engleski

Practical Cross-Layer Testing of HARQ-Induced Delay Variation on IP/RTP QoS and VoLTE QoE

As the LTE PHY/MAC-layer IR HARQ and RLC-layer ARQ protocol, adopted in LTE, may create sudden ramping of the delay when retransmissions occur, the arising question is whether these significantly and dominantly contribute to the overall packet delay or not? With this regard, in addition to the TCP retransmissions, with LTE protocol stack, there are two more retransmitting layers – the PHY/MAC and the RLC, which may impose intolerable additional delay of data blocks and so of the IP and finally the RTP packets, which further on may significantly degrade the end-to- end voice QoE, especially when voice is to be transmitted over LTE. We propose and demonstrate a VoLTE QoS and QoE test procedure based on cross- layer protocol analysis (HARQ/IP/TCP-UDP/RTP) and perceptual speech quality measurements. Specifically, we found out that, for the HARQ RTT value of about 8 ms, only up to 2 HARQ retransmissions is appropriate during any voice packet, otherwise the delay accumulation might not be possibly “smoothed out” by jitter/playback buffers along the propagation path.

Keywords—VoLTE ; voice QoE testing ; protocol analysis

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