6 gsm bss communication flow

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Copyright © 2006 Huawei Technologies Co., Ltd. All rights reserved.

GSM BSS Communication Flow

This unit is the second part of BSS signaling and interface analysis. In the first part, we have learnt basic knowledge of NO7 signaling; in this course, we will continue to learn BSS communication flow, and give the specific explanation based on the actual interface analysis.

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Page2Copyright © 2006 Huawei Technologies Co., Ltd. All rights reserved.

Foreword

Course Purpose:

Grasp the means of interface trace and the method of analyzing it to aid

us for fault handling.

How to learn this course:

Copy the interface trace result files.

Use the maintenance console’s interface trace review function to study,

Attention:

It is strictly forbidden to use the interface tracing function when the

system is busy or the system is under heavy duty.

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Contents

1. Location Update Flow

2. Call Setup (Originating)

3. Intra-BSC Handover

4. Inter-BSC Handover

5. BSC Normal Release

6. BSC Local-end Release

7. Case Analysis

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Location Update Flow

Purpose of location update:

To inform the system the LA in which the MS is to facilitate

paging

When need the MS perform location update？

Getting into a new LA

Periodical location update

First of all, we should know the purpose of location update: To inform the system the LA in which the MS is to facilitate paging. When need the MS perform location update? There are two cases:The first one: Getting into a new LA. This type of location update is called forced location update. The second one: regular update, it is also called periodical location update. Even if the MS doesn’t enter a new location area, GSM system will also require the MS to perform regular location update, thus to grasp the MS current status. For periodical location update, we have two relevant time parameters in GSM network.The first one is periodical location update time limit value in BSC; the second one is location update periodical parameter in MSC.First, let’s talk about periodical location update time limit value in BSC.This time limit value is sent to the MS via system message and is saved in the MS. The MS regular update is completed in accordance withperiodical location update time limit value sent by BSC. For example, if periodical location update time limit value is set as one hour in BSC, the MS will update regularly per hour.

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The second parameter: MSC location update periodical parameter. This parameter is only saved in VLR, and will not be sent to the MS via system message or other means, this is to say, the MS need not know the actual value of this parameter.What’s the meaning of this parameter? For example, we set location update period as three hours in MSC; if MS doesn’t perform one location update to MSC via A-interface within three hours, the MS will be set as deactivation status, I.e., it is set as shutdown status. Actually, MSC location update period is more than BSC location update period. When the MS is power-on, the MS can generally originate location update to MSC via A-interface. In some special cases, for example, the MS is in lift or other network blind area when the MS is originating location update. In this case, the MS will miss one time location update. If MS has continuously missed several times of location update, it will cause location update periodical parameter timer in MSC is overflow, and the MS will be set as forced power-off, this is to say, even if the MS is power-on, MSC will regard the MS is power off or is out of network coverage area. From the above explanation, we know the meaning of location update periodical parameter in MSC and BSC are different. Generally speaking, location update period in MSC should be three times or above of location update period in BSC. If this value is small (for example, period parameter in BSC is set as one hour, and period parameter in MSC is set as 1.5 hours), the MS will be set as forced power-off status in case that the MS misses one location update. In this case, it may occur that the MS cannot be paged by the called when the MS is in power-on status. Therefore, we recommend that location update period parameter in MSC should be set as three times or above of location update period parameter in BSC.

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Location Update Flow

SABM: Set asynchronous balanced

mode command——initial access frame

UA: Unnumbered acknowledgement

response

MS BTS BSC MSC

Channel_req Channel_Required

Channel_Active

Channel_Active_Ack

IMMEDIATE ASSIGN COMMAND

Establish_IND(Location Updating Req) CR(Complete_L3_

information)

Location Updating Accepted

TMSI Reallocation Complete(NOTE 2)

CC

(NOTE 1)

(NOTE 3) Clear_CMD

Clear_CMP

First SABM

UA

In location update flow, the first message that MS originates to base station is “Channel Request”. When BTS accepts this channel request message, it will originate channel application message to BSC, I.e., channel required message. Then BSC perform “channel active” on BTS, and BTS will perform active response “channel active ack”. So far, the specific channel between BSC and BTS has been prepared, and MS can accessed this channel. The next, BSC will assign this channel to the MS. This signaling is called “immediate assign command”. This command is sent from BSC, and gets to BTS, then it is transferred to the MS via BTS.When the MS accepts this “immediate assign command”, MS will get the corresponding channel parameter from this command, and MS will originate access message to BTS. This access message is called “first SABM message”, and it is link layer message used for the MS first time access to the new channel.When BTS accepts this message, it will originate “establish_IND” to BSC. This “establish_IND” contains the specific content—” location updating request”.

