G.hs: Loading Coil Detection

xDSL modems are designed to operate between a Central Office and a customer premises. As such they use existing telephone network wiring between the Central Office and the subscriber. There are several modems in this class which function in generally similar manner. All of these modems transmit their signals usually above the voice band. As such, they are dependent on adequate frequency response above voice band. The telephone lines which are already in place generally have not been tested for adequate response in the intended frequencies of operation. In particular, prior to ISDN deployment, use of loading coils on long telephone lines has been common. Their intended purpose was to equalize the voice band frequency dependent signal propagation. Their effect on xDSL modems however is drastic. Because xDSL modems can not be expected to operate on loaded lines, here we propose a technique to identify their presence.

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