Wireless VoIP
C3
R94922096 謝明龍
R94922088 關尚儒
Outline
Problems to use VoIP on wireless network
Voice over WLAN

MAC method
802.11e
Dual queue scheme
VoIP and 802.11x standards
VoIP on Wireless Network
Wireless network – lower speed , noise

Upgrade physical speed , reduce noises (PHY)

Real-time packet prioritize (MAC)
1AP-to-many Station


Upgrade the capacity of single AP
Admission control
Roaming
Mobile device power
Wireless security
Voice over WLAN
802.11 supplements glossary
802.11a – 5GHz OFDM PHY layer
802.11b – 2.4GHz CCK PHY layer
802.11c – bridging tables
802.11d – international roaming
802.11e – quality of service MAC
802.11f – inter-access point protocols
802.11g – 2.4GHz OFDM PHY
802.11h – European regulatory extensions
802.11i – enhanced security
802.11n – MIMO ODFM PHY
PHY  802.11n
2.4GHz+5GHz (a/b/g)
MIMO+OFDM

MIMO (Multiple-In, Multiple-Out)
IEEE 802.11 MAC
Dual Queue Strategy
Dual Queue Strategy
The 802.11e MAC implementation cannot
be done by just upgrading the firmware of
an existing MAC controller chip only
It is difficult to Upgrade (replace) the
existing APs
Dual Queue Strategy
above 802.11 the MAC controller


Original NIC driver  FIFO queue
New NIC driver  RT + NRT queue
Strict priority queuing
Effect of MAC HW Queue
Dual Queue Strategy
VOIP AND ADMISSION CONTROL
VoIP

codec  G.711
64 kbps stream
8-bit pulse coded modulation (PCM)
sampling rate : 8000 samples/second

A VoIP Packet per 20ms
160-byte DATA + 12-byte RTP header + 8-byte
UDP header+ 20-byte IP header + 8-byte SNAP
header
= 208 bytes per VoIP packet
VOIP AND ADMISSION CONTROL
VoIP Admission Control

assumptions
ACK Packet transmitted with 2Mbps
Long PHY preamble

Packet transmission MAC
DIFS deference
Backoff
Packet transmission
SIFS deference
ACK transmission
VOIP AND ADMISSION CONTROL
VoIP packet transmission time ≒ 981μs

VoIP MAC packet transmission time
192-μs PLCP preamble/header + (24-byte MAC header + 4byte CRC-32 + 208-byte payload) / 11 Mbits/s = 363 μs

ACK transmission time at 2 Mbits/s
192-μs PLCP preamble/header + 14-byte ACK packet /
2Mbits/s = 248 μs

Average backoff duration
31 (CWmin) * 20 μs (One Slot Time) / 2 = 310 μs
VOIP AND ADMISSION CONTROL
Every VoIP sessioin


inter-active  2 senders
one voice packet transmitted every 20ms
Every 20ms time interval

20 (= 20 ms / 981 μs) voice packets
Maximum number of VoIP sessions over a
802.11 LAN is 10
COMPARATIVE PERFORMANCE
EVALUATION
Using the ns-2 simulator


802.11b PHY
Traffic
Voice  two-way constant bit rate (CBR) session
according to G.711 codec
Data unidirectional FTP/TCP flow with 1460-byte
packet size and 12-packet (or 17520-byte) receive
window size.
COMPARATIVE PERFORMANCE
EVALUATION
EVALUATION RESULT
Pure VoIP
Effect of VoIP with different TCP session
numbers
Performance with Dual queue
Unfairness of NRT Packet
Effect of MAC HW Queue
Observation
Compare to our Evaluation

packet drop rate
50 packets for the RT queue size
Downlink is disadvantaged
Simulation results are based on 11 Mbps
EVALUATION RESULT
Pure VoIP
Effect of VoIP with different TCP session
numbers
Performance with Dual queue
Unfairness of NRT Packet
Effect of MAC HW Queue
Observation
Effect of queue size
EVALUATION RESULT
Pure VoIP
Effect of VoIP with different TCP session
numbers
Performance with Dual queue
Unfairness of NRT Packet
Effect of MAC HW Queue
Observation
worst case delay 11ms


Queuing delay with the single queue
MAC HW queue wireless channel access
NRT queues


