Patrick Verkaik Yuvraj Agarwal, Rajesh Gupta, Alex C. Snoeren UCSD - - PowerPoint PPT Presentation

▶
patrick verkaik yuvraj agarwal rajesh gupta alex c snoeren
SMART_READER_LITE
LIVE PREVIEW

Patrick Verkaik Yuvraj Agarwal, Rajesh Gupta, Alex C. Snoeren UCSD - - PowerPoint PPT Presentation

Jun 10, 2023 833 likes •1.19k views

Patrick Verkaik Yuvraj Agarwal, Rajesh Gupta, Alex C. Snoeren UCSD NSDI April 24, 2009 1 Voice over IP (VoIP) and WiFi increasingly popular Cell phones with WiFi + VoIP: iPhone (+ Skype, Fring, iCall, ..) T -mobile UMA and

slide-1
SLIDE 1

1

Patrick Verkaik Yuvraj Agarwal, Rajesh Gupta, Alex C. Snoeren

UCSD

NSDI — April 24, 2009

slide-2
SLIDE 2

2

 Cell phones with WiFi + VoIP:

  • iPhone (+ Skype, Fring, iCall, ..)
  • T
  • mobile UMA and @home

 Voice over IP (VoIP) and WiFi increasingly

popular

>1M downloads of Skype for iPhone in just two days

slide-3
SLIDE 3

3

Internet

CNN.com

802.11 AP

  • Call quality?
  • Impact on data users?

VoIP user Data users

slide-4
SLIDE 4

4

 802.11 designed for data traffic  Substantial per-packet overheads

  • Framing (headers, ACK)
  • Contention (backoff, collisions)

 VoIP:

  • Small packets
  • High packet rate (20-100 pps)
  • Does not respond to congestion

VoIP makes inefficient use of WiFi

slide-5
SLIDE 5

5

 Residual capacity

  • TCP / UDP throughput

 Mean opinion score (MOS)

  • How audio appears to a real person
  • Score: 1 (bad) – 5 (very good)
  • Can be calculated based on: [Cole et al., 2001]

 Voice codec  Network packet loss, delay, jitter

slide-6
SLIDE 6

6

 802.11 b/g testbed:

  • 10 VoIP stations
  • One data station

 Gradually activate more

VoIP stations

TCP/UDP VoIP

AP VoIP users Data user

slide-7
SLIDE 7

7

Downlink MOS (AP→station) Uplink MOS (station→AP)  

Quantization, codec, etc.

slide-8
SLIDE 8

8

Measured TCP throughput Expected from size of VoIP packets

slide-9
SLIDE 9

9

  • Increases contention overhead (collisions)
  • Measured further reduction of residual capacity

 Decrease VoIP packet rate  Use higher speeds (802.11g, 802.11n)

  • ‘Protection’ in the presence of older versions of 802.11
  • VoIP traffic too infrequent for 802.11n aggregation

 Prioritize VoIP traffic (802.11e)

802.11g + lower VoIP packet rate MOS

slide-10
SLIDE 10

10

 Downlink direction:

  • Aggregation across multiple receivers
  • Addresses framing and contention
  • verhead

AP

 Uplink direction:

  • Prioritized TDMA (Time Division

Multiple Access)

  • Addresses contention overhead
slide-11
SLIDE 11

11

 TDMA by VoIP stations:

  • Avoids collisions by serializing channel

access

  • Cycle of 10 TDMA slots, each 1 ms

AP send at t=2 send at t=1 send at t=3 time

 VoIP stations must:

  • Establish TDMA schedule
  • Synchronize clocks
  • Compete with non-TDMA traffic
  • Compete with non-TDMA traffic
slide-12
SLIDE 12

12

channel

  • ccupied

 Problem:

  • Non-VoIP stations unaware of TDMA
  • May prevent VoIP stations from sending on time

4 5 TDMA slots 3 .. ..

 Let VoIP stations prioritize their traffic

  • ..by changing 802.11 contention parameters

VoIP user (TDMA) Data user (non-TDMA) time 6

slide-13
SLIDE 13

13

  • VoIP station 5 must wait..

4 5 3 .. .. time 6

 Data packet overruns TDMA slot 5!

