High Sdcch Assignment Failure Analysis

1 When SDCCH are required?
The requests for the SDCCH in GSM are mainly made in the following two procedures:
A. The procedure for requesting the SDCCH for services such as point-to-point calls, location updating (only SDCCH), call re-establishment, or short messages;
B. The procedure of SDCCH handovers (including both the intra-BSC and inter-BSC handovers)

2 Principles of SDCCH Congestion Rate:
SDCCH congestion rate is one of the important counters that indicate accessibility in circuit service. This counter provides the ratio of failed SDCCH seizures due to busy SDCCH to the total requests for the SDCCH. SDCCH congestion rate indicates the failed requests for the SDCCH for various reasons. SDCCH congestion rate also indicates the status of the SDCCH resource utilization.

3 Definition & Formula of SDCCH Congestion :
The definition and formula of SDCCH congestion rate are as follows:
• Definition:
Ratio of failed SDCCH seizures due to busy SDCCH to the total requests for the SDCCH
• Formula:
SDCCH congestion rate = Failed SDCCH seizures due to busy SDCCH/Total requests for the SDCCH x 100%

4 Factors of SDCCH Congestion Rate in GSM :

4.1 Congestion Caused by Faults on Equipment or Transmission :
The faults on BTS, BSC, and Abis interface, such as broken LAPD link, cause the SDCCH congestion. The alarm "Excessive Loss of E1/T1 Signals in an Hour" also causes the SDCCH congestion.

4.2 Congestion Caused by Insufficient Signaling Resources :
The heavy traffic and burst traffic cause the SDCCH congestion. Proper setting of the number of SDCCHs and TCHs, and the SDCCH dynamic conversion function can relieve the congestion.

4.3 Congestion Caused by Improper Data Configuration :
The SDCCH congestion relates to the relevant parameters of the BSC such as SDCCH Availability, LAC and T3101 (the timer used in the immediate assignment procedure), and T3212 (the timer used for periodic updating). If these parameters are set correctly, the SDCCH congestion can be relieved. In addition, if the assignment procedure is set to Late Assignment, the time of the SDCCH being occupied increases, which may lead to congestion.

4.4 Congestion Caused by Interference :
Interference on the Um interface also causes congestion. For example, if the main BCCH in the serving cell and the TCH in the neighboring cell share the same TRX frequency and BTS BSIC, the handover access on this TCH may be mistaken as random access. As a result, the SDCCH is abnormally allocated and congestion occurs. The excessive receive sensibility can also make the interference signal mistaken as access signal, which leads to congestion.

5 Analysis Procedure of SDCCH Congestion Rate :
SDCCH congestion rate is classified into BSC-level and cell-level according to the statistic object of the counter. The SDCCH congestion rate of a BSC is composed of the SDCCH congestion rate of a series of cells. Procedure of SDCCH congestion rate analysis is as shown below:

SD Optimization.jpg

First, determine the level of the SDCCH congestion rate. If the congestion occurs in a large area, check the traffic volume and the setting of T3212. Then, calculate the SDCCH capacity to check whether it meets the system demand and whether the faults exist on system-level equipment or transmission. If congestion occurs in only a few cells, check the hardware, data configuration, and Um interface quality of the cell.

Below given is the analysis & optimization procedure for SDCCH Congestion Rate :

1 Checking Channel Configuration :
Query the traffic statistics to see whether the traffic volume on the SDCCH and TCH are higher than the normal value. If the congestion is caused by excessive traffic on the SDCCH, for multi-TRX BTSs, enable SDCCH dynamic allocation function or increase the number of SDCCH channels to relieve the congestion. For cells with only one TRX or without extra channels, enable the very early immediate assignment procedure. In the very early immediate assignment procedure, the TCH channel is immediately assigned when the SDCCH has no available resource during the processing of access request. In this case, a TCH can be used as only one SDCCH, which is a waste of the TCH resources.

All the services such as location updating, MS attach/detach, call setup, short messages are performed on the Schiff a certain service causes burst traffic, the SDCCH congestion may occur.
In this case, check whether the abnormity of Channel Requests, Successful Immediate Assignments or Successful SDCCH Seizures is caused by the service of location updating, MOC, paging, or short message. Note that you should check the history traffic statistics when querying these traffic counters to see whether they fluctuate during a certain period.

If the SDCCH congestion is caused by certain burst services such as location updating and short messages, use the following methods according to the specific configuration of the BTSs.

a. For the BTS with multiple TRXs, enable the SDCCH dynamic allocation function or increase the number of SDCCH channels.

b. For the cell with only one TRX or without extra channels, enable the immediate assignment procedure. In the immediate assignment procedure, the TCH is immediately assigned when the SDCCH has no available resource during the processing of access request.

c. Increase the number of TRXs.
It is difficult to avoid the SDCCH congestion caused by network burst services; however, you can take some relief measures such as increasing the number of SDCCHs or enabling the SDCCH dynamic conversion function.

