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XRPi Documentation - Protocols
KISS
Protocol Details
KISS is a simple protocol which encapsulates AX25 frames for transmission over serial (e.g. RS232) lines. The framing method is identical to SLIP. There is no flow control or error handling in the original protocol.
The protocol specifies the following special characters:
Name Hex Dec Purpose --------------------------- FEND 0xC0 192 Frame End FESC 0xDB 219 Frame Escape TFEND 0xDC 220 Transposed FEND TFESC 0xDD 221 Transposed FESC
The FEND characters mark the start and end of the frame containing the encapsulated datagram as follows:
.------.------.-------.------. | FEND | Type | Data | FEND | '------'------'-------'------'
In order to ensure that the FEND character only occurs at the start and end of the frame, FENDs which occur within the unencapsulated data are "escaped" to the two byte sequence FESC TFEND. Likewise FESC is escaped to the sequence FESC TFESC.
It is permissible (but not obligatorry) for two frames to share a FEND:
.------.------.-------.------.------.-------.------. | FEND | Type | Data | FEND | Type | Data | FEND | '------'------'-------'------'------'-------'------'
The first byte of each frame, after the FEND, is a "type" indicator. This byte is broken into two 4-bit nibbles such that the low-order nibble indicates the command number (given in the table below) and the high-order nibble indicates the port number for that particular command. In systems with only one HDLC port, it is by definition Port 0. In multi-port TNCs, the upper 4 bits of the type indicator byte can specify one of up to sixteen ports.
The following commands are defined:
Command Function Comments
--------------------------------------------------------------
0 Data frame The rest of the frame is data received
from / to be sent to the HDLC channel.
1 TXDELAY The next byte is the transmitter keyup
delay in 10 ms units. The default
start-up value is 50 (i.e. 500 ms).
2 PERSIST The next byte is the persistence
parameter, p, scaled to the range 0-255
with the following formula: P=p*256-1
The default value is P=63 (i.e. p=0.25).
3 SLOTTIME The next byte is the slot interval in 10
ms units. The default is 10 (i.e. 100ms).
4 TXTAIL The next byte is the time to hold up the
TX after the FCS has been sent, in 10 ms
units. This command is obsolete, and is
included here only for compatibility
with some existing implementations.
5 FULLDUP The next byte is 0 for half-duplex,
nonzero for full-duplex. The default is 0
(i.e. half-duplex).
6 SetH/w Specific for each TNC. In the TNC-1,
this command sets the modem speed. Other
implementations may use this function for
other hardware-specific functions.
255 Return Exit KISS and return control to a higher
level program. This is useful only when
KISS is incorporated into the TNC along
with other applications.
--------------------------------------------------------------
Only command 0 is allowed in frames from TNC to host. Commands 1 to 6 are used to set TNC parameters, and are usually sent at 5 minute intervals.
Limitations of Plain KISS
In the original protocol, there is no error detection to protect against noise and corruption on the RS232 lines.
More seriously, the host has no way of knowing how much data is queued in the TNC awaiting transmission. A busy channel could prevent the TNC from transmitting, causing the host's FRACK timer to repeat frames, which simply add to the queue.
When the channel clears, the original frame and all the
repeats are spewed out in one huge transmission, causing the
other end of the link to respond with a string of acks. In bad
cases, the AX25 module retries out, so when the channel
clears, the original frame, plus repeats, plus a string of
Checksum-KISS appends a single byte checksum to the "data"
portion of the frame, to detect line errors. Frames that
fail the checksum test are silently discarded. The upper
layer eventually detects the loss and re-sends the frame.
When operating in this mode, the host attaches a 16-bit
"serial number" to each frame (between the "type" and "data"
fields), and the TNC sends an "acknowledgement" frame to the
host when it has transmitted that frame on-air. This enables
the host to know how much data is queued, and to start its
AX25 timers at the correct time. This mode uses the command
code of 12 for both data frames and the acknowledgements.
In this mode, the TNC does not send any data to the host until
it is asked to do so by a POLL command (command number 14).
