Public Safety prioritization

LTE QoS and prioritization for mission critical communication

Public safety (PS) users have the most stringent requirements for network & service accessibility and service quality. Various mechanisms can be used to fulfill these requirements in different network conditions. Those mechanisms must be able to differentiate both users and applications based on certain priorities. 

PS users can have different priority levels, which can also change dynamically. For example paramedic engaged in emergency mission has a higher priority than policemen on a lunch break. LTE networks can be shared serving both PS users and commercial subscriber with same resources (e.g. same carriers), in which case there is also a need to prioritize PS users over regular subscribers.

Key reason for adopting LTE for mission critical communication is of course broadband capability for data applications. At the same time LTE certainly supports also voice calls (both group calls & one-to-one calls). In case of high load, it may be necessary to prioritize certain applications in resources allocation. Typically voice communication is the most important service. Voice and data applications have also different latency, jitter and throughput requirements, which must be managed by network when forwarding the application packets. 

What can the LTE network do for PS users and application even in extreme network load? Below some basic prioritization mechanisms are explained.

Access class barring


If LTE cell is overloaded, users may not be able to get access due to signalling congestion. In the worst case the random access procedure simply fails, because too many UEs try simultaneously establish RRC connection. If the network is suppose to guarantee services for PS users, then access class barring can be activated.

All UEs are randomly allocated to one of the access classes 0 to 9. The access class (AC) is stored on USIM. Additionally UE may be a member of one or more special categories (AC 11 to 15). According to 3GPP TS 22.011 the high priority access classes are following:

  • Class  15        -            PLMN Staff;
  • Class  14        -            Emergency Services;
  • Class  13        -            Public Utilities (e.g. water/gas suppliers);
  • Class  12        -            Security Services;
  • Class  11        -            For PLMN Use.
Emergency calls have access class 10.

Network can activate access class barring (ACB) in order to reduce the load. It is possible that eNB can automatically activate ACB based on certain overload thresholds. Assuming that PS users have for example AC 14, network can reduce the load by barring AC 0-9 users either completely or with certain probability. ACB is optional information broadcasted in system information block type 2 (SIB2) [3GPP TS 36.331].

Admission control


Assuming the PS user can establish RRC connection in a cell, there might be still shortage of available resources for new data radio bearers. In that case allocation and retention priority (ARP) comes handy for PS users and applications. Admission control takes into account the ARP priority of EPS bearer in case there is no capacity for all bearer requests. Furthermore existing lower priority bearer can be pre-empted by higher priority bearer, if the lower priority bearer has pre-emption vulnerability set and the higher priority bearer has pre-emption capability set.

The range of the ARP priority level is 1 to 15 with 1 as the highest level of priority. The ARP priority levels 1-8 should only be assigned to resources for services that are authorized to receive prioritized treatment within an operator domain (i.e. that are authorized by the serving network). The ARP priority levels 9-15 may be assigned to resources that are authorized by the home network and thus applicable when a UE is roaming. [3GPP TS 23.203]

ARP value is valid for both non-GBR bearers and GBR bearers. Typical public safety example could be prioritization of GBR bearer for mission critical push-to-talk (MCPTT) service over regular VoLTE call. The dedicated GBR bearer for MCPTT call could have ARP priority 3 and dedicated GBR bearer for VoLTE could have ARP priority 10. If all GBR resources were consumed in a cell and pre-emption enabled, then a new MCPTT call could pre-empt an existing VoLTE call.

ARP is part of QoS parameters. For default bearers the ARP priority is included in subscription data in HSS for each allowed APN. Pre-emption capability and vulnerability for default bearers is set by MME according to operator policy. ARP priority and pre-emption settings can be also modified by P-GW based on interaction with PCRF. ARP for dedicated bearers is set by P-GW based on subscription or based on interaction with PCRF. [3GPP TS 23.401]

Traffic scheduling


QoS for voice and data applications is managed by traffic schedulers in network elements. Especially eNB scheduler for radio transmission is in key role. Scheduler allocates radio resources in priority order so that GBR bearers can transmit according to the downlink and uplink guaranteed bit rate values.  Non-GBR traffic has both APN level aggregate maximum bit rate (APN AMBR) limit as well as UE specific total maximum bit rate (UE AMBR) limit, but as name applies there is no guaranteed bit rate. Throughput of non-GBR bearers can be differentiated with different scheduling weights in eNB. 

Every default and dedicated EPS bearers has QoS class identifier (QCI), which defines resource type (GBR or non-GBR), latency target, packet loss rate and priority level for scheduling. Original 3GPP Release-8 specified standard GBR QCIs 1-4 and non-GBR QCIs 5-9.

3GPP Release-12 added additional QCIs for public safety applications. GBR QCIs 65 and 66 are respectively for mission critical PTT voice and non-mission critical PTT voice. Non-GBR QCI 69 is for mission critical signalling and non-GBR QCI 70 is for mission critical data. [3GPP TS 23.203



MCPTT service will use QCI 69 bearer for signaling and QCI 65 for voice media. This is similar to VoLTE service, which uses QCI 5 for SIP signaling and QCI 1 for voice media. With these new QCIs PS users get priority scheduling for MCPTT even over the VoLTE service. 

As mentioned in the beginning new data applications are key reason for PS users to use LTE networks. It is possible to offer better throughput for mission critical data applications (QCI 70 bearer) than for example for commercial subscribers' Internet access (QCI 6-9 bearer) by defining higher scheduling weight for QCI 70.

Dynamic QoS modification


Default EPS bearers have QoS defined in HSS (QCI, ARP priority, Aggregate Maximum Bit Rate). It is possible dynamically to modify the QoS of EPS bearers including default bearers. This can be achieved by having a PS application (e.g. dispatcher application in PS control room) that triggers policy change via PCRF. So the scenario for dynamic priority level for PS users is possible with bearer QoS modification. PS users could for example have QCI 6 with ARP priority 8 for generic data connection normally and in case of emergency mission the data connection QoS could be modified to QCI 70 and ARP priority 6.

Conclusions


Access can be guaranteed for PS users even in shared network by using access class barring in case of overloaded cells. This could happen for example in mass events.

Resources for data bearers can be guaranteed by giving high ARP priority for PS users and their applications (default & dedicated bearers) and enabling pre-emption of lower priority bearers.

High voice and video quality as well as responsive data applications can be guaranteed for PS users by using new 3GPP Release-12 QCIs.

Dynamic QoS control for PS users and applications is possible with PCRF. 

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