ering to SS7 for voice. However, 4G mobile protocols specify IP for both voice and data traffic. These networks will use SIP-type protocols for all of their 4G traffic. Most mobile carriers continue to use 3G protocols for voice when they initially implement 4G mobile service. Incompatibilities between Different SIP Implementations SIP is an extremely flexible protocol that offers different ways to implement many IP applications. These differing methods are referred to as Requests for Comments (RFCs). RFCs were originally used only to solicit comments when standards are being developed. Once approved by standards bodies, the final versions of standards are then published as RFCs. SIP has many RFCs, which has been a complicating factor when carriers connect with one another at peering sites and when carriers purchase new IP equipment from manufacturers that use different SIP implementations than the carrier utilizes. In these cases, manufacturers modify their equipment to match the RFC implemented by the carrier for its network. The SIP Forum, a nonprofit organization that advances interop- erability and hosts interoperability testing, has been attempting to simplify inter- networking between carriers as well as between carriers and enterprises. The forum recommends specific profiles (RFCs) to use when connecting enterprise SIP trunks for VoIP service. Most carriers are now transitioning to these and other specified profiles. Interconnecting Carriers and Providing Secure Space for Equipment in Co-Location Facilities Co-location facilities are sites where network service providers house their switches and routers and connect to one another's networks. Co-location facilities are also referred to as carrier hotels. Carrier Networks There are up to 200 carriers located at the very largest co-location sites. Co- location facilities are connected to the Internet, public and private data networks, and the PSTN. Because SO many network providers depend on co-location facilities, secu- rity and sustainability in the face o