1.4. Inter-SSI Communications
In any signalling scheme there may be a requirement, depending on the
physical extent of the network, to divide the railway into a number of
areas (or blocks), each controlled by a separate interlocking. Where
SSI is concerned this distribution of control is further necessitated
by the limited capacity of a single central interlocking processor.
Limited capacity means the signalling area under the control of one
operator will be divided between a number of Interlockings. On this
scale the divisions may be rather small so it is important that
boundaries are not only transparent to traffic in the network, but
also transparent to the signal operator. The less fragmented the
operator's view of the network is the better SSI can approach the
broad aim in railway signalling of relieving the signal operator of
the greater part of the burden of the safety of railway traffic.
In order for the control of a train to pass safely between
interlocking areas some communication mechanism is needed to transfer
information that needs to be shared about the status of the network in
the fringe area. A typical situation is illustrated by the scheme plan
which focuses the discussion below.
East and West communicate to set routes from entry signal
S7 or S9 in East, to the exit signal (S5) in
West---since West controls the tail portion of both routes (just
that over T7, plus overlaps). There are no West to East
routes as those up to S8 are contained in West, and routes
onward from this signal are controlled by East, as is the signal
Here the cross-boundary routes converge before the boundary and
terminate at a common exit signal. It is also possible that routes
will diverge again after the boundary. In general there will be
numerous lines linking the two interlockings. Signal engineering
practice seeks to avoid placing boundaries through points since the
complications introduced significantly increase the communication
overheads. For the same reason boundaries are avoided if there would
be points immediately beyond the signal at the boundary.
Data are transmitted between Interlockings by means of a high speed
communication bus called the
Internal Data Link.
Several Interlockings can be connected to a single bus, but normally
an individual need only exchange data with its nearest neighbours.
are prepared by commands in the Geographic Data and the generic
control program is configured to copy their contents to the link at
least once a major cycle. Two main classes of data are required to be
communicated: continuously required data such as the aspects displayed
by signals in advance of the boundary, and intermittently required
data such as requests from one SSI for another to perform some
signalling function such as moving a set of points or setting a route.
Exactly what data need to be communicated depends on the nature of the
boundary---our concern in
will only be with the complex situation of setting routes that are
divided by Interlocking boundaries. Typically, the inter-SSI
communications these induce occupy about twenty percent of the
capacity of one Interlocking.
- 1.4.1. Setting routes over Voundaries
- 1.4.2. Releasing Sub-routes over Boundaries
- 1.4.3. Implementing Remote Route Locking
Matthew Morley, Edinburgh. Date: 29 November, 1998