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C4FM
C4FM stands for Continuous Four Level Frequency Modulation and is a FDMA (Frequency Division Multiple Access) modulation method.
C4FM is a digital modulation technology. What your radio uses over the air to talk digitally to other digital radios and repeaters.
Wires-X
Wires-X is a technology that runs on top of System Fusion. It provides a network of repeaters. Wires-X has a concept called rooms where multiple repeaters and nodes can connect and talk to each other. A room is similar to a reflector in D-Star, a Talk Group in DMR and a conference in Echolink.
Wires-X provides a directory of connected rooms and manages the connections from Nodes/Repeaters to each room.
In addition, Wires-X provides a way from the radio to change to a different room, search rooms and more. Some radios have more features than others because of touch screens, etc.
There is also the ability to send messages and pictures, put them in a common message store on each node or room for others to read (think of news).
Without Wires-X, System Fusion is just a communication protocol.
YSF
YSF short for Yaesu System Fusion really is not a Yaesu technology when looking at the YSF servers. It is another method of creating a room where other System Fusion users can connect together.
Unlike Wires-X, there is no central management of the rooms that are up and available. There are directories of YSF servers (for example: https://register.ysfreflector.de/ and many of the digital softwares you run on your computer also maintain lists for easy selection.
The important thing to note is that System Fusion repeaters cannot directly access these servers. To get from Wires-X on Yaesu System Fusion to a YSF server, you need a bridge (more on that in a bit).
You can find YSF rooms connected on some repeaters that are running the MMDVM software as their repeater controller. Some of you may also know it as Pi-Star which is a pretty interface over the MMDVM software.
Fusion
This is the name that Yaesu has given their implementation of C4FM. The full name is really System Fusion. It is not a new technology but a marketing term created by Yaesu for referring to their C4FM.
You can learn about System Fusion on the Yaesu website here: http://systemfusion.yaesu.com/what-is-system-fusion/
Bridge
A bridge is a way of connecting 2 different protocols or technology together. It is similar in concept to a cross band repeater that for example takes in a signal on 2 Meters and transmits it back out on 70cm.
Since YSF cannot directly talk to Wires-X, there is a bridge that can talk to both YSF and Wires-X. When it hears something on Wires-X it sends it to YSF and the opposite happens when it hears something on YSF.
There are multiple ways you can bridge between the networks.
One method is simply setting up a Wires-X node with a radio and setting up a YSF/FCS node with a radio on the same frequency. Very similar to what cross band repeat does.
There are some that may be able to make the translation using software. For example, a our XLX reflector translates from Brandmeister DMR TG 20222 to XLX165 Module B all in software digitally. It never decodes the audio.
That is possible because both use the AMBE2 format. For example, if you wanted to take the same DMR and put it on D-Star you would need to drop it to audio first and back because D-Star uses the older AMBE protocol.
Since Fusion, YSF and FCS are all using AMBE2, it can be done easily in software.
It is possible that there is a bridge between Wires-X and YSF and then one between YSF and FCS with no bridge between FCS and Wires-X.
This would mean that when someone keys up on A Wires-X repeater, it would connect to YSF via a Bridge and then YSF would connect with FCS through a bridge.
It can get very complicated and it is important to remember that every bridge adds delay. Too much delay and people are talking over each other.
