D-STAR Fundamental Differences with traditional VOIP Repeater Linking

 A response to a question from K2AAU on DSTAR_DIGITAL at Yahoo! Groups


There is a fundamental difference between D-STAR and the VOIP systems like Echolink, IRLP, WIRES, AllStar/Tiara, ...

On those other systems, the approach is to take analog audio from a receiver, and with a computer/soundcard, convert that audio into a digital format for transmission across the Internet, where it is immediately switched back to analog audio for a transmitter.  The radios and repeaters involved are the same analog FM that has been around since the early amateur radio repeaters in the middle of the last century.  This is what people are familiar with and they approach the connectivity issues from that frame of reference.  Nothing wrong with that, it is the way we humans approach problems, building on prior experience.

D-STAR is a paradigm shift for the amateur community.  It isn't about merely moving voice (audio) from one location to another.  It is, in every sense, a digital mode. 

A D-STAR transmitter (for Digital Voice or DV), translates, compresses, and encodes the "audio" into a digital stream using AMBE, at 2400 bits per second, within the radio.  That digital stream is also given some forward error correction (FEC) consuming another 1200 bits per second.  This FEC is a factor that allows D-STAR transmissions to recover the original audio content and deliver clarity when the signal is very weak (on analog FM it would have a great deal of noise - here is a video comparing the two in real time http://www.youtube.com/watch?v=FyYhLtS-0gE - not english, but clearly shows the difference).  The actual transmitted signal is encoded at 4800 bits per second, so removing the 3600 bps for the audio, there is another 1200 bps that can be used for various ancillary communications, including position reporting (GPS coordinates), short message service, and various contextual pieces of data.  This additional data has been exploited for some fairly sophisticated applications, such as D-STAR TV (sending images), D-RATS (chat, message passing including email gateway to the Internet, TCP/IP tunneling, ...), etc.  This combination of digitally encoded audio and ancillary data is always in the data stream in a D-STAR (DV) signal, whether the audio is silent or not -- it is there, whether the "data" portion contains all zeros or one or more of the listed data items -- it is there - they are inseparable.  This data stream is modulated onto a narrow (6.25kHz.) FM signal using GMSK modulation.

A D-STAR receiver, on the other hand, receives the narrow FM signal, and using GMSK demodulation, extracts the data stream, and processes it through the AMBE chip to deliver the audio and the ancillary data mentioned above.  If you were to listen with a traditional FM receiver you would hear what I call "structured noise." You can tell it is not natural, it has patterns in the noise. 

A D-STAR repeater doesn't convert this datastream to audio and ancillary data, though it does examine the data stream to know what to do with the payload (Callsigns, control bits, and so forth).  In fact, you cannot tap audio out of the D-STAR repeater directly.  In the Icom repeaters, I've been told the audio sections of the radios have not been populated.  The repeater directs this digital stream from the receiver to the transmitter, in digital form, and optionally send the digital stream via the controller to co-located repeaters (based on callsign routing) or to a gateway computer.  Again, the gateway computer keeps everything at the digital level, and based on information from the datastream optionally directs the datastream to remote gateways, which, in turn, pass that digital signal to a controller, which sends it to the appropriate repeater module, and on to the user radio, where the audio and ancillary data are ultimately decoded.

The gateway computer is connected to the controller using Ethernet, and by specification must be "close" to the controller (from a latency point of view).  In most installations the gateway computer is co-located with the controller and repeater decks and the Internet is brought to the computer  at the repeater site. (The Greeks report having separated the computer from the controller by some distance using low latency WiFi style connections, but this is not typical.)  The Internet connection must be stable, low latency, low jitter (inter-packet timing should be stable, it doesn't work well through satellite connections, because those circuits will buffer and burst packets creating high jitter), and have an IP address for the computer that changes infrequently. It also requires a router with certain characteristics (LAN side Class-A address space).

The gateway has many functions to perform including keeping a database of all known callsigns on the D-STAR network and what gateways have most recently heard them. This is why on D-STAR you can tell your radio you want to speak to a specific station (callsign) without knowing what repeater (worldwide) the station is monitoring, the gateway system will figure it out and send your signal (datastream) to the right repeater and ultimately to your desired receiving station.  In order to keep all of this data up to date, the gateway needs sufficient bandwidth to send and receive the datastreams for each of the attached repeaters to the various destinations, including a potential 128 kbps, usually TCP/IP, stream (from 23cm DD mode - Ethernet over the ether), as well as the command and control, data synchronization, etc.

So, as you can see,  this system is much more complex, and powerful, than  the  VOIP pack (IRLP,  Echolink, etc.) that are only concerned with routing audio from one repeater to another over the  Internet.  In D-STAR, there is simultaneous audio and data, that must traverse the whole network.

For these reasons, you will not be able to provide the services that D-STAR users want and expect without a decent, reliable, and moderately high bandwidth Internet connection.  In D-STAR, you need to understand and provision a network, not just a radio link.