Trunking PTT Latency Part 1

part 2 practical example of solving LTR trunked radio system overloading

No practical two-way wireless system has infinite subscriber capacity in finite time. For commercial and public safety systems from analog LTR, Multinet, and Passport trunking up through APCO-25, DMR, NXDN or iDEN trunking there is channel access latency.

PTT latency, that is, the time it takes from the user pressing the PTT switch to getting the “clear to talk” tones is a key source of frustration on a trunking system. Trunked PTT latency is not the same as conventional (non-trunked) users are accustomed to. They must be considered when designing public safety and utility systems and training their end users where safety of life is as stake. There are parallels to these problems in conventional systems particularly during adverse events. We give a qualitative overview of the issues in this article.

These issues apply equally to trunked radio systems on any of the common two-way radio bands. In the United States the most popular bands are VHF (150MHz), UHF (450MHz), 700MHz, 800MHz and 900MHz. Other countries have other frequency bands popular for two-way radio.

A lot happens in the fraction of a second between when a user “keys up” to talk and the clear to talk being issues. Consider this in the light of trunked radio technology being nearly four decades old.

Motorola chose to go with a 3600 baud FSK control channel. The downside is that channel cannot be used for voice at all, but the PTT latency is significantly lower and the contention behavior (two users trying to PTT at same time) is vastly better than for LTR systems.

Motorola Type I and II systems use a simple “connect tone”, which is merely a subaudible sine wave–a CTCSS code essentially, but one that is used across the system. Connect tone is analogous to:

  • SAT tone in AMPS
  • Color Code in Passport
  • Area Code in LTR
  • System Key in Multinet

A brief burst of disconnect tone avoids tail bursts of noise. The repeater transmits to the subscribers a subaudible 150 baud “OSW” during voice transmissions, subaudible data of increasing complexity with newer Motorola trunked systems. This is necessary to allow priority override of transmissions (“calling all cars”) and other advanced signaling. A key advantage of Motorola trunking (and all other trunking except LTR) is that individual subscriber radios can be individually disabled on a repeater. This helps ensure that all radios used are paying the monthly per-radio rate. For public safety and other systems, lost radios can be disabled “stunned”.

Clearchannel LTR uses a 300 baud FSK subaudible signaling, so that voice and data flow on any channel in the system. A consequence of the higher baud rate than DPL or Motorola OSW is that LTR has a more audible background noise, that some users have compared to a diesel truck idling.

Passport likewise has higher “wub wub wub” noise in the background. Without custom programming on the radios, the LTR subscribers have to request a channel each time, there is no “memory” of the last used channel without custom repeater controller software. This means the relatively fragile and lengthy LTR channel request is repeated each and every PTT. As long as a radio is listening on its home channel, priority override transmissions (from a priority talkgroup) can occur. If a user has trunked away from the home channel, priority override won’t work until the transmission ends. Custom repeater software (MultiNet) could drop non-priority calls (cease transmitting) to force users back to home.

Passport systems have optional ESN validation, which occurs upon turn-on (or as soon as the radio gets in range of a system). Keying up on a Passport system takes just a bit longer as the MIN is transmitted along with the desired talkgroup.

From slowest to fastest keying in the discussed systems are: Passport, MultiNet, LTR-Net, ESAS, LTR, Motorola Type II/I.

LTR-Net, ESAS and the constriction of two-way radio: Despite some eye-popping investments including by those turning their Nextel-bought SMR back into trunked radio, I don’t hear about these system anymore. With the most widespread first generation trunked system (LTR), there was a pent up need with no alternatives. That is, the wireless two-way dispatch choices through the early 1990s were:

  • conventional (PL/DPL)
  • trunked (LTR, Motorola Type II/I, MARC/EDACS)
  • analog/digital cellular

Conventional and trunked two-way radio were offered at monthly prices of $15-$25/radio depending on system coverage, competition, etc. with effectively unlimited minutes typically. Cellular by the mid-1990s was settling down to roughly $100/month for light to moderate business users. So you can see how two-way radios had a payback time of easily a year or less through the mid-1990s.

Then the digital cell phone resolution hit, allowing several digital phone conversations to fit in the space of one analog cell phone conversation. PCS and other spectrum auctions of the mid to late 1990s brought a multiplication of spectrum available for voice and soon more and more data with the mobile internet coming at the millennium turn. The goal of the cellular carriers was to push ARPU higher and higher naturally, from $60 to $80 a month. Texting suddenly became something to charge $10/month for. So one could argue that business cell phone monthly prices had stopped falling drastically by about 1995, and more slowly went up and down depending on what addons corporations wanted.

