[Nexus 5 Q&A, Help & Troubleshooting] j'vai:
i wondered if i was conversating over alot of peple's heads in making my case about this;

on another site, it was even asked why did i bring up ehrpd when taking about ecsfb, in relation to sprint's network..

i have to assume, that those who'd ask that, didn't understand the underlying working of sprint's current network, (all it takes is research, one doesn't have to be a rf engineer to understand english, TO connect dots) -

from here A critical examination of CSFB as a solution for providing Voice-on-LTE (PDF)

1st & foremost -

Background to voice-on-LTE

Despite the recent growth of 3G mobile broadband and smartphone appstores, the bulk of cellular industry revenues still come from ordinary telephony. Irrespective of the rapidly-rising volumes of data traffic and the need for more capacity and speed, it is clearly important for operators to retain the ability to deliver a good voice experience, on any new radio network deployment intended for a broad audience.

But unlike previous generations of mobile standards like GSM, LTE does not have dedicated channels for circuit-switched (CS) telephony, instead relying on an end-to-end IP connection from the handset to the core network. Consequently, any form of voice service used on an LTE bearer, by definition, must be some form of VoIP. As far back as 2006 it became obvious that implementing voice telephony was the “elephant in the room” for LTE, but progress on solid and suitable definitions has been glacially-slow. Following a rather panicked flurry of activity, there are now two 3GPP-approved solutions to this problem – VoIMS and CS Fallback - as well as several other non-standardised alternatives.

ok, in a cdma network, this is where ehrpd would come into play, to bridge that, for BOTH voice (1x) & IP (cdma 2000) -

http://http://www.mobilitytechzone.com/topics/4g-wirelessevolution/articles/37366-evolution-options-from-cdma-lte-benefits-ehrpd.htm

This combination aims to enhance the user’s interaction with the network and further drive demand for mobile multimedia services. Furthermore, the LTE evolution calls for a transition to a ‘flat,’ all-IP core network with a simplified architecture and open interfaces. This requirement is defined by the System Architecture Evolution (SAE) — also known as Evolved Packet Core (EPC) — the 3GPP specification for changes to the packet core network architecture.

While GSM/UMTS-based operators have a natural evolution to LTE, many CDMA-based mobile operators also have decided to evolve to the LTE specification. Changes in mobile communications have traditionally been evolutionary, and the deployment of LTE will be the same. The transition for CDMA operators from High Rate Packet Data (HRPD) to LTE will be over a period of several years, as is the case still with the transition from 1xRTT to HRPD. As a result, mobile operators must look for a migration path that will enhance their existing HRPD networks, while addressing LTE deployment requirements and will not require a ‘forklift’ upgrade.

The choice of migration path depends on many factors including radio access strategy, network resource strategy, services enabled, timing and cost. A key goal of LTE is to enhance service provisioning while simplifying interworking with non-3GPP mobile networks. This is essential for CDMA operators that have chosen to migrate to LTE. The following are three basic migration paths to LTE currently available for CDMA operators.

Overlay — In this approach, a complete LTE network is deployed as a second network to the existing HRPD network. However, this will be very expensive, and with an overlay, a subscriber roaming from the HRPD network to the LTE network will experience a loss of continuity for the IP session.

& from the 1st link -

Problems with CSFB

There are numerous drawbacks with the CSFB approach to interim voice and messaging, which Disruptive Analysis believes should prompt operators and standards authorities to look afresh at alternative interim mechanisms. Part of the problem has been that past assessments of the standards have focused mostly on the technical aspects, rather than addressing issues around user experience and behaviour, or impacts on broader application usage and indirect impacts on business models.

The following issues are examined in more details in the next sections of the document:

Additional call set-up latency Requirements on network coverage Side-effects of dropping the data connection during voice calls Impact on data applications, especially on multi-tasking devices Issues relating to SMS support Implementation cost and practicalities Negative impacts on current or potential new LTE business models, eg MVNOs Poor fit with new types of voice application Problematic integration with femtocells

i've read issues, where the newer single chip qualcomm phone (nexus 5 & lg g2 on sprint) would have ither, a strong steady lte signal, but no switch over on voice calls, missing the paging channel altogether, resulting in a voicemail, w/o the phone even ringing..

or vice versa, the phones would park on 3g & stay parked in definately, one would suspect they were in a lte coverage area, & would toggle airplane mode, or reboot, (effectively ripping down the established connection, to rebuild a new one) & getting a strong lte signal!

the older phone with two paths rarely suffered this, regardless of the settings , on home only or automatic, in system select..