Download Report

Lantronix
presents:
How to add 802.11BG
to your Embedded
Device
Gary Marrs
Senior Field Application Engineer
06-26-09
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Agenda for Today
Review of Basic RF concepts
Wireless Security
Implementing a WLAN embedded interface
Troubleshooting a WLAN link
Questions / Discussion
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About Lantronix
Networking solutions for over 19
years (Founded: 1989)
Sales, manufacturing, and
support in Asia, Europe, USA
Annual sales $56M+
Market and technology leader in
device servers, console servers,
and embedded Ethernet
solutions
Over 3,000,000 networkenabled devices and 20,000+
customers worldwide
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Lantronix Headquarters
Irvine, California
NASDAQ: LTRX
What we Do…
Lantronix Device Networking products provides
complete solutions for complex networking
integration.
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Accelerated Time-to-Market:
Lantronix Device Server’s are easy to integrate allowing any device to be
enabled with network connectivity—in a few days using an external box and
in a few weeks using an embedded unit.
Reduced Design Risk:
Lantronix products are already engineered, tested, and EMC-compliant,
eliminating the R&D and investment risk inherent in any new design.
Competitive Edge:
Network enabling a device provides functionality with quantifiable benefits.
Through pre-emptive notification and remote diagnostics, service costs will
drop while responsiveness increases, simultaneously improving both
revenue and customer satisfaction.
@2004 Lantronix Corporation. All Rights Reserved. Confidential
(Wireless Design) is a hard rocky path
fraught with pitfalls for the naive and
uninitiated
Patrick Mannion – Senior Editor EE Times
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Implementing 802.11BG into an Embedded System…
…Comes a reality of challenges:
● Difficulty of integrating chipsets and drivers
● Understanding of RF and TCP/IP networking
● FCC (Federal Communications Commission) and
Worldwide certifications
● Wireless Security
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Which Wireless Technology is Right for Your Application?
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RF technology
Range
Data Rate
Power
Security
Zigbee
10-40 m
Low
Low
Medium
Bluetooth
10 / 100m
Medium / High
Medium
Medium
802.11BG
100m
High
High
High
802.11A
50m
High
High
High
Proprietary Wireless
design
Up to 4 km
Low to Medium
Low to High
Low to Medium
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IEEE 802.11 Wireless
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IEEE 802.11 - Infrastructure Mode
In infrastructure mode (also called Infrastructure BSS), there is a wireless
"station", which is usually a PC equipped with a wireless network interface
card (NIC), and an "access point" (AP), which acts as a bridge between the
wireless and wired networks.
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IEEE 802.11 Modes – Ad hoc Mode
Ad hoc mode (also called peer-to-peer mode or an Independent Basic
Service Set, or IBSS) is simply a set of 802.11 wireless stations that
communicate directly with one another without using an access point
or any connection to a wired network.
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Wireless Configuration
To communicate via Wireless, the computers must have the
same Network Name. The Network name for wireless is called
the SSID.
To add security, you can configure the Network Authentication
and the Data Encryption. WEP, WPA and WPA2 are the
standards for data encryption.
To configure an Access Point – you can enable or disable SSID
broadcast. An enabled SSID broadcast will allow anyone to
find and connect to the access point.
It is a good idea to change the default password on your AP or
Wireless router.
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Basic RF Concepts
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Signal Strength
There are 4 units of measurement used to represent RF signal
strength.
mW – (milliwatts)
dBm – (dB-milliwatts)
Received Signal Strength Indicator (RSSI)
A percentage measurement
Measuring RF energy in mW is not always convenient. Signal
strength does not fade in a linear manner. Due to free space
loss, RF energy fades inversely to the square of the distance.
Example – measure the signal at 20m away from the AP.
Then move to 40m away (double the distance), the signal
power decrease by a factor of 4.
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The dBm is a logarithmic function and mathematically
represents this relationship very well.