Updated 12-31-2011 at 12:55 PM bytelecominfo


GSM radio frequency optimization (GSM RF optimisation) is the optimization of GSM radio frequencies. GSM network consist of different cells and each cell transmit signals to and receive signals from the mobile station, for proper working of base station many parameters are defined before functioning the base station such as the coverage area of a cell depends on different factors including the transmitting power of the base station, obstructing buildings in cells, height of the base station and location of base station. Radio Frequency Optimization is a process through which different soft (Cell Reselect Offset, BTS power) and hard (e.g. Electrical Tilt, Mechanical Tilt, Azimuth etc.) parameters of the Base transceiver stations are changed in order to improve the coverage area and improve quality of signal. Besides that there are various key performance indicators which have to be constantly monitored and necessary changes proposed in order to keep KPIs in agreed limits with the mobile operator.


Optimization is an important step in the life cycle of a wireless network. Drive testing is the first step in the process, with the goal of collecting measurement data as a function of location. Once the data has been collected over the desired RF coverage area, it is output to post-processing software. Engineers can use the collection and post-processing software to identify the causes of RF coverage or interference problems and determine how these problems can be solved. When the problems, causes and solutions have been identified, steps are performed to solve the problems. Network statistics are also an important step in analysis and troubleshooting of RF issues. Every node (BTS, BSC, MSC) has its own counters some of which are incremented/decremented on occurrence of different events e.g. a dropped call due to low signal strength. These statistics are analysed using different graphs and reports and when KPI from the statistics exceed the limit, extensive analysis is carried out to identify and troubleshoot the problem.

Technical details[edit]

GSM call flow[edit]

There are various control channels involved in setting up of a voice call in a GSM network. On Broadcast Channels system information and various parameters along with synchronization and frequency correction information is transmitted. Common Control Channels are used for informing the mobile or the GSM network about a service(voice, data, SMS) initiation and Dedicated Control Channels are used for call setup, authentication, location updating and SMS.

A mobile is informed on a paging channel(PCH) that it has a call or SMS, to which the mobile station responds with a Random Access Channel (RACH) request. The mobile station is notified on an Access Grant Channel (AGCH) that it may tune to a specific Stand-alone dedicated control channel (SDCCH) which is called Immediate Assignment. The user is authenticated and ciphering commands are received on this channel. After successful authentication the mobile station is requested to tune to an assigned traffic channel(TCH). This process is called TCH assignment. Then the user starts to move from one cell to another and the process of smooth transitioning of call from one cell to the other is called a handover. While on the SDCCH or TCH a call may get dropped which is accounted to SDCCH drop or TCH drop respectively.

Key performance indicators[edit]

Paging success rate[edit]

Paging Success by far is the most complex KPI to deal with as the process of paging touches almost all the nodes in GSM system and is influenced by performance of each of them. That’s the reason why this write up on paging looks too interwoven and cross refers to too many things. But the plus point with paging is by the time paging success rate in a network gets improved; almost all the other KPIs too stand improved. In response to an incoming call, the MSC initiates the paging process by broadcasting a "paging request" message on the paging sub channel (IMSI or TMSI of the MS and its Paging Group) and starts timer T3113. A "paging message" consists of the mobile identity (IMSI or TMSI) of the MS being paged and its "paging group number". A Paging Request Message may include more than one MS identification. The maximum number of paged MS per message is 4 when using "TMSI" for identification of the MS (maximum number of paged MS per message is 2 when using IMSI). The BSC receives this page and processes the paging request and schedules it for transmission on the PCH at appropriate time. The MS on its part will analyse the paging messages (and immediate assignment messages) sent on the paging sub channel corresponding to its paging group. Upon receipt of a "paging request" message, MS will initiate within 0.7s an immediate assignment procedure. Upon receipt of a page at the MS, the MS responds by transmitting a channel request on the RACH. BSS in response to the received "channel request", will process it and immediately assign the MS a SDCCH (immediate assignment / assignment reject; done over AGCH). MS Paging response- After receiving the immediate assignment command, MS switches to the assigned "SDCCH" and transmits a "Paging Response". The establishment of the main signalling link is then initiated with information field containing the "PAGING RESPONSE" message and the "paging response" is sent to the MSC. Upon receipt of the "Paging Response" MSC stops the timer T3113. If the timer T3113 expires and a "Paging Response"message has not been received, the MSC may repeat the "Paging Request" message and start T3113 all over again. The number of successive paging attempt is a network dependent choice.

