Transcript Enhancing 802.11 - QoS and Throughput Perspective

Improving IEEE 802.11 WLAN:
QoS and Throughput
Perspective
Sunghyun Choi, Ph.D.
Assistant Professor
School of Electrical Engineering
Seoul National University
E-mail: [email protected]
URL: http://ee.snu.ac.kr/~schoi
Introduction to My Group in SNU

Multimedia & Wireless Networking Lab.
(MWNL)


Started September 2003


Within School of Electrical Engineering, Seoul
National University
One of the youngest groups in SoEE, SNU
1 (+2) Ph.D. & 3 masters students
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Introduction to My Group in SNU
(Cont’d)

Working on WLAN MAC and around

Resource management – power, rate, …

QoS & mobility

TCP/UDP over WLAN

4G wireless network

Cross-layer design

(Sensor networks)
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Contents

Introduction

QoS provisioning

Throughput enhancement

Conclusion
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Introduction to IEEE 802.11 WLAN

Wireless Ethernet with comparable speed


Supports up to 11 and/or 54 Mbps within >100
m range
Enable (indoor) wireless and mobile highspeed networking

Runs at unlicensed bands at 2.4GHz and 5GHz

Connectionless MAC and multiple PHYs
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Limitations of Current 802.11

Lack of QoS support


Low throughput due to large overhead


Best-effort service with contention-based MAC
< 5 Mbps throughput at 11 Mbps 802.11b link
My group is currently working on improving
both aspects

Will show only preliminary results here
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QoS Improvement
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Emerging IEEE 802.11e MAC

New draft standard for QoS provisioning


Expected to be finalized by early next year
Defining a new MAC backward compatible
with the legacy MAC


Legacy 802.11 MAC – DCF (+ PCF)
802.11e MAC – HCF with two access
mechanisms


Controlled channel access
Contention-based channel access (EDCA)
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802.11 Distributed Coordination
Function (DCF)

Carrier Sense Multiple Access with Collision
Avoidance (CSMA/CA)
Immediate access when
medium is idle >= DIFS
DIFS
Busy
Medium
DIFS
PIFS
SIFS
Contention Window
Backoff
Window
Next Frame
Slot Time
Defer Access
Select Slot and decrement backoff
as long as medium stays idle
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802.11e Access Category (AC)
AC2
AC3
Backoff
AIFS[3]
BO[3]

AC1
Backoff
AIFS[2]
BO[2]

AC0
Backoff
AIFS[1]
BO[1]

Access category (AC)
as a virtual DCF
4 ACs implemented
within a QSTA to
support 8 priorities
Multiple ACs contend
independently
The winning AC
transmits a frame
Backoff
AIFS[0]
BO[0]

Virtual Collision Handler
Transmission
Attempt
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Differentiated Channel Access of
802.11e EDCA

Each AC contentds with

AIFS[AC] (instead of DIFS) and CWmin[AC] /
CWmax[AC] (instead of CWmin / CWmax)
Immediate access when
medium is idle >=
AIFS[AC]+SlotTime
AIFS[AC]
+SlotTime
Busy
Medium
AIFS[AC]
PIFS
SIFS
Contention Window
from [1,1+CWmin[AC]]
Backoff
Window
Next Frame
SlotTime
Defer Access
Select Slot and decrement backoff
as long as medium stays idle
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Simulation Results - DCF vs. EDCA

Delay comparison

2 video (1.5 Mbps CBR), 4 voice (36.8 kbps
CBR), 4 data (1 Mbps Poisson)
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Our Software-Based Approach for
RT Traffic Support


IEEE 802.11e is not available yet
Even if it becomes available, many existing
legacy 802.11 APs will be there


Especially, for WISP with many deployed APs,
replacing existing APs costs a lot of money
Software (or firmware) upgrade-based
approach is very desirable at least in the
short term
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System Architecture
TCP/UDP
TCP/UDP
IP
IP
RT+NRT
RT+NRT
RT
NRT
Device Driver
frame processing
frame processing
MAC
MAC
PHY
PHY
(a) Original
driver
Host
AP
(b) Modified
driver
Host
AP
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Measurement Configuration