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Signal Power in Decibels
The power of RF signal levels is commonly measured in
Decibels (dB).
dBm stands for a dB above one milliWatt
To convert between watts and dBs use this formula:
dB = 10 x log10 (power output / power input)
To find dBm – simply use 1 mW for the input power
dBm = 10 x log10 (power output / .001)
ie.. 100 mW = 20 dBm
mW values less than 1mW are represented by negative
dBm values.
Every 3 dB increase is a doubling of power
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Power in Decibels
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Power (in watts)
Power (in dBm)
0.000000000001
-90
=
,001 pW
0.00000000001
-80
=
.01 pW
0.0000000001
-70
=
.1pW
0.000000001
-60
=
1 pW
0.00000001
-50
=
10 pW
0.0000001
-40
=
100 pW
0.000001
-30
=
1 nW
0.00001
-20
=
10 nW
0.0001
-10
=
100 nW
0.001
0
=
1 mW
0.01
10
=
10 mW
0.1
20
=
100 mW
1
30
=
1W
10
40
=
10 W
100
50
=
100 W
1000
60
=
1000 W
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Power (in watts)
Received Signal Strength Indicator (RSSI)
RSSI is generic radio receiver technology metric. IEEE
802.11 defines RSSI as a relative received signal
strength in a WLAN environment, in arbitrary units.
RSSI provides a way for the circuitry on a WLAN NIC to
measure and represent RF energy.
RSSI measurements are unit less and in the range 0 to
255. The maximum value, RSSI_Max, is vendor
dependent. Every chipset vendor scales the RSSI
differently.
The 802.11 standard does not define any relationship
between RSSI value and power level in mW or dBm.
Key Point – you can not compare the RSSI readings
from different WLAN chipset vendors.
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Percentage Signal Strength
The percentage represents the RSSI for a received
packet. The RSSI value is divided by RSSI_Max and
multipled by 100.
Provides a good way to circumvent the complexities of
RSSI readings.
If all vendors used percentage, then percentage would
provide a good cross vendor metric for comparing signal
strength.
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Antennas
An antenna is a device that is made to efficiently radiate
and receive radiated electromagnetic waves. There are
several important antenna characteristics that should be
considered when choosing an antenna for your
application as follows:
Antenna radiation patterns
Power Gain
Directivity
Polarization
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Antenna Radiation Patterns
An antenna radiation pattern is a 3-D plot of its radiation far
from the source. Antenna radiation patterns usually take two
forms, the elevation pattern and the azimuth pattern.
Angle
The elevation pattern is
a graph of the energy
radiated from the
antenna looking at it
from the side.
Sometimes referred to
as the vertical view.
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Sin2 Angle
Antenna Radiation Patterns
The azimuth pattern is a
graph of the energy
radiated from the antenna
as if you were looking at it
from directly above the
antenna as illustrated.
Sometimes referred to as
the Horizontal view.
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Y
Angle
X
Rubber Duck Omni Antenna
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Reflector Grid WLAN Antenna
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Antennas – Power Gain
The perfect antenna is known as an Isotropic radiator –
generates a perfect sphere of energy like light emitted
from a light bulb. Isotropic radiators only exists in
theory
The Gain of an antenna is a ratio of the power input to
the antenna to the radiated power output.
The simplest antenna is a dipole antenna. Consists of 2
equal length pieces of wire that are at a length that is
resonant to the desired frequency.
Dipole antennas exhibit 2.1 dB of gain!!!
The gain of an antenna is specified in dBi or dBd. dBi is
the gain referenced to an isotrope. dBd is the gain
referenced to a dipole.
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Antennas - Polarization
Polarization is the orientation of electromagnetic waves
far from the source.
For Example - a simple dipole antenna produces a signal
polarized in the same plane as the antenna. A vertically
mounted dipole produces a vertically polarized signal.
Ideally, the receiving antenna should be co-polarized
with the transmitting antenna.
A cross-polarized receiving antenna offers significant
rejection of the transmitted signal.
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Types of Antennas
Dipoles and Isotropes are
common omnidirectional
antennas. This means they
generate a field 360 degrees
around them.
Yagi – is a common high gain
directional antenna.
