3G Circuit Switched Call setup message flow

 

 

To be updated according 3GPP release 3G TS 25.331 and 3G TS 24.008 V3.0.0 ...

1. System Information (BCCH) 

The UE reads the System Information that is broadcast on BCCH. The information is not read continuously. It is only read if the information changes 

2. RRC: RRC Connection Request (CCCH) 

The Mobile user decides to initiate a voice call. The first message the UE will send on CCCH is RRC Connection Request. This will contain among other things, Initial UE Identity and Establishment Cause 

3. NBAP: Radio Link Setup Request 

The SRNC sends this message to Node B. It will pass the Cell Id, TFS, TFCS, frequency, UL Scrambling code, etc to Node B.

4. NBAP: Radio Link Setup Response 

Node B allocates the resources and starts PHY Reception. While transmitting the response it includes the Transport layer addressing information that includes the Binding Identity of the AAL2 for Iub data transport bearer

5. ALCAP: Establish REQ 

The AAL2 binding identity (Iub Data Transport Bearer Id) is passed to ALCAP protocol in Node B. The Iub Data Transport bearer is now bounce to DCH. 

6. ALCAP: Establish CNF 

Establish confirm from ALCAP in Node B

7: DCH-FP: Downlink Synchronization 

The Node B and SRNC establishes synchronization for the Iub Data Transport bearer by means of exchange of the appropriate DCH Frame Protocol frames.

8: DCH-FP: Uplink Synchronization 

Once the UL synchronization is achieved, Node B starts DL transmission.

9: RRC: RRC Connection Setup (CCCH) 

RRC Connection Setup message is sent on CCCH with the parameters required to establish DCH. Also the state indicator will be set to DCH for the voice (or CS) call.

10: NBAP: Radio Link Restore Indication 

Once the UE establishes Radio Link, Node B will send RL Restore indication to the SRNC.

11: RRC: RRC Connection Setup Complete (DCCH) 

RRC Connection Setup complete will be sent on DCCH. Integrity and Ciphering related parameters and UE capability information will be sent back to SRNC

12: RRC: Initial Direct Transfer (CM Service Request) 

First NAS message is now sent by the UE. It indicates that a UE originated Voice call is required. The UE identity (TMSI) will also be passed in this message

13: RANAP: Initial UE Message (CM Service Request) 

The NAS message will be forwarded to appropriate CN Domain (CS Domain in this case). Along with the CM service request, it will also include LAI and SAI.

14: RANAP: Direct Transfer (Authentication Request) 

MSC/VLR needs to perform authentication to make sure that the UE is genuine. For this reason it will challenge the UE with a Authentication token and RAND (random number)

15: RRC: Downlink Direct Transfer (Authentication Request)

SRNC transfers the NAS message to the UE

16: RRC: Uplink Direct Transfer (Authentication Response)

UE computes the response (RES) and sends it back in the NAS message

17: RANAP: Direct Transfer [Authentication Response] 

SRNC relays the response to the MSC/VLR. The MSC/VLR will compare the response RES with the expected response XRES. If they are the same then the procedure will continue.

18: RANAP: Security Mode Command 

MSC/VLR sends the Security Mode Command to start Ciphering and Integrity Protection. Ciphering is optional while Integrity Protection is mandatory. The Algorithms, etc are known to the MSC/VLR and the UE and only the ones that are common between them are used.

19: RRC: Security Mode Command 

RRC Forwards the Security Mode command received from MSC/VLR to the UE.

20: RRC: Security Mode Complete 

The UE configures the Ciphering and Integrity Protection and responds back to the network. The response message is Integrity Protected for further safety. Ciphering is started at Ciphering activation time. Since this is a Circuit switched call, the Ciphering will be started in MAC. In case of AM and UM bearers it is started in RLC.

21: RANAP: Security Mode Complete 

The network forwards the Security Mode Complete message to MSC/VLR.

22: RANAP: Direct Transfer (TMSI Reallocation Command)

The network may decide to re-allocate the TMSI to the UE. It sends a DT message which includes the NAS TMSI Reallocation Command.

23: RRC: DL Direct Transfer (TMSI Reallocation Command)

The RNC relays the DT message to the UE.

