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GSM BSS Network PS KPI (Download Rate) Optimization Manual INTERNAL
Product Name Confidentiality Level
GSM BSS INTERNAL
Product Version
Total 38 pages
GSM BSS Network PS KPI (Download Rate)
Optimization Manual
(For internal use only)
Prepared by GSM&UMTS Network Performance
Research Department
Date
2008-02-29
Reviewed by Date
Reviewed by Date
Granted by Date
Huawei Technologies Co., Ltd.
All rights reserved
GSM BSS Network PS KPI (Download Rate) Optimization Manual INTERNAL
Revision Record
Date Version Description Author
2008-02-29 1.0 Draft completed Wang Guanghua
2008-11-01 1.1 Reorganized Geng Haijian
(ID: 00105443)
2008-12-17 1.1 Revised according to review comments Geng Haijian
(ID: 00105443)
2008-11-01 All rights reserved Page 2 of 38
INTERNAL
Contents
1 Overview.........................................................................................................................................8
1.1 Position of the GPRS or EGPRS Network in End-to-End Applications..................................................................8
1.1 Position of the GPRS or EGPRS Network in End-to-End Applications..................................................................8
1.2 Introduction to CQT and DT....................................................................................................................................9
1.2 Introduction to CQT and DT....................................................................................................................................9
1.3 Performance Baseline of the (E)GPRS Network....................................................................................................10
1.3 Performance Baseline of the (E)GPRS Network....................................................................................................10
1.4 Introduction to Channel Resource Management Algorithm...................................................................................11
1.4 Introduction to Channel Resource Management Algorithm...................................................................................11
1.5 Introduction to Link Quality Management Algorithm............................................................................................14
1.5 Introduction to Link Quality Management Algorithm............................................................................................14
1.6 Mechanism of Link Synchronization/Channel Synchronization............................................................................18
1.6 Mechanism of Link Synchronization/Channel Synchronization............................................................................18
2 Thoughts About Identifying Download Rate Problems.....................................................20
2.1 Downloading Large-Sized Files in CQT in Idle Hours to Identify the Problem....................................................20
2.1 Downloading Large-Sized Files in CQT in Idle Hours to Identify the Problem....................................................20
2.1.1 Failure to Assign or Steadily Occupy Four Channels .....................................................................................22
2.1.2 Failure to Steadily Occupy a High Coding Scheme ........................................................................................23
2.1.3 Error Block.......................................................................................................................................................24
2.1.4 High Ratio of Control Blocks...........................................................................................................................25
2.1.5 Abnormal Release of TBF................................................................................................................................30
2.1.6 High Transmission Rate at the RLC Layer But Low Transmission Rate at the Application Layer.................30
2.2 Competition of the Download Rate through CQT..................................................................................................31
2.2 Competition of the Download Rate through CQT..................................................................................................31
2.3 Comparision between DT downloading rates in busy and idle hours....................................................................31
2008-11-01 All rights reserved Page 3 of 38
INTERNAL
2.3 Comparision between DT downloading rates in busy and idle hours....................................................................31
3 Conclusion....................................................................................................................................34
3.1 Problem Identification............................................................................................................................................34
3.1 Problem Identification............................................................................................................................................34
3.2 Routine Optimization..............................................................................................................................................37
3.2 Routine Optimization..............................................................................................................................................37
4 Appendices...................................................................................................................................38
4.1 Appendix 1: Parameter Description........................................................................................................................38
4.1 Appendix 1: Parameter Description........................................................................................................................38
4.2 Appendix 2: Optimization Manual for Parameters on Server and Test PC............................................................38
4.2 Appendix 2: Optimization Manual for Parameters on Server and Test PC............................................................38
2008-11-01 All rights reserved Page 4 of 38
INTERNAL
Figures
GPRS networking.............................................................................................................................9
Payload of the GPRS protocol stack ..........................................................................................10
Channel allocation example.........................................................................................................12
Information shown by the TEMS log when you download large-sized files in ideal
situation............................................................................................................................................21
Messages traced at the application layer when you download large-sized files in ideal
conditions.........................................................................................................................................22
Timeslots assigned in Packet Downlink Assignment message (on the left) and Packet
Timeslot Reconfigure message (on the right) ..........................................................................23
Viewing the Receive block bitmap (URBB) in the Packet Downlink ACK/NACK
message to determine the error blocks ......................................................................................24
PDP context of the MS...................................................................................................................26
MS flow control data reported by the PCU to the SGSN.......................................................27
Application layer data traced at the MS side............................................................................28
Checking whether TBF is abnormally released through the ACK message.......................30
Comparison of the radio transmission environment before(upper) and after (lower) the
network swapping..........................................................................................................................32
Frequent reselections caused by improper neighboring cell configuration and
reselection parameter settings......................................................................................................33
2008-11-01 All rights reserved Page 5 of 38
INTERNAL
Abstract and Abbreviation
Keywords
CQT, DT, download rate
Abstract
Download is an end-to-end service process from the FTP server to an MS. The GPRS network
serves as the transport network layer. This document focuses on this end-to-end process,
explains the approaches of checking the impact of the nodes in this process on the download
rate, and describes the methods of optimizing the GPRS network.
Acronyms and Abbreviations
Acronym and Abbreviation Full Spelling
GPRS General Packet Radio Service
EGPRS Enhanced GPRS
MS Mobile Station
CQT Call Quality Test
DT Drive Test
FR Frame Relay
GBR Guarantee Bit Rate
PCU Packet Control Unit
SGSN Serving GPRS Support Node
BSS Base Station System
BSC Base Station Control
BTS Base Transceiver Station
GB GSN-BSS Interface
Um Radio Interface for GSM BSS
TBF Temporary Block Flow
RLC Radio Link Control
MAC Media Access Control
MCS Modulation and Coding Scheme
LA Link adaptation
IR Incremental redundancy
2008-11-01 All rights reserved Page 6 of 38
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50 gsm bss network ps kpi (download rate) optimization manual[1].doc

  • 1. GSM BSS Network PS KPI (Download Rate) Optimization Manual INTERNAL Product Name Confidentiality Level GSM BSS INTERNAL Product Version Total 38 pages GSM BSS Network PS KPI (Download Rate) Optimization Manual (For internal use only) Prepared by GSM&UMTS Network Performance Research Department Date 2008-02-29 Reviewed by Date Reviewed by Date Granted by Date Huawei Technologies Co., Ltd. All rights reserved
  • 2. GSM BSS Network PS KPI (Download Rate) Optimization Manual INTERNAL Revision Record Date Version Description Author 2008-02-29 1.0 Draft completed Wang Guanghua 2008-11-01 1.1 Reorganized Geng Haijian (ID: 00105443) 2008-12-17 1.1 Revised according to review comments Geng Haijian (ID: 00105443) 2008-11-01 All rights reserved Page 2 of 38