  • .. therefore stations 5 and 6 collide in slot 6

 Solution: prioritize among VoIP stations 5 and 6

VoIP user (TDMA) Data user (non-TDMA) 1.4 ms 0.5 ms 1 ms

slide-14
SLIDE 14

14

Time wrt TDMA cycle (µs)

S t a t i

  • n

1 2 3 4 5 6 7 8 9

 Experiment:

  • CSMA/CA background data traffic
  • Ten TDMA VoIP stations

 TDMA:

  • 10-ms cycle
  • 1-ms slots

Most transmissions should start in own

  • r next slot
slide-15
SLIDE 15

15

 Downlink direction:

  • Aggregation across multiple receivers
  • Addresses framing and contention
  • verhead

AP

 Uplink direction:

  • Prioritized TDMA (Time Division

Multiple Access)

  • Addresses contention overhead
slide-16
SLIDE 16

16

AP aggregated IP packet VoIP packets Voice application aggregator

  • verhear
  • verhear
slide-17
SLIDE 17

17

voice application wireless card softspeak control iptables Softspeak

  • enabled

phone AP aggregator de

  • aggregator

downlink aggregation uplink TDMA register IP address + port wireless driver Atheros, Ralink Skype, T winkle TCP/IP

slide-18
SLIDE 18

18

 Impact of Softspeak on:

  • Call quality
  • Residual throughput

 TCP data traffic, 10-ms voice codec  See paper for:

  • UDP data traffic
  • 20-ms codec
  • Simulation results
slide-19
SLIDE 19

19

Downlink MOS TCP throughput (KB/s) default TDMA aggregation softspeak

Throughput Downlink MOS Uplink MOS When TCP downloads

5x 3.5→3.3 3.7→3.6

When TCP uploads

+50% 1 →3.5 2.9→3.8

 

slide-20
SLIDE 20

20

3x

slide-21
SLIDE 21

21

 Testbed with voice + Web + bulk TCP  When enabling Web traffic:

  • Bulk TCP upload improvement disappears
  • However combined TCP capacity improvement is

preserved

 Exactly as is the case without VoIP traffic

slide-22
SLIDE 22

22

 Softspeak:

  • Protects call quality and data throughput
  • Using TDMA and aggregation
  • Implementable in software based on commodity

hardware

 Source code and audio samples at:

  • http://sysnet.ucsd.edu/wireless/softspeak/
slide-23
SLIDE 23

23

 Abundance of prior work:

  • Prioritizing voice, TDMA, aggregation, AP polls stations

(PCF), …

 Share one or more limitations:

  • Targets framing or contention overhead
  • Replaces CSMA/CA contention mechanism
  • Requires changes to AP or WiFi hardware
slide-24
SLIDE 24

24

 802.11e  QoS extension  Prioritizes VoIP

Uplink MOS 802.11b+e 802.11b TCP throughput (KB/s)

 802.11g  Higher speed

slide-25
SLIDE 25

25

 Goal: agree on TDMA schedule

  • Cycle of 10 TDMA slots, each 1 ms

 However:

  • Stations might not hear each other
  • Unmodified access point

AP Probe request 00:21:00:a9:1e:04 Probe request 00:21:00:23:02:02 Probe response 00:21:00:a9:1e:04 Probe response 00:21:00:23:02:02 reserved prefix random slot# Probe response 00:21:00:23:02:04 Probe request 00:21:00:23:02:04 Station 1 Station 2

slide-26
SLIDE 26

26

time channel

  • ccupied

packet backoff

SIFS + (2 + random) * cwslot SIFS + (2 + random) * cwslot

Short inter-frame space (10 µs in 802.11b) Contention window slot (20 µs in 802.11b)

slide-27
SLIDE 27

27

SIFS + (1 + 0 ) * cwslot SIFS + (2 + random) * cwslot

random cwslot SIFS random

SIFS + (2 + random) * cwslot

random=0 time Short inter-frame space (10 µs in 802.11b) Contention window slot (20 µs in 802.11b) channel

  • ccupied

packet

slide-28
SLIDE 28

28

 Station i periodically modifies its contention

parameters

Slot i+2 Slot i Slot i-1 Slot i+1 Standard 802.11: SIFS + (2 + random) * cwslot SIFS SIFS+ 1*cwslot TDMA slot Backoff station i time

slide-29
SLIDE 29

29

 Stations need a shared time reference  Access points send beacons

  • E.g. every ~100ms
  • Heard by all stations

 To synchronize:

  • Reset TDMA clock after each beacon
  • Note: also counters clock drift
slide-30
SLIDE 30

30

Time wrt TDMA cycle (µs)

S t a t i

  • n

1 2 3 4 5 6 7 8 9

slide-31
SLIDE 31

31

 No retransmission for poor overhearer  Exacerbated at higher 802.11g rates  Mitigating steps:

  • Pick specific destination as receiver:

 Have it associate at lower MAC rate  Helps if it’s a poor receiver  Note: can be dedicated device

  • Poor receivers can simply opt out
slide-32
SLIDE 32

32

TCP sends TCP receives +45% 4x

Overhearing improvements: 7% No overhearing improvements: 20%

slide-33
SLIDE 33

33

TCP sends TCP receives Uplink MOS aggregation default TDMA softspeak

Softspeak maintains uplink MOS Severe degradation uplink MOS