Page 153 - DCAP311_DCAP607_WIRELESS_NETWORKS

Unit 9: Wireless MAN Systems

What Elements Make up a Packet Station? Notes

Figure 9.4 shows an illustration of a typical station setup with a schematic diagram of a station
wiring.
Figure 9.4: Station Setup

TNC (terminal Node Controller)

A TNC contains a modem, a computer processor (CPU), and the associated circuitry required
to convert communications between your computer (RS-232) and the packet radio protocol in
use. A TNC assembles a packet from data received from the computer, computes an error check
(CRC) for the packet, modulates it into audio frequencies, and puts out appropriate signals to
transmit the packet over the connected radio. It also reverses the process, translating the audio
that the connected radio receives into a byte stream that is then sent to the computer.
Most amateurs currently use 1200 bps (bits per second) for local VHF and UHF packet, and 300
bps for longer distance, lower bandwidth HF communication.

Notes Higher speeds are available for use in the VHF, UHF, and especially microwave
region, but they often require special (not plug-and-play) hardware and drivers.

Computer or Terminal

This is the user interface. A computer running a terminal emulator program, a packet-specific
program, or just a dumb terminal can be used. For computers, almost any phone modem
communications program (i.e. Procomm+, Bitcom, X-Talk) can be adapted for packet use, but
there are also customized packet radio programs available. A dumb terminal, while possibly the
cheapest option, does have several limitations. Most dumb terminals do not allow you to scroll
backwards, store information, upload, or download files.

A Radio

For 1200/2400 bps UHF/VHF packet, commonly available narrow band FM voice radios are
used. For HF packet, 300 BPS data is used over single side band (SSB) modulation. For high speed
packet (starting at 9600 bps), special radios or modified FM radios must be used. 1200 bps AFSK
TNCs used on 2-meters (144-148Mhz) is the most commonly found packet radio.

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