1 way audio on internal calls between IP phones

[George Banuelos]

Hi,

In getting ready to add an off-site office to a CUCM 8.6 across the street, a 7941G that was built and registered at the home office was taken and connected to the LAN at the new office.  The CUCM 8.6 DHCP server is used on the publisher at the home office... the phone registered using a different  DHCP server at corporate... this was surprising since the TFTP server is not on the DHCP scope on the corporate DHCP server that it rec'd its IP from - can someone explain how this happened? 

The other question is that the IP phone was able to make outbound and receive inbound via PSTN without a problem (2 way audio OK).  Calls to the Unity were also OK (2 way audio OK).  However, the moment internal calls were made from or to this IP phone, I could hear everything fine but the other parties could not hear me.  This 1 way audio happened on all internal test calls to other IP phones.  The exception is that the internal calls to Unity did receive my audio - test message captured audio fine.  Can someone explain what might be taking place?

Please note that the the Voice vlan is not up yet in the new office so the IP phone was on the data subnet - i don't know if this would make any difference but do add in case it has any bearing.

Thank you!

[George Banuelos]

update... I located this document that might explain a routing problem where 1-way RTP was being trasmitted and received while the opposite direction RTP packets were not... http://www.cisco.com/en/US/tech/tk652/tk698/technologies_tech_note09186a008009484b.shtml what is not clear is how the Unity was able to receive RTP without issue - can someone note possibilities?

Thank you

[William Bell]

George,

Depends on the nature of the routing issue. Is there something different with the Unity server's logical network connection? If you stuck a phone on the same subnet as the Unity server, would you have full-duplex audio? Also, you say you are using Unity. Are you by chance running G729 across the WAN? If so, are you using a transcoder somewhere? That could explain a difference in behavior if the phone affected can reach the transcoder (by IP) but not the Unity server.

Also, taking a step back, if you happen to put Unity in the same subnet as CUCM and you are able to reach CUCM by IP (else you wouldn't get TFTP nor register) then clearly routing to the CUCM/Unity subnet is OK.

I guess my point is that just because you have full duplex audio to Unity is not an indicator that you should have full duplex audio to other IP endpoints.

HTH.

-Bill (http://ucguerrilla.com)

[James Hawkins · Accepted solution]

Re. the DHCP issue I have found that phones which have been registered in a subnet with option 66 or 150 set to provide a TFTP server address seem to remember the TFTP address if they are subsequently moved to a subnet with a DHCP scope that does not offer option 66/150.

One of my customers has small offices around the world where they send phones which are connected to small SOHO routers whose DHCP scopes do not have the option 66/150 set up. This worked just fine until they wanted to upgrade CUCM. We built a new CUCM 9.1 cluster with different IP addresses from their original 6.1 cluster and the remote phones would not swap to the new cluster as we could not set the option in DHCP. We resolved it by using Voipintegration's phone remote control software to set alternate TFTP servers on the phones.

Re. the one way audio it is almost certainly a routing problem between the phones. Put a host that you can ping from in each device subnet and check connectivity using ping, traceroute etc.

[George Banuelos]

Thank you all for the replies.  The scenario posted by James Hawkins explains why the IP phone that was registered at the home office regeistered OK at the new office without a DHCP TFTP reference.  The Unity Connection is on the same subnet as the CUCM.  The IP subnets on the CUCM DHCP scopes may not be routable from the new office, but the new office's subnet is probably routable from the home office where the CUCM is located, thus I could here them they coul not hear me - correct?

I have not yet tried to hook up a phone to the Uinty subnet to provide Bill with a reply, but there is no transcoder that I know of.

Thank you again for all your input!

[Jagpreet.Singh · Accepted solution]

Hi George,

     Every IP phone has a cache memory and retains it in the Device Configuration. When it won't receive the TFTP information, it will default to the cache.

Like James said, the one way audio issue is most likely a reouting problem. Is the Unity Connection in the same Subnet as the IP phones at the home office?

Regards,

Jagpreet