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WCDMA MO/MT Call Flow: Mobile Originated and Terminated Calls

wcdma mobile communication call flow protocol circuit switch

Introduction

This article describes the mobile originated (MO) and mobile terminated (MT) call flows between a mobile device (UE) and a network compatible with the WCDMA standard. It covers the messages exchanged between Layer 3 entities on both sides. It also includes the channels used at Layer 1 to carry these messages over the air.

This article assumes that the initial frequency and time synchronization has already been done between the UE and the network as per WCDMA specifications.

WCDMA mobile originated terminated voice call

Figure: WCDMA mobile originated terminated voice call flow

The figure above illustrates the message sequences between a WCDMA UE and the network for an MO call (Mobile originated call). In a Mobile Terminated (MT) Circuit Switched (CS) voice call, once the RRC connection is established, the MT UE receives the call setup requirement from the network and responds with a call confirm message.

Call Control (CC) Messages for MO Call Establishment

The Call Control (CC) messages exchanged between the CC entity of the UE and the CC entity of the network are summarized below for MO call establishment:

(UE) SETUP >>> (NETWORK) (UE) <<< CALL PROCESSING (NETWORK) (UE) <<< ALERTING (NETWORK) (UE) <<< CONNECT (NETWORK) (UE) CONNECT ACK >>> (NETWORK)

Call Control (CC) Messages for MT Call Establishment

The Call Control (CC) messages exchanged between the CC entity of the UE and the CC entity of the network are summarized below for MT call establishment:

(UE) <<< SETUP (NETWORK) (UE) CALL CONFIRMED >>> (NETWORK) (UE) ALERTING >>> (NETWORK) (UE) CONNECT >>> (NETWORK) (UE) <<< CONNECT ACK (NETWORK)

Fields (IEs - Information Elements) of MO CC SETUP Message

  • CC protocol discriminator
  • TI
  • Message type
  • Bearer capability (RI,1,2)
  • Facility
  • Calling party subaddress
  • Called party binary coded decimal (BCD) number
  • Called party subaddress
  • Low layer compatibility (RI,1,2)
  • High layer compatibility (RI,1,2)
  • User to user
  • SS version
  • CLIR suppression
  • CLIR invocation
  • CC capabilities
  • SI

Fields (IEs - Information Elements) of CALL PROCESSING Message

  • CC protocol discriminator
  • TI
  • Message type
  • Bearer capability (RI,1,2)
  • Facility
  • Progress Indicator
  • Priority granted
  • Network call control capabilities

Fields (IEs - Information Elements) of ALERTING Message

  • CC protocol discriminator
  • TI
  • Message type
  • Facility
  • Progress Indicator
  • User-user (information)