Pactor Software Modem

[Vannessa Rataj]

PACTORis a radio modulation mode used by amateur radio operators, marine radio stations, military or government users such as the US Department of Homeland Security, and radio stations in isolated areas to send and receive digital information via radio.[citation needed]

PACTOR is an evolution of both AMTOR and packet radio;[1] its name is a portmanteau of these two technologies' names.PACTOR uses a combination of simple FSK modulation[citation needed], and the ARQ protocol for robust error detection and data throughput.[citation needed] Generational improvements to PACTOR include PACTOR II, PACTOR III, and PACTOR IV which are capable of higher speed transmission. PACTOR is most commonly used on frequencies between 1 MHz and 30 MHz.[citation needed]

PACTOR was developed in order to improve the reception of digital data when the received signal was weak or noisy.[1] It combines the bandwidth efficiency of packet radio with the error-correction (CRC) and automatic repeat request (ARQ) of AMTOR. Amateur radio operators were instrumental in developing and implementing these digital modes.[citation needed]

PACTOR radio equipment consists of an HF transceiver, a computer and a terminal node controller.[citation needed] Software running on the computer drives the terminal node controller.[citation needed] The most commonly used amateur program for this purpose is Airmail.[citation needed]

PACTOR is used by Amateur Bulletin board system operators to exchange public messages, and open conversations across the world.[citation needed] It is also used by the NTSD (digital) portion of the ARRL's National Traffic System (NTS) to pass digital ARRL Radiograms.[citation needed] Newer PACTOR modes are used to transfer large binary data files and Internet e-mail, particularly via the Winlink global e-mail system.[citation needed]

PACTOR utilizes very rapid time-division duplexing, giving PACTOR communications its characteristic cricket-like chirping sound when listened through a single-sideband (SSB) receiver.[clarification needed]

Depending on the version of PACTOR protocol used and the radio-frequency conditions, PACTOR transmission speeds range from 20 to 5200 bits per second (bit/s; net rate) or 9000 bit/s gross rate utilizing speed 10 (32-QAM).[6][7]

A robust network of PACTOR stations has been established to transfer data between radio stations and the Internet, extending Internet access to sea-based and other isolated users, led by volunteers involved with Winlink, under the auspicies of ARSFI (a Florida-based non-profit organization).[13]

Pactor modes other than level 1 (P1) are not open source,[14][15] but are publicly documented[16] and can be monitored and decoded easily over the air by third parties using free Raspberry Pi software ("PMON for Raspberry Pi")[17] or PMON utility on the modem itself.[18]

Go wireless between your PC and PTC modem with Bluetooth! A Bluetooth enabled PC and the PTC-IIIusb-BT gives full modem functionality from up to 10 meters without wires with its Class II Bluetooth transceiver. The PTC-IIIusb-BT comes standard with PACTOR III.

PACTOR-III is a software upgrade for existing PACTOR-II modems that provides a new data transmission mode for improved speed and robustness. PACTOR-III is not a new modem or hardware device. Most current PACTOR-II modems are upgradeable to use PACTOR-III via a software update since PACTOR-II firmware accommodates the new PACTOR-III software. Both the transmitting and receiving stations must support PACTOR-III for end-to-end communications using this mode.

On an average channel, PACTOR-III is over three times faster than PACTOR-II. On good channels, the effective throughput ratio between PACTOR-III and PACTOR-II can exceed five. PACTOR-III is slightly more robust than PACTOR-II at their lower SNR edges.

The ITU emission designator for PACTOR-III is 2K20J2D. Because PACTOR-III builds on PACTOR-II, most specifications like frame length and frame structure are adopted from PACTOR-II. The only significant difference is PACTOR III's multi-tone waveform which uses up to 18 carriers while PACTOR-II uses only two carriers. PACTOR-III's carriers are located in a 120 Hz grid and modulated with 100 symbols per second DBPSK or DQPSK. Channel coding is also adopted from PACTOR-II's Punctured Convolutional Coding.

The calling modem uses the PACTOR-I FSK connect frame for compatibility. When the called modem answers, the modems negotiate to the highest level of which both modems are capable. If one modem is only capable of PACTOR-II, then the 500 Hz PACTOR-II mode is used for the session. With the MYLevel command a user may limit a modem's highest mode. For example, a user may set MYL to "1" and only a PACTOR-I connection will be made, set to "2" and PACTOR-I and II connections are available, set to "3" and PACTOR-I through III connections are enabled. The default MYL is set to "2" with the current firmware and with PACTOR-III firmware it will be set to "3". If a user is only allowed to occupy a 500 Hz channel, MYL can be set to "2" and the modem stay in its PACTOR-II mode.

PACTOR-III specifications are available from the International Telecommunication Union Publications Sales in Recommendation ITU-R 1798, "Characteristics of HF radio equipment for the exchange of digital data and electronic mail in the maritime mobile service," starting on page 40 of the document.

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I've spent several weeks evaluating the on-air performance of VARA and Pactor-III under various conditions. The variability of the HF channel can skew the results when comparing modems this way so the data was compiled from an average of several test runs before a comparison was made. The tests were conducted minutes apart in alternate succession with modem run times of 5 minutes or more.

Speed calculations were taken from Winlink sessions that include the total session speed and file transfer speed in BPM. Tests were conducted on the 40 and 80 meter bands where multi-path and noise tend to be at their worse. The plots below, which were compiled from more than 60 hours of on-air test data, reveal some interesting results.

The first plot shows that VARA can exceed Pactor-III speeds by a considerable margin under the best channel conditions. They also show that the performance gap falls off to the point where Pactor-III eventually becomes the faster mode as conditions deteriorate.

The next plot shows the results for the 80 meter band. Once again, Vara shines under the best conditions while Pactor-III tends to do better with under worse conditions. As expected, the additional noise and multi-path result in slower overall speeds for both modems. Under the best conditions, speeds on 40 meters were about 1.8 times faster than those on 80 meters.

The next plot reveals a more detailed look at the poor signal performance between the two modems. Pactor-III tends to be the faster mode when subjected to low signal-to-noise ratios which would suggests that the low-speed Pactor modes tend to be more robust.

The next plot is more interesting from a practical point of view. It depicts the total Winlink session time i.e., the time it takes to complete a session from start-to-finish. Pactor-III took less time than Vara under the worse conditions while Vara completed sessions in less time than Pactor-III once conditions improved.

It would be interesting to see how the on-air data compares to the more accurate data that's possible using the controlled environment of a path simulator. If anyone knows of such tests, please pass them my way.

Ferrites: It is important (with any radio installation!) that ferrite chokes be installed at each end of both cables from the modem to radio, and also at both ends of the USB cable from computer to modem (does not apply with a bluetooth connection). The ferrites should be located near each end, use a small cable-tie to prevent them from sliding down the cable. It is also a good idea to add a cable-tie around the ferrite itself to make sure it stays closed. The recommended ferrites are Type-31 material from Fair-Rite.

What this means is that once GPS is connected to the radio for DSC, Airmail can update your lat-lon whenever Terminal window is opened, and Position Reports are updated automatically. Multiple formats are available including MarineTraffic to continue your AIS track offshore, CruisersCafe for Yotreps-style updates, and Sailblogs for a comprehensive tracking/blogging site.

Modems connected to the computer via a USB connection will require software drivers. For the SCS PTC-IIusb, PTC-IIIusb, DR-7400 and DR-7800 Pactor modems, a USB (type-B) connection is provided and the Airmail software installer also installs the required SCS version 2.08 drivers. In most cases this is all that is needed.

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