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=== Wireless M-Bus ===

Wireless M-Bus communication uses frequencies between 868 MHz and 870 MHz. Different physical layer modes are available. The two most common modes are S and T.

In S mode a meter sends messages in regular intervals (e.g. every 15 minutes or every 2hours). Messages sent have a long message header. If the meter is only sending messages and not listening for messages the meter is said to use the mode S1. If the meter is also listening for messages the mode is called S2. The S2 mode requires significantly more power. Therefore devices using S2 are usually not battery powered. A gateway that collects data from meters always uses the S2 mode.

In mode T the meter sends data more frequently (usually every few seconds) with a shorter message header. Again the meter can only send data (T1) or also listen for data (T2). A meter in T2 mode only listens for incoming messages for a short time after having sent a message. A gateway in T2 mode will constantly listen for messages instead.

Several wireless M-Bus frame types exist: SND-NKE, SND-NR, ACC-NR and more. The data frames that meters transmit in regular intervals are called SND-NR (send without request). SND-NR is the most common frame type and it is the only frame type that jMBus supports.

The SND-NR frame contains the secondary address of the sender. This uniquely identifies the sender. Note that the sender may not necessarily be the meter that the data is originating from. The meters secondary address can be specified in the variable data structure as explained below. Keys used to encrypt and decrypt M-Bus messages are associated with a specific secondary address.

Finally the SND-NR frame contains a CI field equal to 0x72, 0x78 or 0x7A and a variable data structure that is explained further down. Often the variable data structure of wireless M-Bus frames only has a short data header instead of a long header (as in wired M-Bus) because the secondary address is already transmitted in the data link layer.

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