3.1 Requirements of a Bus System for Consumer Utility Meters

Of the various possible topologies which might be considered for reliable and cost-effective networking consumer utility meters, only the bus topology (see Section 2.1) is in fact suitable. The requirements which are made by meters on such a bus system will now be explained.

The most important requirement is the interconnection of many devices (several hundred) over long distances - up to several kilometers. Since the data sent by the meters are used for end user billing, a high degree of transmission integrity is required for the bus. On the other hand it is possible to dispense with high speed of transmission, since usually only a relatively small amount of information must be transferred. To ensure this high degree of transmission integrity, the bus must be exceptionally insensitive to external interference, as a result of capacitive or inductive coupling. In order to avoid ground loops, the slave should be electrically isolated.

A further requirement for the bus is low cost for the complete system. The transmission medium which is used should therefore not require shielding, and the cost of individual meters can be minimized by using as few components as possible and by remotely powering the slaves from the bus. In addition the costs of installing and servicing the system must be taken into account: These can be reduced by incorporating protection against reversed polarity, and making provision for the connection of additional facilities (Life Insert) during operation of the bus system.

3.2 The M-Bus in the OSI Model

Since no bus system was available which met the requirements detailed in Section 3.1, the Meter-Bus (M-Bus) was developed by Professor Dr. Horst Ziegler of the University of Paderborn in cooperation with Texas Instruments Deutschland GmbH and Techem GmbH. The concept was based on the ISO-OSI Reference Model, in order to realize an open system which could utilize almost any desired protocol.

Since the M-Bus is not a network, and therefore does not - among other things - need a transport or session layer, the levels four to six of the OSI model are empty. Therefore only the physical, the data link , the network and the application layer are provided with functions.

Layer	Functions	Standard	Chapter
Application	Data structures, data types, actions	EN1434-3	6
Presentation	empty
Session	empty
Transport	empty
Network	extended addressing (optional)	-	7
Data Link	transmission parameters, telegram formats, addressing, data integrity	IEC 870	5
Physical	cable, bit representation, bus extensions, topology, electrical specifications.	M-Bus	4

Fig. 6 The M-Bus layers in the OSI-Model

Because changing of parameters like baudrate and address by higher layers is not allowed in the ISO-OSI-Model, a Management Layer beside and above the seven OSI-Layers is defined:

Management Layer
Application Layer
Presentation Layer
Session Layer
Transport Layer
Network Layer (if address = 253)	Address 253 / Enable Disable CI=$52/$56
Data Link Layer
Physical Layer	Address 254 (255)

Fig. 7 Management-Layer of the M-Bus

So the address 254 and perhaps 255 are reserved for managing the physical layer and the address 253 (selection) for network layer (see chapter 7), which is only used in certain cases.

With this new layer we can directly manage each OSI-layer to implement features, which do not conform to the OSI-Model.