IEEE 802.11e QoS on WLANs

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IEEE 802.11eQoS on WLANs

• Speaker Min-Hua Yang
• Advisor Ho-Ting Wu
• Date 10/25/05

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Overview

• 802.11 background information
• DCF
• PCF
• Qos limitations of 802.11
• 802.11e (QoS support)
• EDCA
• HCCA
• Simulation result

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IEEE 802.11 background information

• WLAN( Wireless Local Area Network )
• A LAN to which mobile users (clients) can connect
  and communicate by means of high-frequency radio
  waves rather than wires.
• WLAN Standard
• IEEE 802.11 (IEEE)
• HiperLAN (European Telecommunications Standards
  Institute, ETSI)

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IEEE 802.11
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IEEE 802.11 Topology

• Basic Service set networks (BSS)
• Independent BSS networks
• Infrastructure BSS
• Extended Service Set (ESS) networks

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The Basic Service Set (BSS)

• The BSS consists of a group of any number of
  stations.
• The basic building block of IEEE 802.11 LAN

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Independent BSS (IBSS) network

• stations communicate directly
• no connection to a wired network
• Ad hoc network

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Infrastructure BSS (BSS)

• Include an access point (AP)
• The AP may have connection to an wired network
• All stations communicate with the AP
• Communication between stations must go through AP
  
• consume twice the bandwidth

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The Extended Service Set(ESS)

• Distribution system (DS) provides logical
  services necessary to handle address-to-destinatio
  n mapping and seamless integration of multiple
  BSSs
• An AP is a STA that provides access to the DS

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Terminology- Interframe Space

• Time interval between frames.
• SIFS Short Interframe Space
• PIFS PCF Interframe Space
• DIFS DCF Interframe Space
• DIFS gt PIFS gt SIFS
• Fixed for each PHY
• Provide priority levels

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IEEE 802.11

• Two coordination functions are defined
• the mandatory Distributed Coordination Function
  ( DCF) based on CSMA/CA
• optional Point Coordination Function (PCF) based
  on poll-and-response mechanism.
• Most of todays 802.11 devices operate in the DCF
  mode only.

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Distributed Coordination Function ( DCF)

• Possible in both Infrastructure and Ad hoc mode
• Known as Carrier Sense Multiple Access with
  Collision Avoidance (CSMA/CA)
• Used During Contention period (CP)
• DCF can support best-effort services , not any
  QoS guarantees.

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Basic Access Mechanism-CSMA/CA

• Sense the medium before transmitting.
• If the medium is not busy, the transmission may
  proceed.
• If the medium is busy, invoke backoff procedure.
• Transmit after the backoff procedure
• If the transmission is not successful, invoke
  backoff procedure.

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Virtual Carrier Sense Mechanism

• Network Allocation Vector (NAV)
• Set NAV based on duration information (available
  in MAC header, RTS, CTS)
• NAV counts down to zero at a uniform rate.
• IF NAV0, medium is idle otherwise, it is busy.

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Basic Access Mechanism-CSMA/CA
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Example of backoff intervals
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IEEE 802.11 DCF (cont.)

• BackoffTime Random() aSlotTime
• where
• Random( ) 0, CW
• CWmin CW CWmax.
• aSlotTime fixed for each PHY
• CWnew (CWold 1) PF 1 (where PF2)
• PF Persistence Factor

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A example of CW
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Other Collision Avoidance Mechanism RTS/CTS

• Hidden Terminal Problem
• Hidden Terminal are STAs that the receiver can
  hear but that cannot be detected by other
  senders.
• In order to solve the problem,a optional RTS/CTS
  is introduced.
• The source sends a short RTS frame before each
  data transmission,and the receiver replies with a
  CTS frame if it is ready to receive.

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Basic access scheme v.s RTS/CTS access scheme
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Point Coordination Function (PCF) Optional in
Standard

• Used during Contention-Free Period (CFP)
• A single AP controls access to the medium, and a
  Point Coordinator (PC) Agent resides in the AP.
• AP polls each station for data, and after a given
  time interval moves to the next station.
• No stations are allowed to transmit unless it is
  polled.
• AP could have a priority scheme for stations.
• PCF is useful for time-sensitive applications.

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PCF(cont.)
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IEEE Legacy 802.11 MAC

• SuperFrame consists of Contention Period (CP) and
  Contention Free Period (CFP).
• PCF used during CFP and DCF used during CP.

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QoS Limitations of 802.11

• DCF (Distributed Coordination Function)
• Only support best-effort services
• No guarantee in bandwidth, packet delay and
  jitter
• Throughput degradation in the heavy load
• PCF (Point Coordination Function)
• Inefficient central polling scheme
• Unpredictable beacon frame delay due to
  incompatible cooperation between CP and CFP modes
• Transmission time of the polled stations is
  difficult to control

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IEEE 802.11e Overview

• New terminology
• QAP QoS Access Point
• QSTA QoS Station
• HC Hybrid Coordinator
• In order to support QoS in 802.11 WLAN , 802.11e
  has defined a new mechanism , namely, Hybrid
  Coordination Function (HCF).
• HCF is implemented by all QAPs and QSTAs
• HCF has two access mechanisms
• Contention based
• Enhanced distributed channel access (EDCA)
• Controlled channel access
• HCF Controlled Channel Access (HCCA)

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IEEE 802.11e MAC frame format
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IEEE 802.11e Overview - Enhanced distributed
channel access (EDCA)

• EDCA defines four Access Categories (AC)
• Background
• Best Effort
• Voice
• Video
• EDCA supports 8 User Priority (UP) values
• Priority values (0 to 7) identical to the IEEE
  802.1D priorities
• Rules
• One UP belongs to one AC
• Each AC may contains more than one UP
• Traffic of higher UP will be transmitted first in
  one AC

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IEEE 802.11e Overview-User Priority (UP)

• 8 User Priority
• Identical to IEEE 802.1D priority tags.

