Meet the 'IP Multimedia Subsystem’

If there is one constant in the technology sector, it is the inevitability of acronyms. They just keep elbowing in — usually right when you thought you had things mostly figured out.

This week’s example: IMS, which stands for “Internet-protocol Multimedia Subsystem.”

If you dabble in cellular and mobile networks, IMS is a fairly common term. Among big telecom suppliers, it’s the buzz.


The cable people paying attention to IMS are those tasked with finding ways to make their voice services work without wires. People call this “fixed mobile convergence.”

Fixed-mobile convergence is the commingling of good cellular stuff (talking untethered) with the good stuff of wired telephony (no dropped calls.)

I first bumped into IMS last month, while listening to a cable technologist detail what it takes to marry wireless with today’s voice-over-IP (VoIP) services.

Ten days later, IMS plopped into a second conversation — this time about innovation. The short version: Whether a company is in a rush to innovate, or just making sure it has a strategy, it’s always wise to examine what already exists. Like IMS.

Web research pulled up a pile of IMS white papers, from all the big names in telecom: Nokia Corp. Motorola Inc. Sony Ericsson. Lucent Technologies.

Here’s a doozy of an acronym mash from one of them: “When a user registers on the MNO’s IMS network, his SSP is downloaded by the CSCF from the HSS.”



From the top: The “sub” in IP Multimedia Subsystem refers to its place, relative to Session Initiation Protocol, or SIP — which warranted a double translation in this column (February 23 and March 8, 2004.)

In short, SIP puts smarts at the end points of a dumb pipe. End points can be cell phones, VoIP phones, handheld organizers, personal computers. They talk right at each other, without much negotiating with the bearing network (meaning the broadband pipe) about the stuff they can do.

Nobody wants to be the dumb pipe. That, in part, is why cable operators like parts of IMS: It makes the network more relevant to SIP-based services.

The “IP Multimedia” part of IMS is probably best explained from the consumer bleachers.

A common example, coming soon to a cell phone near you, is “push to talk.” It’s like a walkie-talkie, except you don’t have to be on Nextel Corp.’s network in order for it to work. In the IMS lingo, it’s abbreviated “PoC,” for “push to talk over cellular.”

Another is “view sharing.” That’s where you’re standing on a mountaintop, or watching your puppy finally perform a “down/stay,” or sitting in the second row of the Van Morrison concert. You call someone: “Check this out!” And they can see what your phone can see.

Or maybe you’re playing pirate pinball on your PDA against someone else on the network. “Push to talk” becomes “push to pester.” (Swim, ye bilge rat!)

Work-related stuff applies, too. SIP, with IMS, lets a video conference call happen like this: You dial in from your PC (using a headset and a webcam.) Bob joins from a laptop near a Wi-Fi spigot at an airport. Jane connects by cell, while walking to her office from lunch. Plunked in, she pulls up the video and all the voices, including her own, while switching (seamlessly) from cell to computer.

My personal favorite is “instant voice messaging.” It goes like this: Instead of calling someone, enduring the five rings, listening to the greeting, and waiting for the voice-mail lady to tell you what buttons to push to leave a message, you just leave the message into your phone. You send it like a text message: Talk. Send. Done.


In the overall cable landscape, IMS moves in PacketCable 2.0 circles. At this point, it’s an architectural discussion. Reason: As with most things that were invented for use by another industry, there are parts of IMS that apply to cable, and parts that don’t.

Much of it involves how much quality control to put in the network, versus at the end points. More control at the end points means less for the network to keep track of. On a practical level, that can translate into more lines served per piece of headend equipment. Maybe that means a softswitch that handles a million lines, instead of 100,000 lines.

On the other hand, more quality control in the network, on a competitive level, means phone tricks that probably perform better than those offered by competing providers — especially competing providers who don’t have a lot of quality control options inside the delivery network.

Much of IMS, for instance, is about invoking quality-of-service (QoS) mechanisms for the services mentioned above.


Say you’re a provider of “over the top” services — meaning, services that ride, undetected, over a cable modem. Verizon Communications Inc. does it with its “VoiceWing” voice service; ditto for AT&T Corp. with “CallVantage.” (“Over the top” video is next.)

If you’re them, and you want to implement a bunch of new phone tricks that work consistently, it helps to have QoS. That requires a deal.

The day Verizon approaches Comcast Corp. for a QoS deal is the day Pepsi approaches Coke for a recipe.

On the other hand, if you’re the broadband pipe, you already have QoS tools — which can be linked to new phone tricks, using something like IMS.

That’s an overview of IP Multimedia Subsystem, or IMS, as it relates to the cable industry.

Next time, more on how it works.