While
successful wireless standards such as IEEE802.11x, GSM and Bluetooth, have been
relatively easy to sell, bringing standardisation into the realms of more
straightforward, low data rate, communications is significantly tougher. Within
this environment, the efforts of the ZigBee Alliance to standardise network and
application-support layers has, arguably, failed to make a convincing
impression. Does ZigBee have a future?
The
nature of RF design and the mentality of the typical RF design engineer being
what it is, means there will invariably be a proprietary system that offers a
better fit for a particular application. However, the attractions of
standardisation for low data rate communications remain compelling. Chief among
these being potential economies of scale, reductions in time to market, and
opportunities to establish widespread ad-hoc interoperability.
ZigBee capabilities and opportunities
The
ZigBee protocol is widely viewed as a companion to WiFi and Bluetooth,
operating in the same 2.4GHz unlicensed radio spectrum and slotting-in neatly
below Bluetooth in the power/data rate continuum. ZigBee also specifies the
lower-frequency 915MHz and 868MHz unlicensed bands available in the Americas and Europe
respectively. At 2.4GHZ, ZigBee provides 16 channels and supports a maximum
data rate of 250kbit/s.
The
standard is managed by the ZigBee Alliance, and takes advantage of the established IEEE802.15.4 PHY
and MAC layers. 802.15.4 provides a convenient and robust radio specification,
delivering benefits such as a straightforward power management strategy
comprising fully operational and stand-by modes only. This provides the basis
for very long battery lifetimes, which can be up to several years, while at the
same time simplifying application design.
Yet some may view ZigBee
as a problem child.
Currently,
the majority of anticipated applications are in domestic controls and utility
metering, as well as industrial, scientific and medical (ISM) equipment. To
penetrate these applications, however, it must entice product developers away
from the RF designs and supporting knowledge and IP embedded in existing
products.
For
example, a wireless key fob for a vehicle or garage door opener system may
already be functioning satisfactorily using a proprietary standard in the
868MHz or 433MHz band. Migrating this to ZigBee would be a significant
undertaking, only considered for a significant improvement in the product, such
as improving security or tamper-resistance.
Developing with ZigBee
and rivals
The
ZigBee Alliance has defined standards from the network layer to the application
support layer, which combine with the 802.15.4 lower layers to provide a
complete wireless stack.
Semiconductor
vendors such as Microchip, Atmel and Freescale have introduced complete modules
for ZigBee development, comprising the 802.15.4 radio and ZigBee upper layers
and integrating the microcontroller for ZigBee firmware and application
hosting.
Using
these modules, engineers are able to complete functioning designs without
specialist design skills such as RF matching and board layout. Development
boards supporting these modules provide a kick-start for application
development. In fact, Meshnetics, which provides software development kits
supporting hardware platforms from a number of vendors, claims a ZigBee network
can be set up for evaluation within one hour.
But
there are many other modules on offer to designers that provide the convenience
of turnkey RF hardware design and an approved radio specification. Designers
using 802.15.4 radio modules, for example, are free to use alternative upper
layers, and the vendors of ZigBee modules provide the flexibility for designers
to make their own choices. On the other hand, some silicon vendors offer
complete transceiver modules based on proprietary radio specifications that are
pre-approved to acceptance criteria in international territories.
The
absence of any license fee is a major advantage to designers using proprietary
products but, on the other hand, greater economies of scale may be available
with a ZigBee-based system.
ZigBee strengths
As
a managed standard designed to provide robust security features, allow large
numbers of nodes, and support interoperability and vendor independence, ZigBee
has convincing arguments in its favour.
It
also embodies features that enable outstanding performance at relatively low
power consumption. For example, support for multi-hop communications and
flexible routing enable effective transmission distances to be extended beyond
the single hop range of the 802.15.4 radio standard without resorting to
increasing transmitter power.
Where
the typical single-hop range is around 30m – but may be between 10m and 100m
depending on environmental conditions, antenna performance and operating
frequency band – the cumulative range of a ZigBee node can be extended to
hundreds, or even thousands, of metres.
In
addition, by supporting data rates up to 250kbit/s, when operated in the 2.4GHz
band, ZigBee can deliver additional savings in transmitter power compared to
networks using slower protocols.
In
sensor networks, for example, where data collection rates may be comfortably
inside this capability, ZigBee’s ability to complete data transactions quickly
allows the transmitter to be powered off for longer periods. Since the power
dissipated by the transmitter is largely independent of its operating speed,
ZigBee delivers power savings by reducing the average energy per bit.
Compared
with some proprietary systems, using a proven ZigBee module also helps with
type approvals. The module vendor will already have overcome the majority of
compliance hurdles, which effectively short-cuts a significant part of the
approvals process. Alternatively, a significant number of proprietary offerings
also carry relevant approvals.
An Achilles heel?
Designers,
therefore, have a number of choices when selecting a platform for low data rate
networking over long distances. The trend towards delivering the radio module
as a complete module is quite commonplace among proprietary, non-ZigBee
systems, as well as within the ZigBee environmen t. Although ZigBee is not the
only system emphasising ease of design, the licensing structure maintained by
the ZigBee Alliance adds to the cost of entry for ZigBee development.
Catalysts over the horizon
On
the whole, ZigBee faces a tougher challenge to gain adherents than either
Bluetooth or WiFi. Potential markets are diverse and also tend to be of lower
total value. A number of well-established products are already in place, even
though these do not offer the same potential for ubiquitous ad hoc
interoperability. Moreover, the associated license fees remain relatively high.
It
is worth remembering that the future of Bluetooth was in doubt for some time,
until governments imposed legislation on the use of mobile phones in cars and
in so doing kick-started the market for Bluetooth headsets. It is hard to
imagine a likely legislative catalyst for ZigBee and a ‘killer application’ to drive
mass adoption seems unlikely, taking into account the characteristics of ISM
markets.
On
the other hand, the standard is now widely understood, development platforms
and silicon are readily available, and demand for wirelessly networked products
in consumer and professional fields continue to grow strongly. If there are
sound technical justifications for ZigBee, reducing the cost of entry may be
sufficient to secure a successful future.