  • 3. INTERNAL Contents 1 Overview.........................................................................................................................................8 1.1 Position of the GPRS or EGPRS Network in End-to-End Applications..................................................................8 1.1 Position of the GPRS or EGPRS Network in End-to-End Applications..................................................................8 1.2 Introduction to CQT and DT....................................................................................................................................9 1.2 Introduction to CQT and DT....................................................................................................................................9 1.3 Performance Baseline of the (E)GPRS Network....................................................................................................10 1.3 Performance Baseline of the (E)GPRS Network....................................................................................................10 1.4 Introduction to Channel Resource Management Algorithm...................................................................................11 1.4 Introduction to Channel Resource Management Algorithm...................................................................................11 1.5 Introduction to Link Quality Management Algorithm............................................................................................14 1.5 Introduction to Link Quality Management Algorithm............................................................................................14 1.6 Mechanism of Link Synchronization/Channel Synchronization............................................................................18 1.6 Mechanism of Link Synchronization/Channel Synchronization............................................................................18 2 Thoughts About Identifying Download Rate Problems.....................................................20 2.1 Downloading Large-Sized Files in CQT in Idle Hours to Identify the Problem....................................................20 2.1 Downloading Large-Sized Files in CQT in Idle Hours to Identify the Problem....................................................20 2.1.1 Failure to Assign or Steadily Occupy Four Channels .....................................................................................22 2.1.2 Failure to Steadily Occupy a High Coding Scheme ........................................................................................23 2.1.3 Error Block.......................................................................................................................................................24 2.1.4 High Ratio of Control Blocks...........................................................................................................................25 2.1.5 Abnormal Release of TBF................................................................................................................................30 2.1.6 High Transmission Rate at the RLC Layer But Low Transmission Rate at the Application Layer.................30 2.2 Competition of the Download Rate through CQT..................................................................................................31 2.2 Competition of the Download Rate through CQT..................................................................................................31 2.3 Comparision between DT downloading rates in busy and idle hours....................................................................31 2008-11-01 All rights reserved Page 3 of 38
  • 4. INTERNAL 2.3 Comparision between DT downloading rates in busy and idle hours....................................................................31 3 Conclusion....................................................................................................................................34 3.1 Problem Identification............................................................................................................................................34 3.1 Problem Identification............................................................................................................................................34 3.2 Routine Optimization..............................................................................................................................................37 3.2 Routine Optimization..............................................................................................................................................37 4 Appendices...................................................................................................................................38 4.1 Appendix 1: Parameter Description........................................................................................................................38 4.1 Appendix 1: Parameter Description........................................................................................................................38 4.2 Appendix 2: Optimization Manual for Parameters on Server and Test PC............................................................38 4.2 Appendix 2: Optimization Manual for Parameters on Server and Test PC............................................................38 2008-11-01 All rights reserved Page 4 of 38
  • 5. INTERNAL Figures GPRS networking.............................................................................................................................9 Payload of the GPRS protocol stack ..........................................................................................10 Channel allocation example.........................................................................................................12 Information shown by the TEMS log when you download large-sized files in ideal situation............................................................................................................................................21 Messages traced at the application layer when you download large-sized files in ideal conditions.........................................................................................................................................22 Timeslots assigned in Packet Downlink Assignment message (on the left) and Packet Timeslot Reconfigure message (on the right) ..........................................................................23 Viewing the Receive block bitmap (URBB) in the Packet Downlink ACK/NACK message to determine the error blocks ......................................................................................24 PDP context of the MS...................................................................................................................26 MS flow control data reported by the PCU to the SGSN.......................................................27 Application layer data traced at the MS side............................................................................28 Checking whether TBF is abnormally released through the ACK message.......................30 Comparison of the radio transmission environment before(upper) and after (lower) the network swapping..........................................................................................................................32 Frequent reselections caused by improper neighboring cell configuration and reselection parameter settings......................................................................................................33 2008-11-01 All rights reserved Page 5 of 38
  • 6. INTERNAL Abstract and Abbreviation Keywords CQT, DT, download rate Abstract Download is an end-to-end service process from the FTP server to an MS. The GPRS network serves as the transport network layer. This document focuses on this end-to-end process, explains the approaches of checking the impact of the nodes in this process on the download rate, and describes the methods of optimizing the GPRS network. Acronyms and Abbreviations Acronym and Abbreviation Full Spelling GPRS General Packet Radio Service EGPRS Enhanced GPRS MS Mobile Station CQT Call Quality Test DT Drive Test FR Frame Relay GBR Guarantee Bit Rate PCU Packet Control Unit SGSN Serving GPRS Support Node BSS Base Station System BSC Base Station Control BTS Base Transceiver Station GB GSN-BSS Interface Um Radio Interface for GSM BSS TBF Temporary Block Flow RLC Radio Link Control MAC Media Access Control MCS Modulation and Coding Scheme LA Link adaptation IR Incremental redundancy 2008-11-01 All rights reserved Page 6 of 38
  • 7. INTERNAL References [1] EDGE DT Download Rate Optimization Thoughts and Cases, at http://support.huawei.com, released on June 26, 2008 [2] TCP/IP Detail Volume 1, by W. Richard Stevens, in April, 2008 [3] GPRS Network Technology, by Motorola Engineering Institute, on June 1, 2005 2008-11-01 All rights reserved Page 7 of 38
  • 8. INTERNAL GSM BSS Network PS KPI (Download Rate) Optimization Manual 1 Overview The first two chapters of this document describe the download process and performance analysis. Readers who are concerned only about the GPRS network optimization can start from chapter 3. The fault identifying tools in this document mainly refer to TEMS and Ethereal/Wireshark. These tools can be used to trace or browse the information at the NE side. The description in this document is based on the condition that the TDM transmission is used over the Abis interface and the FR transmission is used over the Gb interface. 1.1 Position of the GPRS or EGPRS Network in End-to- End Applications For end-to-end applications, an MS functions like the network card of the client to connect to the GGSN through the GPRS network, and then to the Internet. This process is the same as that when accessing the Internet through cables. The difference is that the MS is not connected to the router directly through cables but through the GPRS network. Compared with cables of 100 Mbit/s or higher bandwidth, the GPRS network has a longer RTT delay and smaller bandwidth. In addition, the delay and the bandwidth vary with the actual conditions. From the perspective of the download rate, the target of GPRS network optimization is to increase the bandwidth and reduce the delay (the advantages of a small delay are quite obvious when you download small-sized files). 2008-11-01 All rights reserved Page 8 of 38
