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Boston bombings raise sharing questions for FirstNet

Network-congestion issues during the aftermath of the Boston bombings should spur FirstNet members to ask some hard questions before agreeing to potential sharing arrangements with commercial operators.

By Andrew Seybold

The bombings during the Boston Marathon were a terrible tragedy, and the news media got a lot of things wrong in the first few hours after the blasts. One report, later retracted by the Boston Globe, was that the federal government had ordered all commercial networks to be shut down. No one knew if the bombs had been set off via cell phones, which is one of many ways they could have been detonated. The idea was that the networks were shut down was based on a fear that there could be more bombs, and the networks could be used to detonate them.

The real story was that the networks were not ordered to shut down, but they were so overcrowded that many call requests could not be put through. So many people being denied access to the networks led to the assumption that they had been shut down.

Even so, the fact that the networks were overloaded—as they are in times of major incidents—should be of concern to FirstNet. Because FirstNet is exploring the possibility of sharing the Nationwide Public Safety Broadband Network (NPSBN) with commercial operators, which will in turn make use of excess capacity on the network, the following questions need to be considered in the final network design. These questions are applicable during commercial-network overload, as well as when—and if—commercial networks are ever ordered to shut down.

1)      If the commercial networks were to be shut down or become overcrowded, and the network operators had a sharing agreement to use the FirstNet network, would all of the users on the commercial networks be shifted over to FirstNet, causing increased traffic and congestion on the NPSBN when it is needed most by the first responders?

2)      If commercial users had access to FirstNet and the commercial networks were shut down, does this also mean that the NPSBN would have to shut down, because a commercial device on this network would also be capable of detonating a bomb by remote control?

3)      What—if any—safeguards will be built into the sharing agreements between FirstNet and the commercial network operators, and how will the load between FirstNet first responders and commercial users be monitored and managed? Will it be possible to shut down access to the NPSBN for secondary users (commercial users) when the commercial networks are shut down or are overloaded?

The FirstNet system design is based on public safety having pre-emptive access to the network. In theory, pre-emption would occur in two phases. The first would be to limit the bandwidth and capacity available to non-first-responder users during incidents. The second would be ‘ruthless pre-emption,’ in which a first responder who accesses the NPSBN would be granted that access instantly, even if it meant terminating a commercial (secondary) user’s connection.

This sounds great in theory, but in today’s real world, any type of true pre-emption might not be possible. One of the committees I serve on is made up of some of the brightest LTE engineers in the business. They have participated in the standards body work for LTE, designed LTE systems, worked with LTE systems, and have more knowledge and hands-on LTE experience than any other group I have worked with or talked with. When I raised the following issues, they dug into them, and their top-line answers are shown after the statements.

 

Assertion 1:If the signaling channel is overloaded, then a User (UE) with maximum priority and pre-emptive rights may not be able to access the network.

Top-Line Answer:This is essentially a true statement, especially in a network that is shared with commercial users.

Assertion 2: LTE provides a way around this problem (as stated in Assertion #1) that can be implemented to ensure full priority access when needed.

Top-Line 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 oversimplification.

 

There is much more to the response from this group that I will publish soon in my Public Safety Advocate e-newsletter, but the reason for the response is based on the following overlying characteristics of LTE (or any cellular-like network):

1)      In order to make a call (or get onto the network), the device must send a request for access to the network. The network then verifies that this unit is permitted access and attaches the device to the network.

2)      The signaling channel input is located at each cell site, and it sends the request to the network. If the number of requests for service exceeds the capacity on the signaling channel, some of the requests will not be processed.

If the request for service or pre-emptive service is not delivered to the cell site and transmitted to the network, the network has no way of knowing the request was even made. Further, if the requests overload a number of cell sites, they may not travel from the cell site to the network for processing. In other words, if the LTE signaling channel (RACH) is congested, the level of priority makes no difference. If the cell site does not receive the request for service, it cannot be processed. So in reality, there is no true pre-emption in LTE today.

Note: One of the arguments we used with Congress to have the D Block allocated to Public Safety was the fact that pre-emptive access on commercial networks was neither practical nor feasible to accomplish. The earthquake centered in Virginia and the following hurricane proved our point, because—while the commercial networks remained operational—there were many instances when access was totally blocked because the signaling channels were overloaded.

The bottom line is that implementing the public-safety LTE system will be a real challenge. I am sure the FirstNet is up to it, but the solution will need to be carefully focused on the needs of public safety during incidents when access to the NPSBN will be critical for the public-safety community.

 

Wireless communications consultant Andrew Seybold is a communications advisor to both the National Sheriffs Association, and the International Association of Chiefs of Police.

Discuss this Blog Entry 12

on Apr 23, 2013

The problem with everyone is that they have a short memory as to just what happens to the cellular system during any major event. Lets start with hurricane Katrina. It took out 17 central offices and the major long lines connections out of New Orleans. There was no long distance service for almost a week. The Nextel system came back up fairly fast, but functioned only in the local area. No long distance calls in or out. Then long distance outbound came up and you could call out of the area, but not into it. Funny thing was the PTT was able to be used with very little problem both locally and out of the region. Then the Bell system switches started to come back to life.

