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Because IMS network issues cause IAD attached phone dial outgoing voice no audio.

Publication Date:  2012-10-11 Views:  47 Downloads:  0
Issue Description
Networking: IAD132 - IP PBX – Layer 3 switch - SIP GateWay - SBC - IMS
The IP addresses of the devices are as follows:
IAD - 192.168.5.5
IP PBX - 192.168.4.5
SIP GateWay - 221.179.88.55
SBC - 221.179.66.52
IP PBX and SIP GateWay are SoftCo9500, Layer 3 switch has directly connected to routing function, can achieve public and private network traversal through the NAT. Layer 3 switch upper-level devices are t are public network IP addresses, down-level devices are private network addresses . 
This kind of networking, client reflect when the IAD attached phones dial the IMS end mobile phone, appear that the peer end can ringing but the local end does not have ringback tone, while the peer end off-hook, it could be normal connection, then appeared voice no audio problem.  
Alarm Information
None
Handling Process
Through ping command text, found IAD and SBC can not pingable, but IP PBX and SIP GateWay was pingable, and other adjacent network elements can also ping each other.
This phenomenon is a very strange, packet capture analysis found that the SIP signaling interaction between IAD and IP PBX is completely normal. RTP media stream also sent to the SBC end (221.179.66.52), but did not receive the peer end RTP media stream.
SIP signaling principle can explain this question. The SIP protocol signaling was built through point-to-point. In the entire network, the network between two adjacent network elements is reachable, can send signaling messages to the peer ends. But the RTP media stream is end-to-end. After both ends signaling interworking, establish media path is end-to-end transmission, if the two ends network barrier, the media stream can not be passed it led to voice double unreasonable.
On this issue, is adjacent network elements between which the networking with each other to ping, but the network between the IAD and SBC is unreasonable, which led to the SIP protocol to negotiate the destination IP address of the media stream for the SBC after the address, IAD RTP packet sent to the SBC, SBC but not receive.
The client did not make plan according to the Huawei provided SoftCo the IMS classic networking scene, led to this problem. Put client SIP GateWay into the private network which can solve the problem.
Root Cause
1. First, view IAD to peer end gateway (IAD / TG) network whether is normal. The firewalls, routers and other devices between the devices may result in data no audio, use the ping command to troubleshoot.
2. IAD cooperated with some softswitch, may exist the no audio problem that caused by inconsistent gateway supports codec. Can capture network packets, or use trace command to track signaling.
Suggestions
IAD's one-way audio , no audio problems in manycases are caused by networking reasons, before locating problem, you should first determine the networking, and then determine whether key network elements can ping each other, and to check network elements in the network with can play as NAT address translation. 

END