Proceedings 2002/2003 - IRSE

38
EURORADIO AND THE RBC
Now we have made it to the end, almost! There is
a fourth message confirming that the RBC accepts
the connection – when the train application receives
it, it knows a connection exists. We have an
authenticated safety connection, over GSM-R, at
last!
Now, that initiation message from the train
application can be delivered to the RBC, and the
reply returned from the RBC to the train.
4.5 SET-UP TIME
It is clear that there is a lot of communication
across the air interface that is nothing directly to do
with the application message from the train to the
RBC – and it all takes time. It is possible to estimate
this, assuming that the transmission delay across
GSM-R is about 400ms, as follows:
– Train sets up GSM-R connection to
RBC (ISDN) 5.0s
– Data link layer set-up
set async. balanced extended 0.4
respond with UA 0.4
– Network layer set-up – none 0
– Transport layer set-up
send CR TPDU 0.4
receive CC TPDU 0.4
– Safety layer set-up
send first authentication message 0.4
receive second authentication message 0.4
send third authentication message 0.4
receive fourth authentication message 0.4
giving a total of 8.2s
This is why such good radio coverage is required
– unplanned disconnection and reconnection must
be a rare event if the railway’s performance is not to
be adversely affected. As always, good network
planning by experienced experts is essential.
And if you want a low-cost system, how might this
be achieved? Omitting base stations is quite
effective – deliberately making the system
discontinuous where there is no data to be sent to
the train or RBC. But this set-up time becomes a
limiting parameter, as you must have coverage while
all this is going on. Wouldn’t it be nice to have a
packet system where the set-up could be retained
whether or not the physical GSM-R layer was
connected? Well there is one, called GPRS, but we
come back to that later.
4.6 CONNECTION
So far, we have a connection, but we are doing
little with it. Now we must start sending application
messages end-to-end. And there are a few points to
remember. The first is that buffer sizes are not
infinite. For interoperability, it was necessary to
define a maximum application message size – to
avoid train equipment from one company sending a
20kbyte message to another company’s RBC which
only has a 10kbyte receive buffer.
The size chosen is 1023 bytes. ‘Why?’ is not so
important, though there are reasons. You can send
messages bigger than this, but it will be up to the
application to chop it into digestible 1023-byte
mouthfuls.
In addition, there is another port defined, although
it is not necessarily implemented by all
manufacturers (it is not required for European
interoperability and so is not mandatory). This can
be used to set up a non-safe transport connection
for other applications. Of course, there is still only a
GSM-R data channel at the bottom of all this,
probably working at 4800bps, so care is needed in
allocating channels – it is not quite the way to give
passengers Internet access!
During the course of a connection, there are two
events that will occur with irritating regularity – both
confusingly called ‘handover’. One is the Base
Station handover, a function of GSM cellular
networks, and the other is the RBC handover that is
built in to ETCS.
4.6.1 GSM-R Base Station Handover
Every few kilometres, the train will move from the
coverage of one Base Station to another. There is a
complex handover process initiated by the mobile
involving a change of frequency and time slot. The
effect is to produce a break in communications of
some 200-300ms, during which a receiver will
receive severely corrupted messages. The HDLC
Link layer will detect the errors and reject anything
that looks anything like a frame, but there then
needs to be an error-free period while recreatetransmission
occurs. The effect seen by higher
layers and the application will therefore be a delay,
which gradually decreases to normal.
4.6.2 RBC Handover
The other handover is completely a function of the
application. Eventually the train will reach the
boundary of one area of control and need to pass
control over to the next RBC. The use of two mobiles
on the train is considered the best way to handle the
problem of passing control of the train from one RBC
to the next. The train has a connection with the ‘old’
RBC over one radio. The train dials the next RBC,
sets up a new connection, the RBCs hand over, and
then the train disconnects from the old RBC. To do
this means the new connection set-up must start a
long way before it is needed, but that is OK – it can
go through the same Base Station and network, just
to a different destination.
But what happens at a national border? There is
likely to be an RBC handover at exactly the same
place that the network changes. This really is very bad
news, as it can take over 30s to subscribe to a new
network and, even worse, the process must start long
before a connection is needed, that is the train must
connect to the new network when it is well within the
coverage of the old network. Normally, a GSM mobile
will look round for the network with the strongest
signal strength (depending also on an internal list of
preferred networks), so it will try to subscribe to the
old network. So it is essential to have a way of forcing
the mobile to subscribe to the new network, against
its own natural inclinations. Of course, the new
network must also be available, so it is likely that
new-network base stations will have to be deployed
into the old-network coverage area, requiring an
outstanding level of co-operation between railways –
and of course between licensing authorities