One control channel Multi Frame is made of 51 TDMA frames with a time duration of 235 ms. Each 51 TDMA frame Multi Frame will have 9 Common Control Channel (CCCH) blocks. Each of these 9 CCCH block is made of 4 TDMA frames. Each CCCH block can carry Paging Messages for 2 MS if IMSI based paging is used or 4 MS if TMSI based paging is used. Thus the paging capacity for one 51 TDMA frame Multi Frame will be 9(number of CCCH blocks available per Multi Frame) * 4 (when TMSI based paging is used) = 36 mobiles per 235 ms or 9*2 = 18 mobiles per 235 ms when IMSI based paging is used. Thus the paging capacity of a cell is 153 mobiles per second when TMSI based paging are used and 68 mobiles per second when IMSI based paging are used. This means we can improve the "paging bandwidth" for a cell (if there are too many "paging discards at the cell level") by using TMSI based paging rather than IMSI based (at the expense of increased processor load at the BSC and MSC). When the rate of "paging load" at the BTS becomes higher than what the BTS is able to handle (paging capacity of BTS), BTS will start discarding pages (check for high "page discard" stats at the cell level). Once an MS deciphers its paging group, in an idle mode, it will tune in and check for an incoming page only during broadcast time for its paging group (so further the paging groups are places across multiple 51 frame multi frames, less frequently it will tune in to check for an incoming page and longer will be its battery life.

Immediate assignment success rate[edit]

This section is empty.You can help by adding to it.(March 2017)

Random access success rate[edit]

Random Access Channel (RACH) is used by the MS on the "uplink" to request for allocation of an SDCCH. This request from the MS on the uplink could either be as a page response (MS being paged by the BSS in response to an incoming call) or due to user trying to access the network to establish a call. Availability of SDCCH at the BTS will not have any impact on the Random Access Success. In the transceiver, the timeslot handler in charge of the RACH channel listens for access burst from mobiles (on the time-slot that transmits BCCH). These bursts contain a check sequence (8 bits) that is used to determine if the message is valid.

SDCCH drop rate[edit]

This section is empty.You can help by adding to it.(March 2017)

TCH assignment success rate[edit]

TCH assignment failure is a phenomenon where the MS is not able to use the TCH which is assigned to it for voice call.This could happen due to uplink /downlink interference, faulty radio or faulty antenna system.

Call drop rate[edit]

TCH drop (or a dropped call) could be broadly classified into 3 sub classes:

  • Degradation of the links (Uplink and Downlink): either degradation of Signal Strength which falls near or lower than the sensitivity of the base station (around to -110 dBm) or that of the mobile (around -104dBm) or degradation of quality of the links (Uplink and Downlink) often due to interference.
  • Excess TA (TA>63 or excess path imbalance due to high TA).
  • Other Reasons.

Handover success rate[edit]

Handover in BSS system is controlled an algorithm in the BSC. This algorithm operates on the basis of Measurement Reports (MR) sent in by the MS on SACCH. The inputs that the BSC uses for making a handover decision, from the received MRs from the MS is the DL signal strength, DL quality, and the signal strength of the six best reported neighbours. From the serving BTS, for the same MS the BSC will use UL signal strength, UL quality and TA.

Optimization process[edit]

Optimization process can be explained by below step by step description:

Problem analysis[edit]

  • Analyzing performance retrieve tool reports and statistics for the worst performing BSCs and/or Sites
  • Viewing Reports for BSC/Site performance trends
  • Examining Planning tool Coverage predictions
  • Analyzing previous drive test data
  • Discussions with local engineers to prioritize problems
  • Checking Customer Complaints reported to local engineers

Checks prior to action[edit]

  • Cluster definitions by investigating BSC borders, main cities, freeways, major roads
  • Investigating customer distribution, customer habits (voice/data usage)
  • Running specific traces on Network to categorize problems
  • Checking trouble ticket history for previous problems
  • Checking any fault reports to limit possible hardware problems prior to test

Drive testing[edit]

  • Preparing Action Plan Defining drive test routes
  • Collecting RSSI Log files
  • Scanning frequency spectrum for possible interference sources
  • Re–driving questionable data

Subjects to investigate[edit]

  • Non-working sites/sectors or TRXs
  • In-active Radio network features like frequency hopping
  • Disabled GPRS
  • Overshooting sites – coverage overlaps
  • Coverage holes
  • C/I, C/A analysis
  • High Interference Spots
  • Drop Calls
  • Capacity Problems
  • Other Interference Sources
  • Missing Neighbors
  • One–way neighbors
  • Ping–Pong Handovers
  • Not happening handovers
  • Accessibility and Retain-ability of the Network
  • Equipment Performance
  • Faulty Installations

After the test[edit]

  • Post processing of data Plotting RX Level and Quality Information for overall picture of the driven area
  • Initial Discussions on drive test with Local engineers
  • Reporting urgent problems for immediate action
  • Analyzing Network feature performance after new implementations
  • Transferring comments on parameter implementations after new changes


  • Defining missing neighbor relations
  • Proposing new sites or sector additions with Before & After coverage plots
  • Proposing antenna azimuth changes
  • Proposing antenna tilt changes
  • Proposing antenna type changes
  • BTS Equipment/Filter change
  • Re–tuning of interfered frequencies
  • BSIC changes
  • AdjustingHandover margins (Power Budget, Level, Quality, Umbrella HOs)
  • Adjusting accessibility parameters (RX Lev Acc Min, etc..)
  • Changing power parameters
  • Attenuation Adds/Removals
  • MHA/TMA adds

External links[edit]

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