Summer 2009 Issue

Digital Radio

Digital Connectivity:
It’s About the Network

By Mark Thompson, WB9QZB


Until the early 1980s operating the digital modes meant making a contact using RTTY on HF or VHF. In the early 1990s other data modes, such as AMTOR and PACTOR, using multi-mode controllers, were implemented by companies such as AEA, Kantronics, and MFJ. In the late 1990s new digital data modes—PSK, for instance—began to be implemented using software and sound-card technology. What all of these data modes have in common is that they’re simplex half-duplex based and require no other network technology in order to use them.

Packet radio was introduced in the early 1980s; this was the first time a digital network was implemented in ham radio. Every packet TNC (Terminal Node Controller) was a digipeater that could repeat a digital packet of information. In the early days of packet it became common to digipeat through several TNCs. During band openings it was often a challenge to see how far away you could digipeat and either connect to a remote TNC or BBS (Bulletin Board System). As packet radio became more popular, packet frequencies became very congested, with packets often colliding with one another. Digipeaters often were placed at high locations with the expectation of extending the range of digipeating. Unfortunately, these high-location digipeaters caused collisions called the hidden transmitter effect. The hidden transmitter effect occurs when not everyone on a frequency can hear one another, resulting in TNCs transmitting at the same time with packets colliding with each other causing a reduction of overall throughput. Consequently, high-location half-duplex digipeaters often increased collisions and reduced overall throughput on a frequency. In some areas, full-duplex FM repeaters were put in place to eliminate the hidden transmitter effect, since all packet TNCs could hear all others on a repeater, resulting in the elimination of packet collisions.

Initially, packet radio was primarily a keyboard-to-keyboard activity. Bulletin Board Systems similar to dial-up BBSes soon were developed to allow users to send messages to one another. The ability to send messages from your local BBS to remote BBSes was added, and a message addressing scheme developed to support sending messages worldwide.

Since packet radio grew and evolved before the advent of the internet, a large terrestrial backbone network and HF backbone network was developed in response to the need to route messages between BBSes. Using packet radio on HF was not very efficient, but it did work. Packet radio network backbones were developed to allow a more efficient connection to a remote network node and to allow BBSes to route messages to one another more quickly and over longer distances. Many of the terrestrial backbone networks operated at a higher speed, 9600 baud, than regular packet radio, which operated at 1200 baud.

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