  • 9. INTERNAL Figure 1 shows the GPRS networking. Figure 1 GPRS networking Figure 1 shows a typical GPRS networking mode based on E1 transmission. Each interface supports direct connection. Timeslot cross devices, that is, DXX devices, can be used for transmission on the Gb interface and the G-Abis interface. If the external PCU is installed, the PCU provides the Gb interface. The Pb interface between the PCU and the BSC usually adopts the E1 direct connection mode. 1.2 Introduction to CQT and DT Telecom operators assess the rate performance of a GPRS network through call quality tests (CQTs) and drive tests (DTs). Why do telecom operators choose CQT and DT to assess the network performance? As a fixed-point test, the CQT is conducted in places where the wireless conditions are good and the C/I ratio is steady. The CQT performed in idle hours help check all the network elements and transmission links between the Um interface and the Gi interface. Such CQT can absolutely show the performance of the equipment. The CQT performed in busy hours help check the quality of the resource management algorithms, including the algorithms related to channels, Abis interface resources, and Gb interface resources. The CQT performed in busy hours, however, bring great uncertainty. For example, if another user is using the download service during the test, the download rate will be greatly affected. In this case, the CQT cannot fully reflect the performance of the equipment. This is because the impact of resource allocation is great. The test result can be used only for comparison of the performance before and after the network swapping. The DT result may be affected by large C/I fluctuation and cell reselection. The DT can be used to assess the wireless coverage and interference, the quality of the algorithm for adjusting the coding scheme, and the processing performance of the PCU during cell reselection (as the handover function in the PS domain has not achieved yet). The DT, however, also bring uncertainty. For example, in the case of red light, whether the C/I ratio is good or and whether the signals are in deep fading points have great impact on the average rate in the DT. 2008-11-01 All rights reserved Page 9 of 38
  • 10. INTERNAL The algorithm for adjusting the coding scheme is also referred to as the link quality control algorithm, or IR/LA algorithm. The reason for adjustment is that the required wireless quality, that is, C/I ratio, varies with the coding schemes. For a specific C/I ratio, a proper coding scheme should be employed, to help achieve the optimal balance between the data amount sent per unit time and the retransmission rate, thus maximizing the transmission rate. 1.3 Performance Baseline of the (E)GPRS Network Calculate the theoretical performance limit of the product in the case that resources are sufficient. Assume that the MSS value of the TCP negotiated between the server and the MS is 1460. This is the default size of a TCP/IP data packet sent on the Ethernet. The data packet is encapsulated according to the Ethernet protocols and a 20-byte TCP header and a 20-byte IP header are added. The MTU value of the intermediate network is 1500. That is, no fragmentation is performed. The size of an LLC PDU negotiated between the MS and the SGNN is 506. The MSC9 coding scheme is employed on the Um interface. Figure 2 shows how the data is encapsulated. Figure 2 Payload of the GPRS protocol stack Actually, if the MSS value is 1450 bytes, the data can be fragmented into three SNDCP packets to achieve the highest encapsulation efficiency. When the MSS value is 1460 bytes, the efficiency to the LLC layer is calculated as follows: 1460/ (1460+20+20+13+24) = 94.99%. When the MSS value is 1450 bytes, the efficiency to the LLC layer is calculated as follows: 1450/(1450+20+20+10+18) = 95.52%. For an EGPRS network, when the MCS9 is employed, the theoretical rate of a single channel is 59.2 kbit/s. If four channels are used for transmission, one channel will be used as the control channel. The control information accounts for about 19% of the data on the channel before the Uplink ACK/NACK optimization scheme in uplink extension mode is incorporated, and the control information accounts for about2% of the data on the channel after the preceding optimization scheme is incorporated. Therefore, the data rate is 59.2 x (4- 2008-11-01 All rights reserved Page 10 of 38
  • 11. INTERNAL 2%) = 235.616 kbit/s. Multiply the rate by the efficiency of the LLC layer to get the maximum rate at the application layer in ideal situation, that is, 235.616 kbit/s x 95.52%=225.06 kbit/s. To achieve a rate close to the theoretical limit, do as follows: Download a large-sized file. During the first stage of download, the TCP connection is just established, and the TCP uses the slow start mechanism. Slow start means that the TCP layer sends data at a slow rate to avoid network congestion when the TCP layer does not know the network transmission bandwidth or quality or when the network transmission bandwidth is reducing or the transmission quality is degrading. Therefore, the volume of the initially sent data is insufficient. Each node tries not to discard packets, frames, or blocks. These nodes refer to the IPBB, core network, Gb interface, PCU, G-Abis interface, BTS, and Um interface, as shown in Figure 1. The flow control at each interface cannot be controlled to the extent that the data is not enough for sending. The bandwidth of the radio interface must be guaranteed and a maximum number of channels should be occupied. Currently, the multislot capability of most testing MSs is 10 or 11, and a maximum of four downlink timeslots can be occupied. The channels are not multiplexed by other MSs, and the MCS9 coding scheme can be used steadily. 1.4 Introduction to Channel Resource Management Algorithm Allocating as many channels as possible to the MS is a method for ensuring the radio interface bandwidth. The channel resource management algorithm allocates channels based on the maximum capability of an MS. That is, the number of channels allocated to an MS corresponds to the multislot capability of the MS. In addition, the algorithm balances TBFs among channels when possible. Block resources are allocated in the following principles: The bandwidth for GBR users is guaranteed. The best fairness is achieved. That is, the polling mechanism is used for the TBFs that are multiplexed onto the same channel. The entire channel resource management algorithm consists of channel allocation, dynamic channel conversion/release, and load balance. Channel resources consist of the channel pool of CS (CSD) and the channel pool of PS (PSD). After configuration, static PDCHs are grouped to the PSD, and dynamic PDCHs are grouped to the CSD. Dynamic channel conversion is to group part of CSD channels to the PSD. The dynamic channel conversion can be triggered by the following conditions: inadequate multislot capability, EGPRS MS assigned to a GPRS channel, and load exceeding the Uplink Multiplex Threshold of Dynamic Channel Conversion/Downlink Multiplex Threshold of Dynamic Channel Conversion. Channel allocation is to assign the optimal channel group in the PSD to the MS. Load balance is to redistribute all TBFs within a timed cycle, to balance the load among channels. The channel resource management algorithm is complicated. Here the downloading process of an MS is taken as an example and the configuration is typical. Figure 3 shows the channel allocation process. Configuration: Maximum Ratio Threshold of PDCHs in a Cell = 100; Uplink Multiplex Threshold of Dynamic Channel Conversion/Downlink Multiplex Threshold of Dynamic Channel Conversion = 12/12. 2008-11-01 All rights reserved Page 11 of 38
  • 12. INTERNAL PDCH Uplink Multiplex Threshold/PDCH Downlink Multiplex Threshold = 70/80. Maximum PDCH numbers of carrier = 8. Figure 3 Channel allocation example (1) During initial access (no matter whether the multislot capability of the MS is known), the PSD has only one channel. Thus, the uplink and downlink data are both multiplexed onto this channel. The requirements of the multislot capability of the MS are not met. The multislot capability of the MS supports four downlink timeslots, but only one timeslot is available now. In this case, dynamic channels are converted. Three TCHs are converted into PDCHs. (2) After 4.5 seconds, the load balance flow is triggered, and another channel allocation is performed on this TBF. The amount of data flow cannot be checked at this time. Therefore, the service is considered as a neutral service. Three downlink timeslots and two uplink timeslots are assigned to the MS. Timeslots 5, 6, and 7 are the three downlink timeslots assigned. (3) The service is identified as the download service after 4.5 seconds. In this case, timeslots are assigned to this MS in 4+1 mode. Timeslots 4, 5, 6, and 7 are the downlink timeslots assigned to the MS. (4) Another MS accesses the network. The service is judged as neutral at the initial phase. Timeslots 4, 5, and 6 are assigned to the second MS. Timeslot 6 on the uplink is occupied by the first MS. Therefore, when the second MS accesses the network, try to avoid the 2008-11-01 All rights reserved Page 12 of 38
  • 13. INTERNAL multiplexing of timeslot 6 by the two MSs. Timeslot 5 is assigned to the second MS. Then, timeslots 4, 5, and 6 are assigned for the downlink data transmission of the second MS according to the multislot capability of the second MS. (5) It is found that the multislot capability of the second MS is not met, and another channel is required. In this case, the channel mapping to timeslot 3 is converted according to the priority and the timeslot re-assignment flow for the second MS is triggered. (6) In this case, the total number of TBFs on all channels is eight, and the number of occupied channels is five. Therefore, the multiplexing degree is 8/5 = 1.6 > 1.2. The heavy load triggers dynamic channel conversion. The number of converted channels is calculated as follows: 8/1.2 – 5 + 1 = 2 (Total number of TBFs in the Cell/Threshold of Dynamic Channel Conversion - Number of PDCHs occupied + 1, where 1 is used as it is an integer). The requirements of the Maximum Ratio Threshold of PDCHs in a Cell and Maximum PDCH Numbers of carrier are met. In this case, channels mapping to timeslots 1 and 2 are converted. When load balance is performed on the TBF, this TBF is assigned to timeslots 1, 2, 3, and 4. When channel allocation does not satisfy the multislot capability of the MS, consider the following aspects:  Channel type. If a channel is configured as an EGPRS dedicated channel, it cannot be assigned to a GPRS MS. If a channel is configured as a dedicated channel for EGPRS, and the channel is occupied by an EGPRS MS, the channel cannot be assigned to a GPRS MS. If a GPRS MS occupies an EGPRS dedicated channel, the GPRS MS is moved from the channel.  When the multiplexing of a channel reaches the PDCH Uplink/Downlink Multiplex Threshold, this channel cannot be assigned.  When an exception occurs on the channel, such as out-of-synchronization, the channel cannot be assigned.  Discontinuous channels cannot be assigned.  If Allow E Down G Up Switch is set to Close and a downlink TBF of an EGPRS MS is on this channel, the uplink data of a GPRS MS cannot be multiplexed onto this channel. If Allow E Down G Up Switch is set to Close and an uplink TBF of a GPRS MS is on this channel, the downlink data of an EGPRS MS cannot be multiplexed onto this channel.  When Reassignment TBF for Different Trx is set to Not Allow, no re-assignment will be performed even if the number of initially assigned timeslots does not meet the multislot capability of the MS.  When PS Concentric Cell HO Strategy is set to No handover between underlaid subcell and overlaid subcell or Handover from overlaid subcell to underlaid subcell, the channels in the overlaid subcell cannot be assigned.  When the Abis interface timeslot is unavailable, the channel cannot be assigned.  For an external PCU, if no PCICs are available, the channel cannot be assigned. Constraints of dynamic channel conversion are as follows:  When the value of (number of static PDCHs + converted dynamic channels)/total number of service channels) reaches the value of Maximum Ratio Threshold of 2008-11-01 All rights reserved Page 13 of 38
