Performance Optimizations for Deploying VoIP Services in Mesh Networks

1. Samrat Ganguly et al.IEEE JSAC’06November 2006 Performance Optimizations for Deploying VoIP Services in Mesh Networks Yoonchan Choi Advanced Networking Lab Oct 9, 2007

2. Outline • Introduction • Related Work • VoIP Service in Mesh Network • Improving the VoIP Capacity • Maintaining the VoIP Call Quality • Mobility Support for VoIP Client • Conclusion

3. Introduction • Wireless VOICE-OVER-INTERNET PROTOCOL (VoIP) • Gains significant popularity due to ubiquity of WLAN • Requires wide area wireless coverage for true mobile phone services • IEEE 802.11-based multihop wireless mesh networks • Considered as a practical solution for wireless VoIP service • Has the following benefits compared with wired LAN connecting WiFi access points • Ease of deployment and expansion • Better and wider coverage • Resilience to node failure • Reduced cost of maintenance

4. Introduction • IEEE 802.11-based multihop Wireless Mesh Network for VoIP

5. Introduction • Goals • Increasing the VoIP capacity of mesh network • The number of supported medium quality calls • decreases with the increase in hops • reduces from eight calls in single hop to one call after four hops

6. Introduction • Goals • Maintaining the VoIP quality • Delay and loss characteristics can vary over time along a multihop path between a source and destination • Such variations impact the quality of a voice call • Supporting mobility to VoIP client • Maintaining calls for mobile VoIP clients that handoff to different APs during an ongoing voice call • Preventing packet losses during a handoff

7. Related Work • Packet aggregation schemes • For improving the end-to-end throughput for applications on various types of multihop networks • Methods • The forced algorithm • The adaptive algorithm • Packet aggregation schemes over IP-based networks • Packet aggregator for multiple VoIP streams in wireless networks • Problems • Difficult to attain the delay bound for real-time application • Inefficient use of the bandwidth • No consideration about the characteristics of the wireless mesh network • Not adjustable for the network situation

8. VoIP Service in Mesh Network • VoIP Performance Metric • R-factor • Defines quality of a call • Should provide a value above 70 for medium quality R = 94.2 – 0.024d – 0.11(d – 177.3)H(d – 177.3) – 11 – 40log(1+10e) • d = 25 + djitter + dnetwork • e = enetwork + (1 – enetwork)ejitter • H(x) = 1 if x > 0; 0 otherwise

9. VoIP Service in Mesh Network • VoIP Service Deployment • Using the concept of a layer-2 switch to see the entire mesh as a single element that switches packets between its ports • A port is a mesh node which has at least two interfaces • One in ad hoc mode for the backhaul in the mesh • One in infrastructure mode to connect to clients • Client-side transparency • The clients associate with an AP using a traditional association mechanism • The handoff process involves both layer-2 and layer-3 procedures

10. VoIP Service in Mesh Network • Testbed Hardware / Software Configuration • Consists of 14 nodes based on the Routerboard • 200 series processor boards with 256 MB of RAM, and 512 MB of compact flash • Each node is equipped with two 802.11b wireless interfaces (supports up to 4 mini-pci cards) and has an open slot for a third one (PCMCIA 16 bit) • The testbed is spread over the third floor of NEC Research Laboratories, Princeton, NJ

11. VoIP Service in Mesh Network • Mesh Node • Using the click router on each mesh node • To provide label switching, routing, and aggregation • Label-Based Forwarding • Implementing the label-based forwarding to achieve fast path switching • To support the proposed path switching • Uses TOS field of each IP packet • For packets with labels other than zero, label-based forwarding is used • Packets with label zero follows underlying routing protocol used in mesh network • Each node has an addition table • (in_label, out_label, interface, gateway)

12. Improving the VoIP Capacity • VoIP Packet Aggregation • Small sizes of packets reduce the network utilization • Merges small voice packets from different calls into larger packets to improve channel utilization • Reduces the overhead and increases the number of supported calls • Two mechanism for aggregation in a mesh network scenario • End-to-end scheme • The aggregation is done only at the ingress node for all flows routed for a common destination • Hop-by-hop scheme • Packets are aggregated and deaggregated at every hop by adding a forced delay at every hop

13. Improving the VoIP Capacity • VoIP Packet Aggregation P—packet being queued at a node; A—aggregation packet being prepared; minPackets—number of packets from the same flow that have to be aggregated at the ingress; MTU—maximum transmission unit, in voice packets; find queue of P; 1: if size(queue) > minPackets add all packets from flow(P); if size(A) < MTU find a queue with the same dest go to 1; else send A directly to destination; else if size(A) < MTU add pkts w same nexthop as p; else if aggregation timer is expired aggregate all packets from the queue of which timer is expired also;

14. Improving the VoIP Capacity • Evaluation of VoIP Packet Aggregation • Aggregation performance for randomcalls on a string topology • Traffic statistics of three aggregationalgorithms on three calls over four-hopleaf nodes

15. Improving the VoIP Capacity • Header Compression • A complementary scheme related to aggregation • Packet headers with redundancy may be reduced through compression techniques as has been done with great success for cRTP or ROHC • Zero-length Header Compression (ZHC) algorithm • Eliminates the redundant header • Leverages the VoIP packet aggregation mechanism • Not require context synchronization between two nodes

16. Improving the VoIP Capacity • Evaluation of Header Compression

17. Maintaining the VoIP Call Quality • Routing • To perform call admission within seconds of placing a voice call • Pre-computed paths are used even if the solution is suboptimal • The call admission process should preferably be distributed • Voice call routing approach consists of • Route discovery • Opting for using DSDV to collect popular routes which are then pinned down and used with label-based routing • Adaptive path selection • Pinning down the paths using label-based forwarding

18. Maintaining the VoIP Call Quality • Evaluation of Adaptive Routing • Delay distribution adaptive versus fixed • Only 12% of the time the delay is greater than 200 ms when the path is adaptive • A fixed path provide delays greater than 200 ms 50% of the time

19. Mobility Support for VoIP Client • Mobile IP-Based Scheme • Uses a technique similar to mobile IP where each station has a unique home AP • Difference from Mobile IP • The mobile station does not need to implement any specific protocol • Mobile Location Register (MLR) • Flat Routing-Based scheme • Uses a full-fledged multihop routing infrastructure in the network of APs in the DS

20. Mobility Support for VoIP Client • Handoff Performance Evaluation • Mesh topology used for handoff performance evaluation

21. Mobility Support for VoIP Client • Handoff Performance Evaluation • Timeline describing a typical fast one-hop handoff for flat routing and TMIP schemes when mobile station switches association from N1 to N2

22. Mobility Support for VoIP Client • Handoff Performance Evaluation • Handoff latencies for flat routing and TMIP with background traffic of 2.5 Mb/s and client probing on a single channel

23. Conclusion • Packet aggregation along with header compression can increase the number of supported VoIP calls in a multihop network by 2–3 times • The proposed fast path switching is highly effective in maintaining the VoIP quality • Our fast handoff scheme achieves almost negligible disruption during calls to roaming clients • Main contribution • Proposing and evaluating performance optimizing techniques that are crucial for supporting VoIP over mesh network