VDES Performance Evaluation for Future e-navigation Services

M. Luglio, C. Roseti and F. Zampognaro

∗

University of Rome Tor Vergata, Via del Politecnico 1, 00133 Rome, Italy

Keywords:

VDES, AIS, Satellite, VHF, e-navigation, Simulation.

Abstract:

E-navigation aims at increase safety and efﬁciency of navigation, deﬁning reliable data exchange formats and

communication channels between either ship to ship or ship to shore. New technological advancements start

from the consolidation of Automatic Identiﬁcation System (AIS), which is mandatory in some classes of ships

for the notiﬁcation of the position and to send distress signals. In pair with AIS baseline services, additional

services are gaining momentum and are available in state of the art equipment, including the handling of sen-

ding and receiving Application-Speciﬁc Messages (ASMs). In this direction, the VHF Data Exchange System

(VDES) standard was recently introduced, to improve on both messaging capabilities and system ﬂexibility

(standardizing the use of satellite channels) as well as to allow higher bitrates for application messages with

regard to AIS and ASM. In this paper, we reviewed the main characteristics of VDES, then we carried out

a technical analysis of the new communication standard in terms of channel compositions, supported rates,

access schemes and latency. Finally, we focus on the performance of two possible future VDES applications,

namely “dematerialization” and “towage” through a MATLAB model of the VDES communication channels.

1 INTRODUCTION

The International Maritime Organization (IMO) de-

veloped the e-navigation strategy to increase safety

of navigation through a better organization of data on

ships and on shore, better data exchange and com-

munication between ships or ship to shore (and vice-

versa). According to the ofﬁcial IMO deﬁnition, e-

navigation is “the harmonized collection, integration,

exchange, presentation and analysis of marine infor-

mation on board and ashore by electronic means to

enhance berth to berth navigation and related servi-

ces for safety and security at sea and protection of

the marine environment”. E-navigation has been for

long time associated to Automatic identiﬁcation sy-

stem (AIS) (IMO, 2001) technology, using commu-

nication in VHF bands and offering a transmission

range of up to 10–20 nautical miles, accordingly to

the used equipment.

AIS is an important framework for safety of na-

vigation and it is a carriage requirement deﬁned by

the International Convention for the Safety of Life

at Sea (SOLAS) for big-sized vessels (e.g, 300 tons

and upwards). Because of its effective and useful

technology, the use of AIS is often extended to vessels

not complied with the carriage requirement (i.e., AIS

∗

Authors in alphabetical order

Class-B) and allows other applications such as Aids

to Navigation (AtoN), Application Speciﬁc Messages

(ASM), Search and Rescue Transmitter (SART), Man

Over-Board unit (MOB) and Emergency Position-

Indicating Radio Beacon (EPIRB-AIS). This exten-

ded use of AIS technology has caused signiﬁcant in-

crease in VHF Data Link (VDL) load, which has be-

come an active concern in IMO and International Te-

lecommunication Union (ITU).

Because of increasing general demand of radio

spectrum for digital communication (mobile pho-

nes and data), ITU issued recommendation ITU-R

M.1842 (ITU, 2009) to deﬁne characteristics of an en-

hanced VHF radio systems for maritime mobile servi-

ces. In addition, ITU deﬁned techniques for efﬁcient

and standardized maritime communications at higher

data rates (up to 32-fold) providing the core element

of the upcoming VHF data exchange system (VDES),

which has been standardized in the 2015 with the re-

commendation ITU-R M.2092-0 (ITU, 2015).

VDES aims at ﬁrst complementing/extending and

in the next future (tentatively by year 2019-2022), re-

placing current AIS, by providing a vast gamut of data

exchange channels and methods. Thereby, the AIS ra-

dio channels will be resilient to overload as AIS po-

pulations increase, and new services will be enabled

by the progressive introduction of alternative VDES

channels.

Luglio, M., Roseti, C. and Zampognaro, F.

VDES Performance Evaluation for Future e-navigation Services.