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Then BSC will originate connection request (called CR message) to MSC. This message is connect-oriented message of SCCP layer, I.e., connect request message. This CR message contained specific information is called “complete_L3_information”, and specific content in complete L3 information is “location updating accepted”. In order to establish a connection number at A-interface, MSC will return a CC message, I.e., connect confirmation message. Respond to CR message via CC message to establish a connection number. So far, transmission channel between MS and MSC at A-interface has been established, this is to say, a SCCP connection number has be established. Actually, when MSC accepts CR message, it has already received MS location update request via this message. At this time, MSC will perform relevant processing. Here there is a NOTE1, and we will illustrate NOTE1 later.In normal flow, after MSC completes the relevant processing, it will return a “Location updating accepted” message. This message will finally be transmitted transparently to the MS via BSC and BTS, to tell the MS that “location updating request” is accepted.Generally speaking, when the MS is updating the location, TMSI number of MS will also be updated. This is optional. We can configure MSC parameters to select whether or not update MS TMSI number. “Location updating accepted” message contains a new TMSI number. When the MS accepts “Location updating accepted” message, it can get the new number. After getting the new number, the MS will return a TMSI reallocation completion message to the network side, this is called “TMSI reallocation complete” message. Here is a NOTE2, and the NOTE2 is to describe that TMSI number update is optional. So far, MSC has accepted “TMSI reallocation complete” message, and completed a normal location update process. The relevant resource at A-interface will be released. This release is completed via Clear_CMDand Clear_CMP. The specific release flow will be described later. Here we will not talk about this.We have complementary notes for this page: we can see the red numbers in the brackets-- 2, 3, 4, 5, 6, 10 and 11, what do these numbers stand for?In the above description, we have mentioned that we prepared some interface trace result files. The message number in this page is the SN of the messages in theresult file. To understand the details, you can analyze each message specifically in these result files. The file name is “A-bis_logon”. We also number these. Please refer to “A-bis_logon” result file for specific study.

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NOTE 1: Authentication, Encryption, Identity

After establishing the SCCP connection between BSC and MSC there might be

authentication and encryption in an intact location update flow.

For Huawei MSC, authentication and encryption are all optional. And normally

authentication is mandatory and encryption is optional in practical situation.

Huawei BSS system supports encryption algorithm A5/1 and A5/2.

Identification: The ID in Est_IND message reported by the MS is TMSI. When

VLR cannot identify the TMSI or TMSI authentication fails, MSC will start

Identification flow. That means MSC will send transparent transfer message

“Identity Request” to MS and MS will report “Identity Response” message

which involves IMSI.

Now we will give the description for NOTE1 of location update flow. After establishing the SCCP connection between BSC and MSC there might be authentication and encryption in an intact location update flow. For Huawei MSC, authentication and encryption are all optional. And normally authentication is mandatory and encryption is optional in practical situation. Huawei BSS system supports encryption algorithm A5/1 and A5/2. In addition, identity flow is also optional. When does the identity flow occur?

The ID in Est_IND message reported by the MS is TMSI. When VLR cannot identify the TMSI or TMSI authentication fails, MSC will start Identification flow. That means MSC will send transparent transfer message “Identity Request” to MS and MS will report “Identity Response” message which involves IMSI. Actually, authentication, encryption and identity are some transparent transfer messages in case that we view from BSS side, this is to say, they belong to DTAP message. BSC and BTS will not translate these message, and mainly take charge of the transparent transfer of these

message.

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NOTE 2

To reallocate TMSI or not during location update is optional

in MSC.

If MSC chooses “Reallocate TMSI during location update” to

“No”, MS will not report “TMSI Reallocation Complete”

message.

In location update flow, we can see a NOTE2. This is to illustrate: when

occurring location update in MSC, to reallocate TMSI or not during location update is optional in MSC. This is to say, when the location updates, the MSC can reallocate TMSI number for MS, or not reallocate TMSI number and keep the original number. This selection is completed at MSC side. There is a parameter called “To allocated TMSI or not during location update”. If this parameter is

set as “No”, MSC will not send new TMSI number for the MS. Reviewing the above flow, we can see that it is actually “location updating accepted” message without newly-allocated TMSI. In NOTE3, in this flow we have simplified the release flow. To know more detailed refer to the BSC Normal Release Flow.

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NOTE3

In this flow we have simplified the release flow. To know more

detailed refer to the BSC Normal Release Flow.

In location update flow, we can see a NOTE2. This is to illustrate: when

occurring location update in MSC, to reallocate TMSI or not during location update is optional in MSC. This is to say, when the location updates, the MSC can reallocate TMSI number for MS, or not reallocate TMSI number and keep the original number. This selection is completed at MSC side. There is a parameter called “To allocated TMSI or not during location update”. If this parameter is

set as “No”, MSC will not send new TMSI number for the MS. Reviewing the above flow, we can see that it is actually “location updating accepted” message without newly-allocated TMSI. In NOTE3, in this flow we have simplified the release flow. To know more detailed refer to the BSC Normal Release Flow.

