Devices and architecture: what an intelligent metering system is

From spinning disc to metering system

77 words, section signs not counted. One sentence, section 2 no. 7 of the Metering Point Operation Act (MsbG), and it defines the intelligent metering system¹. For practical purposes a single line will do: modern metering device plus smart meter gateway.

First came the Ferraris meter. An aluminium disc turns in a magnetic field, a mechanical register adds up the turns as kilowatt-hours. How much power flowed, the device knows. When? No idea.

Electronics replaced the disc in the modern metering device (mME after its German name). By law it has to reflect consumption and time of use, generation where needed, and to be capable of being linked in through a smart meter gateway¹. That one word carries the definition. Without a gateway an mME sends nothing at all. It still has to show the connection user consumption and time of use, plus daily, weekly, monthly and yearly values for the past 24 months².

Only the gateway turns the modern metering device into an intelligent metering system¹. The act also knows a plain metering system: any meter "linked into a communication network", such as a meter with its own modem. Since the start of 2026, one that doesn't meet the requirements of sections 21 and 22 may only be installed and used under narrow conditions. For consumption above 100,000 kilowatt-hours and plants above 100 kilowatts, that cut-off comes in 2029³. Besides meters, the gateway can link in other technical equipment. A PV system, for instance.

Section 29 decides who gets what. Final consumers above 6,000 kilowatt-hours a year must have an iMSys. Consumers with a §14a agreement and plants above 7 kilowatts get one with a control unit, and the government's EEG 2027 bill would lower that plant threshold to 2 kilowatts⁴. Every other metering point gets at least an mME, where economically reasonable by the end of 2032, new buildings by completion⁵. Quotas and exceptions are in chapter 1.

And how far has the rollout got? On 30 June 2026, 3.17 million of 54.1 million reported metering locations had an iMSys. That's 5.9 percent⁶. So the typical meter cabinet still holds a meter without a gateway.

A further stage has been announced, politically. For customers outside the mandatory rollout, the coalition wants a "Smart Meter Light" that lets them optimise their electricity bills "at low cost and in a cyber-secure way"⁷. What that would be technically, no bill has defined yet.

One gateway, three networks

The BSI's technical guideline TR-03109-1 describes the gateway through its neighbours. Version 2.0, dated 13 December 2024, knows three communication areas: LMN, HAN, WAN⁸.

The table draws on sections 2.3 and 4 of the guideline^8,11.

Between the three networks the gateway is a firewall that keeps them apart⁸. Most of the time it does unglamorous work. Readings come in from the LMN, get a time stamp, are processed by the rules loaded into the device and go out to authorised recipients in the WAN. Time stamps and billing-relevant values put it under German weights and measures law⁸.

Configuration belongs to one party only, the gateway administrator, GWA for short¹². It loads profiles and firmware and monitors the device. If needed, it wakes the gateway with a signed data packet. The readings? Out of its reach. The guideline puts it bluntly: "The GWA has no insight into the meter readings."⁸

Configure everything, read nothing. An odd division of labour for a box in the basement, and a consistent one.

LMN: where the meters connect

How do readings get in? Two physical interfaces, and the gateway must have both¹¹.

Wired means RS-485. On top of it the guideline stacks HDLC as transport protocol and TLS above that, and the gateway polls the meter actively¹¹. The wireless one runs Wireless M-Bus under EN 13757-4, in one direction only. The meter transmits. The gateway listens, and it ignores packets from any meter the administrator hasn't set up a profile for¹¹.

Several customers can share one device. In an apartment block the gateway has to keep the meters of different connection users apart⁸. German law goes a step further: several metering points in one property may hang off one gateway, even metering points on different grid connections, provided they sit on the same grid node at the same voltage level¹².

So one device can serve a whole building. Possibly several buildings on one node.

HAN: customer, technician, control channel

Three roles meet on the home network. Connection users (CON) check meter readings and their personal log, on a display, a PC or through an energy management system. Then there's the service technician (SRV), who works for the gateway administrator. On site the technician reads the system log and may adjust the device within narrow limits, using rights the administrator has set up in the gateway beforehand⁸.

The third role is the interesting one. CLS, Controllable Local System, covers anything in the building that is to be controlled or read without belonging to the metering system. A heat pump or a wallbox. A battery, an inverter. Even a data concentrator for heat cost allocators⁸.

