IEEE 802.11 overview1 IEEE 802.11 WLAN By Orly Meir & Ilan Bar.
QoS in IEEE 802.11 Networks
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Transcript of QoS in IEEE 802.11 Networks
QoS in IEEE 802.11 Networks
Resources
Introduction
IEEE 802.11
Simple, EffectiveDesigned for Best Effort Service
Real Time Services: Throughput and DelaySensitive
End-to-End QoS Guarantees, IEEE 802.11eEnd-to-End QoS Guarantees, IEEE 802.11e
IEEE 802.11 Architecture
STASTA
STA STA
STASTASTA STA
APAP
ESS
BSS
BSSBSS
BSS
Existing Wired LAN
Infrastructure Network
Ad Hoc Network
IEEE 802.11e
- QAP = QoS AP- QSTA = QoS Station
TCP
802.11 PHY
802.11 MAC
IP
802.3 MAC
802.3 PHY
TCP
802.3 PHY
802.3 MAC
IP
802.11 MAC
802.11 PHY
LLCLLC LLC
Station
AP
server
infrastructure network
Layers
PHY Layer
-HR/DSSS: High Rate Direct Sequence Spread Spectrum
- FHSS : Frequency Hopping Spread Spectrum
MAC Sublayer
PCF (Point Coordination Function)DCF (Distributed Coordination Function)
PCF-Point Coordinator (PC)-Only in infrastructure networks-Designed for delay-bounded services
PIFS
EIFS: Extended IFS
PCF- Centralized: location-dependent errors- Stations must wait for polling: Delay at low
load - AP needs to contend for the channel using
DCF to begin a CFP: variable CFP- Managing large number of stations using
polling affects the applications that use DCF
- No admission control
DCF• Distributed, Contention-based• CSMA/CA• Binary Exponential Back-Off
(CW: Contention Window)• Physical channel sensing• Virtual channel sensing (NAV:
Network Allocation Vector)• Hidden Terminal problem:
RTS/CTS• Timers• Retry limits• Fragmentation
Fragmentation
PriorityNAV: Network Allocation Vector
DIFS: DCF Inter Frame Space
SIFS: Short IFS
PIFS: PCF IFS
EIFS: Extended IFS
time
medium busy SIFSPIFSDIFSDIFS
next framecontention
Tunable Parameters
QoS Mechanisms
Service Differentiation
Priority: classification of traffic
Fair Scheduling: partitioning the bandwidth fairly by regulating the wait times of traffic classes according to given weights
EDCF (Enhanced DCF)
Virtual Collision Handler ~ Internal Collisions
Priority AC For1 0 Best Effort
2 0 Best Effort
0 0 Best Effort
3 1 Video Probe
4 2 Video
5 2 Video
6 3 Voice
7 3 Voice
AC: Access Category
AIFS: Arbitrary IFS
Persistent Factor DCF (P-DCF)
- A persistent factor P is selected; small P means higher priority traffic class
- A uniform random number r is generated every slot in back-off stages.
- A flow stops the back-off and starts transmission only if r > p in the current slot given no transmission occurs in previous slots The back-off interval is a geometric distributed random variable with P
Geometric random variable is the number of trials required to obtain the first failure
Distributed Weighted Fair Queue2 schemes are proposed:• CW for a flow = Difference between actual and expected throughput
A station decreases the CW to get higher priority Lower CW when the actual throughput is lower than theexpected one
• Li’ = Ri/Wi Ri = the actual throughput Wi = the weight Each station adjust its CW by comparing others Li’
SelfishnessMore stations will have small value of CW
Distributed Fair Scheduling (DFS)The back-off interval is based on the packet length and traffic class
For flow i, BIi proportional to:
1. The weight (higher for higher throughput classes)
2. The packet length 3. A scaling factor (to min the probability of
collisions in case different stations have same back off interval)
Distributed Deficit Round Robin (DDRR)1. Each throughput class i at station j is given a service
quantum rate (Qij) equal to its required throughput
2. A deficit counter (DCij) is advanced at the rate Qij in a round robin fashion
3. Once a DCij becomes positive, the ith queue is allowed to send one packet
4. After transmission, DCij will be decreased by packet length each time a packet is transmitted
DCij is used to calculate IFSij (time before transmit or back-off): larger DCij, smaller IFSij
DDRR
Polling in a round robin way
Queues of different throughput classes
Admission Control and Bandwidth Reservation
• Service differentiation does not perform well under high traffic loads
• There is a need to protect existing streams
• A wireless node has no knowledge of exact condition of the network
• With CSMA/CA, bandwidth provision is quite difficult
Measurement-Based Admission Control
- The decision is made on measurement of existing network status (delay, throughput, …)
- Different methods used:- Virtual MAC: the use of virtual MAC frames,
and using a virtual source algorithm to tune the virtual MAC.
- Probe packet: the use of probe packet for ad hoc
- Data probe: the use of data packets
Calculation-Based Admission Control
• Performance metrics or criteria for evaluating the network status
• Permissible throughput propagation• Saturation-based
Scheduling and Reservation-Based Schemes
• ARME (ASSURED RATE MAC EXTENSION
• ): - Extension of DCF - Uses token bucket-based algorithm to detect
overloading condition - improvements mad by adjusting CW
Scheduling/Reservation• AACA:
- RTS/CTS used for reservation - Mainly was for solving hidden terminal problem
Link Adaptation
Dynamically change the transmit rate , specified in the PLCP header of the PHY layer, that depend on channel conditions
Link Adaptation• Received Signal Strength (RSS)
– Each station maintains 12 RSS thresholds and corresponding transmission rate
– Measure RSS and adjust the transmission rate• PER-Prediction
– Decisions are based on Packet Error Rate-Prediction• MPDU-Based
• Success/Fail Thresholds• Code Adapts To Enhance Reliability
Direct Link Protocol (DLP)
• QSTA transmits directly to another QSTA• Set up with the QAP is needed• STAs cannot go into power saving mode
for active duration of the direct stream.• DLP is not applied in Ad Hoc networks• DLP messages can include security
information
Group ACK• Send a group of frames before any ACK to
reduce overhead• GroupAckReq• GroupAck frame with an ACK bitmap• Sender retry unacknowledged frames with
a retry limit• Receiver should keep the state of burst
data received (sender address, bit map, sequence numbers)
Challenges
• IEEE 802.11e and DiffServ• IEEE 802.11e and IntServ• Integration of WLAN and MANET• Integration of WLAN and Bluetooth• Integration of WLAN and 3G wireless
networks
Resources1. “A SURVEY OF QUALITY OF SERVICE IN IEEE 802.11 NETWORKS” By: HUA ZHU, MING LI, IMRICH CHLAMTAC, AND B. PRABHAKARAN THE
UNIVERSITY OF TEXAS AT DALLAS2. www.eecs.berkeley.edu/~ergen/docs/IEEE-802.11overview.ppt 3. www.cs.ucla.edu/classes/ winter04/cs117/chap7wlanRvsd.ppt 4. http://www.it.iitb.ac.in/~kirang/academic/MTP/Firststage/slides.pdf
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