other functionality as well, such as a simple bulletin board system to accept messages while the operator is away. Following the OSI model, packet radio networks can be described in terms of the physical, data link, and network layer protocols on which they rely. Physical layer: modem and radio channel 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 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 Hz for Mark and 2,200 Hz for space (1,000 Hz shift). In the case of Bell 103 modulation, a 200 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 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 (G1NTX) and Jerry Sandys (G8DXZ) in the 1980s. Other