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INTELLIGENT SYSTEMS DECEMBER 2003
Tag: networking
Standards-Based Wireless Networking Alternatives
Selecting the appropriate technology for your application.
Introduction
The primary role of any sensor or sensor-based system is to acquire information, be it
temperature, flow rate, inventory level, machine health, or any of the other parameters we'd like
to measure. While generating sensor data is fairly straightforward and well understood,
conveying data from a sensor to a monitoring or control system remains a challenge due to the
cost and complexity of installing and maintaining communications networks. For wireless
networks in particular, the lack of industry standards has complicated the sensor integration
process and inhibiting broad-based deployment. So, while sensors continue to gain intelligence,
all too often they remain ‘mute’—unable to communicate their data to remote systems
Most sensors are hard-wired into the systems that they are monitoring and controlling, due in
part to the lack of appropriate, reliable, and cost-effective wireless solutions. Wireless
standards, including Wi-Fi™, Bluetooth™ and ZigBee™, have emerged which provide
increased flexibility over wired systems and reduce the risk of integrating proprietary wireless
communications. However, many companies are still not clear about which wireless technology
to use. The advantages of wireless – ease of installation and system flexibility in particular –
have long been touted but concerns over cost and reliability have lingered. With Wi-Fi and
Bluetooth now shipping in the tens of millions of units annually costs have fallen dramatically.
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New networking technology available in ZigBee, the first wireless standard designed specifically
for remote monitoring and control applications, can significantly improve the reach and
reliability of wireless networks
Wireline or Wireless?
Wireline communications protocols, such as ModBus, LonTalk or DeviceNet, do an excellent
job of integrating sensors into their target environments, and typically provide high levels of
reliability and security. Wireline networks are appropriate whenever time-critical or missioncritical data and closed loop control are required. The downside of wireline systems is their
installation cost and inflexibility. Cabling and installation for a building automation project in an
existing facility can run as high as 80% of the total system cost, and can exceed $1,000 per linear
foot in regulated environments such as a typical power plant and $2,000 per foot in a nuclear
power plant. Furthermore, once a cable is installed, it is costly and time consuming to relocate
the cable, even if it only needs to move a few feet.
Proprietary or Standards-Based Wireless?
For sensor-based systems that require the flexibility of a wireless network, and which can
tolerate modest message latency, users can select between proprietary and standards-based
solutions. Since proprietary systems are usually customized to their application, they can offer
benefits in transmission range, very low power consumption and per unit cost. However, they
are not generally more secure than standards based systems, and their proprietary nature means
that they can’t achieve the high unit volumes and aggregated industry investment of standardsbased systems.
The primary drawbacks of proprietary wireless systems include their complexity, single-vendor
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risk, and relatively higher system cost.
The lack of standards for RF
communications and sensor-based
networking has resulted in a highly
fragmented market that forces customers
to either rely on a single vendor or to hire
highly specialized communications
engineers. Since most companies are
forced to recreate the wheel from a radio and networking software perspective, R&D resources
are diverted into the basic functions of developing reliable RF communications platforms for a
variety of sensors. As standards-based wireless products are deployed in large volumes across
multiple applications and throughout multiple industries, a ‘virtuous cycle’ of silicon economics
results in rapidly falling prices as high unit volume drive lower costs that fuel higher unit
volume.
Which Wireless Standard?
Once a company has decided to integrate standards-based wireless communications into their
products, they still need to select the most appropriate wireless technology. Today, users can
select from several wireless alternatives with non-intuitive names such as GPRS, Wi-Fi,
Bluetooth and ZigBee. While the increased market selection is helpful, the market uncertainty
over which technology to use can complicate the product development process.
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Wireless Wide Area Networks
Cellular or paging based wireless communications are the most widely deployed form of
standards-based wireless telemetry today, and include technologies such as CDPD, GPRS,
CDMA/1xRTT and FLEX. Satellite technology is also used for telemetry, particularly in remote
environments. While wireless wide area networks can provide significant geographical
coverage, they typically entail monthly service fees and are more cost-effective when
transmitting small amounts of data infrequently .
Although these technologies can provide reliable wide area communications, the applications are
usually restricted to a single node such as an oil field wellhead or a large commercial air
conditioning unit. Sensor-based applications where wireless wide area networks are well suited
include remote equipment monitoring and mobile asset tracking.
Wi-Fi [Wi-Fi attribute table?]
The recent market success of Wi-Fi, technically known as IEEE 802.11, has created
opportunities for users to include this technology into their product design as costs have fallen
dramatically. Wi-Fi technology is now included as a standard feature in most new laptops, and is
optimized for high data rate applications such as large file transfer, email and Web access. IEEE
802.11 comes in a variety of flavors, including ‘a’ (54 MBps at 5.8 GHz), ‘b’ (11 MBps at 2.4G
Hz) and ‘g’ (22 MBps at 2.4 GHz). This complexity adds to the difficulty users have in selecting
a standards-based wireless platform.
Wi-Fi specifies the physical and media access control layers of the protocol, and relies upon
TCP/IP as the network layer. Wi-Fi’s impressive bandwidth comes at a hefty price in terms of
power consumption; most portable Wi-Fi devices are expected to be recharged routinely, if not
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daily. Wi-Fi works well for human-centric devices such as laptops or PDAs, and line-powered
systems that don’t require support for large networks. Sensor-based applications where Wi-Fi
would work well include line-powered, IP-based video surveillance and data acquisition from
very high resolution sensors.
