Vijay_ICCN-11

Improved QoS in WLAN Using IEEE 802.11e
Vijay B T
Ph.D. Student
Department of Electronics and Communication Engineering
National Institute of Technology Tiruchirappalli India
Under supervision of Dr. B Malarkodi, Assoc. Professor NITT-15
August 20, 2016
Vijay B T (NITT) ICCN - 11 (MAC Layer Protocol) August 20, 2016 1 / 35

Overview
1 Objectives
2 Simulation Methodology
3 Reverbed Modeler
4 Motivation
5 Basic IEEE 802.11
IEEE 802.11 Logical Architecture
MAC Protocol Functionality
MAC for Channel Access Functions
6 IEEE 802.11e MAC Layer
Modiﬁed MAC Architecture
HCF - Inception
Transmission Opportunity (TXOP)
HCF Controlled Access
7 Simulation Results
8 Conclusion and Future Enhancement
9 References

Objectives
Objectives
Cutting edge mechanisms for QoS assistance, specifically Enhanced
Distributed Coordination Function (EDCF) and Hybrid Coordination
Function (HCF), defined within the 802.11e drafts are usually evaluated.
Pre-eminent Objectives are as follows:
Reducing the number of retransmission attempts by providing very
good service differentiation.
Medium Access delay improved by adding management frames.
To minimizing the Data dropped due to unavailability of access to
medium.
Increase the Throughput of the Network by reordering relative
Priorities.

Simulation Methodology
Simulation Methodology
The below ﬁgure shows a system of methods used in a particular Reverbed
Modeler [AE 17.5 PL6] to study of IEEE 802.11e.
Figure: Flow Diagram

Reverbed Modeler
Reverbed Modeler[AE 17.5 PL6]
This scenario studies impact of deploying 802.11e QoS facilities on
network performance, where stations generating application traﬃc for
all QoS (ToS) classes are present.
Figure: Study of IEEE 802.11e [4]
It also highlights how a QAP (11-e capable access point) can
overwrite the EDCA Parameters for each Access Category to re-order
their relative priorities.

Reverbed Modeler
The Project/Scenario Workﬂow
Create Project
Create baseline Scenario
Import or create topology
Import or create traﬃc
Choose results and reports to be collected
Run simulation
View results
Duplicate Scenario
Make Changes
Re-run Simulation
Compare results

Reverbed Modeler
3-Tired Reverbed Modeler Hierarchy
Three domains: network, node, and process
Node model speciﬁes object in network domain
Process model speciﬁes object in node domain
Figure: 3-Tired Hierarchy

Reverbed Modeler
Object Attributes
Here we can do what ever we want like changing routing parameters,
MANET Properties or WLAN Conﬁguration and ...
All objects have attributes that
control aspects of their
behaviors and attributes may
vary from one model to the next
Attribute values may vary
between objects of the same
model type
Right-click on an object and
select “Edit Attributes” to view
or change its attributes
Figure: Edit Attributes

Motivation
Motivation
Customary
The IEEE 802.11 WLAN is being deployed widely and rapidly for many
diﬀerent environments including enterprise, home, and public access
networking.
Feature
The main characteristics of the 802.11 standard are simplicity and
robustness against failures due to the distributed approach of its MAC
protocol.

Basic IEEE 802.11
Basic IEEE 802.11
The following features of Basic IEEE 802.11 are ...
A basic standard for WLAN
Comprise of several component and services and provides station
mobility
Supports network topologies such as BSS, EBSS and IBSS.
Has three diﬀerent PHY layers: DSSS in 2.4 GHz band, FHSS in 2.4
GHz band, IR light
Supports CSMA/CA access method
Support of Privacy and security of data being transferred.
Support data rates of I Mbps to 2 Mbps.

Basic IEEE 802.11 IEEE 802.11 Logical Architecture
IEEE 802.11 Logical Architecture
The logical architecture deﬁnes the network’s operation, which applies to
each station consists of a single MAC and one of multiple PHYs
Figure: IEEE 802.11 entities
The goal of the MAC layer is to provide access control functions for
shared-medium PHYs in support of the LLC layer.

Basic IEEE 802.11 MAC Protocol Functionality
MAC Protocol Functionality
The main purpose of MAC layer is to
1 Provide data services to the user of MAC i.e. higher layer protocols.
2 Control fair access to the shared wireless medium
To understand these functionalities, we consider Basic Service Set
(BSS) composed of an Access Point (AP) and a number of stations
associated with AP.
Standard deﬁnes two methods
1 Frame Exchange Protocol
2 RTS/CTS Mechanism (Optional)
This mechanism is deﬁned to increase the robustness of the protocol
and address problems such as hidden node.

