Circuit ID - Circuit Designations in The United Kingdom

Circuit Designations in The United Kingdom

Each carrier (Public Telecomms Operator) in the UK has its own form of designation. The Post Office/BT system is described here from the original term ‘Engineering Circuit Designation’. Other PTOs have their own scheme and would be suitable for inclusion.

The Post Office or BT system historically used PW and R, optionally followed by a Region and/or Area code, followed by a number of digits between 4 and 6 for rented, analogue private lines. A Region code might have been LR for London Region or ER for Eastern Region, and for an Area L/NW for North West London or CB for Cambridge. In all but the rarest instances these must have been migrated to AX by the Analogue Upgrade project to digitise as much as possible for FDM Offload, (no) dc path uniformity and remote access in maintenance.

Between 2 locations then:

• AX nnnnnn designates an analogue (2w or 4w) presented link.

• KX nnnnnn designates a digitally presented link up to 64 kbit/s.

• NX nnnnnn designates a digitally presented link from 128 up to 1,024 kbit/s.

• MX nnnnnn designates a digitally presented link from 2 Mbit/s upwards

Each of the prefixes may have an addition making MX/GB, KX/INT for example.

There are many specialist designations covering bearers for other purposes, for example for IP and bearers provided for other PTOs. One common other use has been IMUK and IMGB for the 2 Mbit/s link from the public exchange to a customer location for the delivery of ISDN30 in the earlier DASS and more recent I.421 format.

The trend has been that the local UK area suffixes are no longer used though legacy lines with these may still be in place.