Smartphones, 4G, IP, and IMS – Part II: Why is this G like no previous G

Making a phone call is an activity that seems almost as natural as walking, or drinking, but it has a history. About 120 years ago the layout of the phone terminal, with transducers for speaking and listening, was a settled matter of design. In another 20 years the matter of dialing phone numbers was settled as automatic electromechanical telephone exchanges were introduced. A phone call is a blend of humans making accommodations to machines and machines being designed for humans, and much of the early outcome of that blending persists today.

If you look at the software in any mobile phone, the key thing that makes it a phone is the call-state management. This is what keeps track of the state of phone calls (and, in 2.5G and 3G, data connections, too). It enables the user interface of the phone to display the correct state visually and play the right call progress tones (the ones that originate in the handset) at the right time. It also hooks into power management to light the LCD backlight at the right times, the switch that detects if a flip phone is closed, the keyboard lock, and performs other operations that create the user experience of a telephone.

The design of all mobile call state management is based on having one bearer channel to a circuit switch, plus SMS and data. The first implementations of mobile call state management had minimalist requirements: Display the on-hook dialed phone number on a numeric display, play call progress tones the user would find familiar from the landline user experience, and enable control over a small set of switch-based functions like call-waiting, where the call on the single radio channel to the phone is switched to a second call. It is familiar enough to be accepted by customers and accommodating enough to the way the network works to be efficiently deployed.

There are some differences between CDMA, GSM, iDEN, etc. but they are close enough to share a lot of the implementation. 2G, 2.5G, and 3G differ in the richness of the circuit switched signaling (but not so much that users are aware of it), and in whether and how much data you have. GSM and UMTS/WCDMA data and HSPA differ from CDMA data in detail, but, again, there's not much user awareness of the differences. Color LCD displays on phones make text messaging nicer, but the impact on the mobile phone call user experience has been limited to presenting a contact list and call log.

4G is the first fundamental break from the roughly 25 year old model of cellular mobile telephony. In 4G, the radio is part of a network interface on an IP network, like a WiFi radio. There are no bearer channels dedicated to voice calls. You either do IP telephony using SIP, in the IMS case, or you simulate circuit switched telephony on top of IP in the VoLGA case. There is no practical limit to the number of voice calls and other kinds of communication sessions a 4G device can juggle. Packets traversing the carrier's network carrying voice to other endpoints are no different from packets flowing between the endpoint and a third party or private gateway/switch with a telephone network interface. Identity, communications services, media (how voice or video is encoded), etc., all become decoupled from the network.

So we are entering a product development phase where the winners will be, in part, determined by how well they can make compelling new communications features and capabilities based on this change without confusing the end-user. What it means to be a phone is now infinitely malleable.

Surely, at the bottom of this a phone call has the same limitations as a face to face conversation plus the further limitations of not being actually face to face. No, even this can be called into question. The ability to redefine the phone call experience is comprehensive and the potential is limitless. And the potential ranges from the obvious – what the user sees on a screen – down to the subtlest microsecond-level manipulation of the human voice.

But will this potential be realized? That is more in doubt than that the potential exists. So far, the modern general purpose pocket-sized telecomputer surfs the Web and plays games. It will take the realization that new communications products can be created, and sold, to motivate reconsideration of how phones enable communication.

Denial is not a river in Russia, but VoLGA is

IMS has made some headway in network infrastructures in part because, if you have an all-IP network, as with cable or WiMAX, for example, IMS provides specs for two key parts of the network architecture:

• A secure carrier-grade signaling architecture based on SIP, an existing, widely used technology

• Interconnection and inter-operation with the worldwide telephone network.

Neither IMS's Vogon ambitions to make charge-able events out of Web site visits, nor the potentially creative aspects of SIP signaling in enabling novel communications features helped IMS. It was purely practical matters that brought implementations of SIP that happened to be IMS capable into the telephone network.

Will IMS therefore win by default?

The answer will emerge from the way LTE (a.k.a. 4G) networks are engineered. LTE is tipped to be the dominant all-IP mobile wireless network technology, and is therefore the big prize for IMS vendors. You can think of LTE (4G) as 3G without the circuit switched telephony. LTE is faster and simpler than 3G.

The question is: Can you build an all-IP LTE network without IMS? Unfortunately for those network infrastructure vendors with a big investment in IMS, the answer appears to be “Yes.”

VoLGA is a spec related to LTE that describes how to do circuit switched voice with an LTE generic access network (GAN), and how to hand off between an LTE VoLGA call and a 3G network and 3G call. By teaching the 3G GAN controller (GANC) node a few things about how VoLGA handsets want to do circuit-switched signaling over IP, and how to set up gateways for VoLGA calls, you do away with the need for a new kind of signaling for IP calls.

That is, VoLGA is a specialized form of IP telephony built to get signaling and payload between an LTE endpoint and a mobile switching center (MSC), which is connected to the conventional telephone network. From that point, the LTE network and the phone calls on it look like a 3G circuit switched mobile network.

VoLGA is, therefore, a dagger at the heart of IMS. A VoLGA handset doesn't need SIP, much less IMS, to make phone calls on an all-IP LTE network. And an LTE network doesn't need to support SIP and IMS to support LTE handsets that support VoLGA.

This is both good and bad:

It's good because it extends the attractive simplicity of LTE: Fewer, simpler nodes make LTE networks cheaper to build and operate and faster to deploy.

It's good because it does not preclude IP communications services in the handset. In fact, it raises the importance of IP in the handset because the network does not take on responsibility for mediating IP communication.

It's bad because it closes the window on operators' ability to create new service products based on IMS capabilities. Cynics might say this is a deservedly unused non-opportunity.

Nevertheless, it does make adding features like, for example, HD voice more challenging in that they will have to be done in the circuit switched signaling domain for LTE voice calls.

For handset makers, VoLGA marks a clear boundary: LTE handsets are mobile Internet nodes, for sure, but they are also endpoints of the conventional PLMN, with 3G signaling. The IP side, with the possible exception of some messaging services, belongs to the Internet. The VoLGA phone calls belong to the telephone network.

Handsets are, therefore, IP communications devices: They are on an IP network, and anything beyond conventional circuit switched calls is going to be IP communication, and it is not going to be mediated by IMS.

Networks will be simpler, leaving more responsibility and opportunity for endpoint devices: Functions depending on multiple IM/VoIM protocols, one-to-many messaging, presence from multiple services, etc. will not have the excuse that “the IMS system will eventually gateway that stuff.” It's up to the endpoint developer to make it happen.

And what do you do if you have already placed a significant bet on IMS? You take the intent of IMS: To make all modes of communication available to the user and inter-operating, and you implement that intent in an endpoint device that takes the new modes of IP communication and makes them first-class citizens of the handset user experience, and does that using not just an IMS service, but using the services the user may want to use on the Internet.