Cisco BTS 10200 Softswitch User Manual page 10

External Interfaces
The following steps describe in detail the T.38 Fax Relay functionality:
The BCM checks the gateway profiles for their T.38 fax relay capabilites. It selects a common/preferred
Step 1
capability supported by both the gateways.
Prior to establishing a connection, the BCM encodes the T.38 fax Local Connection Option (LCO) in the
SAI_CreateCxData_t . The t38fax LCO information is transmitted by the MGA in the CRCX.
The BCM performs a check to ensure that the gateways are, in fact, MGA and H3A. The BCM initiates
Step 2
an RQNT to the gateway by sending SAI_CCM_OOBINFO_REQ.
Based on these additional attributes in the RQNT, the MGA determines whether it can allow T.38 fax
calls. In order to detect whether T.38 fax calls are supported on the H3A, the BCM checks the trunk
group profile. In addition, the BCM checks the MGA static profile to ensure T.38fax is supported.
The gateway detects fax transmission. It notifies the originating Signaling Adapter of the reception of
Step 3
fax information. The Signaling Adapter in turn informs the BCM about the fax-start event in the
SAI_CCM_SVCFEATURE_IND primitive.
The BCM upon reception of the SAI_CCM_SVCFEATURE_IND primitive looks for the
Step 4
SAI_EI_FAX_START event. Then BCM checks to make sure that the passive_leg is in the
LEG_JOINED state; the remote BCSM state machine is in the active state. It then relays the T.38fax
information to the remote Signaling Adapter in the SAI_CCM_SVCFEATURE_REQ primitive. In the
case, where the passive_leg is not in the LEG_JOINED state, a FAX_FAILURE event is sent in the
SAI_CCM_SVCFEATURE_REQ towards the gateway.
Note
The Signaling Adapter upon receipt of the T.38 fax_start information, invokes the CNM API
Step 5
RemoteModifyCL_ex inorder to initiate a ModifyConnection request to the other Signaling Adapter
with the new SDP information. Two consecutive RemoteModifyCL_ex is sent to the peer Signaling
Adapter; one of them initiates the MDCX in the inactive mode while the other initiates an MDCX with
modified codec.
The Signaling Adapter's CNM upon receipt of the RemoteModifyCL_ex from the remote CNM modifies
Step 6
the SDP information. The codec information is also modified.The ACK/NACK that is received from the
originating gateway is then sent all the way back to the CNM of the terminating Signaling Adapter. This
is accomplished by retaining a copy of the remote Signaling Adapters identification.
The gateway detects an end of fax and transmits it to the other Signaling Adapter. The Signaling Adapter
Step 7
in turn transmits the fax stop information in the SAI_CCM_SVCFEATURE_IND primitive to the BCM.
The BCM upon reception of the SAI_CCM_SVCFEATURE_IND primitive looks for the
Step 8
SAI_EI_FAX_STOP event. It then relays the fax information to the other Signaling Adapter in the
SAI_CCM_SVCFEATURE_REQ. Following this, the call is cleared in a normal manner upon the receipt
of SAI_REL_IND from the H3A.
Cisco BTS 10200 Softswitch T.38 Fax Relay Support
10
Since T.38 fax is handled only in the ACTIVE state, the passive legs are in the LEG_JOINED
state and a valid bcsm_id exists for the remote end.
Cisco BTS 10200 Softswitch T.38 Fax Relay