Table of Contents
Second-Generation Mesh Extender
DFAT Funded Refinement of Mesh Extender
As part of the Pacific Humanitarian Challenge (PHC) grant that we have received from the Australian Department of Foreign Affairs and Trade (DFAT), we are in the process of redesigning the Mesh Extender for use in tropical-maritime environment, and generally addressing the productisation of the Mesh Extender, so that we can easily produce robust and reliable Mesh Extenders that are required for the grant activities, but also so that after the grant, Mesh Extenders can be readily produced for other potential users.
The primary repository for the Mesh Extender is:
- https://github.com/servalproject/openwrt - Modified OpenWRT repository for building firmware images with correct settings. Contains all information required to build a working firmware image.
If you wish to develop on the individual packages, then these are the relevant repositories:
- https://github.com/servalproject/openwrt-packages - MeshExtender2.0 branch, for the custom Serval Project OpenWRT packages required by the above.
- https://github.com/servalproject/serval-dna - MeshExtender2.0 branch
- https://github.com/servalproject/lbard - MeshExtender2.0 branch
- https://github.com/servalproject/flash-rfd900 - master branch
- Full custom-designed injection-moulded housing.
- Designed to meet IP65 or IP66 (although we do not have funds to subject it to certification).
- Integrated flexible power options: 12v/24v automotive, solar (with integrated MPPT), and/or LiFePO4/LiIon/SLA battery (with integrated charge controller). It might also be possible to power with 5V USB – stay tuned.
- microSD slot for Rhizome bulk data storage (replacing the previous energy consuming and unreliable USB memory-stick interface).
- Slot for RFD900/RFD868 radio.
- External UHF and Wi-Fi antennae for improved radio performance.
- Mesh of Things/Internet of Things (MoT/IoT) interface with 2x opto-isolated digital inputs, and 60W load rated relay switched output. These sensor/controller interfaces will be MeshMS-controllable via future software-update.
- 2x Internal ethernet ports (1x giga-bit and 1x 100Mbit). These are not routed to the outside of the IP65/66-designed enclosure, due to the cost that this would entail.
- Option for USB charging port.
- Internal USB port for future accessories.
- Serial EEPROM in power/radio cable to allow control of radio regulatory parameters, without having to configure the Mesh Extender device itself.
Radio Regulatory Control
One of the significant challenges for the Mesh Extender, is that it must be easily importable during the acute phase of a disaster. Problems can easily arise when importing radio equipment in such circumstances, as the customs/border protection officers are typically very busy during such events. It must, therefore, be very easy for them to establish that the hardware conforms with local regulatory requirements.
To facilitate this, Mesh Extender hardware will only operate using Wi-Fi, unless the serial EEPROM in the power cable indicates otherwise. That is, it can be honestly said that a shipment of Mesh Extenders can transmit only in the 2.4GHz Wi-Fi band, if they are shipped separate from their power cables. This allows a single inventory, possibly regionally positioned, regardless of target country (subject to radio band compatibility, which will be discussed later). Kits of power cables, encoding the local regulatory parameters for each target country can then potentially be pre-positioned in-country ahead of time. Or failing this, such cables can be rapidly prepared using a special programmer, and then shipped either together with the Mesh Extenders, or in a separate consignment from the Mesh Extenders themselves.
A secondary use of the serial EEPROM is to inform the Mesh Extender whether it is allowed to accept unsigned firmware updates, or whether it is legally permissible to have arbitrary firmware installed. This is to address regulatory problems in places like the USA, where their national regulators are (unwisely in our view) moving towards such requirements.
See EEPROM memory layout for more information.
It should be noted that all tools required to program the serial EEPROM form part of the open-source source-code of the Mesh Extender system, and thus these measures go no further than necessary in preventing the kind of unintentional or accidental behaviours that national radio regulators typically require. That is, where there is a legitimate operational requirement to change the behaviour of the system, the tools required to do so exist – as indeed they exist for most programmable radio systems already approved for use in the relevant markets.
At present, the only difference in inventory is whether a Mesh Extender is physically fitted with an RFD900 or an RFD868 UHF packet radio. The RFD900 is suitable for transmitting from about 900MHz to 930MHz, and is thus suited for the Americas, Australia, New Zealand, many Pacific nations, and a handful of other countries. In contrast, the RFD868 is designed with the European/African 868MHz band in mind, and is thus suitable for those regions. Unfortunately, there is no single version of the radio that can operate in both bands, and ever were there such a version, it would require different antennae.
Thus, while it is possible to have regionally commonalised Mesh Extender hardware, there will still be two slightly different versions for the EU/African vs Americas/Oceania markets.
It should be noted that the RFD900/RFD868 radio module can be replaced inside a Mesh Extender by simply unplugging and replacing the radio module, so where there was an operational need to repurpose the radios between the differing regions, this could be done at reasonable cost.
Custom-Designed Injection Moulded Housing
The PCB is currently undergoing the third revision to address outstanding errata. We hope to have samples before the end of March.
First revision PCB, front and rear:
This cable is a IP65 D-SUB 25 female to 3-pin and 4-pin IP67 cable, designed to connect the Mesh Extender to each of:
- solar panel OR vehicular 12v/24v supply
- 2-cell LiFePO4 or 6v sealed lead acid battery
It also contains an integrated I2C EEPROM that tells the Mesh Extender which radio frequency and transmit power it is allowed to use, along with other configuration data.
We are engaging with Arkidelo to design and fabricate a low-pressure encapsulation mould for the D-SUB 25 connector, to provide appropriate environmental sealing of the cable head, from which the power input connector, battery connector and (optional) external USB charge ports will emerge. This will also need to include provision for the thumb-tighten screw-ins for the connector. The purchase order for Arkidelo to begin work has been raised, and will take approximately 7 weeks before we receive the first parts, probably early May.
See the bill of materials for details on the components.
See quality assurance for details on testing the cable, and programming the integrated I2C EEPROM.
Bill of Materials
Quality Control and Testing
We need to establish quality control, programming, test, qualification and certification procedures for the Mesh Extender units. To be confirmed.