User`s manual

CHAPTER 7
THEORY OF OPERATION
7-2
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 253
ESTeems on a single channel or frequency. For example
in the United States there are over 50 frequencies
available in the VHF spectrum and 1600 frequencies in
UHF, giving a network density of greater than 16,830
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 bank 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 2020 byte data buffer on
both the receive and transmit buffers in the modem.
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, Operating
Frequency, and HDLC Communications Baud Rate) that
allow communications access to the modem giving over
200 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 RS-232C baud rate to the modem is
9600 baud, asynchronous, full duplex, but this is
misleading since the ESTeems actually communicate to
each other half duplex, synchronous, at 4800 b/s. The
The effective baud rate is a function of the above items
plus the packet length variable in the ESTeem (definable
from 1 to 1010 baud). If the packlength variable is set to
1010 baud the effective baud rate is approximately 4484
baud but the effective baud rate will degrade as the
packlength variable is reduced.
Another item that must be understood is transmission
turn around time. Remember, 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, which 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 45 milliseconds one way which
includes transmitter turn-on time and packet frame
overhead or a total turn around time accounting for the
(ACK) of 90 milliseconds. Therefore total time to send a
data packet is 90 milliseconds plus the time required to
send the data (ie. number of bits sent/4800 b/s). There
are tables in Appendix D that give the user greater detail
on this subject.