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The term telecoms resilience means enabling a telephone subscriber to continue to be served even when one line is out of service. The UK carrier networks are required by Ofcom to be 99.999% resilient. This means there should be no more than 5 minutes per year downtime in any single telephone exchange. Connectivity from the local telephone exchanges to the customer premises is called the "last mile". This can be an ISDN30 connection, delivered through either a copper or fibre cable. This ISDN30 can carry 30 simultaneous telephone calls and many direct dial-in telephone numbers, (DDI's). When leaving the telephone exchange, the ISDN30 cable can be buried in the ground, usually in ducting, at very little depth. This makes any business telephone lines vulnerable to being dug up during streetworks, liable to flooding during heavy storms and general wear and tear due to natural elements. Loss, therefore, of the "last mile" will cause the failure to deliver any calls to the business affected. Business continuity planning often provides for this type of technical failure. Any business with ISDN30 type of connectivity should provide for this failure within its business continuity planning. There are many ways to achieve this, as documented by both the Cabinet Office and the CPNI. == Dual parenting == Dual parenting is where the telephone carrier provides the same numbers from two different telephone exchanges. If the cable is damaged from one telephone exchange to the customer premises most of the calls can be delivered from the surviving route to the customer. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Telecoms resilience」の詳細全文を読む スポンサード リンク
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