karn@mouton.UUCP (11/02/84)
Packet repeaters are actually fairly easy to build, as they can be much simpler than full TNCs. Many of the current TNCs can also function simultaneously as digipeaters, and in many areas the "local digipeater" is just the personal TNC of a ham who happens to be in a good location and makes a point to leave his rig on. If you want to build a "dedicated" digipeater, you can make do with less $$. All a digipeater has to do is receive HDLC frames, check the address, set a bit and retransmit. The old VADCG boards, which without the upgrade kit have too little memory to be truly useful as TNCs, can perform this job easily; the software has been written by KE3Z and WB4JFI, and is available from AMRAD. Many other "ad-hoc" digipeaters have been constructed, but the VADCG board will probably be the easiest route for most people. Phil
neihart@smu.UUCP (11/05/84)
The easiest way to proceed is to find a local voice repeater which is not being used very much and convincing the owner to switch it over to a packet repeater. In most cities there are enough (if not too many!) voice repeaters, but rarely any packet repeaters. A plain old repeater can be used for a packet repeater, since the intelligence is in the PADs (packet assemblers/disassemblers) which are located at the terminal/radio equipment. The repeater just gives one the standard advantage of a repeater -- being able to talk farther than on simplex. The repeater itself is oblivious to the fact that it is retransmitting data. In fact, a repeater could double as a voice and packet repeater during different hours of the day, although I wouldn't suggest this as good practice.
karn@mouton.UUCP (11/09/84)
Regarding the suggestion to use voice repeaters for packet, I have some comments. 1. Voice repeaters are usually not designed for fast, bursty transmissions such as those produced by packet boards. Keyup delays can involve quite a bit of overhead, particularly if mechanical relays are involved. The constant "kerchunking" involved in packet can cause quite a bit of wear and tear on these relay-switched machines (and on their tech-committee chairmen.) 2. Voice and packet users simply cannot coexist peaceably on the same repeater. The "racket radio" users drive the human listeners to distraction (mainly because they can't understand what's being said about them!), and the long-winded voice users can bring data traffic to a standstill. 3. Because of carrier hang timers on most voice repeaters, the TNCs cannot use RF carrier detect for CSMA (not that many people do, even on simplex), but must use modem subcarrier detect. This also has the effect of infuriating the voice users, since they get stepped on by the packet boxes. 4. A voice repeater is very expensive in comparison to a single frequency "digipeater", since it requires cavities and isolation to run in full duplex. 5. The one advantage of using a voice repeater over a single frequency digipeater is that collisions due to one station's inability to hear all others on the channel can be avoided. However, working against this is the fact that a wide-coverage repeater forgoes the possibility of "frequency reuse", in which two pairs of stations who are far enough apart could communicate simultaneously on the same frequency without interference, given enough of a signal-to-interference ratio to get good capture. There is an opportunity for a research project here that would be of real use to amateur packet radio. Do an analysis to find, given an actual set of user stations and antenna QTHs, the optimal power level for each station to make maximum utilization of a channel, considering the effects of digipeating, collisions, etc, and to compare the results to the efficiency of a wide-area duplex analog repeater. One might start by looking at the work of Kleinrock and others and seeing how it might be applied to a practical situation. I remember one paper of his in which he showed that a packet radio network did not have to be "disconnected" (i.e., have pairs of stations that could not directly communicate) to a very great degree before the channel throughput degenerated to the Aloha level. It would be interesting to see how this bears out in an actual situation. In any event, my practical advice is to put up a single-channel digipeater in a good location if you want to provide a utility service to the local packet radio community. 73, Phil, KA9Q