Cisco BTS 10200 Softswitch User Manual page 6

T.38 Benefits
The negotiated fax mode is provided in an extension to the LocalConnectionOption (LCO) parameter
of the CRCX message that the CA sends to MGCP-controlled gateways, which allows the CA to give
the gateway up front instructions on how to handle fax for any given connection. This reduces the
number of messages required between the CA and the gateway, which reduces the time to switch to the
T.38 media format.
The syntax of the fx: option is a preferenced list. The possible values for the fx: option are:
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If the CA does not provide an fx: LCO for the connection, the gateway provisioning makes the decision
on the fax transmission method to use, the default being the CA-controlled mode. This setting for fx:
remains in place for the duration of the connection unless explicitly altered by another fx: setting
transmitted to the gateway.
H.323 Network Capabilities
In the absence of dynamic capability exchange, capabilities are provided by current H.323 networks
during the call setup time. Due to protocol complexities, the Cisco BTS 10200 Softswitch currently has
static provisioning per logical H.323 network cloud. They are provisioned in the H.323-TG-PROFILE
table that can be associated with certain H.323-GW (or H.323-endpoint) entries.
Should the Cisco BTS 10200 Softswitch have to provide an interface to H.323 networks that have
H.323 gateways (or endpoints) with different capabilities, multiple H.323-TG-PROFILEs can be
provisioned and all of them can use the same H.323-GW entry.
The only capability of H.323-TG-PROFILE that is currently supported is the T.38 CA-controlled mode.
The H.323-TG-PROFILE can also be provisioned with a preferred mode, the default value being the
CA-controlled mode.
H.323 Adapter Enhancements
The enhancements made to implement T.38 Fax Relay and the "modify connection" feature in the H.323
adapter (H3A) in Release 3.3 include the following:
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Cisco BTS 10200 Softswitch T.38 Fax Relay Support
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t38—The gateway is to use CA-controlled T.38 media change for fax relay on the connection.
The gateway should use this option if both gateways involved in the connection support T.38.
This is the only mode supported if the gateway needs to interoperate with H.323-based gateways.
pt—Use the existing MGCP fax passthrough functionality to process the fax transmission. Fax
passthrough is used if T.38 is not supported by both of the involved gateways, but fax passthrough
is. The manner in which fax passthrough functions for MGCP is not altered by this feature.
off—This option means that no special handling for fax is performed at the MGW. This involves no
upspeeding, no alteration of silence suppression or echo cancellation settings for the connection.
The fax payload is transported using an inband bearer path.
Implementation of H.245 request mode procedures (refer to the call flowdiagram,
Handling of the ModifyCx command from the basic call module (BCM) when the channels are already
established required implementation of the H.245 sending Empty Capability Set (ECS) messages.
State machine changes required in H3A for H.245 OLC/CAPs for T.38 fax relay and sending ECS
messages.
Implementation of the pass through media case for H.323 to H.323 calls, where any H.245 message
results in "media/connection pass through" messages on the other H.323 side.
Addition of new fields in existing data structures, specifically the new parameters added to the
H323_TRUNK_GRP_PROFILE table.
Cisco BTS 10200 Softswitch T.38 Fax Relay
Figure 1).