Practical cross-layer testing of HARQ-induced delay variation on IP/RTP QoS and VoLTE QoE

Adriana Lipovac,Ines Obradović,Ivan Grbavac,Vlatko Lipovac

doi:10.1186/s13638-021-01970-y

Adriana Lipovac, Ines Obradović + Show 2 more

Open Access

https://doi.org/10.1186/s13638-021-01970-y

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Abstract

As the PHY/MAC-layer IR-HARQ and RLC-layer ARQ error recovery procedures, adopted in LTE, may impose additional delay when their code-block retransmissions occur, the arising question is whether these significantly contribute to IP and consequently RTP packet delays, and finally degrade the overall application-layer end-to-end QoE, especially when voice is transmitted over LTE? With this regard, we propose and demonstrate a VoLTE QoS and QoE test procedure based on PHY/MAC/RLC/IP/TCP-UDP/RTP cross-layer protocol analysis and perceptual speech quality QoE measurements. We identified monotonic relationship between the paired observations: QoE and HARQ RTT, i.e. between the PESQ voice quality rating and the IP/RTP packet latency, for given BLER of the received MAC/RLC code-blocks. Specifically, we found out that, for the HARQ RTT value of about 8 ms, only up to 2 HARQ retransmissions (and consequently no RLC-ARQ one) is appropriate during any voice packet, otherwise delay accumulation might not be accordingly “smoothed out” by jitter/playback buffers along the propagation path.

Highlights

Voice-over-Internet Protocol (IP) (VoIP) has been deployed for quite a long time in wireline networks paving the way to IP telephony [1]
Let us recall that, in addition to the Transmission Control Protocol (TCP) retransmissions, in Long Term Evolution (LTE) protocol stack, there are two more retransmitting layers: the Physical/Medium Access Control (PHY/Medium access control (MAC)) and the Radio Link Control (RLC), which may impose additional delay onto their data frames, and onto Internet Protocol (IP) and, Real-Time transport Protocol (RTP) packets, which, further on, may degrade the end-to-end Quality-of-Experience (QoE), especially when voice is transmitted over LTE [2]
This practically implies that only up to 2 Hybrid Automatic Repeat-reQuest (HARQ) retransmissions are appropriate during any voice packet, otherwise it might be impossible to “smooth out” the delay accumulation by jitter/playback buffers along the propagation path

Summary

Introduction

Voice-over-IP (VoIP) has been deployed for quite a long time in wireline networks paving the way to IP telephony [1]. Lipovac et al J Wireless Com Network (2021) 2021:83 With this regard, let us recall that, in addition to the Transmission Control Protocol (TCP) retransmissions, in LTE protocol stack, there are two more retransmitting layers: the Physical/Medium Access Control (PHY/MAC) and the Radio Link Control (RLC), which may impose additional delay onto their data frames, and onto Internet Protocol (IP) and, Real-Time transport Protocol (RTP) packets, which, further on, may degrade the end-to-end Quality-of-Experience (QoE), especially when voice is transmitted over LTE [2]. As the LTE protocol stack PHY and MAC layers are both with a number of variable parameters and fairly complex, with deployment of VoLTE in particular, providing their services to the network and transport layers through cross-layer design and management, has got new challenges in the everlasting goal to achieve high service throughput and low delay [3]

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