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

"Larry" wrote in message
...
"plano" wrote in
:

I'm surprised the Cap'n can pass through (relay) AIS NMEA sentences,
but who is going to read them at 4800baud? All equiment that accept
AIS do this at 38400.
Also, if there is dense AIS traffic, you would soon run into bandwidth
problems running at 4800, the very reason why AIS uses 38400.
plano




Listen to your VHF radio tuned to the two AIS channels. The traffic is
bursts of data with LOTS of dead time. At some point, you're right, it
could become saturated. But 4800 baud would handle the data I've heard
coming over the two channels very easily in busy Charleston harbor.

Larry
--
http://www.youtube.com/watch?v=fEJmc...elated&search=

"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=

Larry wrote:
"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.


If you think Liverpool, UK is a busy shipping area I suggest you think
again. Think English Channel, Shanghai, Rotterdam, Singapore for
"slightly" busier areas.

Also, range at see that I get is a LOT better than 20 km... The big guys
tend to have their antenna's pretty high up! (My AIS antenna is a Metz
Manta-6 whip mounted 2 m above sea level.)

--
Kees

Larry wrote:
"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

There is another interesting AIS system overseeing the Kiel canal at
http://www.vesseltracker.com/en/kielnok/AreaMap.html

You have to register, which is free to individuals.

See all the convoys and bottlenecks building up, as and when they occur!

Dennis.

"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

Paul wrote:
snip
Short summary: A 4800 bps NMEA link may very well overflow in
not-insanely-busy conditions. Explanation follows:

A couple of weeks ago...
snip

Paul,

Thank you for your most excellent report and very coherent explanation.

It was wonderful!

Jack

--
Jack Erbes in Ellsworth, Maine, USA (jackerbes at adelphia dot net)
(also receiving email at jacker at midmaine dot com)

"Paul" wrote in
:

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.


You can see 90 ships on AIS from a stern rail antenna? How do you plot
these so it doesn't destroy the other navigation? The stern rail antenna
to a ship antenna 100' up would only have a range of 10 miles.

Ok, we can load up the crappy NMEA 1966 data link. I'll say "uncle". All
the more reason the whole damned boat should be on ethernet. But, alas,
how many pleasure boaters ever see 20 ships painted on an AIS display? I
doubt it's very many. I feel sorry for those who have to sail in those
conditions, just like boaters on the ICW in Florida stuck in a ditch so
dense with boats noone can ever have a fun time.

We South Carolinians need to look at ourselves, once in a while, and see
how lucky we are to be able to sail around and never see another boat for
hours, certainly not one that crosses our paths....lucky indeed.

Larry
--
http://www.youtube.com/watch?v=fEJmc...elated&search=

"Larry" wrote in message
...
You can see 90 ships on AIS from a stern rail antenna? How do you plot
these so it doesn't destroy the other navigation? The stern rail antenna
to a ship antenna 100' up would only have a range of 10 miles.

Theoretically yes, but there's also refraction of the VHF signals around the
"edge" of the earth that makes the signals go further. You as a ham should
know that.

Ok, we can load up the crappy NMEA 1966 data link. I'll say "uncle". All
the more reason the whole damned boat should be on ethernet. But, alas,
how many pleasure boaters ever see 20 ships painted on an AIS display? I
doubt it's very many. I feel sorry for those who have to sail in those
conditions, just like boaters on the ICW in Florida stuck in a ditch so
dense with boats noone can ever have a fun time.

Cross the English channel and find out for yourself how busy it is. Or the
Solent area.

We South Carolinians need to look at ourselves, once in a while,
and see how lucky we are to be able to sail around and never see another
boat for hours, certainly not one that crosses our paths....lucky indeed

Yeah, you South Caronlinians are a bunch of pussies compared to the guys who
regularly cross the English Channel... ;-))

Meindert

"Larry" wrote in message
...
"Paul" wrote in
:

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.


You can see 90 ships on AIS from a stern rail antenna? How do you plot
these so it doesn't destroy the other navigation? The stern rail antenna
to a ship antenna 100' up would only have a range of 10 miles.

Ok, we can load up the crappy NMEA 1966 data link. I'll say "uncle". All
the more reason the whole damned boat should be on ethernet. But, alas,
how many pleasure boaters ever see 20 ships painted on an AIS display? I
doubt it's very many. I feel sorry for those who have to sail in those
conditions, just like boaters on the ICW in Florida stuck in a ditch so
dense with boats noone can ever have a fun time.

