The WiMAX Reference Network Architecture can be used in point-to-point and point-to-multipoint network topologies, and is suitable for providing last-mile, building-to-building, and residential broadband connectivity. Another major application, and the main focus of this study, is the use of fixed WiMAX as backhaul for voice and data services. We evaluate voice over IP (VoIP) performance over a fixed WiMAX testbed and quantify the benefits from employing applicationand network-level aggregation. We examine such aggregation schemes using our fixed WiMAX testbed and report the results for both uplink and downlink. If we use objective mean opinion scores (MOS) as the main gauge of overall performance, application-layer aggregation appears to be the best scheme, allowing our fixed WiMAX testbed to sustain nearly three times more flows in the downlink and over two times more flows in the uplink than when no aggregation is used, at comparable MOS values.
WiMAX has been at the center of attention in wireless communications during the last years. Nonetheless, very few testbed or field trial measurement accounts have been reported in the peer-reviewed literature. We fill this gap by exploring scenarios where fixed WiMAX is employed for VolP traffic. VolP packets typically exhibit large header overheads and small total packet sizes. The actual codec payload per packet is very small compared to the total length of headers appended to each voice frame. RObust Header Compression (ROHC) can significantly decrease header size by capitalizing on static or rarely changing header fields. Aggregating multiple voice frames into one packet is another attractive and effective way to increase application goodput and overall bandwidth utilization. We study the effect ofROHC and application layer aggregation on VolP performance in a fixed WiMAX testbed consisting of one base station and two subscriber stations. We find that ROHC increases the number of simultaneous bidirectional emulated VolP flows by 6% when compared to plain VolPe When aggregation and ROHC are employed in unison, they allow for 86% more flows than standard VoIP to be sustained in our testbed.
A growing number of mobile WiMAX deployments are in progress world-wide and the technology is anticipated to play a key role in next generation mobile broadband wireless networks. However, although the theoretical potential of WiMAX technologies is already well established, independent, publicly released, thorough evaluations of WiMAX network performance in the real world are yet to become available. In this study, we consider synthetic Voice over IP (VoIP) performance over the WirelessMAN-OFDMA air interface of a state of the art mobile WiMAX testbed operating at the 3.5 GHz frequency band and quantify the benefits of employing VoIP aggregation and Robust Header Compression (ROHC). Although VoIP aggregation and ROHC have been proposed and thoroughly evaluated through simulation and modeling, this is the first study to present empirical results from employing them over the WirelessMAN-OFDMA air interface of a real-world WiMAX system. Our results indicate that the combined use of VoIP aggregation and ROHC can increase the number of flows sustained without loss by approximately a factor of three.
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