Gokul Blog — A conversation on VoIP, IMS, Cisco and Just about Anything

Deeper analysis of VoIP

No Dial Tone nightmare?

Posted by tggokul on February 15, 2007

There seems to be a Vonage outage and a lot of customers are reporting that they cannot hear the dial tones when they pick up the phone. Russell Shaw is following this news and hopefully we can get some updates from him.

My colleague who is an expert in Cisco’s IPT and IPCC solution was talking about a similar issue few days ago which seemed interesting. He was involved in a huge Call Center implementation( I think around a 1000 agents) in Indonesia. The agents  sit in different locations and these locations are connected to the central location ( where the call manager resides) over WAN.

There was a caveats/assumptions section in the design document ( which was agreed/signed off by the customer and implementor) that said that some of the agents who login from remote sites might not hear the dial-tone the first fifteen minutes. For example if the shift starts at 8.00 AM, some of the agents might not get a dial-tone till maybe 8.15A.M!!!! The reasoning was that there would be a lot of data traversing through the network when all the agents login and some of the registration messages ( from the different ip phones to the callmanager) could get lost.

The central controller has complete control over the endpoints in SCCP ( Cisco’s proprietary protocol) and MGCP;  i.e the endpoints gets all kinds of instructions from the central server ( called call-agent in case of MGCP) including whether it needs to play the dial-tone or not. This is not the case when SIP/H323 is used. Routing/call will never go through but you can atleast hear the dial-tone. ( atleast in most implentations of SIP endpoints. I know a few SIP endpoints that will play the dial tone only when it is registered) From the symptoms I see in Vonage problem, it mostly looks like the central controller was unreachable ( or maybe it was overloaded and dropping packets) and they were using SCCP or MGCP.

Maybe Vonage needs to have a caveats/disclaimer clause in their Service contract that says “the user may or may not hear the dial tone” . That would help 🙂

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