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Abnormal Occasion

BSC receives “Location Updating Rejected” message from

MSC. The possible reason is listed below:

MSC has not configured the CGI which has been configured in

BSC.

Communication failed between MSC and VLR.

MS has not been registered in HLR.

Now let’s talk about location updating abnormal occasions, i.e., location updating rejected. The possible reason is listed below:

A. MSC has not configured the CGI which has been configured in BSC.B. Communication failed between MSC and VLR.C. MS has not been registered in HLR.In the above three occasions, location update will be rejected, this is to say,

MSC will send “Location Updating Rejected” message.

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Contents







7. Case Analysis

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Call Setup Flow (Originating)MS BTS BSC MSC

Channel_req Channel_RequiredChannel_Active

Channel_Active_AckIMMEDIATE ASSIGN COMMAND

Establish_IND(CM Service Req)First SABM

CM Service Accepted

CR(Complete_L3_information)CC

(NOTE 1)SetupCall Processing

ASSIGNMENT COMMANDFirst SABM Establish_IND

Channel_ActiveChannel_Active_Ack

(NOTE 2) Assignment_Req

Assignment_CMPASSIGNMENT CMPAlertingConnect

Connect Ack(NOTE 3)

DisconnectRelease

Release Complete

(NOTE 4) Clear_CMDClear_CMP

Conversation

UA

UA

The first message sent by MS on Um interface when the MS originates a call is ‘Channel Required ’ . Then BTS will forward this message; we can see “Channel required” at A-bis interface. We can understand “Channel_req” as channel request; “Channel required” between BTS and BSC is called channel application. After BSC receives “Channel required”, it will return a “channel active”, which has a very important information—BSC’s description of target channel activated by BTS. After BTS activates this channel, it will return a “Channel active ACK”. Then BSC can assign this target channel, and this signaling message is called “Immediate Assign Command”. What is the channel type of the target channel activated and assigned by BSC? In the normal flow, the MS generally accesses BTS on RACH, this is to say, random access channel. Then BSC will generally activate SDCCH channel—stand-alone dedicated control channel. “Immediate Assign Command” message is sent to the MS via AGCH channel, and one SDCCH channel is assigned in AGCH channel. After MS receivesthis message, it will know its target channel, then it will adjust its work channel to the target channel, and send one “first SABM”. This is to attempt to access link layer of target channel, this is called initialization access message.

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After BTS receives SABM access message, BTS will generate an Establish_IND—establishing indication, to tell BTS that BSC target channel is well established and SDCCH channel is connected. Thisestablish_IND channel contains CM service request. CM service request is call management service request.When BSC receives Establish_IND message sent by BTS, BSC will send one “connect request” message to MSC. Connect request has two functions: Establish SCCP connection number, then load CM service request message to MSC; after MSC receives connect request, it will respond a “Connect confirm”, and this is used for establishing SCCP connection at A-interface. Then there is a NOTE1, which is actually to describe that there are several tasks when MSC receives CM service request: authentication, encryption and identification. These three flows are all optional, and we have illustrated at the above location update flow. Here in NOTE1, there is a MS class mark update flow for dual-band MS, we will talk about this later . Normally, MSC will send one CM service accepted to BSC and BTS, and this means that MS CM service has been accepted, then the MSwill send one “step up”, which contains the called number. After MSC receives this “step up” message, it will perform relevant processing according to the called number and other relevant information. For example, perform number analysis according to the called number, and perform routing selection. Meanwhile, MSC can assign relevant circuit resources at A-interface according to the relevant service information. This assignment is completed via “Assignment request”, which contains A-interface circuit resources. For example, an A-interface circuit is asigned with the information of CIC number (circuit identification code) relevant performance description. When BSC receives “Assignment request”, BSC will send one “Channel active”. There is a NOTE2 at this place. In NOTE2, the assignment has certain rules when MS is applying service or logging on the network. The assignment methods are: early assignment, late assignment, and very early assignment. We will talk about this later for details.Early assignment is used in this flow. And early assignment is the most frequently adopted in GSM system. Its features are: when MS firstly applies channel, it will assign SDCCH channel, then CM service request comes, and then the system will assign TCH channel or service channel, and this service channel assignment is before Alerting tone. This is early assignment method.

