In this paper, we perform an empirical study on commercial 5G. Unlike previous works studying 5G in early deployments, we focus on fully deployed 5G networks that already serve massive numbers of users, and in particular, we perform controlled experiments to analyze TCP and multipath TCP (MPTCP) performances and behaviors over 5G. Our results show that 5G faces a dilemma in maintaining in-network buffer for TCP traffic: on one hand, TCP packets are less likely to accumulate within the network, thanks to the high data rate of 5G radio; but on the other hand, with TCP performance highly sensitive to signal quality, when signal becomes poor abruptly in mobile environment, the in-network buffer easily overflows, leading to packet loss bursts. We also confirm that employing MPTCP can aggregate capacities on 5G and 4G paths, but under the current deployment model where 5G and 4G base stations co-locate and share the same core network, MPTCP does not necessarily enhance communication reliability. In addition to active measurement experiments, we leverage a speedtest service to evaluate commercial 5G from a nationwide perspective. By exploiting over $13k$ crowdsourced speedtest results reported from 197 cities in China, we find that there exist considerable regional differences on 5G performances across the nation, and with the 4G co-locating deployment model, the 5G networks in some populous metropolitan areas have inferior performances. Our study provides insights for people to understand commercial 5G and to optimize upper-layer protocols and applications over 5G.

在本文中，我们对商用 5G 进行了实证研究。与之前在早期部署中研究 5G 的工作不同，我们专注于已经为大量用户提供服务的完全部署的 5G 网络，特别是我们执行受控实验来分析 5G 上的 TCP 和多路径 TCP (MPTCP) 性能和行为。我们的结果表明，5G 在维护 TCP 流量的网络缓冲区方面面临两难：一方面，由于 5G 无线电的高数据速率，TCP 数据包不太可能在网络内累积；但另一方面，由于TCP性能对信号质量高度敏感，当移动环境中信号突然变差时，网内缓冲区很容易溢出，导致丢包突发。我们还确认采用 MPTCP 可以聚合 5G 和 4G 路径上的容量，但在目前5G和4G基站共址共享核心网的部署模式下，MPTCP不一定能提高通信可靠性。除了主动测量实验外，我们还利用速度测试服务从全国范围内评估商用 5G。通过剥削$13克$来自全国197个城市的众包测速结果，我们发现全国5G性能存在较大区域差异，4G同地部署模式下，部分人口密集的大都市区5G网络性能较差。我们的研究为人们了解商用 5G 和优化 5G 上的上层协议和应用提供了见解。