Patch or panel – useful for
their slim profile
Dish or Parabolic – very high
gain directional antenna
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Signal to Noise Ratio - SNR
Defined as the ratio of a signal power to the noise power
corrupting the signal
802.11 environments use this term slightly different. There it
is used as “ the ratio of the power of the data signal to the
power level of the noise floor”.
In this case, noise refers to the background RF radiation
present in the receiver's environment. Every environment has
some noise; sources of interference, like cordless phones and
Bluetooth, increase noise.
Signal-to-Noise Ratio (SNR) compares peak signal strength to
noise. An SNR of 22 dB or more is workable. The higher the
SNR, the more stable and usable the WLAN network is.
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RF Interference - Multipath
Multipath interference – occurs when the desired signal
reaches the receiving antenna via multiple paths, each of
which has a different propagation delay and path loss.
Each of the received signals are summed at the receiving
antenna
Caused by reflections off objects in the environment or
atmosphere.
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RF Interference - Fade
Signal Fading is interference related to a time-varying change
in the path loss of a link.
The movement of objects in the radio path causes reflections
which result in random variations in amplitude and the
frequency of the received signal. Examples are – leaves
blowing in the wind, rain and someone closing a door.
Fade Margin – this is the amount of power in excess of what is
needed to maintain a connection. Or, the amount by which a
received signal level may be reduced without causing system
performance to fall below a specified threshold value.
The greater the fade margin, the more reliable the system will
be under adverse propagation conditions.
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Antenna or Spatial Diversity
Antenna diversity - a receiving station obtains two
observations of the same signal sent from a
transmitting station. The receiver will switch reception
to the antenna currently receiving the stronger signal.
For best results, the antennas are usually placed one
wavelength apart. Only useful for receiving data.
The use of antenna diversity can augment the wireless
connection and minimize the effects of multipath
interference.
The Diversity Gain is the ratio of the signal field
strength obtained by diversity to the signal strength
obtained by a single path.
Diversity gain is usually expressed in dB.
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Free Space Loss (FSL)
Radio waves get weaker with distance
The attenuation associated with distance in an unobstructed
path is called Free Space Loss.
FSL (in dB) is calculated by using the following formula:
(20Log10 x freq. in Mhz) + (20Log10 x dist. in miles) + 36.6
For example - at 2.4 Ghz for 2 miles
FSL = 20Log10 x 2,400 + 20Log10 x 2 + 36.6 = 110 dB
The higher the frequency, the more attenuation will occur
over a given distance.
A 6 dB power change will double or halve the total distance.
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Free Space Loss at 2.4 Ghz
FSL in dB
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3
100
.6
201
.2
402
.3
804
.7
1 .6
km
3 .2
km
8 .0
5k
16. m
1k
40. m
25
80. km
5k
m
50.
1
25.
2
12.
6 .1
3 .0
1 .5
Free Space Loss (FSL)
40
50
60
70
80
90
100
110
120
130
140
150
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Receiver Sensitivity
Receiver Sensitivity is the lowest power level at which
the receiver can detect an RF signal and demodulate
data. Sensitivity is purely a receiver specification and is
independent of the transmitter.
Improving the sensitivity on the receiver (making it
more negative) will allow the radio to detect weaker
signals, and can increase the transmission range.
Receiver Sensitivity is dependent upon transmission
speeds!
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A Link Budget
How many dBs of radio path loss can my Wireless link handle
before communications deteriorates ?
We can easily answer this once we have the following pieces
of information:
Radio transmit power (Ptx)
Transmit antenna gain (Gtx)
Receiver antenna gain (Grx)
Diversity Gain (Gdv)
Receiver Sensitivity (Srx)
Fading Margin (M)
Using the Formula:
LB = Ptx + Gtx + Grx + Gdv – Srx - M
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A Link Budget Example - Downlink
For example, suppose we have the following link. We are
transmitting from a Orinocco AP-700 Access Point to a
Lantronix WiPort at 11 Mbps. Here is the data:
Transmit power = 24 dB
Transmit antenna gain = 2.1 dBi
Receiver antenna gain = 2.1 dBi
Diversity Gain (Gdv) = 0 dBi
Receiver Sensitivity = -82 dBm (of the WiPort)
Fading Margin (M) = 20 dBm
LB = 24 + 2 + 2 + 0 – (-82) – 20 = 90 dBm
This means that the radio link can overcome 90 dB of
attenuation and still work correctly
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A Link Budget Example - Uplink
The next example, we have the return link (called the uplink).