  • 14. INTERNAL PDCHs in a Cell, no dynamic channels can be converted.  When the number of PDCHs of on a carrier reaches the value of Maximum PDCH numbers of carrier, no dynamic channels can be converted.  When the number of occupied Abis interface timeslots on a TRX reaches the value of MaxAbisTSOccupied, no dynamic channels can be converted.  When the dynamic channel is occupied by voice service, this channel cannot be converted.  When the channel conversion property of the concentric cell is set to Only convert at UL, the dynamic channels in the overlaid subcell cannot be converted. The multislot capability of an MS refers to the maximum number of downlink and uplink channels that the MS supports, and the number of timeslots between an uplink channel and a downlink channel. The multislot capability of a common testing MS is 10 or 11, supporting two allocation schemes: the allocation scheme of three downlink channels and two uplink channels or the allocation scheme of four downlink channels and one uplink channel. MSs that support the EDA function and with the EDA function enabled can also support the allocation scheme of two downlink channels and three uplink channels. In two-phase access or 11-bit one-phase access stage, the PCU can obtain the multislot capability of an MS. For 8-bit one–phase access, the PCU does not know the multislot capability of the MS at first. But the attachment request sent by the MS carries the multislot capability of the MS. Thus, the SGSN can obtain the multislot capability information. After the TBF of the MS is established, the PCU can obtain the multislot capability of the MS through the timed RA capability update flow between the PCU and the SGSN. 1.5 Introduction to Link Quality Management Algorithm Link quality management is known as coding scheme adjustment. A high coding scheme provides a high transmission rate, but also requires good wireless conditions. The algorithm for adjusting the coding schemes is to achieve the balance between the transmission rate and error block rate. Coding scheme adjustment is based on the BEP reported by the MS. Such adjustment is a delayed adjustment. Therefore, the purpose of coding scheme adjustment is to follow the rules of link quality change, that is, the envelope, but not the actual link quality changes. In different scenarios, such as in high-speed application scenarios, the extent to which the rules are followed is different. Protocols and algorithms also provide the following interface externally adjustable: BEP Period. This interface is used to control the number of blocks to be filtered when the BEP is reported. The longer the BEP period is, the larger the number of blocks to be filtered and the steadier the filters. The shorter the BEP period is, the larger the proportion occupied by the latest BEP value. It is recommended that the BEP period set to a small value in high-speed applications. Table 1describes the relations between the BEP value and the coding scheme. 2008-11-01 All rights reserved Page 14 of 38
  • 15. INTERNAL Table 1 Mapping between BEP and coding scheme CV_BEP 0 1 2 3 4 5 6 7 MEAN_BEP 0 MCS-3 MCS-3 MCS-3 MCS-3 MCS-3 MCS-3 MCS-3 MCS-3 1 MCS-5 MCS-5 MCS-5 MCS-5 MCS-3 MCS-3 MCS-3 MCS-3 2 MCS-5 MCS-5 MCS-5 MCS-5 MCS-3 MCS-3 MCS-3 MCS-3 3 MCS-5 MCS-5 MCS-5 MCS-5 MCS-5 MCS-3 MCS-3 MCS-3 4 MCS-5 MCS-5 MCS-5 MCS-5 MCS-5 MCS-3 MCS-3 MCS-3 5 MCS-6 MCS-5 MCS-5 MCS-5 MCS-5 MCS-5 MCS-5 MCS-5 6 MCS-6 MCS-5 MCS-5 MCS-5 MCS-5 MCS-5 MCS-5 MCS-5 7 MCS-6 MCS-6 MCS-6 MCS-5 MCS-5 MCS-5 MCS-5 MCS-5 8 MCS-6 MCS-6 MCS-6 MCS-5 MCS-5 MCS-5 MCS-5 MCS-5 9 MCS-6 MCS-6 MCS-6 MCS-5 MCS-5 MCS-5 MCS-5 MCS-5 10 MCS-6 MCS-6 MCS-6 MCS-6 MCS-5 MCS-5 MCS-5 MCS-5 11 MCS-6 MCS-6 MCS-6 MCS-6 MCS-6 MCS-5 MCS-5 MCS-6 12 MCS-6 MCS-6 MCS-6 MCS-6 MCS-6 MCS-6 MCS-6 MCS-6 13 MCS-6 MCS-6 MCS-6 MCS-6 MCS-6 MCS-6 MCS-6 MCS-6 14 MCS-7 MCS-6 MCS-6 MCS-6 MCS-6 MCS-6 MCS-6 MCS-6 15 MCS-7 MCS-6 MCS-6 MCS-6 MCS-6 MCS-6 MCS-6 MCS-6 16 MCS-7 MCS-7 MCS-6 MCS-6 MCS-6 MCS-6 MCS-6 MCS-6 17 MCS-7 MCS-7 MCS-7 MCS-7 MCS-7 MCS-7 MCS-7 MCS-7 18 MCS-7 MCS-7 MCS-7 MCS-7 MCS-7 MCS-7 MCS-7 MCS-7 19 MCS-7 MCS-7 MCS-7 MCS-7 MCS-7 MCS-7 MCS-7 MCS-7 20 MCS-7 MCS-7 MCS-7 MCS-7 MCS-7 MCS-7 MCS-7 MCS-7 21 MCS-7 MCS-7 MCS-7 MCS-7 MCS-7 MCS-7 MCS-7 MCS-7 22 MCS-8 MCS-8 MCS-8 MCS-8 MCS-8 MCS-8 MCS-8 MCS-8 23 MCS-8 MCS-8 MCS-8 MCS-8 MCS-8 MCS-8 MCS-8 MCS-8 24 MCS-8 MCS-8 MCS-8 MCS-8 MCS-8 MCS-8 MCS-8 MCS-8 25 MCS-8 MCS-8 MCS-8 MCS-8 MCS-8 MCS-8 MCS-8 MCS-8 26 MCS-8 MCS-8 MCS-8 MCS-8 MCS-8 MCS-8 MCS-8 MCS-9 27 MCS-9 MCS-9 MCS-9 MCS-9 MCS-9 MCS-9 MCS-9 MCS-9 28 MCS-9 MCS-9 MCS-9 MCS-9 MCS-9 MCS-9 MCS-9 MCS-9 2008-11-01 All rights reserved Page 15 of 38
  • 16. INTERNAL CV_BEP 0 1 2 3 4 5 6 7 MEAN_BEP 29 MCS-9 MCS-9 MCS-9 MCS-9 MCS-9 MCS-9 MCS-9 MCS-9 30 MCS-9 MCS-9 MCS-9 MCS-9 MCS-9 MCS-9 MCS-9 MCS-9 31 MCS-9 MCS-9 MCS-9 MCS-9 MCS-9 MCS-9 MCS-9 MCS-9 Table 1 describes how to select a coding scheme for new blocks. For the GPRS network, the coding scheme that is used to send the retransmitted data blocks is the same as that used to transmit the new blocks. For the EGPRS network, a lower coding scheme is adopted to send the retransmitted data blocks, and this coding scheme depends on the following three factors: the coding scheme for sending the earlier data block, coding scheme that is the same as that for sending new blocks, and link quality management mode (IR or LA). Select one-cluster coding schemes. For example, MCS3, MCS6, and MCS9 belong to one cluster. The extent of lowering is determined by IR or LA. For example, if the LA is adopted, MCS9 must be lowered to MCS3. If the IR is adopted, MCS6 is enough. In IR mode, the MS buffers part of the data of the block that is not decoded just now and then softly-combine and decode it according to the retransmitted data. The data can be decoded in this way. Therefore, it is recommended that you adopt the IR for the test. But the soft combination function of certain MSs is not good, or some MSs do not support such function at alll; in this case, it is possible that the IR function brings a negative gain. The coding scheme is selected on the basis of the following conditions: 1. Wireless environment. Table 2 shows the quality of wireless signals required by the coding schemes. Table 2 Requirements of different coding schemes for the Um interface Coding Scheme Required Receiving Level of the MS (dBm) Required C/I Ratio in TU3 Mode (dB) MCS1 ≥-102 13 MCS2 ≥-101 15 MCS3 ≥-99 16.5 MCS4 ≥-97 19 MCS5 ≥-98 18 MCS6 ≥-96 20 MCS7 ≥-93 23.5 MCS8 ≥-90.5 28.5 MCS9 ≥-86 30 2008-11-01 All rights reserved Page 16 of 38
  • 17. INTERNAL 2. Transmission quality of the G-Abis interface If out-of- synchronization occurs on the sublink, a high coding rate cannot be used. Data blocks encoded according to high coding schemes do not carry the synchronization header. When serious code slipping occurs, the network sides chooses to use a low coding scheme with the synchronization header instead. 3. Number of timeslots on the Abis interface. Table 1 describes the number of 16 kbit/s Abis interface timeslots required by the coding schemes. Table 1 Requirements of different coding schemes for the timeslots at the Abis interface EGPRS GPRS Number of Required 16 kbit/s Timeslots on the Abis Interface MCS1-MCS2 CS1-CS2 1 MCS3-MCS6 CS3-CS4 2 MCS7 3 MCS8-MCS9 4 The timeslot on the Abis interface can serve as: a statistically multiplexed signaling link (one OML for each BTS; one RSL for each TRX); a voice channel (one 16 kbit/s timeslot is required for each channel); a PDCH (several 16 kbit/s timeslots are required for each channel. One is called the main link, and other links are sublinks.) In the case that the Flex Abis function is not enabled, the voice channel and PDCH is bound to one 16 kbit/s timeslot during timeslot configuration. For the MSs that are performing the packet service, if the coding scheme should be adjusted and the Abis resources are required, apply for idle timeslots on the Abis interface. If the application is approved, adjust the coding scheme. When the coding scheme is lowered, the release of Abis resources will not be triggered. When the channel is idle, the Abis resources are released when Timer of Releasing Abis Timeslot times out. In the case that the Flex Abis function is enabled, the Abis timelot is not bound to a channel during channel configuration. An Abis interface timeslot is applied and bound when the channel is activated. That is, the original main link is also grouped into the idle Abis resource pool. Based on the principle that the CS service is preferred, a channel should be preferentially assigned to the CS service. If no Abis timeslot is available, the channel that is earlierly assigned to the packet service will be preempted. Therefore, when idle timeslots are insufficient:  Configure all the remaining timeslots as idle timeslots in the case that the Flex Abis function is disabled. Check whether the customer uses DXX devices for timeslot cross. If DXX devices are used, the number of available timeslots is not certainly the number of timeslots on the E1. In this case, ask the customer about the number of available timeslots 2008-11-01 All rights reserved Page 17 of 38
