Nexus 5 and LG G2 experience temporary Sprint LTE connectivity issues due to Circuit Switched Fallback technology
by Robert Herron
Sprint 4G Rollout Updates
Monday, November 11, 2013 - 11:55 PM MST
S4GRU Staff and Members have been anxiously waiting for over a year for smartphones that would support all three of Sprint’s LTE bands. Since April 2012, Sprint LTE devices have been limited to only one band. Band 25 (1900MHz in the PCS Band). Subsequently, Sprint closed down the Nextel network and picked up another LTE band (Band 26). Also, Sprint purchased Clearwire and picked up another LTE band that they had started to use (Band 41). So Sprint now has use of three LTE bands which will allow it to provide more capacity, better maximum LTE speeds and coverage. With now three LTE bands, Sprint needs Triband LTE phones.
S4GRU and our members really became excited in Mid 2013 when we learned through sources that the first Triband LTE smartphones would be out in late September. Shortly after that, we learned that the upcoming LG G2 would be able to support all three Sprint LTE bands via FCC reports.
Rejoicing and happy tears filled all of us wireless nerds in anticipation of the first Sprint Triband LTE devices. Then we learned through a source that Sprint Triband LTE devices would not support SVLTE (Simultaneous Voice and LTE). To some this was a setback, but the hard core wireless enthusiast was not distracted. We all wanted Triband LTE at any cost. Besides, nerds don’t spend much time on the phone talking with people.
We waited and waited, but nothing released. A few more Triband LTE devices came through the FCC, including the Nexus 5, Samsung Mega 6.3 and the Samsung Galaxy S4 Mini. Then we found out that the Sprint versions of these device would not launch at the same time as their competitor counterparts. Many of our members were screaming about the delays. And we knew there had to be good reason. We just didn’t know what.
The dam finally broke with Google’s release of the Nexus 5 on Halloween. And Sprint finally broke down and released the LG G2 about a week later. S4GRU and dozens of our core members quickly got their hands on their new Triband Nexus 5’s and G2’s and all was happy. For a few minutes. Until they tried to use Sprint’s LTE network where they used to on previous LTE devices.
Some of our members reported that both the G2 and N5 had extremely strong LTE signals in Band 25. The best they have ever encountered. However, there was a very vocal group who were reporting that they could not stay connected to LTE for more than a few seconds. Something was very wrong.
We tried to troubleshoot and figure out the problem with our members. But there were no clear common denominators among the problems that we could ascertain. We could not figure it out. And then we received heads up from internal memos within Sprint as to the problem. Sprint Triband LTE devices use Circuit Switched Fallback (CSFB) on the network.
Sprint Triband LTE phones dropped SVLTE for eCSFB/CSFB
Up until these new Triband devices, previous Sprint LTE devices supported simultaneous voice and LTE (SVLTE). It could do so with two separate transmission paths from the antennas to the chipset. Voice/texting could run via 1xRTT on one transmission path. LTE could run a separate path, allowing data and voice to be used simultaneously.
In contrast, Sprint Triband LTE devices do not support two separate transmission paths. They have one path, shared by voice/SMS and data. We were alerted to this months in advance. However, we did not realize that the network would have to run on Circuit Switched Fallback in order for this to work and what the ramifications of this would be.
S4GRU was told by a source this past summer that Sprint and the OEM’s came to the conclusion that these new Triband LTE devices could not use SVLTE in the conventional way they used to, and it would require a lot of engineering, testing and cost to even attempt such a design change. It was decided to release Triband LTE devices without SVLTE. It may seem that the only drawback for doing that is Sprint Triband LTE devices would not be able to run simultaneous LTE data while on a phone call or when actively transmitting a text. But there is another. And it’s why many early adopters of these new Triband LTE smartphones no longer are being able to connect to Sprint LTE in many places that they used to.
How it works
In previous Sprint LTE phones, when a device was in Sprint LTE coverage it would park in both the LTE and CDMA Sprint networks at the same time. When a voice call came in, it would just go straight through to the device. And signal to the LTE network would be maintained the whole time while the call was active.
In contrast, a Sprint Triband LTE device can only stay on one technology at a time. CDMA or LTE, not both. So when a Sprint LTE Triband device is in Sprint LTE coverage it parks only in LTE. And doing so means it cannot transmit calls without Circuit Switched Fallback (CSFB) on the network side. CSFB and eCSFB (Enhanced Circuit Switched Fallback) are network controls that will allow a single mode/single path network to operate in two modes, both CDMA and LTE.
Here is how it works in the simplest way I can describe. When your Triband LTE device has an LTE signal, it cannot receive or make calls on its own. It is just using LTE data happily. However, what if someone calls you? How does it get through the CDMA network to your device? Via CSFB.
