User's Manual

CHAPTER 7
THEORY OF OPERATION
Revised: 27 March 01
7-2
INTRODUCTION
ESTeem wireless modem products provide a "Wireless
Solution" by eliminating conventional hardwiring of
leased phone lines.
All of the ESTeem models come with the industry
standard RS-232C, RS-422, and RS-485 asynchronous
communications ports to give the user a new dimension
to "Local Area Networking".
Our packet burst, frequency agile communications
products allow the user to create a "Radio Area Network"
of up to 255 users on a single frequency. The packet
burst communications technique was chosen to give the
system very high data integrity in high noise industrial
environments. The ESTeem incorporates a method of
error checking that provides received data accuracy of
greater than one part in 100 million.
Internal Digi-Repeater features allow the user to increase
operating range by relaying transmission through a
maximum of three ESTeems to reach the destination
ESTeem. An ESTeem can operate as an operating node,
a repeater node, or both simultaneously for added
flexibility.
"Private Data Communications" is provided by the use of
an interleaving technique of the modulated data, user
definable commands for unit addressing, network
addressing, and security lock-out of software
programming. If higher security is required, the ESTeem
is compatible with asynchronous Data Encryption
Standard (DES) encryption devices.
The ESTeem has programmable software commands to
allow the user to easily configure the unit for any
application or mission. The ESTeem setup parameters
are saved in non-volatile memory.
When you buy ESTeem products you are getting
equipment designed by the company that holds the
United States and Canadian patent for the wireless
modem. We are proud to say that we design, develop and
manufacture our products in the United States. Each
ESTeem is subjected to a rigorous quality control bench
test before shipping to insure our customers have
out-of-the-package reliability. We also have a dedicated
Customer Support Staff, Field Engineering Services and
Factory Training classes to make sure that your
application problems are solved.
HOW IT WORKS
Now, as you can probably guess, the ESTeem is a
sophisticated piece of technology, however the concept is
easy to comprehend if you understand packet radio.
All packet systems, whether hardwired or radio, share the
same principle of operation; data is taken from your
standard RS-232C, RS-422, or RS-485 asynchronous port
and is transmitted in "Blocks". Think of this block as an
"Electronic Envelope" that we call a packet. The size of
the packet can be defined by the user from 1 to 2000
bytes of information. Reducing the size of the packet
allows the ESTeem to operate better in high EMF noise
environments, because by reducing the packet size you
reduce transmission exposure time on the radio waves
thereby increasing your probability of a successful
transmission.
Once this packet of data is formed, it’s transmitted in a
"burst," one ESTeem to another, hence the term "packet
burst communications". Now, if more than one packet is
required to send the data then the ESTeem goes into full
automatic mode and transmits additional packets.
Before an ESTeem transmits its packet it listens to ensure
that the air waves are clear before transmitting. This
listen before transmit scheme is called "carrier sensed
multiple access," or CSMA.
When a "packet" has been transmitted, every modem in
radio range on the same frequency hears it. To design a
modem to communicate with a network of modems it has
to be "address specific" so only the modem you want to
talk to accepts your information. It's like yelling into a
crowd of 255 people but you want only the person’s name
you called to acknowledge (ACK). Well, very simply,
that's how the ESTeem works. Once the address you're
calling receives your packet, it's checked for accuracy.
Accuracy is probably the single most important part of
any communication device. The ESTeem uses Forward
Error Correction (FEC) and a 32 bit Cyclic Redundancy
Check (CRC) which is a very sophisticated method of
checking the data integrity of the packet once its been
received. The CRC insures data integrity greater than
one part in one hundred million. Once the CRC is
completed on the received packet, the data is outputted to
the user and a positive acknowledgment (ACK) is
transmitted back to the sender.