Earlier this week, I mentioned that there are a number of approaches to blending mobile networks. Some rely on a carrier-centric, top-down approach to integrating disparate networks.Among these efforts are those known almost interchangeably as IP Multimedia Subsystem (IMS) and Fixed-Mobile Convergence (FMC). They use IETF- and Third Generation Partnership Project (3GPP)-sanctioned IP protocols, such as the Session Initiation Protocol (SIP), for voice signaling and calling features within the public network infrastructure. The idea is to eventually support IP to IP sessions over wireline IP, 802.11, 802.15 and Code Division Multiple Access (CDMA)- and GSM-based packet-switched cellular networks.By contrast, today\u2019s cellular networks don\u2019t run voice at the IP layer, using lower-layer protocols for connectivity and signaling. So an IMS infrastructure could potentially take several more years to build out to ubiquity.There are also several enterprise-centric attempts to blend networks. Some attempt to \u201cmobilize\u201d the enterprise business phone; others attack the problem from a cell-phone-centric perspective.The DiVitas Networks setup, described in the last newsletter, attempts to mobilize the desk phone and internal PBX capabilities. It is reminiscent of several other enterprise-centric approaches:* The Motorola-Avaya-Proxim Wireless Enterprise Seamless Mobility solution, which merges Wi-Fi and cellular networks using customer equipment that hands off signals from one type of wireless network to another. Caveats: For now anyway, it works only with Motorola\u2019s own CN620 dual-mode handset; GSM-based networks; and Avaya IP PBXs. Like the DiVitas system, you call the PBX number, not the cell phone number. The cellular carrier has to disable the voicemail function so that all messages are funneled into a central unified messaging system.* A product category called \u201cmobile VPNs,\u201d which has been discussed often in this newsletter. The client\/server software from companies like Ecutel, IBM, NetMotion Wireless and Padcom enables roaming among any kind of lower-layer network while retaining IPSec VPN sessions. Caveat: For now, these systems work with data connections only.* Orative Networks\u2019 Enterprise Software, a cell phone-centric approach. This vendor- and carrier-neutral presence and telephony software allows a mobile phone to behave as a substitute desk phone (in this case, you call the mobile number, not the PBX number, to get the features). The system provides you with context, location information, and directory information about your colleagues to reduce the number of missed calls. Orative recently began integration efforts with Cisco CallManager and Unity integrated messaging systems as a first step toward integrating the enterprise \u201cmobile phone network\u201d with the internal calling system.