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LU9DCE > PACKET 28.12.16 15:44l 112 Lines 5469 Bytes #999 (0) @ WW
BID : 16177_LU9DCE
Read: GUEST
Subj: PACKET RADIO - PART 15
Path: IW8PGT<CX2SA<LU9DCE
Sent: 161228/1334Z @:LU9DCE.TOR.BA.ARG.SOAM #:16177 [TORTUGUITAS] $:16177_LU9DC
From: LU9DCE@LU9DCE.TOR.BA.ARG.SOAM
To : PACKET@WW
INTRODUCTION TO PACKET RADIO - PART 15
by Larry Kenney, WB9LOZ
TIPS FOR BETTER PACKET OPERATION
Here are some tips to help make your packet operating more
enjoyable. Whether it's while making local QSOs, checking into a
BBS or mailbox, or working DX, there are a few things you should
take into consideration that will help eliminate problems and
waiting time, increase your "throughput" and make packet a lot more
fun. ("Throughput" is a word that has come into common usage by
packet operators and means the amount of usable packet information
transmitted or received.)
When connecting to another station, don't use a digipeater or node
unless you have to. Each digipeater you add to the path increases
the time required to get your signal to its destination and to
get an acknowledgement returned. It also increases the chance for
interference and for collisions with other packets. You'll be amazed
at the difference in throughput when comparing a direct connect to
one with just one digipeater in the path.
The packet node network, as discussed in previous articles in this
series, does a great deal to help you get your packets through,
but you must remember that throughput there, too, is affected by
the number of nodes used and by the conditions between you and
the destination station. The big advantage of the nodes is that
the acknowledgements do not have to return all the way from the
destination station before your TNC is satisfied. Packets are
acknowledged from node to node, so that eliminates a large part
of the problems encountered. Getting the original packet through,
however, remains to be as much of a problem for the nodes as it
is for you when using digipeaters. It can take several minutes to
get a packet through when you're working a station some distance
away. Have patience!
Dr. Tom Clark, W3IWI, has determined that for EACH HOP in a packet
path the loss of packets can vary anywhere from 5% to 50% depending
on the amount of traffic. Remember, each digipeater and node adds
a hop, compounding the problem, and you have twice as many hops as
you might think, because of the acknowledgements. You can see how
quickly the path deteriorates as traffic increases and digipeaters
and nodes are added to it.
If you have a choice, use a frequency that doesn't have a lot of
other traffic on it. It makes sense that the more stations there
are on a frequency, the more chances there are for collisions and
retries. A path that will work perfectly without a lot of traffic,
can become totally useless under heavy traffic conditions. Just
one additional station on the frequency can decrease throughput by
about half in many cases.
Another consideration, especially if working over a long distance,
is atmospheric conditions. You might not have experienced this before
on VHF, but with packet's high sensitivity to noise, a slight change
in signal strength can mean the difference between getting your
packets through or not getting them through. Long paths between
nodes are very susceptible to these changing conditions. There
are times, especially on a hot summer day, when it's impossible
to get a packet from one node to the other on what is normally
a good path. At other times, "thermals" can increase your range
dramatically and you're able to use node paths that normally don't
exist. In the San Francisco Bay Area, the fog has a significant
affect on VHF signals. When a fog bank is moving in off the Pacific,
it can act as an excellent reflector. Signals that normally aren't
heard or are very weak can reach signal strengths of 40 over S9.
Multipath is another problem that can greatly affect your packet
signal. Multipath is the term used to describe the receipt of
multiple signals from one source due to reflections off of buildings,
hills or mountains. The "ghost" in a television picture is a form of
multipath. A station with a very strong signal into a digipeater or
node often cannot use that path if multipath causes the signal to
be distorted. Each packet is checked for 100% accuracy and is not
acknowledged unless it is. Multipath reflection can cause occasional
bits to be lost so you can end up with multiple retries and a poor
path even with strong signals.
To sum up, for best results on VHF use the least number of
digipeaters and nodes as possible, use a frequency with low
activity, and be aware of atmospheric conditions and multipath
problems. Remember, by decreasing PACLEN and MAXFRAME in your TNC,
you improve your chances of getting packets through under poor
conditions.
If you use packet on HF, remember to change your transmit baud rate
to 300 and to use a short PACLEN (a value of 40 seems to work quite
well) and a MAXFRAME of 1. The chances of getting a short packet
through the noise and QRM are much better than for a long one.
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