DOI: 10.5220/0006850200670075

In Proceedings of the 15th International Joint Conference on e-Business and Telecommunications (ICETE 2018) - Volume 1: DCNET, ICE-B, OPTICS, SIGMAP and WINSYS, pages 67-75

ISBN: 978-989-758-319-3

VDES supports terrestrial data communication as

well as a satellite component, leveraging VHF ra-

dio channels. The traditional SAT-AIS (ESA, 2018)

was provided as an added option to exploit the ad-

vantage of global satellite coverages for AIS broad-

casting beyond the coastal areas, without increasing

the actual terminal capabilities. On the contrary, the

combined use of terrestrial and satellite channels is

deﬁned in the early stages of the VDES standards, as

a design requirement. This represents an opportunity

to offer worldwide coverage and facilitate the imple-

mentation of interoperable e-navigation and moderni-

zation of the Global Maritime Distress Safety System

(GMDSS) by speciﬁc optimizations coming from the

use of satellite communications.

The evaluation of VDES theoretical performance

as inferred by a critical analysis of the standards is

the ﬁrst focus of the paper. Next, taking as a refe-

rence two VDES-compliant applications deﬁned in

the frame of an ongoing European Space Agency

(ESA) project, named MARVELOWS (ESA, 2017),

a preliminary feasibility study through a simulation

campaign is presented and discussed.

2 VDES OVERVIEW

The VDES aims to provide an effective and efﬁcient

use of radio spectrum, enhancing the capabilities of

AIS and addressing the increasing requirements for

data exchange. New VDES channels and techniques

enable higher data rates than those used for AIS. Furt-

hermore, VDES network protocol is optimized for

data communication so that each VDES message can

be transmitted with a high conﬁdence of reception, on

either terrestrial or satellite VHF channels.

The VDES system supports the unique identiﬁca-

tion and location of all active maritime stations as a

default service. For the purpose of identiﬁcation, the

Maritime Mobile Service Identity (MMSI) is used,

as deﬁned in the latest version of Recommendation

ITU-R M.585 (ITU, 2012). Therefore, VDES com-

prises the functions of the existing AIS, the additional

communication links for the exchange of Application

Speciﬁc Messages (ASM) and further dedicated com-

munication links enabling dedicated higher capacity

VHF data exchange (VDE). Figure 1 depicts possible

VDES communication options and includes in parti-

cular:

– Shore-to-ship (and vice versa) VHF terrestrial

communications for AIS, VDE and ASM servi-

ces;

– Ship-to-ship (and vice versa) VHF terrestrial

communications for AIS, VDE and ASM servi-

ces;

– Shore-to-ship (and vice versa) VHF satellite aided

AIS services;

– Satellite broadcasting services for e.g., VDE mes-

sages of general interest.

Figure 1: VDES/AIS communication framework.

VDES is currently a work-in-progress activity,

where in the period 2019–2020 VDES is expected to

replace AIS and evolved ASM services, to be part in

year 2021 of a generalized maritime access, including

Satellite channels. Therefore, today is the right time

to deﬁne and study advanced services over VDES, ex-

ploiting the additional capabilities introduced by the

terrestrial component (ﬁrst to be introduced according

the the VDES implementation roadmap), and by the

satellite component later on.

3 USE OF VDES FOR FUTURE

E-NAVIGATION SERVICES

The availability of a standardized messaging system

extending current AIS capability will allow in the near

future to deﬁne a wide set of standardized enhanced

e-navigation services, tailored to specif user require-

ments and use cases.

Currently, several use cases associated to speci-

ﬁc services are being deﬁned. For instance we have

the UKCM (Under Keel Clearance Management), de-

ﬁned by the Australian Maritime Safety Authority,

Route Exchange Ship to Ship, Logistic services, etc

(The Nautical Institute, 2015).

