Comparing LMR and Cellular for Mission-Critical Communications

Electrical Storm - Reliability-and-event-overload
Geoff-Peck-Tait-Communications-Chief-Technical-Architect

Geoff Peck | Tait Communications

Chief Technical Architect Geoff Peck compares the performance of cellular and LMR for mission critical communications, and concludes it’s not as simple as it seems.

There is ongoing debate on the topic of cellular versus LMR. Indeed, nothing seems better designed to polarize opinion. This article looks at the issue from where it matters: the end user.
The first thing you notice about the debate is the misinformation – it’s not easy to nail things down simply.

So let’s start with some important points of comparison.

For ComparisonLand Mobile RadioCellular
PurposeDesigned for coverageDesigned for capacity
InfrastructureRelatively few towersMany towers
PowerHighLow
Spectral efficiencyLowHigh
Data CapacityLowHigh
DuplexHalfFull
Group modeGroup modeNo group mode (until 4G)
TalkaroundWhen infrastructure fails4G will have in 2015
Voice qualityHigh (DAQ)Consumer voice quality
EncryptionAES256 standardCellular LTE = AES256
Call typesAll informedPerson direct
Dominant featureVoiceRich services and applications
Reliability99.9%99.99%
Event overloadAvoid event overloadSubject to event overload
CallingPTTDial number

There are some clear differences, like the fact that LMR uses fewer towers, requires higher power and has a larger effective range. These are colored in green. Yellow features are a bit more controversial, while those in red are hotly disputed or frequently misunderstood. It is these that we will explain further here.

Reliability and Event Overload

Electrical Storm - Reliability-and-event-overloadWe have all experienced cellular systems apparently failing. Usually this is call drops at the edge of coverage but during times of extraordinary high use such as natural disasters, they also seem to fail. Or do they?

Technically, cellular systems are designed to 5x9s or 6x9s reliability, which exceeds normal LMR standards by some margin. So the issue is not that cellular systems are less reliable technically. It’s all about perceived reliability.

What a user experiences is a combination of effects:

  • The cellular system is a commercial operation that delivers service on that basis. LMR is designed for mission critical use, with short bursts of guaranteed connectivity. Comparing their reliability is not straightforward because of the different purpose.
  • When we lose a cellular call, often it’s a result of a 3G cell ‘breathing’ its coverage due to loading and failing to hand off your call to an adjacent cell. With LMR there is no concept of a ‘cell’, you either get access or you don’t.

4G LTE systems behave differently again, and so comparing LMR with 3G and using that to imply expected behavior on future 4G LTE systems is risky – like comparing apples with oranges, then assuming what is true for apples is also true for pears.

For example, it’s difficult and expensive to create private 3G systems, but private 4G LTE systems are much more affordable. Provided they are engineered with user outcomes in mind, they can provide levels of performance not easily provided on commercial systems.

Cellular systems seem to fail during natural disasters. Quite apart from the extreme load on public systems during events, natural disasters frequently destroy infrastructure. LMR survives these events better than cellular. For example, a disaster usually affects quite a small region in the context of an LMR system. Statistically, the few LMR towers will end up surviving better than the larger number of cellular towers. This also gives LMR an advantage when power is disrupted, since refueling a smaller number of LMR backup systems is often easier and faster.

LMR systems are effectively circuit switched so you get exclusive access to some resource for a short period. This precious resource can also be consumed fully in an emergency so LMR can be overloaded too.

On balance then, LMR seems a better bet for critical communications in a disaster. But this does not mean that a cellular system is unreliable – it comes down to what service it is designed to deliver. Private cellular systems like LTE can be engineered to deliver whatever you need.

Push to Talk (PTT)

Tait-TP9300-16key-3way-black-with-grey-buttonPTT is the reason LMR systems exist, and is a very effective way for a dispersed group of people to stay in touch with central dispatch and each other. All-informed group modes reflect the Standard Operating Procedures (SOP) of mission-critical agencies.

Using it correctly demands some process. The system works best with short, coherent exchanges – long rambling conversations are not ideal. LMR delivers near-real time PTT. This is an important feature for safety and operational efficiency in mission-critical operations.

Of course LMR systems provide a lot more than voice, and with GPS, can easily be used for location monitoring. Many lives have been saved through the Emergency button.

Voice was originally the reason that cell phones existed, but the iPhone changed that. While advanced data services existed well before Apple ventured into the phone business, the combination of a powerful mobile processing platform coupled with an innovative user interface changed the landscape forever.

Ironically, it is the modern smartphone’s visually-rich display that can get in the way of mission critical communication. Our eyes are drawn to the display and we need both hands to operate the touch screen. This is the very reason the use of cell phones in cars is restricted. Cellular technology created another pathway for PTT but using the original carrier core voice system proved difficult to match the speed of LMR, and all-informed group calls proved challenging.

In 1994 Nextel-Sprint released iDEN, which combined the benefits of LMR and cellular and also gave each handset its own IP-address. iDEN could achieve a call setup and volley time close to LMR. By comparison VoIP (Voice over IP) systems of that time were hopelessly slow – with 5-10 second setups and 2-5 second volley times. Modern cellular systems provide the data bandwidth to deliver an effective standards-based VoIP PTT solution. A big benefit is that users do not waste expensive airtime minutes and in the era of unlimited data plans, agencies could implement low cost dispatch systems.

A bigger advantage is re-use of existing, inexpensive cellular handsets. Rich user interfaces on smartphones continued to drive this by adding features not easily possible on LMR, such as simultaneously following and recording different groups from anywhere on the web. Shifting calls from the analog domain into digitized packets has allowed a vast array of added features, although not all suit mission-critical users.

On balance, PTT for mission-critical communications seems better on a purpose-built device because it has a simple interface (a button) that you can use without taking your eyes or hands off your task.

However PTT delivery would be more valuable if it can work seamlessly over different bearer pathways and may in fact give more resilience than PTT on LMR.
The other critical aspect of deploying smart phone technology that needs attention for mission-critical users is the way users interact with the device. In a critical situation, officers can’t be expected to take their hands and eyes off their task to operate the device.

Adapting hands-free interfaces will make smartphones come very close to a purpose-built LMR radio without even starting to consider the richer functionality and applications you can use them for. So a narrow win for LMR, with the realization that a re-run of the race may tell a different tale.

The Future

It would be easy to conclude that the future belongs to smart phones. In reality, the future will belong to improved mobile platforms that are engineered to fit specific needs. Future devices will build on what we have now in ways that will delight us, but we need to prove these new features in the heat of battle. It’s one thing to love a feature in the showroom, but quite another to see how it works when lives may depend on it. This conservative view suits everyone, but if mission critical outcomes are important to you then a careful approach is needed.

The future belongs to devices that make us work smarter and which understand the mission critical context we operate within and help us like a mobile ‘buddy’. Prudent operators will take advantage of evolving solutions that improve mission critical outcomes and avoid those things that add to the problems. So this has less to do with the cellular-vs-LMR argument – in fact both can provide highly complementary outcomes.

So which is best?

Answering this seemingly simple question is, in fact, not so easy. It takes time to understand the context, the real user needs and where the technology is going. While LMR has an edge right now for mission-critical communications, combining the best of both platforms will continue to give significant advantages.

We welcome your views, comments and suggestions in the Tait Connection Magazine LinkedIn group.

Tait Connection - Issue 4This article is taken from Connection Magazine, Edition 4. Connection is a collection of educational and thought-leading articles focusing on critical communications, wireless and radio technology.

Comments

  1. Very fair and balanced report. Thank you

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