From: gary@ke4zv.uucp (Gary Coffman)
Subject: Re: High Speed links
Reply-To: gary@ke4zv.UUCP (Gary Coffman)
Organization: Gannett Technologies Group
Date: Thu, 24 Dec 1992 22:14:01 GMT

In article <J4NZVB4w165w@n3eop.pgh.pa.us> paul@n3eop.pgh.pa.us (paul) writes:
>A friend of mine is looking to set up a remote digital link that can 
>support at least 19.2K full duplex.  I figure that a pair of 56K GRAPES 
>modems can work, especially if they can go full duplex.

Yes, they are designed to support full duplex operation. The RF board
is arranged so it can be cut into separate receiver and transmitter
sections and housed in different cabinets if desired for isolation.
The data encoder and decoder are on separate PC boards.

>However, the software used is designed for a direct link, or optionally a 
>phone modem.  So, using standard COM ports are required, and they are 
>already set up with NS16550AFN UARTs.  We have done some testing with 
>standard TNC2s full duplex (wired and dual band rigs), so setting CONPERM 
>and TRANS will give acceptable results.
>
>Does operation with the GRAPES modems change the way that TNCs operate, 
>other than the increased speed?  I assume that there is a way to increase 
>the TNC to talk to the computer at 57600 or faster.

Well the software and comport hardware issues are a problem. Remember
that the modem is a synchronous modem and supplies all clocks. So you
need a synchronous comm chip to support it. That typically means an
8530 in the PC, or a buffering TNC. If you wish to ignore AX25 and
just send raw duplex data, then a synchronous comm port is all you 
need. But if you don't have a sync comm port or need to packetize
the data, then there are complications. 

A soupped up TNC2 with our special KISS56 code can support the modem
at 56 kb on the RF side, but we've had problems stuffing the data 
through an ordinary async com port at that speed so the KISS56 code 
handles the data to/from the PC at 19.2 kb and buffers packets in it's 
internal memory. That's ok for a shared channel because it can transfer 
data on the serial line while others are sharing the RF channel at 56 kb. 
But for full duplex contention free channels, it limits thruput. 
Note we suspect the limitation is in the KA9Q async handler and not
in the TNC. Other software may work fine at this data rate. Laplink
and it's ilk run up to 115 kb over the standard async port, but in
half duplex mode of course.

With an 8530 talking directly to the modem, data flow is no problem,
but the PC software has to do the packetizing of the data if needed. 
For 8530 cards, there are a couple of choices. The Ottawa PI card is 
good, but only has half duplex DMA. That would keep it from supporting 
full duplex full speed links. The Gracilis PacketTwin card can support 
full duplex DMA.

For your application, I would suggest that you experiment with 56 kb
wired links between machines running the software you require. If the
software can handle 56 kb, then a soupped up TNC buffer will likely work.

Gary
-- 
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