Link layers for 6LoWPAN
One of the most important functions of the Internet Protocol is the interconnection of heterogeneous links into a single interoperable network, providing a universal "narrow waist". This is equally true for 6LoWPAN and embedded networks, where there are many wireless (and also wired) link-layer technologies in use.
The specialized applications of embedded networks require a wider range of communication solutions than typical personal computer networks, which almost universally use Ethernet and WiFi. Luckily the IEEE 802.15.4 standard is the most common 2.4 GHz wireless technology for embedded networking applications, and has been used as a baseline for 6LoWPAN development. Other technologies used with 6LoWPAN include sub-GHz radios, long-range telemetry links and even power-line communications. The requirements and interactions of 6LoWPAN with the link layer are discussed next, along with an introduction to IEEE 802.15.4, a sub-GHz radio and power-line communications.
There is a set of required or recommended features that a link should provide in order to work with Internet protocols. These include framing, addressing, error checking, length indication, some reliability, broadcast and a reasonable frame size. The issues involved with designing a subnetwork for use with IP are discussed in [RFC3819]. 6LoWPAN is designed to be used with a special type of link, and has its own set of link requirements and recommendations.
The most basic requirements for a link layer to support 6LoWPAN are framing, unicast transmission and addressing. Addressing is required to differentiate between nodes on a link, and to form IPv6 addresses which are then elided by 6LoWPAN compression. It is highly recommended that a link supports unique addresses by default (e.g. a 64-bit extended unique identifier [EUI-64]), to allow for stateless autoconfiguration.
Multi-access links should provide a broadcast service. Multicast service is required by standard IPv6, but not by 6LoWPAN (broadcast is sufficient). IPv6 requires a maximum transmission unit (MTU) of 1280 bytes from a link, which 6LoWPAN fulfills by supporting fragmentation at the LoWPAN adaptation layer. A link should provide payload sizes at least 30 bytes in length to be useful (and preferably larger than 60 bytes). Although UDP and ICMP include a simple 16-bit checksum, it is recommended that the link layer also provides strong error checking.
Finally, as IPsec may not always be practical for 6LoWPAN, it is highly recommended that links include strong encryption and authentication. The 2006 version of the IEEE 802.15.4 standard actually does not include a "next protocol identifier", making the detection of which protocol is being carried difficult. Although partially dealt with in the LoWPAN format using a dispatch value, it is a feature that a link should preferably have. Subnetwork design and link-layer issues are discussed in Section 2.2.
The next sections introduce three link-layer technologies used with 6LoWPAN: IEEE 802.15.4, a sub-GHz ISM band radio and low-rate power line communications.
The IEEE 802.15.4 standard [IEEE802.15.4] defines low-power wireless embedded radio communications at 2.4 GHz, 915 MHz and 868 MHz. The first version of the standard was released in 2003, and was then revised in 2006. More recently the IEEE 802.15.4a standard was released, extending 802.15.4with two new physical layer options: Chirp Spread Spectrum at 2.4 GHz and Ultra Wide-Band at 3.1–10.6 GHz. Work continues on new features such as MAC improvements in IEEE 802.15.4 Task Group 4e (TG4e), active RFID (TG4f), larger networks (TG4a) and specialized PHYs for China (TG4c) and Japan (TG4d). More information is available on these efforts from [IEEE].
In practice IEEE 802.15.4 at 2.4 GHz is used almost exclusively today as it provides reasonable data rates, and can be used globally. The sub-GHz channels are limited geographically with 915 MHz mainly available in North America and 868 MHz in the European Union (EU). That, combined with the limited data rates and channel selection of sub-GHz IEEE 802.15.4, means that there are only a few chips on the market today. Often more flexible sub-GHz chips tend to be used, as explained in the next section. This trend may yet change, with new sub-GHz applications becoming widespread and efforts like the IP500 Alliance, together with improvements in the latest IEEE 802.15.4 standard for sub-GHz channels.
