User`s manual

CHAPTER 7
THEORY OF OPERATION
7-1
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 VHF and UHF
communications products allow the user to create a
"Radio Area Network" of up to 254 users on a single
frequency. The packet burst communications using
Synchronous Data Link Control (SDLC) 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
the ESTeem firmware SDLC, bit compression, and
Manchester encoding techniques. The user can define
over four different security code and communications
parameter groups that allow communication access to the
"Radio Area Network". If higher security is required, the
ESTeem is compatible with asynchronous Data
Encryption Standard (DES) encryption devices.
The ESTeem has over a 100 programmable software
commands to allow the user to easily configure the unit
for any application or mission. The ESTeem setup
parameters are saved in its own 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 problem are solved.
ESTEEM 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 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 1010 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, its 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 persons 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 a 16 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.