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6PACK a "real time" PC to TNC protocol

by Matthias Welwarsky, DG2FEF @ DB0KLN.DEU.EU, Am Pelz 77c, 64295 Darmstadt, Germany
translated by Thomas Sailer, HB9JNX @ HB9W.CHE.EU, Weinbergstraße 76, 8408 Winterthur, Switzerland
converted to HTML and indexed by Alexander Stohr, DG3MMF @ OE9XPI.AUT.EU, Schönauer Str. 5, 88131 Lindau, Germany


During the development of the PC/FlexNet software package, there was a strong desire to use the existing hardware, especially the very widespread TNC2, which populates almost every packet radio station nowadays. Sysops of TheNetNode digipeaters also showed much interest, since many TNN nodes use TNC2 devices connected using a KISS token ring.

Existing TNC protocols have severe disadvantages. The KISS protocol does not allow much influence on the channel access. This prevents the implementation of alternative channel access schemes, such as DAMA or OPTIMA. The so-called "WA8DED Hostmode" does not provide data transparency, and its throughput is very limited.

The basic concept of 6PACK was developed at the end of 1993 by Ekki Plicht, DF4OR, Henning Rech, DF9IC and Gunter Jost, DK7WJ. It was further developed for use in the PC/FlexNet software package by Gunter Jost, DK7WJ, and Matthias Welwarsky, DG2FEF. The protocol then did not support multiple TNCs connected in a ring topology. It only allowed for one TNC per asynchronous serial interface. It was then found that a ring topology required different protocol features than a simple point to point connection.

The current revision of the protocol has many additional features compared to the first version. It now allows a ring topology of up to eight TNCs connected to a single asynchronous serial interface of the computer. The ring wiring is hardware compatible with the existing token rings used by the "TheNetNode" software.

6PACK provides:

  • Data transparency
  • predictable capacity requirements on the ring
  • data and realtime information is distinguished
  • fast response to changing channel usage, even under high loading and multiple TNCs on the ring
  • automatic ring setup replaces a channel number "patched" into the (EP)ROM
  • data is protected by a checksum


The asynchronous serial interface parameters are: 8 data bits, 1 stop bit, no parity bit. The baud rate on the ring must be at least twice the highest HDLC bit rate of the TNCs connected to the ring. RTS/CTS is ignored, RTS however should toggle about every 10ms to reset an eventually connected hardware watchdog. The TNC software should implement another watchdog which terminates any transmission if no further data is received from the PC. To reset this watchdog, any command sent to the TNC will do, for example an LED control command. Bits 6 and 7 of every byte distinguish between channel data and control codes.

76 543210 Meaning
00 xxxxxx channel data
01 xxxccc status/protocol data
1x xxxccc status/protocol data
x data
c channel address

Control codes always consist of only one byte. Code groups of more than one byte are never used. Therefore, control codes require little transmission capacity. Control codes are completely independent from previous or following control codes. This makes the protocol very robust. An exception to this rule is, as in the KISS protocol, the "start/end" command, which always belongs to a data packet, and the channel data itself. Because two bits are used to distinguish between channel data and control codes, only six bits per byte are available for channel data. Therefore, three data bytes have to be encapsulated into four "6PACKs", according to the following scheme. Note that a channel data byte does not correspond directly to a byte on the asynchronous serial interface.

1st 6PACK 2nd 6PACK 3rd 6PACK 4th 6PACK
Codes 00xx.xxxx 00xx.yyyy 00yy.yyzz 00zz.zzzz
Bits 54.3210 76.3210 76.5410 76.5432

The symbols "x", "y" and "z" with the corresponding bit numbers represent the channel data. This rather complex scheme was chosen since it requires the least possible shift operations to extract the channel data from the 6PACKs.

The fast transmission of real time events is achieved by using priority codes. The TNC for example transmits an appropriate command code at every change of the DCD line. Additionally, the TNC sends the DCD state about every second to allow recovering of a potentially lost command code.

The coding of the command codes is shown in Table 1. The code 0xC0 (1100 0000) is not used in 6PACK; this should prevent an accidentally connected KISS TNC from sending.

The data transmission between TNC and PC

The following simple rules govern the data transmission between the TNCs and the PC.

