Simple Sensor Interface protocol
The Simple Sensor Interface protocol is a simple communications protocol designed for data transfer between computers or user terminals and smart sensors. The SSI protocol is an Application layer protocol as in the OSI model.
The SSI protocol has been developed jointly by Nokia, Vaisala, Suunto, Ionific, Mermit and University of Oulu. Currently SSI is being developed within the Mimosa Project, part of the European Union Framework Programmes for Research and Technological Development.
The SSI protocol is used in point-to-point communications over UART and networking nanoIP applications. SSI also provides polling sensors and streaming sensor data. For RFID sensor tags SSI specifies memory map for sensor data.
The criteria for SSI protocol development are:
- general purpose
- simple – minimal overhead
- small footprint on the server side
SSI message structure
An SSI message consists of a 2-byte header and an n-byte payload. The header consists of a one byte address and a one byte message/command type. The different possible values for the message/command type are presented in SSI v1.2 command base.SSI v1.2 command base
Command byte | Direction | Description |
Q,q | C-> | Query |
A,a | <-S | Query reply |
C,c | C-> | Discover sensors |
N,n | <-S | Discovery reply |
Z,z | C-> | Reset sensor device |
G,g | C-> | Get configuration data for a sensor. |
X,x | <-S | Configuration data response |
S,s | C-> | Set configuration data for a sensor |
R,r | C-> | Request sensor data |
V,v | <-S | Sensor data response |
D,d | <-S | Sensor response with one byte status field |
M,m | <-S | Sensor response with many data points |
O,o | C-> | Create sensor observer |
Y,y | <-S | Observer created |
K,k | <-> | Delete sensor observer / listener |
U,u | <-> | Observer / listener finished |
L,l | <-S | Request sensor listener |
J,j | C-> | Sensor listener created |
E,e | <-> | Error |
F,f | <-> | Free data for custom purposes |
The group of commands:
- Q – query
- A – query reply
- C – sensor discovery
- N – discovery reply
- Z – reset
- G – get sensor configuration
- S – set sensor configuration
The group of commands:
- R – request sensor data
- V – data response
- D – data response with status field
For data streaming purposes defined commands are:
- O – create sensor observer
- Y – observer created
- K – delete observer
- U – observer finished
- L – request sensor listener
- J – sensor listener created.
- V – data response
- M – data response with many data points
Point-to-point SSI
Networking SSI
SSI networking in a variable environment is done using nanoIP. In a typical case using SSI, an individual message is not important, and so nanoUDP is used as the message format. If individual messages are important, nanoTCP can be used, as it provides flow control and retransmission at a cost of message size and increase in network traffic.A nanoUDP message consists of a 5-byte nanoUDP header, an n-byte message payload and an optional 2-byte CRC checksum. The header consists of one protocol byte, a 2-byte message length, a 1-byte source port and a 1-byte destination port number. The destination port number should be 0x28 for SSI messages.
Version history
- 0.1 March 14, 2003
- 0.2 April 29, 2003
- 0.3 May 20, 2003
- 0.4 October 2, 2003
- 0.5 December 5, 2003, not compatible with previous
- 0.6 November 3, 2004
- 0.7 December 22, 2004
- 0.8 January 14, 2005
- 1.0 April 11, 2005
- 1.1 October 27, 2005
- 1.2 May 27, 2006, not compatible with previous