Evaluating Opportunistic Delivery of Large Content with TCP over WiFi in I2V Communication

TL;DR

This study evaluates TCP throughput over outdoor WiFi for connected vehicles, revealing how link impairments and mobility cause throughput degradation, and suggests the need for protocol adaptations for vehicular content delivery.

Contribution

It provides an experimental analysis of TCP performance over WiFi in vehicular scenarios, highlighting the impact of mobility and link impairments on throughput.

Findings

WiFi impairments cause TCP duplicate ACKs and congestion window reductions.

Throughput degrades significantly with increased distance and mobility.

Protocol adaptations are needed for reliable vehicular WiFi content delivery.

Abstract

With the increasing interest in connected vehicles, it is useful to evaluate the capability of delivering large content over a WiFi infrastructure to vehicles. The throughput achieved over WiFi channels can be highly variable and also rapidly degrades as the distance from the access point increases. While this behavior is well understood at the data link layer, the interactions across the various protocol layers (data link and up through the transport layer) and the effect of mobility may reduce the amount of content transferred to the vehicle, as it travels along the roadway. This paper examines the throughput achieved at the TCP layer over a carefully designed outdoor WiFi environment and the interactions across the layers that impact the performance achieved, as a function of the receiver mobility. The experimental studies conducted reveal that impairments over the WiFi link (frame loss, ARQ and increased delay) and the residual loss seen by TCP causes a cascade of duplicate ACKs to be generated. This triggers large congestion window reductions at the sender, leading to a drastic degradation of throughput to the vehicular client. To ensure outdoor WiFi infrastructures have the potential to sustain reasonable downlink throughput for drive-by vehicles, we speculate that there is a need to adapt how WiFi and TCP (as well as mobility protocols) function for such vehicular applications.