As part of the D-STAR protocol, linking is an inherent feature. Linking is accomplished by the repeater controller, the gateway server and just recently the reflector.
Original Routing
The original implementation of the controller and gateway allowed for the source radio to specify where its signal should be sent. A user on VHF could send their signal to another port on the same repeater when using only the local controller. When a gateway computer and an Internet connection is added, a user can send their signal to another port on any other gateway connected repeater.
When using this method, only the source radio’s signal path is specified. When someone tries to talk back, they must configure their radio to route their signal to the same repeater and port as the source radio. Unless the source radio operator tells the destination radio operator where they are at, the destination radio operator cannot determine the location to respond to.
To allow for an easier means to reply, the D-STAR gateway implements a “Where is he” function. If I specify that I want to talk to an individual call sign, the Gateway Servers will determine the last repeater and module on which that user was heard and route the signal to that location. One issue with this is that the updating process is not fast, sometime taking an hour to update a position. If the station that you want to contact switch between bands or switches between repeaters, it might take time for the system to find the user’s location.
Many Icom radios implement what has been called the "One touch" or RX-CS button. This button allows the calling user's information to be inserted into call sign memory and therefore program the radio to respond to the caller.
To assist in users in programming their radio, the D-STAR calculator lists all of the repeaters and modules available and allows you to specify the source and destination that you want to talk to.
G2 Multicast
In the G2 software, a new feature was added that extends the original routing concept. In G2, an administrator can specify an alias that includes multiple nodes. For this to work effectively, all repeaters that are in the list need to have the same set of members in their lists. A user then specifies the alias in their UR Call and their signal is routed to all nodes that the administrator has specified (total of up to 11). When a user talks, the repeater sends a stream of data to each of the specified servers. For gateways without very fast Internet access, this can be an issue.
dPlus Linking
A piece of software written by Robin Cutshaw, AA4RC called dPlus allows for gateway servers to connect two nodes together. An administrator can specify the connection with a command line command on the gateway, or it can optionally be controlled by a radio.
When two modules are linked with dPlus, the users need not, actually should not, use any special routing. The configurations should be as if they are talking to a local user on the same node, but with traffic configured to go through the gateway. All traffic on one node is repeated to another node. This is much more analogous to the way that FM repeaters are linked today.
dPlus Linking with Reflectors
dPlus linking with reflectors is essentially the same as linking two nodes together, except the multiple repeaters can all link to the same reflector. This allows for many systems to be connected together and is limited only by the connection speed that the reflector has to the Internet.
FAQs
Q) I want to talk to someone in Australia, what is the best way to do so?
A) The original routing solution would be the best to use in this instance. If you know what repeater that the person is on, then you can specify the gateway in the UR Call. If you only know the call sign, then you can specify their call sign in the UR field.
Q) We have two repeaters that we would like to link together. How can we do it?
A) dPlus would be the first solution for this situation. Users on each repeater would hear users on the other. But it is also valid to create a multicast address in the G2 gateway software and have all of your users utilize it. The advantage would be that the users are under the control of when their signals are repeated.
Q) We have 7 repeaters that we would like to link together for a net.
A) The choice here is not as obvious as many of the others. The large number of repeaters will require significant bandwidth between the gateways. The use of a reflector isn’t dedicated, so it is possible that others might already be on the reflector when you join. Both are valid solutions.
Q) We want to have a regional net with 14 repeaters.
A) This can only be accomplished by using dPlus with a reflector.
Q) I sometimes hear someone say “Linked” or “Unlinked” or “Currently not Linked.” Who and what is that?
A) That is AA4RC, the creator of dPlus. When someone causes your node to be linked, a recorded message is played on the air for all to hear.