Basic IEEE 802.11 MAC for Channel Access Functions
MAC for Channel Access Functions
Standard speciﬁes two Channel Access Functions
DCF (Distributed Coordination Function)
1 Basic access method, mandatory for all STAs
2 Allows sharing of wireless medium between compatible PHY layer
devices through use of CSMA/CA
PCF (Point Coordination Function)
1 Its for controlled access
2 An optional channel access function is designed to support time
bounded services.

Inter Frame Spacing
802.11 MAC protocol are given while values for different IFS are given in
Table
SIFS = Short inter frame space = as in Table dependent on PHY
PIFS = point coordination function (PCF) inter frame space = SIFS
+ slot time
DIFS = distributed coordination function (DCF) inter frame space =
PIFS + slot time
the back-off timer is expressed in terms of number of time slots.
PHY SIFS DIFS Slot time CWmin
802.11a 16 34 9 15
802.11b 10 50 20 31
802.11g 10 50 20 15
Table: MAC values in microseconds for different PHYs [3]

Coexistence of PCF and DCF
In the case of DCF Carrier Sense is performed using both Physical and
Virtual Mechanism
Figure: Coexistence of PCF and DCF

PCF
It is for controlled access to medium
Its an optional channel access function and is designed to support
time bounded services.
Contention free access to the wireless medium is controlled by Point
Coordinator collocated with the AP.
IEEE 802.11 deﬁnes two periods between two consecutive Delivery
Traﬃc Indication Msgs (DTIM) and beacons
Contention Period (CP)
Contention Free Period (CFP)

PCF-Polling
Polling
Polling starts by PC sending a CF-Poll frame to one of pollable STAs.
If the PC itself has pending transmission it could use data frame
piggybacking CF-Poll frame.
The polled STA can respond with Data + CF-Ack frame or with
CF-Ack only, if there is no pending transmission in STA.
Once the frame sequence with one station is complete, PC sends
CF-Poll to another STA in its list of Pollable STA.
When PC has ﬁnished polling all pollable STAs, or CFP duration has
expired, PC broadcasts a CF-end frame to announce the end of CFP.
The NAV of all STAs are set to maximum at TBTT to protect CFP
from unwanted transmission.
Then the AP broadcast the actual CFP duration in the beacon, and
NAV are updated accordingly.
At the end of CFP, all STAs reset, their NAV to zero, when either they
have received CF-End frame or CFP duration expires.
From now on until the next DTIM beacon, all STAs contend for
wireless medium using DCF.

IEEE 802.11e MAC Layer
IEEE 802.11e MAC Layer
The major enhancement of 802.11e
Traﬃc diﬀerentiation
Concept of Transmission Opportunity (TXOP)
Enhanced DCF (contention-based)
HCP controlled channel access (contention free)
Burst ACK (optional)⇒ Not Explore Here
Direct link protocol (DLP)⇒ Not Explore Here

IEEE 802.11e MAC Layer Modiﬁed MAC Architecture
Modiﬁed MAC Architecture
Hybrid Coordination Function (HCF)⇒ IEEE 802.11 Task Group E (TGe)
proposes HCF to provide QoS for real-time applications.
DCF ⇒ A contention-base access for 802.11.
PCF ⇒ An option to support contention-free access in 802.11. [1]
Figure: Upgrade MAC Architecture

IEEE 802.11e MAC Layer HCF - Inception
HCF - Inception
HCF combines functions from the DCF and PCF with enhanced
QoS-speciﬁc mechanisms.
HCF consists of
Enhance DCF (EDCF) for contention-based access
Controlled Access (HCCA) for contention-free access

HCF - Interpretation (1/2)
Interpretation with respect to Modiﬁed MAC Architecture
Hybrid coordinator (HC)
The point coordinator for HCF.
QoS access point (QAP)
An access point (AP) that implements the access point functions speciﬁed
in the IEEE 802.11e standard.
QoS facility
The set of enhanced functions, frame exchange sequences, and
management objects.

HCF - Interpretation (2/2)
Interpretation with respect to Modiﬁed MAC Architecture
QoS station (QSTA)
An IEEE 802.11 station which implements QoS facility and HCF.
QoS basic service set (QBSS)
A basic service set that supports QoS facility speciﬁed in the IEEE 802.11e.
QoS independent basic service set (QIBSS)
An independent basic service set in which one or more of its stations
support the QoS facility.

EDCF – Traffic Category
The EDCF provides differentiated access to the WM for 8 priorities,
identical to IEEE 802.1D priority tag, for non-AP STAs.
Priorities are numbered from 0 (the lowest priority) to 7 (the highest
priority).
The set of MSDUs with the same priority is refer to a Traffic
Category (TC).

