A Case for WiFi Relay: Improving VoIP Quality for WiFi Users

Amit Mondal, Northwestern UniversityCheng Huang, Microsoft Research

Jin Li, Microsoft ResearchManish Jain, Akamai

Aleksandar Kuzmanovic, Northwestern University

Booming of Unified Communications

• Attending meeting in person is nice, but the overhead due to travel is high– SEA CPT• Planned: ~24 hours• Actual: ~42 hours

• Unified Communications promises

– VoIP service double in next 4 years, with an annual growth rate of 26% (Infonetics Research, 2008)

not being there, but better than being there

• Gradually being adopted by enterprises– Cisco acquired WebEx– Google announced acquisition of GIPS, which powers

– Microsoft Unified Communications

Booming of Unified Communications

• Widely validated in the consumer world

Problem due to WiFi

• Large number of WiFi users in enterprises– more than 43% enterprises provide only WiFi

connections to their employees– 36% of organizations use VoIP over WiFi

VoIP calls using WiFisignificantly worse than those not

(field data from Microsoft UC system)

SureCall and WiFi Relay

• SureCall– a measurement and experiment platform• understand problems and experiment solutions

• WiFi Relay– a simple application-layer solution to improve VoIP

QoS over WiFi

SureCall Platform

• A distributed measurement and experiment platform– Agents are installed on volunteers’ machines– Measurements and experiments driven by a master

• Deployment status– 80 unique machines

• across geographical regions, enterprises and homes

• SureCall VoIP measurement– Emulated voice sessions

• 5-minute (per hour), 50 fps (20 ms gap), 60 byte packet

– Detailed packet-level traces collected

Impact on WiFi links of VoIP

WiFi links can significantly degradeVoIP performance

Effectiveness of Redundancy

Packet losses can be effectively mitigated using application layer packet replication

• Passive analysis with voice packet replication– Replication ratio r = 2, 3, 4 or 5

Overhead of Replication

• Typical voice packet size = 60 bytes• Encapsulated with RTP (12 bytes), UDP (8 bytes), IP (20 bytes),

802.11 MAC (28 bytes), PHY (20 us for 802.11g) headers• Airtime

• DIFS + PHY header + ((60 + 76 bytes) / 54Mbps) = 70 usReplication Ratio Air Time (us)

1 70

2 79

3 87

4 96

Replicating audio packet at application layer causes only marginal increase in air time

WiFi Relay Solution

• Nearby wired endpoints as relays• Heavy replication between relays and wireless

endpoints• No dedicated infrastructure

Evaluation

• Evaluated on the SureCall platform– Upgrade SureCall agents to support relay

• Simultaneous direct and relayed VoIP calls between each pair of SureCall agents– apple-to-apple comparison

• Relay node selection based on enterprise internal location database

Improvement with WiFi Relay

WiFi Relay greatly reduces packet loss

Improvement with WiFi Relay

• Mean Opinion Score (MOS)• Calculated from

packet loss rate and jitter (Cole et al., CCR 2001)

• Fixed de-jitter buffer of 100 ms

WiFi Relay significantly improves VoIP quality for WiFi users

Conclusion

• WiFi links greatly degrade VoIP quality• Application layer packet replication is an

effective way to mitigate packet loss• WiFi Relay significantly improves VoIP quality

Cape Town: attend in person

Elsewhere: use WiFi Relay