When the Sprint network tries to forward a call to your device but cannot see it via CDMA, it then checks for an LTE connection to your device. If it sees one, it tells your device to disconnect from LTE for a moment and reconnect to CDMA. Your device then jumps over to take the call on Sprint CDMA and the LTE session is interrupted. This happens very fast and seamlessly. Except for the loss of data availability. If you receive a text, the Sprint network is able to route it to your device via LTE.
Circuit Switched Fallback is a great solution to the issue of Sprint Triband LTE smartphones. But the problem here is that the Sprint network is being upgraded in Network Vision, and not all Sprint parts of the Sprint network can currently support CSFB. And it affects all Sprint Triband LTE phones, not just the Nexus 5 and LG G2.
Why it’s not working and impacting LTE in some places
As everyone reading this article probably knows, Sprint is in the middle of a network modernization program nationwide called Network Vision. It upgrades every piece of network hardware, site equipment, radios, software and network backhaul to every one of Sprint’s nearly 40,000 CDMA sites. And much of Sprint’s legacy network either doesn’t support Circuit Switched Fallback or doesn’t support it in cases where the legacy network equipment is by a different manufacturer than the new Network Vision equipment.
The problem that these early adopters of Sprint Triband LTE devices are encountering is that when their phones connect to the Sprint network they try to connect to LTE. And when it cannot see the CDMA network through CSFB, it then reverts back to Sprint CDMA and stays there. It does this in order to preserve device connectivity for the user to Sprint voice capability. When forcing these devices into LTE Only mode, the LTE works very well ruling out a device problem. They just are unable to use LTE in default mode without being able to have access to CSFB on the Sprint network.
How and when is this problem going away?
The good news is that most of the Sprint network is capable of supporting CSFB in some form or another now. Some markets are not having any problems at all, like Ft. Wayne/South Bend, Puerto Rico/Virgin Islands, most of Chicago and Indianapolis. eCSFB is complete or very close to complete in these markets.
Upgrades to the Sprint network are being handled nationwide by three different OEM’s. Samsung, Alcatel/Lucent and Ericsson. They are in various stages of deployment and are currently impacted differently by region. In places where CSFB is in place and operational, there are no problems with using LTE on a Sprint Triband device.
And Sprint and their OEM’s are scrambling to get CSFB operational in all the other places. Some of the existing networks are capable of supporting CSFB and Sprint is working to get software upgrades in place for these networks to get it operational on them. However, some of the Sprint network has unsupported equipment from Motorola and these cannot be upgraded and will need to be replaced with their new Network Vision equipment to allow LTE and voice to work together via CSFB.
Currently, just over 60% of Sprint sites have their sites upgraded to new Network Vision 3G standards which allow Circuit Switched Fallback capability. However, not all 60% of these sites are currently allowing LTE to work on a Triband device. These all should be capable of using LTE on a Triband device now, or in the next few weeks. Many of these markets will need to have their MSC Switch Center’s software upgraded too for CSFB to work.
Beyond this, Sprint also has another 10% of their sites that have LTE operational but not the 3G upgrades that support CSFB. These 3,000 sites currently have Sprint LTE live, but it cannot be used by Triband devices without CSFB active. But there is hope for these locations. These sites do already have all the hardware needed to install upgraded 3G that will work with CSFB on the network. Sprint is scrambling with their OEM’s to get 3G up and running on these sites as soon as possible. Many have been upgraded recently and they will continue to be upgraded over the next weeks and months. I was told by an unnamed Sprint source that half of these will be CSFB capable in a month and the other half will be between 2-3 months additional beyond that.
Sprint should be in a position that in the next 3 months that their entire LTE network will be CSFB capable and this will go away. As each site gets CSFB capable, Sprint LTE Triband device owners will be able to connect to LTE. And some S4GRU Members have already experienced this and are now reporting some sites reappearing to be used by their Triband LTE devices. This is likely do to a recent enabling of CSFB at the connected site.
What about the last 30%?
The last 30% of the Sprint network is not currently affected by this problem because they have yet to be upgraded with Network Vision or LTE. These sites are in various stages of being upgraded. In internal correspondence, Sprint says they will now take into account CSFB availability before launching new markets.
Network Vision deployment will continue as normal, but OEM’s will now try to launch LTE and CDMA upgrades together at each site whenever possible and install CSFB capability at the network level for all the remaining sites. In cases where they cannot happen together, Sprint will continue to allow the LTE site to go live. But the site will only be discoverable initially to Sprint SVLTE devices. But by the time Sprint is ready to launch the whole market, CSFB will need to be operating before they issue the Press Release so customer expectations are met for all LTE device holders.
The bottom line here is that there are thousands of Sprint Triband LTE early adopters that are currently not able to connect to LTE sites that do not have a CDMA network connection that support Circuit Switched Fallback. But the problem is temporary, and improvements will go live every day around the nation reducing the number of affected sites. It will get better and better every day. However, we do not know how different markets will fare and when. It will be highly variable.
