The LMN (local metrological network, Lokales Metrologisches Netz) is the interface through which meters deliver their measured values to the smart meter gateway. BSI TR-03109-1 defines two variants for this: a wired one and a wireless one. Both are encrypted – without the right tools, the data traffic remains a black box.
Wired: RS-485, HDLC and SML
With the wired LMN, the meter and SMGw are coupled via an RS-485 bus connection, which on the base meter (Basiszähler) is usually implemented as an RJ12 socket. Data is transmitted in the HDLC frame format (DIN EN 62056-46), with SML as the data model; the link is secured by TLS. Several meters can be combined on one line via splitters; the line is terminated with a bus terminator and wired with suitable LMN bus cables.
Wireless: Wireless M-Bus
The wireless LMN variant uses Wireless M-Bus in the 868 MHz band. It is used mainly where cabling is impractical – for example with distributed meter positions or for other utility sectors such as gas and water.
LMN in practice: making it visible and testing it
Development, commissioning and troubleshooting require access to the LMN data stream. DIINNO offers a tiered tool chain for this:
- LMN-USB12V – the mobile entry point for smartphone and PC with integrated 12 V supply;
- LMN-Highspeed – PC adapter with µs-accurate timestamps for timing and protocol analysis;
- LMN-Box – complete platform with control interface for automated test sequences.
On the software side, the LMN-Parser parses the data stream in real time, the LMN-Tester performs one-click tests including a crypto check according to BSI TR-03116-3, and the DIINNO Toolbox turns your smartphone into a meter tool. As a complete package, the complete LMN solution combines hardware and software.
Frequently asked questions
Which connector does the wired LMN use?
Base meters according to the FNN specification (FNN-Lastenheft) typically provide the wired LMN interface as an RJ12 socket. The connection to the smart meter gateway is made with suitable LMN bus cables; with several meters, splitters and a bus terminator are added.
Which protocol runs over the LMN?
With the wired LMN, the data is transmitted over an RS-485 connection in the HDLC frame format (DIN EN 62056-46). SML (Smart Message Language) serves as the data model; the connection between meter and gateway is secured with TLS.
Why is LMN communication so difficult to analyse?
Because it is encrypted and timing matters. Without suitable tools, you only see raw bytes. Specialised adapters and software – such as LMN-Parser for live analysis of the data stream or LMN-Highspeed for µs-accurate timestamps – make the communication visible and testable.
Can I connect several meters to one smart meter gateway?
Yes. The wired LMN is designed as a bus: several meters are combined on one line via splitters, and the line is terminated at its end with a bus terminator. The maximum practical number depends on the gateway used and the installation.