What brought the constriction of traditional two-way radio shop business models in the early 2000s was a confluence of factors including:

  1. highly-reliable, stable radio circuitry lasting 5 years between repairs, mainly to physical wear
  2. #1 hastened the attrition of an aging workforce, where companies like Bearcom and many others offered flat-rate repair, with their business model allowing whole-board replacements on the backs of other radios fixed cheaply
  3. Psycho-social: as cell phones trickled throughout white-collar to grey-collar, the site managers started “forgetting” to turn their radio on, or “not hearing” calls through an “intermittent” radio. Once those excuses worked for the foreman, the rank-and-file started using them. After a few years, they all had cell phones and $100 worksite simplex walkie talkies.

The high-margin two-way radio business that remains includes

  1. industrial customers that pay someone to handle their problems, that don’t want to assign someone to ship/receive radios to Bearcom. They appreciate quick fixes.
  2. medical customers that need help getting radio systems to work throughout their dense and expanding campuses. No Wifi radios yet.
  3. educational physical plant and security; similar to medical they have big and growing campuses. Cell phones don’t always work in the basements they need to ply.
  4. City/county utilities and services: a long mainstay of radio. Just too many radios to replace with cell phones, their accountants wouldn’t like it given great ROI of self-owned two-way radio systems.
  5. the obvious public safety and critical infrastructure that are mainstays for at least another decade.

You will notice the issue–the five categories of high-margin customers remaining in two-way radio ALL run their OWN radio system. The long tail of commercial users dried up considerably over the past decade. Of course, there are still farmers, taxis, construction and other vertical markets. Looking at each of them in turn:

  1. farmers: highly seasonal use patterns. Local cell companies accommodate with seasonal turn on/off of phones. Low minutes used
  2. taxis: quick blip of Nextel Direct connect gives pickup address. Less than minute call upon passenger pickup. Company margin high, demand growing, coverage area expanding. Not a dead two-way radio market, but shriveling
  3. construction: Cheap jobsite repeaters and cheap two-channel radios are all most need. Even the average joe has a cell phone now, after a few months their foreman gets the number.

and so on. As service companies (and every other company type) consolidated in the early 2000s, their workers covered larger areas, outside even the reach of a single two-way radio provider’s repeater network in too many cases. Thus, even the first-generation commercial trunked network systems like LTR-Net (1999) and ESAS (1996) came too late, they effectively came post dot-com.

A key distinction with Trident Microsystems Passport is they came at it from the Motorola angle, going for a higher-end market (actually mid-tier). Now in 2005, the bulk of the two-way radio business is split from low to high tiers:

  1. simplex disposable $100 jobsite radio users (one step above FRS): construction, nurseries
  2. conventional repeater users: small construction companies, other small users that are too big to switch to cellular but small enough in number and usage to bother switching to trunked (yet). They are profitable even if the old repeater has to be replaced with a trunking/conventional repeater.
  3. trunked LTR users: coming off of SMR, these are the bread and butter. Still good for $20/month/radio
  4. networked Passport users: for those taking on the risk, can you charge them extra for the infrastructure costs or will they just go cellular
  5. self-owned users: one-time profit + maintenance

The April 2005 announcement of digital NXDN at IWCE may slow big investments in Passport. Another big manufacturer is said to have commercial digital radio in the works to compete with NXDN.

Physical and Engineering Causes of Trunked Radio System PTT Latency:

Four causes of PTT latency in analog and digital trunked systems are:

  1. Poor SNR (subscriber → tower or tower → subscriber)
  2. System overloading (all channels busy or too many suddenly try to transmit)
  3. Malfunctioning radio/tower radio
  4. Poor RF design or interference (radio or system)

Let us examine these in turn. In Part 2, we will do quantitative simulations of each failure type.

  1. Poor SNR. Wireless communications, particularly of the NLOS variety experienced in typical two-way radio is characterized by deep fading. The frequency of the fading is influenced by wavelength, hence why 800MHz and 900MHz sound so much more “fluttery” in analog systems vs. 150/450MHz. Digital radios use signaling designed to statistically fight fluttering, and so effective system coverage quality for a given DAQ can be much larger, tens of percent larger. All sorts of interesting techniques are employed by radios and repeaters to handle the errors in the subaudible data, which has led to lawsuits over copyright issues. [to be continued]
  2. With the long PTT cycle time of LTR, there is a real chance of two users keying at the same time, effectively jamming each other until one or the other gives up. On systems where too many users are on one home channel, this becomes more problematic and even an ongoing problem. [to be continued]
  3. If a radio is off frequency, or a repeater modulation balance is misaligned, this can lead to significant digital bit error rates, and intermittent performance at any signal level. [to be continued]
  4. In any trunked system, control/home channel performance is precious. If users can’t PTT, the system is useless. Multinet and Passport work around some of this by breaking up home/control channel functionality into two or three channel all the time. Nextel interference, co-channel interference, adjacent channel interference can pop up unexpectedly. Do you monitor your whole system performance? If you have doubts, you can call an expert such as myself for assistance in evaluated degraded system performance, and what you might do even simple fixes that I might implement on a first site visit to improve your multi-million dollar radio system performance. [to be continued]

Reference: EFJ Clear Channel LTR Application Note