This article relates to the Cisco TelePresence MCU 4203, Cisco TelePresence MCU MSE 8420, Cisco TelePresence MCU 4505 and Cisco TelePresence MCU MSE 8510 products.
A. From the point of view of a user dialing into an MCU conference there is little difference - the difference is all in the way that the MCU registers conferences with the gatekeeper. Here is a brief explanation and a few examples:
The MCU service prefix is a number (of any length) registered with the gatekeeper on its own, such that the gatekeeper knows to route any dialed E.164 number starting with that number to the device which registered it, even if the complete number dialed is not itself registered with the gatekeeper. The Prefix for MCU registrations is a number, also of any length, which is pre-pended to any numeric IDs (for conferences or auto attendants) that the MCU wants to register as aliases with the gatekeeper. Therefore, the MCU would concatenate together the Prefix for MCU registrations and the Numeric ID of a particular conference, and register the result as an alias with the gatekeeper.
The two prefixes can be the same or different and you can use one, the other, or both.
Example 1 - MCU has registered a Service prefix 123, but Prefix for MCU registrations is blank. A conference has been created with numeric ID 1000, but the Numeric ID registration - H.323 gatekeeper check box is unselected. An endpoint dials 1231000. Gatekeeper routes call to the MCU because the number started with 123 (it doesn't care about the 1000 part). MCU receives the call, ignores the 123 and decides what to do based on the 1000. The call is connected straight to the conference with that numeric ID. It is important to note that the numeric ID of that conference does not itself have to be registered to the gatekeeper. If the numeric ID of the conference is registered with the gatekeeper, then the endpoint would also have been able to dial straight in with 1000, if it had wanted.
Example 1a - After that call, the same endpoint dials 123 on its own. The call is routed to the MCU, which strips the 123 and is left with nothing. The call is connected to the default auto attendant of the MCU.
Example 1b - The endpoint dials 1231111. The call is routed to the MCU, which is left with 1111 after stripping the prefix. The MCU does not have a conference or auto attendant with the numeric ID 1111, so it follows the action in the Incoming calls to unknown E164 number field and connects the call to the auto attendant, creates a new ad-hoc conference, or simply disconnects the caller.
Example 2 - MCU has Prefix for MCU registrations set to 456, but Service Prefix is blank. A conference is created with numeric ID 1000 and the Numeric ID registration - H.323 gatekeeper check box is selected. The MCU registers an alias 4561000 with the gatekeeper. An endpoint dialing 4561000 will be connected straight into the conference. However, an endpoint dialing only 456, or only 1000, will see its call attempt rejected by the gatekeeper, since no corresponding aliases exist.
Example 2a - As 2, but the Numeric ID registration - H.323 gatekeeper check box for conference 1000 is unselected. No alias for this conference is registered with the gatekeeper. A call to 4561000 is rejected by the gatekeeper.
Example 3 - MCU has registered service prefix 123 and the Prefix for MCU registrations is set to 456. The conference with numeric ID 1000 is still there and has the Numeric ID registration - H.323 gatekeeper check box selected as above. An endpoint can dial into this conference by dialing 1231000 (because of the way that service prefixes work) or by dialing 4561000 (because 4561000 is registered as an alias with the gatekeeper). Unlike in example 1, if the endpoint only dials 1000, the call will be rejected by the gatekeeper, because 1000 is not registered as an alias with the gatekeeper.
Example 4 - as 3, but the service prefix and prefix for MCU registrations are both set to 123.