  • 18. INTERNAL  Calculate the signaling link multiplexing degree according to the traffic model. Increasing the multiplexing degree can conserve Abis interface resources  Enable the Flex Abis function to help realize sharing of the Abis interface resources Abis interface resources of Huawei products are divided into resource pools in the unit of BTS, and all cells of a BTS share the Abis resources. In addition, for cascaded BTSs, part of the Abis interface timeslot resources of the upper-level BTSs should be reserved for the lower-level BTSs. You can set the MaxAbisTSOccupied parameter to specify the maximum number of Abis interface timeslots that can be bound on a carrier. 1.6 Mechanism of Link Synchronization/Channel Synchronization Each PDCH is bound to several Abis interface timeslots. One of the timeslots is the main link, and others are sublinks. At the initial phase after the channel is converted, channel synchronization occurs. This process takes about one second. The PCU sends a synchronization frame through the main link to the BTS. The BTS returns a synchronization frame to the PCU. Thus, the differences of frame number and block number between the two frames can be calculated. Thus, the PCU sends data packets based on the advance. When a synchronization frame is sent, if no synchronization header is found, or the check fails after a synchronization frame is received, the channels cannot be synchronized. Each channel contains several Abis interface links. The synchronization should be performed separately on each link. Each subsequent frame sent on the link contains a synchronization header (the header is not carried when MCS6 or MCS7 is used). The main link also sends signaling frames in CS1, and the sublinks send idle information frames. These frames also contain synchronization headers. Therefore, certain changes to the synchronization headers are acceptable. But if the synchronization headers change once in every ten minutes, the PCU will restrict the frames in high coding scheme. If synchronization of the main link fails, synchronization of the channel fails. If synchronization of a sublink fails, the coding scheme will be adjusted. When the transmission quality on the G-Abis interface is decreased to a certain level, synchronization of the link fails. The transmission quality on the G-Abis interface is reflected by the frame error rate on the G-Abis interface. The frame error rate on the G-Abis interface is calculated through the following formula: frame error rate on the G-Abis interface = (Number of Received Check Error TRAU Frames + Number of Received Out-of-Synchronization TRAU Frames)/ (Number of Sent Valid TRAU Frames + Number of Sent Empty TRAU Frames). If the frame error rate is less than 10e-5, the link is of good quality, and the MS can transmit data steadily If the frame error rate is between 10e-5 and 10e-4, it has certain impact on the data rate the MSs in transmission state. If the FER is higher than 10e-4, the link is quite unstable and tends to be out of synchronization. In this case, an MS can hardly transmit a large amount of data. 2008-11-01 All rights reserved Page 18 of 38
  • 19. INTERNAL Possible causes of a high frame error rate or loss of synchronization on a link on the G-Abis interface are as follows: loss of synchronization of the E1 clock with the Um interface clock of the BTS and unsteady transmission quality. You can check whether the high frame error rate or loss of synchronization is caused by transmission problems by performing local or remote loopback at the BTS side. You can also connect the test tools to both ends to capture data packets for analysis. 2008-11-01 All rights reserved Page 19 of 38
  • 20. INTERNAL 2 Thoughts About Identifying Download Rate Problems As described in section 1.2, compared with the CQT, the DT involves changes to the coding scheme and cell reselection due to C/I fluctuation. Actually, the link quality control algorithm can achieve balance between coding scheme and retransmission rate, and changing the coding scheme changes the actual bandwidth on the Um interface. The impact of the slow start process when the TCP connection is just established is on downloading small-sized files is more obvious than that on large-sized files. Therefore, when the download rate is low, you should perform tests of downloading large-sized files in radio transmission environments where the C/I ratio is good, so as to check whether the transmission on each node is normal. 2.1 Downloading Large-Sized Files in CQT in Idle Hours to Identify the Problem Figure 4 shows the rate recorded in the TEMS log when you download large-sized files in idle hours in good radio transmission environment. 2008-11-01 All rights reserved Page 20 of 38
  • 21. INTERNAL Figure 4 Information shown by the TEMS log when you download large-sized files in ideal situation Dummy control block: The PCU dispatches a block every 20 ms. The sending sequence is as follows: NACK block—>VS block—>PACK block. When the PCU has no such blocks to send, the PCU sends the Dummy control block. The TEMS counts Dummy control blocks as control blocks. That is, the control blocks counted by the TEMS consist of real control message blocks and Dummy control blocks. The messages traced at the application layer of the MS shows that the delay of each block is fixed. The MS returns an ACK message after receiving every two TCP packets without packet loss or disorder. The strict test results of the download process should contain only the interaction messages with the FTP server. No interaction messages with other IP addresses should be included. Figure 5 shows the messages traced at the application layer of the MS. 2008-11-01 All rights reserved Page 21 of 38
  • 22. INTERNAL Figure 5 Messages traced at the application layer when you download large-sized files in ideal conditions The following sections specify how to identify the problems based on the messages traced at the application layer and the TEMS log files. 2.1.1 Failure to Assign or Steadily Occupy Four Channels You can use several methods to check the number of assigned channels through the TEMS. You can check the number of yellow grids shown in Figure 4. Another method is to view the value displayed in GSM data timeslot. These two methods, however, cannot show the accurate number of assigned channels. To obtain the accurate value, locate the latest Packet Downlink Assignment or Packet Timeslot reconfiguration message, as shown in Figure 6. 2008-11-01 All rights reserved Page 22 of 38
  • 23. INTERNAL Figure 6 Timeslots assigned in Packet Downlink Assignment message (on the left) and Packet Timeslot Reconfigure message (on the right) According to the description in section 1.3, for the downloading task performed in idle hours, TS 1, TS 3, and then TS 4 may be assigned. Normally, multiple downlink timeslots can be assigned to the MS. However, if a channel cannot be occupied, check the channel configuration to see whether the PDCHs, consisting of static and dynamic channels, are sufficient. If the PDCHs are insufficient, you should configure enough PDCHs on the BCCH TRX. Then, check whether loss of synchronization occurs on the channel through the alarms related to the PCU. For the external PCU, run the mt pdch show state <cell ID> all command to check the status of all PDCHs in this cell. For a built-in PCU, run the DSP PDCH command to check the channel status. The failure to occupy four channels, including preemption of the channel by the voice service, does not occur in tests in idle hours. Otherwise, the failure is caused by a channel fault. 2.1.2 Failure to Steadily Occupy a High Coding Scheme This situation is classified into the following scenarios:  Long occupation of a low coding scheme. Possible causes are as follows: fixed downlink coding scheme and inadequate Abis interface resources. If the downlink coding scheme is fixed, change the configuration to non-fixed coding scheme. If the Abis interface resources are insufficient, increase the number of available Abis interface timeslots according to the description in section 1.5.  Untimely adjustment of the coding scheme. If downward adjustment is untimely, the block error rate is high. In this case, you can solve the problem by reducing the value of BEP period. If upward adjustment is untimely, another possible cause is that the binding 2008-11-01 All rights reserved Page 23 of 38
  • 24. INTERNAL of the Abis interface timeslot takes a long time. For the test, if the initial adjustment is slow, the Downlink Default MCS Type can be set to MCS9. 2.1.3 Error Block Error block means that the MS does not receive the data block sent by the PCU. There is no standard to determine an acceptable block error rate. For different channel models, the block error rates are different in the same configuration. How to check the block error rate? As shown in Figure 4, BLER/TS(%) is the block error rate calculated by the TEMS on the basis of the total number of received blocks. To check whether a specific data block is received, view the Packet Downlink ACK/NACK message, as shown in Figure 7. Figure 7 Viewing the Receive block bitmap (URBB) in the Packet Downlink ACK/NACK message to determine the error blocks As shown in Figure 7, the analysis of the Packet Downlink ACK/NAC message in the EGPRS network by the TEMS is incorrect. For the GPRS network, the analysis of NACKed Block numbers by the TEMS is correct. The Receive block bitmap (URBB) shows the error blocks. Why these blocks are not received? If the data block loss is not caused by the Um interface factors or G-Abis interface factors, check whether the failure of receiving blocks is caused by the receiving of the system information from the neighboring cell before the receiving of the Packet Downlink ACK/NACK message. The possible causes of the failure to receive data blocks are as follows: 1. A high bit error rate occurs on the Um interface, and the error bits are sequential. Consequently, data block loss occurs. The test is performed in good radio transmission environment. Therefore, the high block error rate is not caused by the Um interface 2008-11-01 All rights reserved Page 24 of 38