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Basic concepts Transmission opportunity (TXOP)

• Time interval permitted for a particular STA to
  transmit packets.
• During the TXOP, there can be a series of frames
  transmitted by an STA separated by SIFS.
• TXOP types
• EDCA TXOP initiation
• Obtained by winning a successful EDCF contention
• Polled TXOP (HCCA TXOP)
• Obtained by receiving a QoS CF-poll frame from
  the QAP

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802.11e EDCF Access Category

• EDCF defines access category (AC) mechanism to
  support the priority mechanism at the non-AP
  QSTAs.
• Each QSTA has four ACs.
• An AC is an enhanced variant of the DCF which
  contends for transmission opportunity (TXOP)
  using the set of parameters such as CWminAC,
  CWmaxAC, AIFSAC, etc.
• Each AC queue works as an independent DCF STA and
  uses its backoff parameter.
• In EDCA, the size of Contention-Window (CW) and
  Inter-frame space (IFS) is dependent on AC

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EDCA Accessing the medium

• EDCA use different IFSs
• SIFS ACKs, between multiple frames with the
  continuation EDCA TXOP
• DIFS / AIFS Used by DCF and EDCA for different
  access categories

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EDCF - Arbitration Interframe Space (AIFS)

• QSTA use AIFS to defer the contention window or
  transmission for each AC
• AIFSAC AIFSNACx aSlotTime aSIFTime
• AIFSN for each AC is broadcast via beacon frame
  containing EDCA Parameter Setelement
• DIFS 2 x aSlotTime aSIFTime

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EDCF Some elements of the Beacon frame
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Default EDCF Parameter Set
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EDCA details

• Each AC has own
• Interframe space AIFS
• Backoff Counter (BO)
• CWmin, CWmax, CW
• TXOP limit
• QSTA listens beacon frames to receive this
  information
• Each QSTA implements own queues for each AC
  traffic
• From the queues the frame with the highest
  priority is sent if internal collision happens

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Legacy 802.11 station and 802.11e station with
four ACs within one station.
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802.11e HCCA Overview - Hybrid Coordination
Function(HCF)

• Designed to increase efficiency by reducing the
  contention on the medium
• Uses polling
• Like PCF
• Thus, HCCA can send polling both under CFP and
  CP
• PCF Only polling in CP
• Specifically assigned transmit times for every
  frame
• Enables QoS guarantees

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802.11e HCCA (cont.)

• Has higher priority than EDCA .Under HCCA,
  HC(Hybrid Coordinator) has full controll over the
  wireless medium
• If HC needs it, it could take over the controll
  of the medium by sending a QoS CF-Poll

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802.11e HCCA (cont.)

• Differences between hybrid coordinator (HC) and
  point coordinator (PC)
• HC can poll QSTAs in both CP and CFP
• HC grants a polled TXOP to one QSTA, which
  restricts the duration of the QSTAs access to
  the medium.

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An example of an 802.11e superframe
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Improve Efficiency 802.11e

• Block Acknowledgment
• Send multiple MSDUs without a bunch of ACKs
• Group ACK
• Direct Link Protocol (LDP)
• No support for DLP in legacy MAC Needs to talk
  through the AP
• Less use of the channel
• The STAs is enable to talk directly to each other
• Sounds easy, lots of things to think about
• Power saving
• Security

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Simulation-QoS , does it work?

• EDCF Parameters for three queues

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Throughput and delay performance for EDCF

• EDCF maintains the throughput of high-priority
  audio and video flows by punishing the background
  traffic.

Throughput(KB/s)
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Comparison of total throughput between EDCF and
DCF

• The total throughput of EDCF is lower than that
  of DCF when the traffic load is larger than
  48EDCF reduces the throughput of low-priority
  flows considerable and therefore results in
  decreasing the total throughput.

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Mean delay of audio,CBR video versus channel load
for EDCF and HCF

• The result show that the HCF controlled channel
  access mechanism can guarantee the minimum delay
  requierment(50ms) for the admitted flows in
  different load rate
• EDCF works very well under low load conditions
  but suffers from delay degradation in high-load
  condition

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Reference

• Qiang Ni, Lamia Romdhani, and Thierry Turletti.
  "A Survey of QoS Enhancements for IEEE 802.11
  Wireless LAN". Wiley Journal of Wireless
  Communication and Mobile Computing (JWCMC), John
  Wiley and Sons Ltd., 2004 Volume 4, Issue 5
  547-566.
• Analysis of IEEE 802.11e for QoS support in
  wireless LANsMangold, S. Sunghyun Choi
  Hiertz, G.R. Klein, O. Walke, B.Wireless
  Communications, IEEE Volume 10,  Issue 6,  Dec.
  2003 Page(s)40 - 50
• Yu-Sun Liu Ph. D. WLAN????
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