The present invention relates to telecommunication systems and more particularly to digital hierarchy systems (SDH). In such systems data is switched within an individual station to the appropriate output port. Also alternate routes are provided between stations by which data can be transmitted from an originating stations port to a receiver stations port so that failure of one route, e.g. due to accidental damage, will not prevent the data from reaching the receiver stations port via the other route.
The data is sent in the form of two packages or so-called virtual containers which essentially are made up of two sections, one being the actual data which is transmitted, i.e. the soiled payload, and the other being data which is concerned with the integrity of the data, referred to hereinafter as overhead or ancillary data.
One of the requirements for a system of the kind outlined above is that there must be some means for checking whether the package is correctly switched (or routed) at each individual station along the overall path of the data. Essentially this is achieved by use of the ancillary or overhead data area at each station in order to include an identity, and other monitoring information, to ensure correct switching.
In our co-pending application no. 9301575 we disclose an arrangement where the package is identified in terms of the channel number of the data, and the receiving stations card number. However there is a potential problem with such an arrangement in situations where the package travels by the alternative route referred to earlier. In the event of failure of the first route the package will acquire a new channel and card number. This means that the destination port of the station will not recognise the package because it will be looking for a package which has an identification which is characteristic of the first route. This problem is a direct result of the use of the alternate route protection identified earlier.
The present invention is concerned with overcoming this problem.