Low Speed Internet — Sub-GHz IP Bridge

A functional low-speed internet link over 915 MHz GFSK using two ESP32-S3 + SX1262 modules (Elecrow ThinkNode M5). Achieves ~40 kbps useful throughput — enough for WhatsApp, Telegram, and lightweight web browsing. Think 56k modem for the 2020s.

Two ThinkNode M5 units running the Sub-GHz bridge — one as Gateway, one as Client.

Real speed test result via the Sub-GHz link: 56 kbps.

How It Works

[Phone/PC] <--2.4GHz WiFi--> [Client M5] <--915 MHz GFSK--> [Gateway M5] <--2.4GHz WiFi--> [Internet Router]
  192.168.5.x     "SubGHz-Bridge"      10.0.0.0/30         192.168.1.x

Two roles, same firmware

Role	ROLE_GATEWAY	Function
Gateway	1	Connects to your home WiFi (STA mode), bridges internet to the 915 MHz link
Client	0	Creates a WiFi AP called SubGHz-Bridge, bridges connected devices to the 915 MHz link

Hardware

• 2× Elecrow ThinkNode M5 (ESP32-S3 + SX1262 + E-Ink display + buttons + buzzer)
• Or any ESP32-S3 board with an SX1262 radio

ThinkNode M5 Pinout

Function	Pin
SX1262 SCK	16
SX1262 MISO	7
SX1262 MOSI	15
SX1262 CS	17
SX1262 RST	6
SX1262 BUSY	5
SX1262 DIO1	4
SX1262 PWR_EN	46
EPD SCK	38
EPD MOSI	45
EPD CS	39
EPD DC	40
Button 1	21
Button 2	14
Buzzer	9
Battery ADC	8
I2C SDA	48
I2C SCL	47

Radio Configuration

Parameter	Value
Frequency	915 MHz (ISM)
Modulation	GFSK
Bit rate	250 kbps
Frequency deviation	80 kHz
RX bandwidth	467 kHz
Preamble	32 bits
TX power	20 dBm
CRC	2 bytes
Whitening	Enabled
Data shaping	BT=0.5
Sync word	4 bytes (0x9C5A3B67)

GFSK was chosen over LoRa for speed: 250 kbps vs ~5 kbps typical with LoRa in the same bandwidth. The SX1262 supports FSK up to 300 kbps.

Protocol Stack

MAC Layer (TDMA)

Time-slotted with Master (Gateway) polling:

Master (Gateway):

1. Dequeue next packet fragment (or send NULL)
2. Build header: ctrl, seq, ack, RSSI, noise
3. Transmit fragment
4. Wait for ACK (up to 60ms)
5. On ACK match → advance to next fragment
6. On timeout/CRC error → retry (up to 3, then abort)

Slave (Client):

1. Listen (up to 1 second)
2. On valid packet → process, build response
3. Transmit back immediately

Packet Format

+--------+------+------+------+-------+------+------------------+
| CTRL   | SEQ  | ACK  | RSSI | NOISE | RSVD | PAYLOAD (0-240)  |
| (1 B)  | (1B) | (1B) | (1B) | (1B)  | (1B) | (variable)       |
+--------+------+------+------+-------+------+------------------+
         \___________ 6-byte header (SG_HDR_LEN) _______________/

CTRL bits:

• 0x40 — NULL type (no data)
• 0x00 — DATA type
• 0x20 — FIRST flag (first fragment of an Ethernet packet)
• 0x10 — MF flag (more fragments follow)

Fragmentation

Ethernet packets (up to 1514 bytes) are split into 240-byte fragments:

• FIRST flag on the first fragment
• MF flag on intermediate fragments
• No MF on the last fragment
• Reassembly buffer: 1600 bytes
• Out-of-order FIRST resets reassembly (desync recovery)

Custom lwIP Netif

A virtual Ethernet interface ("SG") bridges lwIP to the radio:

• sg_transmit(): Queues Ethernet frames from lwIP for radio transmission
• sg_netif_start(): Creates the netif with IP 10.0.0.x/30
• net_rx_task(): Receives reassembled frames and injects them into lwIP

NAPT Configuration

Device	NAPT Interface	Internal Net	External Net
Gateway	SG (10.0.0.1)	10.0.0.0/30	STA (192.168.1.x)
Client	WIFI_AP_DEF	192.168.5.0/24	SG (10.0.0.2)

Performance

Metric	Value
Radio bitrate	250 kbps (GFSK)
Useful throughput	~22-40 kbps
TDMA cycle	~5-6 ms
Peer RTT (ICMP)	51-68 ms
WAN RTT (8.8.8.8)	~256 ms
PER (benchmark)	57-65%
RSSI	-20 to -30 dBm
Noise floor	-106 to -110 dBm
Fragments per 1500B packet	7 (240B each)

Building & Flashing

Using Arduino CLI

# Gateway
arduino-cli compile --fqbn esp32:esp32:esp32s3 SubGHzBridge_M5_v1_2_baseline_gateway
arduino-cli upload --fqbn esp32:esp32:esp32s3 --port COM15 \
    --input-dir SubGHzBridge_M5_v1_2_baseline_gateway/build

# Client
arduino-cli compile --fqbn esp32:esp32:esp32s3 SubGHzBridge_M5_v1_2_baseline_client
arduino-cli upload --fqbn esp32:esp32:esp32s3 --port COM16 \
    --input-dir SubGHzBridge_M5_v1_2_baseline_client/build

Dependencies

• RadioLib v7.7.1+
• GxEPD2 v1.6.7+
• ESP32 Arduino Core v3.3.8+

SDK Configuration Required

CONFIG_LWIP_IP_FORWARD=y
CONFIG_LWIP_IPV4_NAPT=y

Serial Commands

Command	Description
start	Start the RF link
stop	Stop the RF link
bm	Toggle benchmark mode (dummy traffic)
stats	Show live statistics
netif	Show network interface status
ping	ICMP echo to peer (10.0.0.x)
ping8	ICMP echo to 8.8.8.8
dns	DNS query to 8.8.8.8
wan	DNS query bound to AP interface (client only)
reset	Reset statistics counters
reboot	Reboot the device

Button Controls

• B1: Toggle start/stop
• B1 long press: Reset statistics
• B2: Toggle beep on/off
• B2 long press: Run RF selftest
• B1 + B2 >3s: Toggle benchmark mode

Display Shows

• Role (GATEWAY/STA or CLIENTE/AP)
• Battery percentage
• WiFi SSID and IP
• SG link IP
• Link status (LINK UP / BUSCANDO / DETENIDO / BENCHMARK)
• Connected clients count (client only)
• Cycle time, DOWN/UP kbps
• Queue depth, PER, RSSI
• Ethernet TX/RX and retransmissions

Known Limitations

• High RTT (~250ms WAN) limits TCP window scaling and throughput
• Asymmetric PER: Gateway shows high losses, Client shows none
• No IPv6 support
• /30 subnet limits to 2 hosts (gateway + client)