A seamless interaction between mmWave cellular networks and transport protocols (e.g., TCP) will be the key to fully exploit the resources available at mmWave frequencies in an end-to-end connection. While most of the research on mmWave has focused so far on the channel and the PHY and MAC layers, the performance of transport protocols over this kind of links is still relatively unexplored.
In particular, mmWave links are characterized by wide variations in the offered rate with LOS/NLOS transitions. Moreover, the link may be in outage if all the reflections in NLOS are blocked, and packets may be lost. This motivates the introduction of link-layer retransmissions (time diversity), and large buffers to avoid packet losses. We showed that without retransmissions it is not possible to sustain a high TCP throughput in NLOS conditions. However, this also increases the end-to-end latency. Moreover, the undesired consequence is the emergence of the bufferbloat phenomenon, which has a negative impact on both the throughput and the latency. Finally, if there is an extended outage, the loss of multiple packets may trigger a retransmission timeout, and we showed that state-of-the-art TCP congestion control algorithms may require a long time to reach full bandwidth utilization, thus wasting resources.
The performance of the end-to-end connection can be improved using path diversity with Multipath TCP (MP-TCP) on combined LTE and mmWave links. However, we showed that also in this case the available congestion control algorithms cannot fully exploit the resources available in the combined links. Moreover, the distance between the mobile user and the base station affects the choice of the best combination of paths: at large distance it is better to couple a mmWave link with a reliable LTE sub-6 GHz link than with another mmWave link, despite the difference in bandwidths.
Another promising approach is the use of cross-layer information to break the abstract view that TCP has of the end-to-end connection and increase the responsiveness of the protocol to sudden changes in the link quality. We proposed a cross-layer congestion control algorithm (X-TCP) to improve the performance of TCP in uplink scenarios, adapting the congestion window with information related to the mmWave channel, and showed a performance gain in both throughput and latency with respect to TCP CUBIC in randomly generated scenarios.
|M. Polese, M. Giordani, M. Mezzavilla, S. Rangan, and M. Zorzi, “Improved Handover Through Dual Connectivity in 5G mmWave Mobile Networks”, to appear on IEEE Journal on Selected Areas in Communications (JSAC)||2017/09||Mobility, Transport, Simulation|
|M. Polese, R. Jana, M. Zorzi, TCP and MP-TCP in 5G mmWave Networks, in IEEE Internet Computing, vol. 21, no. 5, pp. 12-19, 2017||2017/09||Transport|
|T. Azzino, M. Drago, M. Polese, A. Zanella, M. Zorzi, "X-TCP: A Cross Layer Approach for TCP Uplink Flows in mmWave Networks", accepted for presentation at the MedHocNet 2017||2017/06||Transport|
|M. Polese, R. Jana, M. Zorzi, “TCP in 5G mmWave Networks: Link Level Retransmissions and MP-TCP”, 2017 IEEE Conference on Computer Communications Workshops (INFOCOM WKSHPS)||2017/05||Transport|
|M. Zhang, M. Mezzavilla, R. Ford, S. Rangan, S. Panwar, E. Mellios, D. Kong, A. Nix, and M. Zorzi, "Transport layer performance in 5G mmWave cellular", 2016 IEEE Conference on Computer Communications Workshops (INFOCOM WKSHPS)||2016/04||Transport|