Here are some thoughts about the questions I get around SVLTE.

Notice the pic and remember this day? This was Verizon and Apple answering questions about the iPhone 4. The biggest technical tidbit of the day was that the 3G iPhone 4 would not allow a simultaneous voice and data session. Of course this was dictated by the 1X CDMA network architecture, not the phone.

Here is a good graphic sort of illustrating that difference.  

Yes, The same issue exists for LTE and CDMA voice!

See, there is a confusion based matrix of items to enable the use of data during a voice call. There is some technical detail to get into to understand what the problem is.

Some of the blame goes to the phone OS, some the phone HW and some is on the network.

HSPA/WCDMA 3GPP networks like ATT and Tmobile support this feature and so the remainder is inside the mobile device. 3GPP2 based Verizon and Sprint on the other hand don’t have this capability inherently in the network. 

UE (Mobile Device) UI Capability/function

The device User Interface (UI) manages the integration of voice and data services for us. As you know, for example, the iPhone visually displays a dial pad for voice calls and has other apps for data like browsers, email, FaceTime/Skype etc… Not so obviously, the multitasking nature of the UI and Operating System  that supports multiple execution threads etc, facilitate and encourage data use such as it is so convenient. The interface makes it easy to switch between these tasks and actually encourage parallel data/voice simultaneous use. So Smartphone = Attention Deficit Disorder (ADD) device.

UE (Mobile Device) Hardware (HW)

Beyond the user interface, the device needs to be enabled with the capability to transmit data and voice simultaneously (or apparently so.)

Many USB based devices are data only (much like the old flip phones were primarily voice only) and only have HW to facilitate a data transmission. No voice HW present, but no need either. Result is simple and cheaper device. Smartphones sort of beg dual use, and often have HW to support data transmission and voice encoding/transmission. Now for the caveat (confusing part), of course it’s possible to have a data application for voice like Skype and that would technically eliminate the need for separate HW however the first deployments in the US are only partial data overlays over extensive voice networks, thus most devices base their connectivity on 3G voice and need to have voice compatible HW onboard for those non 4G or 3G data covered areas. Secondly, the quality of voice in voice networks is currently more assured, or higher priority than the existing non assured data services. So in the case of Skype over 4G, your voice packet gets the same priority (currently by default) as my web browsing packet. (With the PCRF this doesn’t have to be true but operators have not really unleashed this tiered type of service over LTE yet.)

So hardware wise, UEs utilize (baseband) processors capable of 3G to in addition to LTE to be capable of communicating with both networks. The first devices had 2 chipsets, 1 for 3G processing and a second for LTE processing. Only recently has there been multi generational (and multi protocol) capable chipsets that are able to process both 4G data and 3G in one chipset. 

To be clear, a device that has a single RF/baseband processing path may only be communicating on one network at a time.

Here is a typical data + voice capable device architecture showing Qualcomm components. 

So if the processing is in place, the next thing in the chain is the Radio Frequency (RF) chips. You can’t physically interface to the network without the capabilities here so it’s essential for the device to be able to interface to the voice and data spectrum bands and modulate/demodulate the signals correctly. This whole subject is so deep, more in a follow up post on this.

 Qualcomm is arguably the market leader in UE components and architecture. The pictures are the previous MSM8960 components which were 2 pieces of silicon for 3G and 4G processing and others for RF. The MSM9600 chipsets that are shipping now are essentially 1 piece of silicon for 3G and 4G processing with a second for the RF interface.

UE Network Communication Protocol

In the US, MetroPCS, Sprint, US Cellular, Verizon are networks that utilize CDMA 1X technology. 1XRTT (IS-95)/CDMA2000 or 3GPP2 based voice is a circuit switched voice protocol, with all the bells and whistles of 1.25MHz channels, RCs, Walsh codes, PNs etc… For high rate data, these networks utilize 1xEVDO (Evolution Data Only) a code and time division protocol and have slots, 1.25MHz channels, DRCs and so on. In summary, CDMA2000 uses 2 separate radio channels with 2 protocols for data and voice communications. ATT, T-Mobile and others have deployed 3GPP based WCDMA networks with 5MHz channels for voice and HSxPA with 5MHz channels for high rate data. 

Just to be doubly confusing, functions and G’s aren’t necessarily the same. There is 3G high rate data and 4G high rate data, and there is 4G voice and 3G voice. 

