parametererized QoS can not be supported.

Enhanced Distributed Channel Access (EDCA)

Basic Elements for QoS

  • Traffic Differentiation
  • Concept of Transmission Opportunity (TXOP)
    1. 一個傳送機會(TXOP)是一個有限的時間區域。在這時間區域內,一工作 站(station)可儘可能地傳送訊框(frames)。
    2. 若訊框(frame)太大而超過傳送機會(TXOP)的時間區域則必須切割分段 (fragment)成較小的訊框(frames)來傳送。
    3. 若傳送機會(TXOP)的時間區域為0時只能傳送一個資料封包。

New Contention Channel Access

  • Enhanced Distributed Channel Access (EDCA)
    1. EDCA提供在802.11中競爭基礎(Contention-Based)頻道存取機制, 排定優先順序(priority)的品質服務(QoS)。此優先順序是以區分訊務 流量類別而實現。802.1D的8個訊務流量類別(Traffic Classes, TC)對 應至4個(語音、視訊、盡力傳送、背景)802.11e的存取類別(Access Categories, AC)。訊務流量類別被指派至不同的資料佇列(Queue),每 個佇列根據下列基礎指定優先順序及使用時間存取媒介:
      • 仲裁內框間隔(Arbitration Inter Frame Spacing, AIFS)
      • 競爭窗框(Contention Window, CWmin/CWmax)
      • 傳輸機會(Transmission Opportunity, TXOP)
    2. AIFS時間短的能提早獲得媒介。傳輸機會(TXOP)為指派給節點傳 輸無線訊框的時間長度。優先順序更高的流量,其等待的AIFS時 槽越小,而且退後等待(Backoff)時間也更短。TXOP的長度對應在 特定優先權所預期的串流類型。
    3. 許可(進入)控制(Admission Control)透過傳輸規格保護優先次序高 的訊務流量類別不會受其他優先次序較低的流量影響。

New Contention Free Channel Access

  • HCF Controlled Channel Access (HCCA)

    HCCA是HCF的一構成要素(component) 同時提供服務 品質(QoS)參數化(parameterized)的支持。

    它除傳承傳統(legacy) PCF的規則外,同時引進一些新 規則。

    HCCA不同於PCF的運作,而對無線媒介(Wireless Medium )提供polled access,這可用在無競爭時期(CFP) 與競爭時期(CP)。

Same Contention Based that EDC but…

  • 8 priority levels per station (identical to IEEE 802.1D priority tags) mapped to 4 access categories (AC)
  • Different Arbitration InterFrame Space (AIFS) per access categories
  • TxOP Burst Limits

EDCA Frame Priorities

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EDCA Implementation Model

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Arbitration InterFrame Space

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Throughput Enhancements

improve channel utilization and efficiency.

Block ACK

Improves channel efficiency by aggregating several acknowledgments into one frame.

2 types of Block Ack mechanisms:

immediate and delayed

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Direct Link Protocol

Ability to exchange data directly between two stations in the network, without traversing the AP.

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Automatic Power-Save Delivery

ps-poll is inefficient for current product, one poll one packet.

Enhancing Power Saving mode in QoS Basic Service Set

QoS-AP automatically delivers downlink frames,which belong to some specified Access Category, to Power-Saving stations

Two types of delivery mechanism

  1. Unscheduled APSD (U-APSD)
  2. Scheduled APSD (S-APSD)

Unscheduled APSD (U-APSD)

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STA will announce its capability for uapsd (ac) through wmm ie . it will tell ap which AC is supported for STA. u can check the (re)association packet to get the information .

Power-saving QoS-STA wakes up and send a “trigger”data frame belonging to “trigger-enabled” AC to QoS-AP

After receiving “trigger” frame, a Service Period (SP) is started

QoS-AP send frames belonging to “delivery-enabled” AC to QoS-STA

如果没有封包,则回一个Null Data(EOSP=1),如果有数据,则回数据帧, 直到最后一帧(EOSP=1)。

Scheduled APSD (S-APSD)

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QoS-STA negotiates a APSD Schedule with QoS-AP

QoS-AP start transmitting the frames of the specified Traffic Stream at Service Start Time and the following periods

QoS STA must wake up at Service Start Time and the following periods to receive frames

WMM Power Save

WMM Power Save是基于U-APSD.

Legacy Power Save

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Figure 8: Wi-Fi legacy power save

Client等待Beacon帧。Beacon帧包含了Traffic Indication Map (TIM)信息, 告知Client当前是否缓存有发往Client的数据,没有没有的话,Client可以 继续休眠直至一个Beacon到来。如果有数据的话,Client向基站发送 PS-Poll帧请求下载数据,AP发送一个Ack帧后,就开始发送第一帧数据,对 于每帧数据,Client都必须先发送一个PS-Poll帧,然后等待AP发送的Ack帧, 当接收到AP发送的帧后,最后要发送一个Ack帧给AP。

这样AP发送一个数据帧,AP和Client都需要发送两个数据帧,以及延迟两个 DCF的时间。

传统的这种省电模式会导致那么需要在AP与Client之间进行快速交换数据的 应用的延迟加长。原因如下:

  1. Client必须等待Beacon帧,不能在更短的时间内初始化传输。
  2. 一次只能发送一个数据帧,Client还不得不在每个接收的数据帧中发送 和接收额外的信号帧。
  3. 休眠时间是由Client的WiFi驱动设置的,而不管当前运行的是那种类型 的应用。这样不利于针对特定应用程序进行调整。

WMM Power Save的改进

WMM Power Save通过增加允许Client休眠的时间和减少Client需要发送和 接收的数据帧个数来改善传统的省电效率。

省电行为是Client与AP之间关联的过程中协商的。针对每个WMM AC传输队 列会设置不同的省电行为(WMM Power Save or legacy power save)。

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Figure 9: Example of a setup for WMM Power Save,

WMM Power Save的行为下,Client不需要等待Beacon帧,可随时发送一个 Trigger帧来启动数据下载。

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Figure 10: WMM Power Save