  • Channel data is always transmitted in whole packets. Every packet is enclosed with a pair of "start/end" commands. The channel number carried in the starting "start/end" command defines the source or destination of the packet. The first data byte carries the TX delay, either the TX delay that should be used by the transmitter, or the one measured by the receiver, depending on the data direction. The last data byte carries the checksum. It is adjustet so that the sum over the whole packet (including TX delay and checksum byte) plus the channel number add up to 0xFF (8bit two's complement arithmetic)
  • Priority commands must be processed before ordinary data. If a TNC receives a priority code with an address that does not match its own, it must immediately retransmit the command, even if it is just sending a packet.
  • A packet of channel data must not be interrupted by another packet of channel data. The TNC may only place its own packets onto the ring either before or after packets from other TNCs.

Each packet of channel data has the following structure:



Start/end of Frame
TX delay in units of 10ms
Channel data
Checksum byte

This structure results after extracting the data from the 6PACKs. The TX delay can be reconstructed only after the second 6PACK with channel data.

H­­­ ­­­L command direction
0100 0ccc start/end of frame PC <--> TNC
0100 1ccc TX underrun TNC --> PC
0101 0ccc RX overrun TNC --> PC
0101 1ccc RX buffer overflow TNC --> PC
011x yccc LED: STA=x; CON=y PC --> TNC
10xy zccc priority message:
x = TX counter + 1
y = RX counter + 1
z = DCD state

PC <--> TNC
TNC --> PC
TNC --> PC
1100 0000 not used (KISS FEND)
1110 0ccc send calibration pattern for 15s
calibration terminated
PC --> TNC
TNC --> PC
1110 1ccc TNC address PC <--> TNC

Table 1

Transmission sequence on the radio port

Transmission on the radio port take place according to the following scheme:

  1. The PC sends a "TX counter + 1" command to the TNC. The TNC then keys the transmitter (i.e. it activates the PTT line) and increments an internal counter. The PC also increments an internal counter associated with that TNC. This counter serves the PC as a PTT indicator.
  2. the PC sends the packet onto the ring
  3. As soon as the TX delay value arrives in the TNC, it decides whether enough time has passed already since the transmitter was keyed on. If not, keying up the transmitter is continued until the specified amount of time passed. When the next data byte arrives, the transmission begins, even if the whole packet has not arrived yet.
  4. If the TNC receives the "start/end" command and the received checksum was bad, it must send an ABORT sequence on the radio port.
  5. As soon as the TNC completes transmitting a packet on the radio port, it sends a "TX counter + 1" command to the PC and decrements its internal counter. Because every packet is preceded by a "TX counter + 1" command from the PC, the TNC always knows how many packets will follow in the current transmission. The transmitter must stay keyed until the last packet is sent.
  6. The PC receives the "TX counter + 1" command from the TNC and decrements its associated internal counter. If this counter reaches zero, then the transmission terminates.

The transmission of the channel data packets from the TNC to the PC takes place similarly, with the exception that the PC does not acknowledge a received packet. Every packet the TNC receives on its radio port is sent onto the ring, preceded by an "RX counter + 1" command.

Automatic configuration of the TNC addresses

Automatic configuration of the TNC addresses is achieved by the following simple scheme.

  1. The TNC receives an address command (1110 1ccc) and sets its own address to the value contained in the command (the ccc bits).
  2. The TNC increments the address contained in the command by one and retransmits the command.

Since every TNC on the ring behaves according to these rules, TNC addresses are allocated sequentially. The command transmitted by the last TNC and received by the PC contains the number of connected TNCs.

KISS versus 6PACK

6PACK provides some advantages over KISS, most notably the ability to transmit real time data and thus the ability to implement the channel access algorithm in the PC.

KISS achieves data transparency by prefixing some data bytes by an escape code. This means that the transmission capacity needed to transmit a data packet depends largely on the contents of the packet. In the worst case, twice the bandwith is needed. With 6PACK, the transmission capacity needed is exactly predictable, if one neglects the real time commands inserted into the data stream.

