IEEE 802.11 for Industrial Applications
Wireless networking is catching the attention
of a lot of people these days. Its impact is growing and spreading
out from its early focus on office network applications into
a host of other areas. In the industrial data communications
space wireless local area networks (WLANs) are attracting
attention in a similar way that wired Ethernet LANs did, albeit
more quickly. Once Ethernet technology became commonplace,
reliable and affordable, the industrial market started looking
at it more seriously, considering how it could meet the unique
and often stringent requirements of industrial applications.
Of course, proprietary wireless systems (point-to-point and
networked) have been around for a while, but cost, lack of
standardization and performance limitations have been an impediment
to their range of implementation. As the cost/performance
ratio of IEEE 802.11 wireless (or Wi-Fi) has improved, manufacturers
and users have begun to develop products and systems specifically
for industrial applications.
Now users are looking to WLANs for solutions to a wider range
of needs. Inexpensive, reliable wireless networks allow industrial
users to enhance data collection, human-machine interfaces
(HMI) and web-based system monitoring, programming and management.
The ability to implement new projects without the time and
expense of running cables is a compelling proposition. Maintenance
departments can readily see the value in providing mobile
access for updating, reprogramming and re-calibrating equipment
over a wireless network.
Basics of the IEEE 802.11 Standard
IEEE 802.11 is a set of standards (first introduced in 1997)
that defines how multiple devices can communicate on a wireless
network. The standard has grown into a set of several standards
with alphabetical suffixes that (as of this writing) extend
from a to v. The standard defines the physical and data link
layers only. As a part of the IEEE family of standards, it
is not surprising that 802.11 WLANs are easily connected to
802.3 (Ethernet) LANs. Higher layer LAN protocols, network
operating systems and internetworking protocols such as TCP/IP
integrate seamlessly.
Under the IEEE 802.11 standard there can be two different
types of devices on the network: stations and access points.
For wireless office networks a station is usually a desktop
PC equipped with a wireless network interface card (NIC) or
a portable computer with built in Wi-Fi or a PCMCIA Wi-Fi
card added. For industrial applications the range of possibilities
is wider. For example, a station could be a Wi-Fi enabled
PDA (personal digital assistant) used as an HMI. Another possibility
is an 802.11 wireless serial server connected directly to
a programmable logic controller (PLC), HMI, or other field
device.
An 802.11 access point is a radio with an interface that
allows connection to a wired LAN. Access points run bridging
software (specified by 802.11d) to facilitate the connection
from wireless to wired network. The access point becomes the
base station for the WLAN. It aggregates access to the wired
network for multiple wireless stations. An access point could
be a standalone device or a card in a PC.
Wireless Network Configurations
The 802.11 standard defines two modes of operation: infrastructure
mode and ad hoc mode. Infrastructure mode makes use of one
or more access points connected to a wired LAN. Wireless stations
communicate with access points to gain access to each other
and/or the LAN. In the Basic Service Set (BSS) several stations
communicate with one access point, which is connected to a
wired LAN. In the Extended Service Set (ESS) two or more access
points connect to the LAN creating a subnetwork.
In ad hoc mode, also called Independent Basic Service Set
(IBSS), access points are not used. Wireless stations communicate
directly with each other in a peer-to-peer fashion. This mode
allows individual computers to set up a network where wireless
infrastructure does not exist.
The original physical layer specification of 802.11 defined
a WLAN operating in the 2.4 GHz ISM band, which does not require
FCC licensing. Three different options were specified: two
using spread-spectrum radio and one using infrared. The infrared
option never gained much traction. The radio options operate
at 1 Mbps and 2 Mbps using frequency hopping spread spectrum
(FHSS) or direct sequence spread spectrum (DSSS) techniques.
The two techniques are not interoperable and provide different
performance characteristics. Frequency hopping has the advantage
of providing better noise immunity but limits the top end
data rate.
802.11b Raises the Bar
Networks based on the original 802.11 had the advantage of
being based on a widely accepted standard, as opposed to earlier
proprietary networks. But it quickly became clear that data
rates of 1 to 2 Mbps were inadequate, especially when the
goal was often to interconnect with Ethernet LANs that operated
at 10 Mbps (10Base-T) and later 100 Mbps (100Base-TX). The
802.11b standard was the first attempt to address these data
rate limitations. The result was a standard that, like the
original specification, utilizes the 2.4 GHz band, but achieves
data rates as high as 11 Mbps, bringing it into the same range
as 10BaseT.
IEEE 802.11b implements the same DSSS modulation scheme used
for one mode of 802.11, but dropped the FHSS mode because
of inherent data rate limitations. Although FHSS provided
superior noise immunity for 802.11, the newer standard compensates
by incorporating several other modulation and coding schemes
that ensure good noise immunity. One of these is dynamic rate
shifting, which causes it to fallback to lower data rates
to compensate for higher noise levels.
