The Curious World of Marine Data Networking

It's a rum do this marine networking business. For someone who has spent the past quarter century dealing with open Internet protocols the strangeness is not so much in the archaic technology of the standards themselves but the way in which they are defined and administered.  The IETF has a variety of funding sources but one of them isn't selling the standards which they define.   Their biggest potential cost, the time of those involved, is funded by the organisations which employ them.  Corporate altruism aside, it makes good business sense to fund a seat at the decision making table.

In the world of marine data networking, standards are controlled by the US-based National Marine Electronics Association (NMEA).  In data networking, these standards cover the entire stack in the OSI model, from physical to application layers.  The International Electrotechnical Commission (IEC) formalises standards for the transmission of marine data based on the NMEA protocols (IEC 61162).  These are aimed at commercial vessels legally bound to conform to IMO requirements.

Unlike Internet standards, marine networking standards are not publicly published.  Anyone wishing to understand or implement the protocols has to buy the standards documents.  Not only this but the NMEA keep strict control of their intellectual property.  The documents aren't redistributable in any form which reportedly includes software source code.  Consequently, NMEA protocols cannot in theory be implemented in open source software.

Open source evangelist Eric Raymond has frankly expressed his opinions about this method of standards control.  Not being familiar with US intellectual property laws and keen to not be blinkered by possible hyperbole from those with hard line views on software freedom, I emailed the NMEA in an attempt to clarify their position on the use of legitimately purchased copies of their standards in open source software.  I never received a reply.

The NMEA has other sources of revenue besides selling standards documents: membership fees, conferences, product certification, training etc.  Whether revenue from standards is a necessary cornerstone of their funding or whether standards remain closed because of entrenched and ideas about how the marine electronics industry does business is not something I wish to speculate on here.

State of the Ark

The NMEA's most up to date data communications protocol is NMEA 2000.  This is based on CAN bus, the vehicle networking protocol from the 1980s which has only recently started to gain widespread use in the leisure boating market.  Still to be found on most marine devices today is the ability to communicate via an older and more widely deployed protocol, NMEA-0183 (a version of which is formalised as IEC 61162-1).  NMEA-0183 encodes data from sensors at the application layer as lines of carriage-return-line-feed-terminated ascii characters and transports them over a serial line (EIA-422 in later standards).

Due to the need to support higher data rates for connections between devices such as radar, sonar, cameras and the multifunction displays they talk to, manufacturers have implemented proprietary protocols (not using NMEA standards) running over ethernet networks.

There also exists an IEC standard for the transport of IEC 61162-1 (NMEA-0183) data over IPv4 networks known as IEC 61162-450. However I have never seen an implementation of IEC 61162-450 in any software or device available in the marine leisure market.

The NMEA is currently working on a new standard for transport of marine data over IP-based ethernet networks.  "Onenet" is due for publication in late 2014.  At the 2013 Southampton boat show and again at the 2014 London boat show I asked on the stands of Raymarine, Garmin and (at Southampton) Navico (who own the B&G, Simrad and Lowrance brands) when we were likely to see OneNet enabled products.  Sadly none of the staff were even aware of OneNet, let alone possible products.

If you don't build it, they will come anyway

PC applications which take marine data from an Internet Protocol ("IP") network using proprietary protocols are not new: Raytech used data from Raymarine equipment and MaxSea could use Furuno data.  Some of the newer tablet and phone apps continue to use proprietary protocols:  The Navionics tablet app communicates with Raymarine plotters (or "Multifunction Displays" (MFDs) as they are increasingly being called) using an undisclosed method agreed between the two companies.  Most of the current crop of mobile and PC apps using marine data from a network are not tied to a particular manufacturer.  The most common method for the transport of marine data over IP (including wireless) networks surprisingly follows no defined standard. 

NMEA-0183 over IP: The Non-Standard

The majority of current apps sending or receiving data over IP do so using the application layer of NMEA-0183.  In other words, the data an application will send or receive will look like a string of ascii characters, starting with a "!" or "$" and ending with an ascii carriage return and line feed.  Very few implement the new features in NMEA-0183 version 4 (e.g. TAG blocks).

