Section: 4.3.17.2 Overview of protection from Potential MTC Related Overload
Content: The number of MTC devices may be several orders of magnitude greater than "traditional" devices. Many (but not all) MTC devices will be relatively stationary and/or generate low volumes of traffic. However, these UEs have the capability to generate normal quantities of signalling. As normal signalling from large numbers of UEs may cause overload independently whether the UE is used for MTC or not, generic functionality for overload and congestion control is required. The total signalling from large numbers of UEs is a concern in at least two situations: - when an application (running in many UEs) requests many UEs to do "something" at the same time; and/or - when many UEs are roamers and their serving network fails, then they can all move onto the local competing networks, and potentially overload the not (yet) failed network(s). To counter these potential problems, the following standardised indications and mechanisms are provided in a generic manner. These permit node specific features to be developed to protect the networks. a) Where applicable, UEs can be configured for enhancements as described in subsequent bullets Post-manufacturing configuration can be performed remotely as described in clause 4.3.17.4. b) For mobile originated services, UEs configured for low access priority provide the E-UTRAN with information indicating that the RRC connection establishment request has low access priority (see clause 4.3.17.4). Clause 4.3.17.4 describes when low access priority is not applicable. c) RRC signalling has the capability of providing 'extended wait timers' when rejecting messages from UEs. These 'extended wait timers' are only used by UEs that access the network with low access priority. d) The MME can initiate rejection of RRC connection establishments in the E-UTRAN for UEs that access the network with low access priority as described in clause 4.3.7.4.1. In addition, MME signalling or O&M can trigger E-UTRAN to initiate Extended Access Barring. These mechanisms are further described in clause 4.3.7.4.1. e) Overload messages from the MME to E-UTRAN are extended to aid the RAN in performing the functionality in bullets b, c and d above. f) UEs configured with a long minimum periodic PLMN search time limit (see TS 24.368[ Non-Access Stratum (NAS) configuration Management Object (MO) ]  [69]) have an increased minimum time in between their searches for more preferred PLMNs. NOTE 1: Following the failure of a more preferred PLMN, UEs configured as above might change to other local competing networks. Expiry of this search timer will lead to the UE re-attempting to access the failed network, and then, if that network has not yet recovered, reaccessing one of the local competing networks. Use of a too short timer for the more preferred PLMN search can both prevent the failed network from recovering, and, impose more load on the local competing networks. g) At PLMN change, UEs configured to perform Attach with IMSI at PLMN change (see TS 24.368[ Non-Access Stratum (NAS) configuration Management Object (MO) ]  [69]) do this rather than a TA update with GUTI (thus avoiding the need to reject the TA update, and to request the IMSI following the subsequent Attach with GUTI). NOTE 2: In the case of a network failure, this reduces the message processing load on a local competing network and hence makes that network more likely to survive the failure of the other network. h) For mobile originated services, UEs configured for low access priority (see TS 24.368[ Non-Access Stratum (NAS) configuration Management Object (MO) ]  [69]) provide a low access priority indication to the MME in NAS signalling that permit the MME to undertake protective measures (e.g. to permit the MME to immediately command the UE to move to a state where it does not need to generate further signalling messages and/or does not reselect PLMNs), as described in clause 4.3.7.4.1. Clause 4.3.17.4 describes when low access priority is not applicable. i) Using Periodic TAU timer value sent by the HSS and/or UE provided low access priority indication (bullet h above), the MME can allocate a long periodic TAU timer value to the UE. A long periodic TAU timer is likely to slow down the rate at which a UE detects a network failure and thus it slows down the rate of movement of UEs from a failed network to other local competing networks (see clause 4.3.17.3). j) Mechanisms for the MME and P-GW to detect congestion associated with a particular APN (see clauses 4.3.7.4.2 and 4.3.7.5). k) The addition of 'back off timers' to EMM and ESM signalling messages (e.g. to rejection messages). These include some time randomisation to guard against a repeat of a load peak. The MME should be able to apply this behaviour on a per-APN basis. as described in clause 4.3.7.4.2 l) Signalling that permits the P-GW to request the MME to generate the above ESM signalling with 'back off timers' (see clause 4.3.7.5). m) An MME overload control mechanism to selectively limit the number of Downlink Data Notification requests the S-GW sends to the MME for downlink low priority traffic received for UEs in idle mode (see clause 4.3.7.4.1a). n) UE configured for specific handling of the invalid USIM state, the "forbidden PLMN list", the "forbidden PLMNs for attach in S1mode list" and the "forbidden PLMNs for GPRS service list" remembers that the USIM is invalid and keeps the PLMN forbidden lists even if the UE is switched off and then switched on. o) When the UE has an activated PDN connection without low access priority or the UE is requested to establish such a PDN connection and the UE is configured with a permission for overriding low access priority the UE doesn't provide a low access priority indication to the MME in NAS MM signalling and also not to the RAN in the RRC requests. In the NAS request for activating a PDN connection this UE always indicates what the upper layers requested, i.e. the UE indicates low access priority in that NAS request unless the upper layers request activation of a PDN connection without low access priority. p) When the UE has an activated PDN connection that is without low access priority or the UE is requested to activate such a PDN connection and the UE is configured with a permission for overriding Extended Access Barring, then the UE ignores any Extended Access Barring (if it is activated in the network) as defined in TS 22.011[ Service accessibility ]  [67]. NOTE 3: It is assumed that the mechanisms described in this entire clause are designed by Stage 3 in a manner that allows extensibility and forward compatibility. q) The eNodeB may use the low access priority indication provided by the UE to steer UEs configured for low access priority to specific MMEs.