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Now let’s talk about early assignment method flow.BSC activates one TCH channel or service channel via “Channel active”. BTS will feed back one “channel active ACK” confirmation message to BSC. Then BSC will send “Assignment command” to the MS. Here “Assignment command” message is used. In the above, “Immediate Assignment command” is used as assigning SDCCH. Please note this difference.After MS receives “Assignment command”, it will send “first SABM” at the new target channel, and this sending is completed at L2 link layer of TCH channel. BTS will respond UA frame to it; and the radio link L2 is established via the response of “first SABM” and “UA frame”. Meanwhile, BTS will send “Establish_IND” (establishment indication). When MS receives UA frame, the MS will send one assignment completion command, this is called “assignment complete” message. After it gets to BSC, BSC will transparently transfer this signaling to MSC. Therefore, the relevant circuit resources at the caller side is prepared. When will it enter into conversation? It mainly lies in the called wait ringing. If the called begins ringing, the called will feed back one “Alerting” message (one ringback voice) to its network side. Then the caller side MSC will send ringback voice “Alerting” to the MS (here Alerting is realized via signaling method). After the MS receives Alerting message, it will send one “connect” message. After the MS receives this Alerting message, it will know the telephone is connected, and wait for the called to hang up. If the called is hanged up, the network side will send one “connect” command to the caller MS to indicate that the called is hanged-up. Now the caller MS will send one “Connect ACK”connection confirmation message. Now it formally start the conversation. The next is release flow, we will elaborate this later.

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NOTE 1: Class Mark Update Flow

1. Authentication, encrytion, identification are optional. See Note 1 in Location Update Flow.2. Class mark update flow:

For dual-band MS, after the SCCP connection is established and before receiving message "CM Service Accepted" there will be a class mark update flow.CR message reports the information of class mark 2. And in the class mark update flow, MS will report class mark 3 information which is related to the dual-band capabilty.

The MS caller call flow has several notes. Firstly, it is NOTE1, and is to describe that MSC can have several processing flows when BSC sends CM service request. 1. Authentication, encryption and identification are all optional. These flows are the same as location update of NOTE1. In actual network, authentication is generally mandatory. 2. Class mark update flow. For dual-band MS, after the SCCP connection is established and before receiving message “CM service Accepted”, there will be a class mark update flow. Class mark is originated by MSC. MSC will send one “Classmark request” message, then BSC will convert this message into “Classmark enquiry”. This is class mark request. Then MS will send one “Classmark change”, and convert it into “Classmark update” at BSS. MSC can get the MS class mark 3 via this message. Here the class mark update is to make MSC get MS class mark 3. CR message reports the information of classmark 2. And in the class mark update flow, MS will report class mark 3 information which is related to the dual-band capability. Therefore, MSC needs class mark update to get the MS class mark 3.

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NOTE 2: Assignment Method

Very Early Assignment

In “IMMEDIATE ASSIGN COMMAND ” ,TCH is directly

assigned. When MSC sends “Assignment _Req”

message, BSC will not assign new channel but

perform “Mode_Modify” flow. After that, MS will

report “Assignment _CMP” message to MSC.

Early Assignment

MSC sends “Assignment_Req” before “Alerting”.

Late Assignment

MSC sends “Assignment_Req” after ”Alerting”.Very Early Assignment

NOTE2 of MS caller flow is to illustrate channel assignment methods.Channel assignment methods in case of MS caller or called are: early assignment, late assignment and very early assignment. In the above courses, we have learnt early assignment. Early assignment method should meet the two requirements: (i). Firstly assign SDCCH channel, and then assign TCH channel; (ii) Assignment of TCH channel is before ringback voice. In interface tracing flow, MSC sends “Assignment_Req”before “Alerting”. Early assignment is a frequently used method. The second channel assignment method is late assignment: MSC sends “Assignment_Req” after “Alerting”; this is to say, after “Alerting”, MSC will assign A-interface circuit, and BSS will prepare BSS side voice circuit.The third method is very early assignment. In Immediate Assignment, TCH is directly assigned. This is to say, the MS TCH channel is not sent at “assignment request” message, but sent at “Immediate assignment request” message. In this case, the MS need perform relevant signaling exchange. When MSC sends “Assignment _Req” message, BSC will not assign new channel but perform “Mode Modify” flow. After “Mode Modify”, MS will report “Assignment _CMP” message to MSC. Thus very early assignment is completed.

What are the features of early assignment, late assignment and very early assignment?

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We will consider this from two aspects: one is connection success ratio, and the second is channel utilization ratio. For very early assignment, TCH channel utilization ratio is lower, because the MS assign TCH channel immediately as originating call. TCH is loaded with a lot of signaling information, therefore, some time is occupied by signaling and this part of time cannot be charged. In another sight of view, very early assignment success ratio is very high; if “Immediate assign command” can assign TCH channel, all of the radio resource has been prepared, this is to say, there will not be interruption caused by the shortage of radio resource. Compared with very early assignment, early assignment channel utilization ratio is higher. But it may occur that immediate assignment is completed and assignment fails (due to unavailable TCH channel) during connection, i.e, connection interruption.Compared with the aforementioned two methods, TCH channel utilization ratio of late assignment is highest. After ringback information, it will assign service channel. This may easily occur the following case: signaling flow of network side is successful, and ringback voice also comes, finally, conversation and connection is not successful.For above reasons, we use early assignment method more frequently in actual application.This slide also gives some descriptions on very early assignment flow. It is very simple. Firstly, assign TCH channel in case of immediateassignment; then BSC will modify channel mode in case that “Assignment request” message of MSC is sent, and the message is called “Mode Modify”; and then BTS’s response is called “Mode Modify ACK”; BSC modifies MS channel mode at radio interface, this is called “Channel Mode Modify”; then MS will respond “Channel Mode Modify ACK”. After these messages, the TCH channel, which is previously used for transfering signaling, is modified to transfer voice. This flow is very early assignment method.