In this direction, the European Space Agency

(ESA) project named “Maritime Applications exploi-

ting Reliable VHF data Exchange LOW cost System”

(MARVELOWS) (ESA, 2017), provided a feasibi-

lity study of VDES-related services starting from a

complete understanding of the users and stakeholders

DCNET 2018 - International Conference on Data Communication Networking

needs. In particular, MARVELOWS project foresees

the development of two upcoming VDES services:

– Dematerialization, which is aimed to share in-

formation among vessels navigating the same ge-

ographical area, typically delivered on paper on

a weekly or even monthly frequency, in addition

to nautical charts exchange and weather forecast

updates.

– Towage, which provides a technological support

for real-time communications between personnel

on the ﬁeld and headquarter during towage ope-

rations. The main information that could be ex-

changed are: start and end of the service, routes

and anchorages, crew list, weather now-casting.

The deﬁnition of the corresponding use-cases

(UCs) and operational scenarios has been then ba-

sed following the IALA Guideline 1117 “VHF Data

Exchange System (VDES) Overview” resulting in the

identiﬁcation of the following combination of IALA

UCs, which for MARVELOWS Dematerialisation

service are:

– UC5 Chart updates and publications;

– UC6 - Route exchange.

and for MARVELOWS Towage service are:

– UC7 Logistic and services.

In all these UCs, it is requested to perform one

or a combination of ship-to-shore, shore-to-ship and

ship-to-ship data exchange, with different communi-

cation requirements, which will affect the conﬁgura-

tion of the VDES system. Such communication re-

quirements are:

– Message size - either short or longer messages are

possible; message size affects the selection of a

speciﬁc VDES channel conﬁguration and access

scheme;

– Message timing - this indicates how frequent send

a message and which scheduling scheme to adopt:

periodic or sporadic/event-drive. This parameter

affects as well the selection of a speciﬁc access

scheme;

– Message reliability - Reliability refers to the gua-

rantee of a message reception. Within some

access schemes, collisions are possible with sub-

sequent transmission failure. It is possible

either to avoid collision (through suitable access

scheme) or implement a retransmission function

at the application layer;

– Real-Time delivery - A time interval may occur

between the data availability and its actual trans-

mission. It is possible to properly tune this time

by properly select the access scheme.

More speciﬁcally, and in relation to message ti-

ming, the following classiﬁcation can be taken as a

general reference from the standards:

– Static information. Every 6 minutes or when data

is amended (on request);

– Dynamic information. Dependent on speed and

course alteration can vary from 2 s (i.e. speed >

23 knots and changing course) to 3 min (i.e. at

anchor or speed < 2 knots);

– Voyage related information. Every 6 minutes or

when data is amended (on request);

– Safety related messages. As required.

4 ANALYSIS OF VDES

VDES deﬁnes VDE terrestrial (VDE-TER) channels,

enabling a seamless two-way data exchange between

ships and between ships and shore in coastal coverage

areas, beyond the capabilities of ASM. The commu-

nication range of VDE-TER is typically 20 – 50 nau-

tical miles (NM) and the supported capacity is up to

32 times higher than AIS. VDE allows data exchange

not bounded to the message structure of ASM, ena-

bling a whole range of new applications which may

require data exchange of higher volume and with dif-

ferent formats. In VDE-TER, data transmission is

made in the VHF maritime mobile band, within the

spectrum allocated for the uplink (ship-to-shore), na-

mely VDE1-A, and for the downlink (shore-to-ship

and ship-to-ship), namely VDE1-B. The spectrum can

be used as 25 kHz, 50 kHz or 100 kHz channels and

transmission leverage Time Division Multiple Access

(TDMA) techniques in a synchronized manner.

In addition to VDE-TER, the VHF Data Exchange

by satellite (VDE-SAT) is deﬁned, to provide data ex-

change between ships and shore via satellite. VDE-

SAT complements the VDE-TER outside the coast

station coverage area, enabling a global coverage

for VDES. Low Earth Orbit (LEO) satellites, with

600 km altitude, are considered at the present for

typical VDE satellite solutions, altought other orbi-

tal conﬁgurations (i.e., GEO) are also possible ac-

cording to the overall system design consideration.

The technical characteristics of communications ship-

to-satellite-to-shore and shore-to-satellite-to-ship are

still under development and they will be reviewed at

World Radiocommunication Conference WRC-19.

