Four general types of communication are in use today, but only two of them adhere to NMEA standards.
The first type are proprietary systems ranging from water-surface temperature sensors or paddle-wheel boat speed sensors that can be plugged into a unit on up to complete networks. Unless they are specified as being NMEA-compatible, the electronic devices in these systems can only be used with units made by the same manufacturer and sometimes only within a certain model series. Most of the display units in these systems can also be used with NMEA-compatible devices; they simply plug into different ports.
The second communication type is NMEA 0183. This method of communication involves "talkers" (devices like speed, temperature or position sensors) that have something to say and "listeners" (units like fishfinders, sonar/GPS combo units and true multifunction displays) that can listen to what the talkers are saying, and pass the information on to us via their displays.
Overall performance is limited because this communication standard is intended to support one-way data transmission at a relatively slow speed from a single talker to one or more listeners, all hardwired to a single bus. Simple information like boat speed, position, water temperature and compass direction can all be passed on to listeners using a separate bus for each talker.
Some marine wiring gurus can work wonders with NMEA 0183, taking it beyond its designed limits, but the wiring required looks like a frightening pile of spaghetti to most do-it-yourselfers. The chief advantage of this communication type is that sensors of any brand name can talk to one or more display units of any brand name as long as they are all compatible with the NMEA 0183 standard.
The third type of communication is NMEA 2000, an updated and vastly improved successor to NMEA 0183. The earlier NMEA 0183 and many proprietary systems are one-way interfaces between talkers and listeners, but NMEA 2000 is a true plug-and-play network with a data-transmission speed 50 times faster than NMEA 0183. You simply lay out an NMEA 2000 "backbone" down the length of your boat using premeasured lengths of cable plugged into T-shaped connectors located close to each electronic device (whether talker or listener), and you install a terminator plug at each end of the backbone. A preformed length of cable just long enough to go from each talker and listener to its nearby T-shaped fitting connects everything to the backbone.
Each talker puts its own "sentence" of information on the network, and you can go into the menu of each listener and tell it which talkers' information you want it to ignore and which you want it to pick up and display. You might have a multifunction display (listener) at the bow, the helm and the stern. You might have a temperature sensor on the transom and another in the livewell. You can set each display to show temperature readings from either or both sensors.
You might discover that the built-in GPS antenna in the new sonar/GPS combo unit you just installed at your helm under a hard top can't reliably receive signals from the GPS satellites; you can install a separate GPS antenna and plug it into the network, and then go into the menu of that new combo unit and tell it to use the position information from the external antenna on the network.
The main downside to NMEA 2000 is that its capacity to carry information is limited. It can carry sentences of information from more sensors than you have room to install on most boats, but the system can't handle huge amounts of data like scrolling sonar pictures or moving radar scans. And, while there is an NMEA 2000 standard for the plug and socket connections on cables and T-fittings, not all manufacturers use them and you sometimes have to install a plug-in adapter to attach devices made by different manufacturers to your backbone. This is worth checking on in advance if you plan to install devices from different makers.
So far we have been talking about moving small amounts of data at different speeds; wheelbarrow loads of data with NMEA 0183 and pickup truck loads with NMEA 2000. But, if we want to move 18-wheeler loads of information, we need to step up to Ethernet, where we have tons more bandwidth and 400 times the speed of NMEA 2000.
Unfortunately, except for a few proprietary systems, this requires stepping back from true networking to just interfacing between units. We can plug two display units together and share scrolling sonar pictures, or we can plug in new high-resolution down- and side-looking sonar systems into one or more units with the appropriate hardware. We can also plug radar scanners and sometimes video cameras and players into our sonar/GPS displays.
We aren't necessarily back to being limited to using only devices made by the same manufacturer in order for them to be compatible, but we must make sure that connectors and cables will plug together and that all devices speak the same form of Ethernet language. NOTE: Most units cannot share enhanced mapping information from plug-in map or chart cards because it is generally licensed for use on one unit only.
This will be greatly simplified when NMEA finishes OneNet (scheduled for late 2014) and standardizes Ethernet communication. It won't be a replacement for NMEA 2000 or 0183, each of which will still have its place on your boat, but it will interoperate with 2000 and carry NMEA 2000 network messages on Ethernet in a standardized manner. OneNet will hopefully also introduce a standard for plug and socket Ethernet connections and a common language for general Ethernet data transfer.
In a nutshell, if all you want are sensor readings and unit-to-unit communication between devices made by the same manufacturer, then a proprietary system will work fine for you. If you only need to link sensors to units and want to mix and match brand names, you can probably get by with NMEA 0183 - although I suggest having the wiring done professionally.
If you want an onboard network with multiple sensors and displays, I suggest going with NMEA 2000 (most new units are compatible with both NMEA 0183 and 2000). If you want to share video, scrolling sonar pictures or use plug-in radar or advanced, high-resolution down- and side-looking sonar systems, then you'll need to make sure your new units have Ethernet ports. Or, if you can stand the wait, you might want to hold back long enough to go with NMEA OneNet.