.
, . , , .
.
- , , . , , . , , , , , , , - , , . , , , .
: -; -; -; -.
.
, . . (Plain Old Telephone Service POTS) , , ISDN.
, .25 frame relay, - . , , , , . .25 frame relay ROBO, , . .25 PAD, , .
, . , , , , , . , ISDN , 64 / 64 / frame relay .
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.
.
PDV
= bt ( , ) + *
PVV
. ( ) ( ) () .