Media and Media Resources
Media resources
An MTP terminates media streams and re-originates them - needed for DTMF mismatch, early offer where the endpoint cannot do it, SIP-to-H.323 interworking, or holding a call. A transcoder converts between codecs when regions do not overlap and lets G.729 calls reach a G.711-only conference bridge or voicemail. Conference bridges can be hardware (PVDM DSPs on a gateway) or software in CUCM; ad hoc conferences use the initiator's capabilities, meet-me uses the bridge's. Annunciators play announcements, and MoH servers stream hold music.
Selection order
Devices get a Media Resource Group List (MRGL); each MRGL contains Media Resource Groups (MRGs) in order; within an MRG resources are also ordered top-down. CUCM picks the first resource that supports the required capability and is reachable. Resources not assigned to any MRG act as a fallback for devices with no MRGL.
Bandwidth and codec control
Regions define audio and video codec pairs plus maximum bit rates between regions; locations hold the bandwidth budget and perform CAC. Music on hold can be unicast (one stream per call) or multicast (one stream per site, requiring multicast routing) - multicast is the answer when WAN bandwidth is the constraint.
Rule of thumb: endpoints that cannot agree on a codec need a transcoder added to their MRGL; DTMF that fails only across a SIP trunk needs an MTP.
Traps: assuming an MTP converts codecs (that is a transcoder); assuming multicast MoH crosses a WAN without multicast enabled; forgetting a software conference bridge still consumes a media resource.