During the first half of 1996, AMRAD members have been thinking in terms of building a multi-media repeater. While some hams have running ATV repeaters for years now, providing both audio and visual information, we at AMRAD want to add a few new twists. The term "multi-media" is trendy because the home computer market is using it for computers with sound cards for audio and CD-ROMs with digital full-motion video. While no one really knows what the AMRAD Multi-media repeater will actually look like, a few characteristics are generally agreed upon. It will have a digital modulation, so that computer networking protocols can be used to transcend the limitations of radio frequency propagation.
The current commercial climate has the local telephone companies reducing rates and growing service areas for ISDN. Cable Television companies also have an eye on customers who want digital bandwidth to "Surf the World-Wide Web". A web-browser program called "Vosaic" has appeared - which hacked the ground-breaking Mosaic browser to add full-motion video to the existing file-based HTTP protocol.
Looking at the present state of affairs and a little bit into the future as well:
Earlier in AMRAD's history, the X.25 standard was modified to suit the amateur radio environment - resulting in AX.25 protocol. Inexpensive boxes called TNCs plugged into the microphone/PTT and speaker jacks of any voice radio and into RS-232 serial ports of a terminal or computer. Bridging the digital and RF worlds in this way was considered revolutionary.
Now we propose to subvert the ISDN protocol and interface standards to our own purposes - and invent RSDN. Inexpensive boxes will be available at consumer commodity prices with an ISDN connector on one side, and jacks on the other side to connect to TV/Video Camera/Video Recorder/microphone/speakers/Ethernet. To use them all we need that's new is a small box to connect ISDN to a radio, or new radios. We need to define RSDN in order for our radios to speak the same language on the air. Bridging the wired and RF worlds in this way will be considered revolutionary.
What will the radio signals be like? Certainly not Bell 202 modem AFSK over FM like packet was. Serious improvement in communications link effectiveness is there for those who use the more direct PSK or even QPSK modulation. Still more significant improvements are gained by using error control coding.
The Store and Forward amateur packet radio network was built using discarded computers from industry. A flood of Xerox-820 Z-80 based single board computers appeared on the surplus market. The IBM PC with its 5-1/4 inch floppy disks resulted in great deals to be had on surplus 8 inch floppy disk drives. The AMRAD newsletter helped many hams get systems running. The influx of cheap computing power built regional messaging systems out of many short VHF radio links, and global messaging via a few HF links.
The present pressure in the home and office computing marketplace today is to upgrade. Currently being discarded as junk are '486 motherboards that only have the ISA bus. Soon the "VL local bus" motherboards will be trash as the only available peripherals will have a PCI bus. What are hams to do with this new cheap computing resource? Answer: Error Control Coding. Phil Karn, KA9Q, has paved the way by providing software to do this. (He has also made it available on his Amateur Radio Digital Communications web page.) The result is systems costing less than the Xerox-820 did are now available that can handle the computation of convolutional coding at speeds of 56 kb/s.
Using discarded PC hardware in this way allows discarded Cellular Phone PAs to be used instead of an expensive 10 Watt radio, or allowing a 25 Watt radio to set up a link instead of a 200 Watt radio. It really seems possible that the amateur radio digital infrastructure might just be able to make the transition from 1200 baud packet to 56 kb/s RSDN.
Getting those properly coded bits on the air should be a priority for AMRAD. It should be useful to have a real-world test of techniques that Mobile Multimedia Communications people are developing. Bridging internet networking technology and cellular telephone technology has been the focal point of the Mobile IP Working Group.
So there is certainly no shortage of ideas to implement a multimedia repeater. Certainly the amateur radio community needs to keep a close eye on the commercial and consumer electronic communications marketplace in order to build affordable systems. It will be OK to adopt existing standards if they work. It would certainly be nice to plug ISDN peripherals into an RSDN box. Is such a system in our future?
73,
Maitland Bottoms, AA4HS
AA4HS Global
Headquarters