What is PDD and what affects it?

PDD is the short Post Dial Delay. It is the time interval between "end of dialing" by the customer and the reception (by the same customer) of the call progress signaling generated by the exchange serving this customer. The call progressing signals can be the dial tone, a recorded announcement, or the abandon of the call.

For call termination with GoIP, the PDD depends on the network condition, SIP INVITE Response, and GSM network.

1. If the network condition is poor, extra network delay is introduced in the SIP communication. This adds to part of the PDD. In this case, the voice quality is likely to be affected as well. Please resolve this issue promptly.

2. SIP INVITE Response has a directly impact on the PDD. If SIP 180 is returned, a ringback tone will be generated locally. The caller can then hears a ringback tone while the GoIP is try to dial out the call. This only affects on the caller's preception, but the actual amount to dial the call is not affected.

If SIP 183 is used, the PDD is then consist of network delay for SIP protocol and the delay of the GSM network. In some case, this could be very long and the caller may hang up the call before he gets a call progress tone back from the GSM network. To improve this, the Ringback mode in the GoIP should be set to "Local Ringing + Early Media". The GoIP will first respond a SIP 180 to a SIP INVITE and then send a SIP 183 once call progress tones is received from the GSM network.