Timewave DSP-232, Operating Manual

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Chapter 5 Packet Operation
The first attempt to avoid collisions was through the use of the
DWAIT
and
RESPTIME
timers.
DWAIT
forced the
TNC to delay the transmission of any packet except for digipeated frames by the time selected. This fixed timer
helped, but packet was still plagued by collisions.
RESPTIME was added to help with large file transfers. Still,
more needed to be done to reduce collisions.
Another attempt to reduce collisions was the introduction of AX.25 version 2 protocol. On VHF packet, most
everyone uses version 2 which is controlled by the AX25L2V2 command. On VHF this helps, but some users on
HF packet turn this command off.
An exponentially distributed random wait method was proposed by Phil Karn (KA9Q) called P-persistent CSMA.
When
PPERSIST is ON , the DSP’ uses the number set in PPERSIST and the time value set by SLOTTIME to
more randomly distribute the transmit wait time. This is more efficient than using the DWAIT time.
As a further attempt to improve packet performance, Eric Gustafson (N7CL) proposed giving priority to
acknowledgment packets (acks); this protocol is controlled by
ACKPRIOR . Check with the experienced packet
users in your area and find out if they are using priority acknowledge or have changed any other parameters.
DAMA Support
AEA has added DAMA (Demand Assigned Multiple Access) to the DSP-232. DAMA allows for a master/slave
connection to reduce packet collisions. At times, a node’s receiver can hear too many signals at once and remote
users can get lost in the noise. A good example of this situation is when ships on the open sea try, but can’t connect
to communication satellites; the stations have a clear shot at the satellite, yet can’t connect.
To solve this problem using existing equipment and still using only one frequency, DAMA has been implemented to
organize all node users’ communications. With DAMA, the master station identifies all slave stations and polls each
of them for information. Slave stations are ‘asked’ by the node if they have any information to send. If a slave
station does, the information is sent and the node ‘remembers’ that this station sent information this time around. If
a slave station does not send information, the master ‘remembers’ and skips this person next time around, coming
back to them at a later time. Those stations replying with information when polled are considered active. Stations
that do not send information are considered inactive. Active stations are given a higher priority than inactive stations
and thus, more opportunity to send information. As the node traffic increases, inactive users fall down through the
priority list. Low priority stations can become active (and have higher priority) at anytime by sending information
when the master station polls. Using DAMA can increase overall channel throughput by organizing stations based
on their state of activity. Stations sending data are given more chances to send data and stations not sending data are
given less chances.
AEA has added the
UPlink
command to the DSP-232 for use with DAMA communications. Information on this
command can be found at the end of this chapter.
CHECK and RELINK
If a station connects to you and then turns its TNC off, you would probably not want to stay connected to the
station. The
CHECK
timer determines the amount of time your DSP will wait before testing the link if no data has
been exchanged.
RELINK sets what happens after the CHECK timer has expired. If RELINK is OFF , the DSP will change to the
disconnected state to terminate the link. If ON , the DSP’ will attempt to reconnect to the distant station to re-
establish the link.
Transparent mode
One of these features is the Transparent mode—it allows any 8-bit binary character to be sent by your packet
station. You usually must use the Transparent mode to transfer binary and executable files to and from other
stations. See Chapter 4 for terminal commands.
September, 05 5-21