Packet radio

Packet radio is a method for digital data transmission in amateur radio and CB radio . The digital information is sent out in short data packets (usually a maximum of 255 bytes ) and reassembled at the recipient. This enables computers to communicate with one another wirelessly and with automatic error correction .

Several intermediate stations can be specified between the data sender and receiver, which receive the packets and send them back unchanged ( digipeater ). As a result, large distances (several 100 km) can be bridged, especially if the digipeaters are in an exposed location. Intelligent network nodes are also able to decide over which sections a data packet must be optimally directed towards the recipient ( routing ).

A similar procedure is used for cell phones with GPRS .

Origin and history

The English term packet radio was coined in Tucson ( Arizona ) at the end of 1981 and has also established itself in German usage. A group of radio amateurs had come together to form TAPR (Tucson Amateur Packet Radio) and were planning to set up a local data network on amateur radio frequencies.

The history of Packet Radio goes back to the 1960s, when the various computers at the University of Hawaii , which were located on different islands, were linked by radio.

In the AMSAT different groups met by radio amateurs and laid a protocol established for data transmission. The main objective was to develop a standardized method for data transmission from and to the planned amateur radio satellite OSCAR 10 . The X.25 protocol had already been introduced for data transmission within the postal network ; in Germany it was common under the name Datex-P . So it made sense to use a similar method for amateur radio. The extended X.25 protocol, which meets all requirements for amateur radio operations, was specified and named AX.25 (A = amateur). AX.25 defines level 2 of the ISO-OSI model .

In 1983, TAPR developed a 'TNC1' computer card (TNC = Terminal Node Controller ) which could send and receive data in the AX.25 protocol. With the further development, the TNC2 on the basis of the Z80 processor, the rapid expansion of the packet radio operating mode began in Germany around 1985 .

In 1993 the TNC3S was developed in Germany on the basis of the 16/32 bit processor MC68302 , with which it was now possible to have two radio devices controlled by one computer at high transmission rates . The computing power of the TNC3S was even sufficient for setting up a packet radio network node (digipeater), so that the computer could be replaced as the central unit of the digipeater. This made the digipeaters very easy to maintain because hard drives no longer limited their service life. As the next step, the TNC4E was developed in Germany in 1997, which provided for the connection of three radio devices and had an integrated network interface for communication with computers and other TNC4Es.

As a cost-effective alternative to TNCs with their own microprocessor, the radio can be connected to a PC via a simple modem or sound card, which implements the digital functions of the TNC using a special program.

Since October 1994, the transmission of digital data has also been permitted on some of the Jedermannfunk (CB radio) channels and is used with the help of TNCs or modems.

In the meantime, the competition from the Internet is making itself felt. New impulses are emerging from converted W-LAN access points. Modified software as well as changed transmission frequencies are currently being tried out as part of the HAMNET project .

Radio links between the digipeaters made it possible to set up an international packet radio network. This network made it possible for radio amateurs to exchange private and personal messages free of charge long before this became common practice on the Internet . The packet radio network in Germany currently consists of several hundred digipeaters with a downward trend.

Since the packet radio network is mainly implemented via radio links, it works independently of the Internet and can therefore also be used for emergency radio operation, even if parts of the Internet fail or are overloaded.

As with every introduction of a new mode of operation in the amateur radio service, Packet Radio caused lively debates, especially among conservative radio amateurs. Similar to the introduction of single sideband modulation in radio communication in the 1960s, the end of amateur radio was also feared with the introduction of packet radio. The networking of some digipeaters with one another via the Internet ( VPN ) triggered the climax of these debates against packet radio. In the meantime, however, the discussion has calmed down.

Today's commercial radio data transmission applications such as GPRS and WLAN are based on the same procedures as the packet radio controllers used by radio amateurs , despite more advanced technology (such as frequency spreading ).

Events

The most important specialist event on the subject of packet radio in Germany is the Packet Radio Conference, which has been known as IPRT for several years - specialist conference for digital data transmission in amateur radio , which takes place every year at the beginning of April in Darmstadt. In addition, packet radio was a recurring topic at other amateur radio events such as Ham Radio or the FM conference in Weinheim. So-called sysop meetings take place again and again, especially between the operators of automatic packet radio stations, so-called digipeaters.

