Q: Can PTT over cellular be used for mission-critical needs?
A: The short answer is yes, but only if you have a mission-critical network and backbone; as well as software, hardware and servers which also meet the required reliability standards.
For customers requiring guaranteed mission-critical communications, strict SLAs or guaruntees on the network's uptime and bandwidth are required. Current examples may include Telstra LANES, Private LTE, and Wi-Fi. Note that each of these needs the appropriate reliability, redundancy and SLA applied as part of the design and selection process.
The Telstra outage on 21 May 2018 affected most of our customers, with the worst affected experiencing two hours downtime.
That's still 99.7% uptime when measured over 30 days, or 99.977% uptime if measured over a year. The last Telstra outage which affected our customers was well over a year before that, and for even longer than this, our fully redundant server model (with hot standby) has served greater than 99.999% uptime to customers. PTT services use mobile data, which has been unaffected by most of the issues Telstra experienced in 2018.
Our year-in-year-out testing shows that the Telstra 4G networks have ample capacity for our application, even in the busiest periods such as Boxing Day at shopping centres, major motorways during peak hour incidents, and New Year’s Eve event areas. During our extensive testing at major events and incidents the Telstra 4G network has provided 100% reliability and performance.
The Telstra mobile network actually provides tighter monitoring and management of faults, faster rectification, and far better redundancy than most two-way radio networks.
However, standard public cellular networks do not come with a service level guarantee, which means that any PTT service using these networks cannot be classed as mission-critical.
Typical mission-critical requirements aim for 99.999% availability ("five nines"), or the equivalent of a maximum of 5.26 minutes of down time per year, on top of other requirements including network hardening and redundancy for reliability in the case of extreme events.
This is extremely arduous for commercial radio network operators. Even the NSW Government Radio Network has stated it "generally" operates at only 99.99% availability, and "Effort to improve availability to the optimal level of 99.999% is continuing" (ref. 1 and ref. 5). Motorola Solutions notes that "Availability of the public safety mobile broadband networks in critical areas should meet the same standards as hardened LMR networks with expected reliability uptimes of 99.995%" (ref 4).
International operators including FirstNet in the US, and TETRA operators in the UK, accept between 99.9% and 99.98% availability (nine hours downtime per year and two hours downtime per year respectively).
For comparison, Telstra's average fixed voice service availability in August 2016 was 99.8% across Australia, 99.95% in Sydney CBD, 99.87% in Perth, and 99.99% in Brisbane CBD (ref 3). Mobile availability is not published, but has a higher level of redundancy than fixed services.
Updated 21 June 2019
1. Public Safety Mobile Broadband, Productivity Commission Research Report, Australian Government. http://www.pc.gov.au/inquiries/completed/public-safety-mobile-broadband/report/public-safety-mobile-broadband.pdf, retrieved 1/2/2017.
2. Defining Public Safety Grade Systems and Facilities, National Public Safety Telecommunications Council, http://www.npstc.org/download.jsp?tableId=37&column=217&id=3066&file=Public_Safety_Grade_Report_140522.pdf, retrieved 1/2/2017.
3. Network Reliability Framework for fixed voice services, Telstra, https://www.telstra.com.au/consumer-advice/customer-service/network-reliability, retrieved 1/2/2017.
4. Official response from Motorola Solutions to the Australian Productivity Commission’s Public Safety Mobile Broadband Cost Benefits Analysis Interim Report, Motorola Solutions, http://www.pc.gov.au/__data/assets/pdf_file/0020/194024/subdr043-public-safety-mobile-broadband.pdf, retrieved 5/2/2017.
5. Response to Productivity Commission Issues Paper: Public Safety Mobile Broadband, August 2015, NSW Telco Authority, http://www.pc.gov.au/__data/assets/pdf_file/0005/192677/sub030-public-safety-mobile-broadband.pdf, retrieved 6/2/2017