Cisco BTS 10200 T.38 User Manual
Feature History
Cisco BTS 10200 Softswitch T.38 Fax Relay
Release Modification
900.03.02.00 This feature was initially introduced on the Cisco BTS 10200 Softswitch
in Release 3.2.
This Feature Module describes the support provided by the Cisco BTS 10200 Softswitch software in
Release 3.2 and 3.3 for ITU-T recommendation T.38 Fax Relay, T.38 Fax Relay is used for transporting
real-time Group 3 Fax documents over the Internet Protocol (IP), commonly used in packet networks.
This feature module includes the following sections:
• Introduction, page 2
–
Fax Basics, page 2
–
Fax Relay Basics, page 3
–
Fax Modes, page 4
• T.38 Benefits, page 4
• External Interfaces, page 7
–
Billing Interface, page 7
–
Alarms/Events, page 7
–
Measurements, page 7
–
Provisioning, page 8
• Related Features, page 11
• Supported Platforms, page 11
• Supported Standards, page 12
• Prerequisites, page 13
• Configuration Tasks, page 13
• Command Reference, page 13
• Troubleshooting Fax Relay, page 17
• Glossary, page 24
• Copyright Notice, page 27
Cisco IOS Release xx.x(x)X
1
Introduction
Introduction
Most current fax devices are Group 3 compliant. Fax Group 3 is a standards-based technology that
consists primarily of International Telecommunications Union (ITU) T.4 and T.30 recommendations.
ITU T.4 pertains to how the fax image is encoded by a fax device, and ITU T.30 details the facsimile
negotiations and communication protocol. T.30 is the standard procedure for fax transmission in the
public switched telephone network (PSTN).
Group 3 fax devices were originally designed for use in the PSTN; however, the PSTN itself was
designed for human speech (35 Hz to 3,500 Hz), so Group 3 fax devices use analog encoding or
modulated signals. Fax machines are digital devices that use a modulated analog signal (audio tones)
to pass digital information through the PSTN.
Gateways in packet networks initially treat voice and fax calls the same. Both types of calls cause the
gateway to load a pre-configured voice compression codec in the digital signal processor (DSP). Voice
compression codecs are usually high compression codecs, such as G729 and G723, so less bandwidth is
used for voice calls. High compression codecs are optimized for voice and do a good job of conserving
bandwidth while maintaining voice quality. However, G.729 and other high compression codecs are not
optimized for fax transmissions. In fact, modulated fax signals usually do not pass correctly when high
compression codecs are used, and fax calls fail as a result.
Cisco BTS 10200 Softswitch T.38 Fax Relay
Fax Basics
Faxes can be transmitted successfully when codecs with lower compression ratios or no compression are
used, such as G.726 and G.711 with no echo cancellation or voice activity detection. This method of
sending faxes through voice codecs is referred to as inband faxing or fax passthrough. The initial analog
modulated signal is encoded and compressed by the codec on the originating gateway and passed across
the packet network as if it were a voice sample. The terminating gateway uncompresses and decodes the
sample and plays it out to the terminating fax machine.
Another technique, known as upspeeding, allows the originating gateway to initially load the configured
voice compression codec into the DSP for voice calls, and changes to a low compression codec if fax
tones are detected.
Fax calls can be divided into two parts: fax negotiation and page transmission:
• Half-duplex fax negotiation occurs at the beginning of a fax call. V.21 modulated high-level data
link control (HDLC) data frames are passed at a speed of 300 bps. These data frames are sent in a
standard sequence between the originating and terminating fax devices. During this exchange, the
two fax devices exchange capabilities and both agree on the fax session characteristics before page
transmission takes place.
Some capabilities that are exchanged and negotiated are page transmission speed (called “training”),
error correction mode (ECM), resolution, page coding, and scan time. Page transmission speed is
the speed at which the fax is going to send its information. Fax devices always try to “train” at the
highest transmission speed possible based on the parameters initially exchanged. The fax devices
retrain to a lower speed if training fails at a higher speed.
