FXS, DID or FXS/DID, FXO, PRI-T1

Can anyone, pls. explain to me what is actually the relationship based on the functionality of FXS, DID or FXS/DID, FXO, PRI-T1 ?
Are the DID Trunks similar to the trunks of PRI-T1, say 24 channles ? Are these 23 b-channels and 1 d-channel for a PRI-T1 are the same as trunks of the DID service?
Dou you still need a DID service offered by telephone companies that enables callers to dial directly an extension on a PBX or packet voice system if you have already a PRI-T1 service?
Do they have the same function between DID Trunks and Voice Channels of PRI T1?
Are the DID Trunks similar to the trunks of PRI-T1, say 24 channles ? Are these 23 b-channels and 1 d-channel for a PRI-T1 are the same as trunks of the DID service?
Dou you still need a DID service offered by telephone companies that enables callers to dial directly an extension on a PBX or packet voice system if you have already a PRI-T1 service?
Do they have the same function between DID Trunks and Voice Channels of PRI T1?
Comments
FXO= [FONT=arial, sans-serif]Foreign Exchange Office. Mainly used for connecting back to the Exchange Office. FXO cards are used for copper lines Example 911
I also use FXO cards to connect into a over head paging system.
PRI=24 channels 1 channel is the D channel and its reserved for the signaling [/FONT]
Dou you still need a DID service offered by telephone companies that enables callers to dial directly an extension on a PBX or packet voice system if you have already a PRI-T1 service?
Do they have the same function between DID Trunks and Voice Channels of PRI T1?