4.1 VDES Channels Conﬁguration

The spectrum allocation for the whole set of VDES

services is reported in ﬁgure 2.

VDES Performance Evaluation for Future e-navigation Services

Figure 2: VDES transmission channels.

All the VDES components (legacy AIS, ASM,

VDE-SAT and VDE-TER) leverage a common frame

structure, which has a duration of 60 seconds and con-

sisting of 2250 slots. A time slot is a time interval of

approximately 26.667 ms. An Hexslot is a group of 6

consecutive time slots (duration = 160 ms). An Uber-

slot is a group of ﬁve Hexslot (duration = 800 ms). A

subframe is a group of 15 Uberslot (duration = 12 s).

Then, a frame is composed of 5 subframe. The basic

frequency allocation plan respects the following rules

to guarantee interoperability:

– 4 channels (1024, 1084, 1025 and 1085) shared

for ship-to-shore and ship-to-satellite services;

– 2 channels (1026 and 1086) exclusively reserved

for ship-to-satellite communications;

– 4 channels (2024, 2084, 2025 and 2085) shared

between shore-to-ship, ship-to-ship and satellite-

to-ship services;

– 2 channels (2026 and 2086) are exclusively reser-

ved for satellite-to-ship communications.

It is important to highlight that the range of fre-

quencies for VDE-TER and VDE-SAT is overlap-

ping, so that speciﬁc mechanisms for VDES termi-

nal to use either terrestrial or satellite slots must be

deﬁned. The coordination between the VDE terres-

trial (ship-to-shore) and VDE-SAT uplink is achieved

using Terrestrial bulletin board (TBB) and Announ-

cement signalling channels (ASC).

4.2 VDES Burst Formats

The VDE-TER allows to alternatively use 25 kHz, 50

kHz or 100 kHz channels. The burst format varies de-

pending on the channel bandwidth as shown in Figure

The data ﬁeld carried on each burst consists of

multiple variable-length datagrams composed in turn

Figure 3: Burst format.

of the following ﬁelds: Datagram Type, Datagram

Size, Destination (optional), Transaction ID (Optio-

nal), Datagram Sequence Number (for multi-segment

datagrams), Source ID, Datagram Payload (variable

size), Data padding and a 4-bytes CRC.

In each of above channels, three different Modu-

lation and Coding Schemes (MCS) are applicable:

– MCS-1. Modulation = QPSK, Coding Rate = 1/2,

communication distance = 50 NM.

– MCS-3. Modulation = 8-PSK, Coding Rate = 3/4,

communication distance = 35 NM.

– MCS-5. Modulation = 16-QAM, Coding Rate =

3/4, communication distance = 50 NM.

VDE-SAT uses 50 kHz channels only. VDE-

SAT downlink supports three different Physical Layer

frame (PL-Frame) formats spread over 90 VDE slots,

as summarized in the Table 1.

Table 1: VDE-SAT downlink formats.

Format Usage Modulation Coding Rate Duration

PL-Frame 1

Bulletin

BPSK 1/2 90 slots

Board

PL-Frame 2

Multicast, ACK

QPSK 1/4 90 slots

Announcements

PL-Frame 3

File segment

8PSK 1/2 90 slots

transfer

VDE-SAT downlink can efﬁciently support multi-

cast of multi-packet data formats and shore originated

unicast data transfer via satellite. VDE-SAT uplink

provides the following types of functionalities:

– Two-way communications; this includes:

•

Shore initiated information polling from ships;

•

Ship initiated inquiry for information from

shore;

•

Ship initiated data transfer to shore

– Transmit Only. Collection of information from

transmit-only VDES terminals. This can be either

event-driven or periodic. The time slot and fre-

quency band for this service should be assigned

by the bulletin board and announcement signal-

ling channels.

Table 2 reports the characteristics of the ﬁve PL-

Frame formats deﬁned for the satellite uplink.

DCNET 2018 - International Conference on Data Communication Networking

Table 2: VDE-SAT uplink formats.