• Page transmission occurs after the training part of the fax negotiation phase is complete using the
previously agreed upon parameters. The page information is coded into scan lines with a standard
resolution of 203 horizontal (H) dots per inch by 98 vertical (V) dots per inch. Fax images are
typically compressed and encoded using Modified Huffman (MH) encoding or Modified Read (MR)
encoding. MH encoding usually compresses fax transmission at a 20:1 ratio. MR encoding typically
provides a 20 percent compression improvement over MH, but is slightly less resistant to error.
Cisco BTS 10200 Softswitch T.38 Fax Relay Support
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Cisco BTS 10200 Softswitch T.38 Fax Relay
When page transmission occurs, a bit rate is used that is substantially higher than the initial 300 bps
used in the call setup negotiation. The bit rate used for the page transmission is confirmed during
training. Following are some of the common rates used in fax page transmission:
–
V.27ter – 2400/4800 bps
–
V.29 – 7200/9600 bps
–
V.17 – 14400 bps
Note The specifications used for page transmission (V.27ter, V.29, V.17) and fax negotiation (V.21) define
how digital data is sent over analog telephone lines in the PSTN. Data modems are also able to use these
specifications even though most data modems have migrated to much faster speeds.
Fax Relay Basics
Fax relay is an important toll-bypass capability that can produce significant cost savings to end users of
packet telephony networks. As more services transition from the PSTN to packet networks, standards
are emerging to ensure interoperability between different vendors’ equipment. Support of ITU-T.38 Fax
Relay in the Cisco BTS 10200 Softswitch is a move toward true voice-data convergence where the end
user can use the same lines for voice and data (fax) in heterogeneous (multi-protocol) networks.
The ITU-T.38 Fax Relay recommendation specifies the messages and data exchanged for facsimile
transmission in real-time between two Group 3 fax terminals communicating over Internet Protocol (IP)
networks based on the H.323 protocol. The fax transmission method prescribed in the ITU T.38
recommendation can best be described as a demodulation/remodulation procedure (demod/remod).
Fax relay is a technique used to overcome the deficiency in high compression voice codecs, such as
G729 or G723, when these codecs try to pass fax traffic. Since a fax call is treated as a regular speech
call, the DSP in each gateway is put into voice mode after which human speech is expected to be received
and processed. If a fax answer tone (CED) or calling tone (CNG) is heard during the call, the DSP does
not interfere with speech processing. It simply allows the tone to continue across the packet network call
leg as if it were a voice transmission.
Introduction
A fax machine, after generating a CED or hearing a CNG, transmits a T.30 Digital Information Signal
(DIS) as part of fax handshaking. DIS is the initial message stating the capabilities of the terminating
fax machine. The terminating gateway's DSP detects the HDLC flag sequence at the start of the DIS
message and initiates a fax relay switchover. It unloads the high compression voice codec and loads a
lower compression fax codec to run the fax call.
Notification is also sent to the DSP on the other side of the packet network so that both DSPs on the fax
call are using the same fax codec. Notification mechanisms differ depending on the fax relay protocol
used. With the lower compression fax codecs loaded, the DSPs demodulate the T.30 HDLC frames,
extract the fax information, and pass it between the gateways using the standard T.38 fax relay protocol.
It is important to note that unlike inband faxing or fax passthrough, fax relay breaks down (demodulates)
the T.30 fax tones into specific HDLC frames, transmits the information across the packet network using
the T.38 fax relay protocol, then converts (re-modulates) the bits back into tones at the far side. The fax
machines on either end simply send and receive tones; neither terminal is aware that the fax relay process
is occurring or that part of the transmission is over the PSTN and part is over a packet network.
The information that follows can be difficult to understand if you aren't familiar with the messaging that
occurs during a typical fax transmission.
Cisco BTS 10200 Softswitch T.38 Fax Relay Support
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T.38 Benefits
Fax Modes
Cisco BTS 10200 Softswitch T.38 Fax Relay
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:
• 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
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
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.
The following assumptions are made for the CA-controlled mode of operation:
• 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.
