Proceedings 2002/2003 - IRSE

EURORADIO AND THE RBC 39
4.7 DISCONNECTING
After all this effort, it comes as a nice surprise that
disconnection is quite painless – one of the
applications decides to disconnect, the command
ripples down the stack, and the protocol layers
‘unpeel’ down to the physical, GSM-R, layer where
one end hangs up.
4.8 OTHER APPLICATIONS
Well, fairly simple. What if you have made use of the
excellent feature whereby a number of transport
connections are provided over the single network
connection? To avoid an impolite interruption to an
ongoing connection, the network connection is not
released until all the transport connections using it are
released. Which, although a fine example of good
manners, could cause a problem if the ETCS application
would like to talk to a new RBC, and there is, say, a
large, non-safety diagnostics dump still being sent to the
last RBC. The solution of course is the same as in any
polite conversation – keep your sentences short.
4.9 NETWORK DISCONNECT
It has also been known for the physical connection
to break, ie for GSM to disconnect. How can this be
handled? At present there is no option but to break
down the other connected layers, and then start
again. However, there are three solutions in the pipeline
than can help shorten the time it takes to recover.
The first is a network reconnection – this has the
benefit of being relatively fast (say 5s to detect loss
of connection, then another 5s to reconnect), as no
reconstruction of the EuroRadio layers is needed.
The second is a proposed modification to
EuroRadio, called ‘fast reconnect’, where the safety
layer is maintained while the lower layers are
reconnected. This will save the three-way authentication
handshake time, but not much else, and may
not be worth developing to save some 1.2s.
A third solution is GPRS, the General Packet Radio
System, which has a completely different way of
working from the normal GSM-R data call.
5 GPRS – THE FUTURE
There is an ongoing debate over GPRS as this
paper is being written, so for completeness let’s look
at some of the upsides/downsides of GPRS whilst
the debate reaches a conclusion. But note, GPRS
does not currently form part of the ERTMS – this is a
look at the future.
As we have seen, conventional GSM sets up a data
call just like a voice call – an end-to-end connection.
The other way of working is to set up a ‘virtual’
connection that looks like a circuit to the upper layers,
but at the lower layers consists of packets of
information, fired off at intervals. Packets have to
contain all the information necessary for the network to
deliver them, but because they are only sent at
intervals, there are gaps between them that allow other
senders to transmit their information (see Figure 7).
This is ideally suited to ‘bursty’ information, such
as short occasional ATP messages, and gives the
enormous benefit of allowing several trains to share
the same channel – good news if you have a dense
railway and are running out of channels.
(application)
SAFETY LAYER
* TCP / UDP *
* IP *
(GSM-R)
Figure 7 – GPRS changes
There is a downside of course – the price you pay
is an increase in transmission delay. How much
depends on how much capacity you build into the
system – the more you pay, the better the
performance. Typically, it is expected to be about
700ms instead of 400ms, but it will vary with load.
However, there is another big plus – connection
set up time effectively vanishes. When a GPRS
mobile switches on, it ‘registers’ with the network –
a time-consuming process similar to setting up a
circuit connection. The difference is it only happens
once, probably at the start of each day. Once
registered, the mobile and the network are both
aware of each other, and can initiate data
transmission with only a very short delay to ensure
no one else is transmitting. In between, the mobile
and the network remember each other, for a
configurable time up to hours. So as long as there is
nothing to transmit, a train could go through a radio
hole (accidental or deliberate) and not notice.
This has many benefits, from base station
maintenance through to a low-cost system with discontinuous
radio coverage, but it will involve some
major changes to the EuroRadio protocol stack, and
should not be expected for maybe one to two years.
However, by having the magic abbreviation ‘TCP/IP’
cast over it, it will inevitably become the most
popular protocol stack in use by the railways!
6 CONCLUSION
We have taken the opportunity in this paper of
explaining a little more of the internal workings of
EuroRadio and the RBC, with the emphasis on
EuroRadio. While the concept of EuroRadio is simple
to understand, there are some aspects of the way it
works that anyone who uses it should be aware of.
Similarly, the RBC is easy to understand on the
surface, but this surface masks a level of complexity
that is not visible from the specifications. Interoperability
demands total compliance with a detailed
internal and external specification for EuroRadio,
whereas it is interface compliance, especially the
train and interlocking interfaces, rather than internal
functionality, that is essential for the RBC.
7 REFERENCES
Unisig document – Subset037 EuroRadio FIS.
Unisig document – Subset064 Symmetric Key
Management System Specification.