MAC sublayer protocol

[15-Jan-1991 0920 <goldstein@carafe.enet.dec.com>]

Steve Johnson asks,

>Agreed.  Am I wrong in assuming that 56k and microwave packet systems
>are purpose-built radios, and do exhibit superior turn-on/off delays as
>compared to low-speed modems connected to standard voice radios?

There are no such "common" systems.  I'd expect most run full-duplex in
a point to point mode, so a MAC protocol (multi-access) is simply not
part of the protocol stack at all!  Even if half-duplex, they still
just need a datalink protocol.  The MAC discussion is for multi-access
systems.

>For technical/installed base/compatibility reasons many (most?) of the
>existing low and moderate speed nets would not be able to (or want to)
>migrate to a new protocol.  However, when dealing with direct 56k user
>access to backbones, as opposed to 1200/2400/9.6k access to a 56k backbone,
>it seems the idea might still have merit.  And for point to point, omni-gain
>type of connections, microwave links it almost seems like a requirement.

Again it's a question of who does what to whom where.  An access channel
needs a MAC protocol; a backbone is normally point to point and doesn't.
Direct 56k user access to backbones is an oxymoron unless the user and
the backbone each have a dedicated link radio to one another, making the
user in effect a backbone node.  The problem we're trying to solve is
ordinary shlubs like me, who don't have dedicated radios, and just run
packet once in a while, not 7x24.

(me)>>Exisiting single-freq. access channels are indeed cruddy due to their
>>basically Aloha nature.  But if we used simple repeaters (even baseband
>>regenerators, like audio-style repeaters), we'd have true CSMA and no
>>HTS.  Then the existing p-persistent pseudo-CSMA would work.

>While doubling the spectrum usage of a 100kHz wide 56k signal seems
>wasteful, I have heard mentioned a narrower beacon being transmitted
>by a central digipeater when it is receiving.  This gives almost the
>same effect allowing CSMA, but does not give collsion detection that
>a full repeater might allow.  It still would support p-persistent
>CSMA as you said, with the effeciency largely a function of turn-on
>and propagation delays.  Doesn't this require a separate receiver to
>at each site to detect 'carrier sense', since without a full repeater
>the site would have monitor both the main frequecy and the beacon
>frequency simultaneously in order to keep from missing incoming packets
>when it has something to send?

CSMA means that everybody hears the channel before transmitting, not
while transmitting.  CSMA/CD means everyone listens _while_ transmitting
too, and can tell if they've been clobbered.  That's VERY tough with
radio; you'd have to compare received with transmitted bits at 2x the
repeater path delay.  Busy Tone Multiple Access (BTMA) means that you
listen to a separate low-bandwidth signal all the time; it's sort of a
halfway point.  Various BTMA schemes have been suggested here but all
need some FDX hardware.

>Are there other reasons besides spectrum usage that split frequency
>full duplex systems aren't used on packet?  It does seem the simpler
>approach if enough spectrum is available.

Yes, there are reasons, but they're pretty much the same reasons that
people prefer AX.25 and Net/Scam, and it took until this year to get a
codeless Tech in Gringonia.  Hams are horrendously conservative and
consider the first working hack to be a standard.  Digipeaters don't
use repeater pairs, and pretend to be repeaters to those who don't
know better.  And in most areas, the FM yak crowd took up all the
reapeater pairs, with waiting lists 'til 2001+, so packeteers are
discouraged from putting on packet repeaters, scared of Charlie et al.

In fact, a repeater is MORE spectrum-efficient than a single-freq node.
Single-freq gives us Aloha, with 17-25% efficiency before congestion
collapse.  Repeaters (with half-duplex leaf nodes) give us CSMA, with
35% or so efficiency and more gracefully self-limiting traffic. SInce
repeaters take half as long (once to receive and once to send) to carry
traffic as digis, they are roughly FOUR TIMES as effective, using TWO
TIMES the bandwidth.  If we really wanted to clean up the 2m mess,
we'd ban digis and insist that all non-direct L2 traffic go through
repeaters.  But the yakkers are content to leave us in our cesspool
around 145.0.

Ham radio doesn't always behave rationally.
   fred k1io