Format Usage Modulation Coding Rate Duration

PL-Frame 1

ACK and short

QPSK 1/3 5 slots

messages

PL-Frame 2

ACK and short

CMP/QPSK 1/3 5 slots

messages

PL-Frame 3

ACK and short

OQPSK 3/4 1 slot

messages

PL-Frame 4

ACK and short

16APSK 3/4 1 slot

messages

PL-Frame 5

Long packet

16APSK 3/4 30 slots

ﬁle transfer

4.3 VDES Shared Medium Access

Access schemes used in the VHF maritime communi-

cations are deﬁned in (ITU, 2014). Different TDMA-

based techniques are available in VDES to access

VHF channels in accordance to both the information

type and update timing requirements.

VDES deﬁnes three modes of operations. The ﬁrst

is named “autonomous and continuous” where VDES

stations must determine their own schedule for trans-

mission, resolve scheduling conﬂicts with other sta-

tions. The second mode is the “assignement” one,

allowing transmission by the reception of an explicit

allocation message. Last, the “polled” mode is based

on sending an response replying to an interrogation

message. Operations in the polled mode do not con-

ﬂict with operation in the other two modes, because

the response is expected on the channel where the in-

terrogation message was received. In order to support

the above operation modes, VDES comprises ﬁve dif-

ferent access schemes.

4.3.1 SOTDMA

The Self-Organized Time-Division Multiple Access

(SOTDMA) is the basic access scheme for AIS and

can be used for VDE messages. Suitable for perio-

dic transmissions (i.e. AIS-like), it allows to embed

in the transmitted data also the indication of the Sta-

tion ID and slot selected for the subsequent frames.

This makes possible for receiving stations to build up

a “map” of which slots are in use in the current and

in subsequent slots, supporting autonomous and con-

tinuous operations.

4.3.2 RATDMA

The Random Access Time-Division Multiple Access

(RATDMA) is suitable for sporadic (i.e. event-driven)

short messages. RATDMA is used when a station

needs to allocate a slot, which has not been pre-

announced. This is generally done for the ﬁrst trans-

mission slot during data link network entry, or for

messages of a non-repeatable type. A VDES station

randomly selects a slot among the unused ones, and

it assumes that such message can collide with other

transmissions.

4.3.3 ITDMA

The Incremental Access Time-Division Multiple

Access (ITDMA) is not used as a stand-alone access

scheme but in combination with others (usually with

SOTDMA). It is useful when there is a temporary

change in the reporting period or to announce a not

periodic message. In other words, ITDMA allows a

station to pre-announce transmission slots for a non-

repeatable message (autonomous and continuous ope-

ration). A station can begin its ITDMA transmission

by either substituting a SOTDMA allocated slot or, by

allocating a new, unannounced slot, using RATDMA.

Either ways, this becomes the ﬁrst ITDMA slot.

4.3.4 FATDMA

The Fixed Assignment Time-Division Multiple Access

(FATDMA) is applicable for base stations on shore,

for instance to support AtoN services. FATDMA allo-

cated slots are used for repetitive messages delivery,

assuming a pre-allocation of channels conﬁgured at

the installation of the VDES terminals; FATDMA sta-

tions broadcast a Data Link Management message to

advice other stations about FATDMA allocation.

4.3.5 CSTDMA

The Carrier Sensing Time-Division Multiple Access

(CSTDMA) is suitable for low-cost transceivers, fully

interoperable with SOTDMA (priority is given to

SOTDMA). Only a slot per frame can be allocated

with this scheme. Using this access method requires

no strict synchronization (which in fact is granted by

GPS units for the previous access mechanisms associ-

ated to Class A devices) but rather on timing derived

by ongoing transmissions by other stations. This re-

duces the complexity of the station, while requiring

to implement a “Listen before transmit” mechanism,

limiting the number of bytes that can be transmitted

in a slot.

5 PRELIMINARY ASSESSMENT

OF VDES PERFORMANCE

On the basis of all conﬁgurations and methods presen-

ted in section 4, it is possible to perform a preliminary

performance assessment of VDES based communica-

tion. Three application-oriented Key Performance In-

dicators (KPIs) were evaluated: nominal bit rate, i.e.,

VDES Performance Evaluation for Future e-navigation Services

the physical throughput that can be achieved (inclu-

ding overhead); user bit rate, which is the throughput

as experienced by the application; Data per frame,

which indicates the actual number of bytes that an ap-

plication can send over a single frame.

These KPIs are necessary both to properly design

the application and to identify the most suitable burst

format and transmission channels for the target ap-

plications. In addition to that, the access mechanism

shall be considered as well, in accordance to the ap-

plication requirements in terms of reliability, and pe-

riodicity. Possible combinations are proposed in the

following analysis as well.

Table 3 proposes the KPI values when consi-

dering VDE-TER downlink channels (VDE-1B) for

shore-to-ship communications. Either SOTDMA or

FATDMA can be used as access schemes and they

present identical values. In fact, shore station can be

assumed as a source of periodic updates (multicast or

unicast) towards the vessels.

Table 3: VDE-1B SOTDMA/FATDMA performance.

Burst Channel Nominal bit User bit Date per

format bandwidth rate (kbit/s) rate (kbit/s) frame (Bytes)

MCS-1

25 kHz 38.4 16.2 54

50 kHz 76.8 33.6 112

100 kHz 153.6 67.2 224

MCS-3

25 kHz 57.6 36.5 121

50 kHz 115.2 75.6 252

100 kHz 230.4 151.2 504

MCS-5

25 kHz 76.8 48.6 162

50 kHz 153.6 100.8 336

100 kHz 307.2 201.6 672

In the VDE-TER uplink, a high variability in both

size and time of sent messages can be assumed. In

case of large ﬁle transfer, ITDMA allows a better efﬁ-

ciency because the use of up to 5 slots per frame. On

the other hand, ITDMA needs to pre-announce ﬁrst

the request of slots. Consequently, actual data trans-

mission is affected by an “access delay” higher than

60 s (frame duration). Corresponding values are re-

ported in Table 4.

Table 4: VDE-1A ITDMA performance.

Burst Channel Nominal bit User bit Date per

format bandwidth rate (kbit/s) rate (kbit/s) frame (Bytes)

MCS-1

25 kHz 38.4 16.2 270

50 kHz 76.8 33.6 560

100 kHz 153.6 67.2 1120

MCS-3

25 kHz 57.6 36.5 605

50 kHz 115.2 75.6 1260

100 kHz 230.4 151.2 2520

MCS-5

25 kHz 76.8 48.6 810

50 kHz 153.6 100.8 1680

100 kHz 307.2 201.6 3135

In case the application data exceeds the allowed

data per frame, the overall application-level transmis-

sion is performed over consecutive frames, then ad-

ding 60 s time contribution per extra frame in the

message delivery time. To opposite, small sporadic

messages (i.e. requests messages, reception acknow-

ledgement, etc.) can leverage RATDMA. This latter

reduces the access delay, while presenting a collision

probability. In this case the KPIs are identical to those

presented for SOTDMA/FATDMA (Table 3). As an

alternative to RATDMA for small sporadic messages

also CSTDMA can be taken into consideration, for

low-cost receivers.

As far as the VDE-SAT downlink is concerned,

the KPIs are reported in Table 5. Similarly to the ter-

restrial component, SOTDMA and FATDMA are the

eligible access schemes.

Table 5: VDE-SAT downlink SOTDMA/FATDMA perfor-

mance.

Burst Channel Nominal bit User bit Date per

format bandwidth rate (kbit/s) rate (kbit/s) frame (Bytes)

PL-Frame 1 50 kHz 3.7 2.1 560

PL-Frame 2 50 kHz 34.1 9.6 2560

PL-Frame 3 50 kHz 51.2 28.8 7681

In the satellite uplink, the number of accessible

slots per frame is constrained by the PL-frame deﬁ-

nition. Therefore, without loss of generality we can

refer to an ITDMA scheme as a representative of all

the other schemes. Table 6 reports the corresponding

performance. It is evident that the number of bytes

hosted in a single frame is much lower than in the do-

wnlink.

