PACKET - Robust PACKET
PACKET, also known as FSK300, AX25 or X25, is a packet based protocol derived from AX.25 and HDLC computer network protocols. Packet radio is a synchronous system in which data is transmitted in ASCII character packets.
There are a number of variants that exist. For HFHigh Frequency (3-30 MHz), there is PACKET 300, and for VHFVery High Frequency (30-300 MHz), there are a number of variants, although the most common is PACKET 1200 and PACKET 9600.
Teletype machines were first developed in the 1930’s. As soon as they became available on the surplus market, hams began to use them on the air.
It wasn’t difficult to get started. You just needed to add a simple modem which was called a “terminal unit.”
It was not a efficient way to send text. First you would turn on the transmitter and it would sit there transmitting a 100% duty cycle carrier. The frequency would shift by 170 HzHertz (Hz), unit of frequency, defined as one cycle per second (1 Hz). in a certain pattern for each key pressed. There was no error detection so you would often see garbled messages.
It used a 5 bit code, commonly known as “baudot.” This allows only 32 different combinations, not enough for all letters and digits. Some of the codes were for control functions like carriage return, line feed, or bell. 26 of the codes were for letters. A “shift” control code was used to get digits or special characters instead of letters. If noise clobbered the shift or unshift code, you would see digits and special characters instead of letters or vice versa.
In 1978 Canadian hams began experimenting with a much different method of sending data over the air. Rather than keeping the transmitter on and sending one character at a time, it was sent in a short package (or “packet”).
Each transmission contained:
- Source address. (e.g. ham callsign)
- Destination address.
- Optional repeater addresses.
- Control / protocol bytes.
- Information part.
- Error checking (FCS, CRC).
With RTTYRadio TeleTYpe we had only a simple modem to convert between digital data and audio tones. When a key was pressed the corresponding code was sent immediately.
We now have some brains in between the terminal and modem to implement the protocol used over the air. This device is called a Terminal Node Controller (TNC).
The operator could take a while to compose a message but the complete message gets sent in a short burst.
This new approach offers many advantages.
- Rather than tying up a radio channel with one person typing a character at a time, a message could be sent in a short burst.
- Each packet contains information about where it came from and where it should be going.
- This allows many people to share one frequency.
- Error detection provided confidence that the data was not corrupted.
- The TNC would send acknowledgements when data was received correctly an perform retries when it doesn’t get thru the first time.
- Data was not limited to printable characters. You can send files such as JPEG images.
American ham radio operators had a disadvantage. The FCC did not allow the ASCII code to be used over ham radio until 1980. Special permission was required to perform the same type of experimentation as the Canadians.
The Vancouver Amateur Digital Communications Group made their design available as a bare printed circuit board. It was necessary to gather up all the other parts which discouraged most.
In 1983, Tucson Amateur Packet Radio (pronounced tapper) introduced their TNC-1 kit which made it a lot easier.
- All parts including modem and power supply.
- Documentation about 2 inches thick.
- $350 not including case.
They threw a lot of hardware at the problem. About 27 integrated circuits. This was later available as the Heathkit HD-4040.
The TNC-2 came along a couple years later. It was smaller and cheaper. MFJ and others produced products based on this design. For a while, everyone was churning out new TNC products.
Today, the 20th Century TNCs are pretty much extinct. You can now get better results and more features at lower cost by connecting your radio to the “soundcard” interface of a computer (e.g. Raspberry Pi) and using software, such as Dire Wolf, to decode the signals.
PACKET-300 uses FSKFrequency-Shift Keying modulation with a 200Hz shift and a 300 BdBaud (unit symbol Bd) is the unit for symbol rate or modulation rate in symbols per second. symbol rate (seldom seen with 600 BdBaud (unit symbol Bd) is the unit for symbol rate or modulation rate in symbols per second.). On amateur frequencies above 30 MHzMegaHertz (MHz) 10^6 Hz, higher speeds such as 1200 and 9600 baudBaud (unit symbol Bd) is the unit for symbol rate or modulation rate in symbols per second. are typically used. In the US, it is illegal for hams to use speeds above 300 baudBaud (unit symbol Bd) is the unit for symbol rate or modulation rate in symbols per second. on frequencies below 28 MHzMegaHertz (MHz) 10^6 Hz due to FCC regulations.
PACKET-1200 uses FSKFrequency-Shift Keying modulation with a 1000Hz shift and 1200 BdBaud (unit symbol Bd) is the unit for symbol rate or modulation rate in symbols per second. symbol rate. There are a number of variations that exist of PACKET-1200, including a PSKPhase-Shift Keying-based satellite version. PACKET-1200 can be seen in the VHFVery High Frequency (30-300 MHz) and UHFUltra High Frequency (300-3000 MHz) bands with indirect FMFrequency Modulation Modulation. FMFrequency Modulation bandwidth is 12 kHzKiloHertz (kHz) 10^3 Hz
PACKET-2400 and PACKET-4800
These are just some more variants of PACKET that operate on 2400Bd and 4800Bd.
PACKET-9600 uses GFSKGaussian Frequency-Shift Keying with a 4800 HzHertz (Hz), unit of frequency, defined as one cycle per second (1 Hz). and 9600 BdBaud (unit symbol Bd) is the unit for symbol rate or modulation rate in symbols per second. symbol rate. PACKET-9600 can be seen in the UHFUltra High Frequency (300-3000 MHz)-SHF band. FMFrequency Modulation Bandwidth is 15 kHzKiloHertz (kHz) 10^3 Hz
|PACKET 1200||PACKET 1200|
| 1200 Zero Crossing
| 1200 Zero Crossing
| 1200 PSKPhase-Shift Keying Satellite
| 1200 FX469 |
PACKET 2400 and PACKET 4800 Variants
| PACKET 2400 FX469
| PACKET 4800 FX469 |
Another Packet 9600 sample:
- Hobby Level Software
- Professional Equipment/Software
- Hoka 300-32
- Wavecom W-Code
- WinRadio Advanced Digital Suite
- WinRadio Universal FSK Decoder
- HF Digital "Tower of Babel"
- RadioRaft PACKET
- WAVECOM Database: PACKET 300
- WAVECOM Database: PACKET 1200
- WAVECOM Database: PACKET 9600
- shoc Database: PACKET 300
- MultiPSK PACKET