World of Ham Radio 1997/WOHR97_AmSoft_(1997-02-01).iso/packet/docs/packet15.doc

Text File | 1997-02-01 | 5KB | 81 lines

INTRODUCTION TO PACKET RADIO - Part 15 - by Larry Kenney, WB9LOZ

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 and will increase your
throughput.  ("Throughput" is a word that has come into use by packet
operators that means the amount of usable packet information received by
the distant station.)

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 acknowl-
edgement 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 and
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.  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.

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, so multiply those
percentages by the number of hops, then multiply by 2 to account for the
acknowledgement, and 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
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.  An example of one path that is very vunerable to
conditions due to its distance is from W6AK-1 on Mt. Vaca to WB6AIE-1 on
Bald Mountain in Yosemite National Park on 145.05 MHz.  Most of the time,
packets go between these two nodes without any problem, but there are
times, especially when it's a hot summer day in the Sacramento Valley, when
it's impossible to get a packet from one to the other.  In the Bay Area,
the fog has a drastic affect on VHF signals.  When a fog bank is moving in
off the Pacific, it can act as an excellent reflector.  Signals that are
not normally heard 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 mutipath.  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 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.

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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