This allows several TNC's to be multiplexed together (usually
with a diode matrix) onto one COM port, which removes the need
for one COM port per TNC. Up to 16 TNC's can be multiplexed
onto one COM port. Each TNC must have a different "address",
i.e. in the upper nybble of the Type field.
A temporary KISS connection may be established over an XRPi
dial-up link using the MODE command in the DUN script. This
is only likely to be of use where phone calls are free!
KISS normally runs over RS232 lines, but can also be run over a TCP link. The other end of the link could be located on the same machine, or on the opposite side of the world.
You could for example use this to link two applications on a single machine, or to make a remote-sited multi-port node appear as one or more ports on XRPi. For more about KISS over TCP, please refer to the KISSTCP page.
KISS can be used to link XRPi with KISS TNC's, or with other
KISS systems via real or virtual COM ports. A typical
configuration in XROUTER.CFG would be as follows:
MTU specifies the largest size for the data portion of an AX25
frame, and the largest IP datagram that can be handled. This
should be set to 256 for KISS TNC's because they usually have
small packet buffers. For KISS links not using TNC's, MTU may
be set larger, up to 1500.
KISSOPTIONS are as follows:
The PORT is configured like this:
CHANNEL is only required if the TNC is not using its default
channel / port.
Serial lines used for KISS must run at 8 data bits. Flow
control must be hardware or none, as XON/XOFF flow control
would interfere with the protocol.
If flow control is used, the cable must contain at least 5
cores, namely TXD, RXD, RTS, CTS and GND. If flow control is
not used, only TXD, RXD and GND are required.
When KISS is used to connect a PC to a TNC, a "straight
through" cable is used, because a TNC is considered a DTE
(Data Terminal Equipment).
When KISS is used to interconnect two applications, some form
of NULL MODEM is required. In the case of "real"
RS232 this could be an actual null modem device, or a cable
that is wired such that the TXDs at each end go to the RXDs
at the other end, and the RTSs at each end go to the CTSs at
the other. "Virtual" COM port pairs include
this functionality as standard.
Most TNC's can be switched into KISS mode using a command
such as "KISS ON", but they have a tendency to randomly drop
back to command mode. Therefore it is more usual to replace
the EPROM with a purpose made KISS EPROM. There are several
versions, such as BPQKISS, JKISS, KISS and 220KISS. These
usually default to channel A, but can be "patched" for other
channels.
For example the BPQKISS EPROM may be patched for different
KISS channels by changing the byte at address 20(hex) in the
PROM as follows:
Once in KISS mode, the only way to switch a conventional TNC
back to normal mode is to send the sequence 192, 255, 192 on
the serial line. This can be done using a terminal porgram
and the numeric keypad, or a simple program such as
KISSOFF.EXE.
KISS With Checksum
KISS With Acknowledgement
Polled Kiss
Dial-Up KISS
KISS Over TCP/IP
Configuring a KISS Link
INTERFACE=1
TYPE=ASYNC <-- Serial RS232
COM=/dev/ttyUSB1 <-- RS232 device
PROTOCOL=KISS <-- Use KISS
SPEED=38400 <-- Baud rate
FLOW=0 <-- No flow control
MTU=256 <-- See below
KISSOPTIONS=NONE <-- Plain KISS
ENDINTERFACE
NONE - Plain KISS (most TNC's use this) (default)
POLLED - For TNCs which send only when polled.
CHECKSUM - Packets protected by checksum. You can
only use this option if your TNC supports it.
ACKMODE - For TNC's which inform XRPi when a frame has
been transmitted on air.
SLAVE - XRPi will act like a polled KISS TNC,
sending only when commanded to do so.
Polled and slave are mutually exclusive.
BPQKISS eproms require POLLED and CHECKSUM, and their use
of ackmode is optional.
PORT=3
ID=144.675MHz User Port
INTERFACENUM=1
CHANNEL=B
ENDPORT
Serial Line Parameters
KISS TNC's
Channel Byte-20h Channel Byte-20h
---------------- ----------------
A 00h I 80h
B 10h J 90h
C 20h K A0h
D 30h L B0h
E 40h M C0h
F 50h N D0h
G 60h O E0h
H 70h P F0h
See Also