  • 25. INTERNAL factors. In addition, in the DT, if the block error rate is high, you can modify the values of BEP Period, BEP Filter and BLER Filter. 2. The data block loss is caused by the error frame or out-of-synchronization frame on the G-Abis interface. 3. The MS is busy and thus cannot receive data blocks. As stipulated by the protocol, the MS must decode the data on the BCCH on the TRX serving one of the six neighboring cells with the strongest signal level within 30 seconds. When the signal fluctuation occurs and a large number of neighboring cells is configured, the MS may frequently resolve the system information of the neighboring cells. In this case, you need to cut down the number of neighboring cells to avoid unnecessary neighboring cells. 2.1.4 High Ratio of Control Blocks In Huawei products, the MS is assigned with only one control channel. The control channel is bidirectional. Thus, the uplink and downlink data is multiplexed onto the same control channel. In this way, you can locate the control channel. As shown in Figure 4, TS 4 is the control channel. That is, real control messages are sent only through TS 4. Before the uplink ACK optimization scheme in uplink extension mode is incorporated, the proportion of the control messages is 17% to 21%. After the uplink ACK optimization scheme in uplink extension mode is incorporated, the proportion of the control messages is about 2%. If a non- control channel transmits control blocks, which are actually Dummy blocks, or the proportion of control blocks on the control channel is large, it indicates that the PCU has no data to transmit, so the PCU transmits the Dummy control blocks instead. The sequence in which the PCU sends data is as follows: NACK blocks (the Packet Downlink ACK/NACK message through which the MS confirms the error blocks); VS blocks (new blocks, the RLC fragments the LLC PDUs according to the bytes born by different coding schemes); PACK blocks (blocks that are not confirmed by the MS). If the preceding data blocks are not available, the PCU sends Dummy control blocks. Note that VS blocks are sent only when the PCU sending window is not full. The possible causes of the high ratio of control blocks may be as follows: 1. The subscribed peak rate is not high. As shown in the GSM PDP context in Figure 8, the peak rate is 128000 octets/s =128000 x 8/1024 kbit/s = 1000 kbit/s, which is greater than the theoretical rate limit. 2008-11-01 All rights reserved Page 25 of 38
  • 26. INTERNAL Figure 8 PDP context of the MS 2. The RLC adopts the acknowledged mode. In this case, the I-frame mechanism is used for data sending. The next frame is sent only when the peer end acknowledges the receiving of the previous frame. In acknowledged LLC mode, connection at the LLC layer should be established and released. This inevitably increases the signaling traffic at the LLC layer. In a word, the acknowledged LLC mode greatly affects the download rate. You can check the operation mode of the LLC layer through the PDP context of the MS. If the acknowledged LLC mode is used, you need to change the mode to the unacknowledged mode at the SGSN side. The subscription information of the SIM card also needs to be modified. 3. The data sending window stalls. This problem usually occurs in GPRS networks, because GRPS networks support only a window with 64 data blocks. Regardless of the cause of error blocks, the RRBP delay of Huawei products is about 200 ms. When an MS reports the receiving of error blocks, 200 ms already passed. If the MS occupies four timeslots at this time, the PCU already sends out 200 ms/(20 ms/block) x 4 = 40 blocks, which easily cause the window to stall. Two methods can be used to identify this problem. You can increase the RRBP frequency to see whether the error block rate is reduced. Or you can measure the amount of received data on the Gb interface in a certain period to check whether the amount of received data is larger than the data sent by the PCU. 4. Improper flow control on the Gb interface. Check the Flow Control MS message traced on the Gb interface to see whether the reported value is smaller than the data sending rate of the PCU. If the reported value is smaller than the data sending rate of the PCU, it means that the flow control is improper. The flow control data on the Gb interface is shown as follows. 2008-11-01 All rights reserved Page 26 of 38
  • 27. INTERNAL Figure 9 MS flow control data reported by the PCU to the SGSN Why flow control is performed on the Gb interface? A GPRS network uses a long narrow transmission channel. The sever keeps sending packets till the window of the server is stalled. The sent packets must be buffered in the GPRS network, but the memory size of the PCU is limited and cannot buffer all the packets. In this case, the SGSN needs to share part of the packet load while ensuring that the PCU has enough data to send. Therefore, the protocol defines that the PCU sends flow control parameters, including the parameters specifying the buffer size and data sending rate, to the SGSN to control the data flow. The SGSN determines each packet to be sent according to the reported flow control parameters. Assume that the PCU sends packets according to the reported rate. That is, the data in the buffer decreases according to the rate. This amount of data decreased in the PCU buffer in the interval between two packets sent by the SGSN can be calculated. If the SGSN sends the packet to the PCU, the amount of the packet is added to the amount of data in the PCU buffer. If the amount of data exceeds the buffer size of the PCU, the SGSN does not send the packet. The mechanism of flow control for Huawei products is as follows: A flow control message is reported when the TC timer expires. If the amount of the data in the buffer is larger than 90% or lower than 10%, a flow control message is also reported. If the SGSN does not respond to the flow control message, the flow control message will be re-sent after the FC timer expires. 5. Insufficient bandwidth on the Gb interface When the SGSN is connected to the PCU in the FR transmission mode, the load is shared by multiple NSVCs. Each NSVC is carried on the BC. The BC consists of multiple 64 kbit/s timeslots on the E1 link. Thus, the physical bandwidth can be calculated, that is, number of the timeslots x 64 kbit/s. It is recommended that you calculate the Gb bandwidth by measuring the Downlink data kbytes sent to FR per NSVC traffic statistics item at the SGSN side for five minutes. Calculate the value of Downlink data kbytes sent to FR per NSVC x 8/(5 x 60) and compare the calculated value with the actual bandwidth. If the calculated bandwidth does not exceed 70% of the actual bandwidth, the bandwidth is sufficient. Otherwise, the bandwidth is not sufficient. 6. Packet loss causes the server to enter the congestion control state. Locate the node where packet loss occurs. You can locate the node by capturing packets at each interface. If TCP packets are lost in the transmission link or at an unacknowledged interface, for example, the Gb interface, you must analyze packet loss by capturing packets through the Ethereal. If a certain packet is lost at the MS side, you can locate the node where the packet is lost by tracing the TCP packet sequence number, as shown in Figure 10: 2008-11-01 All rights reserved Page 27 of 38
  • 28. INTERNAL Figure 10 Application layer data traced at the MS side Figure 10 shows that the packet before packet 92064556 is lost. The size of each TCP packet is 1368 bytes, that is, 558 in hexadecimal. Thus, the sequence number of the lost packet is 92064556 – 558 = 92063FFE. At the GGSN side, the Gi interface and Gn interface can be mapped to mirroring ports. Trace the data of a single user on the SGSN. The traced data can be converted into an Ethereal packet capture file. Lost packets on these interfaces can be located through the TCP sequence number. For packet loss on the Gb interface, you can determine whether packet loss occurs based on the messages traced on the Gb interface at the PCU side and whether the Nu is continuous. Nu indicates the sequence number of an NS-PDU. Resolve the TCP packet header through the PCU. If one or multiple NS PDUs of the TCP packet are lost, the entire TCP packet is discarded. The possible causes of packet loss on the Gb interface are as follows: interface board, transmission link, frame check mode, and FR congestion control parameters if the intermediate network adopts the FR transmission mode. The frame check mode at the SGSN side must be the same as that at the PCU side. As for the intermediate network exists between the SGSN and the PCU, it is recommended that frame check be disabled. The congestion control parameters of the FR include Bc, Be and CIR. In the period of Tc = Bc/CIR, if the data transmission rate is higher than Bc but lower than Bc + Be, the packet may be discarded during transmission; if the data transmission rate exceeds Bc + Be, the packet is certainly discarded. In addition, even if the transmission rate does not exceed Bc, the transmission network discards packets according to certain rules if the bandwidth of the FR transmission network is not insufficient. The possible causes for the PCU to discard an LLC PDU are as follows:  The packet stays in the PCU buffer for more than 30 seconds. The PCU determines that the server will retransmit the packet due to RTO timeout if the packet is not sent after such a period. This event is of extremely low probability.  An inter-PCU cell reselection occurs. This is called flush LL packet loss. The packet loss caused by the above two reasons can be reflected by the following parameters on the PCU side: Number of Downlink LLC PDUs Discarded due to Timeout and 2008-11-01 All rights reserved Page 28 of 38
