a pri is 1.54 mega bits, 23 b channels (barrier) and one dch (data) the d is used for setup and tear down of the call, the barrier channels are used for 64k of voice or data. the old non pri t1's are 1.54 meg 24 channels each 64k and each one used 8 k overhead for setup and tear down.. a single d can switch 23 pris, but i've never tried using a single with a back up on more the 8 pri's...
packet switching is very unique as compared to voice.. each packet has the source address and the dest address with all the subnet mask etc febn becn etc etc for voice if you dial 9 1 213 555 1212 from outside the 213 area code, in my switch i drop the 9, my switch uses it to tell me it's outside the switch (takes me to ld 90) since the npa (area code) is 1213, i delete the 1 and send it to my ld carrier, he already knows it's ld if i send it to him, he routes to a pri to LA main but he deletes the 213, since LA main knows the call is for the 213, LA main (a central office) looks at the 1st 3 digits, 555 and routes the call to a pbx that handles information for that office.... same thing happens if you call me the last step is downtown central office see's my nxx and send me just the last 4 digits, with pri i also get the calling number and name on my dch... so pstn switching is a carry over from the mechanical days and before (manual operators)... now voice is sent as data packets after the setup, and teardown is still removing a 600 ohm short on the far end..
john poole
bellsouth business
columbia,sc