FEATURE: CRITICAL COMMUNICATIONS
“With release 12, 3GPP added the essential elements needed for critical communications traffic support.” demanding requirements, multiple LTE carriers can be combined with LTE aggregation to enable gigabit service delivery. In major public safety agencies across the United States, day-to-day operations already tap the power of commercial LTE services to link computer aided dispatch (CAD) systems with terminals in police and fire vehicles. But no public agency counts on these LTE systems for mission critical operations and provisions exist to fall back to pure voice operation on a PMR system in event of LTE network congestion or failure.
3GPP SAVES THE DAY Before LTE can serve the needs of public safety agencies, 3GPP needed to expand the capabilities supporting quality of service. With Release 12, 3GPP added the essential elements needed for critical communications traffic support. The first feature, Access Class Barring, protects a cell sector when too many users attempt to access the radio resources of the sector. The standards body added a ranked set of five priority access classes to differentiate high priority non-public safety users, security services, utilities and emergency services. The Access Class is incorporated into the user’s SIM, allowing rapid acceptance or barring of access attempts during periods of congestion. After the base station admits the UE connection, the base station must allocate base station scheduling resources. The allocation and retention priority (ARP) feature of Release 12 handles the setup and prioritisation
of data bearer connections. With this feature, lower priority connections may be pre-empted by higher priority connections. As traffic flows between the user equipment and application, the base station’s scheduler software executes Quality of Service (QoS) procedures that ensure guaranteed bit rate and non-guaranteed bit rate traffic is handled properly. In the base station and across the Evolved Packet System (EPS), bearer connections are associated with an QoS Class Identifier (QCI) that reflect the needed traffic characteristics. Before Release 12, the original QCI mechanism helped distinguish between best-effort data and Voice over LTE traffic. With Release 12, 3GPP introduced new QCIs that allow a higher level of priority than Voice over LTE. These QCIs are needed to ensure low latency mission critical push-to-talk transmissions. Taken together, the features of Release 12 provide the basis for mission critical data application support on public and private LTE networks. But full support for MCPTT remained incomplete due to a requirements for group calls and direct device-to-device communications when the network is not available. That condition arises frequently during emergency operations and PMR systems all provide a direct mode of connectivity for users to continue operation. To address this gap, Release 13 efforts addressing MCPTT requirements introduced functionality for group calls, call prioritisation and operation in off-network mode. Multimedia Broadcast Multicast Services (MBMS), the underlying architecture for pushto-talk group calling, gained a new interface to support applications such as MCPTT console systems. But efforts to leverage LTE deviceto-device proximity services to solve the problem of supporting users
that are out of network coverage remain incomplete. Beyond signaling challenges, the limited power transmission possible with LTE UEs raises concerns about the suitability of LTE for direct mode operation. That said, device makers can replicate direct mode functionality with simpler non-LTE mechanisms such as an emulation of TETRA direct mode or even simplex analog capabilities.
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