The 802.15.4 standard provides 20–250 kbit/s data rates depending on the frequency. Channel sharing is achieved using carrier sense multiple access (CSMA), and acknowledgments are provided for reliability. Link-layer security is provided with 128-bit AES encryption. Addressing modes for 64-bit (long) and 16-bit (short) addresses are provided with unicast and broadcast capabilities. The physical layer payload is up to 127 bytes, with 72–116 bytes of payload available after link-layer framing, addressing, and optional security.
The MAC can be run in two modes: beaconless mode and beacon-enabled mode. Beaconless mode uses pure CSMA channel access and operates quite like IEEE 802.11 without channel reservations. Beacon-enabled mode uses a hybrid time division multiple access (TDMA) approach,with the possibility of reserving time-slots for critical data. IEEE 802.15.4 includes many mechanisms for forming networks, and for controlling the superframe settings. An IEEE 802.15.4 reference is provided in Appendix B.
Early 6LoWPAN standardization work was originally aimed at the IEEE 802.15.4 standard [RFC4919, RFC4944] and thus assumed that some 802.15.4-specific features such as beacon-enabled mode and association mechanisms would be used along with 802.15.4 device roles. Based on practical experience with [RFC4944] and industry needs, recent 6LoWPAN standardization has been generalized to work with a larger range of link layers and avoids the assumption of IEEE 802.15.4-specific features. The use of 6LoWPAN with IEEE 802.15.4 is covered in more detail in Section 2.2.
Sub-GHz ISM band radios
Sub-GHz radio technologies using the industrial, scientific and medical (ISM) bands for unlicensed operation are especially popular in low-power wireless embedded applications such as telemetry, metering and remote control. The sub-GHz ISM bands cover 433 MHz, 868 MHz and 915 MHz. The main reasons for sub-GHz popularity are the better penetration of lower frequency, resulting in better range compared to 2.4 GHz, and the 2.4 GHz ISM band becoming very crowded in urban environments.
One example of a popular sub-GHz chip is the Texas Instruments CC1101 transceiver [CC1101]. This transceiver acts as a reconfigurable radio and is capable of 300–928 MHz operation, with a wide variety of modulations, channel and data rates up to 500 kbit/s. Such a chip can also be used with an external power amplifier for increasing range. The features of the chip include carrier sensing, received signal strength indicator (RSSI) support, and frame sizes up to 250 bytes. The system-on-a-chip version, the CC1110, additionally includes a 128-bit AES encryption hardware engine.
This kind of transceiver only provides the physical layer, so the datalink layer is implementation specific and needs to provide e.g. framing, addressing, error checking, acknowledgments and frame length. When designing a link layer for this type of transceiver, the IEEE 802.15.4 frame structure and beaconless mode operation is typically used as a starting point.
Power line communications
6LoWPAN also has interesting uses over special wired communication links, such as lowrate power line communications (PLC). Applications of this technology include home automation, energy efficiency monitoring and smart metering.
One such system from Watteco [Watteco] uses what is called a watt pulse communication (WPC) technique, greatly reducing the complexity of communications. The data rate of the physical layer provided using WPC is 9.6 kbit/s, and the resulting channel over the power system of a house, building or urban area is multi-access and similar to a wireless CSMA channel.
Watteco provides a version of WPC with an emulation of the IEEE 802.15.4 data link layer. This allows 6LoWPAN to be used with PLC in a very similar way to IEEE 802.15.4 and other ISM band radios. With PLC, multihop routing is not an issue as typically all nodes are on the same stable link. Multihop forwarding may be useful to interconnect several PLC subnets, or to integrate PLC and wireless 6LoWPAN islands.
Coming up in Part 4: Addressing, header format, bootstrapping, mesh topologies and Internet integration.
Printed with permission from John Wiley & Sons, Ltd. Copyright 2009. "6LoWPAN: The Wireless Embedded Internet" by Zach Shelby and Carsten Bormann, ISBN: 978-0-470-74799-5. For more information about this title and other similar books, please visit John Wiley & Sons.
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