Glossary Reference

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AIS systems

Automatic Identification System, a device that broadcasts the on board navigational data, and receives such from other vessels via radio signal to help prevent collisions at sea.


Amplitude Modulation. A method of radio signal modulation where the data or audio signal is represented by oscillating the amplitude of the carrier wave. See also FM and SSB.


American Standards Code for Information Interchange. ASCII is a standard for representing characters and symbols using 8 bits. This covers 128 characters comprising of upper and lowercase letters, the numbers 0-9, and some extra symbols, basically almost everything you see on a computer keyboard.

Asynchronous (network)

In the context of data networks, asynchronous means a system without a central clock defining when bits (1's and 0') should be sent. CAN Bus networks such as NMEA 2000 are classed as Asynchronous which is a bit of a misnomer, there is a huge amount of synchronising in an asynchronous network! Networked electronic devices, like musicians in an orchestra, have to follow some kind of beat - and as there is no central 'conductor' asynchronous devices are almost constantly re-syncing to keep in step.


Baud rate by definition refers to the number of symbols, or pulses per second flowing in a data system. However, when used in marine data systems 1 electrical pulse is used for each bit so, at least in this context, the term 'Bit rate' and 'Baud' are the same and used interchangeably.

Bit (b)

A bit refers to the smallest entity of data represented as either a 1 or a zero. Bits are the individual 1's and 0's that make up raw data. There is not much that can be communicated with a single bit, as it only has 2 states but by grouping them together in blocks called bytes we can make more combinations and send useful data.

Bit Rate (bps) (Kbps) (Mbps) (Gbps)

The Bit Rate is the number of bits can flow per second, you may see the term used as bps, or bits/s and most often higher Bit Rates are quoted with a metric prefix such as Kilo, Mega, or Giga:

Kbps (Kilo bits per second) = 1000 bits per second

Mbps (Mega bits per second) = 1,000,000 bits per second

Gbps (Giga bits per second) = 1,000,000,000 bits per second

Obviously the more bits per second generally equates to faster data transfer, but how these bits are used is a consideration when comparing one standard to another, with more modern systems making better use of the available Bit Rate.

Byte (B)

If we combine 'bits' together in groups, we have more combinations. With groups of 8 bits we have up to 256 combinations, so we can count from 0 to 255 with just 8 bits. Each group of bits is called a byte.


In this context a bus is simply 2 or more wires that carry data in the form of electrical pulses that represent the 1's and 0's of binary numbers arranged as a 'back bone' where devices simply 'T' into the data wires in parallel. This topology is used in CAN's controller area networks such as NMEA 2000.

Bitwise Arbitration

A method of resolving data conflicts in NMEA 2000 networks whereby no data is lost in the process. Devices on the network can if need be battle for supremacy by simultaneously sending their identifying data bits in a quick game of 'first to send a 1 loses'. Devices with a lower ID number will always win as they have the highest priority, and the most amount of leading zeros, and the losers just stop transmitting, ready for the next round. No data is lost in the process as the winner just continues sending its data frame.


Controller area network. This is a network of devices whereby there is no central computer, each device follows a set of protocols that allows the network of devices to organise themselves. CAN's use a 'bus' topology where each device's data wires are wired in parallel.

Carrier Wave

When radio devices send a signal, usually a carrier wave of a specific Frequency transmitted and modulated in some way so the data or audio can be represented by the differences between the centre frequency and the side bands either side.

Coaxial Cable

A type of cable used for connecting radio equipment to the antenna, so called as the axis of each conductor co-exist 'co-axial' - essentially in simple terms this is a cable with a central core surrounded by an insulating 'dielectric' shield, surrounded by an electrically conductive outer Shield, and a final plastic outer sheath.

Data Stack

A Data stack is a theoretical model of the 'departments' involved in data communications, each department has a specific role and is referred to as a 'Layer'

Data Link Layer

The 'data link layer' is a set of guidelines within a data communications standard that specifies the language used, how the symbols are used to form sentences, when they can be sent, who 'talks' first, and when. As an example if you're communicating with a friend, the data link layer would define the language, grammar, and etiquette used.


One tenth of a Bell, the Decibel named after Alexander Graham Bell is a metric developed originally for use to measure signal loss on the telephone system. We use it to measure signal loss, and line losses in radio antenna cabling.


Digital Selective Calling, a system that allows radio communications devices, such as VHF, MF and HF to send digital electronic messages.


Method of communication whereby information can flow back and forth at the same time, like the telephone system.


Engine Control Unit. Many marine engines have a small computer to manage the engine, these can be interfaced to other devices on board to view engine data via an electronic adapter.


Frequency Modulation. A radio signal modulation type where the centre Frequency of the carrier wave oscillates to represent the data or audio signal.


Global Navigation Satellite System. This term encompasses all the different radionavigation Satellite systems of which GPS is one.


Global Positioning System, originally Navstar GPS, is a satellite-based radionavigation system owned by the United States government and operated by the United States Space Force.


High Frequency, a band of radio frequencies from 3Mhz to 30Mhz used for long range communications at sea using what is often called a HF radio, or more commonly SSB (single Side band)


Medium Frequency, a band of radio frequencies from 300Khz to 3 Mhz used for medium range communications using what is referred to as an MF radio, although this is usually built into the HF radio and referred to as the SSB radio


Multi Function display, this is a device that can display more than just chart information, such a radar, AIS data, sonar, and even engine data can be displayed. Often these devices are referred to as chart plotters even if they do much more.

NMEA (pronounced Nee-mah )

National Maritime Electronics Association. USA based non-profit association who over the years have set standards for communication between marine electronics, primary NMEA 0183, and NMEA 2000 are the most well known.


Parameter Group Number. A number used in NMEA 2000 to define specific data groups such as engine for example, within which there will be a number of data fields pertaining to engine data such as oil pressure, temperature, rpm etc. There are some network management PGN's too that carry data to help electronic devices establish and introduce themselves to the other devices on the network.

Physical Layer

The Physical Layer is a set of guidelines within a data standard that defines the very basic functionality of how devices communicate. As an example, if you need to communicate with a friend - you would message, email, or phone, and you would need some kind of symbols, in this case the alphabet, to form some kind of communication. These items are defined within the 'physical layer' of a data communication standard.

Within data protocols, the physical layer defines how the individual raw data 'bits' are converted and represented as electrical or radio signals. In the case of wired systems the Physical Layer may also specify the type of cable, voltages, maximum cable lengths, and type of connector. In the case of signals transmitted by radio waves the actual radio frequency, modulation, and power levels are defined in the Physical Layer Standards.


A signal is a series of electrical pulses, (or radio waves for wireless connection) that can carry information. The specific methodology is defined in the 'Physical Layer' properties of the data standard in use.


A method of communication where information can only flow in one direction at a time, some VHF channels are simplex so you must press a button to talk, and release to listen, as apposed to Duplex (see above)


Single Side band. The long range radio equipment covering HF and MHF frequencies is often referred to as an SSB radio. The term derives from a type of radio signal modulation often used at HF frequencies whereby the signal that is transmitted comprises of only the net difference between the audio signal and an internal reference carrier wave; the 'carrier wave' as used in VHF communications for example, is not transmitted. In order for the signal to be comprehendible at the receiving station the carrier wave is electronically re-inserted to provide a reference for the incoming signal, once the incoming signal is compared against this new carrier wave of the exact same frequency as the one produced in the transmitting radio, the signal differences between the two will represent the original audio.


Very High Frequency, this term can refer to a frequency band of 30Mhz to 300Mhz, but it can also refer to the on board VHF Radio that works at frequencies around 156Mhz.

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