For these devices the gateway acts as a TLS proxy. One encrypted channel arrives from the WAN, another leads into the home network. Both end inside the gateway, which hands the data across without processing it⁸. Who may use the proxy? Only an active external market participant, aEMT. A passive one just receives data.

And from outside? Direct access from the internet to the gateway's services, say via the customer's router, is not provided for¹¹.

This proxy channel is where control happens. For control under §14a the act names data communication "via the smart meter gateway and a control unit connected to it"¹³. How a command gets from grid operator to heat pump, and why TAF 11 has nothing to do with it, is for chapters 4 and 5.

Hardware in the field

In the field, more equipment joins the gateway. At least one piece of it changes its name depending on who you ask.

Inside the gateway sits a security module. Keys and random numbers come from it; it signs, checks signatures and keeps the key material⁸. Physically it's part of the gateway. Yet the protection profile treats it as a product of its own, certified separately¹⁴. How that fits together is chapter 3.

Next, the control unit. That's the act's term. Installing and running it, the link to the gateway and to an energy management system included, is a standard service of the metering point operator¹³. The industry says control box, Steuerbox. VDE FNN writes its specification, currently version 1.5 of August 2025¹⁵.

For the CLS adapter, BSI TR-03109-5 has applied since 1 January 2024. It covers communication adapters in the home network that use the gateway's TLS proxy channel and terminate it on their side¹⁶. Inside that channel, back end and adapter speak whatever protocol the application needs; IEC 61850, EEBUS and OpenADR are the guideline's examples¹⁶. The BSI list now holds 26 certificates¹⁷.

Last comes the customer's energy management system. Not part of the metering system. On the home network it shows up as CON when it displays consumption and as CLS when it controls something⁸.

Why the confusion over names? Because the component comes in several designs. Germany's energy agency dena lists four labels in use for it: CLS unit, CLS communication adapter, HAN communication adapter and (FNN) control box. According to dena, the terms reflect the range of implementations, and the FNN control box is the standardised variant. It has digital interfaces via EEBUS and analogue ones via relay contacts, and passes the signal either straight to the device or to an energy management system¹⁸.

Four names, several designs. In a tender, only the guideline number makes it unambiguous.

The WAN question

"The protocols below the network layer are not specified by this technical guideline."¹¹ One sentence, in a note on figure 4.2. With it, the WAN becomes the operator's problem. LTE, a 450 MHz network, powerline over the power cable, fibre? The guideline doesn't choose.

Legally, the bar is low. Besides the route in use, the gateway has to be open to one more kind of remote communication, arranged by the administrator¹². A second medium as an option. No more.

In the other direction, the state can draw lines, and has been able to since December 2025. Section 19(2) authorises the economics ministry to set requirements by ordinance for the own wide area connections of controllable consumption devices, storage and renewable plants, and in certain cases to ban connections that bypass the gateway outright or allow them only through it³. No such ordinance could be found by mid-October 2026.

That one medium falls short was known early. In mid-2020 the Barometer for Digitalising the Energy Transition counted around 14 percent of households with fibre; at the end of 2020, 96.5 percent of the land area had LTE. Its conclusion: "With only one of the communication technologies available to date, sufficient coverage and availability cannot be guaranteed. Hybrid solutions would be needed."¹⁹ And land area isn't a basement.

No help from the protection profile either, by design. Its subjects are confidentiality, integrity and authenticity, and it says plainly: "The availability of the SMGW is not addressed by the PP."¹⁴ Availability is an operations question.

And in a blackout? Public telecom networks aren't blackout-proof across all their components, notes an expert report by BET and WIK-Consult for the federal economics ministry, dated 28 June 2024. For the customer installation, blackout robustness is enough: an orderly restart once the voltage returns. To connect the iMSys, the report recommends the 450 MHz radio network²⁰. German law does list an "uninterruptible, blackout-proof, dedicated wide area communication connection" as an additional service. It ties that service to a statutory ordinance¹³, and none had turned up by mid-October 2026.

How often does the connection fail in the field? There are only indirect signs. A PwC study, reported by the trade press in October 2026, gives a first-visit success rate of 74 percent and names a missing WAN connection among the reasons appointments fail. Sample and method aren't stated²³.

