Mobile-Terminated Calls (MTC) in OpenBTS

The MTC is the most complex operation in the Stage 1 OpenBTS. The MTC is the transaction by which an incoming call is connected to a mobile handset. It is largely implemented in Control/CallControl, but with other parts in SIP/SIPInterface, Control/SDCCHDispatch and Control/FACCHDisptach.

Here is the transaction sequence for the MTC:1. An INVITE arrives at the SIPInterface giving the GSM mobile ID (an IMSI) as the SIP user ID.

1. The SIPInterface (in SIPInterface::checkInvite) creates a call entry in the global transaction table and also initiates the paging process (via Pager::addID).

1. The Pager sends out the mobile ID of the called handset in Paging Request messages on the CCCH (Common Control Channel). The CCCH is a unicast channel monitored by all idle handsets in the cell.

1. The handset, upon seeing its ID in a paging request, starts sending channel requests on the RACH (Random Access Channel).

1. The RACH bursts are received and decoded in L1 and passed up to the AccessGrantController in Control/RadioResource. The RACH burst contains an 8-bit tag.

1. AccessGrantController allocates an SDCCH (Standalone Dedicated Control Channel) and sends a description of this channel, along with the associated 8-bit tag, in an Immediate Assignment message on the CCCH.

1. The handset, still monitoring the CCCH, sees its 8-bit tag in the immediate assignment and goes to the assigned channel.

1. The first message on the SDCCH is a Paging Response from the handset. This message is processed in the SDCCHDispatcher, which checks the indicated service type and passes the message along to the MTCStarter in Control/CallControl, via the PagingResponseHandler in Control/RadioResource.

1. The MTCStarter searches the transaction table for the entry created by the SIPInterface back in step 2.

1. MTCStarter initiates alerting (ie, makes the handset ring) and allocates a Traffic Channel (TCH) for the call.

1. The TCH assignment is sent to the handset in an Assignment Message. The transaction ID for this call is also put onto the TCH channel object as a tag.

1. The handset moves to the assigned channel and sends an Assignment Complete message on the FACCH.

1. The assignment complete message arrives in the FACCHDispatcher, which checks its tagged transaction ID in the transaction table and passes control to the MTCController, also in Control/CallControl.

1. The call setup proceeds in the MTCController.

1. Once the call is fully connected, control passes to the callManagementLoop, also in Control/CallControl.

If things go well, all of this will take 3-5 seconds.

注:Mobile-Terminated Calls (MTC) in OpenBTS(原文出处,翻译整理仅供参考!)