Telecom Tutorials by Samir Amberkar

MO Voice Call [Under UMTS]

Let us take a look at example of Mobile Originated Voice Call (CS-domain). MSC stand for MSC/VLR.

UE

RNC

MSC

GMSC

UE/RRC in idle mode. User makes a call. Protocol stack receive AT command ATD. RRC Connection establishment procedure. RNC is now Serving RNC1 for the UE. CM Service Request thru RRC IDT message SCCP connection req/cnf Iu signaling connection establishment CC - CM Service Request over RANAP CM Service request contain cause for request (voice call, SMS etc.), identity (IMSI, TMSI) etc. MSC/MM may trigger authentication & ciphering procedure at this time. Authentication & ciphering procedure (with HLR's help similar to GSM, check GSM article on voice call). CC - SETUP CC/SETUP message contain called number, bearer capability info, AMR speech capability info etc. MSC validate SETUP request and start resource assignment for carrying user data (in this case it would be voice data) at Iu interface. MSC may also acknowledge UE with CC/Call proceeding message CC - Call Proceeding RNC does Iu bearer setup with ALCAP signaling (as shown in Iu interface article, ALCAP is needed for setting up Iu bearer for CS data. For PS data, ALCAP is not needed). RNC invokes radio bearer setup. RANAP - RAB Assignment Request RB establishment procedure. RAB Assignment Response While RAB setup is in progress, MSC may have already started network side signaling (like talking to HLR to get routable number for dialed mobile number and ISUP call establishment with target MSC, or ISUP call establishment if the dialed number belong to PSTN or another non-GSM/UMTS number). ISUP - IAM ISUP - ACM ISUP trunk establishment (two way or one way from GMSC to MSC). CC - Call Proceeding (alert) Called party answered the call. ISUP - ANM ISUP trunk connected both ways if not connected earlier. CC - CONNECT CC - CONNECT Ack Call is in progress now. UE/RRC in connected/CELL_DCH state (doing measurements and reporting to RNC; this may result in HO). For PS call, RRC would be in CELL_DCH (large amount of data) or CELL_FACH (small amount of data) state.

1: Serving RNC (SRNC) is that one which provide Iu connection for the UE. The terminology comes in UMTS due to presence of Iur interface between RNCs. Drift RNC is the one which has radio connection with UE. Drift RNC (DRNC) act as relay between UE and SRNC. When register to the network, RNC which it registers to becomes SRNC. Later it may drift to another RNC (during active CS or PS connection) and that RNC becomes DRNC if new RNC has Iur connection with SRNC. If Iur connection is not present, the configuration become similar to GSM; hard handover is to be performed in such case.

References: UMTS by Sanchez and Thioune, and WCDMA for UMTS by Holma and Toskala.

Copyright © Samir Amberkar 2010

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