Performance of Bundling and Multiplexing in an

Figure 7.eHSPA protocol stack for VoIP.

Figure 8.LTE protocol stack for VoIP.

In eHSPA systems, due to the soft handover traffic in the up-link direction, there is significant cross traffic between the eHSPA Node Bs over the Iur interface. The load of this traffic can also be reduced by applying multiplexing on the Iur inter-face; this was also investigated in our simulations. As bundling introduces higher extra delay, it is not feasible to apply bun-dling on the Iur interface.

4.1.3 Performance of Bundling and Multiplexing in an

Figure 9.Average link usage if bundling is used, including a reference case without bundling. The differences in throughput are the gains due to the efficiency improvement. Simulated with 20 VoIP connections.

Figure 10.Average link usage if multiplexing is used, including a reference case without multiplexing. The differences in throughput are the gains due to the efficiency improvement. Simulated with 20 VoIP connections.

The simulations investigated the maximum number of con-nections the bottleneck backhaul links can support. To evaluate this, the quality of voice connections was calculated on the MOSc scale [18][19][20]. When the bottleneck link becomes congested, packets will be queued in the IP buffers, and some of them will be dropped. This clearly disrupts the perceived quality and the phone calls will become unintelligible. The

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voice quality on the MOSc scale is a good indicator of the max-imum number of supported VoIP connections, and it also con-siders the effect of the added delays. The simulations were re-peated with different numbers of VoIP connections. The effect of UDP level bundling is shown in Figure 11, the maximum number of packets bundled together was simulated with differ-ent values. While the transport is more efficidiffer-ent when more packets are bundled together, the largest performance differ-ence is between the cases with only two packets bundled to-gether and no bundling at all. The effect of UDP level multiplex-ing is shown in Figure 12. In this case, the maximum time a packet may be delayed was set to different values; the average number of packets multiplexed together is proportional to this parameter. It can be seen that multiplexing is far more effective than bundling at the same protocol layer. Furthermore, when the number of VoIP connections is sufficiently large to congest a link, then the frequency of VoIP packet arrivals is also high;

therefore, a sufficient number of packets can be multiplexed to-gether even in the case of a short maximum added delay. Figure 13 compares the efficiency of bundling performed at different protocol layers. It is clear that the high level bundling per-formed at the RTP layer is by far more efficient than bundling performed at lower protocol layers. Figure 14 compares the ef-ficiency of different multiplexing techniques, and reveals that multiplexing performed at a higher layer is more efficient. Ad-ditionally, multiplexing the soft handover traffic on the Iur in-terface that connects Node Bs is also found to improve perfor-mance.

Figure 11.Voice quality according to the number of parallel VoIP connec-tions on a 2 Mbit/s link when UDP level bundling is used. A reference case without bundling is also shown. The backhaul link can support more VoIP connections if bundling is used.

Figure 12.Voice quality according to the number of parallel VoIP connec-tions on a 2 Mbit/s link when UDP level multiplexing is used. A reference case without multiplexing is also shown. The backhaul link can support more VoIP connections if multiplexing is used.

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Figure 13.Comparison of bundling performed at different protocol layers.

Voice qualities according to the number of parallel VoIP connections on a 2 Mbit/s are shown. A reference case without bundling is also shown.

RTP bundling, performed at a higher protocol layer, enables the most parallel connections.

Figure 14.Comparison of different multiplexing techniques. Multiplexing may be applied only on the gateway–Node B interface, or also to soft hando-ver traffic. Voice qualities according to the number of parallel VoIP con-nections on a 2 Mbit/s are shown. A reference case without multiplexing is also shown. The best performance is achieved by higher level multi-plexing on both interfaces.

Based on these eHSPA results, it can be stated that while RTP level bundling offers the highest bandwidth savings, the best bandwidth–delay trade-off can be achieved with UDP level multiplexing, especially if multiplexing is extended to traffic on the Iur interface. VoIP consumes considerable bandwidth when there are many parallel connections in parallel, thus in the most relevant cases, multiplexing will be efficient even if the VoIP packets are delayed for only a few milliseconds.

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