Performance of VoIP in a 802.11b wireless mesh network D. Niculescu, S. Ganguly, K. Kim, R. Izmailov NEC Laboratories America

802.11 wireless mesh natural extension of one-hop wireless APs useful in home and enterprise advantages: reliability/recovery scalability load balancing D. Niculescu Performance of VoIP in 802.11...

Multihop disadvantages complex implementation requires route maintenance potential performance degradation traffic from the same flow interferes with itself D. Niculescu Performance of VoIP in 802.11...

Testbed Nodes Intel Stargate (Crossbow) linux 2.4 802.11b wireless cards x 2 15 nodes in 30x50m building Voice traffic Zyxel Prestige 802.11 VoIP phones packet generators

Voice quality measure R-score(loss, delay) is a codec specific quality measure The wireless mesh can trade one for the other: Loss can be reduced by retransmitting packets Delay can be reduced by dropping packets Example: G.729a 20 packets / second 20 bytes max R-score 82% loss < 2% delay < 150ms D. Niculescu Performance of VoIP in 802.11...

Challenges of maintaining voice quality in multihop route changes reduced capacity because of self interference high overhead for small voice packets loss < 2% delay < 150ms D. Niculescu Performance of VoIP in 802.11...

How to improve QoS & capacity? Three techniques multiple cards – scalability, self interference label switching – slow routing updates aggregation/compression – overhead D. Niculescu Performance of VoIP in 802.11...

Multiple cards Single channel Case A Case A Case B Case B 1 7 1 7 1 1 7 1 7 1 Case A 1 1 1 1 1 1 4 11 4 11 4 Case A 1 6 11 1 6 11 1 7 1 7 1 Case B Case B 1 7 1 7 1 1 1 6 6 11 11

Voice routing: problems used DSDV and DSR loss based metrics hop count call across entire building (one card) problems temporary long links slow rerouting how reactive? Conclusion: standard routing is not appropriate for real-time traffic

Voice routing: label switching collect most popular DSDV paths overnight maintain best 5 paths for each S-D pair monitor each path with a low rate ping continuously switch to the best path no packets are lost because of the switch D. Niculescu Performance of VoIP in 802.11...

Voice overhead: 802.11, IP, UDP, RTP G.711 G.729 D. Niculescu Performance of VoIP in 802.11...

aggregation & header compression aggregation only: several IP packets become payload for a large IP packet simulation vs. testbed aggregation + header compression: simulation only header compression (cRTP style) provides little benefit 3x aggregation benefit; 2x header compression benefit D. Niculescu Performance of VoIP in 802.11...

opportunistic aggregation controlled delay is introduced at the ingress of a flow only packets queued for next hop are aggregated intermediate nodes do not delay to improve aggregation short flows do not delay long flows example A, B calls aggregated independently aggregation A+B should not delay A network time for flow A A alone 61ms A,B agg 89ms A+B agg 79ms

implementation Click configuration Intel Stargate (PDA sized) linux 2.4.18 / 64M / 400Mhz click modular router (MIT) DSDV, DSR aggregation modules labels stored in IP header packets can use default routing as well current work: transparent mesh D. Niculescu Performance of VoIP in 802.11...

summary experimental investigation of VoIP quality in mesh three techniques explored use of multiple interfaces easiest scalability challenges: available channels, allocation label switching seems best solution for real-time traffic hard to give guarantees with unlicensed spectrum aggregation && header compression work much better together 13x capacity increase with all optimizations D. Niculescu Performance of VoIP in 802.11...