Switching to VoIP

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Components of the PSTN

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PSTN Distribution Frames

Distribution frames surround the CO. They are high-density cross-connection points

where multiple subscribers’ loops are tapped into the feed cables (that is, 50 to 800

pairs in a single cable) supplied by the CO. Usually, all connections to a distribution

frame are copper. Distribution frames allow the telephone company to use high-den-

sity copper cabling that is less susceptible to breakage to feed groups of subscribers

on the edge with a connection to the CO.

Those subscribers usually have a low-density (2–25 pairs) copper cable running from

their buildings to the distribution frame. The frame is the aggregation point for many

customers in a small area. One customer can use a CO loop that was once used by

another customer due to the cross-connect terminals located inside the distribution

frame. New buildings can be built in the area, and they can tap into the distribution

frame rather than having to be connected directly, and usually over a longer dis-

tance, to the CO.

Main Distribution Frames

MDFs are distribution frames that have some smart switching or encoding equipment

in them. Usually, this equipment has been placed there in order to facilitate the use of

DSL (digital subscriber line) for access to the Internet by telephone ...