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:
• Support of sending Empty Capability Set (ECS)—Cisco BTS 10200 Softswitch deployment in
Cisco BTS 10200 Softswitch T.38 Fax Relay Support
4
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
connection parameters on the remote H.323 gateway after the logical channels are established. For
modifying the connection parameters, H3A implements a mechanism called “sending Empty
Capability Set (ECS).” In prior releases of the Cisco BTS 10200 Softswitch software, H3A
supported this capability only for IVR applications.
Note Any gateway that requires closure of both logical channels after sending ECS is not
supported by the Cisco BTS 10200 Softswitch.
• Clearing the call once the T.38 fax relay is completed—after the fax image transfer, the
connection remains established until one of the endpoints goes onhook, then the call is cleared.
• Clearing the call if an H245 request mode rejection is received—if an H245 request mode
rejection is received from the remote media gateway, the call is cleared.
• Seamless integration of various features—connection parameters on the remote H.323 gateway
can be modified after the connection is established by sending empty capability set (ECS).
The CA-controlled mode for T.38 fax relay provided by this feature also imposes several restrictions:
• Only partial support of MGCP fax package (fax start, fax stop, fax failure events only).
• No voice fall back is supported—
–
when the fax call completes the call is cleared.
–
if the T.38 fax invocation fails, the call is cleared.
• CA-controlled mode feature cannot function for H.323-to-H.323 calls transiting the CA.
T.38 Benefits
Capability Negotiation
For the CA-controlled media change, capabilities supported by the gateway are advertised in the SDP
exchange during connection establishment. This provides knowledge early and prevents unnecessary
messaging to switch to a media or voice format that is not supported by both gateways.
During the initial bearer path setup, the Cisco BTS 10200 Softswitch negotiates the T.38 Fax Relay
capabilities of both the originating and terminatinggateways (endpoints).
• If there are no common T.38 Fax Relay capabilities, the inband method is chosen.
• If both sides have more than one capability in common, the common preferred T.38 mode is used.
• If there is a common preferred mode, which is also present among the common capabilities, that
mode is chosen.
• If no common preferred T.38 mode is available, then one of the common T.38 mode capabilities is
chosen.
Since the H.323 stack in the version 3.2 software does not support T.38 capability exchange during
Fast-start information exchanges, the Cisco BTS 10200 Softswitch initiates a Terminal Capability Set
(TCS) procedure to exchange T.38 Fax Relay capabilities along with DTMF capabilities.
MGCP Gateway Capabilities
The Cisco BTS 10200 Softswitch provides default provisioning parameters in the MGW-PROFILE table
for MGCP-controlled media gateways. The MGWs are also provisioned with static capabilities showing
the types of T.38 Fax Relay supported by the MGW.
Cisco BTS 10200 Softswitch T.38 Fax Relay Support
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T.38 Benefits
Cisco BTS 10200 Softswitch T.38 Fax Relay
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:
• 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.
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:
• Implementation of H.245 request mode procedures (refer to the call flowdiagram, Figure 1).
• 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 Support
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Cisco BTS 10200 Softswitch T.38 Fax Relay
External Interfaces
This section contains information about the external operational, management, and administrative
interfaces for the T.38 Fax Relay feature, which include the following:
• Billing Interface
• Alarms/Events
• Measurements
• Provisioning
Billing Interface
Three new fields have been added to the Call Data Blocks (CDBs) to support the T.38 fax relay feature:
• Fax Indicator—The value of this indicator is dependent on whether or not the call involved any fax
transmissions. This indicator can have one of the following values:
–
Not Fax = 1
–
Fax Only = 2
–
Vo i c e & Fa x = 3
External Interfaces
Alarms/Events
Table 1 details the alarms and events that can be generated by the T.38 Fax Relay feature.
For detailed information about Cisco BTS 10200 Softswitch alarms and events refer to the
Cisco BTS 10200 Softswitch Event Messages Reference.
Table 1 T.38 Fax Relay Alarms and Events
Alarm or Event
FAX_T38_START
FAX_T38_STOP
FAX_FAILURE
Measurements
Not Fax (1) indicates that no fax transmissions occurred, the other two values (2 or 3) indicate that
faxes were sent or received. If faxes were sent, the number of pages is indicated by the next field.
