RFWireless M-Bus (WM-Bus) Tutorial: Network, Protocol, and Frame
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This tutorial provides an overview of Wireless M-Bus (WM-Bus), covering its system features, network architecture, protocol stack, and frame types.
Introduction
M-Bus, short for Metering Bus, is a European Standard that defines specifications for the physical layer (layer-1), data link layer (layer-2), and application layer (layer-7). Wireless M-Bus is the radio frequency (RF) version of M-Bus, specified in the standard EN 13757-4.
The following table outlines the key features of a Wireless M-Bus system:
| Specifications | Wireless M-Bus |
|---|---|
| Operating Frequency band | 169 MHz, 315MHz, 433 MHz, 868 MHz, 915 MHz |
| Data Rate | Six different modes are supported. Based on these modes, different data rates are supported, including 4.8 Kbps, 100 kbps, and 32768 kbps. |
| Coverage range | 1 Km in 868 band |
| Modes | Mode S - Stationary Mode Mode T - Frequent Transmit Mode Mode R2 - Frequent Receive Mode Mode C - Compact Mode Mode N - Narrowband VHF Mode 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. |
Wireless M-Bus Network
Figure 1 illustrates typical Wireless M-Bus network components, including meters (water, gas, electricity), a concentrator or data collector, and the utility company.
Devices in the network are categorized as either meters or “other” devices. Meters transmit consumption data, while “other” devices collect data transmitted by remote meters wirelessly. These “other” devices can also send commands to retrieve consumption data.
Wireless M-Bus Protocol Stack
Figure 2 shows the Wireless M-Bus protocol stack, which consists of four layers: RF, Physical Layer, Data Link Layer, and Application Layer.
- RF Layer: Defines the frequency band and transmit spectrum mask for WM-Bus products (meters and other devices like concentrators or data collectors).
- Physical Layer: Defines the maximum transmit power and 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: Defines the application protocol, ensuring interoperability between different vendor devices within a single Wireless M-Bus network. Different application profiles are defined at this layer.
Wireless M-Bus Frame Format A and Frame Format B
The EN 13757-4:2013 standard specifies different packet formats, namely format-A and format-B.
Figure 3 depicts these frame formats. Format-A consists of a preamble, payload blocks, and a postamble. The preamble contains header and sync information and is used for synchronization between the transmitter and receiver. The payload contains multiple blocks, with block-2 and block-n being optional. The postamble is a short bit sequence added at the end of frames in mode-S, mode-T, and mode-R2.
The preamble length varies depending on the mode:
- Mode-S: 6 bytes
- Mode-T: 6 bytes
- Mode R2: 12 bytes
- Mode C: 8 bytes
- Mode N: 4 bytes (GFSK) or 8 bytes (4GFSK)
- Mode F: 12 bytes
The following table details 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 Wireless M-Bus
