We normally use a layer shipboard data network architecture as a reference model. The current model (Rødseth Ø.J., Christensen M.J.; Lee K. 2011) has five layers as shown below. The first version of the model was developed during the MiTS project (Rødseth Ø.J., Øgård O., Hallset J.O., Haaland E. 1992) using four layers: Instrument, Process, System and Administrative. The modernized version has added a general ship layer, including the administrative layer as one of its components, and an off-ship layer.
The instrument layer serves to interconnect relatively simple devices and would in practical terms be an industrial fieldbus, NMEA 2000 or NMEA 0183. Each network on this layer would typically cover a limited number of devices, dependent on physical area and function. Several isolated instrument networks may be used to implement redundant process segments, e.g., to implement redundant propulsion or steering systems. An instrument network may or may not support redundancy in itself.
The next layer is the process layer which is used to integrate all functionality in one process segment, e.g., navigation, ventilation or engine control. Again, redundancy may be required and this could be done by implementing two independent process network, by supplying redundancy in the network itself or both.
The top layer in the trusted part of the system is the Integrated Ship Control (ISC) layer. This is used to interconnect different process segments as required and also to bridge over to administrative functions where this is needed. As the ISC layer potentially connects to all process segments, one single error on the ISC layer may propagate down into several systems if no safeguard is put in place. Thus, one will need some form of gateway (GW) or Firewall (FW) between the ISC layer and each of the process segments.
In the figure we also indicate the use of a special database function combined with similar FW/GW safeguards to interconnect the ISC layer to the higher and more vulnerable layers.
The first version of the MiTS architecture is shown below (Rødseth Ø.J. 1994). It contains only three network layers: The instrument layer integrates sensors and actuators for each process segment, the control layers corresponds to the ISC layer in the new model and the administrative layer is now part of the general ship layer.
The higher complexity of the current model is a good indicator of the significant developments that have been made in integrated ship control since 1994.
A new ISO standard is under work by ISO TC8/SC6, dealing with the installation of ship data networks (ISO, 2012). This is currently in draft form, but will probably be published as final standard mid 2013.
The ship network architecture is similar to the one presented above and the standard deals mostly with high level design and operational issues. Thus, it is on a higher level than many of the other standards presented here and of a more general nature.
The LWE standard was finished in 2011 (IEC, 2011), in a sense as a result of a more than twenty year long process as discussed in the history section. It is intended for use mainly in the navigation system on the process layer, although it can also support instrument layer integration and also on ISC layer, if desired.
It uses the IEC 61162-1 sentence format (ship related NMEA 0183 sentences) with some additions from the NMEA 0183 sections related to AIS base stations. The latter components are a set of constructs to group and further characterize the sentences.
Messages consist of a number of sentences with additional data such as sentence number, grouping information, sender and receiver identities and put into an UDP datagram. The maximum size of the datagram is about 1200 bytes.
The UDP datagram is sent as a multicast to one of 16 predefined addresses. Eight of these are default addresses based on the type of sentences in the datagram. The default selection can be overridden by configuration.
There are a number of restrictions on network equipment defined in the standard that are intended to ensure easy integration and safety of operation. This includes bridging to other networks and what types of routers to use in the system. The general properties of the standard and the rationale behind the technical design is explained in (Rødseth Ø.J., Christensen M.J.; Lee K. 2011).
NMEA 2000 was introduced in 2001 (NMEA, 2001) as an alternative to NMEA 0183 for low cost bus type systems with much faster data transmission capabilities. It is based on the Controller Area Network (CAN) standard (ISO, 2003). The nominal data rate is 250 kbps at bus lengths up to 200m. This translates to about 130 kbps for user payload. An overview of the standard can be found in (Luft L. A., Anderson L., Cassidy F. 2002) which is also available as a resource.
NMEA 2000 has also been internationally standardized as IEC 61162-3 (IEC, 2008). This standard is intended for ships in international trade that has to conform to the requirements of the SOLAS convention (IMO, 2009). It requires a two-bus redundant system with backup of all vital components such as power supplies. This is supported by NMEA 2000, but is optional in that specification. The main reason for requiring redundancy is generally higher safety standards for SOLAS ships. As a bus type network represents a single point of failure in itself, it was decided to require redundancy also on network level. This is not a requirement in IEC 61162-1.
In August 2012 several news flashes have told that NMEA is working on the so called "OneNet" which is a standardized method of transporting NMEA 2000 messages over Ethernet. The new standard will also increase the device capacity, supporting 65000 devices instead of CAN bus' about 50 and will introduce power over Ethernet as well as other features. One of the news articles can be found on Digital Ship.