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Abnormal Situation

Abnormal Occasion 1: MSC has sent “Assignment_Req” and

BSC does not send “ASSIGNMENT CMD” but sends a

“Assignment_Fail” message. Possible reasons:

A. BSC has not available TCH.

B. The CIC assigned by the MSC is not idle in BSC.

Abnormal Occasion2: BSC has sent “ASSIGNMENT CMD

message but MS sends “ASSIGNMENT FAIL”. Possible reason:

Downlink bit error rate is too high.

Abnormal Situation in case of MS originating call flow:Abnormal Occasion 1: MSC has sent “Assignment_Req” and BSC does not send “ASSIGNMENT CMD” but sends a “Assignment_Fail” message. Possible reason:

A. BSC has not available TCH.B. The CIC assigned by the MSC is not idle in BSC, and it may be in fault status.

Abnormal Occasion2: BSC has sent “ASSIGNMENT CMD message but MS sends “ASSIGNMENT FAIL”. Possible reason:

Downlink bit error rate is too high.

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Contents







7. Case Analysis

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Intra-BSC HandoverMS BTS1 BSC BTS2 MS MSC

Channel_Active

Channel_Active_Ack

Measure Report from MS

HANDOVER COMMAND

Handover AccessHandover_Detect

PHYINFOPHYINFO

First SABM

Establish_IND

HANDOVER COMPLETEHandover_Performed

(NOTE 2)

(NOTE 1)

UA

The third call flow is Intra-BSC handover, this is to say, the MS handover from one base station to another base station, or handover from one cell to another cell within the same BSC. There are many handovers, such as, intra-BSC handover, inter-BSC handover, etc. Inter-BSC handover can be sub-divided into intra-MSC handover and inter-MSC handover. The typical handover is intra-BSC handover. Above all, let’s study intra-BSC handover flow. What’s the handover for? There are many causes, such as poor quality of voice, lower receiving level, interference or equipment fault, or network load, etc. The MS will report measurement report to BSC during conversation, and BTS2 will also report measurement result to BSC. From these measurement result, BSC can get the causes that trigger this handover. Supposing that BSC finds the handover target cell is in another BTS, this is to say, MS handover is from BTS2 to BTS1 and the two BTS are in the same BSC.

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Firstly, it is handover trigger. Then BSC will send a “channel active” to the new target BTS, and then BTS will return one “channel active ACK”; thus a new TCH channel is established via these two messages. BSC shall send the relevant message of the new TCH channel to the MS, and the message is called “handover command”. The sending of this message is completed in the previous TCH channel, because the previous TCH channel is still occupied by the MS. After the MS receives “handover command”, it will get the relevant parameters of the target channel such as frequent point number, time slot number, etc; the MS will forward these to the new target channel. The first message sent by the MS in the target channel is called“handover access”. After BTS1 receives this message, it will convert this into “handover detect”, and notify BSC that the handover is detected.The next is NOTE1, and this is mainly to illustrate the difference between synchronous handover and asynchronous handover. The flowin the slide is asynchronous handover.In asynchronous handover, BTS1 will send “physical information” frame to the MS. After MS receives the “physical information” frame, it will send one “first SABM”, and this is also a initialization access frame. But this initialization access frame is sent at the target channel, and it is L2 information. BTS will also return a UA frame to confirm the SABM.TCH channel link layer is established via SABM and UA frame. When BTS1 receives SABM, it will send an Establish_IND, and this flow is the same as the above. When MS receives UA frame, MS will also send a “handover complete”, thus the handover is completed.When the handover is completed, BTS shall send a “Handover_performed” to MSC, for MSC internal statistics; BSC shall start the previous channel release flow at the same time. The previous MS conversation at BTS2 has some A-bisinterface login and other resources, and these problems are described in NOTE2. NOTE2 is to illustrate that BSC will start the local-end release flow for the previous channel, and the release cause: after the handover is completed, the channel resources will not be used, and BSC starts the local-end release flow.

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NOTE1:Asynchronous Handover and Synchronous Handover

Intra-BSC asynchronous handover:

BTS1 and BTS2 do not belong to the same BTS. Thus the target

BTS shall send PHY INFO message.

Intra-BSC synchronous handover:

BTS1 and BTS2 belong to the same BTS. Actually they are two cells

of one BTS. In such occasion there will not be PHY INFO message,

After “Handover Access” message, MS will send SABM directly.

