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PA2AGA > HDDIG 18.06.00 14:53l 190 Lines 7181 Bytes #-9439 (0) @ EU
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From: PA2AGA@PI8HGL.#ZH1.NLD.EU
To : HDDIG@EU
Date: Sun, 18 Jun 00 00:20:04 MET
Message-Id: <hd_2000_168F>
From: pa2aga@pe1mvx.ampr.org
To: hd_broadcast@pa2aga.ampr.org
X-BBS-Msg-Type: B
From: "David Findlay" <nedz@bigpond.com>
Subject: Packet Radio
> As I wrote (or tried to write): this is not practical at 10 Mbit/s.
> The packet system at 1200 and 9600 bps works that way, but at 10 Mbit/s
you
> are really limited to point-to-point links. Dedicated equipment at each
> end of the link.
>
> See it as a leased line, not as an ethernet. You can build a large
network
> out of leased lines (like the Internet), but when you are not somehow on
> the network (= have a link to another node that is on the network) you can
> do nothing.
Would mobile stations be possible? or would you have to drop to 115Kbps or
56Kbps?
David Findlay
------------------------------
Date: Sat, 17 Jun 2000 15:34:44 +1000
From: "David Findlay" <nedz@bigpond.com>
Subject: Packet Radio
>You will have the problem that it will be both non-free in the sense that
>you cannot use it for anything you like (limited by license conditions),
>and non-free in the sense that it will cost *a lot* to build, and someone
>has to pay for it. If not the users and/or contributors, who else?
You would pay for the maintinence of your equipment. If everyone broadcasts
on the same channel, then you don't rely on any specific links.
>Most attempts to do what you are trying to do (there have been many, at
>least on paper and in late-night chats) have failed because of this.
Could you give me some examples of networks that have failed? URLs?
>Worse: 10 of those 100 users, 7 of them from the group "that paid for it",
>will call you at 23:00 because "their link is down", or will spread
>messages that "the link has now been down for a week and it is all so much
>better managed in <fill in your neighboring state>".
If it used my system, it wouldn't be my problem. And noone would complain
because no one pays for it. They pay their own gear.
David Findlay
------------------------------
Date: Sat, 17 Jun 2000 10:49:16 +0300
From: Paul Keinanen <keinanen@sci.fi>
Subject: Packet Radio
On Sat, 17 Jun 2000 12:52:46 +1000, "David Findlay" <nedz@bigpond.com>
wrote:
>Would mobile stations be possible? or would you have to drop to 115Kbps or
>56Kbps?
The problem is multipath or more precisely selective fading, in which
the direct and reflected signal cancel each other _at_ specific_
wavelengths. If you are using a narrow band modulation method, this
can ruin your signal completely. In a mobile environment, you may have
several signal dropouts every second.
One way around this is to use interleaving and since maybe 1 to 10 %
of time the signal is corrupted in a mobile environment, the error
correction can take care of these burst errors. The problem is that if
you stop _at_ the signal null e.g. due to traffic light, there is not
going to be any communication.
The other approach is to transmit on different wavelengths, of which
some are always taken out by selective fading, but most wavelengths
will get through and the ECC can reconstruct the message. There are
various ways of doing this. In (frequency hopping) spread spectrum,
the signal is transmitted at a large number of wavelengths sequently
one wavelength at a time, in COFDM a large number (up to several
thousand) wavelengths are transmitted simultaneously.
Unfortunately none of these techniques are used by hams (except for
some scattered tests).
Paul OH3LWR
------------------------------
Date: Sat, 17 Jun 2000 07:30:44 GMT
From: nomail@rob.knoware.nl (Rob Janssen)
Subject: Packet Radio
David Findlay <nedz@bigpond.com> wrote:
>Would mobile stations be possible? or would you have to drop to 115Kbps or
>56Kbps?
I would not attempt mobile 10 Mbits/s operation. But you are welcome to
try.
Rob
--
+----------------------------------+--------------------------------------+
| Rob Janssen pe1chl@amsat.org | WWW: http://www.knoware.nl/users/rob |
| AMPRnet: rob@pe1chl.ampr.org | AX.25 BBS: PE1CHL@PI8WNO.#UTR.NLD.EU |
+----------------------------------+--------------------------------------+
------------------------------
Date: Sat, 17 Jun 2000 07:35:13 GMT
From: nomail@rob.knoware.nl (Rob Janssen)
Subject: Packet Radio
David Findlay <nedz@bigpond.com> wrote:
>>You will have the problem that it will be both non-free in the sense that
>>you cannot use it for anything you like (limited by license conditions),
>>and non-free in the sense that it will cost *a lot* to build, and someone
>>has to pay for it. If not the users and/or contributors, who else?
>You would pay for the maintinence of your equipment. If everyone broadcasts
>on the same channel, then you don't rely on any specific links.
But then you rely on everyone's capability to hear another. That will be a
costly mistake, that you will only discover when you are beyond the stage
of initial experiments...
>>Most attempts to do what you are trying to do (there have been many, at
>>least on paper and in late-night chats) have failed because of this.
>Could you give me some examples of networks that have failed? URLs?
Few people have the motivation to document a failure.
>>Worse: 10 of those 100 users, 7 of them from the group "that paid for it",
>>will call you at 23:00 because "their link is down", or will spread
>>messages that "the link has now been down for a week and it is all so much
>>better managed in <fill in your neighboring state>".
>If it used my system, it wouldn't be my problem. And noone would complain
>because no one pays for it. They pay their own gear.
The only problem is that your system does not work in practice.
It is re-invented every decade, and people try to set it up only to
discover that it starts to break down once the usage of the channel gets
beyond a certain level, and the number of stations that do not hear another
(and thus happily transmit at the same time) goes up.
The current breed of packeteers that is wandering into this trap is using a
mode called APRS. From what I understand about your intentions, you should
just drop your datarate expectations to 1200 or 9600 bps and dive into the
APRS thing. It works the way you think a network can work, and you will
quickly see what the limitations are.
Only it does not use established network protocols like IP (with
multicasting), but it builds on the amateur packet protocol AX.25 in
datagram mode, with an ad-hoc definition of datagram contents.
(position reports, weather reports, short messages, etc)
Search for APRS on the WWW.
Rob
--
+----------------------------------+--------------------------------------+
| Rob Janssen pe1chl@amsat.org | WWW: http://www.knoware.nl/users/rob |
| AMPRnet: rob@pe1chl.ampr.org | AX.25 BBS: PE1CHL@PI8WNO.#UTR.NLD.EU |
+----------------------------------+--------------------------------------+
------------------------------
Date: 17 Jun 2000 03:28:30 GMT
From: ebruchac@aol.comnojunkma (Ed Bruchac )
Subject: PSK31 for FD??
Anyone planning on running PSK31 for Field Day next weekend? Any suggestions
To be continued in digest: hd_2000_168G
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