There has been no official liaison with IEC as far as we know so it is not currently clear how the proposed standard will position itself with regards to IEC 61162-3 (IEC version of NMEA 2000) and IEC 61162-450 (Light Weight Ethernet) or if the ambition is also to make it into an international standard.
NMEA 0183 started to get wide international adoption around 1987 with its 1.5 edition (NMEA, 1987). It has since been continuously updated and was at time of writing at edition 2.1.
NMEA 0183 is a serial line standard allowing up to 10 receivers to connect to one sender. Data is sent as text messages with a distinct format, well known to those working with integration of bridge equipment. An example is shown below.
$GPGGA,092750.000,5321.6802,N,00630.3372,W,1,8,1.03,61.7,M,55.2,M,,*76 $GPGSA,A,3,10,07,05,02,29,04,08,13,,,,,1.72,1.03,1.38*0A $GPGSV,3,1,11,10,63,137,17,07,61,098,15,05,59,290,20,08,54,157,30*70
The speed was originally limited to 4.8 kbps, but is commonly used on many other network rates, up to over 100 kbps. The limitation is in the receivers ability to process data as well as in the physical interfaces used. Later versions of NMEA 0183 contains sentences for many ship applications as well as for AIS shore data network operation and management. Part of this is a TAG (Transport Annotate and Group) mechanism that adds additional character structures to the basic NMEA 0183 sentence format. Below is a brief example that shows explicit grouping of the last two messages shown above. The TAG construct has also been adopted in the IEC 61162-450 standard.
NMEA 0183 was also adopted by IEC and in 1995 the first transliterated version of NMEA 0183 was published as IEC 1162 (later issued as IEC 61162) (IEC, 1995). The IEC standard has been updated as the NMEA standard evolved and is currently in edition 2. However, the IEC standard restricts the specification in NMEA 0183 somewhat. It only allows 4.8 kbps (although IEC 61162-2 also allows 38.4 kbps), it restricts sentence types to those used on ships and it does not include the TAG construct.
A new edition of IEC 61162-1 is under developments and it will not include all the descriptions from the NMEA 0183. Only lists of allowed sentence formatters will be included as well as other relevant specifications for IEC.
IEC 61162-2 includes all provisions of IEC 61162-1, but allows an alternate transmission speed of 38.4 kbps. This is mainly to support very fast transmitting devices such as gyros.
Rødseth Ø.J., Christensen M.J.; Lee K. (2011). Design challenges and decisions for a new ship data network, ISIS 2011, Hamburg, 15th to 16th September 2011.
Rødseth Ø.J., Øgård O., Hallset J.O., Haaland E. (1992). Integrated ship control and open systems (Authors' draft), IFAC symposium on Control Applications in Marine Systems, Genova 8-10 April 1992
Rødseth Ø.J., Haaland E. (1993). MITS: An Open Standard for Integrated Ship Control (Authors' draft), Proceedings of ICMES 93, Hamburg September 1993
Rødseth Ø.J. (1994). Integrerte skipsstyringssystemer - nye løsninger (Integrated ship control - new solutions), Harlans Seminar 1994, Sjøkrigsskolen i bergen, September 1994.
ISO, (2012). ISO 16425-CD:2012, Ships and marine technology Installation guideline for ship communication network of improving communication for shipboard equipment and systems (Committee Draft).
IEC, (2011). IEC 61162-450 Ed. 1.0:2011, Maritime navigation and radiocommunication equipment and systems - Digital interfaces - Part 450: Multiple talkers and multiple listeners - Ethernet interconnection
NMEA, (2001). NMEA 2000 Standard for Serial-Data Networking of Marine Electronic Devices, National Marine Electronics Association, Version 1.000, September 12, 2001.
Luft L. A., Anderson L., Cassidy F. (2002). NMEA 2000 - A Digital Interface for the 21st Century, Presented at the Institute of Navigation s 2002 National Technical Meeting January 30, 2002 in San Diego, California
ISO, (2003). ISO 11898-2:2003, Road vehicles - Controller area network (CAN).
IEC, (2008). IEC 61162-3:2008 Ed1.0, Maritime navigation and radiocommunication equipment and systems - Digital interfaces - Part 3: Serial data instrument network
IMO, (2009). The International Convention for Safety of Life at Sea (SOLAS), International Maritime Organization IMO, London, United Kingdom. As amended up to 2009.
NMEA, (1987). NMEA 0183 - Standard for Interfacing Marine Electronic Navigational Devices, Version 1.5, National Marine Electronics Association, 5 December 1987.
IEC, (1995). IEC 61162-1:1995 Ed. 1, Maritime navigation and radiocommunication equipment and systems - Digital interfaces - Part 1: Single talker and multiple listeners
Last updated 2012-08-29 by Ø.J.Rødseth @ MARINTEK