Editing Packet radio (section)

=== Physical ===
{{ listen
| filename = Packet-radio-simple-stardado.ogg
| title    = Example of packet radio transmission in 1,200 bits per second
}}

Modems used for packet radio vary in throughput and modulation technique, and are normally selected to match the capabilities of the radio equipment in use.
Most commonly used method is one using audio frequency-shift keying ([[AFSK]]) within the radio equipment's existing speech bandwidth.
The first amateur packet radio stations were constructed using surplus [[Bell 202]] 1,200 [[bit/s]] modems, and despite its low data rate, Bell 202 modulation has remained the standard for [[VHF]] operation in most areas. More recently, 9,600 bit/s has become a popular, although more technically demanding, alternative. At [[High frequency|HF]] frequencies, [[Bell 103]] modulation is used, at a rate of 300 bit/s.

Due to historical reasons, all commonly used modulations are based on an idea of minimal modification to the radio itself, usually just connecting the computer's audio output directly to the transmitter's microphone input and receiver's audio output directly to the computer's microphone input. Upon adding a ''turn the transmitter on'' output signal ("PTT") for transmitter control, one has made a ''radio modem''.
Due to this simplicity, and just having suitable microchips at hand, the ''Bell 202'' modulation became standard way to send the packet radio data over the radio as two distinct tones. The tones are 1,200&nbsp;Hz for Mark and 2,200&nbsp;Hz for space (1,000&nbsp;Hz shift). In the case of ''Bell 103'' modulation, a 200&nbsp;Hz shift is used. The data is differentially encoded with a [[NRZI]] pattern, where a data zero bit is encoded by a change in tones and a data one bit is encoded by no change in tones.

Ways to achieve higher speeds than 1,200 [[bits/s]], include using telephone modem chips via the microphone and audio out connectors. This has been proven to work at speeds up to 4,800 bit/s using fax [[ITU-T V.27|V.27]] modems in half-duplex mode. These modems use [[phase-shift keying]] which works fine when there is no [[amplitude-shift keying]], but at faster speeds such as 9,600 bit/s, signal levels become critical and they are extremely sensitive to [[group delay]] in the radio. These systems were pioneered by [[Simon Taylor (photographer / author)|Simon Taylor]] (G1NTX) and Jerry Sandys (G8DXZ) in the 1980s. Other systems which involved small modification of the radio were developed by James Miller (G3RUH) and operated at 9,600 bit/s.

1,200 bit/s AFSK node controllers on 2 meters (144–148&nbsp;MHz) are the most commonly found packet radio. For 1,200/2,400 bit/s UHF/VHF packet radio, amateurs use commonly available narrow band FM voice radios. For HF packet, 300 bit/s data is used over single sideband ([[Single-sideband modulation|SSB]]) modulation. For high speed packet (9,600 bit/s upwards), special radios or modified FM radios must be used.

Custom modems have been developed which allow throughput rates of 19.2&nbsp;kbit/s, 56&nbsp;kbit/s, and even 1.2&nbsp;Mbit/s over amateur radio links on FCC permitted frequencies of 440&nbsp;MHz and above. However, special radio equipment is needed to carry data at these speeds. The interface between the "modem" and the "radio" is at the ''intermediate frequency'' part of the radio as opposed to the audio section used for 1,200&nbsp;bit/s operation. The adoption of these high-speed links has been limited.

In many commercial data radio applications, audio baseband modulation is not used. Data is transmitted by altering the transmitter output frequency between two distinct frequencies (in the case of FSK modulation, other alternates exist).

The 2.4&nbsp;GHz "Wi-Fi" band partially overlaps an amateur radio band, so commercial Wi-Fi hardware can be adapted and used by licensed amateur radio operators at higher power levels, although restrictions on amateur radio limit the appeal of using packet radio to connect to the internet. US FCC regulations do not allow amateur radio communications to be encrypted or private, in addition to other content restrictions.<ref>Security & Data Integrity on a Modern Amateur Radio Network – By: Paul J. Toth – NA4AR "HSMM and Information Security," by K8OCL CQ-VHF Fall 2004 – preview via CQ-VHF website "Data Encryption is Legal," N2IRZ, CQ Magazine Aug 2006 – preview from the Summer 2006 TAPR PSR http://www.arrl.org/files/file/About%20ARRL/Committee%20Reports/2004/July/HSMM.pdf</ref>

{{see also|High-speed multimedia radio}}