When Marconi was sold to Ericsson in January 2006, Telent plc retained System X and continues to support and develop it as part of its UK services business.
The first local digital exchange started operation in 1981 in Woodbridge, Suffolk (near BT's Research HQ at Martlesham Heath).
Developed by BT at Martlesham Heath and based on the Monarch PABX, the first example was put into service at Glenkindie, Scotland, in 1979, the year before the first System X.
[2] Several hundred of these exchanges were manufactured by Plessey[3] and installed in rural areas, largely in Scotland and Wales.
DLEs and DMSUs operate in major towns and cities and provide call routing functions.
This resilient configuration allows for hardware changes to fix faults or perform upgrades without interruption to service.
The CPUs in an R2PU processing cluster are quadruplicated to retain 75% performance capability with one out of service, instead of 50% if they were simply duplicated.
It also receives dialled information from the subscriber and passes this to the exchange processors so that the call can be routed to its destination.
Incoming traffic on the 30 channel PCM highways from the Concentrator Units is connected to Time Switches.
The Time Slot inter-connections are held in Switch Maps which are updated by Software running on the Processor Utility Subsystem (PUS).
The DMSU network in London has been gradually phased out and moved onto more modern "NGS" switches over the years as the demand for PSTN phone lines has decreased and BT has sought to reclaim some of its floorspace.
The NGS switch referred to is a version of Ericsson's AXE10 product line, phased in between the late 1990s and early 2000s.
The Processor Utility Subsystem (PUS) controls the switching operations and is the brain of the DLE or DMSU.
The advanced features of this fault-tolerant system help explain why these are still in use today – like self fault detection and recovery, battery-backed RAM, mirrored disk storage, auto replacement of a failed memory unit, and the ability to trial new software (and roll back, if necessary) to the previous version.
This is a very rare occurrence, as due to the design of System X it will isolate problematic hardware and raise a fault report.
Should the base cluster or switch be irrecoverable via restarts, the latest archive configuration can be manually reloaded using the restoration procedure.
This can take hours to bring everything fully back into service as the switch has to reload all its semi-permanent paths and the concentrators have to download their configurations.
The space switch is a more complicated entity, but is given a name ranging from AA to CC (or BB within general use), a plane of 0 or 1 and, due to the way it is laid out, an even or odd segment, designated by another 0 and 1.
Mark 1 DSS is controlled by a triplicated set of Connection Control Units (CCU's) which run in a 2/3 majority for error checking, and is monitored constantly by a duplicated Alarm Monitoring Unit (AMU) which reports faults back to the DSS Handler process for appropriate action to be taken.
It is an optical fibre-based time-space-time-space-time switching matrix, connecting a maximum of 2048 2 Mbit/s PCM systems, much like Mark 1; however the hardware is much more compact.
The Mk2 DSS unit is a lot smaller than the Mk1, and as such consumes less power and also generates less heat to be dealt with as a result.
In the early 2020s, BT commenced rationalisation of its SystemX estate to save power, cost, floorspace and improve reliability – the ageing Mk1 switches were becoming a maintenance headache.
This major software revision is expected to last until network operators retire their SystemX estate, even though retirement plans are invariably taking longer than planned due to the inability of IP-based networks to handle legacy services, especially machine-to-machine communications.
MTS links are 'nailed up' between nodes by re-purposing individual 64 kbit/s digital speech channels across the switch into permanent paths for the signalling messages to route over.
However, the omission of Marconi from BT's 21CN supplier list, and the shift in focus from telephony to broadband, led to much of the System X estate being maintained.
The switched telephone network (both PSTN and ISDN) is due to be turned off on 31 January 2027, after customers are moved to voice over IP services.