SMART ACK Modes
- Matthias Kassner
SMART ACK defines two modes depending on the desired level of optimization:
SMART ACK Direct Mode
This mode requires that there is a direct link (without repeaters) between the SMART ACK Sensor and the SMART ACK Controller.SMART ACK Repeater Mode
This mode allows the use of repeaters between the SMART ACK Sensor and the SMART ACK Controller. The SMART ACK Sensor communicates directly with one designated repeater (the Repeater Postmaster) which provides the Mailbox functionality for the SMART ACK Sensor.
Direct Mode
SMART ACK Direct Mode requires that there is direct connection from the SMART ACK Sensor to the SMART ACK Controller. SMART ACK messages cannot be transported from the SMART ACK Sensor to the SMART ACK Controller (or vice versa) via repeaters.
In SMART ACK Direct Mode, the functionality of the SMART ACK Postmaster and the functionality of the SMART ACK Controller are implemented in the same device which thereby becomes the Controller Postmaster.
Repeater Mode
SMART ACK Repeater Mode allows using repeaters for the processing of SMART ACK messages. To do so, it separates the functions of SMART ACK Postmaster and SMART ACK Controller and allows implementation of the SMART ACK Postmaster functionality on a repeater which thereby becomes the Repeater Postmaster.
Comparison of Direct Mode and Repeater Mode
SMART ACK Repeater Mode allows SMART ACK communication in situations where one-level or two-level repeating is required to transport messages between the SMART ACK Sensor and the SMART ACK Controller.
SMART ACK Repeater Mode introduces however several limitations:
Setup Complexity
The setup of SMART ACK Repeater Mode is complex and usually requires a skilled installer. The main complexity is the analysis of the network topology to select of a suitable repeater for the implementation of the SMART ACK Repeater Postmaster functionality. This repeater must have reliable communication both with the SMART ACK Sensor and the SMART ACK Controller as it will be a point of failure for the communication between those two devices.Operation Complexity
The repeater acting as SMART ACK Repeater Postmaster will be a single point of failure failure for the communication between SMART ACK Sensor and the SMART ACK Controller.
Failure of the communication between the SMART ACK Sensor and the SMART ACK Repeater Postmaster (for instance due to a temporary obstruction in the line of sight) might however not be obvious to the SMART ACK Controller if there are other repeaters that can forward telegrams from the SMART ACK Sensor to the SMART ACK Controller.
From the perspective of the SMART ACK Controller, both the SMART ACK Sensor and the SMART ACK Repeater Postmaster will appear to work correct, but the SMART ACK Communication between the SMART ACK Sensor and the SMART ACK Repeater Postmaster will not work.Device support
Not all EnOcean devices support the SMART ACK Repeater Postmaster. Use of this functionality therefore requires careful equipment selection.Security
The security material required for communication with the SMART ACK Sensor will be in possession of the SMART ACK Controller. This allows the SMART ACK Controller to securely (encrypted and authenticated) provide data to the SMART ACK Sensor.
Repeaters implementing SMART ACK Postmaster functionality do not possess the security material used by the SMART ACK Sensor. Repeaters will store the pre-processed (encrypted and authenticated) data from the SMART ACK Controller in their Mailbox for retrieval by the SMART ACK Sensor. Without the security material used by the the SMART ACK Sensor, they can however not authenticate the request for retrieval to verify if this is indeed originating from the SMART ACK Sensor. This enabled denial-of-service attacks against SMART ACK communication by a rogue device that is constantly issuing retrieval requests.
Due to these reason, the use of SMART ACK Repeater Mode is only recommended for specific cases after careful analysis.