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NOTE2

After reporting “Handover_Performed” message to MSC, BSC

will start the local-end release flow for the previous channel (in

BTS 2).

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Abnormal Occasion

In case of intra-BSC asynchronous handover, when BTS1

has sent PHY INFO message to the maximum time but the

MS still can’t access the new channel, BTS1 will send CONN

FAIL IND message. Its causing value is “handover access

failure”. When BSC receives this message, it will start local-

end release flow to release the new channel in BTS1.

In intra-BSC handover flow, there is an abnormal occasion: In case of intra-BSC asynchronous handover, when BTS1 (new cell) has sent PHY INFO message to the maximum time but still can’t the MS access the new channel, BTS1 will send CONN FAIL IND message. Its causing value is “handover access failure”. When BSC receives this message, it will start local-end release flow to release the channel in BTS1.

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Contents







7. Case Analysis

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Inter-BSC Handover

MS BTS1 BSC1 BTS2 MSMSC

Measure Report from MS

BSC2(NOTE 1)

Handover_Required Handover_RequestChannel_Active

Channel_Active_AckHandover_Request_Ack

Handover_CMDHandover_Detect

Handover_Detect

HANDOVER COMMAND Handover Access

PHY INFO

PHY INFOFirst SABM

Establish_INO

HANDOVER COMPLETEHandover_CmpClear_CMD

Clear_CMP(NOTE 2)

UA

Now let’s study “IV. Inter-BSC handover”. We have learnt intra-BSC handover flow. What are the differences of the two flows:Message at A-interface during intra-BSC handover is very few, and only “handover_performed” is sent to MSC after completing handover, to report to MSC that a handover is successfully completed. Message at A-interface during inter-BSC handover is large; when there is a new BSC, the previous BSC will report the relevant message to MSC and MSC will transit this to the new BSC; this is to say, for inter-BSC handover, many of messages at A-interface are transparent transfer message. Here let’s have a look at A-interface: first of all, it is measure report, then BSC1 send “Handover_required” to MSC; MSC will send “handover_request”to the target BSC; target BSC will perform “channel_active” to prepare the channel; and then the target BSC will return “Handover_request ACK”; the previous MSC will send channel number and relevant handover parameters in “handover_request ACK” message to the MS via “handover command”. Now the MS will access via the new channel after receiving “handover_command”.For synchronous handover, the MS can directly handover to the new channel and send “handover_access” message for direct access. Generally speaking, inter-MSC handover are asynchronous handover. At this time, the new BTS2 will send “PHY INFO”. The MS will send SABM to the target channel, and wait for BTS feed back UA frame for confirmation.

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After the target BTS receives SABM, it will send “establish_IND” to the target BSC (BSC2). After the MS receives the UA, it will send a “handover_complete”to BSC2; then BSC2 will send “handover_complete” message to MSC, and MSC will clear the previous A-interface resources; then BSC1 (the previous BSC) will release the local-end resources. This is inter-BSC handover flow.Here many A-interface messages are involved. Compared with intra-BSC handover flow, the message content are basically the same, and are transparently transferred to the target BSC via A-interface (or transferred from the target BSC to the previous BSC).We have not saved the trace result files of inter-BSC handover due to lab environment. Please refer to intra-BSC handover to see the internal message.

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NOTE1: Two Different Handover Occasions of Inter-BSC Handover

Intra-MSC inter-BSC Handover:

Two BSCs belong to the same MSC.

Inter-MSC Handover:

Two BSCs do not belong to the same MSC.

The A-interface messages are the same for the two occasions

above.

Inter-BSC handover has two notes: NOTE1: two different handover occasions of inter-BSC handover: (i). intra-MSC inter-BSC Handover, this is to say, two BSCs belong to the same MSC; (ii). Inter-MSC Handover, this is to say, two BSCs do not belong to the same MSC. The A-interface trace messages are same for the two occasions above; BSC doesn’t know whether the handover target cell are in the same MSC, therefore, the processing flow of the two occasions for BSC are totally the same. NOTE2: When BSC1 receives Clear_CMD message from MSC, it will start local-end release flow for the previously occupied channel in BTS1. Now the release flow is completed by BSC1, and this is called local-end release flow.

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NOTE2

When BSC1 receives Clear_CMD message from MSC, it will

start local-end release flow for the previously occupied channel

in BTS1.

Inter-BSC handover has two notes: NOTE1: two different handover occasions of inter-BSC handover: (i). intra-MSC inter-BSC Handover, this is to say, two BSCs belong to the same MSC; (ii). Inter-MSC Handover, this is to say, two BSCs do not belong to the same MSC. The A-interface trace messages are same for the two occasions above; BSC doesn’t know whether the handover target cell are in the same MSC, therefore, the processing flow of the two occasions for BSC are totally the same. NOTE2: When BSC1 receives Clear_CMD message from MSC, it will start local-end release flow for the previously occupied channel in BTS1. Now the release flow is completed by BSC1, and this is called local-end release flow.

