ZigBee/IEEE 802.15.4 Summary

Introduction
1.1 Evolution of LR-WPAN Standardization
The cellular network was a natural extension of the wired telephony network that became pervasive
during the mid-20th century. As the need for mobility and the cost of laying new wires increased,
the motivation for a personal connection independent of location to that network also increased.
Coverage of large area is provided through (1-2km) cells that cooperate with their neighbors to
create a seemingly seamless network. Examples of standards are GSM, IS-136, IS-95. Cellular
standards basically aimed at facilitating voice communications throughout a metropolitan area.
During the mid-1980s, it turned out that an even smaller coverage area is needed for higher user
densities and the emergent data traffic. The IEEE 802.11 working group for WLANs is formed to
create a wireless local area network standard.
Whereas IEEE 802.11 was concerned with features such as Ethernet matching speed, longrange(
100m), complexity to handle seamless roaming, message forwarding, and data throughput
of 2-11Mbps, WPANs are focused on a space around a person or object that typically extends
up to 10m in all directions. The focus of WPANs is low-cost, low power, short range and very
small size. The IEEE 802.15 working group is formed to create WPAN standard. This group
has currently defined three classes of WPANs that are differentiated by data rate, battery drain
and quality of service(QoS). The high data rate WPAN(IEEE 802.15.3) is suitable for multi-media
applications that require very high QoS. Medium rate WPANs (IEEE 802.15.1/Blueetooth) will
handle a variety of tasks ranging from cell phones to PDA communications and have QoS suitable
for voice communications. The low rate WPANs(IEEE 802.15.4/LR-WPAN) is intended to serve
a set of industrial, residential and medical applications with very low power consumption and cost
requirement not considered by the aboveWPANs and with relaxed needs for data rate and QoS. The
low data rate enables the LR-WPAN to consume very little power.
1.2 ZigBee and IEEE 802.15.4
ZigBee technology is a low data rate, low power consumption, low cost, wireless networking protocol
targeted towards automation and remote control applications. IEEE 802.15.4 committee started
working on a low data rate standard a short while later. Then the ZigBee Alliance and the IEEE
decided to join forces and ZigBee is the commercial name for this technology.
ZigBee is expected to provide low cost and low power connectivity for equipment that needs
battery life as long as several months to several years but does not require data transfer rates as high
as those enabled by Bluetooth. In addition, ZigBee can be implemented in mesh networks larger
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than is possible with Bluetooth. ZigBee compliant wireless devices are expected to transmit 10-75
meters, depending on the RF environment and the power output consumption required for a given
application, and will operate in the unlicensed RF worldwide(2.4GHz global, 915MHz Americas or
868 MHz Europe). The data rate is 250kbps at 2.4GHz, 40kbps at 915MHz and 20kbps at 868MHz.
IEEE and ZigBee Alliance have been working closely to specify the entire protocol stack. IEEE
802.15.4 focuses on the specification of the lower two layers of the protocol(physical and data link
layer). On the other hand, ZigBee Alliance aims to provide the upper layers of the protocol stack
(from network to the application layer) for interoperable data networking, security services and a
range of wireless home and building control solutions, provide interoperability compliance testing,
marketing of the standard, advanced engineering for the evolution of the standard. This will assure
consumers to buy products from different manufacturers with confidence that the products will work
together.
IEEE 802.15.4 is now detailing the specification of PHY and MAC by offering building blocks
for different types of networking known as ”star, mesh, and cluster tree”. Network routing
schemes are designed to ensure power conservation, and low latency through guaranteed time
slots. A unique feature of ZigBee network layer is communication redundancy eliminating ”single
point of failure” in mesh networks. Key features of PHY include energy and page link quality detection,
clear channel assessment for improved coexistence with other wireless networks.
1.3 ZigBee vs. Bluetooth
ZigBee looks rather like Bluetooth but is simpler, has a lower data rate and spends most of its time
snoozing. This characteristic means that a node on a ZigBee network should be able to run for six
months to two years on just two AA batteries. (HOW?)
The operational range of ZigBee is 10-75m compared to 10m for Bluetooth(without a power
amplifier).
ZigBee sits below Bluetooth in terms of data rate. The data rate of ZigBee is 250kbps at 2.4GHz,
40kbps at 915MHz and 20kbps at 868MHz whereas that of Bluetooth is 1Mbps.
