Isolation is one of the greatest barriers to educational development faced by teachers and students alike in Australia. After six years of country teaching, I have a good understanding of the problems experienced by people on both sides of the teacher's desk.
Computer communications technology has brought with it some very interesting possibilities for the country and isolated teacher. Simply by connecting a computer to the telephone line, through a modem (Figure 1), teachers can obtain access to other computer networks and log into databases and information services. These sources of information are either not available to the country teacher, or restricted to vacations or other periods when it is possible to visit the city.
The telephone line is now being pressed into service as a direct teaching link whereby students enrolled with distance education institutions are able to talk, and exchange data and word processed files with teachers with whom contact would otherwise be impossible.
Figure 1: Computer communications by telephone line
Packet radio permits these opportunities to be extended to students by allowing them to communicate through a computer using a shortwave radio channel. In Australia our internationally famous Schools of the Air make relatively little use of computer technology, although they stand to benefit greatly from doing so. Teachers and students of radio based instructional systems could exchange word processed documents, scanned files and even compiled computer programs over the shortwave radio.
In short, packet radio replaces the telephone line with a radio channel. Rather than connecting the modem to a phone line, a specially modified modem known as a Terminal Node Controller (TNC) connects the computer to the microphone and speaker circuits of a radio transceiver (Figure 2).
Figure 2: Computer communications by radio
A modem converts voltage levels generated by the computer into tones. These tones, usually two, respond to the high and low voltages sent to it by the computer's serial port, and represent the data to be transferred to the remote computer. The modem also receives tones sent to it from the remote computer, which is similarly connected to a modem, and converts these tones into the voltage levels required by its computer.
A TNC undertakes the functions of a modem and some additional processes. Upon reception of the data voltages from the computer, the TNC strips off the start and stop bits from each character and assembles all these characters into one long stream. Addressing and error checking information is attached and this packet of data is sent to the TNC's internal modem and then to the radio. The TNC also switches the radio to transmit, provided that the channel is clear, and proceeds to send this packet of data to the remote user. The remote system receives the audio from its radio, which in turn passes it directly to the receiving TNC. Here the process is reversed. The data is checked for accuracy, using a CRC error correction method, and then reassembled into the format required by the computer for processing.
The CRC error detection protocol will signal the sending TNC of any errors in the received information and request a repeat of the offending packet. Only when the data is correct, will the TNC permit it to pass on to the computer.
The protocol used, AX25, is a modified form of the X25 link layer computer communications standard. The basic difference is found in the addressing information. The average amateur radio operator's callsign comprises five to six alphanumeric characters. However, the address field specified in the X25 protocol is restricted to one byte of information, that is only one character. Therefore AX25 allows for an extended address field which permits the use of the normal amateur callsign which is necessary according to international licensing conditions.
This addressing information is important in ensuring that data addressed to a particular station, and in this case a student's home station, is received and processed by the appropriate TNC. Where it is possible for any station listening on the air to receive data addressed to another, it is also possible through software control to disable that function. Therefore, if required, test data may be sent to an individual student without its reception and decoding by others on the frequency.
Figure 3: Short wave radio reflection by the ionosphere
Errors can occur by changes in the ionosphere causing fading and multiple path distortion, resulting in loss of signal or distortion of the tones (Figures 3 and 4). Therefore it is important to have an error check to ensure the integrity of the data.
Figure 4: Disruption of ionospheric reflections
Compared with the cost of a satellite earth station, the packet radio alternative is extremely cheap. A suitable shortwave transceiver could be as low as $500 second hand and the TNC could be purchased for approximately $400. The antenna could cost as little as $20. The operator does not pay for the time spent when the system is in use, unlike using a satellite system where there are expensive transponder hire charges. The financial outlay ceases once the packet radio system is up and running.
In an age of ever changing technology it is very easy for existing systems to be ignored in favour of the latest developments. Shortwave radio has been with us since Marconi and has continued to provide a very reliable communications system, even though it has the occasional hiccup due to sun spots and flares. Packet radio has brought shortwave radio back into the new technology arena and has the potential to provide a cost effective and reliable data transmission system to isolated communities and homesteads without the aid of expensive satellite systems.
|Please cite as: Diggens, M. (1990). Enhancing distance education through radio-computer communication. In R. Atkinson and C. McBeath (Eds.), Open Learning and New Technology: Conference proceedings, 113-116. Perth: Australian Society for Educational Technology WA Chapter. http://www.aset.org.au/confs/olnt90/diggens.html|