H323 callflow

[Nguyen KeThuc]

Hi all,

I have CUCM topo as-following:

IP Phone----SCCP---CUCM---H323 GW---ISDN E1----PSTN--- External User.

I understand that when an IP phone call to another internal IP phone, RTP packet will be sent by IP address of these IP Phones, right. One question is if these IP Phones make outbound call to PSTN, each call will be assigned one time slot in the E1 channel, the IP addresses  (source and destination ) belong to GW and IP phones. So, How does the GW differentiate which RTP packet for which time slot? are there any mapping table between IP Address (or Destination port RTP packet) and time slot?

Some one explains for me.

Thanks,

Thuc

[paolo bevilacqua]

To differentiate, UP port address is used, moreover in RTP header there is a session field..

[Nguyen KeThuc]

Thank you, Paolo

[arunkum3 · Accepted solution]

Hi Nguyen,

The way I understood your question is that you are looking for a table that maps RTP ports to E1 channels (much like your arp table for MAC/IP mapping). The answer is No, there is no such table.

The reason is because we are speaking about a VOIP entity (RTP ports) and a TDM (E1 channels). Both these entities are different. The way these two entities are bridged in a Voice GW is with the CCAPI (Call Control Application Programming Interface).

The CCAPI creates a separate session ID for each leg of the call because there might be different requirements for each those call legs (E.g., a conference or transcoding resource might be needed). There is no CLI command that gives us a mapping table, but we can make out and co-relate with the help of debugs.

Hope I have answered your question.

Regards,

Arun Kumar

[Nguyen KeThuc]

Hi Arun Kumar,

One question : Are there any mapping tables between Called number ( or calling number or IP address internal phone) and time slot in E1?

Thuc