If faxes were received, the number of pages is indicated by the third field.
• Fax Pages Sent—The number of fax pages that were sent during this call (0 through 999).
• Fax Pages Received—The number of fax pages that were received during this call (0 through 999).
Table 2 lists the T.38 specific traffic counter values reported by the Cisco BTS 10200 Softswitch.
For detailed information about Cisco BTS 10200 Softswitch measurements refer to the Cisco BTS 10200
Softswitch Operations Guide.
Cisco BTS 10200 Softswitch T.38 Fax Relay Support
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External Interfaces
Provisioning
Cisco BTS 10200 Softswitch T.38 Fax Relay
Table 2 T.38 Fax Relay Traffic Counters
Measurement Name Measurement Description
FAX_PAGES_SENT Number of fax pages sent.
FAX_PAGES_RECV Number of fax pages received.
This section describes provisioning changes that resulted from the addition of the T.38 Fax Relay feature
to the Cisco BTS 10200 Softswitch software. For more information on provisioning commands, refer to
the “Command Reference” section on page 13.
H323_GW—This table defines the capabilities of each H.323 protocol gateway. The parameters added
for T.38 fax relay include:
• FAX-RELAY-CAP—the T38 fax relay option for this gateway. The value is one of the following:
–
T38_CA_FAX—CA-controlled fax mode. This is the default value.
–
INBAND—Inband fax mode.
H323_TRUNK_GRP_PROFILE—This table is responsible for providing information that is used on
a per-call basis for any H.323 trunk group. The parameters added for T.38 fax relay include:
• FAX_T38_CAMODE_SUPP—if set to "Y." CA-controlled mode is supported for T38 fax relay
• FAX_INBAND_SUPP—if set to "Y," the inband mode is supported for T38 fax relay
• FAX_PREF_MODE—this is the preferred mode for T38 fax relay. The value is one of the following:
–
FAX_T38_CAMODE—CA-controlled mode is the preferred mode (This is the default value.)
–
FAX_INBAND—Inband mode is the preferred mode
MGW_PROFILE—This table provides templates for defining a Media Gateway by hardware vendor.
It identifies the specifications and settings necessary for communications between the Call Agent and
each type of Media Gateway. The parameters added for T.38 fax relay include:
• FAX_T38_CAMODE_SUPP—if set to "Y" (the default value), the CA-controlled mode is
supported for T38 fax relay
• FAX_INBAND_SUPP—if set to "Y" (the default value), the inband mode is supported for
T38 fax relay
• FAX_PREF_MODE—this is the preferred mode for T38 fax relay. The value is one of the following:
–
FAX_T38_CAMODE—CA-controlled mode is the preferred mode (This is the default value.)
–
FAX_INBAND—Inband mode is the preferred mode
Cisco BTS 10200 Softswitch T.38 Fax Relay Support
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Cisco BTS 10200 Softswitch T.38 Fax Relay
N
)
)
age/t38, SendRecv
p
N
p)
p)
Call Flow
The following call flow diagram (Figure 1) illustrates the interactions between the Cisco BTS 10200
Softswitch software modules for the CA-controlled T.38 Fax Relay implementation.
Figure 1 CA-Controlled T.38 Fax Relay
External Interfaces
CRCX(PCMU, t38)
RQNT (fxr / t38())
TFY (fxr / t38(start))
MGA
CNM API
EstablishCx_ex (PCMU, T38 )
RTP Cut thru (H323 fast start)
Service feature Ind(fax-start
CNM API
RemoteModifyCL_ex (in active)
H3A
Service feature Req(fax-start)
H245 Request mode(t38
MDCX (inactive)
MDCX (L: a:im
TFY (fxr / t38(stop))
CNM API
RemoteModifyCL_ex (send recv, t.38
SDP)
ÍÎ Fax information exchange
CNM API
repareResponse (ACK/NACK)
Service feature Ind(fax-sto
Service feature Req(fax-sto
Cisco BTS 10200 Softswitch T.38 Fax Relay Support
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