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Can you help us resolve compatibility problems with Cisco and Avaya?

I belong to a midsized organization that is spread as three independent operating units around the world (Europe, North America and Middle East). The unit I belong to has immediate needs to purchase a PABX (for 150 users), as there are critical support issues that need to be increased. We decided to go in for a Cisco IP PBX solution, mainly because our core infrastructure is built around Cisco, and Avaya does not have a full fledged office in the region and support may be limited. The other group has just started reviewing their options on IPT and prefers Avaya primarily because they are already using Avaya. Their core infrastructure is also built around Cisco (with dual 46xx switches in HSRP mode).Now the other group is saying that Avaya and CCM will have interoperability issues and that H323 / SIP is not supported by Cisco hence we should be looking at Avaya. I'm a bit confused at this stage and have looked about but have found nothing available to confirm SIP / H323 compatibility by Cisco. Is there any advice you can give me on this.
The newer VoIP/IP Telephony systems are based on SIP technology and this will give you the most options for phones and other devices that you may want attached. There are problems with SIP to H323 communications in some enterprises. There are gateways available to address these issues, but it will add another level of complexity and there will be one more box to administer. There is also work in the IETF to address some SIP interoperability issues as different vendors interpreted the standards differently.

Personally, I like the new SIP technology better as it sets up virtual connections for the duration of the call and in Voice, rerouting and retransmissions are not acceptable. The key here is what your routers and switches will understand and pass as prioritized traffic. With the older H.323 systems, prioritization is accomplished through setting the QoS bit, in SIP it more closely mimics the POTS network.

One other key note is that all of the major VoIP manufacturers recommend that you audit your infrastructure and network health prior to adding any VoIP due to the sensitive traffic requirements for those systems.

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