Protocol Analysis of Wireless Audio Transceivers

Low-level analysis of the USB communication protocol used by a Corsair wireless audio dongle. The project covers descriptor enumeration, interface mapping, USB traffic capture on Arch Linux, and active techniques to stimulate isochronous audio streams for inspection.

→ Tools Used

Hardware

• Device: Corsair wireless headset USB dongle
• USB spec: USB 1.10, Full Speed (12 Mbps)
• Logical interfaces: 4 (numbered 0–3)
• Default Endpoint (EP 0): handles all global control transfers

Interface & Endpoint Map

Endpoint	Direction	Transfer type	Max packet	Interval	Purpose
0x00	Bidirect	Control	64 bytes	N/A	Device config, volume/mute
0x81	IN	Isochronous	144 bytes	1 ms	Microphone audio stream → host
0x02	OUT	Isochronous	288 bytes	1 ms	Headphone audio stream → device
0x83	IN	Interrupt	32 bytes	1 ms	Physical button events → host

Interface 0: AudioControl

Manages mute and volume for both the microphone and headphone channels via internal Feature Units. No active endpoints; all requests go through EP 0.

Interface 1: AudioStreaming (Microphone)

• Alternate Setting 0: no active endpoints (bandwidth saving when mic is idle)
• Alternate Setting 1: PCM, Mono, 16-bit, 8000–48000 Hz

Interface 2: AudioStreaming (Headphones)

• Alternate Setting 2 (active): PCM, Stereo, 16-bit, 8000–48000 Hz
• Larger max packet size (288 bytes) reflects the higher stereo bandwidth requirement

Interface 3: HID

Captures physical hardware events (mute button, volume wheel, pairing). Interrupt transfer guarantees low-latency response.

Methodology

1. Environment setup: USB traffic capture on Arch Linux

Load the usbmon kernel module to enable USB bus monitoring:

sudo modprobe usbmon
# Persist across reboots
echo "usbmon" | sudo tee /etc/modules-load.d/usbmon.conf

Set permissions so Wireshark can capture without root:

/etc/udev/rules.d/99-usbmon.rules

SUBSYSTEM=="usbmon", GROUP="wireshark", MODE="0640"

sudo udevadm control --reload-rules && sudo udevadm trigger

Identify which USB bus the dongle is on:

lsusb
# Example output: Bus 002 Device 003: ID 1b1c:1b3c Corsair Corsair Gaming Keyboard
# → capture interface: usbmon2

2. Idle state analysis

Unlike a mouse or keyboard, the audio dongle drops to zero USB traffic when no audio is playing or no buttons are pressed. This is by design: isochronous endpoints only activate when the OS opens the audio stream.

Monitor device state continuously:

sudo watch -n 1 lsusb -v -d 1b1c:0a51

3. Stimulating isochronous traffic

To force the USB audio stream and observe isochronous packets in Wireshark:

# Find the ALSA card number
aplay -l

# Inject pink noise directly to the hardware (replace X with card number)
speaker-test -D hw:X,0 -c 2 -r 48000

This forces the USB driver to initialize periodic isochronous OUT transfers, visible in Wireshark as a continuous burst of packets.

4. Software-triggered USB re-enumeration

To capture the full descriptor negotiation (Get Descriptor → Set Configuration → address assignment) without physically unplugging the device:

# Find the device port path
lsusb -t
# Example: 3-1

# Logical power-cycle loop
while true; do
  echo 0 | sudo tee /sys/bus/usb/devices/3-X/authorized > /dev/null
  sleep 1
  echo 1 | sudo tee /sys/bus/usb/devices/3-X/authorized > /dev/null
  sleep 5
done

Each cycle forces the kernel to destroy and rebuild the device’s logical structure, producing a clean, isolated enumeration sequence in Wireshark per loop.

Tools Used

Tool	Purpose
Wireshark	USB packet capture and analysis
usbmon	Linux kernel module for USB bus monitoring
lsusb	USB device enumeration and descriptor inspection
ALSA / aplay / speaker-test	Audio stream stimulation
sysfs	Software-controlled USB authorization cycling

Installation guide

Windows

Note: usbmon, lsusb, and sysfs are Linux-only. On Windows you can only install Wireshark natively; the rest require WSL 2.

# Wireshark (includes USBPcap for USB capture)
winget install WiresharkFoundation.Wireshark

For usbmon, lsusb, and ALSA, install WSL 2 with Arch Linux or Debian and follow the sections below inside the WSL terminal.

# Install WSL 2
wsl --install

Debian / Ubuntu

# Wireshark
sudo apt update
sudo apt install wireshark

# lsusb
sudo apt install usbutils

# ALSA tools (aplay, speaker-test)
sudo apt install alsa-utils

# usbmon is part of the kernel, just load the module
sudo modprobe usbmon

# Add your user to the wireshark group
sudo usermod -aG wireshark $USER

Arch Linux

# Wireshark (CLI + GUI)
sudo pacman -S wireshark-qt

# lsusb
sudo pacman -S usbutils

# ALSA tools
sudo pacman -S alsa-utils

# usbmon is built into the kernel, load with:
sudo modprobe usbmon

# Add your user to the wireshark group
sudo usermod -aG wireshark $USER

After adding yourself to the wireshark group, log out and back in for the change to take effect.