Monday, May 19, 2008

Digital Mode of the Week: PACTOR

PACTOR® (from Latin "the mediator," also a possible play on PACket plus amTOR) was developed by German hams in the early 1990s. It was originally intended to deal with the limitations of packet radio and AMTOR over noisy and fading HF circuits. It has become something of a de facto standard for HF e-mail systems, not only in amateur bands but also in commercial networks used by ships at sea and by nongovernmental organizations working in isolated areas.

PACTOR was originally based on the clever idea of using the good features of HF packet (robust error checking) and of AMTOR/SITOR (tight sync, short packet lengths), while eliminating the bad ones. The data bursts are longer than SITOR's, greatly reducing the timing demands on equipment. The protocol is better suited to HF than AX.25 packet, meaning fewer retries. Much of the time, PACTOR outperforms both modes in real-world band conditions.

The original mode is called PACTOR-I (Roman numeral one). The company has been speeding it up and adding features ever since, to the point where PACTOR-I is now rather slow and primitive by comparison.

PACTOR-I is a half-duplex, synchronous, ARQ mode which uses connections. The initiating station operates in "master" mode, while the called station is the "slave." Stations then take turns as information sending and receiving stations. The receiving station does a Cyclic Redundancy Check (CRC) on the packets, and transmits a brief ACK/NAK Control Signal (CS). In order to speed things up, a system called "memory ARQ" is used to compare packets and reduce repetition. In the original SCS equipment, this feature used an extremely clever analog algorithm.

There is also a PACTOR-I FEC mode. This does not use connections, but something resembling the "unproto" mode in packet. Frames are repeated and padded out with character 21 if nothing is in the send buffer.

PACTOR-I packet lengths are 96 bits in the slow mode (100 baud) and 192 at 200 baud. The modems are able to choose the appropriate data rate based on error responses from the receiver. PACTOR uses online data compression to further speed up throughput. Text is ASCII using Huffman coding (which prints as garbage unless decoded), falling back to straight ASCII when necessary or for calling.

PACTOR-I modulation is frequency-shift keying (FSK/AFSK), 200-Hertz shift. Like packet, the bits are in the state transitions, so it can be successfully tuned in either USB or LSB without changing polarity. For this reason, amateur mailboxes often list PACTOR frequencies by their center of intelligence, between the two original FSK tones, which then are +/- 100 Hz. LSB dial/window reading will then be the center frequency plus the audio center of your modem or software. On USB, you subtract the audio center.

PACTOR-I has been released for use in any 3rd party products, including modems and multimode sound card programs. It can be considered a standard. Everything else, however, remains completely proprietary to SCS, the German company started by PACTOR's inventors, or its licensees. The company has also worked with large commercial networks such as Globe Wireless to adapt PACTOR into even more proprietary modes.

SCS modems are high-end products for professional use, and they are priced accordingly. There has been some criticism from hams that these prices are a bit out of reach for amateurs. SCS has answered with a simplified PACTOR modem, the PTC-IIex, that lists for "only" 614 Euros as opposed to 1025 Euros for the full-featured version. Of course, either of these prices is a steal compared to the staggeringly expensive WAVECOM and HOKA multimode packages that will do all PACTOR's modes.

Out in the real world, PACTOR-I is used mostly for calling. Some people are still reporting traffic in it, but otherwise you'll be able to tell it's PACTOR and grab a callsign or two, then things will get weird in a hurry as the modems adapt. They'll start to switch quickly through a truly bewildering number of highly advanced modes that remain available only in boxes made or licensed by SCS.

PACTOR-II adds several more compression and coding features, and switches the modem to differential phase-shift keying (DPSK) to save spectrum. The sound changes from the well known lazy brrrrrrp brrrrrrp brrrrrrp to various hisses and buzzes usually otherwise heard in advanced military modes. Throughput increases from 100/200 baud to a best case 1200 bits/sec using compression.

The current hot setup is PACTOR-III, which adds yet more features to the firmware in existing SCS modems. Users can try these features for 20 connects, then a license is required. This one goes at a screaming best case throughput of 5200 bits/sec, though in doing so it becomes very wide indeed (2.4 kHz), with 18 tones and a physical bitrate of 3600/sec.

Here's a list of PACTOR modulations:

FSK, 200 Hz, 100/200 baud

PACTOR-II (uncompressed)
2 tone DBPSK, 200 b/s physical, 100 b/s throughput
2 tone DQPSK, 400 b/s, 200
2 tone 8-DPSK, 600 b/s, 400
2 tone 16-DPSK, 800 b/s, 700

PACTOR-III (uncompressed)
2 tones, 200 b/s physical, 76.8 net data rate
6 tones, 600 b/s, 247.5
14 tones, 1400 b/s, 588.8
16 tones, 3200 b/s, 2039.5
18 tones, 3600 b/s, 2722.1

Pactor-III also has many submodes depending on various combinations of tones and DBPSK vs DQPSK.

In all modems, the maximum speed level can be set by the user.

Legal note: PACTOR® is a registered trademark of SCS, Germany.