## 5 Wireless Han
### Not Based on IEEE Standards
#### 5.2.6 Wireless M-Bus  
As continuation of the Metering Bus (M-Bus) standard for cable applications, the Wireless M-Bus (WM-Bus) has been developed, to properly operate with WSN scenarios, as partially done in some home automation context [73]. The absence of modularity is one of the main reasons why standardized routing protocols are not available for Wireless M-Bus at the present.  
The M-Bus benefits asymmetric network topologies with data collectors or gateways with higher performance on the one side and low-cost or low-power metering devices on the other side.  
Presently, it only supports star network topologies or point-to-point but it does not for mesh or multi-hop topologies those are not yet possible [65]. The Wireless M-Bus (EN 13757-4:2005 and EN 13757-4:2011) has been lately recommended by the Open Metering System group 4 for metering scenarios and proposed for use in smart meters. The energy needs for WM-Bus transceivers are low due to a low-overhead protocol, transmission-only modes and long-range sub-GHz broadcast bands. Whereas the first document EN 13757-4:2005 approved the use of the 468 MHz ISM and 868 MHz bands, the following standard version EN 13757-4:2011 added new extra communication modes at 169 MHz with reduced data rates [74]. The lower 169 MHz frequency band allows higher transmission range thanks to the intrinsically lower path losses. In the meantime, lower data rates increase the receiver sensitivity, allowing a decrease of the transmission power at the transmitter or a longer transmission range for an identical transmission power. The established WM-Bus modes are classified as following:  
* T form: 100 kb/s data rate from meter to gateway, frequent transmission mode (several times per second or per minute), 868 MHz. In T2 mode, the transmitter requires an acknowledgement (ACK) while T1 does not.
* S form: 32.7 kb/s, stationary mode (several transmissions per day), 868 MHz. In S2 mode, the transmitter requires an ACK while S1 does not.  
In both of the communication modes, the meter initiates the transmission to the concentrator, which in turn is always in reception mode. Once the transmission of the first packet is done, the sub-meter activates a reception window, waiting for the concentrator request or command. The transmission session cannot be activated by the concentrator for the reason that the meters mostly remain in sleep mode in order to save the life of the battery, corresponding with what generally happens in low rate communication systems [75].  
The WM-Bus protocol defines several specifications for the N mode, as follows:  
* Nc form: 2.4 kb/s, 169.431 MHz. N2c needs ACK, N1c does not.
* Na form: 4.8 kb/s, 169.40 MHz. N2a needs ACK, N1a does not.
* Ng form: 38.4 kb/s, 169.437 MHz. Ng always requires ACK.  
The WM-Bus can reach longer distance transmissions when compared to IEEE 802.15.4. Sensor nodes equipped with ZigBee/802.15.4 transceivers can only cover a few dozens of meters. If greater distances are required to be covered employing ZigBee/802.15.4 technology, then a multi-hop data transfer strategy can be expected which, in exchange, raises the power requirements for the sensor nodes. WM-Bus not only allows long ranges, but it also offers a relaying method to cover longer distances with multi-hop techniques as defined in EN 13757-5 [73].