Andrew Seybold's Take on Building the NPSBN

If you're looking for some weekend reading and education on FirstNet, Andrew Seybold recently released the final part of a four-part series on what it will take to construct the first ever Nationwide Public Safety Broadband Network (NPSBN). 

We recommend starting with Part 1 and reading all the way through, but here is an excerpt from the fourth installment as a teaser:

Issues Faced by FirstNet

• How many eNodeBs (cell sites) will be needed to cover the United States?
  • How many will be needed within a specific city, county, or region to be able to cover the geography and handle the load on the network? LTE capacity is a function of how many users there are in a single cell sector or site. Where demand will be heaviest, more cells will need to be built closer together.
• How many EPC or cores will be built and placed where within the network? For the sake of redundancy, more than one will be needed, but what is the correct number? Further, how far will the nearest core be from a major metro area?
  • If the nearest core to Los Angeles, New York, or other major metro area is a thousand or more miles away, and even if the backhaul is redundant fiber connections, there is the possibility of a communications failure between the core and the city. If this happens, and Los Angeles, for example, loses connectivity with the core, we would have on the order of 180 useless cell sites and no service in that area until connectivity was reestablished. I believe that in addition to the nationwide core(s), we may need to permit major metro and regional areas to install their own cores, closer to their own area of operation. More than a year ago there was a sub-committee of waiver recipients that analyzed this issue and concluded that multiple cores could be employed. The recommendation was that in addition to the nationwide core(s), if a given area wanted to have its own core it would have to fund its operation and its integration into the network. I am in full agreement on both points.
• What sites will be selected? Will they all be commercial sites or will FirstNet also employ some key Public Safety sites as well?
  • My belief is that wherever possible, FirstNet should use Public Safety sites first and then use commercial sites to fill in coverage and capacity as needed. If the system is designed to use the usually more hardened Public Safety sites and there are failures at the commercial sites, there would at least be a backbone of key sites over which users could communicate. Many commercial sites already have shared towers and facilities with all network operators, so if there is a failure and a site(s) is not working, none of the commercial operators would have service. If the NPSBN is also located on these sites, Public Safety will lose service to the same geographic area. I know FirstNet does not have a lot of money for this network but it can make wise choices, for example, in my area, the same hilltop or mountaintop hosts a county installation and a commercial cell site installation. In cases like this, I believe that the location of the eNodeBs should be on the Public Safety site—not on the commercial site.
  • Commercial networks are designed with at least three levels of sites depending upon the population they serve and the area they cover. Top-level sites are full-blown sites with back-up batteries and generators. Second and third tier sites are used to fill in for capacity and coverage. Many times these sites do not have the same power back-up capabilities since the main sites can, in times of failure, provide some basic coverage for sites that are out of commission. By balancing Public Safety and commercial sites, FirstNet should be able provide even better coverage during times of disaster.

 

Priority Access
Next up is perhaps one of the most important aspects of this network. I am assuming here that in order to afford this network, FirstNet will rely heavily on commercial network partners and piggyback on their facilities, use their backhaul where needed, and in other ways maximize the commercial network operators’ assets. I see no way around this, and in return, the network operators will be able to use NPSBN excess capacity to help manage their own networks and demand for services. However, as I wrote in a recent article for Urgent Communications, I have some serious concerns about network sharing. This started as a discussion among a group of us about sharing the network with commercial users and the requirement that some form of absolute and pre-emptive priority be given to the Public Safety community.

Unlike commercial networks where capacity can be predicted (e.g., Times Square before and after the theater), incidents that will require first responders and where they will occur cannot be predicted. A quiet neighborhood can suddenly erupt in a major incident as easily as in a known trouble area of a city. A major accident can occur along any stretch of highway anywhere in the nation, and hurricanes, tornadoes, and wild fires can hit almost anywhere. Therefore, when planning excess capacity availability, FirstNet must ensure that there really is a way for Public Safety to gain full and immediate access to all of the network capacity at any time in any area of their operation.

In the Urgent article, I started with the premise that the cellular networks in Boston after the bombings had been shut down, which was an early assumption that was not correct. However, what did happen was that the networks were so overloaded with traffic that it appeared to many that they were not operational and thus must have been shut down. The result was the same. The public, reporters, and Public Safety did not have assured access to any of the commercial networks.
The next logical point was to wonder whether commercial network customers who would have sharing rights with the Public Safety network would have been moved to the NPSBN, thus adding to its congestion. Moreover, if the commercial networks had been shut down to foil a bomber’s attempt to set off a bomb using cellular technology, would the NPSBN have been shut down too, just when it was needed the most, because it had commercial users on it?

Based on these discussions, I went back to a committee of LTE experts that is run by a very smart consultant. This is a unique committee made up of some of the best LTE engineering minds in the nation. Members of this group work for different, competing companies. They have come together for the benefit of the Public Safety community and have shared their expertise with many organizations including the Public Safety Spectrum Trust, the waiver recipients, and the APCO broadband committee. They give freely of their time and as a group they are involved in every phase of LTE from working with the standards bodies to working with their own companies and thus with the network operators. They have studied and learned as much as they can about LTE, and in some cases have been involved with LTE before it became a standard. So after my discussions, I decided to ask this group to comment on two assumptions that need to be answered in order to ensure that Public Safety will, in fact, have complete and total access to the NPSBN whenever and wherever it is needed. The two assumptions I started with were:

Assumption #1: If the signaling channel is overloaded, a User (UE) with maximum priority and pre-emptive rights may not be able to access the network.
The short answer: This is essentially a true statement, especially in a network that is shared with commercial users.

Assumption #2: LTE provides a way around this problem (as stated in Assumption #1) that can be implemented to ensure full priority access when needed.
The Short Answer: Mitigation tools exist in the 3GPP standards, but due to a wide range of potential scenarios and causes, to characterize this as solved would be an over-simplification.

The team’s full report will be made available in the near future but the bottom line is this:
Using today’s LTE, if the signaling channel for an eNodeB or group of eNodeBs receives too many requests for connection, the signaling channel (RACH) will, in fact, become overloaded and the eNodeB will not even know that a user with any level of priority is trying to access the network. Further, if a number of eNodeBs all have their signaling channel overloaded, it is also possible that the part of the core network that processes priority service requests will be overloaded and will not be able to process the requests.

Are there solutions on the horizon? The group concluded this was perhaps the case, but as mentioned at the beginning of this segment, we need to deal with what is here and available today, having some faith that enhancements will be developed over time, passed by the standards bodies, tested, and proven to mitigate some of the issues. In the meantime, it will be important for FirstNet to find ways to load-balance the network between Public Safety and secondary users. This information should be used to balance the amount of secondary traffic on the network with the need for Public Safety’s absolute need to access the network at any given time.