There is no standard transport layer but in practice the options are limited anyway.  Most software apps and hardware devices use either TCP, UDP or offer the user a choice of either.  There is no standardisation for maximum UDP datagram size or whether one or several sentences may be carried in a single datagram.  There is no common statement that Nagle algortihm should be disabled for tranmission of marine data over TCP.

There is no standardisation for data transfer at the network layer except a de facto standard that IPv4 is used.  My own multiplexer program kplex is the only marine data application I am aware of which supports data in NMEA-0183 format over IPv6.  TCP is inherently unicast.  UDP is most commonly used for broadcast.  Even there there is no standardisation: Some applications use the broadcast address of the zero network (255.255.255.255), others the network broadcast address.  Most applications don't care which broadcast or unicast address is used as they simply listen for any UDP packets the system receives (ie bind to INADDR_ANY).  Very few applications use UDP multicast.  The only ones I know of are the ones I have put the code in to support it myself: kplex and OpenCPN.

There is no common standard for device addressing and no standard for service discovery.  Navico have developed and released a service discovery protocol described in their "GoFree Tier 1" specification.  This is a combination of bonjour and service information announced using JSON strings send over UDP multicast.  Few of the major applications described as working with "GoFree" currently make use of the service announcements: instructions for iNavX and similar applications involve getting the (dynamic) IP address of the MFD via the MFD's menus then connecting the mobile device to that.  Service discovery in OneNet is rumoured to be similar to this.  Perhaps unsurprising given that Navico is one of the biggest players in the industry and none of their major competitors has proposed such a protocol.

So why use a 30 year old protocol instead of the "current" NMEA 2000 at the application layer?  I am open to suggestions but here is my speculation.

1. Prior to disassociating the application layer from the lower layers of the protocol stack, it was easier and cheaper to present NMEA-0183 to a PC over a serial port (or via a serial to USB converter) than it was to present NMEA-2000 (via hardware incorporating a CAN controller).  Many applications developed prior to the widespread use of IP networks on boats were written to take NMEA-0183 from a serial interface.  Simply using the application layer of the already-implemented data communications protocol involves the least implementation effort when network communication is added.
2. NMEA-0183 is the most common format for data generally available on a pleasure boat.  Although NMEA 2000 has become more common over the past 5 years the ubiquity of NMEA-0183 means that most MFDs will have an NMEA-0183 interface in addition to any NMEA 2000 interface.  Many boats have older equipment which don't support NMEA 2000.  NMEA 0183 interfaces are still more common on some types of new equipment (VHF radios, AIS receivers).
3. Many of the details of the application layer of NMEA 0183 are known.  The details of NMEA 2000 are considerably less widely known.

The latter point needs discussing further.

NMEA 0183: An Open and Shut case

As previously mentioned, the NMEA's protocols are "closed".  To legitimately use them they must be purchased and not re-distributed in any way, including by open sourcing code written to implement them.  However many of the details of NMEA-0183 up to and including version 3 have been widely publicised on the Internet.  The document I consulted in writing kplex and possibly the best known publicly available source of information on NMEA-0183 is the document assembled by Eric Raymond.  In this he states that none of the content was directly derived from NMEA standards but was taken from a variety of other publicly available sources.  It was written as part of the gpsd project, but also in part as a political protest against the NMEA's closed standards.

As a consequence of the details of NMEA-0183 being publicised it has been widely implemented in apps where the developers have not purchased or cannot use (because the application is open source) the official standard.  NMEA 2000 has not been publicly revealed in this way.  The features added to NMEA-0183 in version 4, although referred to in some publicly accessible presentations and errata are not publicly described in detail and remain largely unsupported by most apps using NMEA-0183 style data over a network.

Are we there yet?

So what are we left with? The leisure market at least has ignored IEC-61162-405.  Products implementing OneNet look to be 2 years away.  We are left with the application layer of an antiquated and arguably poorly defined protocol which is generally implemented based on non-primary unofficial sources transmitted over an unspecified transport layer in an IPv4 network with whatever method of addressing happens to exist.

The quite remarkable thing is that it mostly seems to work.