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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?

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5 Replies 5

Suresh Hudda
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Let me describe you in simple words..

FXS is a port where we connect analog phone or FAX machine directly, so this is a analog phone port.

FXO is a single analog trunk line which connects to Cisco router or 3rd party gateway on FXO port, when we buy a fxo line then we get one single number (full number)  from service provider, this trunk line can be used by internal phones to dial outside (pstn) but it will allow only one single call(incoming/outgoing) at a time.

Now comes to PRI, e.g. T1, which has multiple channels , as you said 23 b-channels so in that case 23 calls (incoming/outgoing) can be flow simultaneously. Now, comes to DID, actually DID is range of numbers on PSTN which reserved for company who purchase that DID, so if you have 1 T1 pri then you can purchase any numbers of DID e.g. 50, 100, 200 etc so you can use these DID numbers for user to call them directly from outside world(pstn). Suppose if you don't buy DID range then you will get one DID (pilot) number from service provider and in that case you can only allows one user/extension to get calls directly from outside world and if caller want to speak to another user then that user has to transfer the call to another user always (if you have IVR/AA then you can use it to transfer calls to specific user as well) so without DID you can't connect a pstn call directly to employees, and if they make outgoing call then only pilot number will be displayed on called person's phone.

So it depends on a company whether they want to give each user a DID number.

If you have DID numbers then you can also allows a analog phone /FXS port to receive call directly from pstn.


Thank you so much for your reply. Would it be ok with you if you can interpret the attached pictures.

The router has 4 FXO and 4 FXS/DID ports where  a total of  8  x RJ11 phonelines are plugged on each port, 4 of which are terminated from  fxo  telephone sockets  with telephone numbers for each port as 236-4247, 236-4273, 236-4857, 236-4396 and the other 4 on telephone sockets marked DID 0, DID 1, DID2, DID3.

There is also T1 connections.

Appreciate so much if you can give an insight what are suppose to be happening with this set up in the real world.

Thank you.

here is the router picture

fxs/did tel. socket

4 FXO lines, these lines should be coming from service provider and working as analog trunk.

4 FXS/DID; these ports can directly connect to analog phones or fax machines, or it can be used to connect up to four direct-inward-dial (DID) analog trunks (suppose you have integration with another gateway (third party) then, one end should connect on FXO port and another end must connect on FXS port). For exact connectivity you need to trace the cable physically.

T1 connection must be using for PSTN calling, then it should be going to MUX of service provider, but yes it can be used as TIE trunk as well between two routers. 


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