Core Issue
When using satellite links to connect two remote sites through a central hub site, the traffic travels a path similar to this diagram:
site1-------- central hub site --------- site 2
As a result, there is a double round trip delay, typically over 1.5 seconds for each data packet. For Voice over IP (VoIP), this typically does not affect the overall voice quality. However, there could be a considerable delay during the call setup phase. This could result in the call originator waiting an extended period before receiving a ringback tone.
The VoIP is configured using a Cisco 2600 series router with voice module at both the ends.
Issuing the debug voip ccapi inout command on either gateway reveals a long gap between cc_api_caps_ind and cc_api_caps_ack.
In this situation, H.323 fast start was not used. Link speed is 128 Kbps.
There are quite a few round trips with the H.323 call to establish the speech path. That takes place at alerting time, prior to ringback. Ringback tone is then played all the way from the terminating gateway over IP to the originating gateway to the calling party, thereby adding to the delay. With double hops across satellite links and long round trip delay times, it might take a while for the calling party to hear ringback.
Low link speed and congested links may also contribute to the delay. Low Latency Queueing (LLQ) is the preferred method of classifying and prioritizing VoIP signaling and media traffic above normal data traffic.
Resolution
H.323 fast start could alleviate this issue. Implement fast start by issuing the call start fast or h323 call start fast command, depending on the Cisco IOS Software version, in voice-service configuration mode.
Note: Some older Cisco IOS Software versions do not support fast start. For a workaround, you can upgrade the Cisco IOS Software version. Also, you can make use of another VoIP signaling protocol, such as Media Gateway Control Protocol (MGCP), or another lower round trip delay network infrastructure. In addition, ensure that LLQ is applied to ensure priority not only for the voice packets, but also for the signaling traffic.
For the Transmission Control Protocol (TCP) port numbers to prioritize and for further information on quality of service, refer to VoIP over PPP Links with Quality of Service (LLQ / IP RTP Priority, LFI, cRTP).
