Thread物联网标准



Intelligent devices around the home make our lives easier and more enjoyable. Connecting these devices and enabling them to communicate and share information is essential for the smart home of tomorrow. Wireless communication technology provides an elegant solution for the communication links between these objects. Wireless networks in the smart home need to satisfy specific requirements such as the ability to provide reliable communication and at the same time consume very little energy. Thread uses as its RF (Radio Frequency) connectivity protocol the IEEE 802.15.4 communication standard [IEEE802154] which is specifically designed for low-rate, low-power WPANs (Wireless Personal Area Networks). To further satisfy the connectivity requirements of the smart home, Thread employs IPv6 connectivity that allows devices to communicate with one another, access services in the cloud, or interact with the user through Thread mobile applications. The need to unify IPv6 and 802.15.4 technologies was resolved by the development of a layer that provides smooth adaptation between the IPv6 networking layer requirements and 802.15.4 link layer capabilities. This layer is called 6LoWPAN and is illustrated in Figure 1.
Figure 1. Thread Communication Stack
Using IPv6as the network layer protocol requires that a minimum MTU (Maximum Transmission Unit) of 1280 bytes must be supported over the link [RFC 2460]. Considering the fact that [IEEE802154] defines a maximum PHY (Physical) packet size of 127 bytes, it becomes apparentthat an adaptation layer is needed. To satisfy these requirements, the 6LoWPAN adaptation layer employs several different techniques such as packet fragmentation and header compression (for the IPv6 header and transport headers such as UDP (User Datagram Protocol)) to ensure the successful delivery and reception of IPv6 packets coming to and from devices in the home.

IEEE 802.15.4
[IEEE802154] is a standard for wireless communication that defines the PHY and MAC (Media Access Control) layers and was issued by the IEEE (Institute for Electrical and Electronics Engineers). Designed with low power in mind, this wireless communication protocol is suitable for applications usually involving a large number of nodes, most of which can function on battery power for many years. One of the characteristics derived from the need for low power and limiting the BER (Bit Error Rate) is enforcing smaller sized packets to be sent over-the-air. These can be up to a maximum of 127 bytes at the PHY layer. The MAC layer payload can be as low as 88 bytes, depending on the security options and addressing type as illustrated in Figure 2.

IPv6 over 802.15.4
To efficiently send IPv6 packets over 802.15.4, one needs to concentrate on the issues that arise from the design of the underlying low power MAC and PHY protocols: the small payload support and the low reliability of the transmission of packets.
6LoWPAN stands for “IPv6 Over Low Power Wireless Personal Networks”. It is designed specifically to handle the limitations when sending and receiving IPv6 packets over 802.15.4 links. In doing so, it has to accommodate for the 802.15.4 maximum frame size that can be sent

over-the-air. In Ethernet links, a packet with the size of the IPv6 MTU (1280 bytes) can be easily sent as one frame over the link. In the case of 802.15.4, 6LoWPAN acts as an adaptation layer between the IPv6 networking layer and the 802.15.4 link layer. It solves the issue of transmitting an IPv6 MTU by fragmenting the IPv6 packet at the sender and reassembling it at the receiver. 6LoWPAN also provides a compression mechanism that reduces the IPv6 headers sizes sent over-the-air and thus reduces transmission overhead. The fewer bits are sent over-the-air, the less energy is consumed by the device. Thread makes full use of these mechanisms to efficiently transmit packets over the 802.15.4 network. The methods by which fragmentation and header compression is accomplished is described in detail by [RFC 4944] and [RFC 6282].
Another important feature of the 6LoWPAN layer is the ability to provide link layer packet forwarding. It provides a very efficient and low overhead mechanism for forwarding multi hop packets in a mesh network. Thread uses IP layer routing with link layer packet forwarding. It makes use of the 6LoWPAN link layer forwarding capabilities to forward the packet by not having to send it up to the network layer. Thread makes full use of the ability of the MAC layer to provide addressing based on short addresses (16-bit length) to further reduce the information bits needed to be sent over-the-air to provide efficient packet forwarding. This saves processing cycles and improves power consumption at the same time while still using an IP based routing protocol.
Looking at the OSI (Open Systems Interconnection) model one can notice that the MAC is considered to be Layer 2 and the IPv6 Network is considered to be Layer 3. Being an adaptation layer, 6LoWPAN sits between these two and provides the necessary mechanisms and interfaces for them to interconnect.
To sum up, the 6LoWPAN adaptation layer provides the following:• IPv6 packet encapsulation
• IPv6 packet fragmentation and reassembly
• IPv6 header compression
• Link layer packet forwarding
Each of these capabilities is described in more detail in the sections that follow.