docs: add rpi tcpip doc (#42)

Co-authored-by: Brendan <[email protected]>

Author: epsi1on

docs/Doxyfile

@@ -201,7 +201,8 @@ EXAMPLE_RECURSIVE      = YES
 # that contain images that are to be included in the documentation (see the
 # \image command).
 
-IMAGE_PATH             = ../examples/ncurses
+IMAGE_PATH             = ../examples/ncurses \
+                         images
 
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docs/images/rpi_tcpip_link.png

@@ -142,3 +142,7 @@ Examples are:
    ```text
    0.0.0.0 0.0.0.0 10.10.3.32
    ```
+
+# Example: create TCP/IP link between two Raspberry Pi
+
+This example demonstrates how a tcpip link could be created between two RPIs: [Example LINK](md_docs_2rpi__tcpip__link.html)

docs/main_page.md

@@ -0,0 +1,105 @@
+# longer range TCPIP via NRF24L01 for a pair of Raspberry PIs
+
+## Intro
+
+This tutorial is trying to use `NRF24L01` to create a TCP/IP link between two Raspberry Pi boards.
+Due to long range of NRFs, some of those have 1800 meter wireless range, it would be good to have a TCPIP link between two RPi via NRFs so it would be possible to have a TCPIP connection between two RPi in longer range. The onboard wifi of RPi cannot do long ranges like 50 meter even in clear sight. 
+
+![schematics](rpi_tcpip_link.png)
+
+## Hardware Configuration
+
+1. wiring: Here is how I did connect the module to the RPi as described in the main RF24 documentation [here](https://nrf24.github.io/RF24/#autotoc_md227).
+2. Noise on 3.3v on RPi: Put some capacitor or L-C filter to reduce the noise on 3.3v supply from RPi.
+3. Shielding PA/LNA module: Shield your radio module, if it has none. See more details [in the RF24 Common Issues document](https://github.com/nRF24/RF24/blob/master/COMMON_ISSUES.md#my-palna-module-fails-to-transmit)
+
+## Software Configuration
+
+1. Enable SPI from [`raspi-config`](https://www.raspberrypi.com/documentation/computers/configuration.html#raspi-config). Select "Interface Options" -> "SPI" -> "Yes" -> "Ok", then exit and reboot the RPi (`sudo reboot`).
+2. Install nRF24 library stack on each machine. See more detail in the [RF24 docs](https://nrf24.github.io/RF24/md_docs_2linux__install.html). 
+
+```text
+sudo apt-get update
+sudo apt-get upgrade
+wget https://raw.githubusercontent.com/nRF24/.github/main/installer/install.sh
+chmod +x install.sh
+./install.sh
+```
+Installer will promp which modules you want to install. I did installed all modules: "RF24", "RF24Network" "RF24Mesh" "RF24Gateway". Also please select `SPIDEV` driver during installation.
+
+after installation done, and if there are no errors in the process, there will be these directories inside the RPi:
+
+```text
+~/rf24libs/RF24
+~/rf24libs/RF24Network
+~/rf24libs/RF24Gateway
+~/rf24libs/RF24Mesh
+```
+
+Next we need to choose a master/primary node (as discussed [here](https://github.com/nRF24/RF24Gateway/issues/41)). so one RPi will be primary, and another one will be secondary. we'll use the official example named `ncurses` in [RF24 repo](https://github.com/nRF24/RF24Gateway/tree/master/examples/ncurses) to establish the network.
+this code is already cloned to local device in process of installation. so we need to have some edits on the code. On the Master/Primary machine no need to do edits, but on the secondary machine we need to edit `~/rf24libs/RF24Gateway/examples/ncurses/RF24Gateway_ncurses.cpp` file, first lines of method `main()`
+
+Before edit (first lines)
+```cpp
+int main()
+{
+
+    gw.begin();
+    //mesh.setStaticAddress(8, 1);
+
+    //uint8_t nodeID = 22;
+    //gw.begin(nodeID,3,RF24_2MBPS);
+
+    //uint16_t address = 0;
+    //gw.begin(address,3,RF24_2MBPS);
+```
+after edit:
+```cpp
+int main()
+{
+
+    //gw.begin();
+    //mesh.setStaticAddress(8, 1);
+
+    uint8_t nodeID = 3;
+    gw.begin(nodeID);
+
+    //uint16_t address = 0;
+    //gw.begin(address,3,RF24_2MBPS);
+```
+
+Again, the above edit is only done in the secondary machine, the primary machine needs no edits.
+
+Next, we need to recompile the ncurses example and run it in the terminal:
+
+```text
+cd ~/rf24libs/RF24Gateway/examples/build
+make
+```
+
+### Primary machine config
+
+```text
+sudo ip tuntap add dev tun_nrf24 mode tun user pi multi_queue
+sudo ifconfig tun_nrf24 10.11.2.2/24
+```
+
+### Secondary machine config
+
+```text
+sudo ip tuntap add dev tun_nrf24 mode tun user pi multi_queue
+sudo ifconfig tun_nrf24 10.11.2.3/24
+```
+
+### Run the ncurses example on both machines
+
+```text
+cd ~/rf24libs/RF24Gateway/examples/build/ncurses
+./RF24Gateway_ncurses
+```
+
+Done. The primary machine IP is `10.11.2.2`, and the secondary machine IP is `10.11.2.3`.
+One could ping machines from each other.
+
+The resulting latency when pinging primary machine from secondary is about a few milliseconds (or even less than a millisecond), and the speed is about `10kB/s` (equal to 100K bits per second).
+