The M-Bus is a standardized field bus for building control systems. It is a two- wire bus concept for the remote reading of meter readings from smartmeters in buildings: for gas, water, electricity and heat. In addition to the wired version, there is also a version that operates with radio transmission, the wireless M-Bus.
The star-shaped master- slave concept underlying the M-Bus is based on the OSI reference model and specified in European standards( EN), the transport-oriented layers in EN 13757-2 and the application-oriented layers in EN 13757-3. The slaves are the meter-reading devices that are queried by the master, a personal computer. The logic levels used by the master for polling are 36 V for logic "1" and 24 V for logic "0". The slaves send their telegrams with current modulated data to the master. A master-slave segment can comprise up to 250 slaves. The transmission rates are relatively low and range between 300 bit/s and 9.6 kbit/s.
The protocol architecture of the M-Bus is based on the OSI reference model. The physical layer can be implemented via the classic category 3 telephone cable. The layer above is the Data Link Layer( DLL), which provides secure connections. This functional layer can be extended by the Extended Link Layer (ELL) above it. This is followed by the Authentication and Fragmentation Layer( AFL), the Transport Layer (TPL) and the Application Layer( APL). The layers above the link layer are activated by special control characters in the M-Bus telegram.
The advantage of the M-Bus is that the consumption meters are read electronically. Each individual consumption meter can be addressed separately and read out individually. The data held in the master can be sent to the energy company via the Internet or the telephone network, where it can be directly processed in terms of programming. With the Wireless M-Bus there is a radio-technical alternative to the M-Bus.