Zigbee Wireless Technology and the IEEE 802.15.4 Radio
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Transcript of Zigbee Wireless Technology and the IEEE 802.15.4 Radio
ZIGBEE WIRELESS TECHNOLOGY AND THE IEEE
802.15.4 RADIO – ENABLING SIMPLE WIRELESS
Jon T. Adams
Freescale Semiconductor, Inc.
ABSTRACT
ZigBee wireless mesh technology makes wireless sensor and
control network applications practical. Cost-effective, simple-to-
use, capable of very long product functionality from a pair of
standard alkaline cells, it is meant for developers who want
either to get rid of the tether that their product must use or
provide a new level of functionality to their products with
wireless communications. The ZigBee Specification [1] takes
advantage of the IEEE STD 802.15.4 [2] wireless protocol as the
basic communications method, and expands on this with a robust
mesh network, applications profiles and device descriptions as
well as interoperability and compliance testing.
1. INTRODUCTION
ZigBee wireless mesh technology has been developed to address
sensor and control applications with its promise of robust and
reliable, self-configuring and self-healing networks that provide
a simple, cost-effective and battery-efficient approach to adding
wireless to any application, mobile, fixed or portable. A typical
IEEE 802.15.4-based, ZigBee-compliant device is shown in
Figure 1.
Figure 1. Typical IEEE 802.15.4/ZigBee Device (including
antenna, RF data modem, applications processor, all necessary
passives and 16MHz crystal, about 15x40mm) - Courtesy
Freescale Semiconductor.
The ZigBee Alliance released their specification to the public in
June 2005, and since then the playing field has become much
simpler for product designers who want to add wireless to their
sensor or control application. An open and growing industry
group of more than 180 companies from product/system OEMs
to applications developers to semiconductor companies, the
Alliance has worked hard to provide a technology that takes best
advantage of the robust IEEE STD 802.15.4 short-range wireless
protocol, adding flexible mesh networking, strong security tools,
well-defined application profiles, and a complete
interoperability, compliance and certification program to ensure
that end products destined for residential, commercial and
industrial spaces work well and network information smoothly.
2. IEEE 802.15.4 STANDARD
The IEEE standard brings with it the ability to uniquely identify
every radio in a network as well as the method and format of
communications between these radios, but does not specify
beyond a peer-to-peer communications link a network topology,
routing schemes or network growth and repair mechanisms.
Figure 2. ZigBee Device Construction
The IEEE 802.15.4 standard, released in May 2003, was selected
by the ZigBee Alliance as its “wheels and chassis”, upon which
ZigBee networking and applications are constructed. This is not
without its challenges, as the Alliance does not control the IEEE
specification. However, many of the same people who sit in the
IEEE 802.15 Working Group are deeply involved in the ZigBee
standard; this relationship has meant that both the IEEE and the
ZigBee specifications track one another fairly well. Figure 2
shows the relative organization of the IEEE radio with respect to
the ZigBee functionality.
1.1 RF Link
The IEEE standard specifies the RF link parameters, including
modulation type, coding, spreading, symbol/bit rate, and
channelization. Currently, the standard identifies 27 channels
spread across three different frequency bands.
Frequency Band (MHz)
868.3 902-928 2400-2483.5
# of Channels 1 10 16
Bandwidth (kHz) 600 2000 5000
Data Rate (kbps) 20 40 250
Symbol Rate
(ksps)
20 40 62.5
Unlicensed
Geographic Usage
Europe Americas
(approx)
Worldwide
Frequency
Stability
40 ppm
76 Texas Wireless Symposium 2005
Table 1. IEEE 802.15.4 Frequency Allocations and Basic PHY
Parameters
The IEEE specification was designed for very low
duty cycle (nominally about 0.1% transmitter duty cycle or less),
intermittently communicating devices in a large, dynamic
network. While feasible, high duty-cycle applications like voice
or low-rate video cannot benefit nearly as much from power
consumption savings as much of the power consumption savings
is due to the device being able to spend most of its time in a
quiescent state.
