Intelligent Transport Systems - Telenor

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on IP technology and implemented as a closed
IP network.
However, a PRT system in itself can also be
viewed as a (closed) IP network where the vehicles
moving about in the networks resemble
packets that move about in an IP network. The
Internet has proven its reliability and fault tolerance
by using protocols with distributed control
or no control at all. IP has thus become an
important metaphor for distributed control of
large systems.
The IP Metaphor
IP networks have some nice properties such as
distributed control, no single point of failure and
gradual decrease in performance with increasing
load. Probably the most basic properties of an IP
network is that all information is split into packets
and the purpose of the network is to route
such packets from source to destination. Routers
make the decisions where packets should go
based on information embedded in each packet
and partial information of the state of the network.
Hence, packet switching and partial network
information are important keywords. When
a fault occurs or a line is not working in the
direction for the packet to be sent, the packet
will be routed around the problems.
The well known problems of delays and packet
losses is normally an effect of the Internet being
susceptible to demand bursts exceeding the
capacity (bandwidth) of specific segments or
routers, or to unstable hardware. An IP network
designed as a closed network with a maximum
capacity, calibrated for a defined demand, and
with controlled hardware, should meet the reliability
and safety levels analogous to those
required for transport of personnel and goods.
Accordingly some of the properties of IP networks
may be used for the design of PRT control
systems as well. Regard the following IP
analogy for PRT:
Each vehicle acts as a packet and each intersection
in the network (in PRT normally conceived
of as a merger or demerger) acts as a router. As
routers in an IP network, each intersection is
numbered and only needs to know the direction
to send vehicles given their destination. This will
work for the problem of getting a vehicle from A
to B. However, as with IP networks, the real
solution for PRT is more complex.
Safety
Things may happen in the IP domain that would
not be acceptable in the PRT domain: retransmissions,
packet losses, etc. This indicates that
there will be parts of the IP analogy that do not
make sense at all for PRT. Collision detection
has to take place a priori, and definitely not a
posteriori. Thus, it has to take the form of detection
of a collision possibility, issue a warning,
and take measures to avoid it. A control system
for a PRT circuit has to be designed so that
errors are more or less impossible, i.e. highly
improbable conforming to a framework of public
transport. “Packet retransmission” and other
relaxed methods of IP are to be excluded. Further,
it implies that monitoring the results is one
thing – making decisions in advance to avoid
them is a very different but essential thing.
Two principles should apply:
• Only make decisions – in advance – that are in
accordance with the rules and the present situation;
• Always monitor the present state of the system,
and take appropriate action if there is
anything wrong, factual or expected.
Transmissions, Terminals
and Stations
In IP, the transmission of packets within a segment
is for all practical purposes instantaneous.
Therefore, it is satisfactory to transmit a single
packet at a time. This is not the case for PRT, as
vehicles spend time in a network segment. There
may therefore be a number of vehicles within a
given segment at a time. This requires co-ordination
between the vehicles within a segment, e.g.
by vehicle-to-vehicle anti-collision systems.
However, according to the principles outlined
above, the vehicles within the network segment
should manage this by themselves.
As in IP networking, dynamic alternative routing
must also be implemented, at least in potential
bottlenecks.
When a packet in an IP network has reached its
destination, everything is fine and the application
or operating system of the receiving host
(the terminal relative to the network) can take
over. In PRT however, when a vehicle enters its
destination, the problem of stations begins. In
terms of the IP analogy, a station would be a terminal
running “an application named station” as
far as the network concerns handed over and forgotten.
Stations do still pose some real problems,
as we see in Anderson’s paper, as does the distribution
of vehicles between stations according to
demand. IP may not have the most relevant
answers to this problem complex.
A Hybrid Approach
A hybrid approach for PRT – relative to IP –
would be to make vehicles act as routers. This
requires that each vehicle also has available the
entire routing table for the present network. For
IP this does not make sense as the Internet for all
Telektronikk 1.2003