A Survey of Quality of Service in IEEE
802.11 Networks
Hua Zhu, Ming Li, Imrich Chlamtac, B.
Prabhakaran
The University of Texas at Dallas
Presentation Structure
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IEEE 802.11 overview
QoS schemes for 802.11
Design challenges
Future Work
Conclusions
Introduction
• IEEE 802.11 advantages
• Broadband bandwidth capability
• Low deployment cost
• Internet services access anytime,
anywhere
• Mobility and connectivity
Introduction (cont’d)
• IEEE 802.11 disadvantages
– Best effort services
– No build in QoS modification of
existing standards required
– Shared medium
So what is the challenge?
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End to end QoS
Protocol interoperability
Multihop scheduling
Full mobility support
Seamless vertical handoff among multiple
mobile/wireless interfaces
802.11 Family
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802.11a
802.11b
802.11c
802.11d
802.11e
802.11f
802.11g
802.11h
802.11i
54 Mbps
5GHz
U-NII, OFDM
22 Mbps
2.4GHz
ISM, HR/DSSS
Bridge Operation Procedures
Global Harmonization
MAC enhancements for QoS
Inter Access Point Protocol (roaming)
36/45 Mbps 2.4 GHz
ISM, OFDM
Dynamic Frequency Selection
Security
802.11 MAC – Medium access protocols
• Contention-based distributed coordination function (DCF)
– Based on CSMA/CA instead of CSMA/CD
• Optional point coordination function (PCF)
– Wireless channel is divided to superframes
– Superframe consists of contention free period (CFP) and
contention period (CP). At the beginning of CFP point coordinator
(AP) contends for access of the wireless channel.
– If an AP acquires the channel a polling / granting policy is applied
for transmission.
802.11 MAC Protocols – PCF problems
• Substantial delay at low load (polling/granting
policy, even in an idle system).
• AP needs to contend for the channel at the
beginning using DCF  Effective period of
contention free polling may vary.
• Difficult management of the polling for large
number of interactive streams without harming the
applications using DCF contention.
• Central no distributed approachLocation –
dependent errors.
802.11 MAC Protocols – DCF
– Based on CSMA/CA
– Carrier sensing in both PHY & MAC layer (physical &
virtual CS)
– MAC Protocol Data Unit = 34 bytes MAC Header +
Payload + 32 bit CRC
– If MAC fr_length>RTS_threshold then RTS & CTS are
used by stations to solve the hidden terminal and
capture effect problems
CSMA/CA – RTS/CTS scheme
Common Tunnable Parameters
802.11 QoS Mechanisms
– Service differentiation
– Admission control and bandwidth reservation
– Link adaptation
Service differentiation mechanisms
802.11 DFC
Fair-scheduling-based
Priority-based
Contention window
EDFC
802.11e
P-DFC
(Persistent
Factor
DFC)
Backoff algorithm
DWFQ
Distributed
Weighted
Fair Queue
Interframe space AIFS
DFS
Distributed
Fair
Scheduling
DDRR
Distributed
Deficit
Round Robin
Service Differentiation
– Priority based
• Binds channel access to different traffic classes by
prioritized contention parameters
– Fair scheduling based
• Partitions the channel bandwidth fairly by regulating
wait times of traffic classes in proportion according
to given weights
– Tunable parameters :
• Contention window (Cwmin/Cwmax), backoff
algorithm, interframe space AIFS
Service Differentiation mechanisms EDCF
• Enhanced DCF (EDCF)
– Part of upcoming 802.11e standard
• Priority of traffic categories based on
– AIFS (Arbitrary Interframe Space)
– Max/min Contention Window
– Multiplication factor for backoff window
– Combination of above parameters is permitted
according to the service provider needs
– Although all traffic categories keep using the same
DCF access method they have different probabilities of
winning the channel contention by differentiating
contention parameters.
Service Differentiation mechanisms –PDCF
• Persistent Factor DCF (P-DCF).
– Each traffic class is associated with a persistent factor P
– In backoff stage a uniformly distributed number r is
generated.
– Each flow stops backoff and starts transmission only if
r>P in the current slot time, given no transmission
occurs in previous slot times.
– Therefore the backoff interval is geometrically
distributed random variable with parameter P
Service Differentiation mechanisms DWFQ
Algorithm 1
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Distributed Wighted Fair Queue
(DWFQ) –algo 1
Backoff CW of any traffic flow is adjusted
based on the difference between the actual
and expected throughputs.
