PACKET RADIO: State Machines

Steve Wolf, W8IZ@W8IZ

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From: Steve, W8IZ @W8IZ.#NEOH.OH.USA.NA
Subject: State Machines
Date: December 26, 1989

As of late 1989, only one brand of TNC, the MFJs, really look at what
they are hearing. If an MFJ hears noise, it knows it is noise and will ignore
it. If it hears a packet signal, it will listen to it. With this type of
TNC, you can run with the receiver squelch wide open.

The rest look at any audio they areceiving as being a packet signal.
Any open squelch will hold up the action of the TNC until the timeout timer
forces a packet to be sent. With these TNCs, you have to run with a closed

Two problems arise with the TNCs that do not recognize packet.

First, in some radios the squelch circuits are slow. When a signal
comes on the air, it takes some time for the receiver to open its squelch and
begin receiving the packet. In some cases, this delay is so long that the
transmitting station has begun to send data and the packet is lost. This can
be avoided by having the sending station add some TXDelay. This is a delay
inserted after the sender puts the signal on the air and before the data is
sent. It is a wasteful dead carrier (kind of like a beacon).

More important, the TNC is deaf for a certain period of time. When the
TNC decides to send a packet, it checks to see if there is a signal on the
air. With the squelch closed, there very well may be a signal that is now on
the air but has not opened the (slow) squelch. Since the TNC has not yet
heard this signal, it assumes the frequency is not in use and begins it's key-
up. It no longer listens to the frequency and a collision occurs.

A State Machine allows a non-MFJ TNC to listen to a radio with the
squelch open and differentiate between packets and noise. The throughput goes
up and the channel collisions go down.

TAPR State Machines are bolt together, one hour kits that allow this

On the NCARC PBBS, two older radios are used. On 223.7, a Midland 13-
509 has a bearable receiver squelch which, with the State Machine, rivals the
new radios. On 145.09, the Clegg FM-28 had a poor (VERY poor) squelch delay
that forced many users to use a TXDelay value they really did not need. The
State Machine is a great help. All users are encouraged to pare down the
TXDelay values they are using to the minimum needed to insure the board copies

On the transmit side, the Midland and the two meter amplifier both
contain relays that force a much longer TXDelay the board must provide (20
ms). This delay (I hope) will be brought down some with the addition of solid
state switching. The parts for this should be available at Dayton in April of
1990. However, there will always be a need for a longer TXDelay on the boards
side as some users have older radios with squelch operated TNCs. They
need the board to send a carrier to allow their slow squelch circuits to open.
A typical TXDelay improvement should be about 45%.

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