"Larry" wrote in message
...
"plano" wrote in
:
If averaged over 10 minutes or so, maybe 4800 would offer enough
bandwidth in certain locations, but you just cannot risk that say 20
(long) AIS sentences are received more or less simultaneously. There
is no way 4800 baud (1/8th!! of 38400) will handle that. Any idea
how much traffic one can expect say in the English Channel? It's the
worst case scenario that counts, not the average in Charleston harbor.
plano
You can watch the Irish Sea:
http://www.aisliverpool.org.uk/index.php
Finest AIS system on the internet.
But, of course, YOUR AIS isn't this good. Your range is about 10-12
miles with a 50' antenna listening to these 12W transmitters. This
limited range limits the number of AIS packets you must process in your
small system. That in itself reduces the load considerably.
Click on [Pan and Zoom] on the Liverpool AIS system. Zoom out until the
scale in the lower left bottom corner says 10 mi - 20 km on your screen.
Now, pan out of the Liverpool ship channel by the docks. Cruise the pan
out the channel into the Irish Sea, a busy place out from Liverpool. Go
off in the direction of the maximum density of ships you can find at the
time. Using the scale in the lower left hand corner as a RADIUS from
your boat, the actual range of your own AIS receiver in any at-sea
situation, how many ships can you get inside that 40 km circle around
your cursor? In Liverpool harbor, with a lot of ships docked but still
transmitting away on AIS, I can get, maybe 15 in range. Out at sea,
where we are concerned about this problem, the ships are spread out more.
If I center my boat 20 km N of Amtwch, the peninsula sticking out to the
East of Liverpool, at this moment I would be painting 7 or 8, tops.
Point is your boat-mounted AIS receiver's 20 km horizon ISN'T going to
paint all those ships you can see on this chart of Liverpool and the
Irish Sea, a very busy place for shipping. If 8 ships are transmitting
full AIS data into my system on 4800 baud every other second, it won't
tax the 4800 baud bandwidth anywhere near its limit to the point where it
would jam or nav data packets from the other instruments would be slowed
down to a crawl. It just won't happen, unless we put up a 1000' mast to
get more packets......
Larry
--
http://www.youtube.com/watch?v=fEJmc...elated&search=
Short summary: A 4800 bps NMEA link may very well overflow in
not-insanely-busy conditions. Explanation follows:
A couple of weeks ago I sailed out of Sausalito to see the Queen Mary 2
enter San Francisco Bay. While still in the slip, I saw over 75
simultaneous AIS targets, out to about a 25 NM range. My antenna for the
AIS receiver is just a 6dB (short) whip, mounted on the stern rail, so with
a masthead antenna the range (and number of ships seen) would have been much
greater. I don't know what the burst data-rate was, but let's assume that
on the average each ship is transmitting a "dynamic information"
message-type at 10-second intervals (2 seconds is the fastest update-rate,
12 seconds is the slowest rate for a ship under way)
Ignoring the less-frequent "Static" messages, each message is 256 bits (a
"dynamic" message is 168 bits, plus 88 bits of overhead).
The radio-channel bit-rate is 9600 BPS (from the AIS spec). I don't know
how well the timeslot-assignment method fills the available slots, but the
maximum would be about 375 active ships (256 bits every 10 seconds, times
375 ships = 9600 BPS).
A "dynamic" message gets encapsulated into a 47-character NMEA message.
This is 517 bits (each ASCII character is 8-bits + start-bit + two stop-bits
= 11 bits).
375 active ships, each transmitting one message every 10 seconds would
create an NMEA serial data stream of 37.5 * 517 = 19387.5 BPS. Either this
is a coincidence, or my math is about right. A 19.2 kbps link should be
able to handle full-capacity AIS. It would only take about 93 active ships
to fill a 4800 BPS NMEA link, and this assumes even spacing of the messages,
or very deep buffers.
I can easily see overrunning the capacity of a 4800BPS link, especially if I
had a mast-top antenna. I have a dual-channel receiver, but having one of
the single-channel receivers should cut these data rates in half.
-Paul