Southern Linc offers ‘Basic MCPTT’ and ‘High Performance MCPTT,’ shuts down legacy iDEN system for LTE

Southern Linc—the southeast U.S. regional carrier that also provides communications for the Southern Company utility—is leveraging its 800 MHz LTE network to provide two flavors of mission-critical-push-to-talk (MCPTT) service to customers, according to Southern Linc officials.

Southern Linc was long known for its iDEN network, but the carrier recently completed a transition to LTE that began in 2016, according to Alan McIntyre, director of engineering for Southern Linc.

“At the end of March, we shut down iDEN, except for a few key sites. Otherwise, it’s really turned off,” McIntyre said during an interview with IWCE’s Urgent Communications. “We transitioned our LTE network from a 1.4 MHz-channel bandwidth to a 3 MHz-channel bandwidth at the same time. We really wanted to get the full benefit of LTE and maximize the efficiency that you get with our spectrum, and we were able to do that.”

McIntyre said the Southern Linc LTE system, known as CriticalLinc, has “worked beautifully” to support the Southern Company’s SCADA needs for fixed data for several years. With 1,100 active cell sites—and 200 more sites planned for deployment—the new Southern Linc LTE network provides better coverage than the legacy iDEN system. In addition, the LTE features make it more practical to deploy multiple network cores, he said.

“We have two core networks—one in Birmingham and one in Atlanta—and, even within each city, there’s a lot of redundancy between nodes and cards,” McIntyre said. “So, even if something fails in Birmingham, it really almost has to fail twice before it would go to Atlanta. They’re both active though—SCADA devices in Georgia are homed to the Atlanta core, and SCADA devices in Alabama are homed to the Birmingham core

“So, we have traffic on both, but each one could easily handle all of the traffic on the network. We have Ericsson full cores, and the capacity of those is much, much larger than what we’ve got [on the network] right now.”

Southern Linc has incorporated other techniques to help ensure reliability, including a mix of fiber and microwave to provide redundant backhaul paths from every cell site, McIntyre said. In addition, Southern Linc uses a combination of hydrogen fuel cells, generators and batteries to provide each site with enough power to maintain operation for about a week in case of a commercial outage, he said.

This network reliability provides the foundation to Southern Linc’s push-to-talk-over-cellular (PoC) services, according to McIntyre. The carrier’s website states that the “Basic MCPTT” and “High Performance MCPTT” offerings make Southern Linc the “first carrier in the country to bring you public-sector-grade Push to Talk on a public LTE network.”

With the “Basic MCPTT “service, Southern Linc customers can make private MCPTT calls, MCPTT call alert and a host of group-calling capabilities. For an extra $10 per month, Southern Linc customers with a Sonim XP8 rugged smartphone can enjoy “High Performance MCPTT,” which includes the key features of emergency calling, mapping and text messaging with pictures, video and voice recording.

“LincMessage is a text-messaging capability that allows you to text within the PTT app rather than having to close the app, go to your text-messaging application, text, and come back to the push-to-talk app,” Southern Linc spokesperson Lynda Swaney said during an interview with IWCE’s Urgent Communications.

Swaney said that Southern Linc has about 60,000 subscribers that utilize voice services. More than half of these are public-sector users, including transportation, corrections and private-EMS personnel, she said.

Southern Company crews have been using the “High Performance MCPTT” service for months, McIntyre said. Southern Linc currently providing push-to-talk services on the WAVE 7000 platform from Motorola Solutions, he said. Ericsson has implemented Evolved Multimedia Broadcast Multicast Service (eMBMS) on the Southern Linc network to enable broadcast communications that is designed to be much more efficient in delivering services like group calling, but that capability is still being optimized.

“We really worked with Motorola and Sonim to do an integration of the WAVE client into the Sonim device,” McIntyre said. “Instead of an over-the-top experience, it’s really an embedded, integrated experience.

“That really helped with the KPIs [key performance indicators], as far as access time and getting those down. That level of integration was a development effort that we took with Motorola and Sonim—and Qualcomm was a piece of that, as well, when we started talking about eMBMS, because there’s the Qualcomm middleware piece that you have to have.

“It was a pretty big effort to integrate all of that into one package, so that it is kind of that public-sector-grade LMR experience—that’s what we were shooting for. I think WAVE offered the best chance of getting there.”

Many industry experts have noted the difficulties associated with meeting the stringent latency requirements associated with the 3GPP standard definition for MCPTT. When asked about these, McIntyre said, “We’re really close. We’re good on two out of three, and on the other one, we’re pretty darn close at hitting it.”

Probably the greatest challenge in meeting the 3GPP standard for MCPTT is providing direct-mode communications via Proximity Services (ProSe). Not only are ProSe chipsets scarce, the communication range provided by a working ProSe service on an LTE device transmitting at 0.25 watts with an internal antenna is expected to pale in comparison to an LMR portable with a large external antenna that can leverage 3 to 5 watts of power.

When asked about Southern Linc user’s options when they are outside the coverage area of the CriticalLinc network, McIntyre cited roaming agreements with multiple carriers and a Sonim direct-mode accessory that leverages LoRa technology operating on unlicensed spectrum in the 900 MHz band.

“We’re working with Sonim,” McIntyre said. “There’s a direct-mode accessory that you can add to both the XP8 and XP5S [Sonim devices]. In fact, it’s very similar to what we had with some of our iDEN devices … You can get a mile or so [communication range].”

Southern Linc’s LTE system includes priority-and-preemption capabilities, and McIntyre emphasized that the feature is not just beneficial to Southern Company personnel.

“I wouldn’t say that Southern Company has carte-blanche priority over everybody. It’s balanced out,” McIntyre said. “Just like we consider that our folks are dealing with life and death, public safety is, as well.”

When asked about interoperability or roaming with FirstNet, the nationwide public-safety broadband network (NPSBN) being built by AT&T, McIntyre said “we certainly would be supportive of that” but expressed skepticism that an agreement could be reached in the near future.

“I think they’d rather compete with us than talk to us on those sort of things,” he said.

In contrast, McIntyre indicated that he is more optimistic about Southern Linc’s relationship with pdvWireless, which is seeking FCC permission to transform interleaved 900 MHz LMR spectrum into a contiguous 3×3 MHz band to support LTE deployments led by utilities and other enterprises. Southern Linc has been working with pdvWireless “through multiple angles,” because it would like to see the ecosystem for embedded LTE equipment—for instance, line-monitoring devices—grow, so better economies of scale can improve pricing of the gear.

“If there are other utilities … doing LTE, it would be great, because I think there’s a lot of sharing that we could do—there’s a lot of synergy—and it could really help drive volumes of those type of products, which would helpful,” McIntyre said.

“So, from a [pdvWireless] standpoint, we’re really supportive of what they’re trying to do, because I think it really would build an ecosystem of other utilities doing what we’re doing.”

Southern Linc also is very interested in developments occurring in the 3.5 GHz Citizens Broadband Radio Service (CBRS) band, which could be used to augment the carrier’s current data rates that typically max out at about 20 MB/s downlink and 10 MB/s uplink, McIntyre said.

“We’re still actively participating in 3GPP, and we’ve done several carrier-aggregation combinations between our 800 MHz [spectrum] in Band 26 and CBRS in Band 48. We’re certainly interested in 3.5 GHz,” he said. “It’s a good opportunity for decent amounts of bandwidth that we can add to what we already have.”