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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 in Figure 1.4 which focuses the discussion below.

Figure 1.4: 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 itself.
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. Outgoing IDL telegrams 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 Chapter 5 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

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Matthew Morley, Edinburgh. Date: 29 November, 1998