05-06-2013, 12:29 PM
ZigBee/IEEE 802.15.4 Summary
[attachment=54800]
Abstract
This document gives the motivation for the ZigBee alliance and explains the physical, medium
access and routing layers of ZigBee.
Introduction
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.
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.
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.
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.
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.
Network Topologies
Figure 2.1 shows 3 types of topologies that ZigBee supports: star topology, peer-to-peer topology
and cluster tree.
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.
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.
IEEE 802.15.4 PHY
The PHY provides two services: the PHY data service and PHY management service interfacing to
the physical layer management entity (PLME). The PHY data service enables the transmission and
reception of PHY protocol data units (PPDU) across the physical radio channel.
The features of the PHY are activation and deactivation of the radio transceiver, energy detection
(ED), link quality indication (LQI), channel selection, clear channel assessment (CCA) and
transmitting as well as receiving packets across the physical medium.
The standard offers two PHY options based on the frequency band. Both are based on direct
sequence spread spectrum (DSSS). The data rate is 250kbps at 2.4GHz, 40kbps at 915MHz and
20kbps at 868MHz. The higher data rate at 2.4GHz is attributed to a higher-order modulation
scheme. Lower frequency provide longer range due to lower propagation losses. Low rate can be
translated into better sensitivity and larger coverage area. Higher rate means higher throughput,
lower latency or lower duty cycle. This information is summarized in Figure 3.1.
[attachment=54800]
Abstract
This document gives the motivation for the ZigBee alliance and explains the physical, medium
access and routing layers of ZigBee.
Introduction
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.
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.
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.
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.
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.
Network Topologies
Figure 2.1 shows 3 types of topologies that ZigBee supports: star topology, peer-to-peer topology
and cluster tree.
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.
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.
IEEE 802.15.4 PHY
The PHY provides two services: the PHY data service and PHY management service interfacing to
the physical layer management entity (PLME). The PHY data service enables the transmission and
reception of PHY protocol data units (PPDU) across the physical radio channel.
The features of the PHY are activation and deactivation of the radio transceiver, energy detection
(ED), link quality indication (LQI), channel selection, clear channel assessment (CCA) and
transmitting as well as receiving packets across the physical medium.
The standard offers two PHY options based on the frequency band. Both are based on direct
sequence spread spectrum (DSSS). The data rate is 250kbps at 2.4GHz, 40kbps at 915MHz and
20kbps at 868MHz. The higher data rate at 2.4GHz is attributed to a higher-order modulation
scheme. Lower frequency provide longer range due to lower propagation losses. Low rate can be
translated into better sensitivity and larger coverage area. Higher rate means higher throughput,
lower latency or lower duty cycle. This information is summarized in Figure 3.1.