Re: Screen shots
"John D. Hays" <john@...>
Hi John,toggle quoted messageShow quoted text
We've been thinking about some of these ideas as well. In the design of the UDR56K-4 we really are shooting for the "sweet spot" between data rate, bandwidth, propagation, and utility.
70cm was chosen as the band because we can run at 56 K bauds and up to 100 Khz of bandwidth (Part 97.307) which is sufficient for many data transfer applications like text email. The band has decent propagation (compared to 33/23 cm) and is less crowded than 2 meters in most areas (especially in the 420-440 segment). Lower loss cable, connectors, etc. and reasonable gain antennas are easier to work with than at higher frequencies. A lot of people have said why don't you put it on 2 meters? Well, we would have to cut back to 19.2 K bauds and the band is too popular with little room for additional modes.
So the main difference between DV and DD, as far as the bits go, is 1 bit flag in the header and the payload. So theoretically one could do a 4800 bps DD signal through a DV repeater. Where it gets tricky is we don't know where in the Icom chain things might fall apart:
So much of this is just a black box on a platform like Icom G2.
Clearly on open source gateway and repeater controller software we could adapt to multiplex DD and DV on the same repeater or half-duplex channel, but we won't know on the Icom stuff until we're able to test. (We do have access to some RP2C / G2 gateway systems for testing.)
One thing about DD is it is callsign addressed, so if the reflector code would only pass DV traffic with CQCQCQ (or the reflector designator) in the UR field, one could multiplex without affecting links or reflectors.
The other concern with very low rate DD is buffering traffic from higher speed systems, e.g. trying to squirt a 56 or 128 K DD packet through a 4800 bps channel should work, but timings for TCP ACK packets, etc. would get a little crazy.
John D. Hays
On Sun, May 27, 2012 at 9:27 AM, john_ke5c <ke5c@...> wrote: