Journal of Transportation Technologies, 2013, 3, 214-219
http://dx.doi.org/10.4236/jtts.2013.33022 Published Online July 2013 (http://www.scirp.org/journal/jtts)
Reliable Train Network with Active Supervisor
Mai Hassan, Ramez M. Daoud, Hassanein H. Amer
Electronics Engineering Department, American University in Cairo, Cairo, Egypt
Received March 19, 2013; revised April 20, 2013; accepted April 30, 2013
Copyright © 2013 Mai Hassan et al. This is an open access article distributed under the Creative Commons Attribution License,
which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
ABSTRACT
In this paper, a new reliable hierarchical model is suggested for a two-wagon train Networked Control System. Each
wagon has a Controller that carries the control load and an Entertainment server that handles the entertainment. A su-
pervisory controller runs on top of the two controllers and the two entertainment servers. Contrary to a similar model in
the literature, the Supervisory node replaces a Controller as soon as it fails (Active Supervisor). All system states are
analyzed and simulated using OPNET. It is shown that, for all states, this architecture has zero control packets dropped
and the end-to-end delay is below the maximum target delay. A comparison between this Active model and the other
model in the literature is presented. It is found that the entertainment in this new architecture is kept available for the
passengers in more of the system states when compared to the architecture previously presented in the literature.
Keywords: Ethernet; Train; Networked Control Systems; Active Supervisor; Hierarchical Architecture
1. Introduction
In industrial and transportation systems, Networked Con-
trol Systems (NCSs) are currently widely applied [1-4].
Previously, deterministic protocols that ensure meeting
critical real-time delays and no packet loss for the small
control packets such as Controller Area Network (CAN),
PROFIBUS and PROFINET, were used [5,6]. Ethernet
(IEEE Std. 802.3) [7], despite its non-deterministic na-
ture, is a promising protocol that is being applied in NCS
[8,9]. Packet Scheduling and reformatting the Ethernet
packet were the main approaches to overcome the non-
deterministic nature of Ethernet [10,11]. Furthermore,
Rockwell automation, the ODVA, EtherNet/IP, TT Eth-
ernet and FTT Ethernet have implemented different mo-
difications to the protocol, some of which are in the
course of standardization [12-16].
Train operation, safety, collision avoidance and ex-
change of information are the main tasks to be handled
by the train networks [17]. As the demand for more and
more entertainment services on board of trains is in-
creasing, Ethernet became a promising technology.
Due to its large bandwidth, Gigabit Switched Ethernet
was successfully tested as a one-wagon train network to
carry both control and entertainment loads within a
wagon [18]. The entertainment load is represented as
video streams and running Wi-Fi applications [18]. The
network model was further enhanced to increase its reli-
ability at the Server level [19]. In [19], the authors used
one server to handle each type of load. The control load
was handled by the Control Server (Controller) and the
entertainment was handled by the entertainment server.
The entertainment server acted as a backup for the Con-
troller; it would handle the control load in case of the
Controller failure [19]. Simulations proved that the re-
quirements on the control packet end-to-end delays were
met in both [18,19]. The reliability of the network was
further enhanced and its performability was calculated
[20,21].
In this research, a two-wagon train control network
using unmodified Ethernet is presented. A hierarchal
structure at the Server level, including an active supervi-
sor, is simulated. As soon as a controller fails, this active
supervisor replaces it and carries its load. The system is
modeled using OPNET network simulation tool [22].
The control packets are sampled at different sampling
periods [23]. Furthermore the entertainment load is
simulated as compressed DVD video streaming and 4
different Wi-Fi applications which are web-browsing,
FTP, database and email access. Additionally, the net-
work is simulated in all possible faulty server states as
well as the fault-free state. It will be shown that the ar-
chitecture with an active supervisor will function cor-
rectly irrespective of server failures. The architecture will
then be compared to the one presented in [24].
The rest of this paper is organized as follows. Section
2 summarizes the previous work done in the field of
C
opyright © 2013 SciRes. JTTs

M. HASSAN ET AL. 215
hierarchal Ethernet train networks. In Section 3, the
newly proposed model will be illustrated. Simulated
scenarios and their outcomes shall be discussed in Sec-
tion 4. Section 5 concludes this paper.
2. Background
In [24,25], the authors proposed a Gigabit Ethernet train
network using the unmodified IEEE 802.3 standard; a
hybrid model was introduced in which multiple control
sampling periods were used. More details about this hy-
brid model are presented below.
2.1. Hybrid Network Model
In the IEC 61375 Standard (Trains network Standard),
different sensor/actuator sampling periods are specified
[23]. In [18,19], only one sampling period is simulated
per network scenario; one scenario uses the 1 ms sam-
pling rate while the other scenario uses the 16 ms sam-
pling rate. According to [23], 16 ms is the sampling pe-
riod of the majority of train sensors/actuator and 1ms is
the smallest sampling period in a train network. In [24],
the authors formulated the network to contain different
sampling periods, specifically combination of 16 ms and
1 ms.
A train wagon contains a total of 250 sensor/actuator
nodes. In [18,19], only 1:1 sensor:actuator ratio is simu-
lated, but in [24,25], the network contained more diverse
combinations of sensors:actuators to simulate a more
realistic scenario. The sensor/actuators were divided into
3 different groups with different sensor:actuator ratios.
2.2. Passive Supervisor
In [24], the authors presented a hierarchal control struc-
ture including having a Passive Supervisor node in addi-
tion to the 2 servers per wagon, resulting in a total of 4
servers and a supervisor. This node is assumed to be the
most reliable node in the network; it acts as a backup for
any controller after all other Servers/Controllers have
failed.
In case the Controller fails in one of the wagons, the
Entertainment Server drops its main functionality (han-
dling the entertainment load) in order to handle the
wagon’s control load. For the Entertainment Server to
handle the control load, the sensors send 4 streams of
their data to the Controllers and the Entertainment Serv-
ers. Only the Server handling the control load is the one
responsible for making the control decision and sending
the control action to the corresponding actuator. If both
Servers in a wagon (Wi) fail (the Controller Ki and the
Entertainment Server Ei), the Entertainment Server (Ej)
of the other wagon (Wj) drops its entertainment load and
handles the Wi control load [24].
If three of the four Servers fail, then the remaining op-
erational server in a wagon (Wj) (either Controller Kj or
Entertainment Server Ej) handles its own wagon load.
This is also the point at which the Supervisor starts to
come into action and handles the control load of the other
entirely-failed wagon Wi. The sensors of both wagons
start to send their data to the Supervisor node after the
failure of three Servers. In case of the failure of all four
servers, the Supervisor node will handle the control load
of both wagons; this is again under the assumption that
the Supervisor will be the last to fail among all Servers/
Controllers.
3. Proposed Network Model
The same network architecture presented in [24] is used
in this study for comparison purposes. The network con-
sists of 2 single wagon networks interconnected via a
switch (Intermediate Switch) and a 10 Gb link to a Su-
pervisor Node. This 2-wagon model represents the main
train building unit such as the Siemens Desiro diesel or
the Siemens electric multiple unit (DMU or EMU) [26].
The two-wagon train unit network model is illustrated in
Figure 1.
There are 60 seats per wagon [27]. In each wagon, all
nodes are connected to the wagon’s Main Switch (MS)
via Gigabit Ethernet fabric. The forwarding rates of the
two Main Switches and the Intermediate Switch are 6.6
Mpps [24]. This rate is much lower than 38.2 Mpps, the
forwarding rate of the commonly available switches in
the market such as the Cisco Catalyst 3560 Gigabit
Ethernet switch [28].
Each wagon has 250 nodes (Sensors/ actuators) di-
vided into 3 groups with different sensor:actuator ratio as
in [24,25]. Group 1 (G1) has a 1:1 sensor:actuator ratio,
Group 2 (G2) has a 2:1 sensor:actuator ratio and Group 3
(G3) has a 3:1 sensor:actuator ratio. There are 60 nodes
in G1 running at a sampling period of 1ms, 150 nodes in
G2 and 40 nodes in G3. Nodes in G2 and G3 are running
at a sampling period of 16 ms [24,25]. To simulate the
worst end-to-end delay for the control packets, the sen-
sors/actuators are located to ensure maximum propaga-
tion time.
Moreover, the same entertainment services are run in
the form of 60 Wi-Fi nodes (one laptop per seat), running
4 different applications as in [24]: web-browsing, FTP,
database and e-mail access. These nodes/laptops are
connected to the wagon MS via a wireless router. Also,
60 Compressed DVD video streams are running at a rate
of 5 Mbps connected to the MS of each wagon [29].
In the fault-free case, there are 2 operational Servers in
each wagon; one Control Server or Controller (K) and
another Entertainment Server (E). They handle the con-
trol and the entertainment loads of the wagon respec-
tively. A watchdog signal of 32 bytes is sent every 1ms
in between all four Servers and each other as well as with

M. HASSAN ET AL. 216
Figure 1. Two-wagon train model.
the Supervisor node. This watchdog enables all Servers
and the Supervisor to be aware of the status of the other
Servers. Also, there are 4 cameras per wagon located at
each door to enhance safety [30]. They send video sig-
nals directly to the Supervisor for safety monitoring pur-
poses by the train driver. The reliability of the Supervisor
is assumed to be the highest as in [24] to ensure it has the
lowest probability of failure and therefore the longest
lifetime for comparison purposes.
Active Supervisor
In this research, unlike the previous system, the Supervi-
sor acts as an Active Supervisor. It is the primary
backup for the Controllers (Ks) in each wagon. If the
Controller in either wagon fails, the Supervisor handles
the control load of the wagon. Additionally, in case the
Controller of the second wagon fails, the Supervisor in
such case will handle the control load of both wagons.
The fault-tolerance relation between the Controllers in
both wagons is no longer present, i.e., they no longer
carry each other’s control load. Furthermore, the Enter-
tainment Servers do not act as backups to the Controllers
and do not handle any control load unlike the presented
case in [24]. In regard to entertainment services, the same
conservative approach followed in [24] is still applied. If
the Entertainment Server fails, the entertainment services
are dropped due to the high safety requirements in train
operations. However, the Entertainment Server does not
drop the entertainment load to handle any control load.
The sensors send their data only to their corresponding
Controller and to the Supervisor. So, for example, the
sensors in wagon 1 only send their data to Controller 1
(K1) and the Supervisor as these are the only nodes to
handle the control load.
4. Simulation Outcomes
In [24], the simulations presented the outcomes for the
unique states which the network experiences. The same
approach is used in this research. However, after analyz-
ing the unique states of functioning Servers at a time,
only 10 scenarios need to be simulated using OPNET
network simulator. Please note that the simulation of
only the unique states means that all possible scenarios
are accounted for by the simulations, because they are
represented by one of those unique states.
4.1. Simulated Scenarios
In [24], 11 unique states were simulated. In this research
however, only 10 states are needed. Table 1 shows the
10 unique states that have been simulated using OPNET.
The scenarios simulate all the possible combinations of
operational Servers that the network can go through. The
main measuring metric for network performance is the
control packet end-to-end delay. As shown in Table 1,
all end-to-end delays are below the sampling periods of
the corresponding group, thus, fulfilling the delay re-
quirement [31]. Also, in the column labeled Entertain-
ment, the entertainment services are on in the specified
wagons.
In [24], there were 11 scenarios simulated while here,
only 10 states are simulated. This is due to the fact that,
in [24], Scenario EiS appeared twice. In the first case the
Entertainment Server (Ei) was carrying the control load
of Wagon Wi, while in the second case, it was carrying
the control load of the other Wagon Wj. As the Enter-
tainment Server in this research does not handle any con-
trol load, consequently, both cases end up being identical.
In the active case, the supervisor node carries the control
load of both wagons while each entertainment server
handles its own wagon entertainment load.
All the results were obtained after a 95% confidence
analysis. The results shown represent the mean value of
the maximum packet end-to-end delay obtained from all
runs. The maximum deviation (Δ) from these means is

M. HASSAN ET AL. 217
0.411 µs. Furthermore, the delays for the door cameras
and the video streaming were below the acceptable delay
requirements. As per [32], the OPNET results presented
in this research are comparable to hardware implementa-
tion outcomes.
For completeness, Table 2 has the corresponding data
for the Passive Supervisor architecture presented in [24].
Figures 2-4 illustrate a sample of the OPNET results for
the Active Supervisor architecture.
In all figures, the x-axis is the simulation time in sec-
onds and the y-axis is the delay in seconds. The red dots
in the graphs are the delay from the sensor to the con-
troller and the delay from the controller to the actuator
are the blue dots.
Table 1. Total end-to-end delay (µs)—active supervisor.
Server
Handling
Control Load
Scenarios
Simulated
Wi W
j
G1 G2 G3 Entertainment
KiKjEiEjS Ki K
j 26.55 14.24 21.14 Wi, Wj
KiEiEjS Ki S 17.12 12.22 19.12 Wi, Wj
KiKjEiS Ki K
j 17.98 13.22 15.99 Wi
KiEiS Ki S 14.41 11.63 16.45 Wi
KiEjS Ki S 9.77 11.50 9.50 Wj
KiKjS Ki K
j 15.07 12.18 15.71 -
EiEjS S S 17.02 12.17 17.98 Wi, Wj
KiS Ki S 11.85 11.76 12.54 -
EiS S S 9.36 11.33 9.10 Wi
S S S 12.4 11.3 9.2 -
Table 2. Total end-to-end delay (µs)—passive supervisor
[24].
Server
Handling
Control Load
Scenarios
Simulated
Wi W
j
G1 G2 G3 Entertainment
KiKjEiEjS Ki K
j 33.2 34.6 20.5 Wi, Wj
KiEiEjS Ki E
j 17.3 16.8 17.7 Wi
KiKjEiS Ki K
j 18.6 15.1 17.4 Wi
KiEiS Ki E
i 15.3 16.7 9.2 -
KiEjS Ki E
j 14.1 16.5 9.2 -
KiKjS Ki K
j 15.2 14.3 16.6 -
EiEjS Ei E
j 15.2 14.3 16.7 -
KiS Ki S 10.5 13.2 12.4 -
EiS Ei S 10.5 13.2 12.3 -
EiS S Ei 10.2 14.9 12.5 -
S S S 12.4 11.3 9.2 -
Figure 2. Fault-free scenario (KiKjEiEjS)—G2.
Figure 3. One controller and one entertainment server in
different wagons (KiEjS)—G3.
4.2. Outcomes Comparison
When comparing the outcomes with the results in [24], it
can be noticed that, in the fault-free scenario (KiKjEiEjS),
the delay is generally lower in the Active supervisor ar-
chitecture. This is due to the fact that the sensors send
their data to their corresponding controller and the super-
visor node only rather than 4 different streams to all
Servers (2 Controllers and 2 Entertainment Servers).
In other scenarios such as KiEiEjS, in [24], the Con-
troller node (Ki) carries the control load of wagon Wi
while the Entertainment Server (Ej) has dropped its en-
tertainment load and is handling the control load of Wj.

M. HASSAN ET AL. 218
Figure 4. Supervisor only (S)—G1.
In this research, since Ej does not drop its entertainment
load, S handles the control load of wagon Wj. Hence, the
passenger can still enjoy the on-board services and will
not be affected by the failure that occurred.
Also, in case of EiEjS in the active model, the super-
visor S handles the control load of both wagons but the
entertainment services are still running in both wagons.
In [24], each of the Entertainment Servers carries its own
wagon control load after dropping the entertainment load.
Therefore, the delay in the Active Supervisor case is
somewhat higher when compared to the Passive super-
visor case.
Comparing another scenario such as “S”, the delay is
the same in the active or passive models since all the
entertainment is dropped in both cases and the sensors
only send their data to the supervisor. When monitoring
the forwarded traffic by the intermediate switch it was
verified that the same amount of traffic (133.9 Mbps)
was forwarded.
The main benefit when comparing the active supervi-
sor case to the passive supervisor case presented in [24]
is that the passenger will only be affected by a failure
when the entertainment server of the wagon fails. Con-
sider scenarios KiKjEiEjS, KiEiEjS and EiEjS; in these
three scenarios, the entertainment is functional in both
wagons. On the other hand, in the passive scenario, the
entertainment is functional in both wagons in the fault
free scenario only. Also, only one wagon will experience
the failure of the entertainment in scenarios KiKjEiS,
KiEiS, KiEjS, and EiS. When comparing with the passive
scenario, the passengers will enjoy the entertainment ser-
vices in one wagon only in scenarios KiEiEiS and KiKjEiS.
Table 3 shows a comparison between the Active Su-
pervisor architecture and the passive Supervisor archi-
Table 3. Number of states with enabled entertainment.
Entertainment
Enabled in Passive Supervisor Active Supervisor
2 Wagons 1/18 States 4/16 States
1 Wagon 4/18 States 8/16 States
tecture with respect to the number of states that have the
entertainment enabled in either one or two wagons. Due
to the symmetric nature of the network, the states:
KiEiEjS, KiKjEiS, KiEiS, KiEjS, KiS and EiS are dupli-
cated. Consequently, in the Active Supervisor architec-
ture, the 10 states are expanded to 16 and, in the Passive
Supervisor architecture, the 11 states are expanded to 18.
Note finally that, in the Active Supervisor architecture,
when the controller of a wagon fails, its only backup is
the Supervisor node. In [24], for each failing controller,
there are 4 other machines that act as backups.
5. Conclusions
Ethernet is an interesting technology in the field of Net-
worked Control Systems. The use of Gigabit Switched
Ethernet on-board of trains has already been reported in
the literature. Previously, a hybrid network model was
proposed for a two-wagon network model. Furthermore,
a hierarchal structure at the controller level was proposed.
However, the supervisor node was a passive one and it
only handled the control load as a last resort.
In this paper, a new role was defined for the supervisor.
As soon as either Controller fails, it acts as a backup for
that failed Controller and handles its control load; there-
fore, it became an active node. For safety purposes, no
other node acted as backup for any failing Entertainment
Server; the entertainment was dropped when the Enter-
tainment server failed.
All possible combinations of operational Servers/
Controllers were simulated using OPNET. It was shown
that the control packet end-to end delays met the control
requirements and that no packet was dropped. The net-
work was proven to function properly even after the fail-
ure of all Controllers and Entertainment Servers; the Su-
pervisor was able to successfully carry the control load of
both wagons. It was also shown that this architecture has
the advantage of keeping entertainment services opera-
tional for a longer period when compared to other hier-
archical architectures in the literature.
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