By now the Federal Network Agency treats this as a compliance question. Installing an iMSys alone is not enough, it says; the devices must reliably record quarter-hourly values and transmit them to the authorised parties⁶. Without a working WAN connection, that can't happen.

Which leaves this. For the LMN the guideline prescribes cable and radio standard. For the WAN, nothing. It does describe a network diagnostics service, through which the gateway reports its modem's signal quality and connection data to the administrator or a market participant. Optional, though. The manufacturer merely declares whether a device implements it⁸. With the right device the problem can be measured. Solved it isn't.

Gas, water, heat: two ways into the gateway

On paper the route is clear. On the LMN, the guideline explicitly lists meters for gas, water, thermal energy and electricity⁸. And the act requires the iMSys to receive readings securely from meters of all four utilities, heat metering devices included¹².

Publicly described projects take another route. Submetering, the consumption metering inside multi-occupancy buildings, enters the gateway there through the home network. A data concentrator or adapter collects the values of heat meters and heat cost allocators and talks to the gateway as a CLS, through the TLS proxy. As early as 2023 a metering point operator was rolling out 500 such "CLS adapters for submetering", close to 200 of them already running²⁴. The BSI list now includes a "CLS Adapter Submetering 2" certified under TR-03109-5¹⁷.

The BDEW sees it the same way. For submetering and existing installations, the association wrote in October 2024, the route via CLS channel and control unit "will continue to be necessary"²⁵.

A contradiction of the specification? No. The same guideline that puts heat meters on the LMN counts data concentrators for heat cost allocators among the CLS on the home network⁸. Both routes are specified, although the LMN route for submetering only arrived with version 2.0 of the guideline⁸. So far, publicly described projects build the second.

And what does the law demand? Less than many slide decks claim.

How connectable is the field, really? A consultancy source from June 2026 keeps it short: the systems are meant to be interoperable and connectable, but "in practice they often aren't". CLS connections work in pilot projects, it says. Productive rollout is still to come³¹. A named multi-utility project over the gateway with published production figures for 2026 could not be found.

Open ground, then, even though two services already sit on the act's list of what metering point operators must offer: transmitting submetering data via the gateway, and connecting the main meter of another utility¹³.

Where the data goes

Readings leave the gateway in one of two ways. Directly, in a star pattern, to each authorised recipient in the WAN. Or indirectly via a market actor⁸. If the route runs through the administrator, the gateway encrypts the content with the recipient's key¹¹. The administrator carries the data but can't read it.

For processing, the act has a target picture. Plausibility checks and substitute values happen inside the gateway, and the values go straight to authorised parties, once the BSI deems that technically feasible and the Federal Network Agency sets it out in a determination. Until then, the Federal Network Agency's determinations let processing and sending run fully or partly outside the gateway: at the metering point operator, and for gas permanently and at other authorised parties too³². The target is in the statute book. For now, the work stays in the back end.

Meter data management lives there, and so does dispatch in the formats of market communication. That's chapter 7.

And the grid operator? It also receives the actual feed-in from TAF 9 and grid status data from TAF 10⁸, the subject of chapter 4. Without a control system for the low-voltage grid, all of that just fills an archive. The BET and WIK-Consult report reaches the same point from another direction: for control in system-critical situations, distribution system operators' control systems would have to be extended so they observe the grid state, detect congestion automatically and send control commands to very large numbers of devices at once²⁰.

Four checks are worth running on your own installed base:

1. Document the home network topology per device type

   Find out, for every gateway model you use, whether HAN-CON and HAN-CLS are separated physically or logically. Then write the separation into new tenders, because the guideline only recommends it.

2. Set a WAN strategy per site class

   Basement location, metal cabinet, mobile coverage, distance to the local substation: group your meter sites by features like these and set a primary and a fallback medium for each class. Openness to a second medium is required by law anyway. Where your gateway model offers the network diagnostics service, use it to check your assumptions in operation.

3. Tie component names to guidelines

   Write "CLS communication adapter under TR-03109-5" or "control box under the FNN specification" into your specifications, not just "control box".

4. Plan multi-utility metering via CLS

   Reading heat or water through the gateway? For now, expect the CLS route with a certified adapter, and look at the bundled offer under section 6 MsbG as a business model.