EnOcean Frame, Telegram, and Subtelegram: Key Differences

This article explains the differences between EnOcean Frames, Telegrams, and Subtelegrams within the context of EnOcean technology.

Introduction:

EnOcean technology is a wireless, self-powered energy harvesting technology. The standard specifications are defined in ISO/IEC 14543-3-1X. The EnOcean stack encompasses the physical, data link, and network layer specifications. It operates on various frequency bands worldwide, including 868 MHz, 315 MHz, 902 MHz, 928 MHz, and 2.4 GHz (for IoT applications). The EnOcean communication protocol is packet-based and utilizes three primary data units: frames, subtelegrams, and telegrams.

An EnOcean device typically consists of a transmitter and a receiver, each incorporating protocol layers such as the physical layer, data link layer, network layer, and upper layers.

EnOcean Frame

The encoded data at the output of the physical layer is referred to as a frame. It’s transmitted in a serial sequence, bit by bit. The frame contains control and synchronization information (PRE, SOF, INV, and EOF) essential for the receiver.

The transmitter’s encoding process converts a subtelegram into a frame. Conversely, the receiver’s decoding process extracts the subtelegram from the frame, stripping away the control and synchronization information.

Figure 1: EnOcean protocol stack showing protocol layers with data units at transmitter and receiver.

EnOcean Telegram

A complete telegram consists of up to three subtelegrams. Bidirectional communication requires telegrams to have addresses, which are assigned through an encapsulation mechanism. A telegram type field (RORG) with a value of 0xA6 indicates encapsulated telegrams.

EnOcean Subtelegram

As the figure below shows, subtelegrams are managed by the data link layer. The EnOcean Radio Protocol (ERP) is primarily designed as a unidirectional protocol, generally operating without handshaking. To ensure reliable transmission, three identical subtelegrams are transmitted within a specific time interval.

Subtelegram structure as specified in EnOcean Radio Protocol-1, V1.2

The fields of an EnOcean subtelegram are described below:

• RORG-CHOICE: Specifies the type of subtelegram.
• DATA: The payload of the subtelegram.
• TXID/SourceID: A unique 4-byte transmitter identity.
• STATUS: Indicates the status of the subtelegram transmission from a repeater and provides information about the integrity control mechanism. This field is absent in switch telegrams.
• HASH/Checksum: A data integrity check value calculated from all the bytes in the subtelegram.

The total length of an ERP1 telegram is 21 bytes (maximum). The subtelegram length is not explicitly transmitted within the structure but is determined by counting the total bytes between the RORG and HASH fields.