vice Used by Enterprise to Connection IP Enterprise to Connection IP PBX to PBX to PSTN Bandwidth IP Bandwidth only had to be Bandwidth of the trunk between the SP requirements measured within the enterprise. and the enterprise needs to be measured and can be the primary limiting factor in the number of calls supported. Echo cancellation parameters Echo cancellation parameters need to be cancellation could be tuned to specifics of optimized to support the widest variety particular companies' network on of media endpoints. their dedicated media gateways. Signaling All SIP signaling would be termi- SIP-based signaling traverses the border requirements nated at the enterprise, and only between the enterprise and the SP. TDM signaling would interconnect with SP. DTMF inter- All DTMF interworking would DTMF parameters and interworking fea- working occur onsite and in control of the tures are dependent on the DTMF inter- enterprise. connect types offered by the SP. Echo Return All levels could be controlled at ERL and Gain/Loss db levels need to be Loss (ERL) and the enterprise and optimized for set to accommodate the widest number Gain/Loss db the specific enterprise. of customers. levels Codec selection The selection of codec used The codec used by the SP affects the within the enterprise is not choice of the codec used by the enter- influenced by the codec used prise because some codecs might by the SP. require additional hardware and others do not. Delay The IP delay budget could be Additional consideration should be entirely used within the given when determining a delay budget enterprise. for the delay for the media to transition the SP network and its media gateway delay. 68 SIP Trunking Another type of media gateway used by a few SPs is a video-enabled media gateway. This device can transfer video that comes across SIP trunk networks to video that can be uti- lized in other networks, such as 3G-based cellular networks. These media gateways can be an expensive component of SP networks; as such, they are normally