24: RRC: UL Direct Transfer (TMSI Reallocation Complete)

The UE takes the new TMSI and responds with the Complete message

25: RANAP: Direct Transfer (TMSI Reallocation Complete)

The RNC relays the message to the CN domain

26: RRC: UL Direct Transfer (Setup)

The UE now sends the 'Setup' message in UL Direct Transfer message. This will include all the required parameters for setting up the voice call. It will include the number that UE wishes to be contacted and the bearer capability

27: RANAP: Direct Transfer (Setup)

The network relays the message to the MSC/VLR

28: RANAP: Direct Transfer (Call Proceeding)

The MSC/VLR sends Call Proceeding to the UE indicating that it is now starting with the RAB establishment procedure.

29: RRC: DL Direct Transfer (Call Proceeding)

The network relays it to the UE.

30: RANAP: RAB Assignment Request 

The CN initiates establishment of the Radio Access Bearer using the RAB Assignment Request message. This message includes the QoS of the call being established, the Transport Address, Iu Transport association, etc.

31: ALCAP: Establish REQ 

SRNC initiates the set-up of Iu Data Transport bearer using ALCAP protocol. The request contains the AAL2 Binding Identity to Bind the Iu Data Transport Bearer to the RAB. (Note that this is not done in case of PS RAB)

32: ALCAP: Establish CNF 

The CN responds with the ALCAP Establish CNF

33: NBAP: Radio Link Reconfiguration Prepare 

SRNC requests Node B to prepare establishment of DCH to carry the RAB. It passes the TFS, TFCS and Power Control Information in the message.

34: NBAP: Radio Link Reconfiguration Ready 

Node B allocates the resources and responds with the Ready message. It sends back the AAL2 address and the AAL2 binding Id for the Iub data transport bearer.

35: ALCAP: Establish REQ 

SRNC initiates setup of Iub Data Transport Bearer using ALCAP protocol. The request contains the AAL2 Binding Identity to bind the Iub Data Transport Bearer to DCH.

36: ALCAP: Establish CNF 

The Node B responds with the Establish Confirm.

37: DCH-FP: Downlink Synchronization 

The Node B and SRNC establish synchronism for the Iub Data Transport Bearer by means of exchange of the appropriate DCH frame protocol frames. SRNC sends the DL Synchronization frames.

38: DCH-FP: Uplink Synchronization 

The Node B responds with the UE Synchronization frames.

39: NBAP: Radio Link Reconfiguration Complete 

Finally the SRNC instructs the Node B of the CFN at which the new configuration will come into effect. 

40: RRC: Radio Bearer Setup 

SRNC sends the RB Setup message to add the new DCH's. The message will be received using the old configuration.

41: RRC: Radio Bearer Setup Response 

After the activation time the UE will respond with complete message using the new configuration.

42: RANAP: RAB Assignment Response 

The SRNC responds with the response to the MSC/VLR. 

43: ISUP: Initial Address Message 

MSC/VLR sends the Initial Address Message to the PSTN. The message tells the PSTN to reserve an idle trunk circuit from originating switch to the destination switch. 

44: ISUP: Address Complete Message 

The ACM message is sent to indicate that the remote end of the trunk circuit has been reserved.

45: RANAP: Direct Transfer (Alert)

The Alert message is sent to the SRNC. This message contains the ACM received from the PSTN.

46: RRC: Direct Transfer (Alert)

The Alert message is forwarded to the UE. The Alert message will initiate the ringing tone on the handset.

47: ISUP: Answer Message 

When the person that is being called picks up his phone, an Answer message is sent to the MSC/VLR. 

48: RANAP: Direct Transfer (Connect)

The MSC/VLR sends the Connect message to the SRNC via Direct Transfer message. The Connect message indicates that the End User has answered the call. 

49: RRC: DL Direct Transfer (Connect)

The SRNC forwards the Connect message to the UE.

50: RRC: UL Direct Transfer (Connect Acknowledge)

The UE confirms the reception of the Connect message using the Connect Acknowledge and sending it via Direct Transfer

51: RANAP: Direct Transfer (Connect Acknowledge)

The Network forwards the Connect Acknowledge to the MSC/VLR. The call has now been successfully established.