If actual_thru<exp_thru then decrease CW in
order to increase flow’s priority and vice
versa
Service Differentiation mechanisms DWFQ
Algorithm 2
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Distributed Wighted Fair Queue
(DWFQ) –algo 2
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Calculation of Li=Ri/Wi
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Ri actual throughput, Wi corresponding weight of
I station
Comparison of Li with all others and adjust
CW for station I, ie if Li<all othersdecrease
CW of I
Service Differentiation mechanisms -DFS
• Distributed Fair Scheduling (DFS)
– Differentiate the backoff interval (BI) based on
the packet length and traffic class and station
with smaller BI transmits
– BIi=Ρi x scaling x factor x Li/Φi
• Li packet length, Φi weight, Pi random variable
uniformly distributed in [0.9,1.1]
• Pi is introduced to minimize the collision caused by
multiple stations with the same BI
Service Differentiation mechanisms -DDRR
• Distributed Deficit Round Robin (DDRR)
– The i throughput class at the j station is assigned
with a quantum rate (Qi,j) equal to the thoughput it
requires and a deficit counter (DCi,j) that
accumulated at the rate of Qi,j and is decreased by
the packet length whenever a packet is transmitted.
– DCi,j is used for Interframe Space (IFSi,j)
calculation
– IFSi,j is the waiting time before transmission or
backoff starting
A larger DCi,j results in a smaller IFSi,j
Service Differentiation mechanisms –
Conclusion and Comparison
• Fair scheduling based
 Fairly allocation of bandwidth among traffic classes.
 Prevent starvation of specific class.
 Often require a substantial modification of existing 802.11
standards.
• Priority based
 Require less modification of the existing DCF access
method.
 Provide better QoS support for real time applications.
• Service differentiation does not perform well under
high traffic loads due to the inefficiency of 802.11
MAC.
QoS mechanisms for Admission Control
and Bandwidth Reservation (MAC)
• Necessity in order to guarantee QoS in high
traffic load.
A wireless node has no knowledge of the exact
network condition.
With contention based CSMA/CA, bandwidth
provisioning is almost impossibleonly soft QoS
guarantee.
• In general admission control requires less
modification than bandwidth reservation in
802.11 standards.
Admission Control and Bandwidth Reservation
Admission Control Approaches
• Measurement-based (admission control based
on measurements of existing network status)
Virtual MAC [Barry et al]: Channel is passively
monitored by virtual MAC frames and local service
(throughput/delay) is estimated by the measurement
of virtual frames.
Probe Packets [Valaee & Li]: Admission procedure
is based on a sequence of probe packets for ad hoc
networks
Data Probe Packets:[Shah et al]: Data packets for
measuring the network load
Calculation based approaches
• Calculation-based (there are certain perfomance
metrics or criteria for evaluating network status)
Permissible throughput: [Kazantzidis et al]:
Admission decision criterion is permissible
throughput. AODV routing protocol
Saturation-based[Zhu et al]: Prediction and
prevention of saturation using piggybacked
information (number of active stations,
corresponding bit rates, average packet lengths) for
each station. Admission control decisions are made
dynamically at both source/destination in a fully
distributed way.
Bandwidth reservation
Flow reservation and priority allocation [Li and
Prabhakaran]: Optimizing the usage of priority
resources.
ARME [Banchs & Perez]: Based on an extension of
DCF. A Token-bucket algorithm is used to detect
overload and improve performance via adjustment of
CW.
AACA[Liu et al]: ACCA adopted the RTS/CTS access
method on a common channel solely for reservation
purposes. After successful reservation a pair station
transmit without interruption in the reserved channel.
QoS Mechanisms for Link Adaptation
(PHY)
802.11 specifies multiple transmission rates that
are achieved by different modulation techniques
in the PLCP header of PHY layer.
Rate adaptation and signaling are open.
Link adaptation mechanism has to maximize the
throughput under dynamically changing channel
conditions.
So an obvious solution is to focus on switching
transmission rates specified at PLCP, without
modification of existing standards.
QoS Mechanisms for Link Adaptation
(PHY) –cont’d
Novel idea
Adjust the length of DSSS pseudo noise (PN) in
802.11b with slight modifications of 802.11b
Metrics used in Link Adaptation (PHY)
Channel signal to noise ratio / carrier to
interference ratio (SNR/CIR).
Received power level.
Average payload length
Transmission acks
Combinations of above
Received Signal Strength RSS
Pavon and Choi
Assumption:
Transmission power is fixed.
Linear relationship between average RSS and
SNR.
Then:
A rate adaptation algorithm at every station
maintain its own 12 RSS thresholds and
corresponding rates. Based on the measured
RSS a station dynamically switches to an
appropriate transmission rate.
MPDU based
Qiao et al
A combination of metrics is used including:
SNR
Average payload length
Frame retry count
The proposed algorithm pre-established a table of
best transmission rate for decision making.
Success/Fail thresholds
Cheville et al
Transmitted frames ACKs are used as a metric
of channel condition.
If number of consecutive successful exceeds S,
transmission rate is increased, otherwise
transmission rate is decreased.
ACKs are used to indicate transmission success
or fail.
Code Adapts To Enhance Reliability
Mullin et al
CATER adaptive PN algorithm gives a
throughput improvement under high bit error
rate (BER) channel conditions at 802.11b.
But due to signaling overhead the throughput
under low BER channel conditions is lower than
the standard 802.11b
Challenges and future work
Wireless Internet and Interoperability
IEEE 802.11 WLANs have been successfully
applied as the last mile technology where there
is a need for wireless/mobile users (Wireless
Hotspots).
