each frame by an address. The AX.25 implementation of HDLC includes sender and destination station call-sign plus four-bit Secondary Station Identifier (SSID) value in range 0 through 15 in the frame address. At ITU WARC2003 the radio amateur station callsign specification was amended so that the earlier maximum length of six characters was raised to seven characters. However AX.25 has a built-in hard limit of six characters, which means a seven-character callsign cannot be used in an AX.25 network. AX.25 lacks an explicit port (or SAP); the SSID often assumes this role. Thus there can be only one service per AX.25 station SSID address, which is often kludged around with varying degrees of success. Some amateurs, notably Phil Karn KA9Q, have argued that AX.25 is not well-suited to operation over noisy, limited-bandwidth radio links, citing its lack of forward error correction (FEC) and automatic data compression. However, a viable widely adopted successor to AX.25 has yet to emerge. Likely reasons may include: a large existing deployment of recycled narrowband FM radios and especially existing APRS applications, easy availability of cheap, low-power FM transmitters, especially for the 430 MHz UHF band, to match existing legacy radio gear, new radio level modulations would need different radio gear than what is currently in use and the resulting system would be incompatible with the existing one thus requiring a large initial investment in new radio gear, adoption of newer line codings potentially including forward error correction takes more effort than the 1,200 bit/s AFSK of Bell 202. Previously sufficient small 8-bit microprocessors with 128 bytes of RAM would not be enough, and new ones might cost US$30 instead of US$3. Phil Karn did demo decoding of this new modulation of his by running it on a Pentium II machine some 10 years later, mid-level embedded microprocessors are capable enough to do the same with under US$50 system cost. Despite these limitations, an extension to the AX.25 protocol, supporting forward error correction, has been created by the TAPR. This extension is called FX.25. Small gadget transmitters do not need to know what is being transmitted. There is only a need to monitor channel occupation by radio receiver RSSI (Received Signal Strength Indication) to know when not to send. Transmitting interleaved Reed-Solomon FEC signal in some smart modulation needs a lot fewer resources than reception of the same signal, thus a sufficient microprocessor might cost just