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Ground Start Signaling
November 2009 289
Ground Start Signaling
Brooktrout modules support two types of ground start signaling:
Foreign Exchange - Office (FXO)
Foreign Exchange - Subscriber (FXS)
When operating in FXO mode, the module assumes the far end of the
connection is an FXS termination. When operating in FXS mode, the
module assumes the far end is an FXO termination. The timing
diagrams on the pages that follow use the labels “FXO” or “FXS”
rather than “Rx” or “Tx” for hookswitch states to illustrate this point.
FXO Ground Start
To configure a Brooktrout module for FXO ground start signaling,
the host must issue an L4L3mENABLE_B_CHANNEL with an
IISDN_ROBBED_BIT_DATA structure that contains the following
settings:
in_trunk_type = IISDNttFXO_GDSTART
out_trunk_type = IISDNttFXO_GDSTART
timed_answer_supervision = 1 (timer expiration indicates far
end answer)
fgb_fgd_mode = 0 (not used)
The remaining settings are described in the context of incoming and
outgoing calls in the subsections that follow.
Incoming Call Processing
During an incoming call, the host receives an L3L4mPRE_SEIZE
message if send_preseize_event = 1 in the IISDN_ROBBED_BIT_
DATA structure. This message is sent when the module detects ring
ground at the far end (FXS end). The module then collects dial pulse
digits from the network; the maximum number of digits the module
expects to receive must be specified in the max_incoming_digit_
count field. The digits are reported to the host in an L3L4mSETUP_
IND message. To answer the call, the host must issue an
L4L3mCONNECT_REQUEST message.