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sensd - A WSN Internet GW, hub, agent, proxy & cloud


Robert Olsson Jens Laas



We've outlined, designed and implemented and very simple concept for WSN data sharing, including data collection, storage and retrieval using standard text tools. The concept restricts Internet access to WSN motes and acts agent not to expose motes directly for robustness and security reasons. Low level, physical or link WSN protocol can be used. including 6lowpan, RIME etc and any type of Radio Duty Cycling (RDC). sensd works on the application layer. A TCP connection initiates an implicit "subscribe". The M2P model is currently supported.

Key concepts

  • Agent. sensd works as an agent and does not allow direct Internet access to motes. Recall motes are constrained in most aspects and can not support many connections, has weak security etc.

  • Hub. Share the data from the root or sink node over TCP. In effect sensor data can be sent over Internet to be shared over among TCP active listeners. The TCP connection initiates an implicit "subscribe".

  • Proxy. The support proxy functions over ipv4 and well as ipv6. Sensd can forward to a proxy on a public IP. The typical case is when GW is behind a NAT.

  • WSN RP "rendez-vous point". A concepts where data from various WSN nets
    are merged. This models a "cloud service" functionality for WSN networks. sensd can be used both to forward data to RP. It can also work as the RP. RP receiving WSN data and allowing multiple TCP listeners.

  • All programs are written C, script in Java and bash. Designed for small footprint and with minimal dependencies. sensd runs on Raspberry Pi and Openwrt.

  • This work introduces a simple tag format for sensor data. The overall idea is that data tagging is an "agreement" between producers and consumer. Tags are simple are description of the data. Example T=25.2. where T= is the tag 25.2 the value. Most likely this a temperature. But we can't be sure since we don't know since this is an agreement between the producer and the consumer. Tags are also used for identification. Example tags, E64= Where globally unique ID can used. Another more relaxed example is TXT= a user description. See docs.

  • Geotagging and timestamping is supported via tags.

  • Ecosystem support. There are telphone apps to for data monitoring and and plotting. Android app can act as WSN-agent and forward to proxy/RP.

  • The concept also includes a mapping to URI (Unified Resource Identifier) to form a WSN caching server similar to CoAP using http-proxy.

  • Copyright. Open-Source via GPL. Projecet used


This is collection of software to implement data monitoring and data collection from WSN Wireless Sensor Networks. The goal is to have a very simple, straight-forward and robust framework.

The scenario: One or several motes is connected to USB or serial port to gather received information from connected WSN motes. Data can be visualized in several ways.

  • Sensor data report can be transmitted and propagated throughout the Internet. sensd acts as server and sends incoming report to active listeners.

  • Data is kept in ASCII with tagging and ID information. Data is conveniently handled, copied and viewed with standard text utilities of your OS.

  • Last mote report is cached into the file system suitable for URI use. The Format is SID/TAG. Typical tags are EUI64 and unique serial numbers. The different TAGS are left for mote user to define. Although the TAGS used in our example setup are included in this draft for example purposes.

Both formats can easily be stored or linked directly in web tree to form a URI to format WSN logging/datafile or caching service.

A daemon that reads WSN mote reports from USB/serial and stores data in a ASCII data file. Default is located at /var/log/sensors.dat

Addtional components

Datafile logging

Below is and example of the anatomy of a sensors.dat file we are currently using in our WSN data collection networks.

2012-05-22 14:07:46 UT=1337688466 ID=283c0cdd030000d7 PS=0 T=30.56  T_MCU=34.6  V_MCU=3.08 UP=2C15C V_IN=4.66

2012-05-22 14:11:41 UT=1337688701 ID=28a9d5dc030000af PS=0 T=36.00  V_MCU=2.92 UP=12C8A0 RH=42.0 V_IN=4.13  V_A1=3.43  [ADDR=0.175 SEQ=33 RSSI=21 LQI=255 DRP=1.00]

Each line is a mote report. They start with date and time and are followed by a set of tags. The tags is different for different motes. In other words they can send different data. Essential is the ID which should be unique for each mote.

The information with brackets is information generated by the receiving mote and is not a part the motes data. Typically RSSI (Receiver Signal Strength Indicator) and LQI (Link Quality Indicator)

Internet sensor data

Start sensd with the -report option. This enables reports to be transmitted over IP to remote listeners. Default TCP port 1234.

Server side example:

sensd -report -p 1234  -D /dev/ttyUSB0

Client side. Example using netcat:

nc server-ip 1234

URI format

URI (Unified Resource Identifier) displays the node ID and the tags in a file tree. It is easy to export this into a web tree to form a URI similar to a CoAP gateway.

Example: In our case we have a unique sensor ID followed by the different data fields represented by "tags".



Read Temp from a sensor:

cat /tmp/WSN1-GW1/281a98d20200004a/T

And it's very easy to link this tree into a web-server.

GPS support

Positioning support has been added via GPS device connected to serial or USB port. Tags added when enabled GWGPS_LON & GWGPS_LAT. GPS code from.

Getting the source and building

Code is stored in github. Typically procedure below is the very straight- forward unix way:

git clone
cd sensd

Put your binaries after your preference:

Pre-built binary versions

For x86: Sensd and friends are available in Bifrost/Linux packages. Those packages are statically linked and can be used on most x86 Linuxes. 32-bit compiled.


The WSN data logging and caching concept is in actual use with Contiki, RIME broadcast application.


One can use netcat to listen to reports:


nc 1235

To save in file use nohup:

nohup nc 1235 > /var/log/sensors.dat

As sensd used TCP and ASCII encoding. tetlnet and web-browsers can be used as well.


WSN sensor daemon and supporting utilities







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