Hardware

The mesh routers are based on the PC Engines Alix2c2 embedded PC board with a size of 152.4 × 152.4 mm. The board features a 500 MHz AMD Geode LX800 Processor and 256 MB DDR DRAM. It provides two Ethernet ports (Via VT6105M), two Mini PCI slots, two USB 2.0 ports, and a CompactFlash socket.

Each router is equipped with three or more IEEE 802.11b/g network interface cards. Currently, we use one LogiLink WL0025 dongle based on the RT2501U architecture with a RT2571W BB/MAC IC and RT2528 RF IC on each mesh router. The cards feature an on-board R-SMA connector and a 4 dBi Hi-Gain antenna. The antennas are mounted at the side panels of the router using extension cables. Additionally, all mesh routers are equipped with two Atheros-based MiniPCI cards (WLM54SAG). We discuss our choice of wireless NICs in detail in Section 4.6 of the corresponding technical report.

The primary Ethernet port is used to boot the operating system over the network and to mount the root file system. The mesh routers have no need for local persistent memory and store their data on a central server.

The enclosure is a customized TEKO AUS23 with the dimensions of 198 × 178 × 90 mm. Keeping everything in a single, small form factor box has been a core objective and excluded alternative solutions. Two intermediate levels keep the components separated from each other.

The mesh routers are mains powered, but in locations that lack access to the power supply system, Power over Ethernet (PoE) adapters are used. These adapters offer the advantage, that a reboot of unresponsive routers can be done remotely by deactivating and reactivating the specific Ethernet port of the switch. 

Each mesh router is equipped with a MSB-A2 sensor node. The sensor node is connected via USB which provides unlimited power supply and is used to communicate with the mesh router. The USB interface is also used to write firmware images to the flash memory of the sensor node. In contrast to over-the-air flashing, this approach minimizes the possible points of failure that can interrupt the flash process and thereby render sensor nodes unresponsive.

The MSB-A2 sensor nodes use an LPC2387 microcontroller produced by NXP Semiconductors. The frequency of the 32-bit ARM7 TDMI-S core based microcontroller can be dynamically configured at runtime to up to 72 MHz, depending on the sensor network application and energy requirements. The Chipcon CC1100 transceiver uses the ISM band at 863 to 870 MHz with a maximum data rate of 500 kbit/s. The MSB-A2 nodes therefore set up a parallel testbed that is fully orthogonal to the frequency band used by WLAN.

The sensor nodes are equipped with a Sensirion SHT-11 temperature and humidity sensor. By using either simple general purpose input/output pins (GPIO) or an on-board mini USB port, extensions may be connected to the MSB-A2. In addition to the internal 512 KiB flash memory microSD-cards can be used. Virtually unlimited data storage is available via the Ethernet backbone.