I had been given a bunch of sat phones to take to the New Orleans area by one of the Federal agencies. These worked good until about Wednesday. Then everyone else with a sat phone filled up the capacity of the sat service and you were back to nothing.

Hurricane Sandy that came through the East coast and making itself known in New Jersey basically did the same thing. All the cellular service went down. It was many days later that there was even some limited service.

Given these events, it would be very wise to not have to rely on the cellular carriers for for much of your public safety backbone at this point. Will the carriers bring their network up to the standards that the public safety systems demand? We will have to sit back and wait to see. My money is on that it will never happen.

NF2G (not verified)
on Apr 23, 2013

LTE seems to be too complex for first responder use. If a radio needs more attention than push-talk-release-listen, then it should not be in the field.

The issue of the system being unaware of a priority call is not a problem with "low tech" radio systems. As a former dispatcher, I know from experience that, if I hear a heterodyne, then somebody is trying to get through. I will make every effort to receive that communication. There is no excuse for being unaware that a priority user is attempting to communicate. We should not be seeking to build that problem into future communication systems.

Mark Pallans (not verified)
on Apr 23, 2013

This may be oversimplifying the issue but -- the government has the GETS system which prioritizes land line phones during an emergency. Can't the same be done on the wireless side?

Anonymous (not verified)
on Apr 25, 2013

WPS is the wireless variation to GETS. However, it relies on cellular service working.

on Apr 25, 2013

Mark; GETS only works if you can draw dial tone. If the central office is too busy, then no dial tone/no access. Same with what Andy was talking about. Priority service IF and ONLY IF you can 'draw dial tone' [get your request into and processed by the system].

resham (not verified)
on Apr 23, 2013

Dispatcher: One Adam Twelve, One Adam Twelve See the... Beep Beep Beep, I'm sorry your call cannot be completed at this time please try back in a few minutes.

on Apr 23, 2013

As usual Andy's comments are on point. I would venture to add another concern... PS systems are specifically designed to provide as close to ubiquitous coverage of an agency's operating area as possible within technical and financial limitations. If FirstNet lives up to this heritage it will likely provide coverage in areas where the commercial carriers don't despite their decades of head start. This could pose yet another challenge to the concept of sharing and since sharing is viewed a potentially critical source of funding Andy's emphasis on the issue is well founded. Since prior protocols like GSM insured the ability for network preemption it is disappointing to learn that LTE lacks such a foundational capability.

canningt (not verified)
on Apr 24, 2013

Very interesting clarifications. I have been a strong advocate of allowing commercial sharing of the PSBN in order to mitigate the PS cost of implementation, however these observations cause me to have serious second thoughts about the advisabilityof this approach.

on Apr 25, 2013

I live in Boston and the network was shut down. As a former Senior RF Engineer with the regional RBOC, I cannot ever remember shutting down the system. Since I now (alone with cellular/LTE) design TETRA networks, TETRA has fully duplex interconnect to the PTSN. Same features as cellular - if TETRA had been employed, PS would have has access to the outside network despite the commercial network being down - not to mention features not available on any domestic (P25 or otherwise) network. PS quality PTT, ANI/Man-Down/Auto Man-Down, SMS, 4:1 channel ratio (if 25 KHz were not given up 450-470 MHz), emailing, texting, AVL, web access (28 kbps), but if TEDS is employed, very usable bandwidth for large data requirements. An impressive and effective gateway for interoperability to the agencies not in the same spectrum. Not mentioning being TDMA, all those "ears" especially the media, second guessing every move would be mute. Never forget watching CNN with one hand on their smartphone, the other phone on some audio streaming of PS network. LTE would have been a disaster being part of the commercial network. It is fine for data, but please do think of using it for any mission critical PTT. US you are behind what 150+ countries have already put in place.

Anonymous (not verified)
on Apr 29, 2013

If I recall priority worked at the switching level but not at the bsc software rf level. Thus if traffic blocked at radio level no calls can grab necessary channel for call completion from tower to bsc and bsc to switch.

Anonymous (not verified)
on May 2, 2013

"If the LTE signaling channel (RACH) is congested, the level of priority makes no difference." Why can't FirstNet users have a separate LTE signaling channel from commercial users? Public safety agencies having their own "door" into the cellular network should help address other issues relating to priority and pre-emptive service that could be accomplished over a "common" wireless network for both users.

Anonymous (not verified)
on May 7, 2013

If the LTE signalling channel approaches saturation, the network can be configured to drop all commercial traffic. Additionally, the number of commercial users can be restricted to allow for sufficient "headroom" in the event of a sharp instantaneous increase in public safety traffic. Also, public safety administrators can intervene when an emergency occurs and force all commercial traffic off the network for the duration of the incicent. Bear in mind that any network can experience overload on either the control channel or the bearer channels, including existing P25 public safety LMR networks. The NPSB LTE network will have more spectrum available than the combined spectrum assets of public safety entities at any given geographic location. These spectrum resources can be reused at closer intervals than traditional narrowband LMR networks and can be dynamically assigned to avoid interference, something that is not possible with narrowband LMR networks.

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