User Manual Part 2

CHAPTER 7
THEORY OF OPERATION
Revised: 9 Oct 03 7-2 EST P/N AA104
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.
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.