PLMN and Cell Selection

PLMN Selection

There are two modes for PLMN selection, automatic and manual.

The UE performs the registration on the PLMN if the UE is capable of services which require registration.

UE-NAS maintains a list of PLMNs in priority order. For each PLMN, associated RATs may be set. Selection of the PLMN is done by manual or automatic. This requests the AS to select a cell belonging to this PLMN.

UE scan all RF channels in the E-UTRA bands according to its capabilities to find the available PLMNs. On each carrier, the UE search for the strongest cell and read its system information, in order to find out which PLMNs the cell belongs to.

The search for PLMN is stopped or requested by NAS. The UE optimize PLMN search by using the stored information e.g. carrier frequencies and cell related parameters from previously received measurement control information.

If successful registration is achieved, the UE indicates the selected PLMN.

If the registration cannot be achieved because no PLMNs are available and allowable, the UE indicates “no service” to the user, waits until a new PLMN is available and allowable and then repeats the procedure.

Cell Selection

The UE uses one of the following two cell selection procedures:

Initial Cell Selection

This requires no prior knowledge of which RF channels are E-UTRA carries. The UE scans all RF channels in the E-UTRA band according to its capabilities find a suitable cell. On each carrier frequency, the UE need only search for the strongest cell. Once suitable cell is found this cell is selected.

Stored Information Cell Selection

This procedure requires stored information of carrier frequencies and optionally also information on cell parameters, from previously received measurement control information elements or from previously detected cells. Once the UE has found a suitable cell the UE selects it. If no suitable cell is found the Initial Cell Selection procedure should be started

The cell selection criteria S is fulfilled when:

Srxlev > 0 and Squal > 0

where:

Srxlev = Qrxlevmeas – (Qrxlevmin + Qrxlevminoffset) – Pcompensation Squal = Qqualmeas – (Qqualmin + Qqualminoffset)

LTE Identifiers - PART1

EPS Bearer - PART3

This article is about ARP,UE-AMBR and APN-AMBR QoS level parameters:

The ARP shall contain information about the priority level (scalar), the pre-emption capability (flag) and the pre-emption vulnerability (flag). The primary purpose of ARP is to decide whether a bearer establishment / modification request can be accepted or needs to be rejected due to resource limitations (typically available radio capacity for GBR bearers). The priority level information of the ARP is used for this decision to ensure that the request of the bearer with the higher priority level is preferred. In addition, the ARP can be used (e.g. by the eNodeB) to decide which bearer(s) to drop during exceptional resource limitations (e.g. at handover). The pre-emption capability information of the ARP defines whether a bearer with a lower ARP priority level should be dropped to free up the required resources. The pre-emption vulnerability information of the ARP defines whether a bearer is applicable for such dropping by a pre-emption capable bearer with a higher ARP priority value. Once successfully established, a bearer's ARP shall not have any impact on the bearer level packet forwarding treatment (e.g. scheduling and rate control). Such packet forwarding treatment should be solely determined by the other EPS bearer QoS parameters: QCI, GBR and MBR, and by the AMBR parameters. The ARP is not included within the EPS QoS Profile sent to the UE.

for e.g. Video telephony is one use case where it may be beneficial to use EPS bearers with different ARP values for the same UE. In this use case an operator could map voice to one bearer with a higher ARP, and video to another bearer with a lower ARP. In a congestion situation (e.g. cell edge) the eNB can then drop the "video bearer" without affecting the "voice bearer". This would improve service continuity.

Each APN access, by aUE, is associated with APN  Aggregate Maximum Bit Rate (APN-AMBR).

The APN‑AMBR is a subscription parameter stored per APN in the HSS. It limits the aggregate bit rate that can be expected to be provided across all Non‑GBR bearers and across all PDN connections of the same APN (e.g. excess traffic may get discarded by a rate shaping function). Each of those Non‑GBR bearers could potentially utilize the entire APN‑AMBR, e.g. when the other Non‑GBR bearers do not carry any traffic. GBR bearers are outside the scope of APN‑AMBR. The P‑GW enforces the APN‑AMBR in downlink. Enforcement of APN‑AMBR in uplink is done in the UE and additionally in the P‑GW.

Each UE in state EMM-REGISTERED is associated with UE Aggregate Maximum Bit Rate (UE-AMBR).The UE‑AMBR is limited by a subscription parameter stored in the HSS. The MME shall set the UE‑AMBR to the sum of the APN‑AMBR of all active APNs up to the value of the subscribed UE‑AMBR. The UE‑AMBR limits the aggregate bit rate that can be expected to be provided across all Non‑GBR bearers of a UE (e.g. excess traffic may get discarded by a rate shaping function). Each of those Non‑GBR bearers could potentially utilize the entire UE‑AMBR, e.g. when the other Non‑GBR bearers do not carry any traffic. GBR bearers are outside the scope of UE AMBR. The E‑UTRAN enforces the UE‑AMBR in uplink and downlink.

The GBR and MBR denote bit rates of traffic per bearer while UE-AMBR/APN-AMBR denote bit rates of traffic group of bearers.

The ARP parameter of the EPS bearer can be modified by the P‑GW (e.g. based on interaction with the PCRF) to assign the appropriate pre-emption capability and the pre-emption vulnerability setting.The ARP pre-emption vulnerability of the default bearer should be set appropriately to minimize the risk of unnecessary release of the default bearer.

The HSS defines, for each PDN subscription context, the 'EPS subscribed QoS profile' which contains the bearer level QoS parameter values for the default bearer (QCI and ARP) and the subscribed APN-AMBR value. The subscribed ARP shall be used to set the priority level of the EPS bearer parameter ARP for the default bearer while the pre-emption capability and the pre-emption vulnerability information for the default bearer are set based on MME operator policy. In addition, the subscribed ARP shall be applied by the P-GW for setting the ARP priority level of all dedicated EPS bearers of the same PDN connection unless a specific ARP priority level setting is required (due to P-GW configuration or interaction with the PCRF).

This is all about EPS Bearer and its QoS.