Table 6: VDE-SAT ITDMA/RATDMA performance.

Burst Channel Nominal bit User bit Date per

format bandwidth rate (kbit/s) rate (kbit/s) frame (Bytes)

PL-Frame 1 50 kHz 38.4 1.6 27

PL-Frame 2 50 kHz 76.8 1.6 27

PL-Frame 3 50 kHz 67.2 50.4 168

PL-Frame 4 50 kHz 134.4 100.8 336

PL-Frame 5 50 kHz 134.4 100.8 336

6 SIMULATION

Ad-hoc simulations are executed using the Matlab

software with the aim to characterize the models of

MARVELOWS applications over VDES. In particu-

lar, VDES channel structures have been implemented

giving the ﬂexibility in selecting the channel band-

width (i.e. 25 kHz, 50 kHz and 100 kHz), the burst

format and the access scheme. Only VDE-TER chan-

nels have been considered. In the simulation models,

the trafﬁc generation model allows the speciﬁcation

of the message size (or of a probability distribution

of message sizes) and the message transmission sche-

duling. Mobility models was not implemented (i.e.,

ships entering and exiting the coverage area), because

for the proposed initial assessment the total number

of active ships in a given instant is sufﬁcient.

DCNET 2018 - International Conference on Data Communication Networking

We set up two different simulation scenarios: i)

transmission of short periodic messages, representa-

tive of applications falling in the proposed Towage

service, where periodic notiﬁcation are expected; ii)

the transmission of larger objects associated to future

data-intensive applications, such as those related to

the Dematerialisation. For the ﬁrst category of ser-

vice, it is considered of paramount importance to eva-

luate the overall load of the system and the maximum

number of users able to successfully transmit short

messages without losses. Here, the assumption is that

messages ﬁll a single slot and then data delivery is

entirely performed within the current frame if no con-

gestion occurs. For the second category of service,

instead, data transmission can require multiple slots

and then can span over consecutive frames depending

on the message size, the maximum number of slots

eligible per frame and the number of simultaneous

transmissions. In this second case, an autonomous

and continuous operational mode with different sized

messages sent in parallel by multiple VDES terminals

is assumed.

6.1 Simulation Results

The ﬁrst test envisages a terrestrial 25 kHz channel for

SOTDMA ship-to-shore communications. The fre-

quency of these messages are assumed in the range

of 1 – 6 min, while the number of active ships is in-

creased from 500 to 9000 during the simulation.

Simulation output records the average number of

slots occupied by the transmissions over the 2250

available. The results are shown in ﬁgure 4: with

growing number of active ships, the average num-

ber of busy slots increases with a linear trend, unless

the limit of slots is reached (i.e., at 2250 slots) when

about 6000 ships are active. Nonetheless, when ap-

proaching (and exceeding) this saturation limit, the

number of eligible slots for transmission is reduced,

so that the competition lead to an additional access

delay and messages loss, due to collisions of the ini-

Figure 4: Simulation of AIS messages channel occupation.

tial RATDMA access. It is important to highlight that

such service can co-exist with AIS legacy transmissi-

ons, alleviating the AIS channels by delivering addi-

tional information on the new VDE channels. Furt-

hermore, VDES data channels can leverage satellite

transmissions, to completely overcome VHF terres-

trial ranges limits of 10–20 NM. In this way, VDES

opens the way to a completely new classes of applica-

tions and overcome limitations of present AIS based

services.

