Wireless M-Bus tutorial | network, protocol stack, frame

The Wireless M-Bus tutorial covers WM-Bus system features, network, protocol stack and Wireless M-Bus frame types.

M-Bus stands for Metering Bus which is a European Standard. It defines specifications for layer-1 (physical), layer-2 (data link layer) and layer-7 (application layer). The radio version of M-Bus is known as Wireless M-Bus. It is specified in the standard EN 13757-4. The table-1 below mentions all the wireless M-Bus system features.

Specifications Wireless M-Bus
Operating Frequency band 169 MHz, 315MHz, 433 MHz, 868 MHz, 915 MHz
Data Rate There are six different modes supported by Wireless M-Bus. Based on this different modes there are different data rates supported viz. 4.8 Kbps, 100 kbps and 32768kbps.
Coverage range 1 Km in 868 band
Modes Mode S - Stationary
Mode T - Frequent Transmit
Mode R2 - Frequent Receive
Mode C - Compact
Mode N - Narrowband VHF
Mode F - Frequent Receive and Transmit
Security utilizes AES-128 encryption algorithm for data security
communication uni-directional, Bi-directional between meters and other devices
Standard EN 13757-4
Applications Used for remote monitoring of water meters, electricity meters, gas meters. This is used for customer billing purposes by the utility companies.

Table-1: Wireless M-Bus System Features

The figure-1 depicts typical wireless M-bus network components viz. meters (water, gas, electricity), concentrator or data collector and utility company.

Wireless M-Bus network

The devices in the network are categorized into meters or "other" devices. The function of meters is to transmit consumption of data. The function of "other" devices is to collect data transmitted by remote meters wirelessly. The "other" devices can also send commands to retrieve consumption of the data.

Wireless M-Bus Protocol Stack

Wireless M-Bus protocol stack

The figure-2 depicts wireless M-Bus protocol stack. As shown there are four layers in wireless M-Bus system viz. RF, Physical Layer, Data Link Layer and Application Layer.
• RF Layer: Defines frequency band and transmit spectrum mask to be used for the WM-Bus products i.e. meters and other devices (i.e. concentrator or data collector).
• Physical Layer: Defines maximum transmit power, lowest receiver sensitivity of the meter.
• Data Link Layer: Defines frame formats A and B. The frame carries preamble, link layer data payload and application layer data payload. It utilizes AES 128 CBC encryption for the payload data.
• Application Layer: It defines application protocol which makes different vendor devices interoperable together in a single Wireless M-Bus network. The different application profiles are defined at this layer.

Wireless M-Bus Frame Format A and Frame Format B

The standard EN 13757-4:2013 specifies different packet formats viz. format-A and format-B.

Wireless M-Bus Frame formats

The figure-3 depicts frame formats supported by Wireless M-Bus viz. format A and format B. The format-A consists of preamble, payload blocks and postamble. Preamble consists of header and sync information. The preamble is used as synchronization between the transmitter and the receiver. There are multiple blocks in payload part in which block-2 and block-n are optional. Postamble consists of short bit sequence which are added at the end of frames in mode-S, mode-T and mode-R2.

The preamble length varies based on different modes. The Mode-S uses 6 bytes, Mode-T uses 6 bytes, Mode R2 uses 12 bytes, Mode C uses 8 bytes, Mode N uses 4 bytes (GFSK) or 8 bytes (4GFSK) and Mode F uses 12 bytes as preamble lengths. The table-2 below mentions frame format-A and frame format-B.

Frame Format type Description
Frame Format-A • Block-1: { L-field (1 byte), C-field (1 byte), M-field (2 bytes), A-field (6 bytes), CRC field (2 bytes) }
• Block-2: { CI-field (1 byte), Data field {((L-9) mod 16)-1) bytes}, CRC-field (2 bytes) }
• Block-n: { Data field ( 16 bytes or ((L-9) mod 16) bytes ), CRC-field (2 bytes) }
Frame Format-B • Block-1: { L-field (1 byte), C-field (1 byte), M-field (2 bytes), A-field (6 bytes), CRC-field (2 bytes) }
• Block-2: { CI-field (1 byte), Data-field (115 bytes or (L-12) bytes, CRC-field (2 bytes) }
• Block-3: { Data-field: (L-129) bytes, CRC-field (2 bytes) }

Wireless M-Bus Frame Terminologies

L-Field: Length Indication
C-Field: Communication Indication (Request, SEND, RESPONSE EXPECTED, ACK etc...)
M-Field: Sending Device Manufacturer ID
A-Field: Address of sending device, consists of { ID number(4 bytes), version(1 byte), device type code(1 byte) }
CI-field: Control Information which indicates protocol used at upper layer
CRC-field: Cyclic Redundancy Check

Reference: EN 13757-4


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