The standard is aligned to specific unlicensed bands
available in different geographic regions. The single 868.3 MHz
channel for use strictly in the European Union is limited to a
0.1% transmitter duty cycle by regulation. The 900 and 2400
MHz bands have no regulatory duty cycle restrictions.
The nominal transmitter power output specified is
0.5mW (-3dBm), again to limit power consumption. However, it
is allowed to increase the output power through external
amplifiers to whatever the regional regulatory limits are. The
nominal receiver sensitivity is specified by Packet Error Rate
(PER). The specification requires 1% PER at -85dBm receive
power level for the 2400 MHz band and -92dBm for the sub-
GHz bands (as measured at the chip’s antenna terminals). This
represents a receiver with a noise figure substantially worse than
20dB, though most radios in production are between 7 and 9 dB
better than this. Thus, manufacture of the radio receiver using
low-cost CMOS processes is very feasible. Also, receiver noise
figure is a significant fraction of the overall receiver current, so
while an extremely sensitive and high performance receiver is
not impractical from an engineering point of view, it may act to
substantially reduce battery lifetime in a portable application.
Figure 3. 15.4 Performance vs other Common PHYs
The modulation mode used by 802.15.4 is phase-shift-
key (PSK) based, chosen because of its strong ability to be
recovered even in very low signal to interference environments.
PSK modulation has been employed by NASA for decades in
deep space mission telecommunications, - its use in most
commercial and consumer applications was traditionally limited
due to complexity and cost, with Frequency-Shift Keying (FSK)
commonly employed. As Figure 3 indicates, the Bit Error Rate
performance of PSK with respect to FSK (as well as the
modulation techniques of other standards) allows 802.15.4 to
enjoy significantly better link margins than other common
systems, giving it added robustness in noisy or marginal
propagation environments.
The channels below 1 GHz use BPSK modulation
(Binary PSK), while the 2400 MHz band employs O-QPSK
(Offset Quadrature PSK). QPSK is spectrally efficient, but
requires a linear transmitter due to the state transitions through
zero. So, the developers chose O-QPSK which avoids the zero
state and thus allows for a constant envelope transmitter,
significantly decreasing transmitter complexity and inefficiency.
Finally, 802.15.4 employs direct-sequence spread
spectrum to provide coding gain and added resiliency against
multipath. 16 chips per symbol are used for the sub-GHz
frequencies, while 32 chips per symbol are used for the
2400MHz band. Interestingly, the spreading code used allows a
standard FSK receiver to successfully demodulate the
transmitted signal, although at significantly reduced link margin.
However, in particularly cost-sensitive applications, it may be
advantageous to sacrifice link margin for cost.
1.2 PHYsical Layer
The 15.4 PHY contains specific primitives that manage the radio
channel, and control packet data flow. This is a packet radio
specification, and the PHY defines 4 different frames that have
unique functions: Data, Acknowledgement, Beacon and MAC
Command. The Data frame can carry up to 104 bytes of payload.
The Acknowledgement frame is used by a receiving station to
“acknowledge” to the transmitting station that a data packet was
received without error. The Beacon frame is used by stations
that may be implementing significant power saving modes, or by
Coordinator and Router devices that are attempting to establish
networks. The MAC Command frame provides some unique
abilities to send low-level commands from one node to another.
While ZigBee networking uses all four frame types, it
is the Data and Acknowledgement frames that are most pertinent
to the robustness and reliability of data transmission. All frames
share generally common structure, as well as mechanisms such
as a 4-octet (32-bit) preamble to allow the receiver to center and
synchronize on the incoming packet, as well as the Frame Check
Sequence (FCS), a 16-bit cyclic redundancy check value that
allows the receiving station to validate that the payload is error-
free. The Data frame contains a Data Sequence Number, a
cyclical 8-bit value that increments for each unique packet the
transmitter sends. The destination device will respond to a
successful receipt of a given Data frame with an
Acknowledgement frame containing the same Data Sequence
Number, allowing the sender to send multiple packets without
waiting for an acknowledgement for each one first.
The PHY uses Carrier Sense Multiple Access (CSMA)
with Collision Avoidance (CA) to access the radio channel. This
means that a radio with data to transmit will first listen to the
channel and if the channel is clear, then transmit its packet.
