By Michael Gordon
Back in ancient history the incredible Icom IC-2 walkie talkie or handy talkie came into existence. Compact, reliable, rugged and simple. The frequency was set with three thumbwheels; the first two digits assumed, so you might have "652" meaning 146.52. You had an offset switch of plus or minus, and a switch to engage the offset. No PL (Private Line, low frequency audible tone0.
Problem: You could inadvertently (or vertently) "key up" more than one repeater if you were in range of more than one repeater.
Solution: Private Line. A tone, such as 103.5 Hertz, that the walkie-talkie transmits any time it is transmitting. Each repeater on a particular frequency will listen for its tone and this way you key up or activate the repeater you intend to use and the others will ignore you.
It might be obvious, but you can use that same PL tone to unsquelch your own radio, so you are not bothered by chatter all day long and only someone that transmits the code will open your squelch. Of course, if you do that then you might think the channel is free and start blurting out a transmission right on top of a conversation.
Problem: Limitation on coverage of a repeater especially in mountain country.
Solution Linked repeaters. What you speak on one of the repeaters is linked to the other repeater or repeaters and their coverage areas.
Digital modes, as implemented for D-Star, are still FM audio, but voice is coded in a "vocoder" into digital form, and then the digital form itself modulates as audio the FM carrier. This seems like rather a lot of massaging to end up with audio anyway BUT it allows to merge actual data into the stream and do some interesting magic using the internet.
A typical D-Star radio takes four parameters: UR, RPT1, RPT2, and MY.
UR Your, or You Are; the station I wish to speak to. A special case exists CQCQCQ and that means I will talk to anyone. If I put a callsign in UR, everyone will see that a transmission is taking place but only the radio having UR in its MY will break squelch and be audible. Interestingly, if the repeaters are set up properly, it will route this call through the linked D-Star network and pop out on the repeater where the destination callsign was most recently heard. Unfortunely, the reverse path is not automatic; the recipient upon hearing your call must push a button, RS>CS or something like that which automatically populates UR, RPT1 and RPT2 on his radio to permit replying. That's an ordeal that I think not many people pursue.
Instead, you might LINK your friendly neighbborhood repeater (NU7TS, lets say) with a repeater near your friends (KF6RAL for instance) and so this is just like linked analog FM repeaters; what either of them hear, both of them repeat.
The actual procedure I might get around to in tables below. Linking repeaters allows for two groups of people to have a nice net or round-table conversation, where by putting your callsign in UR, and on your radio doing likewise but put my callsign in your UR, we can talk but only to each other. We will still hear anyone else on the channel that has CQCQCQ in their UR destination callsign field. But if several people have specific callsigns, or everyone, then the channel would be busy but no one would HEAR anything except their buddy with whom they are conversing. Not only that, but because of the capture effect of FM, the station that you do not hear, but is stronger, will silence the station you are trying to hear.
MY This is my own callsign; owner of the radio. You can optionally add a slash and up to 4 characters; this will be sent and could be your state, or model of radio, or 4-letter nickname. There's a trend to put the model of radio (/id51, /id52, /705 and so on).
RPT1 is a callsign and the equivalent of PL (Private Line). All radios hear it but only the repeater that owns this callsign will respond. It also includes a "module" designator, a better word would be BAND designator, A is gigahertz, B is UHF and C is VHF.
Why is it needed? You would think that since NU7TS hears you on 440 (70cm) it already knows you are using the "B" channel or module. While true, it can also be there's a personal station, NU7TS, that is not the repeater! AC7O station, and AC7O Repeater. So the module designator in the 8th character position is essential for repeaters to distinguish the repeater from the person. Also, this is pushed through linked repeaters so that the other end can reverse it and find YOU on the "B" channel when otherwise it has no idea which channel/port/module/radio to actually use.
RPT2: Tells the repeater that you wish to include this resource in the repeating. Very often, it will be the same callsign as the repeater, but with a "G" in position 8, telling the repeater to include the LINK destination (whatever it is, and if anything is linked).
