t always offers a SIP interface; although in rare cases, this interface might also be H.323. All IP trunk for PSTN access SP offerings that 144 SIP Trunking Table 8-1 CUBE Protocol Conversion Support Protocol In-Leg Out-Leg Support H.323-H.323 Fast Start Fast Start Bidirectional Slow Start Slow Start Bidirectional Fast Start Slow Start Bidirectional H.323-SIP Fast Start Early Offer Bidirectional Slow Start Delayed Offer Bidirectional SIP-SIP Early Offer Early Offer Bidirectional Delayed Offer Delayed Offer Bidirectional Delayed Offer Early Offer Unidirectional became available in the past 2 years have been SIP, but a handful of older H.323 offerings in the installed base preceded the industry transition to SIP. These are no longer actively sold to new customers but exist in several current service provider networks. Many enterprise networks have not yet migrated to SIP internally, and even for those who have the variation of SIP used by the enterprise's call agent might also not be of the same vintage, or compatible in message use, with the service provider's SIP offering for all call flows. For example, there are variations in the use of the SIP REFER or REINVITE messages for modifying calls during a transfer. There are also multiple ways in SIP to put a call on hold and to resume it. Because of these variations, SIP trunk service providers offer an explicit UNI specifica- tion of which message types, formats, and fields are valid on their service offering. For the enterprise to comply with this UNI, it is often easier to place a border element at the edge of the network to normalize all the variants from different enterprise applications and endpoints than to try to configure each individual application or endpoint to comply with the UNI. It is especially SO if the enterprise connects to multiple different SIP trunk providers, perhaps for least-cost routing or for redundancy purposes. Using a CUBE to interconnect to the SIP trunk also enables small businesses and enter- prises to take advantage immediately of S