Optimization ReportTelefonica Movistar Guatemala

Technology: WCDMA

Category: Parameter change result

Parameters changed: cfnOffsetMarginSrbDchDl

Prepared: Sergio Rivera C

Ericsson Internal | Sergio Rivera C | 2014-05-23 | Page 2

› Problem characteristics– Statistical High Drop Speech (Other Reasons)– Statistical High number of pmDchFramesTooLate counter events

› WCDMA Network specific characteristics– cfnOffsetMarginSrbDchDl RNC parameter value = 0– Very limited IubLink transmission resources – Transmission by E1s (generating a high number of

pmDchFramesTooLate events)– Transmission by IP (no pmDchFramesTooLate events in the IubLink)

Problem Description And Network State

High number of nodeBs in our market uses Transmission by E1s

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› ALEX parameter definition

cfnOffsetMarginSrbDchDl = 40 { 0..255 } Additional operator margin added to the activation time (CFN) calculated by the RNC when using SRB on DCH in the downlink. Not used when establishing the first RAB.

Unit: 1 radio frame (10 ms)Takes effect: At next synchronized reconfiguration of the connection.

› RAB Release

› When switching between different RAB combinations and setting the activation time (decided by CFN), it is possible to add an extra margin to the calculated value by the parameter cfnOffsetMarginSrbDchDl when the SRB is on DCH DL. The CFN offset (Δ CFN) is then calculated as:

• Δ CFN = calculated value + cfnOffsetMarginSrbDchDl

Background

In our network:

Initial

cfnOffsetMarginSrbDchDl=0

Changed to

cfnOffsetMarginSrbDchDl=80 (8 radio frames)

Ericsson Internal | Sergio Rivera C | 2014-05-23 | Page 4

› ALEX table Speech Drop Rate VS cfnOffsetMarginSrbDchDl values

Background

Ericsson Internal | Sergio Rivera C | 2014-05-23 | Page 5

› Speech RAB release due to high Iub Link delay› Measuring the Iub Link delay, sending a ping in the Iub Link

Analysis

Healthy node transmissionIub Link Ping Maximum Delay 8 ms

Bad node transmissionIub Link Ping Maximum Delay 233 ms

and Time outs!

Delays up to 800 and 900 ms were found in the network

RAB Speech specially don’t support long transmission delays, which turns into a high number of RAB speech releases by the RNC (because RNC doesn’t know it has to wait more time due to poor Iub Link Transmission, before a RAB release)

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› Trend BEFORE and AFTER parameter change (one RNC)

Analysis

A very good Drop Speech improvement after changing RNC cfnOffsetMarginSrbDchDl (now RNC waits 800ms to decrease the impact of poor node transmission) , Drop Speech rate decrease from 1.6% Average to 1 % Average

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› Trend BEFORE and AFTER parameter change (all Network)

Analysis

The vertical dark blue lines show the RNCs cfnOffsetMarginSrbDchDl change value, the three changes with significant improvement in Drop Speech Rate KPI.

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› Increasing the parameter cfnOffsetMarginSrbDchDl has a good result in Drop Rate Speech Rate if the network has a problem of high delay in IubLinks, very poor transmission.

› In our network we increase cfnOffsetMarginSrbDchDl from 0 to 80 (800ms) in all RNCs (covering rural and urban areas) with a very good KPI Drop Speech Rate impact, improving from 1.8 ~ 2.0 % to less than 1% in the whole network.

› High number of pmDchFramesTooLate were a common problem, due to lack of transmission resources in the whole network.

› The amount of pmDchFramesTooLate in the IubLink are not impacted or solved, just RNC give more time to the RAB Speech to survive against this delays.

Conclusions