Wi-PTP: A C repository from huhanwj

Wi-PTP

Abstract

Emerging mobile applications involving distributed control and sensing call for accurate time synchronization over wireless links. This paper systematically studies the performance of Precision Time Protocol (PTP) in today's Wi-Fi networks. We investigate both software and hardware PTP implementations. Our study uncovers the root causes of software PTP synchronization errors. We show that with fine-tuned system configurations and an online calibration procedure, software PTP can achieve reasonable accuracy with off-the-shelf Wi-Fi devices. Hardware PTP requires a PTP hardware timestamper clock not contained in Wi-Fi NICs. We propose a method to make use of the hardware TSF counter to emulate the PTP clock. Rigorous tests traversing various conditions show that both software and hardware PTP implementations can achieve a 1-μs level of accuracy in today's Wi-Fi networks.

The above figure illustrates how Wi-PTP works.

Usage

The ath9k directory contains the modified ath9k driver based on Linux Kernel 4.19.37. The ptp directory contains the adjusted linuxptp.

Build & Replace ath9k Driver

Replace drivers/net/wireless/ath/ath9k in the Kernel source tree with this repo's ath9k. We recommend rsync to do this:

    rsync -avP <repo path>/ath9k <kernel source>/drivers/net/wireless/ath/

Build ath9k driver.

    cd <kernel source>

    make M=drivers/net/wireless/ath

Replace the ath9k driver.

WLAN_DEV=wlan0

ifdown ${WLAN_DEV}

rmmod ath9k
rmmod ath9k_common
rmmod ath9k_hw
rmmod ath

cd <kernel source>/drivers/net/wireless/ath
insmod ./ath.ko
insmod ./ath9k/ath9k_hw.ko
insmod ./ath9k/ath9k_common.ko
insmod ./ath9k/ath9k.ko

sleep 1

ifup ${WLAN_DEV}

Note: please replace the wlan0 above with the actual ath9k interface.

Check for the PTP device.

    ls /sys/class/net/${WLAN_DEV}/device/ptp

The above command should output a ptp<X>.

Build linuxptp

    cd <repo path>/ptp

    make

After building the linuxptp, the ar9300+ wireless cards can be used to perform PTP as ethernet cards.

Citation

Use this bibtex to cite the paper:

@inproceedings {273763,
author = {Paizhuo Chen and Zhice Yang},
title = {Understanding Precision Time Protocol in Today{\textquoteright}s Wi-Fi Networks: A Measurement Study},
booktitle = {2021 {USENIX} Annual Technical Conference ({USENIX} {ATC} 21)},
year = {2021},
isbn = {978-1-939133-23-6},
pages = {597--610},
url = {https://www.usenix.org/conference/atc21/presentation/chen},
publisher = {{USENIX} Association},
month = jul,
}

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