  • 29. INTERNAL Number of Downlink LLC PDUs Discarded due to FLUSH (actually, it indicates the number of lost packets). Packet loss due to the G-Abis interface is of low probability. If error bits exist on the G-Abis interface and the BTS does not detect the error bits and sends the data over the Um interface, the data is lost as it fails to be assembled into an LLC PDU. The LLC layer usually uses unacknowledged mode, which may also lead to packet loss at the application layer. Only unacknowledged ports lead to packet loss at the application layer. Even if packet loss occurs on the Um interface or G-Abis interface, the RLC layer still sends the packet to the MS when the RLC uses the acknowledged mode. Congestion control: The congestion control mechanism includes slow start and congestion prevention. The implementation method is that the TCP layer of the sending end maintains a congestion window and slow start threshold. The congestion window is initially set with one data packet. In certain TCP applications, this window is set with multiple packets, but the size cannot exceed 4380 bytes. The sum of the congestion window threshold and maximum sequence number of an acknowledged data packet is the upper limit of the sequence number of the packet sent through the TCP. When the congestion window is lower than the slow start threshold, the congestion window grows exponentially. When the congestion window is greater than slow start threshold, the congestion window grows linearly each time a packet is acknowledged. When packet loss occurs, the threshold of slow start is reduced by half. When data is re-sent due to timeout, the threshold of slow start is reduced by half, and the congestion window is set with one data packet. 7. Weak transmission quality on the uplink leads to slow return of the TCP ACK message. The server sends data only after receiving the ACK message. As a result, data transmission is slow. The following situations may lead to the weak transmission quality on the uplink: − Improper adjustment of the uplink coding scheme. Currently, the uplink coding scheme is adjusted according to downlink coding scheme. If interference exists on the uplink or the uplink level is weak, improper adjustment of the coding scheme may lead to failure of data transmission on the uplink. The uplink level can be checked through the uplink and downlink balance traffic statistical item. The uplink interference can be checked through the Analyzed Measurement of Interference Band traffic statistical item. In the case that the uplink coding scheme is adjusted according to downlink coding scheme, enter the super user mode, choose Configure BSC Attributes > Software Parameters > Support EGPRS uplink MCS Dynamic Adjust, and check whether the Support EGPRS uplink MCS Dynamic Adjust parameter is set to Dl Ack with Downlink Signal Quality. If the value of Support EGPRS uplink MCS Dynamic Adjust is the same as the downlink signal quality in the DL ACK message, you can lower the uplink coding scheme by three classes by entering the super user mode, choosing Configure BSC Attributes > Software Parameter > DSP Control Table 2, and setting bit 5 to 1. − High RRBP frequency. Enter the super user mode and choose Configure BSC Attributes > Software Parameters > RRBP Frequency for EGPRS Downlink TBF(Blocks). The default value of RRBP Frequency for EGPRS downlink TBF(Blocks) is 20 . It is recommended that you set the parameter to a value larger than 12. If the frequency is high, the MS cannot transmit uplink data blocks. 8. Improper settings of TCP parameters on the server or test PC. If the TCP window is set to a small value, the TCP window easily gets congested. If the MSS is set to a small value, the utilization rate is low. If the MSS is set to a large value, and the MTU on the intermediate network is small, the IP packet is fragmented during transmission. The parameter settings are described in section 4.2. 2008-11-01 All rights reserved Page 29 of 38
  • 30. INTERNAL 2.1.5 Abnormal Release of TBF Abnormal release of TBF does not necessarily lead to packet loss, because the PCU saves the data that is not sent or that is sent but not acknowledged by the MS in 30 seconds after the abnormal release of TBF. The MS initiates a TBF reestablishment request shortly. The TLLI does not change at the time. The PCU can obtain the context of the MS according to the TLLI and then sends the remainder to the MS. How to determine whether a TBF is abnormally released? According to the TBF release flow, when a downlink TBF is normally released, the FAI in the Packet Downlink ACK/NACK message is set to 1. When an uplink TBF is normally released, the FAI in the Packet Uplink ACK/NACK message is set to 1. If the FAI is not set to 1, it indicates that the TBF is abnormally released. If the network side sends a Packet TBF release message to release the TBF, you can infer that the TBF is abnormally released. If the cause value indicates that the release is due to a normal event, you can infer that the abnormal release is caused by N3105 timeout. Figure 11 Checking whether TBF is abnormally released through the ACK message The impact of abnormal TBF release on the transmission rate is as follows: During the release, data cannot be transmitted. The possible causes of abnormal TBF release are as follows: 1. Timeout of N3101 and N3103 2. Timeout of N3105 3. Preemption of the control channel 4. Cell reselection 5. Internal processing faults 2.1.6 High Transmission Rate at the RLC Layer But Low Transmission Rate at the Application Layer This is usually caused by the incorrect operation of the tester. During the test, the tester must 2008-11-01 All rights reserved Page 30 of 38
  • 31. INTERNAL disable applications or services (for example, automatic update) that automatically initiate requests to accesses the Internet. Otherwise, when these applications or services attempt to access the Internet, the transmission rate at the application layer is affected. Sometimes, it is difficult to identify the applications or services that automatically initiate requests to access the Internet. You can check the data captured by the Ethereal to see whether all the data involves interaction with the IP address of the server. If other IP addresses exist, enter the IP address in the Internet Explorer to identify the network. 2.2 Competition of the Download Rate through CQT After the faults in downloading large-sized files in idle hours in the CQT are rectified, based on the test method, you can determine whether to modify the settings of certain parameters to obtain better performance for competition. In the scenario of downloading small-sized files multiple times, the following factors may affect the downloading process: initial slow start at the TCP layer, whether the TBF is released during the period from the end of a download process to the start of the next download process, and detection of the service type during download. In this case, check whether the release delay of the uplink/downlink TBF and the extended uplink TBF functions are enabled. It is recommended that you set bit 1 of DSP Control Table 2 to ON. Assume that you are to download files in busy hours. If the multiplexing degree after network swapping is higher than that before network swapping, or the average number of occupied channels is smaller than that before network swapping, check the parameters related to channel allocation. Increasing the number of PDCHs can solve this problem. 2.3 Comparision between DT downloading rates in busy and idle hours Compared with the CQT, the DT involves radio transmission environment fluctuation and cell reselection. Cell reselection results in TBF release in the original cell and TBF re- establishment in the new cell. 1. Radio transmission environment fluctuation: Compare the C/I ratio before and after the network swapping. Figure 12 shows the comparison of C/I Worst before and after the network swapping. 2008-11-01 All rights reserved Page 31 of 38
  • 32. INTERNAL Figure 12 Comparison of the radio transmission environment before(upper) and after (lower) the network swapping Before the network swapping, the cells with the C/I lower than 30 account for 60% of all the cells. The cells with the C/I greater than 30 account for 40% of all the cells. After the network swapping, the cells with the C/I lower than 30 account for 80% of all the cells, and the cells with the C/I larger than 30 account for 20%. This indicates that the C/I falls after the network swapping. To deal with C/I decrease, you must check every cell along the testing route to locate the cell where the C/I worsens, and then eliminate the interference. The fluctuation of the radio transmission environment is usually unpredictable, but predictable in the scenarios such as the high-speed railway. In such scenarios, because the C/I changes fast, you must properly reduce the value of BEP Period. In normal scenarios, the recommended value is 5. In high-speed scenarios, the recommended value is 4. 2. Cell reselection Problems involved in cell reselection are as follows: disconnection delay caused by cell reselection, rate increase after cell reselection, service type detection during access to the new cell after cell reselection, packet loss during cell reselection, and number of cell reselections during the DT. − For disconnection delay caused by cell reselection, it is recommended that you enable the NACC and Packet SI status functions to reduce the delay caused by cell reselection. − Rate increase after cell reselection. The application layer enters the congestion control state due to disconnection after reselection. Congestion control slows down data packet transmission. The low transmission rate in turn causes insufficient data at the application layer. However, disconnection during reselection is inevitable. To reduce the risks of this situation, a good transmission quality before reselection is 2008-11-01 All rights reserved Page 32 of 38
  • 33. INTERNAL required. That is, the low data transmission rate before reselection and disconnection during reselection lead to severe congestion at the application layer. Therefore, the wireless coverage and reselection parameters must be optimized to quicken the cell reselection to a better radio transmission environment from a degrading radio transmission environment. − Service type detection after reselection: This can be checked by the service type continuation function. The idea of this function is as follows: If an MS leaves the original cell while performing the download service, the service type remains unchanged after it enters the new cell. The MS is assigned with 4+1 channels in the new cell. − Whether data loss occurs during reselection. Inter-PCU cell reselection may lead to data loss. Therefore, for comparison DTs, try to avoid inter-PCU cell reselection along the DT route. − Number of reselections during DT. To minimize the number of cell reselection times, you should optimize the reselection parameters and neighboring cell configuration. For the cells in which the MS stays a short time, it is recommended that you avoid arranging the testing route along such cells, especially those cells where ping-pong reselections occur. Configure the neighboring cells that are missing. As shown in Figure 13, the neighboring relations and reselection parameters need to be optimized. For details, see section 4.1. Figure 13 Frequent reselections caused by improper neighboring cell configuration and reselection parameter settings 2008-11-01 All rights reserved Page 33 of 38
