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)

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.

EPS Bearer - PART2

In this article we will discuss about Bearer Level QoS Parameters:

The EPS bearer QoS profile includes the parameters QCI, ARP, GBR and non-GBR. The QoS parameters which are applied to aggregated set of EPS bearers are APN-AMBR and UE-AMBR.

The QCI is scalar that is used as a reference to node specific parameters that control packet forwarding treatment ( e.g. scheduling weights, admission thresholds, queue management thresholds, link layer protocol configurations etc) and that have been pre-configured by the operator owning the node (e.g. eNodeB). The characterstics describe the packet forwarding treatment that an SDF aggregate  receives edge-to-edge between UE and PCEF in terms of the following characteristics:
1. Resource Type (GBR and non-GBR)
2. Priority
3. Packet Delay Budget (PDB)
4. Packet Error Loss Rate (PELR)

The one-to-one mapping of standardized QCI values to a standardized characterstics is as below:

The Resource Type determines if dedicated network resources related to a service or bearer level Guaranteed Bit Rate (GBR) value are permanently allocated (e.g. by an admission control function in a radio base station). GBR SDF aggregates are therefore typically authorized "on demand" which requires dynamic policy and charging control. A Non GBR SDF aggregate may be pre-authorized through static policy and charging control.

The Packet Delay Budget (PDB) defines an upper bound for the time that a packet may be delayed between the UE and the PCEF. For a certain QCI the value of the PDB is the same in uplink and downlink. The purpose of the PDB is to support the configuration of scheduling and link layer functions (e.g. the setting of scheduling priority weights and HARQ target operating points).

Every QCI (GBR and Non-GBR) is associated with a Priority level. Priority level 1 is the highest Priority level. The Priority levels shall be used to differentiate between SDF aggregates of the same UE, and it shall also be used to differentiate between SDF aggregates from different UEs. Via its QCI an SDF aggregate is associated with a Priority level and a PDB. Scheduling between different SDF aggregates shall primarily be based on the PDB. If the target set by the PDB can no longer be met for one or more SDF aggregate(s) across all UEs that have sufficient radio channel quality then Priority shall be used as follows: in this case a scheduler shall meet the PDB of an SDF aggregate on Priority level N in preference to meeting the PDB of SDF aggregates on Priority level N+1 until the priority N SDF aggregate's GBR (in case of a GBR SDF aggregate) has been satisfied.

The Packet Error Loss Rate (PELR) defines an upper bound for the rate of SDUs (e.g. IP packets) that have been processed by the sender of a link layer protocol (e.g. RLC in E‑UTRAN) but that are not successfully delivered by the corresponding receiver to the upper layer (e.g. PDCP in E‑UTRAN). Thus, the PELR defines an upper bound for a rate of non congestion related packet losses. The purpose of the PELR is to allow for appropriate link layer protocol configurations (e.g. RLC and HARQ in E‑UTRAN). For a certain QCI the value of the PELR is the same in uplink and downlink.

 continues..

EPS Bearer - PART1

Before knowing EPS Bearer we need to know about the PDN connectivity service. The PDN connectivity service is nothing but the Evolved Packet System provides IP conectivity between a UE and PLMN external packet data network. The PDN Connectivity Service supports the traffic flow aggregate(s), consisting of one ore more Service Dara Flows (SDFs).


Now coming to EPS Bearer,

 An EPS bearer is the level of granularity for bearer level QoS control in the EPC/E-UTRAN. That is, all traffic mapped to the same EPS bearer receive the same bearer level packet forwarding treatment (e.g. Scheduling Policy, Queue Management Policy, Rate Shaping Policy, RLC Configuration etc). Providing different bearer level packet forwarding treatment requires seperate EPS bearers.


One EPS bearer is established when the UE connects to a PDN, and that remains established throughout the lifetime of the PDN connection to provide the UE with always-on IP connectivity to that PDN. That bearer is referred to as the default bearer. Any additional EPS bearer that is established for the same PDN connection is referred to as a dedicated bearer.


Every dedicated bearer is associated with a TFT. A unidirectional EPS bearer is either associated with an UL TFT or a DL TFT that matches the unidirectional traffic flows and a DL TFT or an UL TFT in the other direction that blocks all traffic flows.

 

For of E-UTRAN, the decision to establish or modify a dedicated bearer can only be taken by the EPC, and the bearer level QoS parameter values are always assigned by the EPC. Therefore, the MME shall not modify the bearer level QoS parameter values received on the S11 reference point during establishment or modification of a dedicated bearer. Instead, the MME shall only transparently forwards those values to the E-UTRAN. Consequently, "QoS negotiation" between the E-UTRAN and the EPC during dedicated bearer establishment / modification is not supported. The MME may, however, reject the establishment or modification of a dedicated bearer (e.g. if the bearer level QoS parameter values sent by the PCEF over a GTP based S8 roaming interface do not comply with a roaming agreement).

The distinction between default and dedicated bearers should be transparent to the access network (e.g. E-UTRAN). An EPS bearer is referred to as a GBR bearer if dedicated network resources related to a Guaranteed Bit Rate (GBR) value that is associated with the EPS bearer are permanently allocated (e.g. by an admission control function in the eNodeB) at bearer establishment/modification. Otherwise, an EPS bearer is referred to as a Non-GBR bearer.

A dedicated bearer can either be a GBR or a Non-GBR bearer. A default bearer shall be a Non-GBR bearer.