Bluetooth [Bluetooth attribute table / text box?]
Bluetooth, officially known as IEEE 802.15.1, is also enjoying market acceptance and has been
successfully deployed in some sensor network trials in the 2.4 GHz band. Bluetooth’s peak data
throughput of 720 KBps makes it a solid solution for high data rate applications, especially
where large network support, distances >10 m and battery power are not needed. Bluetooth
chipsets, now on their third and fourth generation, are targeted primarily at the cell phone and PC
peripheral industries, and as a result, their high unit volumes have driven prices down
considerably.
Bluetooth specifies an integrated protocol stack, including the physical, media access control,
network and application layers. Bluetooth’s support for voice and ad hoc networks contributes
to its 250 KB system overhead for the protocol stack, which increases its system cost and
integration complexity. In addition, Bluetooth’s current network limitation of seven nodes per
‘piconet’ restricts its applicability for large sensor network deployment. Sensor applications
where Bluetooth would work well include personal healthcare monitoring and visually oriented
equipment monitoring where both data and video are required.
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ZigBee and IEEE 802.15.4 [ZigBee attribute table / network topology images?]
ZigBee, based on IEEE 802.15.4, is a low-power, low-data-rate wireless networking standard
designed specifically for remote monitoring and control applications. Ratified in May 2003,
IEEE 802.15.4 is a simple but powerful packet data protocol that provides high reliability
through acknowledgement, error checking, prioritized communications, direct sequence spread
spectrum, the ability to change frequencies to avoid interference, and user-selectable security
levels. IEEE 802.15.4 specifies the physical (PHY) and media access control (MAC) layers, and
defines three license-free frequency bands including 2.4 GHz, 915 MHz and 868 MHz, offering
users an alternative regional frequency if the 2.4 GHz band is not optimal for a particular
application. Per node transmission ranges of 30–100 m are possible, and transmission distance
can be extended through the use of power amplifiers and multi-hop mesh networking.
ZigBee specifies the network, security and application layers atop the IEEE 802.15.4 PHY and
MAC layers, though it is possible to deploy proprietary networking schemes on top of IEEE
802.15.4 hardware. ZigBee’s network layer supports star, mesh and cluster-tree topologies,
which provides for increased range, reliability and flexibility when deploying wireless networks .
While the IEEE 802.15.4 standard supports 255 nodes per physical network, ZigBee’s 64-bit
network address support extends this to over 65,000 nodes per logical network. The ZigBee
protocol also specifies key management for the IEEE 802.15.4 integrated 128-bit AES
encryption security model. Sensor-based applications where ZigBee would work well include
wireless lighting and HVAC control systems, AMR and submetering, industrial data acquisition
and battery-powered security monitoring.
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Wireless Network Topologies
In addition to selecting a specific wireless technology, users also need to determine the most
appropriate network topology for their application. The logical shape of a network, or topology,
determines how different nodes in a network are logically connected to each other and how they
communicate. Network topologies include star, mesh, and hybrid architectures. Each has its own
strengths for various applications. Star topologies work well for relatively simple or low-power
applications. In a star topology, all wireless nodes are connected to a network coordinator or
gateway. When physical obstructions or strong RF interference blocks communication the
normal fix is to move the affected node(s), which is unacceptable for most sensor network
applications once they are deployed.
Mesh networks use a decentralized, multi-hop architecture in which each node is in direct
communication with its immediate neighbors. Mesh topologies can provide an extended
coverage area by relaying information from node to node. If a single node fails for any reason,
including the introduction of strong RF interference, messages can be automatically routed along
one or more alternate paths. Optimal route selection is usually determined by measured signal
strength between any two nodes, and the cumulative number of hops required to transmit a
message from one node to another. In addition, the routing algorithms can either be proactive or
reactive, depending on the application requirements. With proactive routing, network routes
between specific nodes are constantly maintained, which can speed data transfer but can become
complicated to manage in very large networks. With reactive routing, network routes are
discovered and updated as needed, based on the current condition of the network and the
demands of source nodes generating the data. Reactive routing can simplify network
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deployment by minimizing the complexity of routing tables, but can lead to message latencies
that are impractical for near real time applications, as a typical mesh network hop can take 50ms.
Mesh topologies can provide the industrial-strength reliability required by applications such as
equipment monitoring and control.
A hybrid cluster-tree configuration combines mesh and star topologies and is frequently the best
solution for low-data-rate applications that require both long-battery life at the node and high
network reliability. Battery-powered sensors in a home or on a single floor of a commercial
building could be linked in a star topology to line-powered wireless appliances on each floor or
in each home, which could then be linked in a mesh topology.
Summary
Selecting the most appropriate networking technology for a specific application can be
challenging, and one size does not fit all. However, once an application’s communications
requirements are clearly defined and the various attributes of the networking alternatives
understood the most appropriate networking solution is usually easy to identify. In some cases, a
hybrid approach may be called for, with a low-power, short-range subnet, such as ZigBee
aggregating sensor data for wide area communications across a GPRS/GSM or Wi-Fi network.
With a growing selection of wireless networking alternatives, users are no longer confined to
wired installations, and with cost-effective and reliable wireless products emerging based on
global standards, users are no longer restricted to proprietary wireless approaches.
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About the author
George Karayannis is Vice President of Sales and Marketing at Helicomm, Inc., Carlsbad, CA,
and can be reached at george@helicomm.com or 760-918-0856.