If one continues the comparison and compares 6PACK rings to KISS token rings, the advantages of 6PACK become even clearer. With a KISS token ring, the response time of the system depends on the loading of the ring. Since the token ring does not transmit DCD and PTT states, each TNC needs to implement its own channel access algorithm, and usually a rather primitive SlotTime/p-Persistence algorithm is used. Also, there is no timing relation between the receipt of a packet over the air and the arrival of the packet in the PC. The PC does not know when the packet is actually sent. (Note by the translator: When the FRACK timer expires and the PC retransmits a frame, the original frame may still wait for transmission, since the channel was busy all the time. Then the retransmission of the frame produces additional unnecessary load on an already congested channel)

This usually leads to the transmission of frames with outdated sequence numbers and to strange effects if one tries to implement a DAMA master on a KISS token ring. With high HDLC bit rates and high load on the ring, the transmitter may be interrupted between packets, which leads to a much increased collision probability.

6PACK provides no mechanisms which would allow the TNC to implement its own channel access. Therefore, the channel access has to be done in the PC. This is a significant advantage, especially on half duplex links.

Short response times, which are needed by, for example, a DAMA slave, cannot be achieved with KISS. The transmitter keying is always delayed by the time it takes to transmit the packet on the asynchronous serial interface.

With 6PACK, real time data and channel data are distinguishable. Therefore, the delay of real time data does not depend on the ring loading, and it can be calculated from the ring bit rate and the number of TNCs on the ring. Each TNC delays the byte by between 10 and 20 bits, which is 260 to 520 µs at 38.4kBit/s. The longest possible delay at 38.4kBit/s is therefore 8 * 520 µs = 4.2 ms.

This means that it does not take more than 4.2 ms to key the transmitter of a particular TNC, or to communicate the DCD state to the PC. This way, even a ring of TNCs may be controlled quite precisely, but the additional delay of a ring may increase the collision probability slightly on a half duplex channel.

A flaw of 6PACK is that the TX delay may be longer than requested. This happens every time a packet is delayed more than the requested TX delay compared to its preceding "TX counter + 1" command. However, measures should be taken to prevent the peer station from complaining about the too long TX delay.

Last, but not least: Unlike with KISS, the TNC LEDs can be controlled by the host PC directly. They are used just like the LEDs on the RMNC3 card.

Some remarks from the translator


Figure 1 shows the wiring of a TNC ring.

FlexNet allows different channel access algorithms to be used. It may use a modified p- persistence algorithm, DAMA, or OPTIMA in the future. FlexNet allows even multiple data rates and modulation schemes to share one radio channel. For example, combined 1200 Baud AFSK / 9600 Baud FSK user accesses can be found in Europe, since it is often not possible to get different frequencies for both modes, because the 70cm and 23cm bands are very crowded. In this case, a 1200 baud AFSK and a 9600 baud FSK modem are connected to the same transmitter in parallel. Thus the channel access algorithm spans multiple channels. In PC/FlexNet, the channel access for radio channels is done in the L2 kernel. So L1 drivers and 6PACK TNCs must not implement their own channel access algorithm.

TX delay is the time between keying the transmitter and starting to send the first packet. This time allows the radio to start the transmitter. HDLC flags are usually sent during this time. TX delay is specified in 10ms units. FlexNet also monitors the TX delay of the other stations on the channel, and complains if a station uses an excessively long TX delay (more than 100ms in excess to what the transmitter needs). PC/FlexNet L1 drivers and thus 6PACK TNCs should listen for a series of HDLC flags preceding a data packet and report the duration of this sequence in the TX delay byte of the received data packet. Note that only error free reception of the flags should be accepted. In the unlikely event that the hardware does not support the measuring of the TX delay, zero should be returned. Zero may also be returned for every packet other than the first of a particular transmission. If, due to overload on the ring, the required TX delay cannot be met, the sending station must make sure the peer does not complain about the long TX delay. This can be done by inserting an ABORT sequence, for example.

Note that for the sake of fast retransmission of priority commands, hardware FIFOs in the asynchronous serial interface should not be used.

The translator would like to thank Matthias Welwarsky DG2FEF and Gunter Jost DK7WJ for the assistance, and Esther Oswald for correcting this translation.

Related literature

[1] Karn, Phil KA9Q, and Mike Chepponis K3MC, The KISS TNC: A simple Host-to-TNC communications protocol, 1990

[2] Welwarsky, Matthias DG2FEF, DAMA und OPTIMA, TheFirmware Version 2.7, 1994