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NET-2.TXT
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1992-07-02
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NET-2.TXT
TXDs AND DCD
------------
Affecting all network configurations is TXD. This is the period following
transmitter key-up and beginning of the packet. Its purpose is to allow
time for the transmitter to become fully turned on before the packet commences.
Most TXDs are set for 300 milliseconds or longer. Simply multiplying
.3 seconds times 1200 baud gives insight into the affect TXDs have on
throughput. It's not unusual to see synthesized node radios with TXDs set to
.5 seconds. If shorter node radio TXDs were used, a significant increase in
throughput is theoretically possible. Therefore the type of radio selected
for node use is important.
Crystal controlled radios have faster receive to transmit (R/T) cycling times
than synthesized units. Factors that affect the R/T time is the time it takes
for the crystal oscillator (or the PLL) and associated stages to stabilize
when switching from receive to transmit. And, on the receiving end, the time
it takes for the squelch circuit to react and lock onto incoming packets.
The best radios having the shortest TXDs are those designed specifically for
data use. These include the GLB NETLINK 220 and those recently introduced by
Kantronics. Reports indicate 5 to 20 millisecond TXDs are possible with these
radios. In terms of cost and availability, the second best radios are the GE
MASTR II or Motorola Micor and Mitrek series of commercial radios that can be
found on the used market. These radios are stable, rugged and, designed to
exist in hostile electronic environments as typically found at many shared
node radio sites. TXDs of 100 milliseconds and less are achievable.
While much shorter TXDs are attainable on the radios selected for internode
traffic (with significant increase in node-to-node network throughput), radios
on user access nodes are not so fortunate. The bulk of amateur radios sold
these days are synthesized units with slower R/T switching time. It does little
good to have the local node set up with a short TXD if it means the users
can't access it! As a result, one has to drop back and penalize user access
performance by increasing the TXD time to a value that allows typical access.
On a simplex network without a backbone trunk, this means optimum TXDs cannot
be used. However if there is a backbone trunk, shorter TXDs can be used "IF"
it is agreed users will not be allowed direct access. In this case, the
network flow would be to the local area access node and then automatically
routed onto the backbone trunk toward its final destination.
Another throughput improvement for node and user radios alike would be to
operate the receivers with open squelch. Depending on individual squelch
circuit characteristics, a savings from 5 to 20 miliseconds could be realized.
At first this may not seem like much, but on a busy circuit, the shorter
response time is quite significant. A simple TNC DCD modification will allow
open squelch operation. The major advantage of node DCD modification is that
the TNC will react only to valid packets. Sometimes on shared sites with
commercial services, intermodulation problems are prevalent. The noise in the
node receiver created by intermod could hold off packet transmissions if DCD
isn't performing properly. To this end, the TAPR DCD kits for the various
TNCs are highly recommended as they will improve the performance of both user
and node TNCs.