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The key source of echo in the PSTN is the 2-wire/4-wire (2W/4W) hybrid circuit used on subscriber access line cards.

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Posted by waqasahmad

Two to four wire circuit is basically called Hybrid Circuit and it is used at the subscriber or end user side and it is the line card as well defined

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Posted by HamidAliKhan

On the subscriber side.

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Posted by waqqas1

In the PSTN the 2-wire/4-wire (2W/4W) hybrid circuit is often used on subscriber access line cards.

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Posted by sagitraz