There are many advantages of being an early adopter. However in this instance, for those who are very dependent on their new found Sprint LTE service, this may be too big of a burden to bear. These folks will need to use a Sprint single LTE band device until CSFB is working in their area or, as some have threatened, use another wireless carrier.
At S4GRU, we believe that knowledge is power. This is the explanation of what’s going on, and what is being done about it. Now use the info to determine what’s best for you. Most of our members will likely just endure it and then reap the rewards once CSFB can be brought online in their area.
A parting point in all this is Sprint is promising some advantages to a single transmission path with Circuit Switched Fallback. Sprint says in their memo that Sprint Triband LTE devices with CSFB will have improved battery life and better edge of cell radio performance. We’ll be glad to enjoy those benefits when they are fully realized.
EDIT: Since the initial publishing of this article, it was discovered that Triband LTE devices were capable of sending/receiving texts via LTE. It is only voice calls that require Triband LTE devices to shunt back to the CDMA network via CSFB. The article has been edited to make this clarification.
Initial LTE devices were data only (like USB dongles and MiFis), then LTE devices with voice/text services use either SVLTE or CSFB. Finally, Voice over LTE (VoLTE) will be enabled in the coming years that will allow simultaneous voice and data without need of falling back to 3G/CDMA networks. But VoLTE is still at least 18 months or more from being instituted on a large scale.
Sprint Internal Memo regarding Circuit Switched Fallback issues:
eCSFB Gap Talking Points
Sprint’s Network Vision Plan (Spectrum)
- Sprint’s Network Vision plan includes access to three spectrum bands, (1.9, 800 and 2.5), and LTE handsets with a single radio.
- Access to diverse spectrum assets allows Sprint to maximize capacity and coverage.
Current Sprint LTE Devices (2 Radios)
- Current Sprint LTE devices are Simultaneous Voice and LTE (SVLTE)—using two radios, one for voice and one for LTE data. This allows CDMA calls and LTE to operate at the same time.
- Dual Radio SVLTE devices can degrade the user’s quality of experience – two antennas may slow throughput at the cell edge and reduce battery life.
Future Sprint LTE Devices (1 Radio)
- CSFB and eCSFB (Circuit Switch Fall Back and enhanced Circuit Switch Fall Back) are features that enable single radio functionality in the handset.
- eCSFB Benefits:
- With eCSFB implementation, Sprint will be able to offer fast LTE data speeds and 3G voice supported on single radio devices. This provides a more cost effective option for Sprint’s long-term business plans to support additional spectrum bands.
- eCSFB is expected to improve handset battery life compared to SVLTE.
eCSFB Gap Issue – What you Need to know:
- Smart devices require eCSFB to “fall back” to 3G to accept a voice call if the user is connected to the LTE network.
- All markets need to be upgraded with eCSFB capability.
- eCSFB Gaps are only in non-incumbent markets where a site has 4G LTE and no 3G upgrade.
4G LTE integration with 3G is different by OEM type:
- Incumbent markets – LTE 4G only sites can communicate with either legacy 3G or NV 3G sites.
- Non-Incumbent – LTE 4G can only communicate with NV 3G sites; so individual sites need both NV upgrades.
LTE devices – Customer Experience
- When 4G LTE is deployed in a non-incumbent market on sites without upgrade to 3G service, there is a gap.
- Existing Devices – No Impact
- eCSFB Devices – Phone becomes 3G only device
- No coverage visible on Sprint.com maps but different experience for users with existing devices versus new eCSFB devices.
- Sprint coverage tools will display LTE coverage. For new LTE smart device users this could be confusing since they would expect LTE and 3G coverage with the device they purchased, but would not experience LTE until the 3G network is upgraded.
New LTE Markets
- When locking new LTE markets for announcement/ launch, eCSFB deployment status will be a factor in the analysis. New LTE markets will not be announced unless the eCSFB gap risk is low.
- Complete the build quickly in eCSFB gap impacted markets.
- CSFB – Circuit Switch Fall Back. It allows for single radio devices. (ERC)
- eCSFB – Enhanced Circuit Switch Fall Back Next generation CSFB. (ALU, STA)
- SVLTE – Simultaneous Voice & LTE. Allows CDMA calls and LTE to operate simultaneously
- Incumbent – An incumbent market is one in which the OEM stays the same as before Network Vision
- Non-incumbent – A market in which the OEM changes as well as all of the equipment.
- ERC - Ericsson
- ALU – Alcatel Lucent
- STA - Samsung
- NV – Network Vision
- OEM – Original Equipment Manufacturer (ERC, ALU, STA)
- Gap – 4G LTE on sites without appropriate upgrades to 3G service
- 1.9 GHz Spectrum - Provides initial capacity and coverage for voice and data services.
- 800 MHz Spectrum - Improves coverage and in-building penetration for all services.
- 2.5 GHz Spectrum - Increases data capacity to support higher data demands and more users.