A short presentation of D-STAR (Digital Smart Technologies for Amateur Radio)

D-STAR (Digital Smart Technologies for Amateur Radio) is a digital voice and data protocol specification developed as the result of research by the Japan Amateur Radio League (JARL) to investigate digital technologies for Amateur Radio in 2001. While there are other digital on-air technologies being used by amateurs that were developed for other services, D-STAR is one of the first on-air protocols to be widely deployed and sold by a major radio manufacturer that is designed specifically for amateur service use.

D-STAR transfers both voice and data via a data stream over the 2 meter (VHF), 70 cm (UHF), and 23 cm (1.2 GHz) Amateur Radio bands either simplex or via repeater. One of the interesting features about the D-STAR protocol is the fact the system uses Amateur Radio call signs not only as an identifier, but for signal routing.

In the most common configuration, the most vital part of a D-STAR system is the gateway server, which networks a single system into a D-STAR network via a trust server. The trust server provides a central, master database to look up users and their  associated system. Allowing Amateur Radio operators to respond to calls made to them, regardless of their location on the D-STAR Network. While almost all documentation references the Internet as the connection point for a network, any IP network connectivity will work, depending on signal latency. Additionally, D-STAR can provide a zero infrastructure support system utilizing point-to-point “backbone” 10 GHz connections. Currently, the global D-STAR trust server is maintained by a group of dedicated D-STAR enthusiasts from Dallas, Texas — the Texas Interconnect Team.

The D-STAR protocol specifies two modes; Digital Voice (DV) and Digital Data (DD). In the protocol, the DV mode provides both voice and low speed data channel on 2 meters, 70 cm and 23 cm over a 4800-bit/s data stream. In the protocol, the DV mode uses a data rate of 4800 bit/s. This data stream is broken down to three main packages: voice, forward error correction (FEC) and data.

The largest portion of the data stream is the voice package, which is a total of 3600 bits/s with 1200 bits/s dedicated to forward error correction, leaving 1200 bit/s for data. This additional data contains various data flags as well as the data header, leaving about 950 bit/s available for either GPS or serial data. This portion of the data stream does not provide any type of error  correction, which has been overcome by implementing error correction in the application software.

While there are various techniques of encoding and transporting a DV signal, the focus of D-STAR’s design was the most efficient way to conserve RF spectrum. While D-STAR’s "advertised" occupied bandwidth is 6.25 kHz, tests reveal a band plan of 10 kHz spacing is adequate to  incorporate the D-STAR signal as well as provide space for channel guards.

In addition to DV mode, the D-STAR protocol outlines the high speed Digital Data (DD) mode. This higher speed data, 128 kbit/s, is available only on the 23 cm band because it requires an  advertised 130 kHz bandwidth, only available at 23 cm in world-wide band plans. Unlike the DV mode repeaters, the DD mode module operates as an "access point" operating in half duplex, switching quickly on a single channel. As with the DV mode, there is a portion of the data stream used for signal identification with the data header as well as various system flags and other D-STAR related items. Once this portion of the data stream is taken into consideration, the 128 kbit/s is reduced to approximately 100 kbit/s — still more than double a dial-up connection speed with significant range.

Another consideration is the data rate specified at 128 kbit/s is the gross data rate. Therefore, the system developers are challenged by the area coverage/potential user issue. Meaning the higher the elevation of the system, the more potential users and the slower the system will become as all the users split the data bandwidth.

Finally, there is an issue from the days of packet radio. While technically, the opportunity for "hidden transmitter" issues does exist and collisions do occur, the T/R switching is very fast and this effect is handled by TCP/IP as it is for WiFi access points.

The simplex channel eliminates the need for duplexers at a repeater site if only the DD mode system is installed. It is still recommended to have filtering, such as a band-pass filter, in place to reduce possible  interference from other digital sources close to the 23 cm band as well as reduce RF overload from nearby RF sources. While some DD Mode system owners would like more sensitivity or more output power (10 W), at the time of print, no manufacturer has developed pre-amps or RF power amplifiers with an adequate T/R switching time to boost the signals.

Radios currently providing DV mode data service use a serial port for low-speed data (1200 bit/s), while the DD mode radio offers a standard Ethernet connection for high speed (128 kbps) connections, to allow easy interfacing with computer equipment. The DD-mode Ethernet jack allows two radios to act as an Ethernet bridge without any special software support required. This allows standard file sharing, FTP, TELNET, HTTP/ Web browsing, IRC chat or even Remote Desktop Connections to function as if connected by wire.

In a Gateway configuration, ALL users must be registered in the network. This provides the DD mode sysop a layer of authorization, meaning that if someone wants to use a DD-mode system, and they have not received authorization to use the gateway, their DD mode access will be denied. Any gateway registered user, on the common network, can use any DD-mode system, even if the  registration was not made on that system. While we are not able to use encryption in the Amateur Radio service, security can be implemented in standard software or consumer routers and firewalls.

A D-STAR repeater system consists of at least one RF module and a controller. While any combination of RF modules can be installed, typically a full system includes the three voice modules (2 m, 70 cm and 23 cm) and the 23 cm DD mode module.

A computer with dual Ethernet ports, running the Gateway software is required for Internet access to the global network. An additional server, as shown in the diagram, can be incorporated for local hosting of e-mail, chat, FTP, Web and other services. In a D-STAR system installation, the standard repeater components (cavities, isolators, antennas, etc.) are not shown but are required as with any analog system. Some groups have removed analog gear and replaced it with D-STAR components on the same frequency with no additional work beyond connecting the power and feed lines.