In recent years BT have started a programme of rationalisation of their System X estate to retire exchange cores (processors & switches) and re-parent concentrators onto other exchanges. Newer types of exchanges known as "Super DLEs" are being implemented as part of this process. These switches are in fact disused Mark 2 trunk switches (DMSUs) that have been converted to host concentrators, and are being used to offload the older and more costly Mark 1 switches. SystemX software is being modified by Telent to cater for the high numbers of concentrators being hosted on exchanges now.