OPTIMIZE CPICH AND CELL PARAMETERS IN 3G+ AND LTE E-UTRAN

I-             UTRAN CELL PARAMETERS OPTIMIZATION

There are numerous configurable base station parameters which influence strongly the system and determine the capacity of the network:

·         CPICH power

·         Antenna setting (tilt, azimuth, height, antenna pattern)

·         Soft handover parameters

1-  THE CPICH

1.1        RADIO NETWORK ACCESS

CPICH stands for Common Pilot Channel in 3G and some other CDMA communications systems. It provides to cell:

• Initial system synchronization

• Channel estimation for the dedicated channel

After turning on the power and while roaming in the network, a mobile phone determines its serving cell by choosing the best CPICH signal. In WCDMA FDD cellular systems, CPICH is a downlink channel broadcast by NodeB with constant power and of a known bit sequence. Its power is usually between 5% and 15% of the total NodeB transmit power. A common the CPICH power is 10% of the typical total transmits power of 43 dBm.
The Primary Common Pilot Channel is used by the UEs to first complete identification of the Primary Scrambling Code used for scrambling Primary Common Control Physical Channel (P-CCPCH) transmissions from the Node B. Later CPICH channels provide allow phase and power estimations to be made, as well as aiding discovery of other radio paths. There is one primary CPICH (P-CPICH), which is transmitted using spreading code 0 with a spreading factor of 256. Optionally a NodeB may broadcast one or more secondary common pilot channels (S-CPICH), which use arbitrarily chosen 256 codes, written as Cch,256,n where 0 < n < 256.

A UE searching for a NodeB will first use the Primary and Secondary Synchronization Channels (P-SCH and S-SCH respectively) to determine the slot limits and frame timing of a candidate P-CCPCH. The code of the P-SCH is the same for all cells and all MNO. There are 512 set of 16 Scrambling code (For 1 frame  = 15 secondary codes for S-SCH + 1 primary scrambling code for P-CPICH). Searching for the primary scrambling code is reduced to the set of 8 possible Primary Scrambling Codes per group. Indeed, there are 64 groups of 15 S-SCH codes (1 frame per group of S-SCH code). From 512 choices, only 1 scrambling primary code will be chosen within the 8 possible. At this point the correct Primary Scrambling Code can be determined through the use of a matched filter, configured with the fixed channelization code, looking for the known CPICH bit sequence, while trying each of the possible 8 PSCs in turn. The results of each run of the matched filter can be compared, the correct PSC being identified by the greatest correlation result.

Once the scrambling code for a CPICH is known, and the mobile synchronized, it can receive on the P-CCPCH (BCH transport channel) system information of the serving cell. The channel can be used for measurements of signal quality, usually comprising of RSCP and Ec/Io. Timing and phase estimations can also be made, providing a reference that helps to improve reliability when decoding other channels from the same NodeB.

Thus, CPICH power determines the cell coverage capacity. Incerasing or decreasing the CPICH power will enlarge or shrink the cell coverage area. Therefore, by suitably adjusting the CPICH power of the NodeB, the number of users per cell can be balanced among neighboring cell, which reduces the inter-cell interference, stabilizes network operation and ease radio resource management. But CPICH value too high can also decrease cell coverage capacity because of interference with neighboring cell (pilot pollution), while a too low value can create “hole coverage”, area where CPICH power is weak for the mobile phone decode the signal. Mobiles in this area are so not covered.

1.2        COVERAGE OPTIMIZATION BY CPICH

CPICH value parameter optimization can provide good cell coverage, especially indoor coverage:

• Increasing CPICH power (With up to 2dB) may perform enhanced indoor coverage as long as it won’t bring congestion, and we are not in a soft handover area. A scope need to be defined when planning a network. For indoor coverage, depending on if you want to cover first wall (window) or a deep penetration, you may respectively increase your CPICH power of a scope of 10dB or up to 25-30dB.

• Closely monitor your congestion level when increasing CPICH power. If there are lots of users in the area, your cell may be congested.

• It would always better to add more sites and possibly repeater, especially if your buildings belongs to a dense area with important clients and it is in a corporate setting without affecting performance on the ground outside. No miracle can be done without sufficient NodeB. We can’t absolutely expect indoor coverage with 2-3 Km distance between 2 NodeB. This distance must be reduced to 700m, 1km max to expect indoor coverage.

LTE : Résultat des améliorations qu'on a longtemps taillées à l'UMTS

Le mobile crée les besoins d'aujourd'hui ...

On demandera toujours beaucoup plus, toujours encore plus à notre mobile, notre tablette... On voudra aller toujours plus loin.