We South Carolinians need to look at ourselves, once in a while, and see
how lucky we are to be able to sail around and never see another boat for
hours, certainly not one that crosses our paths....lucky indeed.

Larry

Larry and Jack,

I took a detailed look at my NMEA logfile to see if I could sanity-check
last night's calculations. It turns out that what I actually was receiving
is fairly close to the estimates -- but not quite as bad:

(and I lied -- on-board VALIS I have a SR-161 single-channel receiver. The
two-channel receiver is at home for the time being, as part of my
development platform)

During a ten-minute period on the Bay, just inside of the Golden Gate
Bridge, I received signals from 76 ships (a few of these were actually
base-stations). The maximum range was 21 statute miles, but remember that
the surrounding hills are pretty high in most directions. About half of the
ships were moored.

During this ten minutes there were 1401 AIVDM (AIS) NMEA sentences sent from
the AIS receiver, totalling 68581 bytes, or 754391 bits. This gives an
average datarate of 1257 bps. Using a dual-channel AIS receiver the rate
would double to 2515 bps. So, perhaps, even with the burstiness of the
data, a 4800bps NMEA link would work -- but the margin isn't comfortable,
and I have to think that with a mast-top antenna I might easily exceed the
link capacity.

In any case, it sounds like we all agree that 4800bps serial is not the
shining path to the future!

I am displaying the AIS targets on my PocketPC, which is usually velcro'd to
the navstation next to the chartplotter. I have a Bluetooth link from the
AIS receiver and the NMEA mux which carries the rest of the nav-data. My
Raymarine RL70 and RL80 chartplotters (now obsolete) don't support AIS, thus
the PocketPC.

The PPC, running a program I wrote, has a display (for AIS) that looks more
or less like the NASA "AIS Radar", with no chart overlay. It still gets
pretty crowded, but I can turn off ship name display and then it is usable
during crowded conditions. The PPC calculates CPA, TCPA, sounds an alarm if
appropriate, etc -- all the stuff that I expect my new chartplotters to do
well, on a much larger screen. The PPC program also shows other nav data,
using dials, numeric displays, etc. I mainly use it as a "black box" to log
filtered nav and AIS data, so I can postprocess it later and remind myself
(when I am programming) of how much I enjoy sailing. I have some
interesting (to me, at least) Google-Earth tracks created from the PPC
logfiles, posted on the VALIS blog: http://www.sailvalis.com/wordpress_1/ .
The most recent posting has the tracks for VALIS and the AIS-equipped ships
heard during the Queen Mary 2 excursion. I use the PPC rather than a laptop
for this in order to keep the power consumption under control.

-Paul

Thank you for sharing this info with us ... Very interesting and
impressive visualization!

Unfortunately, I have some older Raymarine equipment (RC530+) , that is
not compatible with any AIS-system, that I know of. Would you know, if
this is correct?

If you should recommend a well working AIS-system that can be used on a
notebook running Windows XP ... What would be your advice? - Software
and hardware? Pls note I run the RNS-5 application on the notebook using
C-Map charts, and I sail in Scandinavian waters (12V-DC/230V-AC) - if
that is of any importance for your advice. TIA!

--
Flemming Torp
Gimle/DEN-61

"Paul" skrev i en meddelelse
...

SNIP

Larry and Jack,

I took a detailed look at my NMEA logfile to see if I could
sanity-check last night's calculations. It turns out that what I
actually was receiving is fairly close to the estimates -- but not
quite as bad:

SNIP

The PPC, running a program I wrote, has a display (for AIS) that looks
more or less like the NASA "AIS Radar", with no chart overlay. It
still gets pretty crowded, but I can turn off ship name display and
then it is usable during crowded conditions. The PPC calculates CPA,
TCPA, sounds an alarm if appropriate, etc -- all the stuff that I
expect my new chartplotters to do well, on a much larger screen. The
PPC program also shows other nav data, using dials, numeric displays,
etc. I mainly use it as a "black box" to log filtered nav and AIS
data, so I can postprocess it later and remind myself (when I am
programming) of how much I enjoy sailing. I have some interesting (to
me, at least) Google-Earth tracks created from the PPC logfiles,
posted on the VALIS blog: http://www.sailvalis.com/wordpress_1/ . The
most recent posting has the tracks for VALIS and the AIS-equipped
ships heard during the Queen Mary 2 excursion. I use the PPC rather
than a laptop for this in order to keep the power consumption under
control.

-Paul