ZigBee uses a basic master-slave configuration suited to static star networks of many infrequently
used devices that talk via small data packets. It allows up to 254 nodes. Bluetooth’s
protocol is more complex since it is geared towards handling voice, images and file transfers in
ad hoc networks. Bluetooth devices can support scatternets of multiple smaller non-synchronized
networks(piconets). It only allows up to 8 slave nodes in a basic master-slave piconet set-up.
When ZigBee node is powered down, it can wake up and get a packet in around 15 msec whereas
a Bluetooth device would take around 3sec to wake up and respond.
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IEEE 802.15.4WPAN
The main features of this standard are network flexibility, low cost, very low power consumption,
and low data rate in an adhoc self-organizing network among inexpensive fixed, portable and moving
devices. It is developed for applications with relaxed throughput requirements which cannot
handle the power consumption of heavy protocol stacks.
2.1 Components of WPAN
A ZigBee system consists of several components. The most basic is the device. A device can be a
full-function device (FFD) or reduced-function device (RFD). A network shall include at least one
FFD, operating as the PAN coordinator.
The FFD can operate in three modes: a personal area network (PAN) coordinator, a coordinator
or a device. An RFD is intended for applications that are extremely simple and do not need to send
large amounts of data. An FFD can talk to RFDs or FFDs while an RFD can only talk to an FFD.
2.2 Network Topologies
Figure 2.1 shows 3 types of topologies that ZigBee supports: star topology, peer-to-peer topology
and cluster tree.
2.2.1 Star Topology
In the star topology, the communication is established between devices and a single central controller,
called the PAN coordinator. The PAN coordinator may be mains powered while the devices
will most likely be battery powered. Applications that benefit from this topology include home
automation, personal computer (PC) peripherals, toys and games.
After an FFD is activated for the first time, it may establish its own network and become the
PAN coordinator. Each start network chooses a PAN identifier, which is not currently used by
any other network within the radio sphere of influence. This allows each star network to operate
independently.
2.2.2 Peer-to-peer Topology
In peer-to-peer topology, there is also one PAN coordinator. In contrast to star topology, any device
can communicate with any other device as long as they are in range of one another. A peer-to-peer
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Figure 2.1: Topology Models.
network can be ad hoc, self-organizing and self-healing. Applications such as industrial control
and monitoring, wireless sensor networks, asset and inventory tracking would benefit from such a
topology. It also allows multiple hops to route messages from any device to any other device in the
network. It can provide reliability by multipath routing.
2.2.3 Cluster-tree Topology
Cluster-tree network is a special case of a peer-to-peer network in which most devices are FFDs and
an RFD may connect to a cluster-tree network as a leave node at the end of a branch. Any of the
FFD can act as a coordinator and provide synchronization services to other devices and coordinators.
Only one of these coordinators however is the PAN coordinator.
The PAN coordinator forms the first cluster by establishing itself as the cluster head (CLH)
with a cluster identifier (CID) of zero, choosing an unused PAN identifier, and broadcasting beacon
frames to neighboring devices. A candidate device receiving a beacon frame may request to join
the network at the CLH. If the PAN coordinator permits the device to join, it will add this new
device as a child device in its neighbor list. The newly joined device will add the CLH as its parent
in its neighbor list and begin transmitting periodic beacons such that other candidate devices may
then join the network at that device. Once application or network requirements are met, the PAN
coordinator may instruct a device to become the CLH of a new cluster adjacent to the first one. The
advantage of this clustered structure is the increased coverage area at the cost of increased message
latency.
2.3 LR-WPAN Device Architecture
Figure 2.2 shows an LR-WPAN device. The device comprises a PHY, which contains the radio
frequency (RF) transceiver along with its low-level control mechanism, and a MAC sublayer that
provides access to the physical channel for all types of transfer. The upper layers consists of a
network layer, which provides network configuration, manipulation, and message routing, and application
layer, which provides the intended function of a device. An IEEE 802.2 logical page link control
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Figure 2.2: LR-WPAN Device Architecture.
(LLC) can access the MAC sublayer through the service specific convergence sublayer (SSCS).
Chapter 3 describes the physical layer of IEEE 802.15.4. Chapter 4 explains the MAC layer of
IEEE 802.15.4. Chapter 5 gives the routing mechanisms that are going to be used in the ZigBee.