Continuing our discussions from last week, today we're going to look at how IP Multimedia Subsystem (IMS) architectures and Unlicensed Mobile Access (UMA) technology can complement each other.To begin, let's look at the similarities of the two technologies that may lead the casual observer to think that these are competing technologies.Both IMS and UMA provide users with access to multimedia services, including voice, video, and text-based applications. Both rely on IP for transport. Both can connect a wireline subscriber to a wireless subscriber, and both offer session control and security when handing off a call (or session) between Wi-Fi and Mobile (GSM\/GPRS) networks. And both technologies are in use today providing commercial applications; for example, UMA is being used by British Telecom's Fusion service and IMS components are being used by multiple mobile providers to offer applications like push-to-talk.Next, let's look at how the technologies are different. First, UMA does not rely on Session Initiation Protocol (SIP) for session control, while IMS does.Second, UMA is built to provide seamless session hand-offs only between a Wi-Fi and mobile Network; it relies on the mobile network to provide connectivity to global telephony networks. IMS controls sessions (via SIP) in both mobile and fixed wireline networks; consequently, only IMS can offer users SIP-like presence and availability information.Third, while UMA does offer authentication and security for call admission control it does NOT contain the user specific profiles that allow for differentiation services authentication by user profile. Rather the UMA's authentication is device-specific. Since per-user authentication can vary on a single device based on user profile, this affects how sessions can get screened based on the user's unique profile.Fourth, UMA is considered by mobile service providers as a supplemental technology to offer mobile services more efficiently to users who are in range of a wireless access point while IMS is considered by both wireline and wireless operators as the future protocol of choice that will eventually provide all multimedia services with session control.Next time, we'll explain this fourth point when we show why the technologies are complementary.