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7. Case Analysis

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BSC Normal Release Flow

MS BTS BSC MSC(NOTE 1) Disconnect

Release

Release Complete

CHANNEL RELEASE

Clear_CMDClear_CMP

First DISCDeactive_SACCHRelease_IND

RF_Release_ReqRF_Release_Req_ACK

UA

Now we will study “V. BSC Normal Release Flow”. What is BSC normal release flow? Actually, BSC performs normal release after the conversation is over and the channel will not be used. There is another flow called local-end release flow. Local-end release flow: BSC performs local-end release after handover is completed and the original resources will not be used. Firstly of all, let’s have a look at BSC normal release flow. The reference file is “A-bis_whole”; in off-hook flow, we make labels of the number at each message for the corresponding result file.In case of off-hook, the first message is “disconnect” message. In the above Figure, if the MS is the caller, the “disconnect” message is sent to the caller from MSC, and this message indicates that the release flow is for the called terminal off-hook initiatively; after the called terminal off-hook initiatively, MSC will send “disconnect” to the caller MS, to release the relevant resources; the caller MS will return a “release”, and MSC will return a “release complete”. After the three messages, the release of CC resources (call control management resources) between MSC and MS is completed. In the above basic course, we have mentioned that the application layer mainly has CC, MM and RR. Here the release is CC resources, this is to say, call control management layer resource is firstly released.

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After the higher resources between the MS and MSC is released, MSC will send a “clear command” message to release A-interface circuit resource; BSC will return a “clear complete” to indicate that the resources at A-interface is released completely; after BSC receives “clear command” message sent from MSC, BSC will start channel release. First of all, a “channel release” is sent to the MS, and this message is transparently transferred by BTS, and it is RR layer message, for releasing RR layer resources of the MS.Then, BSC will send a “deactive SACCH” message, for releasing SACCH logic channel of BTS, and also for releasing TCH channel related to SACCH. Here we will talk about UM interface and A-bisinterface; first of all, we will talk about UM interface: when MS receives “channel release” message, the MS will send a “disconnect” message, then BTS will return a UA frame; this “disconnect” and UA is L2 message, for releasing L2 link resource between MS and base station. At A-bis interface, after BTS receives “deactive SACCH”, it will release the relevant TCH and SACCH logic channel, and respond a “release_IND” to BSC. Finally, BSC will send a “RF_release_req (RF resources release request), for releasing RF resource of BTS; after BTS release iscompleted, it will respond a “RF_release_req_ACK”, thus the relevant resources are completely released.

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NOTE1：Two Occasions of Release

NSS originating release

MS originating release

In these two different occasions all the messages are the same

except the direction of the first 3 messages.

In BSC normal release flow, there is a NOTE1. NOTE1 is the notes for two occasions of release: (i). NSS originating release; (ii). MS originating release. In these two different occasions, all the messages are same except the direction of the first 3 messages. The three different directions messages are “disconnect”, “release” and “release complete”.

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Contents







7. Case Analysis

36


Local-end Release Flow

MS BTS BSC MSC

Deactive_SACCH

Release_Confirm

RF_Release_Req

RF_Release_Req_ACK

Release_Req(LOCAL END) (NOTE 1)

Now we will talk about “VI. Local-end Release Flow”. We have talked about the cause of BSC local-end release flow: after the MS is handover to the new channel, the previous channel will not be used again, and BSC will originate the release. The first message is “deactive SACCH”, to release SACCH logic channel, and to release relevant TCH channel at the same time. The second message is “release request”, which includes the causing value of local end release. BTS will respond a “release confirm”. The next is “RF_release_req”, then BTS will respond “RF_ release_req_ACK”, thus local-end release is completed. In actual maintenance, we seldom analyze local-end release flow, because the release flow is fixedly performed inside BSC, and it seldom has problem. If you want to further analyze BSC local-end release flow, you can refer to “A-bis_whole” corresponding result. We also make labels of the number on each message. You can refer to the contents of the relevant message.

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Contents







7. Case Analysis

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Case Description

BSC’s ASSIGNMENT CMD message has been sent but the BTS

sends several EST IND messages. Later MS sends ASSIGNMENT

FAIL message and the causing value is Protocol error

unspecified.

Typical case analysis:

BSC’s ASSIGNMENT CMD message has been sent, but the BTS sends

several EST IND messages. Later MS sends ASSIGNMENT FAIL message

and the causing value is Protocol error unspecified.

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Case Analysis

By analyzing the messages we can find that MS has reported

several EST IND messages before ASSIGNMENT FAIL message.

When BTS receives a SABM frame it will report an EST IND

message to BSC. So we know now that the MS must have sent

many SABM frames to the BTS.

BTS can decode the SABM frame sent by MS but MS cannot

decode the UA frame sent by BTS. And this causes that the BTS

reports many EST IND messages.