EDCF - Access Category (1/4)
EDCF deﬁnes access category (AC) mechanism to support the priority
mechanism at the non-AP QSTAs.
An AC is an enhanced variant of the DCF which contends for
transmission opportunity (TXOP) using the set of parameters such as
CWmin[AC], CWmax[AC], AIFS[AC], etc.
The parameter set is speciﬁed in the “EDCA parameter set element”
of beacon frames.[2]
AC CWmin CWmax AIFS
0 CWmin CWmax 2
1 CWmin CWmax 1
2 (CWmin + 1)/2 - 1 CWmin 1
3 (CWmin + 1)/4 - 1 (CWmin + 1)/2 - 1 1
Table: Default QoS Parameter set

EDCF - Access Category Timing Diagram (2/4)
The timing relationship for an EDCF is shown in Figure
Figure: Timing Diagram

Virtual Contention with in a station is shown below ﬁgure
An QSTA has four ACs.
Collision between ACs within a QSTA is called internal collision.

Internal collision resolution: High priority AC wins the right of
transmission, but low priority AC back off as if it experiences a
collision.
The mapping from 8 priories to 4 ACs are :
User Priority AC Traffic Type
1 0 (BK) Background (BK)
2 0 (BK) Spare or Standard
0 1 (BE) Best Effort (BE)
3 1 (BE) Excellent Effort (EE)
4 2 (VI/AVI) Controlled Load (CL)
5 2 (VI) Video < 100ms latency (VI)
6 3 (VO) Voice < 10ms latency (VO)
7 3 (VO/AVO) Network Control (NC)
Table: Mapping Virtual Contention

IEEE 802.11e MAC Layer Transmission Opportunity (TXOP)
Transmission Opportunity (TXOP)
Definition : which enables a station to transmit multiple frames
consecutively within a burst after it gains the channel.
A TXOP is defined by a starting time and a maximum duration.
Two types of TXOP: EDCF TXOP and Polled TXOP.
An EDCF TXOP begins when the wireless medium is determined to be
available under the EDCF rules, and the length of TXOP is specified in
beacon frames.
An Polled TXOP begins when a QSTA receives a QoS(+)CF-Poll from
HC, and the length of TXOP is specified in the QoS(+)CF-Poll.

IEEE 802.11e MAC Layer HCF Controlled Access
HCF Controlled Access
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 QSTA’s access to the medium.
Figure: Timing Diagram

Simulation Results
Simulation Results WLAN Delay (1/3)
The WLAN end-to-end delay values measured by STAs Voice-1, Video-1
Best Eﬀort-1 and Background-1 are compared. When all STAs belong to
BSS-1, each reports similar end-to-end delays.
Figure: WLAN Delay

Simulation Results
Simulation Results WLAN AP Connectivity (2/3)
When the mobile STAs roam
with BSS-1 to BSS-2, they
can be now make use of
EDCA to contend for the
medium since their current
AP is often a QAP.
Prioritized solution for the
higher-layer traﬃc straight
away impacts on the
measured delay values.
Figure: WLAN AP Connectivity

Simulation Results
Simulation Results WLAN Throughput (3/3)
VI AC reports throughput
only starting around 250
seconds in the simulation
QSTAs handle all higher
layer traffic along with all
traffic received from the
medium as BE traffic.
Figure: WLAN Throughput

Conclusion and Future Enhancement
Conclusion and Future Enhancement
Choosing the concept of varying levels of services for diﬀerent traﬃc
types, i.e. voice, video and data.
Which often supports a unique priority to access the radio channel.
Fidelity and Flexibility of IEEE 802.11e QoS mechanisms.
Future Enhancement
A couple changes to IEEE 802.11 have recently been published 802.11aa
and 802.11ae.By providing support for multicast transmission, improved
audio video streaming, coping with inter-network interference, and better
prioritization of management frames.

References
References
Katarzyna Kosek-Szott, Marek Natkaniec, and Szymon Szott
What’s New for QoS in IEEE 802.11?
Journal of IEEE Network, 2(1):95–104, 2013.
Mangold, Stefan and Choi et al.,
Analysis of IEEE 802.11 e for QoS support in wireless LANs
IEEE Wireless Communications, 10(6):40–50, 2003.
Daqing Gu and JinyunZhana,
QoS Enhancement in IEEE 802.11 Wireless Local Area Networks
IEEE Communication Magazine,41(6): 120–124, 2003
Adarshpal S. sethi , Vasil Y. Hnatyshin.
The Practical OPNET User Guide for Computer Network Simulation.
CRC press Taylor and Franscis Group, 2011.

References
The End
Any Questions!
Thank You

Vijay_ICCN-11

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