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Call Setup Success Rate (CSSR)

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Table of Contents

 Call Setup Success Rate (CSSR) Before diving into the KPI in Telecom, we should discuss some background information. 

Call Setup in Telecommunication is a procedure in which a virtual circuit is established across the telecommunications network by using a signaling protocol. 

Call Set-up Time can be understood in two ways;

  • The overall length of time needed for the purpose of establishing a circuit-switched call
  • Data communication- the overall length of time needed for establishing a circuit-switched call, that is, from initiating a call request to the beginning of a call message, between terminals


Definition: Call Setup Success Rate or CSSR

Call Setup Success Rate is one of the KPI that directly influences customer satisfaction about the telecom service provided by a network and its operator.

It is used by network operators to evaluate the performance of networks. CSSR is used to describe the fraction of attempts made while making a call resulting in a successful connection to the dialed number. The fraction in the process is the percentage of all the attempted calls. 

A call is successfully connected, in terms of telecom can be defined in many ways. Let’s say, the number you dialed is busy, but the fact that the call is connected to the dialed number, the connection is deemed successful. 

The CSSR is exceptionally high and above 99.9% in the case of a land-line network.

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How to calculate Call Setup Success Rate?

CSSR = Outgoing and incoming call setup/ Total no. of call attempts

1 is the ideal value of CSSR. The value implies that the network should be able to accept 100% of the calls dialed. 

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What are the procedures for Call Setup?

There are two procedures used for a Call Setup to achieve successful CSSR. 

Immediate Assignment: 

This process is used to establish a connection between MS (mobile station) and the Network.

To initiate the process the MS sends a CHANNEL REQUEST message on the RACH (Random Access Channel) to the BTS (Base Transceiver Station) requesting a signaling channel, SDCCH (Standalone Dedicated Control Channel). 

The message includes- information field, establishment cause, and random reference. 

The establishment cause provides the reasons why MS is requesting a signaling channel.

The reasons such as 

  • Emergency call
  • Call re-establishment
  • Originating speech call
  • Location updating

Next comes, the process of signalization between the MS and the Network to activate the signaling channel.

To confirm the successful establishment of SDCCH, an Establish Indication message is sent from MS to BTS. Then it is again sent to BSC (Base Station Controller). 

After successful establishment, further coordination processes like authentication, ciphering, etc. are performed on SDCCH. 

Assignment Procedure: 

This procedure is performed to occupy a radio resource. 

The MSC (Mobile Switching Centre) initiates the process by sending an ASSIGNMENT REQUEST message to the BSC. In the message, it requests an assignment of a Radio Resource. 

Between the BTS and BSC, a process of signalization occurs to assign and activate a Radio Resource (RR); Traffic Channel- TCH. 

If MS successfully captures the TCH, then BSC sends the message: ASSIGNMENT COMPLETE.

Now that we have established two ways a Call Setup is achieved, the next step is to understand what the reasons are for poor CSSR.

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What causes a poor Call Setup Success Rate (CSSR)?

The factors you need to check to prevent poor CSSR are: 

Low Signal Strength: This simply means that the call setup failed because of the weak signal. No re-selection could happen because handover has not occurred. 

No access to SDCCH: As mentioned in the first procedure of setup, a BSS receives a channel request (in the form of RACH) from a source requesting resources for transactions in the network. After validating the request, BSS attempts to allot a channel (SDCCH) for the source. 

After the availability of the SDCCH channel is confirmed the BSS sends an immediate assignment to MS. It indicates, via AGCH, the SDCCH sub-channel. There are two ways the SDCCH can fail.

  1. Valid RACH or SDCCH Congestion: IF the SDCCH is unavailable BSS will respond to MS by rejecting immediate assignment and terminate the network transaction. The call setup is deemed unsuccessful due to SDCCH Congestion. 
  2.  Phantom RACHs: In this case, the received RACH is generated from an unknown source. It failed to continue the process of the transaction after BSS allotted the SDCCH. 

Phantom RACHS symptoms are carried when the channel request detects overshooting cells, handover access burst from distanced MS, deficiency in hardware, UL/DL imbalance path or when MS moves out of the range. 

CM Service rejects: The CM service requests for a message and MS informs the Network about the types of services it requires. Following this, BSS will initiate a connection request BSSMAP message and send it to MSC for approval. 

MSC responses in two possible ways: 

  • Connection confirmed- link between MS-BSC-MSC is successfully established or,
  • Connection Refused- terminating the network transaction. 

TCH Congestion: After establishing the link between MS-BSC-MSC, MS issues an assignment request to the BSC. It requests for a TCH assignment to the MS. The BSS will try to allot a free TCH for MS voice messaging. 

After the MS receives the Assignment command confirming the availability of TCH for MS it will respond with an Assignment Complete to the TCH. BSS will send the message to MSC to complete the signal activity. 

Hardware Issues: This also plays a role in the failure of CSSR. If any BTS hardware is functioning improperly it can affect the overall performance of the side.

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Solutions for the Issue causing poor CSSR

Low Signal Strength: RX level will increase by boosting the signal strength. This can lead to a better handover and thus, re-selection can happen.

No access to SDCCH: Check the SDCCH Blocking and Congestion before confirming that there is a problem with SDCCH. 

More slots should be assigned for SDCCH if SDCCH blocking is more than 1% or Congestion is more than 2%.

A TCH can also be assigned by utilizing the parameter Immediate Assign Mode passing the SDCCH.

TCH Congestion: You can enable the features of TCH queuing, Directed Retry, and Congestion Relief.

  • When you enable the feature TCH queuing, MS queues in the original SDCCH and waits for the next available TCH. (However, if the queuing timer expires, BSS will terminate the transaction leading to an unsuccessful setup). 
  • When you enable Directed retry, MS performs a handover to TCH of another cell. 

Fix Hardware Issue: When starting the drive-test, ensure the cell site is free of hardware alarms. Check the path balance; from the radio to the antennas.

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