Design considerations and tests that the Mesh Potato Hardware will have to pass
Requirements: Standard electromagnetic interference tests in compliance to the regulatory domains where the device is supposed to be used. (like FCC, RFI, CE)
Test: Send device to EMI test laboratory.
Wireless range, stability and routing throughput
- WiFi interface must provide a transfer rate of >3MByte/sec for 24 hours
- Device must provide >60KByte/sec stable bidirectional transfer speed at a
range of 1km in free space with clear fresnel zone.
- Must run without any malfunction in a large (100 mesh nodes or more) in
real life mesh deployment for 72 hours.
- Transfer TCP traffic from PC-A connected to LAN port of MP-A via MP-B
to MP-C, traffic terminates at PC-B, connected to the LAN port of MP-C.
- Outdoor test. Download from /dev/zero from busybox webserver in Mesh Potato A to Mesh Potato B and vice versa.
- Connect to large real-life mesh network like Freifunk Berlin.
Comment: Wireless range depends on free space loss, antenna gain, transmit power, noise level on radio channel and receiver sensitivity. Challenge in a real life mesh network are MAC-timer skews and erroneous MAC TSF timestamps.
CPU and IO-performance
Requirement: Device must be capable to route 20 voice calls with Speex codec 8kbps and 5 audio fragments of 20ms per voice packet while running a routing protocol with 500 nodes in the routing table. Load average on the CPU must be less than 0.80.
Test: Can be performed in a community mesh network such as Freifunk Berlin.
Static electricity on RF input
Requirement: Static electricity potential building up at the antenna (for example by lightning in close proximity) or at high altitudes can destroy the WiFi receiver or make it deaf. This is a very common problem which has been observed during many real life deployments. This can be avoided by a DC decoupling capacity in the RF path and a inductivity together with anti-parallel diodes between the RF input and ground at the the input of the WiFi interface.
Test: Apply static electricity to RF input.
Reversed DC polarity on DC input
Requirement: The device must survive any attempt to connect power with reversed DC polarity.
Test: Connect reversed polarity from a laboratory DC power supply with current limit.
Comment: A very common user mistake is to reverse DC polarity when connecting a DC operated device to a power source (battery, power supply). Without a protection circuitry the device will be utterly ruined. There are two common methods to avoid destruction of devices under such circumstances:
- A combination of a fuse and a diode that shorts the reversed potential
against ground and hence blows the fuse before the reversed potential can cause any damage. Problem is that users may think that the device is defective and throw it away. Or they replace the broken fuse with tin foil or wire - upon the next attempt to connect with reversed polarity the device will be ruined. These issues can be solved by the use of a polyswitch (self-resetting polymeric fuse).
- A diode (preferrably of Schottky type not silicon to minimize diode
forward power loss) in series with the DC supply path. The device won't start but also won't be damaged until the right polarity is provided. This is the preferred method, but will slightly increase the power consumption of the MP, because of diode forward loss.
Too high AC (from mains grid) or DC voltage connected to DC input
Requirement: The MP must survive any attempt to power the device with 230 Volts AC on its DC input.
Test: Connect AC to DC input (perform with extreme caution!)
Connecting AC to the DC input is a dangerous endevour. I have seen people do that in Bangladesh for example. If something doesn't work they take the open end of a cable and plug it into the next AC socket. Making the MP proof against this abuse and protecting users is not trivial.
Solution to protect the device:
Ship the device with fitted connectors. Add a fuse and a 18 Volt varistor to the DC input of the MP. This will protect the device against any attempt of connecting too high AC or DC voltage, to a DC cable with open end. AC will be rectified by the Schottky diode in series and the varistor will short every voltage higher than 18 Volts against ground and subsequently blow the fuse.
Note: Even with this protection it can be dangerous for people touching ground conductors connected to the device. If the AC potential is connected to the ground of the MP (a 50% chance) everything connected to the ground (Computer connected to LAN port, Telephone(s) connected to the FXS port) of the MP will be life. This can be handled by more complex circuitry at higher cost.
Requirement: The device must withstand the temperatures building up in the housing at high environment temperatures when exposed to strong direct sunlight. (Sub-Sahara conditions) The housing must not loose stability or melt. SoC and other chips (DC/DC IC, diodes) must not overheat. This condition is also critical for electrolytic capacitors in the DC/DC conversion circuitry. Low environment temperatures are not a issue (even at high altitudes) because the device will heat itself from energy losses of about 1.5Watt in the electronic circuitry.
Test: Heat up the housing of the MP under full load to a temperature of 80 degrees Celsius and sustain the condition for considerable time. Monitor frequency stability and system stability.
Solution: Attach heatsink to the SoC chip if the temperature becomes critical. Use a electronic design which is power effective. Outdoor housing must be plain white. Ventilation holes on the bottom of the device. Choose plastic with good material properties.
Humidity, rain and hail, mechanical stability, UV irradiation
Requirements: Housing must be waterproof and robust against strokes and harsh environment. Connectors and sockets must be protected again rain and mositure by the housing. Connectors must be gold plated and protected against stress (force pulling on cables)
Test: Put device under harsh environment conditions.
Solution: Choice of plastic, design and strength of material. The Ubiquiti Nanostation is a good example of a well made design.
Power consumption and DC input range
Requirement: The device must consume less than 3 Watts when supplied with 12,5 Volt DC. MP must operate between 9 and 18 Volt DC. MP consumption must not exceed 4 Watt consumption within voltage input range.
Test: Laboratory power supply
AC/DC power supply shipped with MP
Requirement: Temperature stability and extended voltage range (>350V AC) and/or overvoltage protection
Comment: This is a non-trivial issue and not easy to come by. In developing countries power generators are often overloaded and voltage per phase of three-phase A.C. current may exceed 300 Volts. (I have measured 330 Volts in Jaganathpur/Bangladesh after a power supply blew up) Hence all standard switched-mode power supplies are susceptible to damage by excess voltage.
Requirement: Should survive connection with another FXS port (or Central Office) line for 30 seconds.
Requirement: Should be robust to lightning strike on phone line connected to FXS port.
Requirement: Optional: Consider using a standard IP04 type plug-in FXS module to allow easy interchange of FXS port should it fail in the field. This will also allow construction of low cost wireless-only mesh potatoes.
Requirement: Should be robust to short circuit applied for 30 seconds.
Requirement: Sustain ringing (if maximum power consumption state) for 5 minutes.
VOIP Performance Tests
Requirement: Develop an automated test to make calls from and Asterisk server to MP FXS port. Use another Asterisk box to ensure FXS call is answered (i.e. simulate telephone going off hook for 10 seconds). Repeat for 24 hours over thousands of calls and check for loadav and memory leaks.
Requirement: Repeat CPU load test from Milestone 1 (route 15 calls while running a FXS call on MP). This will test additional CPU load of FXS driver.