Size = 50 or 100  increase as the number of
TCP flows increases
Size = 500  almost no change in delay
EVALUATION RESULT
Pure VoIP
Effect of VoIP with different TCP session
numbers
Performance with Dual queue
Unfairness of NRT Packet
Effect of MAC HW Queue
Observation
Unfairness

between upstream and downstream TCP
flows with the queue sizes of 50 and 100
Queue size for the AP should be large
enough - This is good for us
EVALUATION RESULT
Pure VoIP
Effect of VoIP with different TCP session
numbers
Performance with Dual queue
Unfairness of NRT Packet
Effect of MAC HW Queue
Observation
Delay of downlink voice packets

increases linearly proportional to the MAC HW queue
size
Another effect

with the MAC HW queue size of 8, the worst delay is
observed with a single VoIP session
Large MAC HW queue size is still aceptable

<25ms
Brief Summary
Driver of the 802.11 MAC controller
Strict priority queuing
Bottleneck of TCP in WLAN  downlink
VoIP and 802.11e QoS
standards
What’s the difference between
Wireless/Wired VoIP?
Mobility

Roaming
Security

Hidden UA
Quality of Service

Guarantee of voice quality
Hidden Node Problem
Quality of Service
QoS problems
802.11e QoS standard
A non-standard solution –
Dual Queue Strategy
QoS Problems
Dropped Packets
Delay
Jitter
Out-of-order Delivery
Error
VoIP requires strict limits on jitter and
delay
Quality of Service
QoS problems
802.11e QoS standard
A non-standard solution –
Dual Queue Strategy
IEEE 802.11e
A draft standard of July 2005
It defines a set of QoS enhancements for
WLAN applications
and enhances the IEEE 802.11 Media
Access Control (MAC) layer
Coordination Function
For stations to decide which one has the
right to deliver its packets
802.11: DCF & PCF
802.11e: EDCF & HCF
Original 802.11 MAC
Distributed Coordination Function (DCF)
Point Coordination Function (PCF)
Distributed Coordination Function
(DCF)
Share the medium between multiple
stations
Rely on CSMA/CA and optional 802.11
RTS/CTS
How DCF works?
DCF Limitations
When many collisions occur, the available
bandwidth will be lower
No notion of high or low priority traffic
A station may keep the medium
If the station has a lower bitrate, all other
stations will suffer from that
No QoS guarantees
Original 802.11 MAC
Distributed Coordination Function (DCF)
Point Coordination Function (PCF)
Point Coordination Function (PCF)
Available only in "infrastructure" mode
Optional mode, only very few APs or Wi-Fi
adapters actually implement it
Beacon frame, Contention Period, and
Contention Free Period
How PCF works?
802.11 MAC Layer Framework
802.11e MAC Protocol Operation
Enhanced DCF (EDCF)
Hybrid Coordination Function (HCF)
Enhanced DCF (EDCF)
Define Traffic Classes
High priority traffic has a higher chance of being
sent than low priority traffic
A "best effort" QoS
Simple to configure and implement
802.11e MAC Protocol Operation
Enhanced DCF (EDCF)
Hybrid Coordination Function (HCF)
Hybrid Coordination Function (HCF)
Works a lot like the PCF
Main difference with the PCF: Define the
Traffic Classes (TC)
Stations are given a Transmit Opportunity
(TXOP)
The most advanced (and complex)
coordination function
QoS can be configured with great
precision
Conclusion
Paper References 1
Jeonggyun Yu, Sunghyun Choi, Jaehwan Lee, “Enhancement of
VoIP over IEEE 802.11 WLAN via Dual Queue Strategy”
Moncef Elaoud, David Famolari, and Ahbrajit Ghosh, “Experimental
VoIP Capacity Measurements for 802.11b WLANs”
Mustafa Ergen, “I-WLAN: Intelligent Wireless Local Area Networking”
Gyung-Ho Hwang, Dong-Ho Cho, “New Access Scheme for VoIP
Packets in IEEE 802.11e Wireless LANs”
Sai Shankar N, Javier del Prado Pavon, Patrick Wienert, “Optimal
packing of VoIP calls in an IEEE 802.11a/e WLAN in the presence of
QoS Constraints and Channel Errors”
Paper Reference 2
Experimental VoIP capacity measurements for
802.11b WLANs
Enhancement of VolP over IEEE 802.11 WLAN
via dual queue strategy
An experimental study of throughput for UDP
and VoIP traffic in IEEE 802.11b networks
Admission control for VoIP traffic in IEEE 802.11
networks
How well can the IEEE 802.11 wireless LAN
support quality of service
Web Site References
http://www.ieee.or.com/Archive/80211/802_11e_
QoS_files/frame.htm
http://en.wikipedia.org/wiki/IEEE_802.11
http://www.cs.nthu.edu.tw/~nfhuang/chap13.htm
#13.1
http://www.eettaiwan.com/ART_8800360909_67
5327_3f3ffd7b_no.HTM
http://it.sohu.com/2003/12/11/09/article21675098
5.shtml