study on admitted ce congestion on rnc sidex

Admitted CE Congestion in RNC 2012

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Study on Admitted CE Congestion

on the RNC Side.

Document Type: Case Study

Document Number: Huawei-3G-22092012-0001)

Prepared By Reviewed By Authorized By

Bipin Tiwari Deep Dhiman Hiren Joshi

Release On :-


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Problem Description

• Admitted CE congestion on the RNC side increases the number of RRC and RAB

setup failures.

Observation

• After dynamic CE resource management is enabled (at Node B), the mechanism

for CE admission and consumption on the RNC side is different than at Node B

side.

-> The NodeB allocates CEs based on actual conditions, whereas the RNC does

not adjust the credits of CE admission based on the adjusted CEs by the NodeB.

In this way, there are remaining CEs on the NodeB side

-> RNC calculates the credit resource usage of admitted HSUPA UEs by using

guaranteed bit rate (GBR) which ensures HSUPA user experience.

• As a result, the RNC may reject new UE access attempts even if the actual CE

usage of the NodeB is low. This limits the RNC's capability to perform admission

control based on credit resource usage


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1. What is CE and how the admission is performed at RNC?

CEs are used to measure the channel demodulation capability of NodeBs. CEs are

classified into uplink and downlink CEs.

-> One uplink CE needs to be consumed by an uplink 12.2 kbit/s voice service (SF =

64) plus 3.4 kbit/s signaling traffic.

-> One downlink CE needs to be consumed by a downlink 12.2 kbit/s voice service

(SF = 128) plus 3.4 kbit/s signaling traffic.

At RNC Call admission control is based on NodeB Credit Resource %%LST UCACALGOSWITCH:;%%

RETCODE = 0 Execution succeeded.

List CAC Algorithm Switch

-------------------------

CAC algorithm switch = NodeB Credit CAC Switch::ON

The NodeB credit resource is a concept on the RNC side. It is referred to as the

channel element (CE) on the NodeB side.


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2. How ‘NODEB Credit usage’ is calculated for admitted user at the

RNC for HSUPA User?

RNC calculates the credit resource usage of an admitted HSUPA UE by using the

following formula:

Credit resource usage = Max (Credit resources required for ensuring the GBR, Credit

resources required for transmitting one RLC PDU)

Assuming DCERM is enabled (dynamic CE resource management) In the preceding

formula,

GBR stands for guaranteed bit rate and RLC PDU stands for Radio Link Control packet

data unit. The RNC performs this calculation to ensure HSUPA user experience.

However, the actual CE usage of the NodeB is lower than the calculated credit

resource usage of the RNC. This is because the NodeB calculates the actual CE usage

based on the UE rate. The UE rate is lower than the GBR or the rate at which an RLC

PDU is transmitted in most cases due to the high penetration rate of smart phones

with a 2 ms HSUPA transmission time interval (TTI). As a result, the RNC may reject

new UE access attempts even if the actual CE usage of the NodeB is low. This limits

the RNC's capability to perform admission control based on credit resource usage.

3. How HSUPA PS Call Admission is performed based of UL CE?

PS call admission based on UL CEs when we have MBR (Dynamical CE switch

close) and GBR (Dynamical CE switch open).How exactly admission is performed

at the NodeB & RNC sides.

Kindly refer below diagram:

Above diagram shows, when Dynamic CE is enabled (At NodeB End) the SF

calculation /UL CE is based on the GBR parameter at RNC END).


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*DCCC (dynamic channel configuration control)

* DCERM (dynamic CE resource management)

4. Describe the difference in CE calculation & impact for 2ms & 10

ms TTI Hsupa user?

CE consumption: For TB blocks with the size of 354 bits, 2 ms TTI subscribers use

SF4 and consume eight CEs; 10 ms TTI subscribers use SF32 and consume one CE.

Usually the GBR is 32 kbit/s or 64 kbit/s. Therefore, the number of CEs for 2 ms TTI

admission is greater than that for 10 ms TTI admission. This decreases the number of

admitted subscribers in the 2 ms TTI, comparing to 10 ms TTI.