Concerning the second test, a VDE-TER channel

is used to allow transmission of large data messages,

i.e. associated to images, navigation-related publica-

tions or chart updates related to the Dematerialization

service. The goal of the test is to identify the sy-

stem limit in sending relatively large-amounts of data,

when the number of active ships increases. The follo-

wing setup is considered:

– 250, 500, 750 or 1000 active terminals;

– message size is 250 Kbytes, which represents for

instance the size of a good quality compressed

image;

– each active ship transmits the message each 1 to 2

hours as sporadic guaranteed messages using ter-

restrial 100 KHz channel and MCS5;

– simulation duration of 2 days;

The ﬁrst result presented is the evolution of slots

occupation over time, shown in ﬁgure 5 for the dif-

ferent amount of active ships. Due to the necessity

of delivering data without contention, a SOTDMA

access scheme was enabled during these tests. The

average number of slots occupied for transmission is

577, 1157, 1732 and 2179 when the number of active

terminals goes from 250 to 1000 in steps of 250. The

system shows a moderate load for up to 750 terminals,

with a practical saturation of slots measured when the

terminals are 1000.

When there is a large availability of slots (which in

practice is experienced up to 750 terminals), the mes-

Figure 5: Slots used for VDE transmission as function of

time.

VDES Performance Evaluation for Future e-navigation Services

sage transmission time is predictable and solely due

to the access to the channel and the channel bitrate.

In this situation, the messages delivery time is 80 mi-

nutes in average. On the contrary, when the number of

active terminals is 1000, the effect of the access me-

chanism (SOTDMA)results into an average delivery

time of 90.42 minutes. In this case, the experienced

delay in average is similar to the average transmis-

sion intervals deﬁned for the test (i.e., 90 minutes).

In fact, if looking at ﬁgure 6, where the probability

distribution function of the message full reception is

reported (for all messages generated during the simu-

lation), it is possible to verify that few objects are for

sure delivered exceeding the 120 minutes. This means

that, according to the application transmission period

(which was deﬁned within 1 hour to 2 hours), some

data objects can be ready for the transmission while

the transmission of the previous one is not yet com-

pleted: this can create problems to the application it-

self.

The modeling of the MARVELOWS applications,

allowed to identify the target performance as function

of the number of active terminals (expected into a gi-

ven coverage area) and the channels in use. These as-

pects shall be carefully considered for the deﬁnition

of future e-navigations systems. For instance, the ex-

pected maximum and average time for the delivery of

a data object must be taken into account in designing

the application, so that transmission periods and data

sizes are carefully tailored to avoid communication

overlaps.

As support for future applications deﬁnition, the

combined use of satellite channels for VDES will pro-

vide a signiﬁcant help, since some service can be mi-

grated on satellite frequencies which offer wider co-

verage (e.g., not limited to the VHF terrestrial co-

verage of 10–20 NM) and may be less congested by

Figure 6: Probability Density Function of messages deli-

very times in saturation conditions (message load exceeding

100%, 1000 terminals).

the lack of legacy communications. This aspect will

be addressed in future work, improving the simulation

models and scenarios considered.

7 CONCLUSIONS

This paper proposes a detailed characterization of the

upcoming VDES technology with the aim to provide

a preliminary assessment of supported performance.

VDES signiﬁcantly improves on traditional AIS, by

introducing additional channels and transmission me-

chanisms. Therefore, higher data rates and more re-

sources foster the deﬁnition of new maritime data ex-

change applications. We proposed two possible ad-

vanced services leveraging VDES: Towage and De-

materialisation. The former represents a sort of en-

hancement of traditional AIS dealing with short mes-

sages, while the latter aims to extend data exchan-

ges to larger messages. In this regard, VDES of-

fers a good ﬂexibility in conﬁguring the service allo-

wing different channel bandwidths, burst formats and

access schemes. Nevertheless, the deﬁnition of new

maritime services requires a ﬁne-tuning and tailoring

to the VDES link layer characteristics. In fact, appli-

cation requirements need to meet timing and format

constraints imposed by the standards. Simulation re-

sults provide a clear indication on such limits in terms

of maximum number of served terminals and message

delivery times. Awareness on such performance fra-

mework is deemed of paramount importance in the

deﬁnition of future services, when the satellite com-

ponent will become available and will increase the

overall system capacity and service coverage out of

the coastal areas.

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VDES Performance Evaluation for Future e-navigation Services