3G/UMTS Call Re-establishment

The total number of mobile subscriptions in the world is expected to reach five billion soon and the number is ever increasing. This has given a good opportunity to venture capitalists and business people to invest in the mobile business sector. With the number of network operators increasing in most of the countries, investors want to gain as much market share as possible which has led to severe competition between network operators. There are different benchmarks and key performance indicators (KPIs) to highlight how better a network operator is compared to the other. These KPIs have become a marketing jargon and a way to attract more customers.

One of the KPIs is Call Setup Success Ratio (CSSR). CSSR represents the percentage of calls that were successfully established, maintained and released. It combines Radio Access Bearer (RAB) and Radio Resource Connection (RRC) signaling establishment. CCSR gives the statistics as how good the Network has Accessibility. Another important KPI is Retainability which indicates the percentage of successful connections which are terminated by the customer. Retainability will drop if the call is abnormally released. Retainability measures dropped call rate to identify and evaluate abnormal call disconnection. Call drop rate can be compared between different operators for speech only or packet only or over Iur Link etc.
In cases, when:

• there is no neighbor Cell relation defined or existing
• the UE loses the L1 synchronization (poor coverage area, for instance when driving in tunnels)
• the UE encounters layer 2 RLC unrecoverable error issue

Then the above would result in dropped calls. This would be a system abnormal release and would lead to high drop call rate and Retainability KPI degradation.
However, one of the 3GPP Releases introduced the feature Call Reestablishment which will allow the operator to increase the CSSR and Retainability KPIs. With Call Re-establishment, the call which would have been dropped could be saved, i.e. re-established when the UE encounters the above failures. The UE will down-switch to common channel (FACH) from Dedicated Channel (DCH) state and will send a Cell Update with cause Radio Link Failure or RLC Unrecoverable Error. Upon receiving the Cell update, the network would realize that UE has encountered a failure and thus it would restore the lost RAB and reply the UE with an RRC Cell Update Confirm Message. The UE would then reconfigure and come back to DCH state.

In 3G Network, it is common to have network topologies with Serving and Drift RNCs. The RNCs are connected with the Iur link. Call Reestablishment can also work over Iur. The UE could encounter the failure(s) in cells belonging to a Drift RNC (DRNC) and send a Cell Update to the DRNC. The DRNC would encapsulate the RRC Cell Update Message in a RNSAP Message with type “UplinkSignallingTransferInd” and send it to the Serving RNC. The SRNC, upon receiving the UplinkSignallingTransferInd message, would realize that the UE has encountered a failure and thus it would restore the lost RAB and encapsulate the RRC Cell Update Confirm in a RNSAP Message with type “DownLinkSignallingTransferInd” and forward this message over Iur to the DRNC. The DRNC will transmit the RRC Cell Update Confirm to the UE.

One more difference between Call Reestablishment in DRNC and Call Reestablishment in SRNC is that CellUpdateConfirm will be carried by SRB0 UM (Signaling Radio Bearer Unacknowledged Mode) in downlink for Iur/DRNC case as compared to SRB1 UM for Call Reestablishment in SRNC. For the DRNC case, the CellUpdateConfirm has to be entirely shipped in Common Control Channel (CCCH), whereas in the SRNC case it can be shipped in conjunction with DCCH->CCCH mapping.

Call Reestablishment is possible for both Circuit Switched (CS) and Packet Switched (PS) RABs. There are dedicated timers that govern the establishment of CS and PS RABs. The timers are broadcasted in System Information (SIB1) from the Network to the UEs. If the Network does not support Call reestablishment, it would set the timer values to 0. The UE in this case will not send cell Update to the Network requesting for reestablishing the call. When the Network supports the Call Reestablishment, the default value of the timer is typically 8 sec for CS RAB and 30 sec for PS RAB. The UE stores these timer values and when it encounters the failure it down-switches from DCH to FACH. The UE would then indicate in cell Update message whether the CS and PS RAB timers have expired or not. When the timers have not expired the network will initiate the call Reestablishment. One important thing that the network has to do before going forward with the Call Reestablishment is to suspend the Radio Connection Supervision (RCS) timers or else the RCS timer might timeout before the Call Reestablishment is completed.

The sequence of Call reestablishment flow is as follows:

• Delete all old Radio Links
• Setup New Radio Link in the cell where the cell Update has originated from
• Send CellUpdateConfirm Message to the UE.
• UE replies with one of the response message (Transport Channel/Radio Bearer Setup/RadioBearer Release/Physical Channel Reconfiguration Complete)

The CellUpdateConfirm Message will contain the information about the RABs that has been restored. In case of Multi-RAB, if only the CS or if only the PS could be restored, then the CellUpdateConfirm message has also the release information for the RAB that could not be restored or has been lost. The UE in this case should reply with RadioBearer Release Reconfiguration Complete.