Linux + HostAP driver for Intersil chipsets

one RTP (1.448 Mbps CBR) + one FTP
Server
switch
Console
Host AP
Client
Implement dual queue
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One-Way Delay of RTP Traffic
Original
Modified
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Percentage Gain in Performance
Parameters
Comparison
Test 1
Original
Throughp
ut
Jitter of
RTP
Two
Queue
Percentage
gain
Percentage Gain in Performance Parameters
10%
RTP troughput
TCP
3.851
3.703
-3.84%
RTP
1.448
1.448
0.00%
-10%
Avg.
2.9
2.6
-10.34%
-20%
Max.
4.0
3.0
-25.00%
min.
2.0
2.0
0.00%
TCP throughput
Jitter Avg.
-30%
-40%
Avg.
30.7
20.2
-34.20%
Max.
32.0
23.0
-28.13%
-50%
min.
32.0
18.0
-40.00%
-60%
Max delay
variation of RTP
27.0
18.0
-33.33%
One-way
delay of
RTP
Jitter Min
0%
Jitter Max.
One-way delay Max.
One-way delay Avg.
Max delay variation
One-way delay Min.
-70%
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Limitations and Future Work

Limitations of the current approach




Running on top of legacy MAC with a single
FIFO queue
AP cannot prevent/control contention from
stations
Downlink RT transmission could be severely
delayed due to the uplink contentions
How to handle this situation is an ongoing effort
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Throughput
Improvement
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IEEE 802.11n Initiative



A new standardization effort to achieve
over 100 Mbps throughput over WLAN
Via both PHY and MAC enhancement
We are considering the MAC improvement
for throughput enhancement
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Frame Size Affects Throughput

802.11 MAC/PHY have big fixed overheads


MAC header, IFSs, ACK, and Backoff
PLCP preamble & header
DIFS
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Busy
PPDU
Channel usec
Backoff - 9 usec x [0,CW]
SIFS (16 usec)
ACK
>= 24 usec
PLCP
PLCP
MAC
Payload FCS
Preamble
Header
Header
16 usec >= 4 usec 24 octets Variable 4 octets
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10
0
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Throughput (Mbps)
Theoretical Throughput for 54
Mbps
40
35
30
25
20
15
10
Preferred
Operation
Range
5
0
Datagram Size (octets)
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Packet Size Statistics
This statistics is from the measurement taken in the 802.11
standard meeting room in the morning of July 22nd 2003
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Frame Aggregation

Aggregation of multiple frames in order to reduce
the fixed overheads relatively!

Multiple frames are aggregated above the MAC
SAP

The aggregated frame is transmitted via a data frame
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Frame Formats
Original
octets:
802.2 LLC
802.11 MAC
octets:
2
2
Frame Duration
Control
/ ID
With aggregation
6
6
6
Addr 1 Addr 2 Addr 3
octets:
6
Destination
Addr.
3
5
variable
LLC
SNAP
IP
Header Header datagram
2
Seq.
Control
6
Source
Addr.
variable
Data
2
4
FCS
variable
Type IP datagram
1
1
1
2
2
2
variable
variable
802.2 LLC octets: 1
DSAP SSAP Control Reserv Count
Etherframe
Etherframe
Size 1 ... Size n
...
with
(0xdd) (0xdd) (0x03) ed
(n)
1
n
aggregation
octets:
802.11 MAC
2
2
6
6
6
Frame Duration Addr
Addr
Addr
Control
/ ID
1
2
3
2
variable
4
Seq.
Data
FCS
Control
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Theoretical Throughput w/
Aggregation (w/o channel error)
40
35
30
20
15
10
5
80
0
90
0
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00
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30
0
40
0
50
0
60
0
70
0
0
10
0
20
0
Throughput (Mbps)
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Datagram Size (octets)
11a w/o aggregation
11a w/ aggregation
# of aggregated datagrams
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Theoretical Throughput w/
Aggregation (w/ channel error)
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Performance Measurement
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Implement frame aggregation in real platform
Linux & Intersil-based platform (.11b)
Measure the throughput performance of UDP
and TCP traffic
Note: Frame aggregation is only applied when
there are multiple frames in the queue
Traffic generator
AP
STA
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Measurement Results - UDP
Throughput performance of packet aggregation
with fixed rate UDP
Packet aggregation, RTP, 10Mbps
8
7
6
Throughput (Mbps)

5
4
3
2
original
aggregation
1
Theoretical
0
0
200
400
600
800
1000
Packet Size (bytes, application-level)
1200
1400
1600
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Measurement Results - TCP
Throughput performance of packet aggregation
with TCP
Packet Aggregation, TCP
7
6
5
Throughput (Mbps)

4
3
2
original
aggregation
1
Theoretical
0
0
200
400
600
800
1000
Packet Size (bytes, application-level)
1200
1400
1600
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Summary and Future Work

Shown that frame aggregation is a good
way to improve 802.11 MAC throughput

Via both analysis and measurements

Frame aggregation can be done above the
MAC SAP very easily

Needs further measurements/simulations
for more realistic scenarios
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Concluding Remarks

IEEE 802.11 WLAN is becoming real
popular these days

There is still a big room to improve the
current 802.11 systems

Important to consider how any improved
system co-exists with legacy systems
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