Device-to-device communication on Android

Research note behind BlueLib's move from "Bluetooth library" to "device communication library". It lists every way an Android app can exchange data with another device, what the platform requires for each, and where BlueLib stands.

Status legend: Shipped in BlueLib, Planned (fits the ByteConnection model, on the roadmap), Out of scope (needs Google Play services, system privileges, or is not a data link).

1. Wired

Link	Android API	Requirements	Status
USB serial (CDC-ACM, FTDI, CP210x, CH34x)	UsbManager host mode, bulk + control transfers	USB OTG, per-device user consent	Shipped
USB raw bulk (vendor class, printer class)	UsbManager	Bulk IN/OUT endpoint pair	Shipped
USB serial PL2303	UsbManager	Vendor init sequence, several chip revisions	Planned
USB HID (barcode scanners in HID-POS mode, custom HID)	UsbManager interrupt endpoints	HID report parsing	Planned
USB accessory (AOA)	UsbManager.openAccessory	Accessory acts as USB host; firmware must speak AOA	Planned
Native UART /dev/tty* (RS-232/RS-485/TTL)	java.io file streams + stty	Vendor or rooted image grants device file access	Shipped
RS-485 half duplex	Same as serial	Adapter with automatic direction control	Shipped via USB/UART; manual RTS direction not supported
Ethernet (RJ45)	Same as TCP/IP below	USB Ethernet adapter or built-in port	Shipped (through TCP)
GPIO, I2C, SPI	None public (Android Things was retired in 2022)	Vendor SDK on industrial boards	Out of scope
CAN bus	No platform API	USB-CAN adapters that speak SLCAN over CDC-ACM work as a serial link	Shipped as raw serial; SLCAN framing planned
Audio jack (FSK, "Square" readers)	AudioRecord/AudioTrack	Modem in software; jack missing on most phones	Out of scope

Notes

• Why BlueLib ships its own USB serial drivers. The well-known usb-serial-for-android library is published on JitPack only. A Maven Central library that depends on it would force every consumer to add the JitPack repository, so BlueLib implements the four chip families that cover nearly all cables (ported from the Linux cdc-acm, ftdi_sio, cp210x and ch341 drivers) and verifies the register values in JVM tests.
• RJ45 means two things. Ethernet (TCP/IP) or RS-232/RS-485 wired on an RJ45 connector. See the TCP/IP, USB and serial guide.
• Native UART access is a property of the device image, not of the app. Typical paths: /dev/ttyS0..9 (Rockchip, Allwinner), /dev/ttyMSM* (Qualcomm), /dev/ttyHS*, /dev/ttyAMA* (Raspberry Pi).

2. IP networking (Wi-Fi, Ethernet, cellular)

Link	Android API	Requirements	Status
TCP client	java.net.Socket	INTERNET permission	Shipped
TCP server (device connects to the phone)	java.net.ServerSocket	INTERNET; phone must be reachable	Planned
UDP unicast / broadcast / multicast	DatagramSocket, MulticastSocket	INTERNET; multicast needs WifiManager.MulticastLock	Planned
Service discovery (mDNS/DNS-SD, Bonjour)	NsdManager	INTERNET	Planned
SSDP/UPnP discovery	UDP multicast	MulticastLock	Planned
Join a device's own access point (ESP32 provisioning, IP cameras)	WifiNetworkSpecifier (API 29+)	User approval dialog; app-scoped network	Planned
Wi-Fi Direct (P2P)	WifiP2pManager	NEARBY_WIFI_DEVICES (API 33+) or location (older)	Planned
Wi-Fi Aware (NAN)	WifiAwareManager (API 26+)	Hardware support, NEARBY_WIFI_DEVICES	Planned
Local-only hotspot	WifiManager.startLocalOnlyHotspot	Location/nearby permission	Out of scope
WebSocket, HTTP/REST, MQTT, CoAP	OkHttp, Paho, Californium	Application protocols on top of TCP/UDP	Out of scope (use a dedicated client)
Google Nearby Connections	Play services	Proprietary, Play services only	Out of scope

Application protocols that ride on a byte stream

These work today with connectTcp, connectUsbSerial or openUart plus sendBytes, and are candidates for dedicated framers:

Protocol	Transport	Framing needed
ESC/POS (receipt printers)	TCP 9100, USB, Bluetooth SPP	None (command stream)
ZPL / TSPL (label printers)	TCP 9100, USB	None
Modbus RTU	RS-485 / RS-232	CRC-16 + inter-frame silence; planned ModbusRtuFramer
Modbus TCP	TCP 502	MBAP header (length prefixed, 6 byte header)
NMEA 0183 (GPS, marine)	Serial 4800/9600	Line based: DelimiterFramer.crlf()
AT commands (modems, HC-05, ESP-AT)	Serial	Line based: DelimiterFramer.crlf()
SLIP / COBS (embedded links)	Serial	Byte stuffing; planned framers
RFC 2217 (Telnet COM port control)	TCP	Lets the app set baud rate on a remote serial port; planned

3. Short-range wireless

Link	Android API	Requirements	Status
Bluetooth LE (GATT client/server, advertising, scanning)	android.bluetooth.le	See the Bluetooth guides	Shipped
Bluetooth Classic (RFCOMM/SPP, L2CAP)	BluetoothSocket	BLUETOOTH_CONNECT	Shipped
NFC reader (NDEF, ISO-DEP, MIFARE, NFC-A/B/F/V)	NfcAdapter reader mode	NFC hardware, foreground activity	Planned (request/response, not a stream)
NFC host card emulation	HostApduService	Declared service	Planned
UWB ranging	androidx.core.uwb	UWB hardware, out-of-band parameter exchange	Out of scope (ranging, not data)
Bluetooth Channel Sounding	android.ranging (API 36+)	See the roadmap	Planned (ranging module)
Thread / Matter	Google Home APIs, Thread network APIs	Play services, border router	Out of scope
Zigbee, Z-Wave, LoRa, Sub-GHz	No platform API	USB or serial dongle (e.g. CC2652 with ZNP over serial, LoRa modules with AT commands)	Shipped as serial link
Infrared	ConsumerIrManager	IR blaster, transmit only	Out of scope

4. Recommended next steps

Ordered by value for the device types BlueLib users connect to most (printers, scales, PLCs, IoT modules):

1. UDP and TCP server — same ByteConnection model; unlocks devices that push data and broadcast discovery.
2. NsdManager discovery — find printers and ESP32 devices by name instead of IP address.
3. Modbus framers (RTU with CRC-16, TCP with MBAP) — the most common industrial protocol.
4. PL2303 driver and multi-port selection (FT2232/FT4232, CP2105/CP2108).
5. WifiNetworkSpecifier helper — device provisioning over the device's own access point.
6. USB attach/detach events as a Flow — today apps poll usbDevices().
7. NFC as a separate request/response port, since it does not fit the stream model.