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Fire Alarm Signalling May Be Affected

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As a part of the requirement to provide a greater capacity BT are fitting new equipment at all telephone exchanges over the next 5 years. This £400M investment will make some small changes to the way phone lines are connected. The new system is titled 21st Century Network (21CN). Most existing equipment will function normally and you may notice a better quality of line. But the changes may effect alarm transmissions for some equipment.

The characteristics of 21CN that have the potential to affect the operation of Customer Premises Equipment (CPE) are:

· Maximum off-hook loop current reducing from 40mA to 25mA (no change to published minimum of 25mA as set out in SIN 351 and 352)

· Ringing implementation moving from predominately unbalanced to predominately balanced (balanced ringing applies ringing voltage to both legs in the pair rather than one)

· Echo Cancellation: always used unless high-speed fax or modems are detected

· Audio Path Discontinuities: with the introduction of jitter buffers, either during a voice call or when modems or faxes are detected

· End-to-End Delay: round trip delay increasing for all calls.

Testing Existing Equipment

Experience indicates that End to End delay needs care when considering existing alarm transmitting arrangements. BT have set up the 21CN CPE Compatibility team to test as wide a range of CPE as is possible and appropriate within time and resource constraints, in order to identify CPE that may exhibit any differences in operation. FIA working group FDWG2 are working with BT.

Information on tested devices: www.switchedonuk.org/a/xls/switchedontestdbexternal231007.xls

Further information can be found at:

· General information about 21CN can be found at: www.btplc.com/21cn

· Information about 21CN and migration can be found at: www.switchedonuk.org

· BT Supplier Information Notes (SINs) can be found at: www.sinet.bt.com

· The UK Transmission Plan can be found at: www.nicc.org.uk

Information provided by the FIA - www.fia.uk.com

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