Last week we began a look at IP Multimedia Subsystems and Unlicensed Mobile Access. Today, we'll introduce a little about how IMS works as a starting point to compare IMS with UMA. And for the "tech-heads" who are reading this, please accept our apologies in advance if we oversimplify these complex network architectures.IMS was born out of the wireless (cellular\/mobile) community so that wireless service providers have a common way to connect and offer mobile applications between the applications, the wireless networks, and the wireline networks of multiple providers. It was developed by the Third Generation Partnership Project (3GPP) as a multivendor approach to solve session control issues, particularly for IP-centric applications developed over a wireless broadband infrastructure. Although developed initially for wireless applications delivery, wireline infrastructures were also top of mind as the standards for the IMS architecture evolved.IMS uses Session Initiation Protocol (SIP) to create, modify and terminate IP sessions. Our longtime readers will recall that SIP is the service providers' planned protocol of choice for VoIP and multimedia services session control.IMS is not a protocol per se; rather it is an architectural model with several working parts. These parts include:* Home Subscriber Server (HSS), which provides user directory information and manages the association between users.* Call Session Control Function (SCSCF), which, among other things resolves addresses (like converting Larry's telephone number to an IP-address).* Applications servers (AS) that provide middleware and "special use features" like converting speech to text or a network command.* Media Gateway Control Function, which offers a gateway for call control between unlike systems.* Media Gateway, which offers connectivity between unlike systems.Today, IMS is being used to offer features like push-to-talk in the mobile networks, and it is in trials or labs for most Tier-1 wireline carriers.Next time, we'll cover UMA.