Wireless Meter Bus, WM-Bus Technology

- tutorial about the Wireless Meter Bus or WM Bus used for applications including remote meter readings.

Wireless Meter Bus Includes:
Wireless M bus basics

The Wireless Meter Bus is a variant of the M-Bus or Meter-Bus which is a European standard defined under EN 13757.

The Wireless Meter Bus or Wireless M-Bus is aimed at remote reading of gas or electricity meters as well as any other types of meters that may need to be read remotely.

The Wireless M-Bus is defined under specification EN 13757-4.

Meter-Bus origins

Wireless Meter Bus has its origins within the Meter-Bus standards. This is a field bus standard aimed at applications for collecting meter data for gas, electricity, water, etc..

The bus is specified in a European Norm specification: EN 13757. This specification is divided into five parts, one of which addresses the Wireless Meter Bus, WM Bus.

The standard is entitled: Communication systems for meters and remote reading of meters.

Meter Bus Standard EN 13757 Parts
Standard Specification	Details
EN 13757-1	Part 1 of the standard defines the data exchange, detailing the basic communication between meters and a central data collector. It provides an overview of the communication system.
EN 13757-2	This part of the Meter Bus standard addresses the physical and link layers for the physical data transmission using wired connections. It also describes the protocol used for the transmission of data.
EN 13757-3	Part 3 of the standard addresses the dedicated application layer. It describes the standard application protocol used to enable a multivendor capability, thereby enabling devices from several different vendors to operate in a single system.
EN13757-4	Section 4, EN 13757-4 addresses the wireless system. It is entitled Wireless meter readout (Radio meter reading for operation in the 868 MHz to 870 MHz SRD Band. This part of the standard addresses the Physical and Data Link Layers for wireless devices.
EN 13757-5	This part addresses relaying. It includes a variety of proposals for relaying data frames as a means of overcoming issues related to range between the meter and the data collection points.

Communication modes

There are different requirements for differing applications for WM Bus. Dependent upon the application, one of a number of communication modes can be selected.

Wireless M Bus Communication Modes
Mode	Communication	Details
S1	Unidirectional	This is a Stationary mode and within this, metering devices send their data several times a day. In this mode, the data collector may save power as the meter devices send a wake-up signal before sending their data.
S2	Bidirectional	This is essentially a bidirectional version of the S1 mode.
T1	Unidirectional	This is the Frequent Transmit mode and metering devices periodically send data to collectors that are in range. The interval between transmissions is configurable and may range between several seconds to several minutes.
T2	Bidirectional	Again this is a bi-directional version of the T1 mode. In this mode the data collector may request dedicated data from the metering devices.
C1	Unidirectional	This is known as the Compact mode. It is similar to the 'T' mode but allows for the transmission of more data within a given energy budget and the same duty cycle. This particular form of mode is useful for walk-by or drive-by data collection
C2	Bidirectional	Again, this is a bi-directional version of C1. In this mode the data collector may request dedicated data from a metering device.

Wireless M-Bus RF layer

The physical layer for the Wireless Meter Bus needs to be relatively straightforward to enable the system to operate reliably and over long periods of time.

The Wireless M-Bus has several options that may be chosen for the radio parameters. A total of three different data rates are specified along with one way or two way communication.

Wireless M-Bus uses the 868 MHz license free ISM band within Europe. The different modes and functions are allocated different frequencies and also data rates that can be used.

A variety of forms of modulation are used dependent upon the mode of operation. Modulation types include Manchester, Non-return to zero- NRZ and 3-out-of-6 encoding.

Wireless M-Bus Parameters
	Meter Device Nodes
Data Collection Nodes	S1, S2	T1, T2	C1, C2	Collection Node Transmission
S1, S2	X			868.30 MHz, 32.768 kcps, Manchester encoding
T1, T2		X		868.30 MHz, 32.768 kcps, Manchester encoding
C1, C2		X		868.30 MHz, 32.768 kcps, Manchester encoding
C1, C2			X	869.525 MHz, 50 kcps, NRZ encoding
Metering device transmitter settings	868.30 MHz, 32.768 kcps, Manchester encoding	868.95 MHz, 100 kcps, 3-out-of-6 encoding	868.95 MHz, 100 kcps, NRZ encoding