Packet radio operation

With the packet radio operating mode radio amateurs can exchange data with one another with their usual VHF radios. In order to increase the range, a network of digipeaters was set up nationwide in numerous countries. The data packets are passed on from digipeater to digipeater. Mobile stations can continuously broadcast their current location via packet radio via APRS .

The intercontinental connection to packet radio stations is established through gateways , which are packet radio digipeaters that convey messages between shortwave connections and the local VHF network. This way, messages from all over the world get into each local mailbox. A number of gateways can be reached via the Internet.

Up-to-date information on rare amateur radio stations on shortwave is distributed by DX clusters via the packet radio network.

For communication between amateur radio satellites and the ground stations, the packet radio protocol with FSK modulation (G3RUH) and 9600 or 38400 bit / s is mainly used.

The integration of Packet Radio in the global WinLink 2000 network enables the transmission of e-mails and information retrieval from the Internet. The target gateway only needs to have an Internet connection.

technology

TNC2400 packet radio modem

To operate a packet radio station you need

a simple computer
a commercially available amateur radio or CB radio device
a modem or a packet radio controller (especially often a Terminal Node Controller (TNC) ).

AFSK mode

Until 1988, transmission at 1200 baud (around 150 characters per second) was the standard. The frequency of a low-frequency auxiliary carrier was keyed between 1200 Hz and 2200 Hz (see AFSK ) and then modulated onto the actual high-frequency carrier using frequency modulation with a frequency deviation of less than 25 kHz .

The bandwidth of the low frequency carrier was about 3000 Hz.

Since the low-frequency carrier was demodulated at the receiver like a normal voice signal, this operating mode could be carried out without interfering with the receiver using a PC with a sound card and corresponding software .

Example of a 1200 baud transmission

There are only a few packet radio digipeaters that work with 1200 baud AFSK. With the emergence of the APRS operating mode, this technically simple transmission method is revived, especially in the 2-meter band. APRS is now also used in CB radio and has established itself there on 27.235 MHz (FM 24) with 1200 baud. Server connections are possible at cbaprs.de:27235 and cbaprs.at:27235.

FSK mode

In 1989, the British radio amateur James Miller (G3RUH) developed a modem to transmit data at 9600 baud (around 1200 bytes per second). The baseband signal generated in these modems is freed of constant components by a scrambler. The method was originally intended for communication with amateur radio satellites, but has now become the general standard for packet radio in amateur radio.

In contrast to the 1k2 mode, the high frequency is now keyed directly in frequency (see FSK ). One channel in a 25 kHz channel grid is sufficient for the radio signal.

In the receiver, the signal must be picked up directly at the demodulator in order to avoid phase or amplitude distortions in subsequent filters or amplifiers. Many modern receivers have their own data output for this purpose .

The transmission speed can be increased depending on the available bandwidth. In this way, data rates of several megabits per second are now being realized in amateur radio; speeds of up to 1.2 Mbit / s are common for experiments and for the connections between the network nodes. Experimental work is also carried out in the gigahertz range at speeds of up to around 10 Mbit / s using a multi-level FSK.

For user access in the UHF range (70 cm), speeds of 76.8 kbaud and more are sometimes used, which of course require a correspondingly wide channel. In the 70 cm band, two channels with a bandwidth of 200 kHz each are reserved for broadband radio data transmission.

Other transmission technologies

In some regions, experimental work was carried out with converted telephone modems. The achievable speed was around 20-30 kbit / s in full duplex mode. Experimental multi-stage modulation methods such as quadrature amplitude modulation (QAM) or quadrature phase shift keying (QPSK) have also been used, but due to the high technical requirements and costs, these have not yet been able to establish themselves in the packet radio sector.

literature

Phil Karn , Harry Price, Robert Diersing: Packet Radio in the Amateur Service. In: IEEE Journal on Selected Areas in Communications, 1985, pages 431-439.

Web links

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