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Introduction
Overview
Computer requirements
Packet Engine Pro
Configure AGWPE
Download and Install
Basic AGWPE Setup
2 Radio Setup
2 Card Setup
Sound Device Setup
Basic Device Settings
Rename Sound Device
Additional Settings
Using the Tuning Aid
Problems?
Program Behavior
Receiving
Transmitting
Connections
Firewalls
AGWPE Features
AGWPE on a Network
Baud Rates & Modes
Remote Control
TCP/IP Over Radio
Tips and Tricks
Traffic Parameters
Compatible Programs:
Setup Help
Radio Interface
Getting Started
Kits and Pre-assembled
USB SignaLink
Receive Audio Cable
Transmit Audio Cable
PTT (TX Control) Cable
2 Radio Modification
About Packet
Packet Overview
Exchange Modes
TNCs and AGWPE
What
To Do with Packet
Common Frequencies
Frame Headers
Further Reading
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Deciphering Packet Frame Headers
AGWPE follows the
AX.25 protocol that is the
standard for ham radio packet,
however, AGWPE does not use the standard TNC2 packet frame display status
protocol, but a more comprehensive custom protocol developed by the
program author, George
SV2AGW.
If your terminal program (e.g. AGWterm) is set to
display packet frame headers, you will see these status indicators and
can follow what AGWPE is doing and how it is interacting with another
station. This can sometimes be helpful in
diagnosing connection problems.
Listed below are the explanations
for each type of packet frame header.
Note that within the packets,
there may be these additional indicators:
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PID is the
"protocol identifier". PID=F0 means the
frame contains text -- "F0". If the frame contained TCP/IP data, the ID
would be "CC". Other identifiers are used
for NET/ROM , Flexnet, Apple Talk, etc.
-
P and
F mean Poll and
Final. The P and F "bits" are used to indicate when a reply or ack
(acknowledgement) is desired. For example, when the sending station
includes a P bit in the packet header, it means "respond
immediately to this packet". If the P bit is missing, the destination
station will wait for additional packets until one appears with the P
bit and then acknowledge all of the packets at once. The sending station
controls how many packets AGWPE will send without a P bit by setting the
MAXframe parameter. For example, if MAXFrame is set to 5, four (4)
packets will be sent without the P bit and the fifth will contain the P
bit. The destination station will respond to the fifth packet with
an ack that includes the F bit to indicate the last number of the packet
frame sequence which is received.
All packets frames are identified by
number, the first number being 0 (zero) and the last being 7. After 7,
the numbering sequence restarts at 0 (zero). The numbering system is
used to track packets, so that each receiving station can compare how many
packets were sent to it and how many it has received. If a packet is
missing, the receiving station will request a re-transmission of numbered packets.
Packet frame headers:
<UI pid=F0 Len=32> - an unconnected frame (e.g. beacon)
sent to no station in particular but to everyone, i.e. . The
PID is "F0"
since it contains simple ASCII text. (UI frames are used in APRS, for
example.)
Len=32 means the packet contains 32
characters.
<SABM P> - a request from station A
to station B for a connection. The P bit is used to request an immediate
reply. (Set Asynchronous Balanced Mode)
<UA F > - the reply from station B to
station A accepting the connection request. It is also used to accept a
disconnection <DISC> request. The F
is used to tell station A that
all packets were received successfully. (If station B could not accept
the request, it would send a DM frame;
see below.)
<DM F > - a refusal to accept the
original connection request, perhaps because another connection in in
progress.
<DISC P> - a request from either
station to disconnect from the other. (This is accepted with a UA
frame).
<I P R3 S0 pid=F0 Len=28 > - this in
an information ( I ) packet containing data
(text, etc). The P requests an immediate
acknowledgment. If the P were absent, then
the receiving station would delay its acknowledgement until it received
a frame with a P in it.
R3 indicates that the station last received
the other station's packet #2 and is ready to receive
#3. S0 indicates that this packet being
sent is # 0 (zero). Len=28 means the packet
contains 28 characters.
<RR P/F R1 > - this is a "ready
to receive' (RR) packet simply acknowledging receipt of packet #0 and
ready to receiver #1.
<REJ P/F R1 > - this is reject (REJ) packet
meaning that the packet just received was out of sequence or a
duplicate or not received accurately; ready to receive packet #1 instead.
The REJ can also be sent by a TNC to indicate its buffer is full and it
is not ready to receive (NRN)]. In addition, some
AX.25
implementations, e.g. BPQ-DOS node,
use a REJ packet instead of
P; immediately after the last packet in
a sequence (maxframe), a REJ is also sent to force an
acknowledgement by the receiving station.
Another frame that is rarely seen is a
FRMR (Frame
Reject). It would be sent if the packet received had an invalid control
field or an illegal data field or a data field that was too long or other
problem. In general, you will not see such a frame unless
you
are writing your own AX.25 code and testing it.
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