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Impact

After the HSUPA 2 ms TTI feature is activated,

1. Admitted CEs for HSUPA 2 ms TTI users are more than those for HSUPA 10 ms TTI

users. After many HSUPA 2 ms TTI users are admitted, admitted CE congestion

occurs easily on the RNC side.

2. The statistics on the NodeB side show that there are remaining CEs. However,

CEs used for admission are congested on the RNC side.

5. How to overcome the Admitted CE congestion under RAN 12?

After the High Speed Uplink Packet Access (HSUPA) 2 ms transmission time

interval (TTI) feature in the live network is activated and dynamic channel

element (CE) resource management is enabled, the statistics on the NodeB

side show that there are remaining CEs. However, CEs used for admission are

congested on the RNC side.

This problem occurs when the following conditions apply:

� On the RNC side, the HSUPA 2 ms TTI switch is turned by running the

following command:

SET UCORRMALGOSWITCH: MapSwitch=MAP_HSUPA_TTI_2MS_SWITCH-1;

� On the NodeB side, dynamic CE resource management and 2 ms TTI license

are activated by setting the following license control items:

HSUPA TTI Function = Yes

DYNAMIC CE = Yes

Impact:

Admitted CE congestion on the RNC side increases the number of RRC and RAB

setup failures. The statistics on the number of used CEs on the RNC side are

consistent with those on the NodeB. Therefore, it may be mistakenly regarded that

the CE usage is low.


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Solution:

In the BSC6900 V900R012, the workaround is as follows:

� Turn on the admitted CE-based TTI dynamic adjustment algorithm switch (it

is turned on by default) to relieve admitted CE congestion after the 2 ms TTI

license is enabled.

SET UCORRMALGOSWITCH: DraSwitch=DRA_BASE_ADM_CE_BE_TTI_RECFG_SWITCH-1;

� Set the threshold for the reserved spread factor (SF) of uplink TTI handover

credit (in the live network, the default value is SF8), so that HSUPA 2 ms TTI

users are handed over to 10 ms TTI earlier.

Cell: ADD UCELLLDR: CELLID=xxx, ULTTICREDITSFRESTHD=8SF4;

NodeB: ADD UNODEBLDR: NodeBName=xxx, ULTTICREDITSFRESTHD=8SF4;

� Verify the threshold for the 2 ms TTI rate to ensure that HSUPA 10 ms TTI

users can be handed over to 2 ms TTI only in the case of high rate.

SET UFRC:BEHSUPA2MSTTIRATETHS=D1280;

� Disable 2 ms TTI retry to prevent 10 ms TTI users from being handed over

back to 2 ms TTI.

SET UFRC: RETRYCAPABILITY= TTI_2MS-0;


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1. What is CE Overbooking Feature, why it is required &

Advantage?

RNC may reject new UE access attempts even if the actual CE usage of the NodeB is low. This

limits the RNC's capability to perform admission control based on credit resource usage. To address the issue CE Overbooking feature is available in RAN14.

This feature applies only to uplink CE resource.

With this Features, the NodeB calculates the actual CE usage of admitted UEs,

includes the calculated CE usage in a private information element (IE)

in a measurement report, and sends the report to the RNC every measurement

report period. The RNC then directly uses the reported CE usage as the credit

resource usage. This maximizes the RNC's capability to perform admission control

based on credit resource usage.

Advantage:

When the penetration rate of smart phones with 2 ms HSUPA TTI is high, the average

HSUPA service throughput is low, and this feature provides the following benefits:

reduces the credit resource usage of admitted UEs.

Increases the number of admitted UEs and the number of HSUPA UEs with a 2

ms TTI.

Increases the cell uplink throughput when Uu and Iub resources are sufficient.

study on admitted ce congestion on rnc sidex

Documents

nodeb credit usage

nodeb credit resource

actual ce usage

admitted ce congestion

credits of ce admission

dynamic ce resource

uplink ce needs

downlink ce needs