MAC sublayer protocol
- To: firstname.lastname@example.org
- Subject: MAC sublayer protocol
- From: uunet!aupair.cs.athabascau.ca!rwa
- Date: Tue, 15 Jan 91 15:52 MST
In <9101151744.AA06967@hpnmdlc1.hp.com> Glenn Elmore <email@example.com> wr
> fred k1io writes in response to Steve Johnson:
>> [good stuff deleted for brevity]
> If the repeater is located such that it *only* radiates to<->from the
> same set of U users which would otherwise be trying to CSMA it alone I
> agree. BUT, very often the repeater is located at a high level site.
> This means that it not only gobbles the spectrum from a much larger area
> but also is likely to increase the number of users. The result can be
> that 10*U(or whatever) users now vie for the same channel and we are
Some trivial solutions:
1) put the repeater lower (saves $ too)
2) run lower power and/or more squelch (another $ saver)
3) okay, put up ten times as many repeaters. Why assume
a scarcity when none need exist ?
and a slightly less trivial solution
4) The above discussion assumes a "no-brains" full duplex repeater.
Ok, maybe it's doing some regeneration. Now let's assume a few more
When the repeater decides that a packet might be arriving, it
starts sending SYNC chars on the Tx side BUT it starts assembling
the incoming packet at least until it has the calling station id -
WITHOUT simultaneously retransmitting the incoming octets. Then
it looks up the calling station's id, and start repeating the
packet if the call is on the approved list. Otherwise, it fakes a
collision on the output side.
The induced latency is only the time between the start of the SYNCs
and the end of the calling station id, which in a standard AX.25
packet is very near the head of the packet. A little FIFO is all
that's logically needed. If the packets are of a good size (a
given, since otherwise all the other latencies are going to kill us)
this won't hurt much, and if not - well, it's *still* better than the
Mind you, I find the concept of a closed repeater to be one of the most
distasteful ideas in hamdom. If the ham population is dense enough
(in the *spatial* sense of the word) to require this sort of
technique, I suppose keeping track of who can use the repeater would
be a bearable administrative overhead (sigh). If the other sense of
the word applies, ghods help us.