However, if the channel is busy, either due to another 802.15.4
station transmitting, or due to interference from a non-802.15.4
station (microwave oven, Wi-Fi access point, etc.), the radio will
hold off from the channel for a random period of time before
again checking the channel for occupancy. In a system where all
stations can hear one another, CSMA-CA can provide nearly a
36% channel usage, but in practical environments where all
stations cannot hear one another, the channel usage efficiency is
77
as low as the traditional ALOHA mechanism, about 18%. Again,
this was understood when the standard was created, and is
acceptable given the requirements for system simplicity.
Figure 4. IEEE 802.15.4 Timing and Minimum Latency
The PHY manages all symbol and bit level timing, as
well as transmit-receive switching times, intra-packet timings
and acknowledgement delays. The PHY can be configured to
automatically acknowledge (or not) every packet received
successfully, depending on the application.
Figure 4 demonstrates the timing involved in a data
transaction between two devices. The sensor wakes up either on
an event or at the end of an interval, checks the channel,
transmits its message, awaits the acknowledgement, then may go
back to sleep or first receive data intended for the node before
going back to sleep.
1.3 Medium Access Control (MAC) Layer
The 802.15.4 MAC contains over two dozen primitives that
allow data transfer, both inbound and outbound, as well as
management by higher-level entities of the RF and PHY. In all
systems currently on the market, the MAC is implemented in
software that runs on some sort of MCU core, but over time it’s
practical to see that as the standard is proofed out that the MAC
could be implemented in a state machine or an embedded core
exclusively dedicated to MAC functions. Since system cost and
power consumption will remain driving factors for the standard
and products based upon the standard, the market will have a
strong influence on the systems architecture over time.
IEEE 802.15.4 uses a 64-bit unique address for every
radio node. Similar to other IEEE standards including 802.11
and 802.3, the address consists of a 24-bit Organizationally
Unique Identifier which identifies a specific entity that “owns”
the address block, and in the case of 802.15.4 a 40-bit sequence
that is allocated by the IEEE in blocks of 224. The 64-bit address
is used in peer-to-peer communications, where no greater
network is available. 802.15.4 expects that in a typical network,
where there is a network coordinator and client devices, that the
coordinator will issue 16-bit addresses to each of the devices that
are part of the network. This 16-bit address is shorter, easier to
manipulate, and represents less overhead in the packet frame.
Figure 5. ZigBee/802.15.4 Mesh Network and Device Types
There are two physical types of device specified in
802.15.4, and three logical types. The two physical types are the
Full Function Device (FFD) and the Reduced Function Device
(RFD). While either device may act as a sensor node, control
node, or composite device, only the FFD may perform routing
tasks for a network. FFDs may, depending on their location in a
network, have child devices for which the router performs
routing functions. RFDs do not route, and therefore cannot have
child devices. Figure 5 is a graphical representation of the
connectivity practical in a ZigBee network using 802.15.4
devices.
There are three logical devices envisioned in an
802.15.4 network, the Coordinator, Router, and End Device. An
End Device exists at the “edge” of the network, thus an End
Device is the end of routing environment for the network. End
Devices may be simple sensors, or complicated control devices,
but the main thing to remember is that they cannot propagate the
network beyond them. End devices are often constructed from
RFDs, primarily for cost reasons. The Router and Coordinator
are potentially identical devices, and differ in function only
based upon the network’s initial power-up sequence. For
example, Devices A, B and C are FFD devices, and all exist
within radio range of one another. Device A is powered up for
the first time and per the 802.15.4 standard, first searches the
available RF channels to look for an existing network. It checks
each channel for 802.15.4 activity, and broadcasts a network
beacon request, waiting for response from a standing network. If
nothing is heard, A will select a quiet channel and attempt to
start a network on that channel. A has become the network
coordinator.
