User's Manual

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Chapter 2 LTE eNB Overview

LTE eNB System Description v2.0 19

Admission Control (AC)

The eNB provides capacity-based admission control and QoS-based admission

control for a bearer setup requested from EPC so that the system is not overloaded.

 Capacity-based admission control

There is a threshold for the maximum number of connected UEs (new

calls/handover calls) and bearers that can be allowed in eNB. Call admission is

determined depending on whether the connected UEs and bearers exceed the

thresholds.

 QoS-based admission control

The eNB determines whether to admit a call depending on the estimated PRB

usage of the newly requested bearer, the PRB usage status of the bearers in

service, and the maximum acceptance limit of the PRB (per bearer type, QCI,

and UL/DL).

RLC ARQ

The eNB performs ARQ function for the RLC Acknowledged Mode (AM) only.

When receiving and transmitting the packet data, RLC transmits SDU by dividing

it into units of RLC PDU at the transmitting side. Also, the packet is retransmitted

(forwarded) according to ARQ feedback information received from the receiving

side for increased reliability of the data communication.

QoS Support

The eNB receives QoS Class Identifier (QCI) in which QoS characteristics of the

bearer are defined and GBR, MBR, and the Aggregated Maximum Bit Rate (UE-

AMBR) from the EPC. It provides QoS for the wireless section between UE and

eNB and the backhaul section between eNB and S-GW.

Through air interface, it performs retransmission to satisfy the rate control

according to GBR/MBR/UE-AMBR values, priority of bearer defined in QCI, and

scheduling considering packet delay budget, and the Packet Loss Error Rate

(PLER).

Through backhaul interface, it performs QCI-based packet classification, QCI to

DSCP mapping, and marking for the QoS. It provides queuing depending on

mapping results, and each queue transmits packets to the EPC according to a strict

priority, and so on.

In Element Management System (EMS), besides to the QCI predefined in the

specifications, operator-specific QCI, and QCI-to-DSCP mapping can be set.

SYNC Handler Function

The eNB provides Synchronization (SYNC) protocol function to the backhaul

section between eNB and MBMS-GW for each Temporary Mobile Group ID

(TMGI) of the MBMS bearer from MME.