Fault analysis

By analyzing the messages, we can find that MS has reported several EST IND message before ASSIGNMENT FAIL message.

When BTS receives a SABM frame, it will report an EST IND message to BSC. So we know now that the MS must have sent many SABM frame to the BTS. We can conclude: MS uplink level in the newly required TCH channel is good, but the downlink level is poor.

BTS can decode the SABM frame sent by MS but MS cannot decode the UA frame sent by BTS. And this causes that MS reports many SABM frame, and the BTS reports many EST IND messages.

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Case Analysis

Conclusion:

In the practical field, when many ASSIGNMENT FAIL messages appear, the main reason might be the pure downlink receiving power level or receiving quality.

Connection between TRX and SCU (CDU).

Case realize:

In the lab we can simulate. Disconnect the TX antenna to make the

downlink signal pure. Trace the A-bis signaling we can find that BTS

sends several EST IND messages to BSC for the newly assigned TCH but

can’t succeed. And then MS sends ASSIGNMENT FAIL via the previous

SDCCH and the causing value is “Protocol error unspecified”.

In order to verify our conclusion, we perform the following:

In the lab we can simulate. Disconnect the TX antenna to make poor downlink receiving power level or receiving quality, and trace the A-bissignaling, we can find that BTS sends several EST IND messages to BSC for the newly assigned TCH, but can’t succeed. And then MS sends ASSIGNMENT FAIL via the previous SDCCH and the causing value is “Protocol error unspecified”. In the field when it appears many ASSIGNMENT FAIL messages, the main reason might be the poor downlink receiving power level or receiving quality.

Finally, the field personnel check the BTS, and find that connection between TRX and SCU (CDU) is loosen. After fixed it, the above fault disappear. Thus it verifies that our conclusion is correct.

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Appendix: Power Class

Phase 2 MS output Power Class and the exact power relations.

power class

GSM900 Max peak power

DCS1800 Max peak power

Tolerance(dB)_normal

Tolerance(dB)_extreme

1 ------ 1W(30dBm) +/-2 +/-2.5

2 0.25W(24dBm) 2 2.5

3 5W(37dBm) 4W(36dBm) 2 2.5

4 2W(33dBm) 2 2.5

5 0.8W(29dBm) 2 2.5

In the above interface trace, we often see the MS classmark. In MS classmark, there is a “power class” parameter. MS power class is to stand for MS max transmitting power. Generally speaking, the new MS is phase 2 MS. Now let’s talk about Phase 2 MS output Power Class and the exact power relations.Power class of 1800M MS is generally 1, and its transmitting power is 1W or 30dBm; power class of 900M MS is generally 4, and its transmitting power is 2W or 33dBm. In different power classes, transmitting power has a tolerance; the tolerance is generally 2dB and the extreme is 2.5dB.

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Power Control Level

Phase 2 MS

Power control

level and the

exact power

relations.

power control level

output power (dBm)

Tolerance_normal

(dB)

Tolerance _extreme

(dB)

Power control level

output power (dBm)

Tolerance_normal

(dB)

Tolerance _extreme

(dB)0-2 39 2 2.5 29 36 2 2.53 37 3 4 30 34 3 44 35 3 4 31 32 3 45 33 3 4 0 30 3 46 31 3 4 1 28 3 47 29 3 4 2 26 3 48 27 3 4 3 24 3 49 25 3 4 4 22 3 410 23 3 4 5 20 3 411 21 3 4 6 18 3 412 19 3 4 7 16 3 413 17 3 4 8 14 3 414 15 3 4 9 12 4 515 13 3 4 10 10 4 516 11 5 6 11 8 4 517 9 5 6 12 6 4 518 7 5 6 13 4 4 5

19-31 5 5 6 14 2 5 615-28 0 5 6

In power control, we can often see another important parameter—power control level, or power level. This parameter is called MS power level.In the front page, we have talked about power class. MS power class is a static parameter, and each MS has its own fixed power class.Power level is adjustable; MS “power level” can be adjusted by power control algorithms, and this is subject to the distance between MS and base station, transmission condition, etc. Power control is to control power level of MS and base station.Here we will focus on Phase 2 MS Power control level and the exact power relations.

Max transmitting power of 900M MS is generally 2W, this is to say, power class is 4; its power control level is generally 5, its transmitting power is 33dBm, so its max power control level value is 5. As performing power control, the value will be adjusted down from 5. For 1800M MS, its max transmitting power is 1W (30dBm).

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Let’s have a look at the right part of the film. For 1800M MS, its power control level should be below 0, 1, 2, and 3; when power control level increase one level, MS transmitting power will lower down 2 dB. For 900M MS and 1800M MS, when power control level increase one level, MS transmitting power will lower down 2 dB. There is a toleranceproblem, I.e., power control level error; the general tolerance is 3 dB, and max tolerance is 4dB or 5dB. For different power level, the tolerance are different.

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