DAREN of WV News September 19, 2010 (.949]
Kenneth Harris WA8LLM (304) 679-3470 wa8llm@yahoo.com
WOOD COUNTY EMERGENCY COMMUNICATIONS
P.O. BOX 3328, PARKERSBURG, WV 26103
http://www.wc8ec.org
DAREN News, are articles and information about the Digital Amateur Radio "Emergency" Network of WV, which operates on 145.69 MHz. Anyone having any information about DAREN, or Amateur Radio in general, and would like to share it with others, may send it to WA8LLM @ PKBGWV on 145.69 MHz, or to my E-Mail: wa8llm@yahoo.com. A State Wide DAREN NET is operated every Saturday from 7:00 pm to 10:00 pm, and lately, before and after those hours. The DAREN Net Announcement, showing the MAIN NCS, ALTERNATE NCS, NODEs accessible to both, and the previous week's check-ins, are posted by Thursday on most of the Main county DAREN PBBS'. The DAREN News can also be found on the wc8ec.org website.
Gene Roy, N8ER, had a question that he would like explained. Some of the local Amateurs in his area would like to know how the TNC on a Packet Radio system does error checking and correcting. To explain how it works will take a little bit of time, but here's how it works. To start off you need to know a little bit about the makeup of a Packet system. The word packet comes from the fact that information being transmitted is in the form of small transmissions called packets. Each packet is has a header, just like an ARRL RADIOGRAM, but a lot more complicated. The header of a packet transmission has six blocks that tells where the message is coming from, who it is going to, and how it's going to get there. It has a start and a stop block at the beginning and end of the header. It has a block that tells what the packet is going to do, like connecting, disconnecting, asking for a re-transmission, or acknowledging a message. The fifth block in the header is used for error checking and is called the CHECK Block. When a message is ready to be transmitted the Central Processing Unit in the TNC performs a mathematical process called a CRC which stands for Cyclical Redundancy Check. The mathematical process uses a very complicated formula. Basically the process counts all the ones and zeros that makes up the traffic in the packet, does the CRC magic, and places a number in the Packet Header CHECK Block. When the receiving station receives the packet transmission, the receiving TNC uses the same mathematical formula as the transmitting station. It looks at the CHECK Block of the received transmission, and compares the two numbers. If the numbers are the same, the receive station sends a message back to the transmitting station that it received the packet error free. If the numbers in the CHECK Block don't match, the receiving station tells the transmitting station that it REJECTS the packet and wants it sent again. The two stations will send messages back and forth until the numbers in the CHECK Block are the same. There are a couple of things you need to know, (1) Packet Radio is not 100 percent error free, errors do slip into packet transmissions. The data in a packet transmission may be error free when the receiving station receives it, but a static burst or other interfering signal may have changed the number in the CHECK Block. Since the CHECK Block numbers don't match a re-transmission will be asked for. (2) An interfering signal may also distort the data in the packet transmission, but may still have the same about of ones and zeros data bits. If the receiving station counts the ones and zeros in the message, does the CRC magic, and comes up with the same CHECK Block number, it will acknowledge that it received the information "error Free" when in fact the transmission was garbled. But, for the most part Packet Radio is error free, but not always. I hope this answers some questions about Packet Radio, and hope it's not as clear as mud.
There were 60 check-ins to this week's State Wide DAREN Net. Terry Mills, KC8TUE, needs only one more check-in to make 350. There were three new participates to the net this week, Jack Wilson, KD8OSV, Jason Stull, KD8OSU, and Kenny Roberts, WV8EER, all of Parkersburg. I want to welcome them to the DAREN system, and hope they can participate as often as possible.