We are transmitting from a Lantronix WiPort to a Orinocco Access
point at 11 Mbps. Here is the data:
Transmit power = 14 dB
Transmit antenna gain = 2.1 dBi
Receiver antenna gain = 2.1 dBi
Receiver Sensitivity = -84 dBm
Diversity Gain (Gdv) = 2 dBm
Fading Margin (M) = 20 dBm
LB = 14 + 2 + 2 + 2 – (-84) – 20 = 84 dBm
This means that the radio link can overcome 84 dB of attenuation
and still work correctly. However, it is 6 dBm worse than the
downlink and will only have a range of half as far.
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Analysis of Link Example
This formula provides an estimate of performance. It allows
you to compare competing solutions and estimate service
coverage.
This formula does not account for cable or connector losses.
These can be significant and should be considered for real
applications.
You must calculate the link in both directions. Typically, an
access point will have higher gain than a remote station
adapter. The weaker link of the two, will be the limiting
factor.
In clear free space, the Down link example is good for around
314 meters. However, the uplink is good for only 157 meters.
Which is what we should have expected – half the distance.
These examples assume that there are no obstructions. If
obstructions or interference exist (which is very likely), then
the link margin will be less.
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Link Adjustments
What can you do to get more link (or fade)
margin?
Since the IEEE- 802.11 and the FCC regulate the
spectrum, you can not increase power output!
You can shorten the distance or minimize interference /
obstructions. Not Practical !!!
You can use an antenna with more gain. This is a
reasonable choice but it does require some paper work
(and possible testing) with the FCC.
The most practical solution is to increase Receiver
sensitivity by selecting a slower transmission speed.
Simpler modulation schemes provide better receiver
sensitivity.
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Wireless Security
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Wireless Security
Wireless security presents a challenge because anyone in ear
shot range can potentially receive your data. There are 2
pieces to a secure WLAN connection.
Encryption and Authentication = A Secure Environment
Encryption is the scrambling of data prior to transmission
Authentication determines if client radio is authorized to
connect to the wireless network
Both require additional processing power that can tax an
embedded system
Built-in security is a good option
Large enterprise environments need a better way to handle
Key management.
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Wired Equivalent Privacy - WEP
Most WLANs in use today use WEP. WEP is vulnerable to
attack for a number of reasons.
WEP uses RC4 algorithm for encryption with a 40 bit or 104
bit key. The initialization vector (IV) is 24-bit.
For authentication there is Open or Shared Key. Open is
essentially no Authentication.
With Shared Key, WEP is used for authentication. A four-way
challenge-response handshake is used. It is possible to derive
the keystream used for the handshake by capturing the
challenge frames in Shared Key authentication. Both
Authentication mechanisms are weak.
In August 2001, Fluhrer, Mantin and Shamir published a
cryptanalysis of WEP that explained how to recover the RC4
key after eavesdropping on the network in as little as one
minute.
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WiFi Protected Access - WPA
WPA was created in response to several serious weaknesses
researchers had found in WEP. Intermediate solution until
802.11i was finalized.
WPA supports 2 modes. A less secure "pre-shared key" (PSK)
mode. Also supports an enterprise authentication mode that
is designed for use with an 802.1x authentication server.
Data is encrypted using the RC4 stream cipher, with a 128-bit
key and a 48-bit initialization vector (IV).
One major improvement in WPA over WEP is the Temporal
Key Integrity Protocol (TKIP), which dynamically changes keys
as the system is used. When combined with the much larger
IV, this defeats the well-known key recovery attacks on WEP.
By increasing the size of the keys and IVs, reducing the
number of packets sent with related keys, and adding a
secure message verification system, WPA makes breaking into
a Wireless LAN far more difficult.