There is an urgent demand for e2e QoS
guarantee to be provided in wire-cum-wireless
heterogeneous networks.
Interoperability between IEEE 802.11 and
DiffServ or IntServ.
Wireless Internet and Interoperability
802.11 and DiffServ
[Park & Kim]
 A proposed architecture for e2e QoS across
wired WAN, wired LAN and WLAN.
 Protocols and drafts used
DiffServ (RFC 2475), IEEE802.1d, IEEE802.1Q,
IEEE802.11. IEEE 802.3.
 802.3 MAC frame carry the user priority via the
802.1Q VLAN tag. User priority is forwarded
through 802.1D MAC bridge to 802.1e and used
by EDCF to differentiate flows.
802.11 and DiffServ
[Park & Kim] cont’d
 There is a necessity for mapping between DiffServ
Code Point (DSCP) and Traffic Category Identification
(TCID) defined in 802.1e.
 Direct mapping
 When IP packets are encapsulated in MAC frames
they are placed in priority queues without
preemption
 Hierarchical mapping
 IP packets are classified and shaped according to the
priority of DSCP values before being forwarded to
802.1e priority queuesmore accurate e2e QoS.
802.11 and IntServ
[Liu & Zhu]
 Integration of RSVP and WRESV (WRESV is a
proposed MAC layer flow reservation and admission
control protocol in IEEE 802.11WLAN).
 Message mapping at Access Point are implemented by
cross-layer interaction and user priorities are mapped to
802.11 MAC with 802.1p.
 WRESV is working with most of the existing MAC
schedulers (DCF, EDCF, DFS).
 This scheme also considers support of both node
mobility and QoS in handoff.
Support of Full Mobility
 Mobility is supported through extended service set
(ESS) for roaming among multiple AP. This roaming
capability is achieved through Mobile Station (MS)
beacon scanning in a channel sweep.
 802.11 WLAN service is only available for low
mobility devices in isolated hot spots.
 Recent efforts have been made to extend 802.11
WLANs into outdoor cellular networks to provide fully
mobile broadband service with ubiquitous coverage
and high speed connectivity.
Support of Full Mobility - Examples
 [Leung et al] claim that without standard modification
the DCF access method with RTS/CTS is feasible for
large outdoor cellular coverage (service area 6 Km).
 Beam transmission instead of in all directions extends
the coverage of 802.11 [Vivato Inc]. Cisco Aironet,
Motorola Inc Canopy Radio and Proxim may reach up
to 10 Km.
 But a cell with large outdoor coverage does not
guarantee high speed connectivity due to unavoidable
channel contentionthroughput may degrade in
overcrowded cell.
QoS and Mobility Management in Hybrid
Wireless Networks
 Seamless horizontal handoff and roaming among
802.11 WLAN supporting QoS anytime anywhere.
 Vertical handoff between WLAN, mobile and ad hoc
networks (MANET), Bluetooth, Universal Mobile
Telecommunications System (UMTS) and Wideband
Code Division Multiple Access (WCDMA)
WLAN – MANET Integration
 [Lamont & Wang]:
 Routing within MANET is handled by the
Optimised Link State Protocol (OLSP).
Handoff between MANETs & WLANs is supported
through automatic node detection and node
switching capabilities of the mobiles.
Functionalities of OLSP are extended to support
Mobile IPv6
WLAN – Bluetooth Integration
 [Conti & Dardari]:
 Analytical model for evaluation of the interference
between IEEE 802.11 & Bluetooth.
In the proposed model PHY and MAC layers are
considered, and the model can be easily
implemented.
Performance is evaluated by packet error
probability in terms of the relative distanced
between the two systems for different conditions.
WLAN – 3G Integration
[Jaseemudin]:
A mobile node is maintaining two connections in
parallel:
Data connection through WLAN.
Voice connection through UMTS.
[Park &Yoon]:
Vertical handoff between WLANs and CDMA
Real time traffic takes into account handoff delay
Best effort traffic takes into account throughput.
WLAN – 3G Integration cont’d
[Buddhicot & Chandranmenon]:
Combination of the features of high rate
small-coverage WLAN and wide-coverage
low rate 3G to improve the QoS and
flexibility of wireless service.
A loose integration approach is realized with
an IOTA gateway and a new client software
in order to support seamless mobility, OoS
guarantees and multiprovider roaming
agreements.
WLAN – 3G Integration IOTA gateway
WLAN – 3G Integration cont’d
Integration of WLANs and 3G/4G requires a
low call dropping probability in the 3G/4G
networks.
[Lou & Li]:
Adaptive allocation scheme termed
measurement based preassignment in order to
prevent handoff failure in wireless cellular
networks. A periodic measurement of traffic
status within a cell help to adjust the number
of reserved channels for handoff.
Summary
Classification of QoS schemes
Link adaptation in the PHY layer.
Channel access coordination in the
MAC layer.
Admission control strategies in MAC &
higher layers.