Maximum REN Loading
The total REN for a subscriber's line is simply the sum of the RENs of all devices connected to the line; this number expresses the overall loading effect of the subscriber's equipment on the central office ringing current generator. The local telephone company usually sets a limit on the total REN, typically 5 or less.
If the total allowable REN load is exceeded, the phone circuit may fail to ring or otherwise malfunction. For example, Call waiting, caller ID and ADSL services are often affected by high REN. 20th century equipment tends to contribute to a larger REN than new equipment. In extreme cases, the telephone service provider may temporarily disconnect an overloaded line to reduce load.
Some analog telephone adapters for Internet telephony require analog phones with low REN, but researching RENs online is difficult because manufacturers and sellers seldom provide that information. The AT&T 210 is a basic phone which does not require an external electrical connection and has a REN of 0.9B.
Read more about this topic: Ringer Equivalence Number
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