PACKET
18
Getting Out of Transparent
Getting into Transparent Mode is easy, just type a
t
in Command Mode. But since Transparent
Mode allows the sending of all characters you can not get out of Transparent Mode by just typing a
Ctrl-C
(COMANND character) as in Convers Mode. In order to get out of Transparent Mode you
must follow a special sequence or use a modem break if your program supports one. The special
sequence must be followed precisely. This example assumes the COMMAND character is Ctrl-C and
CMDTIME is 1 second:
Wait at least 1 second since the last character was sent from the computer to the TNC
Type a
Ctrl-C
Within 1 second type a second
Ctrl-C
Within 1 second type a third
Ctrl-C
Wait 1 second and the
cmd:
prompt should appear
If the guard time of one second before and after the three Ctrl-Cs is not there, the TNC assumes
that they are data and sends them to the radio. Don't get impatient, one second can seem longer
than you think it should.
HF Packet Operation
All of the information previously presented applies to HF Packet as well as VHF Packet, but the ma-
jor differences all center around the timing parameters.
The reason for this is the band conditions on HF generally cause more of what is called the hidden
station concept. A simple example will demonstrate:
If I'm located in Lawrence, Kansas, and I'm talking to a station in Boston, Massachusetts, all goes
very well as long as we are the only two stations on frequency. In the real world, there are many
other stations on at the same time. Now, I send a packet of data to my friend in Boston and a sta-
tion in San Diego has data to send also. He doesn't transmit because he hears my signal. The Bos-
ton station receives my packet and I quit transmitting. Now the Boston station keys up to send his
acknowledgment back to me but the San Diego station also keys up to send his data because he
can't hear the Boston station. Here we have a collision. I hear both of them, so the acknowledge
doesn't arrive.
On the other side of the coin, if the Boston station sends me a packet, I may receive it fine, then
the San Diego station keys up to send a packet of data. Perhaps this is several long packets which
cause him to transmit for 5 seconds. I didn't transmit because I heard the San Diego station. Now,
the San Diego station stops sending, I start my DWAIT or SLOTTIME wait, and a station in Seattle
keys up to answer the San Diego station. Again I don't transmit because I hear the Seattle station.
By this time, perhaps 8 seconds or so have gone by. The Boston station didn't receive my ack – of
course not! I haven't sent it yet because the channel isn't clear! But, his FRACK time is expired and
he thinks I missed it. Even if I acknowledge right now, he will still have to go through the
POLL/RESONSE sequence since his FRACK time expired.
What we see here, is the need on HF, with more hidden stations and slower baud rates, to in-
crease our FRACK time to give the receiving station a reasonable opportunity to ack our packets
before we retry. Otherwise, we have lots of retries and very little data.