EnOcean Protocol Stack: Physical, Data Link, and Network Layers Explained

This document explains the EnOcean protocol layers, specifically focusing on the Physical layer, Data Link layer, and Network Layer. We will cover the functions of the EnOcean protocol stack layers and describe the data flow through them.

Introduction

EnOcean is an energy-harvesting wireless technology primarily designed for building automation. However, its applications extend across various industries, including:

• M2M (Machine to Machine) communication
• IoT (Internet of Things)
• Smart Homes

The frequency bands utilized in EnOcean technology include 868 MHz, 315 MHz, 902 MHz, 928 MHz, and 2.4 GHz (for IoT). EnOcean employs both ASK (Amplitude Shift Keying) and FSK (Frequency Shift Keying) modulation schemes.

The specific modulation used depends on the frequency band, as defined by the standard. This results in two physical layer variants in EnOcean technology. ASK is typically used with 315 MHz and 868 MHz frequencies, while FSK is used with 902 MHz and 928 MHz frequencies.

EnOcean protocol layers

The figure above illustrates the EnOcean protocol stack, which encompasses all the OSI layers except the Session layer. EnOcean specifications define radio protocol #1 and radio protocol #2 in two versions for ASK and FSK respectively. Let’s delve into the functions of each EnOcean protocol layer.

The following points summarize data flow across the lower three layers of the EnOcean protocol stack: Physical, Data Link, and Network Layer.

• EnOcean Transmitter Operations: Addressing of telegrams, medium access control using CSMA/CA or LBT (Listen Before Talk), subtelegram structure formation (including checksum/CRC-8 mechanism), encoding of subtelegram, and radio transmission.
• EnOcean Receiver Operations: Radio reception, decoding of subtelegram, subtelegram structure decoding, checksum/CRC checking, and telegram decoding for upper layers.
• Application Layer: Manages equipment profiles and handles RPC (Remote Procedure Call) and RMCC (Remote Management and Control Call).
• Presentation Layer: Handles radio telegram processing and encryption/decryption.
• Transport Layer: Manages Smart Ack and remote management.

EnOcean Physical Layer

The Physical Layer is responsible for the following services:

• Encoding/Decoding: Encoding at the transmitter and decoding at the receiver. This includes bit inversion operations to recover data bits and assist in subtelegram recovery at the Data Link Layer.

• Radio Transmission/Reception: Transmitting and receiving radio signals at the transmitter and receiver, respectively.

• Frame Formation:

  • (FSK Specification): {Preamble, Synchronization word, Length, Data_PL}
    • Preamble: 16 bits
    • Synchronization Word: 16 bits
    • Length: Number of data bytes following
    • Data_PL: Payload data bytes
  • (ASK Specification): {Preamble, SOF, DATA, SYNC/EOF}
    • Preamble: 8 bits
    • SOF (Start Of Frame): 4 bits
    • DATA: Payload
    • SYNC/EOF: Indicates whether another byte follows (“01”) or not (“10” indicates the last byte)

Let’s examine the Physical Layer operations performed on the input data (one or more subtelegrams) received from the Data Link Layer. The following operations occur at the transmitter:

• Frame Formation: This layer forms frames according to the ASK or FSK format after receiving data from the Data Link Layer.

• Bit-to-Symbol Conversion: Converts bits to symbols based on the chosen modulation scheme (ASK or FSK), according to the standard’s modulation parameters. ASK modulation uses either 315 MHz or 868 MHz RF carriers, while FSK modulation uses 902 MHz or 928 MHz, based on country-specific allocations.

• ASK Specifics: The protocol is inverted before transmission, and an 8-to-12 coding scheme is employed.

• FSK Specifics: A GFSK (Gaussian Frequency Shift Keying) filter is used, adhering to national regulations.

At the EnOcean receiver, the reverse operations are performed to recover the subtelegrams, which are then forwarded to the Data Link Layer.

EnOcean Data Link Layer

The Data Link Layer provides these services:

• Subtelegram Structure Formation

• Control Sum Calculation

• Subtelegram Timing Mechanisms

• Medium Access Control: Using CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance) or LBT (Listen Before Talk)

• Subtelegram Structure: { RORG, DATA, TXID, STATUS, HASH/CHECKSUM }

  • RORG: Subtelegram type
  • DATA: Payload
  • TXID: 4-byte unique identifier
  • STATUS: Identifies if the transmission is from a repeater, and the type of integrity mechanism used
  • HASH/CHECKSUM: Data integrity check value for all preceding bytes in the subtelegram

Let’s understand Data Link Layer operations performed on the telegram received from the Network Layer.

• The telegram received from the Network Layer is packed into a subtelegram structure. An 8-bit checksum or 8-bit CRC is calculated and appended as shown in the structure above.
• Checksum Calculation: The checksum is calculated by a simple sum of all the bytes of subtelegram except the “HASH” value field.
• CRC-8 Calculation: The CRC-8 is calculated using the generator polynomial X⁸ + X² + X + 1 and the algorithm described in the EnOcean specification. This “last byte” is critical for data integrity checks at the receiver.
• To ensure transmission reliability, three identical subtelegrams are transmitted over a fixed period.
• Similar to TDMA, all transmitters send subtelegrams at different time intervals, within a defined Tx maturity limit (40 ms). This helps prevent subtelegram collisions.
• Transmitters also implement the LBT (Listen Before Talk) algorithm to avoid channel collisions. However, LBT can consume more energy, so it’s often avoided in energy-harvesting wireless devices.

EnOcean Network Layer

The Network Layer provides the following services:

• Telegram Addressing: Encapsulation at the transmitter and decapsulation at the receiver
• Switch Telegram Conversion: Choice/status processing
• Repeating: Status processing

Let’s understand the Network Layer operations performed on the message, telegram, or data received from upper layers.

• The Network Layer manages the number of repeater hops during telegram transmission. A maximum of two hops is allowed between the transmitter and receiver. The Status byte indicates the subtelegram’s status during transmission, including repetition status, and identifies the sender: original transmitter or the first/second repeater.
• The Network Layer handles telegram addressing using encapsulation at the transmitter and de-encapsulation at the receiver. It inserts a “DESTID” field (four bytes) preceding the TXID in the original telegram before transmission.
• Redundancy is used during transmission if no other secured mechanism is used, such as acknowledgement. A maximum of two repetitions are used.

NOTE: This document provides a generic overview of the functionalities implemented at the various EnOcean protocol layers: Physical, Data Link, and Network. For complete details on all protocol layers and their functions, fields, sizes, and order, refer technical specification documents: EnOcean Radio Protocol V1.2 and V1.3, available on the EnOcean website.

References

• EnOcean Certification Specification (part 1a), Air Interface (ASK) V1.4, Released on Feb.10, 2019
• Visit www.enocean.com for information about EnOcean technology.
• EnOcean Alliance