Mission Critical Reports: Public Safety Grade LTE: Myth or Reality?

By Joe Ross, Steve Sidore, Scott Edson and Ted Pao

Recently, there has been some debate on the meaning and definition of “public safety grade.” The recent hurricanes underscore the need for clarity on what it means and what public safety needs with regard to reliable data. A National Public Safety Telecommunications Council (NPSTC) document published in 2014 provided a definition that the overall system design enables system and service to achieve 99.999 percent availability. What does availability of this magnitude mean in lay terms? Availability at 99.999 percent (five nines) results in net outage of five minutes per year. Availability at 99.99 percent (four nines) results in net outage of roughly 53 minutes per year. Both factors are better than the general commercial carrier availability of 99.0 and 99.9 percent availability (between 88 and 8.8 hours respectively per year)

This creates a significant difference in expectations because the devil is in the details of how availability is calculated versus measured in an operational network. The First Responder Network Authority (FirstNet) is highlighting the importance of data in public-safety operations, and as public safety wrestles with a 25-year commitment decision with the selected vendor, it becomes one of the cornerstones of the eventual solution that will last a generation.

If FirstNet and data communications are ever expected to become “mission critical,” public safety must be able to rely on data communications as much as LMR, built to five nines availability, which is needed to achieve public safety grade. So, a definition is less material than whether FirstNet will be as reliable as public-safety radio communications. If in five or 25 years, broadband data is only slightly more reliable than existing commercial networks, the mission-critical element of broadband data will not occur. Public safety needs a concerted effort to work toward public safety grade, defined as 99.999 percent service availability. Five nines, among many other requirements, is mandatory for broadband to replace LMR.

While not the desired level of reliability, four nines of availability simply is not achievable without substantial network hardening. Network transport services — the connection between cell sites and the core network — generally have service level agreements (SLAs) that only guarantee 99.9 percent service availability. For higher service availability, multiple unique connections to the cell site are required.

Likewise, a power-related event caused by equipment failures or major weather events is not going to deliver four nines. These events frequently have average downtime durations of a full day. A single event could then cause an entire region to experience less than 99.9 percent reliability. It is not feasible that a commercial carrier could achieve nationwide 99.99 percent availability without hardening the majority of its network.

When public safety builds systems to public safety grade, the availability applies to the system itself. Not every site is guaranteed to achieve 99.999 percent availability, but overall, across all aggregated sites, the construction and commitment are generally 99.999 percent. Each site, connectivity and core that support each other must be designed collectively for greater than 99.999 percent availability. So, a purpose-built network for a city or county achieves 99.999 percent availability in that city or county. A nationwide commitment, on the other hand, could mean that areas where it is difficult to achieve a high degree of availability — areas that experience frequent hurricanes, for example — could be sacrificed because of expense.

For example, if AT&T’s network in Los Angeles County failed for an entire day, it would have little impact on AT&T’s nationwide compliance but would result in, at best, 99.7 percent availability in Los Angeles County. AT&T and FirstNet might consider this a success, but an outage affecting Los Angeles County serving 10 million residents for a day would cause major problems and put lives at risk.

While it may be difficult and challenging to achieve four nines, much less five nines of availability, public safety needs most sites for each region, city and county to be hardened to ensure that local public-safety officials can rely on FirstNet wherever they live and work.

Read the full story in the October issue of MissionCritical Communications. https://www.rrmediagroup.com/repository/files/PublicSafetyGradeLTE.pdf