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IEEE802.11i / WPA2 Security
WPA2 is the approved Wi-Fi Alliance interoperable
implementation of 802.11i.
WPA2 makes use of the Advanced Encryption Standard (AES)
block cipher (referred to as CCMP); WEP and WPA use the
RC4 stream cipher.
Like WPA, WPA2 supports 2 modes. A pre-shared key mode
(PSK), also known as personal mode and 802.1X for
enterprise authentication.
PSK mode is used in small networks that don’t require the
complexity of an 802.1x authentication server. The key can
be entered as 64 hex digits or as a pass phrase of 8 to 63
ASCII characters.
PSK mode provides authentication via a pre-shared key, or
password.
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IEEE802.11i / WPA2 Security
Enterprise authentication mode uses the Extensible
Authentication Protocol (EAP) and typically requires a RADIUS
or other authentication server for strong authentication.
802.1X includes several different types of EAP protocols for
enterprise security.
EAP-TLS
EAP-TTLS/MSCHAPv2
PEAPv0/EAP-MSCHAPv2
PEAPv1/EAP-GTC
EAP-SIM
LEAP is a Cisco protocol used in hospitals and other large
enterprise applications. LEAP has exposed weaknesses and is
not part of the 802.11i specification.
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Implementing a
Wireless LAN Solution
for an embedded device
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Implementing an 802.11B WLAN
There are 3 options for implementing a WLAN interface for
an embedded application.
1. The first option is to buy a chip set (usually 2 or more
chips) and integrate the hardware and software into
your existing application and uP.
2. The second option is to integrate a PCMCIA (PC Card) or
a Compact Flash (CF) card with your existing uP and
operating system.
3. The third option is to buy an off-the-shelf solution that
is designed to integrate directly with your existing
controller.
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Tradeoffs for the Chip set solution
Advantages
Can provide a cost effective solution for high volume
applications
Allows the most customizations of the network and radio
Interface.
Disadvantages
Requires RF technical expertise to complete the hardware
design
Requires extensive software integration to interface to the
existing uP.
Requires complete FCC testing and approval before product
can be released
Time to Market – 14 to 18 months minimum.
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Implementing a CF or PC card design
Must support the full CF or
PC card interface here.
32 bit
MicroController
TCP/IP
Stack
CF or PC
Card SW
Driver
+3.3 V
and gnd
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CF or PC
WLAN Card
Tradeoffs for the PC Card / Compact Flash (CF) solution
Advantages
Makes sense if you are going to implement an OS that already
supports that particular card
Some cards offer higher output power
Disadvantages
Requires a fair amount of storage and processing on the host
CPU. The embedded CPU must support a full TCP/IP stack.
Requires software integration to interface to the existing Driver.
In some cases the driver is hard to get.
Can require extensive FCC testing and approval before product
can be released
Some or all of the encryption or key generation will be relegated
to the host CPU
Time to Market – 10 to 12 months minimum.
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Tradeoffs for the off-the-shelf solution
Advantages
Can easily integrate with existing uP firmware. Does not
require any changes to existing firmware code if you are
already using a serial interface.
Design can be completed in 4 to 8 weeks (quick time to
market).
Does not require highly skilled RF HW and SW engineers on
staff to implement.
The OEMs FCC compliance can be leveraged to gain approval.
In most cases it is only a paper work project if the design
uses a 3 dBi antenna or less.
Disadvantages
Limited options to customize RF interface.
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Implementing an off-the-shelf solution
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Lantronix WLAN Modules - Advantages
• MatchPort BG, MatchPort BG Pro & WiPort
• Fast time to market. Wireless connectivity in as little
as 60 days. Reduces project risk
• Eliminates the need for RF & networking expertise
• Frees up resources to focus on your application solution
• Built-in WEP, WPA & WPA2 security. Support for PSK or full
enterprise security.
• Both Wire and Wireless in one module. POE options for wired
Ethernet
• Supports serial to WLAN or Ethernet bridging to WLAN.