IEEE 802.11g Steps Up
IEEE 802.11g takes a big step forward without cutting ties
to its siblings. The standard specifies a WLAN that operates
on the 2.4 GHz band at data rates as high as 54 Mbps, but
is backward compatible with the earlier standard. It incorporates
at least two modes of operation, one that is compatible with
the slower 802.11b and another that operates at the higher
data rate. Systems can incorporate 802.11b and 802.11g equipment
and they will interoperate. However, when connected into the
same network the 802.11g equipment will operate at the 11
Mbps limitation of the 802.11b equipment. To overcome this
problem separate b and g networks can be created and linked
together through a router or access point (if it is equipped
with the necessary capabilities). This keeps slower 802.11b
traffic separate and allows the 802.11g WLAN to operate at
the higher data rate.
IEEE 802.11a an Alternative
Another member of the 802.11 family—the 802.11a version—takes
a slightly different approach by operating in the 5 GHz band.
Like the 2.4 GHz band, 5 GHz does not require licensing and
has the added advantage of being less congested. The maximum
data rate for 802.11a is 54 Mbps, the same as for 802.11g.
While 802.11a WLANs have some advantages, the downside is
that they are not directly compatible with the b and g versions.
In order to connect 802.11a to either of the other networks
special bridging equipment must be used.
The 802.11 Data Link Layer
Like 802.3 (Ethernet), the 802.11 data link layer is made
up of two sub-layers: the Logical Link Control (LLC) sub-layer
and the Media Access Control (MAC) sub-layer. Both 802.3 and
802.11 use the same LLC, specified by 802.2, one reason why
integrating 802.11 and 802.3 networks is relatively simple.
The 802.11 MAC sub-layer is also similar but does different
in the way the shared radio carrier is accessed. While Ethernet
uses Carrier Sense Multiple Access with Collision Detection
(CSMA/CD), 802.11 uses a variation called Carrier Sense Multiple
Access with Collision Avoidance (CSMA/CA).
In CSMA/CA a station that intends to transmit ‘listens’
for traffic on the radio carrier frequency and sends if it
is clear after a random delay period. If the receiving station
receives the packet intact it sends an acknowledgement (ACK)
to confirm the packet has been received. If the transmitting
station does not receive an ACK it assumes a collision occurred
and transmits again after a random delay period.
Another aspect of the 802.11 data link layer that is different
than Ethernet is the use of a packet fragmentation and CRC
error checking with each packet. Ethernet implements these
functions at higher protocol layers whereas 802.11 fragments
packets and uses CRC at the data link layer. This allows the
WLAN to send smaller packets that are less likely to be corrupted
by interference, decreasing the need for re-transmissions.
802.11 Range, Security and
Other Considerations
IEEE 802.11 devices communicate via radio signals that must
penetrate solid objects to reach other network nodes. These
signals are attenuated to varying degrees depending on the
type of material and its thickness. The transmitter power
output, the type of antenna used and the amount of attenuation
through materials affects the useable range. Other factors
also affect range and overall performance. Electromagnetic
noise, the amount of network traffic, other radio devices
operating in the same frequency band (e.g. portable phones,
etc) and reflections (multipathing) are factors. In an infrastructure
network the number of access points and their coverage pattern
also affect how well the system operates. Typically an 802.11
device operating indoors will have a range from 100 feet minimum
to about 500 feet maximum. Outdoors, some products, using
high gain antennae may achieve line-of-sight ranges of five
miles or more.
Security is a significant concern for WLAN users, and industrial
users are not exempt. Whether security threats originate intentionally
or unintentionally, wireless systems are more susceptible
than wired systems. IEEE 802.11b uses Wired Equivalent Privacy
(WEP) protocol to encrypt transmitted data. Designed to provide
the same level of security as that of a wired LAN, WEP operates
at the physical and data link layers of the network and has
been found to be somewhat lacking. IEEE 802.11g originally
implemented a more robust security standard called Wi-Fi Protected
Access (WPA), a scheme designed to improve on WEP’s
limitations. It has better encryption algorithms and uses
a technique called authentication. WPA was considered an interim
standard. IEEE’s 802.11i standard (which was adopted
recently) incorporates WPA as well as additional security
features. It is expected to replace WPA.
Industrial Applications Challenge WLANs
Applying WLANs to industrial applications presents added
challenges compared with home or enterprise applications.
Industrial environments often produce significant amounts
of electrical noise. Variable frequency drives, competing
radio systems, radar and microwave sources and welders are
a few examples of industrial noise sources. Signal attenuation
and reflections also can compromise signal coverage in industrial
buildings and worksites. Transmitter power levels, receiver
sensitivity and access point placement is critical. Reliability
of individual components and the overall system can affect
plant safety, security and downtime costs. Industrial users
demand performance guarantees. These guarantees extend to
system characteristics such as data latency and corruption
levels.
In response, many manufacturers are marketing equipment designed
to address these challenges. For example, stations and access
points targeting the industrial market implement higher transmitter
power levels. Industrially focused equipment increasingly
offers weatherproof enclosures, industrial mounting options
and connectors and other robust features. Manufacturers often
include software to perform RF site surveys to assess the
consistency and reliability of plant coverage. Some access
points include remote management software.
The list of 802.11 modems, serial servers, repeaters, access
points and other equipment grows daily. Quality and ruggedness
continues to improve. At the same time the 802.11 standard
continues to evolve while maintaining backward compatibility.
Industrial equipment manufacturers and users are embracing
wireless networking in concept and practice, and finding success
in the process. IEEE 802.11 compliant WLANs are a key part
of that trend.
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