Device B is powered on for the first time, goes through
the same process, and upon requesting a network beacon on A’s
channel, receives A’s beacon and recognizes that there is a valid
802.15.4 network on that channel. It completes its scan, and if
no other networks are discovered, returned to A’s channel and
attempts to associate with A’s network. If A allows this
association, then B becomes a member of A’s network and
receives, as the first device in A’s network, the network address
of 1. If the network topology allowed by A lets B become a
Router, then B can assume that role since it’s also a FFD.
Next, C is powered up for the first time. When C
requests a beacon on the network channel, both A and B will
attempt to respond, with the CSMA-CA mechanism preventing
78
them from both responding simultaneously. C will hear one of
the network beacons, and attempt to associate with the network.
As C can hear both A and B, the association imperative is to join
networks as close to the Coordinator as practical. Therefore, C
will attempt first to associate directly to A. If that fails (perhaps
the path really is marginal), then C will fall back to attempting to
associate with B. B, being a router, does not “own” the network
but has been allocated a block of addresses by A to distribute.
Depending on network parameters, B may accept C’s association
request or may require A to make the decision. Once C has been
accepted into the network, C will trade its 64-bit address for a
16-bit local address. Assuming C is allowed to be a router, C
will get the next router address from A, which will depend
directly on the ultimate network topology allowed by A. If C
joins at B, then C will get address 2. Figure 4 shows a typical
ZigBee network address structure and the initial tree routes that
are created.
The ZigBee specification does not take advantage, at
this time, of all the functionality specified in the 802.15.4 MAC.
For instance, there is the ability to use, instead of CSMA-CA
channel access, Time Domain Multiple Access, This is
implemented by a system of regular timing beacons, superframes
and allocation of specific time slots to different stations. At this
time, this feature is not used within any ZigBee profile, but there
are currently proprietary applications that take advantage of this
function.
3. ZIGBEE NETWORKING
ZigBee networking is natively mesh-based. Cost-effective, long-
battery-lived radios cannot use high transmit power to ensure
successful transfer of data. Instead, the network must be more
clever – the most robust route between source and destination
may not be the obvious, shortest physical path route, but instead
a route that requires other radios to “relay” the information.
The ZigBee networking specification provides great
flexibility to the systems developer, providing tools to design a
network that can meet a variety of needs. It is important to note
that the network cannot be all things to all developers - a
network may be developed along a number of somewhat
orthogonal dimensions including power consumption, latency,
topology, RF frequency, expandability, and capacity, as well as
other, more interrelated factors.
The ZigBee networking specification provides
networking mechanisms that allow a developer to create star,
tree and mesh network topologies, depending on network
requirements. Once formed, a wireless network can be subject to
interference, propagation changes, continued growth, unintended
usage and security issues. How does a ZigBee network respond
to these factors? Wireless networks bring with them added
flexibility in deployment and modification, but do not carry the
generally assured connectivity that twin strands of copper wire
provide. By no means does that make a wireless system less
useful or robust. The wireless system architect must understand
the types of environments in which the wireless network is
expected to be used, the usage patterns, and (at least) the most
common failure modes. With this information, mitigations may
be crafted that improve the reliability and robustness to levels
that are “good enough”.
Figure 6. Typical ZigBee Address Allocation and Tree Routing
Structure for Lmax = 3, Cmax = 20, and Rmax = 6
ZigBee networking (NWK) sits atop the IEEE RF/PHY/MAC
and provides the required functionality to create and manage
mesh networks. When a node is activated for the first time, the
NWK commands the MAC to search all channels for an
available network. Once it finds one, the MAC can provide that
information to the NWK layer and let the end application
determine whether to join, or it can join the network
automatically. Once joined, the network’s ZigBee Coordinator
or one of the Routers assigns a 16-bit address to the node
according to rules based upon the parameters contained in what
is called the ZigBee “Stack Profile”.
In Figure 6, a small network is described to show how ZigBee
allocates 16-bit IEEE802.15.4 addresses. ZigBee networks
strongly use the concept of generations of family: Device 5 is
the Child of Device 1, and the Parent of Device 10. Also, Device
2 has a total of 9 descendants (4 Children, 5 Grandchildren).
