industrial wireless book special edition - Networking ...

Internet of Things
unique in that it also addresses severe resource
constraints.
IEEE 802.15.1 (Bluetooth) is another wireless
link technology that falls under the WPAN classification.
Intended to serve as a cable
replacement technology, Bluetooth supports
relatively high throughput for a limited number
of nodes within a small range. IEEE 802.15.3
pushes WPAN capabilities further, with greater
throughput and support for more nodes.
Although both are intended for battery
operation, they only target lifetimes of several
days to several weeks.
In contrast, 802.15.4 is intended for low data
rate applications in which numerous nodes
must be low cost and have multi-year lifetime
on modest batteries.
The 802.15.4 standard supports up to 64,000
nodes within a PAN compared to a small handful
with other WPAN links. 802.15.4 has also
reduced complexity, intended to function with
eight bit microcontrollers providing 8 Kbytes of
RAM or less. Although IP over Bluetooth using
the Bluetooth Network Encapsulation Protocol
has been around for several years, it is typically
used to provide a point-to-point connection
over a single radio hop.
Researchers have developed numerous mesh
network layers over 802.15.4, as open source
projects (such as TinyOS, industrial forums
(ZigBee and WirelessHART), or proprietary
offerings (Dust Networks, Sensicast and
Millenial Net). Each has defined its own set of
incompatible packet formats tied to particular
MAC features, routing algorithms, and
addressing. Many address only the individual
802.15.4 subnet, leaving all further communication
protocols to be defined via ad hoc
gateways. 6LoWPAN potentially offers unification
of this disparate activity and enable
embedded 802.15.4 devices to be incorporated
into Ethernet, Wi-Fi, General Packet Radio
Service, and other environments within a
uniform IP framework.
Many embedded TCP/IP stacks provide IP host
functionality and are widely used in wired and
powered settings. However, few embedded IP
stacks directly address the issues related to
supporting IP over low power mesh topologies
in LoWPANs.
Within the IETF, the mobile ad hoc networks
(MANET) working group and related research
activities, tremendous effort has been devoted
to reactive and proactive routing protocols for
mobile devices. This work has assumed capable,
high bandwidth links and powerful hosts with
high, random mobility. As such, it used conventional
IP datagrams and frame formats and
hasn’t attend to the impact of resource
constraints.
Work in the IETF Auto configuration (AUTOCONF)
working group is devoted to developing solutions
for stateless address auto configuration and
Neighbour Discovery in settings in which IP
connectivity is naturally viewed as a collection
of overlapping partial broadcast domains.
The Routing Over Low power and Lossy links
(ROLL) working group was recently chartered
to address routing in LoWPANs (independently
of the link layer).
Until recently, extending IP out to wireless
industrial networks was thought to be impractical,
if not impossible. Vendors embraced
proprietary protocols because they presumed
that IP, which is memory and bandwidth
intensive, couldn’t be scaled down to operate
on the microcontrollers and low power wireless
links used in these environments. Recent
efforts within the IETF make IP over low power
communication links now feasible, including
IEEE 802.15.4. These developments make IP
attractive for use in low power devices,
everything from handhelds to instruments.
Jonathan Hui Ph.D, Arch Rock Corporation
David Culler Ph.D, University of California, Berkeley
Samita Chakrabarti, IP Infusion
From the IPSO paper 6LoWPAN: Incorporating IEEE
802.15.4 into the IP architecture
6LoWPAN Neighbour Discovery: a high level overview
The 6LoWPAN Neighbour Discovery protocol
is an extension to the standard IPv6
Neighbour Discovery protocol. 6LoWPAN ND
mostly uses the same standard ND messages
with some additional extension of options.
A few new messages have been added to
consider ND support for mobility, fault
tolerance, bootstrap, header compression
etc. The goal of this protocol is to support
both IP-routed and link-layer mesh networks.
IPv6 provides 128-bit address space which
can support huge number of unique IP
addresses. The core of IPv6 protocol functionality
is the Neighbour Discovery Protocol which
is mainly used for address resolution, address
auto configuration, router discovery and
neighbour reachability.
The IETF standardisation effort for 6LoWPAN
Neighbour Discovery has taken the following
issues related to low power wireless links into
the design considerations:
● A continuously changing wireless link;
● Assigned short-addressing;
● The flat network model of 6LoWPAN;
● The use of both link layer mesh and IP layer
mesh routing techniques;
● The lack of native multicast;
● Deep sleep cycles of 6LoWPAN nodes;
● The mobile nature of wireless nodes
● The limited energy and computational
capacity of nodes.
The new 6LoWPAN ND specification takes
these 6LoWPAN issues into account, providing
optimised ND mechanisms. In order to do so
certain assumptions are taken into account.
For simplicity, IPv6 addresses have a direct
mapping to link layer MAC addresses, and
because of the nature of low power wireless
networks a subnet is extended to cover whole
6LoWPAN islands. The use of multicast is
avoided, and the operation of hosts is kept as
simple as possible.
Protocol overview and operations
6LoWPAN ND defines new terminology which is
useful for understanding how it works. The
nodes in 6LoWPAN networks are 6LoWPAN
Hosts, 6LoWPAN Routers and Edge Routers.
Hosts send and receive packets but do not
route; 6LoWPAN routers perform IP routing
within the LoWPAN, and Edge Routers route
between 6LoWPANs and other IP networks.
A 6LoWPAN IPv6 subnet includes all the nodes
which share the same IPv6 prefix covering a
whole LoWPAN or Extended LoWPAN. Figure 1
shows the 6LoWPAN architecture with different
kinds of IPv6 LoWPANs and network elements.
A LoWPAN is defined by the nodes sharing
the same IPv6 prefix, usually with a single
Edge Router as shown in the figure. A LoWPAN
may be connected to other IP networks over a
backhaul link, e.g. xDSL or GPRS.
Fig. 1. The Model for 6LoWPAN architecture
The Extended LoWPAN in the figure is made
up of two Edge Routers and their nodes.
Extended LoWPANs use a backbone link, e.g.
Ethernet, to coordinate information about the
LoWPAN, which is also part of the LoWPAN
IPv6 subnet. Finally, an Ad Hoc LoWPAN is also
shown which can operate without a backbone
infrastructure. In Fig. 1, IP routing is used as
an example within the LoWPANs with both
Host and Router nodes.
After the underlying wireless link technology
has been initialised, 6LoWPAN-ND provides
basic mechanisms for a new node to bootstrap
itself onto a 6LoWPAN using IPv6 stateless
auto configuration.
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