A Complete Mobile
Originated Circuit Switched Call Setup is shown in the MSC below:
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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 bounf to DCH.
6. ALCAP: Establish CNF
Establish confirm from ALCAP in
Node B
7: DCH-FP: Downlink
Synchronisation
The Node B and SRNC establishes
synchronisation for the Iub Data Transport bearer by means of exchange of the
appropriate DCH Frame Protocol frames.
8: DCH-FP: Uplink Synchronisation
Once the UL synchronisation is
acheived, 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 paarmeters 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 orginated 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 Integity 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 Informnation 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
Synchronisation
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 Synchronisation
frames.
38: DCH-FP: Uplink Synchronisation
The Node B responds with the UE
Synchronisation 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
Reponse
After the activation time the UE
will respond with complete message using the new configuration.
42: RANAP: RAB Assignment Reponse
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 inditiate 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
indictaes 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.
Further Reading:
[1] 3GPP TS 25.331: RRC Protocol
Specification
[2] 3GPP TS 25.413: RANAP Protocol
Specification
[3] 3GPP TS 25.433: NBAP Protocol
Specification
[4] 3GPP TS 24.008: Mobile radio
interface Layer 3 specifications; Core Network Protocols; Stage 3
[5] ISUP (ISDN User Part)
Tutorial: http://www.pt.com/tutorials/ss7/isup.html
[6] ITU-T Recommendation Q.2630.1:
"AAL2 Type 2 Signaling Protocol".
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