• Compact form factor
• Future proof technology allows us to easily productize new
Wireless devices
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WiPort Wireless Embedded Device Server
• IEEE 802.11BG wireless radio
• Capable of Wired or Wireless communications
Serial to 802.11BG
Serial to 10/100 Mbps Wired Ethernet
Bridge 10/100 Mbps to 802.11BG
• Built-in WEB Server. 1800 KB Flash Memory
available for OEM WEB pages
WPA & WEP & 802.11i PSK security
• 2 High Speed Serial Ports (920 Kbps)
• Up to 11 digital I/O Pins
ModBus ASCII/TCP or RTU/TCP (optional)
RS485 support included
• Power 3.3 VDC @ 460 mA max
• Operating Temperature of -40 to +70 C
• FCC CFR47 part 15 Compliant
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WiBox – External Wireless Device Server
External Wireless Device Server
• IEEE 802.11bg wireless radio
•
Adhoc or Infrastructure modes
•
Supports bridging
• 1800 KB Flash Memory available for OEM WEB pages
• 2 High Speed Serial Ports (920 Kbps)
•
Port 1 – RS232
•
Port 2 – RS232, RS422 or RS485
• Power 9 to 30 Vdc (2W max)
• Operating Temperature of 0 to +60 C
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MatchPort BG
Optimized 802.11 b/g Radio module
• Capable of Wired or Wireless communications
WEP, WPA & WPA2 PSK built-in security
Serial to 802.11BG
Serial to 10/100 Mbps Wired Ethernet
Bridge 10/100 Mbps to 802.11BG
Exclusive AES end-to-end encryption option
2 Serial ports - 921 Kbps Data Rate
802.11 Power Saving Radio Sleep Mode Options
Web Server with Java Applet Support
Operating Temperature of -40 to +70 C
• FCC CFR47 part 15 Compliant
Compact: 45mm x 45mm with 40-pin 2 mm Header
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MatchPort BG Pro
Optimized 802.11 b/g Radio module
• Capable of Wired or Wireless communications
Serial to 802.11BG
Serial to 10/100 Mbps Wired Ethernet
Bridge 10/100 Mbps to 802.11BG
WPA, WEP & 802.11i (AES-CCMP encryption)
PSK
WPA2 Enterprise/EAP Protocols
• 2 Serial Ports (230 Kbps)
• Up to 5 GPIO digital I/O Pins
• Power 3.3 VDC @ 460 mA max
• Operating Temperature of -40 to +70 C
• FCC CFR47 part 15 Compliant
On-board U.FL antenna cable connector
Variety of available cables to meet customer needs
Pin Compatible with all MatchPort
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Trouble shooting WLAN
NetStumbler – a useful tool for pinpointing details of a
wireless network, helping you configure, secure, optimize and
discover.
http://www.netstumbler.com/
Ethereal/Wireshark – Very useful for troubleshooting Ethernet
and 802.11bg networks.
http://www.wireshark.org/
Airsnort - a wireless LAN (WLAN) tool which recovers
encryption keys. Passively monitors transmissions, computing
the encryption key when enough packets have been gathered.
Fluke – hand-held tools for WLAN analysis.
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Network Diagnostic Utilities
IPCONFIG –
Shows the entire TCP/IP configuration present in a host
computer
Type in: C:\IPCONFIG
PING –
Test connectivity across the network and confirm that an
IP address is reachable
Type in: C:\PING <ip address>
ARP (ARP.EXE) –
Used to display the ARP cache which holds the IP to MAC
address translation
Type in: C:\ARP –a to see ARP cache
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References for Wireless Networking
Implementing 802.11, 802.16 and 802.20 Wireless Networks,
Ron Olexa. Elsevier Communications Engineering Series
802.11 Wireless LAN Fundamentals, Pejman Roshan, Jonathon
Leary. Cisco Press.
802.11 Wireless Networks – The definitive Guide. Matthew S.
Gast. O’Reilly
“You believe you understand what you think I said…” Joshua
Bardell
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Thank you…
Questions??
@2004 Lantronix Corporation. All Rights Reserved. Confidential