Improving IEEE 802.11 WLAN: QoS and Throughput Perspective

1
Improving IEEE 802.11 WLAN QoS and Throughput
Perspective

• Sunghyun Choi, Ph.D.
• Assistant Professor
• School of Electrical Engineering
• Seoul National University
• E-mail schoi_at_snu.ac.kr
• URL http//ee.snu.ac.kr/schoi

2
Introduction to My Group in SNU

• Multimedia Wireless Networking Lab. (MWNL)
• Within School of Electrical Engineering, Seoul
  National University
• Started September 2003
• One of the youngest groups in SoEE, SNU
• 1 (2) Ph.D. 3 masters students

3
Introduction to My Group in SNU (Contd)

• Working on WLAN MAC and around
• Resource management power, rate,
• QoS mobility
• TCP/UDP over WLAN
• 4G wireless network
• Cross-layer design
• (Sensor networks)

4
Contents

• Introduction
• QoS provisioning
• Throughput enhancement
• Conclusion

5
Introduction to IEEE 802.11 WLAN

• Wireless Ethernet with comparable speed
• Supports up to 11 and/or 54 Mbps within gt100 m
  range
• Enable (indoor) wireless and mobile high-speed
  networking
• Runs at unlicensed bands at 2.4GHz and 5GHz
• Connectionless MAC and multiple PHYs

6
Limitations of Current 802.11

• Lack of QoS support
• Best-effort service with contention-based MAC
• Low throughput due to large overhead
• lt 5 Mbps throughput at 11 Mbps 802.11b link
• My group is currently working on improving both
  aspects
• Will show only preliminary results here

7
QoS Improvement
8
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)

9
802.11 Distributed Coordination Function (DCF)

• Carrier Sense Multiple Access with Collision
  Avoidance (CSMA/CA)

10
802.11e Access Category (AC)

• 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

11
Differentiated Channel Access of 802.11e EDCA

• Each AC contentds with
• AIFSAC (instead of DIFS) and CWminAC /
  CWmaxAC (instead of CWmin / CWmax)

12
Simulation Results - DCF vs. EDCA

• Delay comparison
• 2 video (1.5 Mbps CBR), 4 voice (36.8 kbps CBR),
  4 data (1 Mbps Poisson)

13
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

14
System Architecture
15
Measurement Configuration

• Linux HostAP driver for Intersil chipsets
• one RTP (1.448 Mbps CBR) one FTP

Implement dual queue
16
One-Way Delay of RTP Traffic
Original
Modified
17
Percentage Gain in Performance Parameters
Test 1 Test 1 Comparison Comparison Percentage gain
Test 1 Test 1 Original Two Queue Percentage gain
Throughput TCP 3.851 3.703 -3.84
Throughput RTP 1.448 1.448 0.00
Jitter of RTP Avg. 2.9 2.6 -10.34
Jitter of RTP Max. 4.0 3.0 -25.00
Jitter of RTP min. 2.0 2.0 0.00
One-way delay of RTP Avg. 30.7 20.2 -34.20
One-way delay of RTP Max. 32.0 23.0 -28.13
One-way delay of RTP min. 32.0 18.0 -40.00
Max delay variation of RTP Max delay variation of RTP 27.0 18.0 -33.33
18
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 on-going effort

19
Throughput Improvement
20
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

21
Frame Size Affects Throughput

• 802.11 MAC/PHY have big fixed overheads
• MAC header, IFSs, ACK, and Backoff
• PLCP preamble header

22
Theoretical Throughput for 54 Mbps
Preferred Operation Range
23
Packet Size Statistics
This statistics is from the measurement taken in
the 802.11 standard meeting room in the morning
of July 22nd 2003
24
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

25
Frame Formats
Original
With aggregation
26
Theoretical Throughput w/ Aggregation (w/o
channel error)
27
Theoretical Throughput w/ Aggregation (w/ channel
error)
28
Performance Measurement

• 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
29
Measurement Results - UDP

• Throughput performance of packet aggregation with
  fixed rate UDP

30
Measurement Results - TCP

• Throughput performance of packet aggregation with
  TCP

31
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

32
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