The communications protocols for voice on 3G on CDMA networks uses Qualcomm’s EVRC (Enhanced Variable Rate Coder) and on WCDMA networks is AMR (Adaptive Multi Rate) coding. The coding converts sound to data bits. Just to stay confusing, 4G voice, under the Voice over LTE (VoLTE) framework uses AMR also, but the rest of the voice protocol is very different than 3G. 

Data story is similar with 3G high rate data on 1X CDMA 3Gpp2 based networks is EVDO (now Rev A is most common) and on WCDMA 3GPP based networks is HSPA (now HSPA+ is becoming the most common.) For 4G we will only focus on LTE which happens to be 3GPP based.

There’s a lot of complexity in communicating the bits back and forth from the network when you compare 3G and 4G and CDMA 1xRTT with WCDMA/HSPA. Due to technical and cost constraints, most devices up to date have been either CDMA 3GPP2 capable or WCDMA 3Gpp capable. As an example, Qualcomm Gobi based devices have support for just about every situation in one component whereas Marvell only supports 3G WCDMA/HSPA and LTE.

A strong desire exists for every device to support every scenario, however the complexity and risks sort of dictated this approach of multiple networks, separate devices. See this seamless mobility chart, key point is you need both protocols, 3G and 4G for now.

Wireless mobile networks have had this multilayered architecture to support 3G circuit switched voice and high rate packet data at the same time for a while now. Adding in LTE is another layer of complexity that manifests itself as a battery and size constraint for today’s UE designs (more HW + more communicating layers in the devices), and in fact Steve Jobs specifically mentioned it as a barrier to LTE implementation in the iPhones.

UE Parallelism/simultaneous use

So now we are clear a device that has a single baseband processing path may only be on one network at a time. That was very long windup and I apologize. Back to the problem of the Verizon iPhone vs the ATT iPhone and voice and data simultaneously…

A mobile device that only supports a single network (voice or data) device will have to pause communicating with the data network if a voice call is to be made or received and vice-versa.  

The first generation of 3G/LTE capable devices have 2 physical components, a 3G path processor and a LTE path processor.  Qualcomm, TI, Marvell and others are beginning to ship components to enable the 2nd generation of LTE devices, those with 2 paths and shared silicon (fewer components.) The first devices are hitting the shelves. The benefits will be improvements to battery life and form factor, however neither will be as best they can be yet. Have a look at a slide from Qualcomm presentation comparing various architectures based on heat (energy waste) output. So it takes 2 radios to have a 3G and LTE flow from the device at the same time in order to support true simultaneous data and voice use.

The next step in device design will either be VoLTE and/or a software defined radio approach that allows more effective conservation of resources but that’s a topic for another day. Back to voice+data.

OK so the ability to make 3G voice calls and use data at the same time can be broken into smaller pieces.

• For 3G 1X or 3GPP2 based networks like Sprint, Verizon, MetroPCS etc…, 1X (voice) + EVDO (data) is called SVDO. Some devices support this but most networks do not…As an example, the Verizon iPhone either communicates data or voice at one time.
• For 3G WCDMA or 3GPP based networks, the voice and data bearers can be controlled from a single control point, (before HSPA it was on the same channel) therefore the voice and data can flow effectively in parallel. I won’t go deeper but this is the end effect although there are some finer points.
• 4G LTE based networks need to have Simultaneous Voice and LTE (SVLTE) capability on both network and devices to operate in parallel.  It’s a specific function that allows better control of the voice and data over the networks. 

Today’s 3G + LTE networks carry voice over 3G networks as circuit switched voice. In a circuit switched paradigm, voice is carried in a point to point stream and requires real time connection between the two points, much like 2 tin cans with string, or a wired phone. Voice over packet, such as voice over IP networks use a connectionless paradigm, no open line is required. The world is starting to prefer it because this method also allows the use of the time in between the sound pulses to carry other data. Skype is and example of voice over packet (sound is quantized and sent over packets on the internet.) 

So the next iPhone launch with Verizon (3GPP2 based) will require specific upgraded network functionality like enhanced High Rate Packet Data (eHRPD) to be deployed as well as a multitasking OS, and multiple connectivity chipsets to enable simultaneous use whereas ATT (3GPP based) will have an easier time and only require the UE capabilities.

As of right now, the Qualcomm MSM9600 chipsets appear to support the SVLTE functionality and based on their use in The New iPad it appears Apple is headed down this path so it will be up to Verizon and others to deploy the necessary network upgrades.

Hope this helps.

Interesting slides on VoLTE….

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