  • 34. INTERNAL 3 Conclusion 3.1 Problem Identification When rate problems are identified during test, you are advised to download large-sized files in good radio transmission environment in idle hours to identify the problem. Then, compare the current test method with others, find out their differences, and check if the differences can be optimized. 1. Download large-sized files in places where the C/I is good and in idle hours. The test result should be similar to that shown in Figure 4. Otherwise, you must identify the problem. − The occupied channels are insufficient. It takes the MS about 4.5 seconds to occupied four downlink channels. − Symptom: failure to assign more channels − Cause 1: Check whether the channels are sufficient by examining the channel configuration. − Cause 2: Check the multislot capability of the MS. You can check the multislot capability by checking the Packet resource request, 11-bit access request, attach request, and the downlink data on the Gb interface when Huawei SGSN is connected to Huawei PCU. − Cause 3: Loss of synchronization occurs on the channel. You can check whether loss of synchronization occurs by checking the relevant alarms. − Cause 4: Abis interface resources or PCIC resources (in external PCU mode) are insufficient. − Symptom 2: failure to occupy multiple channels steadily − Cause 1: The channels are pre-empted by voice services. − Cause 2: Loss of synchronization occurs on the channel. You can check whether loss of synchronization occurs by checking the relevant alarms. − Low coding scheme 2008-11-01 All rights reserved Page 34 of 38
  • 35. INTERNAL − Symptom 1: long time occupation of a low coding scheme − Cause: Inadequate Abis interface timeslots − Solution 1: When the Flex Abis function is not enabled on the Abis interface, set all the timeslots that have not been configured to idle. − Solution 2: Increase the signaling link multiplexing degree to increase the transmission rate on the Abis interface. − Solution 3: Enable the Flex Abis function. − Solution 4: Increase the transmission rate − Symptom 2: fluctuation of the coding scheme − Cause: Bit error on the G-Abis interface. You can check the bit error through the G-Abis Interface Measurement traffic statistical item. − Cause 1: A transmission fault occurs. You can check for transmission faults through local loop and remote loop at the TMU side. − Cause 2: A fault occurs on the interface board. − Block error − Cause 1: Bit error on the Um interface. You can check whether the C/I ratio on the air interface is deteriorated. − Cause 2: Error frames and out-of-synchronization frames on the G-Abis interface. You can identify this cause through the G-Abis Interface Measurement traffic statistical item. − The MS is busy with other events. For instance, in the case that the number of neighboring cells is large, and signals fluctuate greatly, if changes take place in the six neighboring cells with the strongest signals, the MS decodes the system messages at the time when it receives data blocks. As a result, blocks are lost. − High proportion of control blocks − Cause 1: The MS subscription capacity is weak. You can check the MS PDP context to identify the subscription capacity. − Cause 2: The acknowledged mode is used at the LLC layer. You can check the MS PDP context to identify the mode. − Cause 3: The sending window stalls. This situation occurs only in GPRS networks. You can identify this cause in the following ways: (1) Check whether the amount of data sent on the Gb interface is larger than that sent on the Um interface during a certain period. (2) Increase the RRBP frequency and check whether the situation is eased. − Cause 4: The performance of server is poor. You can check the server performance by tracing the messages on the server. − Cause 5: The flow control performed on the Gb interface is improper. You can trace the data on the Gb interface to check whether the flow control rate reported by the PCU is higher than the sending rate on the Um interface. − Cause 6: Packet loss on the Gb interface and upper-layer NEs. You can use the Ethereal to check packet loss at the MS side. Packet loss exists (packet loss usually occurs on the unacknowledged interface) Then, capture the packets on 2008-11-01 All rights reserved Page 35 of 38
  • 36. INTERNAL each interface to check on which interface packet loss occurs. − Abnormal release of TBF. By checking whether the value of the FAI cell in Uplink ACK is 1, you can determine whether the uplink TBF is normally released. By checking whether the value of the FAI cell in Downlink ACK is 1, you can determine whether the downlink TBF is normally released. In addition, if the TBF release is caused by the PACKET TBF RELEASE message sent by the network side, then the TBF release is abnormal. If the release cause cell indicates normal release, the abnormal release of the TBF is caused by N3105 timeout. − The following are some possible causes of the abnormal release of TBF: − N3101 and N3103 timeout as the uplink wireless conditions worsen − Control channel being preempted − Windows failing to slide for several times − High rate at the RLC layer but low rate at the application layer − Cause: The MS performs other PS services during the test. You can use the Ethereal to check for interaction data with other IP addresses, and identify the applications or services that automatically initiate requests to access the Internet according to the IP addresses. Then, disable these applications or services. 2. Compared with the CQT in idle hours, the CQT on downloading small-sized files in busy hours may lead to lack of resources, including Abis interface resources, channel resources, and PCIC resources for (in external PCU mode), and the channel may be multiplexed by certain number of MSs. Compared with the CQT on downloading large- sized files, the CQT on downloading small-sized files may lead to lack of data at application layer at the initial phase of data sending. This is because of the slow start process during data sending at the TCP layer. In addition, timeslots are re-assigned after the service type is identified. This also lowers the transmission rate at the beginning of download. Other problems are as follows: − Symptom 1: Compared with the test before the network swapping, the average number of occupied channels is small. The causes of failure to assign channels are as follows: no available channels, no Abis resources, and no PCICs (in external PCU mode). Check the channel occupation during the test to check whether the channels are sufficient, and check the traffic statistics and resource budget to see other resources. If channels are insufficient, lower the threshold of dynamic channel conversion. − Symptom 2: Compared with the test before the network swapping, the multiplexing degree is high. In this case, you can lower the threshold of dynamic channel conversion and the channel multiplexing threshold. 3. For DTs in both idle and busy hours, even if the problems described in 1 and 2, the adjustment of coding rate caused by the changes of radio transmission environment, disconnection for a period after cell reselection, and rate increase after cell selection are all solved, the following problems may exist: − Symptom 1: The C/I ratio after the network swapping is worse than that before the network swapping (based on the C/I WORST message exported from the TEMS). In this case, network optimization is required. − Symptom 2:Cell reselection occurs frequently, especially in cells where the MS stays a very short time. In this case, you must optimize the neighboring cells and 2008-11-01 All rights reserved Page 36 of 38
  • 37. INTERNAL reselection parameters. − Symptom 3: Slow rate increase after cell reselection. This is due to the low rate before cell reselection, and disconnection afterwards, which causes the server to enter the congestion control state. In this case, you must optimize the reselection parameters. 3.2 Routine Optimization In routine optimization, you must consider factors that affect the transmission rate. The key tasks in routine optimization are as follows: 1. Optimize the wireless conditions and neighboring cells in the same way as optimization in the CS domain. 2. Ensure and assign resources. Identify the bottleneck of resources. The bottleneck must be the most precious resources, which in most cases, are wireless resources. In this case, you must guarantee transmission resources on interfaces, such as the Gb interface. In addition, CS and PS services share the wireless and Abis resources now. You must optimize assignment of these resources and the management parameters of these resources to maximize the bandwidth for PS services while ensuring the bandwidth for CS services. 3. Settle the problems that are common on the entire network, such as high bit error rate at the Gabis interface, and packet loss at the GB interface. Section 4.1describes the common parameters that affect download rate. For the swapped network, you must obtain the following information:  Network structure. For example, whether DXX devices are used on the Abis interface for timeslot cross and whether the Gb interface uses the FR transmission mode.  Resource allocation, including the number of timeslots assigned to the Abis interface for each BTS by the original network, relevant resource management parameters, number of timeslots assigned to the Gb interface, and FR congestion control parameters when the Gb interface uses the FR transmission.  Traffic statistics on the Gb interface and Um interface of each PCU on the original network. The statistics are used for traffic comparison.  Test log. After the network swapping, make routine observation on the following traffic statistics and alarms:  Check whether an alarm related to out-of-synchronization channel is generated.  Check whether an alarm related to out-of-synchronization with the remote end is generated on the Gb interface.  Check the G-Abis Interface Measurement, TBF establishment success rate, call drop rate, and PS Channel Measurement. 2008-11-01 All rights reserved Page 37 of 38
  • 38. INTERNAL 4 Appendices 4.1 Appendix 1: Parameter Description Set t i ngs of Downl oad- Rel at ed Par amet er s. xl s 4.2 Appendix 2: Optimization Manual for Parameters on Server and Test PC GPRSEGPRS Test FTP Par amet er Opt i mi zat i on Manual V1. 0. doc 2008-11-01 All rights reserved Page 38 of 38