Packet-Radio is a low-cost way of data communication via radio.
1982, Packet-Radio was defined in the USA by the TAPR group. Meanwhile, it grew to a word-wide amateur-operated data network.
Packet-Radio is operated exclusively by amateur-radio operators (HAMs). There exists no administration or organisation. Packet-radio is free for all amateur operators.
With 1200 Baud operation, any FM-radio-transceiver (amateur-radio or CB-radio) will do. Just connect the Packet-Radio-Controller to microphone and speaker. With CB-radio, 1200 Baud is used most.
Radios with non-removeable microphones cannot be used.
9600 Baud data speed is standard with amateur-radio equipment.
All SYMEK Packet-Controller (TNC) can be operated with any computer which has a serial (RS232, V24, COM) interface. Ideal are IBM-compatible PC (AT, 386, 486, Pentium etc.), Laptops, Notebooks etc.. Atari-ST, Amiga etc. are suited as well. For all these computers, there are terminal programs available to send and receive data.
The TNC converts the audio-signals of the radio into digital information and transfers them to the computer and vice versa. Further, the TNC generates the data packets with the proper protocol and analyses the received data packets. The TNC is connected to the computer via a serial RS232-cable to the COM-interface. At the radio, the three connections microphone, speaker and PTT (push-to-talk contact) are used. Depending on the desired communication speed, SYMEK offers a variety of different TNC-types.
Packet-Radio ist a method to transmit and receive digital data via radio. In general, the data source is a computer, the Packet-Radio-Controller (TNC) converts the data to tones which are transmitted via the radio link. The TNC of the distant station receives these tones and converts them back to computer data. Data flow is not continuous but data is combined to packets. Because this, the operating-mode is called 'Packet-Radio'.
With every data packet, not only the useful data but also the addresses of receiver and sender aree transmitted. So, it is possible that more than one connections share the same frequency without interference. Every receiver picks out the packets which are addressed to it. Every transmitter checks if the frequency is in use before accessing the channel ans transmitting data. So, collisions can be minimised and the frequency can be used by many users at the same time.
The data packets contain checksums which make it possible to recognise errors. Only 100% error-free packets are accepted by the receiver and displayed. Good packets are acknowledged. Unacknowledged packets are repeated until they arrive correctly at the receiver.
Packet-Radio-traffic may be established between two partner stations but it is also possible to extend the range by adding repeater stations in-between the two stations. Using the 'classic' AX.25 digipeating, there may be up to 7 digipeaters between the connected stations. Assuming, these digipeaters are located on mountain tops, this gives an enormous range for the connection.
The nodes of the world-wide Packet-Radio-network are permanently in contact with each other. Haveing access to any one of the nodes is all you need to access ALL stations of the net. For TNC3 packet-radio-controllers, there exists a software called X-Net which performs all functions which are necessary to perform the autorouting functions of a intelligent net node.
Packet-Radio is used in Amateur-Radio since over 15 years and is proven and reliable. There are thousands of SYMEK Packet-Radio-Controller in use all over the world.
To connect a TNC to a radio with 1200-Baud, it is only necessary to use four wires: Ground, microphone, speaker and PTT.
For FM radios, the 'AFSK-modulation' (Audio Frequency Shift Keying) is normally used so far. A audio-carrier is shifted in frequency according Bell 202 standard (tones 1200 and 2200 Hz). A minimum of adjustment is required. No modifications of the radio are necessary. The speed is 1200 Baud i.e. 1200 Bit/sec., this is equivalent to approx. 150 characters per second.
Amateur-radio uses more and more the 9600 Baud FSK modulation defined by G3RUH. It is much faster compared to 1200 Baud and can be used with most modern amateur radio transceivers.