Tuesday, March 29, 2011

Voice and LTE

So, I posted earlier about LTE, the "4G" standard that's rapidly becoming the standard for mobile communications - with one or two prominent exceptions, just about everyone who's rolling out a next generation network is rolling out LTE.

LTE is "All IP", that is, all communications between the handset and the rest of the world (data, voice, text messages, etc) go over what's essentially an Internet connection, with the lower levels of LTE providing the Internet connection. This raises an obvious question - how do you make a phone call over it?

First of all, unless you're interested in the technical side, you're not going to see a difference. You still have a keypad you dial numbers on, and when you get an incoming call you get a ring tone, as usual.

What's going on underneath is that the operator will be using something called "Voice over IP" (another subject I've written about.) This is the same type of thing that's going on if you've ever used a service like Skype, the phone dialer thing on GMail, that MagicJack thingie, or Vonage. Your voice gets encoded into a digital form, transmitted over the Internet, and then decoded and played back, in real time, either to the end user (if they're on the Internet too) or to a line on the phone network.

This is all great in theory, and when the designers of LTE sat down to figure out how their next generation system would work, they kind of assumed that the whole "Voice over IP" thing was already done and dusted and they didn't need to work on that, they could just concentrate on the Internet connection side. After all, the group standardizing on LTE, called the 3GPP, had indeed released its own Voice-over-IP system some years back, called IMS (IP Multimedia System.) IMS is more than VoIP, and in theory as it was already standardized and integrated into the GSM family of systems, of which LTE is the latest version, there was nothing more that needed to be done.

Some operators, including Verizon, disagreed. They felt IMS was too high level and didn't address low level issues necessary to make a phone call.

Let's explain that by using an example. Let's suppose that a hundred people use a single Internet connection to access the web, in, say, the space of a minute. It's very unlikely they'll all try at once, but it is likely that some seconds of that minute will have more people trying to access the Internet than at other times. If you were to graph the amount of data coming through the Internet connection then, you'd see something that looks like a mountain range, rather than a nice neat curve or a straight line.

Now, during the times a lot of people are trying to get data, the network is likely to get congested - that is, there'll not be enough capacity to support all of the data being sent. So what happens? Well, each user's connection is effectively slowed down until there's enough capacity to support everything that still needs to be done.

On the web, this doesn't matter. You can read a website regardless of whether it took five seconds to load, or six.

But voice isn't like that. The system cannot slow down one person's connection if the network becomes congested because that would result in the conversation being stalled, or chunks of audio becoming missing. The designers of IMS did some things to reduce the chance of this happening, by letting the quality of calls drop in the event of network congestion, but this isn't a desirable path to follow.

What Verizon and others want is for, instead, anyone on a voice call to be able to reserve a certain amount of data throughput, so that the amount of capacity makes a difference only in whether you can make a call to begin with, not whether you can still make sense of what someone's telling you a few seconds into the call.

What they're working on is coming called Voice-over-LTE (VoLTE.) The concept builds upon IMS and allows bandwidth to be reserved for each call.

Many operators are holding off implementing voice on their LTE networks until the VoLTE work is finalized. Once this is done, it will be possible to buy any LTE phone that supports your operator's frequencies, and know it will work, just as you can with a GSM or UMTS phone today. But it also means you need to be aware that LTE phones today are only half ready. They generally rely on a separate 3G or 2G system to make and receive regular phone calls, and will not work with VoLTE once it's rolled out.

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