Why the Rush to Voice over LTE?

FirstNet is hard at work developing a business plan and an architecture model for the first Nationwide Public Safety Broadband Network (NPSBN). When this network was first envisioned, it was to be a fully interoperable data and video network designed to augment, not replace, Public Safety voice over existing Land Mobile Radio (LMR) systems. Yet even before it becomes operational, some are pushing to add voice to the network. The question in my mind is why the rush to voice.

When Long Term Evolution (LTE) technology was chosen by the Public Safety Community then approved by the Federal Communications Commission (FCC), it was envisioned that the network would be used for data and video services and would be designed to provide access to and from first responders in the United States regardless of where they were. It was also envisioned that the network would be the foundation for interoperability on a nationwide basis, helping to fix the issues that have been hounding Public Safety for more than thirty years but only became known to the public because of the communications failures during the 9-11 attacks, Katrina, and several other major and widespread incidents.

During the long and often contentious discussions with Congress and the FCC that finally resulted in the assignment of the 700-MHz D Block to Public Safety, the formation of FirstNet and the initial funding for the system ($7 Billion), the use of voice on this network was somewhere in the future and not the primary reason Congress approved the re-assignment of this spectrum to Public Safety, nor was voice thought to be an integral part of the network for many years to come. Yet before we even get the network up and running, the idea that voice will be included from day one has gained momentum and now seems to be expected by many.

Today, commercial network operators are using their 2G and 3G systems for voice and LTE is being used for data and video. Yet Voice over LTE (VoLTE) is on all of the network operators’ drawing boards. However, dial-up voice for voice phone calls, what they consider an important voice service, is a far from being a requirement for the Public Safety community. One little known or acknowledged fact about the reallocation of the D Block to Public Safety is the requirement that Public Safety return the T-Band (470-512 MHz) to the FCC for repurposing. This requirement and the burden it is causing the eleven major metropolitan areas affected was placed on the Public Safety community because a single vendor in the wireless broadband business (not in the Land Mobile Radio business) was able to convince some congressional leaders that Voice over LTE for Public Safety is just around the corner and that those now using the T-Band could easily move down into the Public Safety LTE spectrum.

The Public Safety community has often stated that Voice over LTE is something that will happen sometime in the future but LMR systems need to be maintained and even expanded for the foreseeable future. APCO, NPSTC, and others have published documents about the importance of maintaining existing LMR systems, yet for whatever reason, there is more attention than ever being placed on adding voice to the NPSBN. Recently, the Open Mobile Alliance (OMA) proposed an open standard for push-to-talk (PTT) over LTE while Motorola, Harris, Twisted Pair (recently bought by Motorola), AT&T, Verizon, Sprint, and a handful of others are all talking about and demonstrating push-to-talk over LTE for Public Safety. Even in the upcoming Public Safety grade NPSTC report, push-to-talk over LTE is discussed and characteristics are being mapped. The Public Safety Communications Research (PSCR) group in Boulder has been very vocal with the standards organizations and has had some success placing a work item on the 3GPP standards agenda. It appears as though the ground swell for voice over LTE is growing in spite of the dangers of assuming that push-to-talk for Public Safety will, in fact, become a reality sooner rather than later over the Public Safety LTE network.

I would like to see everyone back off and wait until several things happen. First, let’s get the network in place for data and video services. Let’s see how much capacity is really needed by Public Safety, especially during incidents that are confined to a single cell sector (which I believe will be the majority of incidents). Let’s wait and see how much spectrum is available for adding partners that will assist FirstNet in funding the network build-out and then reselling excess capacity. At the moment, we don’t know how much, if any, excess capacity there will be in major metropolitan areas. Until Public Safety is up and running with data and video services, we won’t know how much capacity Public Safety will need either for day-to-day routine patrols or for incidents. Adding voice before we have some metrics to help determine usage patterns does not make sense to me. To simply assume that we will have capacity could result in instances of system blocking, which could impact not only data and video services but voice services as well.

Next there is the fact that, today, there is no VoLTE standard for push-to-talk. Permitting multiple and incompatible flavors of PTT on the Nationwide Public Safety Broadband Network does not make sense simply because it would set Public Safety back in terms of voice interoperability. Today there are still a number of Voice over LMR interoperability issues, but they are fewer and further between than they have been. Until and unless there is a common PTT standard, my vote is to not permit any form of voice communications on the Public Safety LTE network.

Adding dial-up voice to the Public Safety network does not make any sense to me either. It will radically change the system architecture and cost more money (we don’t have enough to build out the network as it is). Commercial operators provide robust dial-up-voice services on their 2G and 3G networks today. Moving forward, they will be adding VoLTE to their own LTE networks so they can refarm their 2G and then 3G spectrum, upgrading it to LTE to gain more capacity and provide better broadband services for their customers.

Beyond dial-up voice, moving to on-network push-to-talk over LTE will have to be done in several steps and no one is sure of the timeframe. The initial step will be to offer non-Public Safety grade PTT services over LTE and tie the LTE network to the various Public Safety LMR networks in order to provide the first level of interoperability. This will require that the Public Safety community, FirstNet, and the vendors agree on a common and fully interoperable PTT standard over the NPSBN that will be presented as non-Public Safety grade PTT for administrative purposes and not for dispatch or mission-critical use.

Next will probably be what is considered as Public Safety grade PTT services using the LTE network, still interconnected to existing LMR networks. This version of PTT will also need to follow the standard chosen for the Public Safety system, and this type of PTT voice service for use for dispatching routine and incident traffic needs to be more robust and well thought out. Many of today’s PTT over LTE systems require that each device that is in PTT mode be assigned one or two resource blocks to keep them live. By design, LTE is based on the allocation of resource blocks on a per-user basis. A user will be assigned more resource blocks for streaming video than for simply using the network for data services, and a PTT session will need even fewer resource blocks. However, using most of the current PTT technologies, these blocks will need to be assigned to a device to keep it “alive” even when it is in listen mode. Therefore, given an incident within a single cell sector and the need for both uplink and downlink data and video services, a number of PTT users, all requiring resource block allocations, could have an impact on the amount of capacity left for data and video services.

Please click here to read the full article