Only IEEE802.15.4 Full-Function devices may “procreate”, and
at Layer Lmax no device may procreate. ZigBee devices that may
procreate are considered ZigBee Routers (ZR), those that may
not or can not procreate are ZigBee End Devices (ZED). Layer 0
shall contain only the ZigBee Coordinator (ZC). It is the
Coordinator that determines the network addressing according to
the following three network parameters:
Layers (Lmax). This specifies the maximum
“radius” that any ZigBee network may have. For an
Lmax=3 network as depicted above, this means that no
node may be more than three physical RF hops from
the Coordinator. From the “family” concept, Device
11 is the “great-grandchild” of device 1.
Children (Cmax). Defines the total number of
nodes in Layer n that may be directly connected to a
parent Router in Layer n-1. For example, node 2, a
Router, may have a total of 20 children, but in the
above diagram has only 4, including 3 Routers and 1
End Device.
Routers (Rmax). The maximum number of
children that may also function as Routers. In the
above example of 20 children per parent, that means
that 6 of those children may also act as parents to their
79
own children, while the remaining 14 (Cmax-Rmax) may
not be parents. For any procreating Parent, the first R
address blocks are reserved for Router children, while
the remaining Cmax-Rmax addresses are reserved for
End Device children.
Most products that are mains-powered are also IEEE 802.15.4
Full Function devices and therefore have the ability to be
parents. However, as defined above, any device in the outermost
layer may not procreate by network rule, whether they are a Full
Function device or not.
The equation defines the maximum number of nodes in a given
ZigBee network. The example network above, with Lmax = 3,
Cmax = 20, and Rmax = 6, can have a total of 861 nodes. There is
no limit defined to Lmax, Cmax and Rmax except that Ntotal cannot
exceed approximately 216. Again, referencing Figure 6, the
number next to the upper right corner of each device box is the
ZigBee Coordinator-allocated IEEE 16-bit network address.
In a network with Lmax = 6, Cmax = 6, and Rmax = 6 (all children
may be Routers), the maximum number of nodes in the network
is over 55,000 and the physical radius (even using only 10m per
hop) could be 120m or about 400’, potentially covering several
acres of office or factory floor. For another network Lmax = 10,
Cmax = 60, and Rmax = 2, where there’s a lot of sensors and a well
placed routing infrastructure, the maximum number of nodes is
well over 61,000, Networks that are physically larger than this
are generally broken into multiple sub-networks, just like
Internet addresses are divided into subdomains, partially for ease
of use and also for added robustness and network capacity.
Figure 7. Physical Network of Figure 5 after Growth of Mesh
Routes
What Figure 7 shows is that while the initial network address
allocation also created a default tree network, very soon
afterward the network routing devices (1, 2, 5, 7, 8, 9) begin to
learn additional routes between themselves, creating a mesh
network that may actually do the majority of the routing,
depending on routing performance. For instance, the sensor 14
default route to the device 1 home security panel is 14>8>2>1, a
total of 3 hops. However, there’s a shorter route 14>8>1 which
probably has a higher reliability since it has one less hop to it.
Once the mesh route is learned, it is probable that data from
occupancy sensor 14 will travel via the mesh route to the
security panel.
3. CONCLUSIONS
This paper has described the fundamentals of ZigBee
networking and its reliance upon the mechanism of IEEE
802.15.4 as the “wheels and chassis” for ZigBee. RF, PHY and
MAC properties specified by the IEEE have been contrasted and
compared to other common radio protocols available today. The
ZigBee network functionality and its use of the IEEE MAC
mechanisms to establish and create the network, as well as the
methods by which a ZigBee network grows and manages
connectivity, have been described. ZigBee devices have a real
potential to solve the challenge of bringing simple, effective
wireless connectivity to low-rate sensors and control devices at
an effective cost.
4. REFERENCES
The following references are available publicly.
[1] Various authors, ZigBee Specification, ZigBee Alliance, 14
December 2004.
[2] McInnis, M. editor-in-chief, 802.15.4 – IEEE Standard for
Information Technology, Institute of Electrical and Electronic
Engineers, New York, 1 October 2003.
Lma
(Rmax)(n-
1)Ntotal = 1 + Cmax
n=1
80