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

6LoWPAN: IP for Wireless Sensor Networks and Smart Cooperating Objects 51-3
51.2.6 Lessons Learned From Maintaining the Internet
New protocols require a deep understanding and a profound knowledge of the internals. By decades
of iterations, IP and further protocols are matured, wide ranging, and well understood. This results in
reduced educational, implementation, installation, and maintenance costs.
51.3 Introduction in 802.15.4
Developments of new protocols for WSN and SCO have just started and are in progress. Based on IEEE
802.15.4 [IEEE154], which defines the PHY and MAC communication layers as depicted in Figure 51.1,
various upper layer protocols are defined by large industrial consortia, e.g., WirelessHART, ISA SP100,
ZigBee. Open standards and proprietary industrial consortia standards will be in a close competition.
Key features of all protocols are
• Long battery life through low-power concepts
• Self-healing networks
• Scalability and support of large networks
• Low-cost node
• Low data rate
IEEE 802.15.4 is a standard for WPANs with low data rates (LR WPAN). While technologies like Wi-Fi
(802.11) and Bluetooth have been developed for high data throughput, the focus of IEEE 802.15.4 networks
was power-aware and low-cost design, which results in low data rates (up to 250.kbps).
IEEE 802.15.4 standard distinguishes between two device classes: full function devices (FFD) and reduced
function devices (RFD). In accordance to IEEE 802.15.4, a device is “any entity (RFD or FFD) containing an
implementation of IEEE 802.15.4 MAC and PHY interface to the wireless medium.” A coordinator is an FFD
with network device functionality that provides synchronization and other services to the network. A PAN
coordinator is the principal controller of a personal area network (PAN). An IEEE 802.15.4 network has one
PAN coordinator.
The main difference between RFDs and FFDs is the requirements for resources as RFDs are only
applied for simple applications. An FFD can operate in any topology and can run as PAN coordinator,
coordinator, and device. RFDs are limited to star topologies and end devices. They are communicating
directly with a coordinator, while FFDs can both interact with any type of device (RFD and FFD) and
operate in star and peer-to-peer networks. In star topology, all devices are communicating with a PAN
coordinator. In peer-to-peer topologies—also known as mesh networks—all devices can communicate
with each other (cf. Figure 51.2).
For the communication of FFDs and RFDs, 802.15.4 describes a beacon-enabled mode. RFDs can
stay in low-power sleep states for predefined time slots. They only wake up to receive the periodical
beacons of the coordinator (FFD). Additional to the beacon signal, further information is included if
Application
Transport
Network
Application specific
profile
DPWS/SOAP/WSDL
XML Schema/HTTP/...
IPv4, IPv6
TCP/UDP
...
IETF 6LoWPAN
Profile
ZigBee
Gateway/proxy required
Wireless-
HART
(draft)
ISA SP100
(draft)
Profile
Bluetooth
PHY
MAC
IEEE 802.3,
802.11a/b/g...
IEEE 802.15.4
IEEE 802.15.4e (+TDMA
and channel hopping)
802.15.1
FIGURE 51.1
802.15.4 defined PHY and MAC layers.
© 2011 by Taylor and Francis Group, LLC