The Industrial Internet of Things is predicated on large-scale, distributed sensor/control networks that can run unattended for months to years with very low power consumption. The characteristic behavior of this type of network entails very short bursts of message traffic over short distances using wireless technologies, often described as a low-rate, wireless personal area network (LR-WPAN). We keep the data frames short to lessen the possibility of radio interference forcing the need to retransmit. One such LR-WPAN approach uses the IEEE 802.15.4 standard. This describes a physical layer and media access control that are often used in the industrial control and automation applications referred to as Supervisory Control and Data Acquisition (SCADA).
In the IoT, local “edge” devices, typically sensors, collect data and send it to a data center — “the cloud” — for processing. Getting the data to the cloud requires communicating using the standard IP protocol stack. This can be done by directly connecting the edge devices via the Internet to the data centers — the “cloud model.” Or, we can communicate from the edge devices to a collection point known as a border gateway to have the data relayed from there to the data center — the “fog model.”
This article will describe characteristics of IEEE 802.15.4 networks, specifically the Internet Engineering Task Force (IETF) IPv6 over Low-power Wireless Personal Area Networks (6LoWPAN) implementation. This implementation supports both the cloud and fog models.
IEEE 802.15.4 PHY Layer
The IEEE 802 standards family is broken out into a number of task groups including 802.3 (Ethernet) and 802.11 (Wi-Fi), as well as 802.15 (Wireless PAN). In particular, IEEE 802.15.4 (15.4 for brevity) is the responsibility of Task Group 4, which is responsible for various characteristics of the protocol including RF spectrum and the physical layers. The 15.4 standard has been expanded to include Radio Frequency identification (RFID) PHYs, ultra-wideband (UWB) PHYs, and is also being discussed as a possible solution for both car-to-car and car-to-curb communications.
802.15.4 only addresses the physical (PHY) and media access control (MAC) layers — in the OSI network model, layers one and two. It leaves the upper layers to the implementer. At layer three and above, there are a plethora of offerings including Zigbee, Z-Wave, Thread, and 6LoWPAN. Each of these implements the remainder of the OSI protocol model to deliver services such as routing and discovery as well as APIs for user applications.
In general, 15.4 supports data transfer rates at 20 Kbit/s, 40 Kbit/s, 100 Kbit/s (soon), and 250 Kbit/s. The basic framework assumes a 10-meter range at 250 Kbit/s. Even lower data rates are achievable to further limit power consumption. In spite of the 10-meter (32-foot) range specification, in the 2.4GHz ISM band, typical achievable ranges for IEEE 802.15.4 radios are on the order of 100 feet indoors, and 200 – 300 feet outdoors. In the sub-GHz frequencies, practical implementations of the protocol have been demonstrated at ranges of over 6.5 km (4 miles) with appropriate antennas in the 900 MHz ISM band.
At the physical layer, IEEE 802.15.4 manages the RF transceiver and channel selection, as well as energy and signal management facilities. There are six PHYs currently defined, depending on the frequency range and data performance required. Four of them use Direct Sequence Spread Spectrum (DSSS) frequency hopping techniques. Chirp spread spectrum (CSS) is in use in the Ultra-Wide Band (UWB) and 2450 MHz frequency bands. Parallel Sequence Spread Spectrum (PSSS) is available only with the hybrid binary/amplitude shift keying modulation technique found in the European 868 MHz band.
The frame size for 15.4 is 133 bytes including PHY, MAC, and the data payload. The format for this frame can be seen in Figure 1. By keeping the frame relatively short, we can limit the amount of time needed to transmit it while simultaneously limiting the probability of radio interference due to the normal operation of industrial equipment.
IEEE 802.15.4 MAC Layer
The IEEE 802.15.4 MAC layer (OSI Model layer two — data link layer) is responsible for:
- Joining and leaving the PAN;
- Carrier Sense Multiple Access with Collision Avoidance (CSMA-CA) for channel access;
- Guaranteed Time Slot (GTS) transmissions;
- Establishing a reliable link between two peer MAC entities;
- Beacon transmissions for a coordinator;
- Synchronization to the beacons.
In addition, the MAC layer supports the use of symmetric encryption using the AES-128 encryption algorithm. There are also options for SHA-based hashes and access control lists to limit the transfer of sensitive information to specific nodes or links. Finally, the MAC computes a freshness check between frame receptions to help minimize the potential for old frames that may have been traveling over a circuitous path from being delivered late to the upper-layer protocols.