Cisco BTS 10200 Softswitch 사용자 설명서 - 페이지 4
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페이지 인쇄T.38 Benefits
Fax Modes
Existing H.323-based VoIP networks cannot interoperate with MGCP-controlled media gateways.
Starting in this release, the Cisco BTS 10200 Softswitch software provides a gateway interface that
supports the H.323 protocol. This allows MGCP-controlled gateways to interwork with H.323 VoIP ports.
In the Call Agent (CA)-controlled mode the Cisco BTS 10200 Softswitch can mediate fax control
signaling using T.38 fax relay attributes between two dissimilar gateways; one an MGCP-controlled
gateway, the other an H.323-controlled gateway. The CA drives T.38 media changes through MGCP
command interaction with the gateways involved in the call. This requires that the CA be upgraded with
the functionality provided in this release to support the T.38 media change protocol inherent in the T.38
fax relay feature.
For the CA-controlled mode of operation, the following approach applies:
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The following assumptions are made for the CA-controlled mode of operation:
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Because the T.38 Fax Relay protocol is standards based, Cisco media gateways and gatekeepers can also
interoperate with third-party T.38-enabled gateways and gatekeepers in mixed vendor networks where real
time fax relay capabilities are required.
H.323 is just one of several native signaling protocols supported by the Cisco BTS 10200 Softswitch.
The H.323 interface allows the Cisco BTS 10200 Softswitch to function as a switching platform for
heterogeneous networks. The Cisco BTS 10200 Softswitch can be used without restricting the network
to any single protocol, such as MGCP, SIP, or H.323.
T.38 Benefits
The Call Agent-controlled mode for T.38 fax relay provided in Release 3.3 also offers several benefits:
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Cisco BTS 10200 Softswitch T.38 Fax Relay Support
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The call is initially established as a voice call.
When a V.21 preamble is detected at the terminating fax machine the CA receives an MGCP NTFY
message:
if both gateways support T.38, the CA initiates an H.323 Mode Request procedure to switch
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both of the media gateways to T.38 fax relay mode for the connection.
To minimize additional delay in switching to T.38, the existing voice port is reused instead of
establishing a new connection through the network. The process of switching from voice to T.38
fax relay includes shutting down RTCP and using the RTP port for the UDP transport of T.38.
if both gateways do not support T.38, or an attempt to switch modes fails, the connection is lost
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and the call is cleared.
On completion of the fax image transfer, the connection remains established until one of the two
endpoints goes on-hook, then the call is cleared.
When a V.21 preamble is detected, the T.30 protocol is persistent enough to handle the delays added
by the MGCP messaging necessary to accomplish the fax switchover.
Detection of V.25 ANS or V.8 ANSam indicates the presence of a non-voice call.
A modem call is assumed and an attempt is made to enable modem passthrough. If a V.21 preamble
is then detected on the terminating fax, an attempt is made to enable T.38 processing, provided both
endpoints involved support T.38 fax relay.
Support of sending Empty Capability Set (ECS)—Cisco BTS 10200 Softswitch deployment in
heterogeneous networks mandates that various CA features, like Call Forwarding No Answer
(CFNA), work seamlessly in H.323 networks. Most of these features require that H3A modify the
Cisco BTS 10200 Softswitch T.38 Fax Relay