I suspect the design includes putting some other callsign and "G" but what would happen, if anything, is unclear and not perhaps useful.
Engaging these functions requires a command in position 8 of the UR callsign. Each of these commands targets whatever repeater is identified by RPT1.
Identify current link state: "I" (India) in 8. 7 spaces and then I.
Unlink any existing links: "U" (uniform) in 8. 7 spaces and then U.
Link to a repeater or reflector: "L" (Lima) in 8, target repeater or reflector in 1-7 typically module in 7. So in "UR": REF029CL transmit for about one second since the data rate is a bit slow. Then switch UR to CQCQCQ to actually have a conversation. This is a bit easier to have memories configured in advance.
Echo: "E" in position 8 of UR. Concurrent with sending the "E", speak a few words. The repeater will echo whatever you sent, a few seconds of it anyway, so you can hear what you sound like.
The Icom IC-705 and some other radios allow to access the AMBE vocoder and use the radio's internal computer to directly interface with some gateways and some reflectors through the internet protocol.
In essence, the radio becomes its OWN "gateway", taking the place of the "G" module on a repeater. The radio becomes a repeater and can link to reflectors and other gateways.
Terminal mode versus Access point mode
Terminal mode uses the radio's microphone to digitize the audio and immediately hand it off to either its own internal gateway mechanism or to an external gateway. If you use the external gateway, basically you've got a $1300 microphone and doesn't seem all that useful although for now it seems that's the only way to reach an actual D-Star Reflector. Several kinds of reflector exist and the XLX reflectors are multi-protocol; you can have conversations with some participants using DMR, others using Fusion and some using D-Star.
Access Point Mode is a mini-repeater. The Icom IC-705 (or similar equipped radio) listens on radio frequency and provides internal or external gateway services. This could be useful in a temporary repeater situation for many people using radios to share a gateway.
Documentation is nearly nonexistent and conflicted where it exists. Icom's documentation is both thorough and nearly useless at times since it does not explain when you would use various features; what problems are solved, what benefits accrue or obtain from doing it a certain way.
Youtube videos are all over the place and not all that helpful although if you watch a dozen or so you might glean an idea.
Internal Gateway stuff
With an expensive radio and the path blocked to the D-Star repeater I mean to use its capability and NOT purchase more hardware when the radio already has the AMBE vocoder, computer and network stack built in!
Gateway Repeater (Server IP/Domain) is where you put the IP address or DNS name of the target gateway, repeater or reflector. Usually a reflector.
Terminal/AP Call Sign This is the callsign of the radio itself, not YOU the operator, but the radio. Often it will be your actual callsign but with a letter in position 8 that on an actual repeater would be the module (or port or band). You can use any letter but probably ought to avoid command letters. I use a "T" for Terminal
Example: "AG7MG T" (T in position 8).
When it is configured correctly; on the reflector dashboard you will see this gateway callsign and module in the list of connected gateways; it sees the radio as a gateway. In "Last Heard" will be your own callsign once you have send a transmission (including just key the microphone).
RPT: This is where you put the callsign and module (channel, room) you wish your radio to join. Oh by the way, YOU and YOUR RADIO are not the same. YOU will be your callsign, maybe with /bob or /705 or whatever; your radio is the gateway often the same callsign but with an identifier in position 8 to distinguish the radio's gateway from you
Example: "XLX120 D"
https://www.cnharc.org/repeaters/digital-voice/d-star-linking-and-un-linking-on-a-repeater-gateway/ A list of linking and unlinking commands for various types of repeater and reflector, also makes mention of callsign routing with slash in the UR field.
http://www.cnharc.org/repeaters/digital-voice/new-call-sign-routing-ccs-explained/ CCS7 Call Sign Routing explained.
D-STAR Uncovered (2008) Details of the D-Star voice and data protoools and how things like UR, MY, RPT1 and RPT2 are mapped to the actual data packet. It also reveals a slash in the UR is a request to route the packet to a repeater and on arrival changes UR to CQCQCQ. But for anyone to reply the UR must be entered; it is uni-directional routing and a repy route is not automatic.