It has been a long time since my last post; I have been busy with work, training and studying. but here we are again. As you may know Analog Devices like FXS, ATA, VG224 connect to CUCM through MGCP and SCCP. I have seen that most of us don’t know when to use MGCP or SCCP; this is why today we are going to highlight differences between both protocol features and briefing on the configuration of each. before going into details I want to make this very clear; when most or all of your devices are Analog phones and you are not worried about Fax Machines then you should use SCCP as it gives you more features and flexibility for your Analog Phones. but when most or all your devices are Faxes and you are not worried about your Analog Phones features then you should use MGCP.
Also I have to highlight that you can mix your configuration with both SCCP and MGCP for port by port behavior (Ports connected to Analog phones configured as SCCP, Ports Connected to Faxs configured as MGCP, you can not control the same port with two protocols) but you should be warned that such configuration can be complex, and Hard to troubleshoot as you may get conflicted on CUCM as which port is running what protocol So you have to be careful with your configuration. Lets now go to details about both SCCP and MGCP features:
SCCP Configuration are considered Complex when compared to MGCP, give more features for Analog phones as it allows you to define FAC (Feature Access Code) on SCCP Gateways and use these features on your Analog Phones with FAC Almost every feature you can use on your IP Phone can be used with your Analog Phone only by dialing a code that is preconfigured to map to that feature using FAC. When it comes to FAX; SCCP supports Fax Pass-through or Cisco Proprietary NSE based T.38 and still there is some issues with Faxes when ports configured as SCCP. SCCP cannot run Standard based T.38 like an IP-based Fax Server. Also one of the main benefits of SCCP that you can use SRST natively in case of CUCM failure which requires a turn around solution with MGCP if you would like to have an SRST-like Functionality.
MGCP Configuration are considered simple when compared to SCCP, give more support and flexibility for Fax and Modem devices as MGCP supports Standard based T.38 which means that you can run IP Based Server Faxes through SIP Trunk ITSP and there is no known issues for Fax machines when running MGCP, On the other hand MGCP doesn’t Support FAC for Analog Phones which means many fewer features for your Analog Phones comparing when it runs on SCCP.
You can get call preservation & SRST-like functionality on your MGCP FXS endpoints. Treat it like you would an MGCP PRI which you want call preservation enabled (i.e. ‘call application alternate default’) . Then you also need to have lower preference dial-peers pointing to your SRST gateway via SIP or H323. Just make sure you create dial-peers with ‘service mgcpapp’. if you configure your dialpeer without service mgcpapp it will get matched when you go offhook in normal mode and it will go randomly to CUCM or your SRST gateway.
Why not using H323 for your VG224 gateway?
Answer is simply because VG224 Gateway with H323 configured are bad for both Analog and Fax devices (doesn’t Support FAC for Analog Phones and doesn’t support standard based T.38 for Faxes).