We have already written about the modulator/demodulator, commonly called modem. In digital radio transmissions Hams use a TNC (Terminal Node Controller), an interface between computer and transceiver (RTX), which performs the coding and decoding of messages in packets. The TNC contains a modem, which transforms logic signals into audio signals. Obviously to communicate with the TNC (through a serial port RS232) a PC will need a terminal emulator program.
The TNC is essentially an intelligent modem; inside of it you can find a CPU, an EPROM that contains the working program, and some RAM memories. The CPU is normally a Z80 type processor, its power being similar to the one of the old ZX Sinclair Spectrum. The TNC managing program is devoted only to control transmission errors; the PC does all the rest.
In its simpler version a TNC resembles externally to a telephone modem; and effectively inside of it we can find many parts in common: it is connected with the PC through a classic RS232, exactly like an external modem. The difference with a traditional modem is essentially in its analogic interface, which instead of a couple of telephone wires is made of a microphone (to send data), a loudspeaker or headphones (to receive data), and in the command to switch the radio between sending and receiving (the PTT or Push To Talk).
The most sofisticated and recent TNCs are able to manage more radios simultaneously, and are real communication controllers; this means that they are able to demodulate and transmit every kind of digital signals, using all kinds of Ham codes (telegraphy or CW, radio-fax, RTTY, AMTOR, etc.)
About their speed, first TNCs were born with 1200 baud speed and are presently the more widespread; nowadays are common even 9600 and 19200 bauds. TNCs hardware and firmware perform a "safe" connection, that is virtually error free, between two data terminals, normally two computers. The implemented protocol is the syncronous HDLC, the one used in X25. The connection in AX25, as in its predecessor X25, is performed through a virtual circuit, which is an abstraction of the physical one (electric line or radio channel) used only by the ones who are physically at its endings. In a few words the protocol gathers data in "packets" labeled with adresses both of sender and receiver; packets are sent at fixed times and validated by the receiving station.
Thanks to the address, even if more stations are on the same radio channel, every one ignores the traffic not related to it, behaving like (apart of a certain delay) if it was connected only to its corresponding station, through the virtual circuit. But TNC functions are not only these: as the radio channel is always usable, TNC can manage radio station without the operator being present. For this reason firmware includes some sofisticated functions which are not present in traditional modems, and that remain active even if the PC connected to the TNC is switched off. To the main protocol managing function are thus added other functions like: mailbox, digipeater, node, gateway.
The MAILBOX works like a small BBS (Bullettin Board System): if a Ham calls and we are not at home, the TNC will answer with a welcome message and will permit, through a command menu, to read or send messages, even to other Hams. You will have a digital post-box always on line.
The DIGIPEATER (digital repeater) is the one that performs packet hopping between stations, to enormously widen the operating range of the transmitter. Whether we are or not operating on the terminal, another Ham can use us as digipeater almost transparently. This function can of course be disabled.
An evolution of the digipeater is the NODE,
which is not limited to only resend packets, but allows connected stations to
operate as if being there, using all the resources of the remote station.
The GATEWAY is a particular kind of node that operates on two frequencies simultaneously, allowing the packets to jump from one frequency to the other, or from a band to another (i.e. HF to VHF or VHF to VHF).
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