User's Manual

CHAPTER 7
THEORY OF OPERATION
Revised: 27 March 01
7-3
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 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. For example
in the United States there are 1600 frequencies (12.5 kHz
channel spacing) in UHF, giving a network density of
greater than 408,000 users in a given cell or geographical
area. Once you are out of radio range, you can construct
another cell of users.
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).
By using this communications technique only one
frequency channel is needed with a very narrow
bandwidth (this is called narrow band FM modulation)
thereby saving valuable radio spectrum space.
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 19,200
baud, asynchronous, full duplex, but this is misleading
since the ESTeems actually communicate to each simplex
over the RF link, at 19,200 bps (25 kHz channel spacing
- Model 192C/F) or 9,600 bps (12.5 kHz channel spacing
- Model 192C/F). The effective baud rate is a function of
the above plus the packet length variable in the ESTeem
(definable from 1 to 2000 bytes). If the packlength
variable is set to 2000 bytes the effective baud rate is
approximately 18,000 baud (25 kHz channel spacing) or
9,000 baud (12.5 kHz channel spacing) the effective baud
rate will degrade as the packlength variable is reduced.
Another item that must be understood is transmission
turn around time. Remember that the ESTeem sends a
data packet and waits for an (ACK) from the destination
modem before another packet is transmitted. All radio
transmitters have a fixed delay time, this is the amount of
time it takes the transmitter to stabilize once it is
energized before it can send data. In the ESTeem the
delay is approximately 15 milliseconds
one way
which
includes transmitter turn-on time and packet frame
overhead or a total turn around time accounting for the
(ACK) of 30 milliseconds. Therefore total time to send a
data packet is 30 milliseconds plus the time required to
send the data (i.e. number of bits sent/19,200 b/s).