Transcript ppt - Iowa State University

QoS in IEEE 802.11 Networks
Resources
Introduction
IEEE 802.11
Simple, Effective
Designed for Best Effort Service
Real Time Services: Throughput and Delay
Sensitive
End-to-End QoS Guarantees, IEEE 802.11e
IEEE 802.11 Architecture
ESS
Existing
Wired LAN
AP
STA
BSS
AP
STA
STA
BSS
Infrastructure
Network
STA
Ad Hoc
Network
STA
BSS
BSS
STA
STA
STA
IEEE 802.11e
- QAP = QoS AP
- QSTA = QoS Station
Layers
server
Station
TCP
infrastructure network
AP
TCP
IP
LLC
IP
LLC
LLC
802.11 MAC
802.11 MAC
802.3 MAC
802.3 MAC
802.11 PHY
802.11 PHY
802.3 PHY
802.3 PHY
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
NAV: Network Allocation Vector
Priority
DIFS: DCF Inter Frame Space
SIFS: Short IFS
PIFS: PCF IFS
EIFS: Extended IFS
DIFS
DIFS
medium busy
PIFS
SIFS
contention
next frame
time
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)
Priority
AC
For
1
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
Virtual Collision Handler
~ Internal Collisions
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 Queue
2 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 the
expected one
• Li’ = Ri/Wi
Ri = the actual throughput
Wi = the weight
Each station adjust its CW by comparing others Li’
 Selfishness
 More 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
Queues of different throughput classes
Polling in a round robin way
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
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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
Resources
1. “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 DALLAS
2. 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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