I was wearing a unique piece of jewelry at NDN community meeting, Mar 2017: a pair of ESP8266 units that communicate with each other over the NDN testbed.
They are ugly, but it is a nice way to demonstrate my creation in a Named Data Networking community meeting.
Two Witty Cloud boards are tied to my wrists, and powered by a USB powerbank in my pocket.
One of them runs a ndnping client, and the other runs a ndnping server.
The client sends Interests to a router in Arizona, the Interests (under a multicast prefix) are flooded through the testbed, and reach the server which is connected to a router in Memphis.
A year ago, a Kickstarter campaign CHIP - The World's First Nine Dollar Computer caught my attention: it's a $9 computer smaller than a banana.
Unlikely the Raspberry Pi, it comes with onboard storage so I don't need to buy a separate SD card, it has WiFi instead of wired Ethernet so I don't have to run wires everywhere, and it is compatible with my existing VGA monitor through a $10 adaptor so I don't have to buy another HDMI monitor.
Therefore, I snapped two of these little computer along with one VGA adapter during the compaign.
A few more hours later, I start to discover a limitation of C.H.I.P software:
The Linux kernel comes with CHIP operating system has very limited features.
$ sudo modprobe fuse
modprobe: FATAL: Module fuse not found.
Obviously, the solution to this problem is to compile my own Linux kernel with more features.
The compilation can be done on the C.H.I.P itself.
I managed to do that when the CHIP is powered by a 5V 1A phone charger plus a 1500mAh LiPo battery.
I had the compilation running under screen(1) and attended to it intermittently, and finished in a day.
While hackers do good most of the time, we occasionally do evil and play a prank.
The ESP8266, unlike JSON, allows me to do evil.
Thus, I programmed the microcontroller for an evil purpose: slow down the WiFi.
How it Works
802.11 WiFi typically operates in infrastructure mode, where a router acts as an access point, and other hosts (stations) connect to the router on a wireless frequency (a channel).
One property of the wireless channel is that, at any moment, only one party (station or access point) can be transmitting.
If multiple senders are transmitting at the same time, the wireless signal will be jammed, and the recipient is unlikely to receive the packet correctly.
In this case, the sender would have to transmit the packet again at a later time.
Packets can be transmitted at different speeds on the wireless channel.
With 802.11g standard, the maximum speed is 54Mbps, and the minimum is as slow as 1Mbps.
The sender (station or access point) dynamically chooses a speed for every packet depending on its perception of wireless channel quality.
Usually, we prefer to transmit at a higher speed, so that the wireless channel can be freed as soon as possible for other senders to use.
However, if the sender and recipient are far apart, high speed transmission is less likely to succeed because signals can be faded, and a slower speed is necessary to increase the chance of a successful transmission.
Witty Cloud board is an ESP8266 development board that has a unique two-PCB design: the top PCB carries the ESP8266 microcontroller and an AMS1117 voltage regulator, and the bottom PCB carries a CH340G USB-serial chip.
The top PCB can operate independently.
It has a pair of 8-pin male headers that expose ESP8266's GPIO pins, which can be used to control peripherals such as LEDs and buttons.
The bottom PCB is needed only for flashing the ESP8266.
It has a pair of 8-pin female headers.
The top PCB should be inserted into the bottom PCB when the firmware is being flashed from a computer, or when we want to use Arduino's serial monitor.
Sometimes, we may want to simultaneously connect ESP8266's GPIO pins to peripherals, and keep the ability of flashing the firmware or monitoring the serial console from a computer.
Can we achieve that?
From electric point of view, when the male headers on the top PCB are inserted into the female headers on the bottom PCB, each of the 16 pins on one PCB is connected to the corresponding pin on the other PCB via a wire.
Thus, if we connect the two PCBs with 16 jumper wires, everything should work.
But it's tedious to connect 16 wires every time.
Can we connect the two PCBs with less jumper wires?
Let me find out.
One of my favorite electronic elements is the photoresistor, an element whose resistance decreases with increasing incident light intensity.
I played with a photoresistor as part of an electronic building blocks toy kit when I was in elementary school, and made a geocaching trackable out of that experience.
But I want a deeper understanding of the photoresitor: what's the correlation between its conductivity and the light intensity?
Recently I acquired some Witty Cloud boards.
This board is built around an ESP8266 microcontroller; a photoresistor (aka Light Dependent Resistor, LDR) is connected to the analog input port of the ESP8266.
With one line of code in Arduino (analogRead(A0)), we could read the light intensity as a number between 0 and 1023.
However, what's the unit of this number, and how does it translate to the standard units such as lumens?
The readings taken are not in any useful units of light intensity.
Photoresistors are not carefully calibrated sensors.
If you wanted to make a light meter, with absolute measurement of light intensity in meaningful units, you would need to create a lookup table that related readings with readings taken from a properly calibrated light meter.
I get addicted to ESP8266 ever since Losant sent me a builder kit which includes an Adafruit Feather HUZZAH.
Witty Cloud GitWits is one of the cheapest ESP8266 device you can get on the market.
It has a microUSB port with USB-Serial interface, which means no soldering!
The board also contains an RGB LED and a button, which allows me to use it without a breadboard.
So, I decide to go for it!
I ordered the Witty Cloud board from AliExpress, a Chinese E-commerce site.
Ordering experience isn't so good.
For some reason, they cancelled my order due to "difficulty in verifying payment".
I had to upload a scanned copy of my passport and credit card statement, before they would ship my items.
After getting past those non-technical steps, the Witty Cloud development board is in my mailbox 2 weeks later.
There's no doubt I'm excited!
The board is very small and lightweight.
Its dimension is 31mm x 30mm x 18mm.
Its weight is roughly the same as an Adafruit Feather HUZZAH.
Witty Cloud board has two PCBs connected via a pair of 8-pin headers.
Losant is a simple and powerful IoT cloud platform.
The platform provides a MQTT broker that sensor devices can connect to and report state in real time; the reported states are stored in the cloud, and can be visualized in several formats.
Recently, Losant sent me a Builder Kit which contains an ESP8266 WiFi chip on Adafruit Feather Huzzah, a TMP36 temperature sensor, and some other components.
Following the official instructions, 1.5 hours later, my room temperature shows up on a webpage.
I'm excited, and I decide to leave it running continuously so that I can monitor the activities of @TheTucsonHeat.
However, Losant platform shows the device as "offline" from time to time, and sometimes it never comes back online unless I press the hard-reset button.
This is mostly because of the bad WiFi in my apartment: 1000ms huge delay, 10% or more packet loss, etc.
Since MQTT is based on TCP, it's severely affected.
To make things worse, our WiFi connects to the Internet through two levels of Network Address Translation (NAT) gateways; it seems that the ESP8266 sometimes is unable to detect the TCP connection to Losant broker is broken, and thus does not take recovery actions.
As a computer networking student, the first thing coming through my mind is the end-to-end principle: the reliability of an application should be ensured by the end hosts, not by the network.
To test whether my temperature sensor remains connected to Losant, I should send a message to the Losant platform, and let the platform send back a reply.
And that's the idea of LosantPingPong: an end-to-end connectivity test between a connected device and the Losant platform.