Build VPN between SPA400 and SPA9000 can not hear sound from inside to outside phone

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Aug 14th, 2009
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Hi All,

I am config the connection between SPA400 to SPA9000. My problme is when I make called between outside mobile phone to internal phone. I can hear outside phone but the outside phone can not hear sound from the inside phone.

Do you know any config I am missing?


Andy Hung

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Patrick Born Tue, 08/18/2009 - 14:35
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Hi Andy,

One-way audio is usually a result of network address translation (NAT) issues when a session border controller (SBC) has not been deployed by the service provider.

However, from your report, I'm assuming that you use your mobile phone to call a PSTN number > SPA400 == VPN == SPA9000 > internal phone.

Do you perhaps have a firewall somewhere that could be inhibiting traffic?

The following requires unrestricted access:


     UDP 5060-5063

     UDP 16384-16482

Consider removing the VPN and then testing in order to eliminate the possibility that the VPN is causing a problem.




andyhung232 Tue, 08/18/2009 - 19:38
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Hi Patrick,

Thanks a lot, If the remove the VPN between the SPA400 and SPA9000, the call for both in/out is fine. For this case is I need build the VPN between them.

I will check the ports that you provide to me and let you know the result.



andyhung232 Tue, 08/18/2009 - 19:54
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Hi Patrick,

Checked the policy is allow any.


Andy Hung

William Childs Thu, 08/20/2009 - 03:38
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What kind of service do you have on each end of the vpn tunnel? For example, your vpn connection adds overhead to your phone call. Therefore, your packets may be too big for your service to send all of the packets. You can calculate your need bandwidth by doing this: If you are using G.711 with a 20ms sampling size (default for SPA400) then you are sending 160 bytes per packet (formula ---- Bytes_Per_Packet (BPP) = (sample size of 20ms or .02) * (Codec bandwidth of 64000 or 64kbps) / 8 (bits in a byte))

BPP = (.02 * 64000) / 8

BPP = 1280 / 8

Bytes Per Packet = 160

if you add the overhead for ethernet, IP, and the rest of the upper layers of the OSI model then you will ultimately add 40 more bytes per packet

ethernet = 20

IP = 20

UDP = 8

RTP = 12


+ 60 Bytes

Now we are up to 220 bytes per packet

Now we have to add in the over head for IPSec (assuming you are using IPSec for your vpn)

IPsec = 50 - 57 bytes

If you add the 50 to the 220 you have already you get 270 bytes per packet. Now we have to calculate the bandwidth needed for your phone call. This is done by using this formula

Total_Bandwidth (TB) = Packet_Size * Packets_Per_Second (PPS)

PPS is determined by sample size divided by 1 second

sample is 20 ms and 1 second is 1000ms. so 1000ms/20ms = 50ms

TB = 270 bytes * 50 PPS

TB = 13500 bytes per second

To get bits per second then multiply by 8

8 * 13500 = 108000 bits per second or

108 kbps for 1 way audio with an uncompressed codec (G.711)

You may choose to get around this issue by switching codecs on the SPA9000 and SPA 400 to use G.729 which has a bandwidth consumption rate of 8 kbps vs G.711 that is 64 kbps. As you know you are able to download much faster than upload which may explain your reason for 1 way audio (you can hear the caller, but the caller can't hear you). The downside of using G.729 is call quality. If it is acceptable, then this is your option, if not you may need to increase your upload speed to accomodate more packets because of G.711. Good luck with your issue and please post your results.




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