Electronic Systems Technology ESTEEM192S Wireless Modem User Manual part 7
Electronic Systems Technology Wireless Modem part 7
Contents
user manual part 7
CHAPTER 7
THEORY OF OPERATION
INTRODUCTION
ESTEEM HOW IT WORKS
SPECTRUM UTILIZATION
PACKET PROTOCOLS
FLOW CONTROL
DATA PRIVACY
EFFECTIVE BAUD RATE
CHAPTER 7
THEORY OF OPERATION
Revised: 11 Feb 99 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 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.
It's safe to assume that the data you receive is good data or
you get nothing at all using the CRC technique. If no ACK
is returned after a given delay, the sender assumes the
packet was not received and "retries" the transmission. The
CHAPTER 7
THEORY OF OPERATION
Revised: 11 Feb 99 7-3
number of retries are user definable from 1 to 255, allowing
the unit to automatically retry sending the packet.
SPECTRUM UTILIZATION
The ESTeem uses a "listen before transmit" or Carrier
Sensed Multiple Access (CSMA) scheme. This means only
one unit in a network is allowed to transmit at a time. By
fixing each user's communication window and allowing the
computer in the ESTeem to be the Air Traffic Controller,
many individual users can share one frequency. The
ESTeem firmware can support up to 255 ESTeems on a
single channel or frequency.
The CSMA technique is a very efficient way to manage
your network of ESTeems and prevent communication
bottlenecks. In addition, an anti-collision software scheme
is used to recover data if two or more units transmit at
exactly the same time. When this feature is added the
technical term for this technique is now called CSMA-CD
(collision detection).
PACKET PROTOCOLS
By using CSMA no polling station or token is required in
the ESTeem network. When an ESTeem has information to
send it will check to see if the channel is clear before
transmitting its packet and await an (ACK). The ESTeem is
a Master/Master system, meaning any ESTeem can
communicate with any other ESTeem.
FLOW CONTROL
The ESTeem supports hardware and software flow control,
which allow different devices on the network to
communicate at different baud rates. In addition to flow
control the ESTeem also has a 4000 byte data buffer on
both the receive and transmit buffers in the unit.
DATA PRIVACY
Data privacy in the ESTeem is provided by three levels of
data encoding in the firmware and by the user being able to
define over four security and communications parameters
(Unit Address, Network ID, and Operating Frequency) that
allow communications access to the modem giving over 100
million combinations. If higher security is required, the
units are compatible with asynchronous Data Encryption
Standard (DES) encryption peripherals.
EFFECTIVE BAUD RATE
The maximum input baud rate to the modem is 57,600
baud, asynchronous, full duplex, but this is misleading since
the ESTeems actually communicate to each other simplex
over the RF link, at 85.5kbps, 171kbps, 344kbps or
688kbps selectable by the user. The effective baud rate is a
function of the above plus the packet length variable in the
ESTeem (definable from 1 to 2000 bytes. The effective
baud rate will degrade as the pack length variable is
reduced.