PACKET RADIO: Line Lengths


(This text from the W8IZ packet radio bulletin

board. It's formatted to fit a 80 character screen.)


From: Steve, NO8M@NO8M

Date: September 4, 1990

Subject: The Reason Bulletins MUST Be Short


Every time a bulletin passes through a PBBS, another forwarding header

is added to its list of headers. You do not normally see these headers. If

you would like to take a look, find a bulletin from California and instead of

reading it with the "R" command, use "RH".

Almost all bulletins are passed on VHF. A California bulletin must pass

through many, many systems on its way here. By the time the bulletin makes it

to NO8M, it may have more stored in the headers than in the message.

If your ALLUS is long to start with, it will be huge by the time it gets

to California. Actually, it will not make it. Many stations will not forward

bulletins over a certain length.


When busy nodes are used to transfer a message, the node itself may

cause the message to be cancelled. The nodes allow the sending station to

send information until it's memory is filled. (The node might have many other

users using it at the same time! Backups do occur!) It then tells the PBBS

to back off by sending a RNR (Receiver Not Ready) packet. The bulletin board

will either sit there and wait or count the RNR as a NACK (Negative

Acknowledge) and decrement its retry counter. When the counter goes to zero

the PBBS disconnects. Hence, the node may never allow the message through.


There is some HF forwarding that occurs. When a long bulletin faces HF

forwarding, it will normally not make it due to the changing band conditions.

300 baud packet is MUCH slower than the 1200 baud we use on HF. HF may just

not allow the forwarding.


If a longer bulletin can not be forwarded, it will lock out any bulletin

that came in before itself. The bulletin board sends bulletins it just

received first. This is called First In, First Out or FIFO.

The board looks at all the bulletins that are to go and starts at the

one it most recently received. It then works backwards. If it constantly

hits one that can not be forwarded, then all the ones before it are locked

out, also.


On NO8M, we are forwarding bulletins less than 2.5K in length to the

Caribbean. We are forwarding bulletins of any length through VHF routes. I

am in favor of limiting VHF forwarding to bulletin of 5k or less.

Should a bulletin be received that are over 10k in length, I will

manually mark it as being forwarded effectively killing it off here.

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