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The Analysis of SMS Delay

Issue Description:

There is SMS delay issue in the network.

According to the customers’ description, MS receives no SMS for a long time. After customers

make a call the MS will receive a lot of SMS which should be received several hours ago.

Reason:

The SMS center sends once SMS.

1. SMS center will not send SMS if there is no paging response from MS. It may caused by

paging lost or some other reasons.

2. When SMS center does not receive the paging response, it will inform VLR to set the flag

which means the MS is out-of-service and do not send following SMS paging.

3. Once the MS activated by location update, MOC, MTC or other services with

authentication the VLR will reset the flag and SMS center resends the SMS. So if one

SMS missed all the following SMS will be delayed. This is the root cause of SMS delay.

Analysis:

ZTE optimization team did the test on April 25th. The test cell is IIB234 sector2 on BSC ISB995.

MS1 did PS upload service and MS2 sent SMS to MS1. SMS delay issue was captured twice.

One was MS error and the other one was paging lost.

The analysis is as follows:

Case1

The MS1 received the paging request when doing PS service, but there is no paging response.

So the SMS center stopped sending SMS paging. This is caused by MS issue.

Case2

In the normal case MS1 receive the packet paging request of SMS sent by MS2 at the same

time. Then comes the paging response and receive the SMS, as the following figure shows.

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In this abnormal case, the MS1 haven’t received any paging request before or after SMS sent.

The MS looks like out-of-service.

According to the paging retransmission setting, the MSC should sent paging twice after each 6

seconds, but it failed as before.

There are several cases in which MS cannot receive the paging messages when doing PS

services:

1. A interface sends paging message to CCCH while MS is in idle mode, the delay time is

0-2s for the affection of paging group. If the MS is in PS service in the 2s the CCCH paging

message will be lost.

2. A interface sends paging message while MS accesses in UL service, there is no IMSI and

the Paging Coordinate function cannot be supported at this time. So the paging message

can only be sent on CCCH which the MS also cannot receive.

3. A interface sends paging message while MS is releasing PS service, the paging

message will start Paging Coordinate function on PACCH at this time. If the MS has

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already released and come back to CCCH, the paging message cannot be received.

Suggestion:

1. According to TP, there is similar SMS delay issue in NSN network. And this issue had

been improved after changing paging retransmission time from 3 to 4. In ZTE CN the max

paging retransmission time can be set to 3. So we suggest increasing the paging interval

time to improve. The current setting is 6s, the suggestion value is 8s.

2. The SMS center sends SMS to MS only once. If the strategy can be modified that the SMS

center sends SMS twice, this issue will also be improved. It is the similar method as NSN

did which means increases another 3 paging request.

3. The paging success rate in current network is about 95%-96%, ZTE optimization team will

continue to improve it. We plan to do more test and analyze abnormal paging lost. But the

paging success rate cannot reach 100% so SMS delay can only be improved but not

avoidable.