wilamowski-b-m-irwin-j-d-industrial-communication-systems-2011

51-6 Industrial Communication Systems
Internet
IPv4/IPv6 core network
Wireless sensor network
6LoWPAN
Enterprise network
IPv4/IPv6 core network
Wireless sensor network
6LoWPAN
Wireless sensor network
6LoWPAN
Subnetwork
IPv4/IPv6 core network
IP router
6LoWPAN/IP edge router
FIGURE 51.4
6LoWPAN WSN integration.
The reader should keep in mind that 6LoWPAN networks will not take the role of transit networks.
Moreover, 6LoWPANs are applied as the sensing parts placed at the edges of existing networks
(see Figure 51.4). Hence, routers connecting 6LoWPANs and IPv6 networks are called edge routers.
To deploy IPv6 on 802.15.4, 6LoWPAN concentrates on two major points: fragmentation and header
compression. For a comprehensive specification, 6LoWPAN also includes rules for stateless address
autoconfiguration, address mapping, and security.
Stateless Address Autoconfiguration. Because of the expected high number of nodes within WSNs, stateless
address autoconfiguration is required. 6LoWPAN examined how to use the EUI-64 device identifier,
assigned to every 802.15.4 device, to generate an identifier for the IPv6 interface.
Addressing Mapping. As 802.15.4 does not support multicast natively, a mapping of IPv6 multicast to the
802.15.4 link layers have to be examined. For an optimized header compression, further mapping even
of unicast IPv6 addresses is required.
Fragmentation. The maximum PHY layer packet size of IEE 802.15.4 is 127 bytes. The maximum MAC
layer frame size is 102 octets. If link layer security is enabled, further overhead is imposed and 81 octets
are left for upper layers. This is far below the minimum packet size (MTU) of 1280 octets required by
the IPv6 specification. Thus, 6LoWPAN must provide a fragmentation and reassembly layer between
the MAC and the IP layer.
Header Compression. Due to the limited packet size of 802.15.4, protocol headers have to be reduced
to provide a suitable payload to header ratio and to avoid protocol overhead. IPv6 basic headers have
a size of 40 octets. UDP headers use 8 octets and TCP headers need 20 octets. This leaves 33 octets for
UDP and only 21 octets for TCP communication. Thus, a header compression scheme is defined by the
6LoWPAN working group.
Security. The application of WPAN implies requirements for data security, e.g., for personal health
records. The working group has also considered the secure exchange of messages and analyzed how the
available link layer security of 802.15.4 can be used to ensure secure IP traffic.
Routing. As stated above, 6LoWPAN does not focus on routing of IPv6 on 802.15.4. Hence, the 6LoWPAN
does not provide a full specification for mesh routing. Nevertheless, existing routing over IP protocols
and routing under IP over MAC layer in a mesh network are examined and specific frame types are
defined to be used by the mesh routing under protocols. For more detailed information about route over
(IP) and mesh under (IP) routing, please refer to the ROLL working group [IETFROLL], which is still
examining different strategies for routing concepts in the time of writing this section.
A common goal of all efforts in the 6LoWPAN working group is the minimization of required bandwidth
and power consumption, protocol overhead, packet overhead, and processing requirements due
to the limited resources. Thus, the presented issues could not have been solved one after the other but
requires cross layer design. For example, address autoconfiguration has to collaborate with header compression
schemes and network management as well. As key concept, 6LoWPAN elides any redundant
information/header fields, which already exist in the lower layers or may be derived by their combination.
© 2011 by Taylor and Francis Group, LLC