Cisco T.38 User's Manual
Contents
1. a Cisco BTS 10200 Softswitch T 38 Fax Relay HI T 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 The call is initially established as a voice call e 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 voic2. 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 Basics Fax calls can be divided into two parts fax negotiation and page transmission e 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 e Pa
3. Cisco the Cisco Certified Internetwork Expert logo Cisco IOS Cisco Press Cisco Systems Cisco Systems Capital the Cisco Systems logo Cisco Unity Enterprise Solver EtherChannel EtherFast EtherSwitch Fast Step Follow Me Browsing FormShare GigaDrive HomeLink Internet Quotient IOS iPhone IP TV iQ Expertise the iQ logo iQ Net Readiness Scorecard iQuick Study LightStream Linksys MeetingPlace MGX Networking Academy Network Registrar Packet PIX ProConnect ScriptShare SMARTnet StackWise The Fastest Way to Increase Your Internet Quotient and TransPath are registered trademarks of Cisco Systems Inc and or its affiliates in the United States and certain other countries All other trademarks mentioned in this document or Website are the property of their respective owners The use of the word partner does not imply a partnership relationship between Cisco and any other company 0705R Cisco BTS 10200 Softswitch T 38 Fax Relay Support EE Cisco BTS 10200 Softswitch T 38 Fax Relay E Copyright Notice Cisco BTS 10200 Softswitch T 38 Fax Relay Support 8
4. Installation Procedures Cisco BTS 10200 Softswitch Command Line Interface Reference Guide e Cisco BTS 10200 Softswitch CORBA Installation and Programmer s Guides e Cisco BTS 10200 Softswitch Operations Manual e Cisco BTS 10200 Softswitch Billing Interface Guide Supported Platforms The T 38 Fax Relay features provided in Release 3 3 of the Cisco BTS 10200 Softswitch are available on a wide variety of Cisco platforms including the following e Cisco AS5300 e Cisco AS5350 e Cisco AS5400 e Cisco AS5800 e Cisco AS5850 amp Note Not all features provided in Releases 3 2 and 3 3 are available on all Cisco platforms Consult the Cisco Feature Navigator web site http www cisco com go fn to determine what platforms and IOS software images can accommodate the Cisco BTS 10200 Softswitch features that you require Cisco BTS 10200 Softswitch T 38 Fax Relay Support a Cisco BTS 10200 Softswitch T 38 Fax Relay W Supported Standards Determining Platform Support Through Cisco Feature Navigator Cisco IOS software is packaged in feature sets that support specific platforms To get updated information regarding platform support for this feature access Cisco Feature Navigator Cisco Feature Navigator dynamically updates the list of supported platforms as new platform support is added for the feature Cisco Feature Navigator is a web based tool that enables you to determine which Cisco IOS software images support a specif
5. active MDCX inactive H CNM API RemoteModifyCL_ x send recv t 38 D p MDCX L a image t38 SendRec ACK NACK gt Fax information exchange CNM API prepareResponse ACCK NACK Fax transmission starts NTFY fxr t38 stop Setvice feature Ind fax stop Service feature Req fax stop Normal Call release Cisco BTS 10200 Softswitch T 38 Fax Relay Support a Cisco BTS 10200 Softswitch T 38 Fax Relay E External Interfaces Step 1 Step 2 Step 3 Step 4 Step 5 Step 6 Step 7 Step 8 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 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 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
6. below 20 If the counter s is higher than 20 investigate the packet loss If the network appears to be lossy it is not reasonable to expect T 38 fax relay to work reliably Further investigation is probably needed to ensure that QoS is provisioned end to end so that the voice and T 38 fax relay traffic has priority and is never lost during congestion Cisco BTS 10200 Softswitch T 38 Fax Relay Support O Cisco BTS 10200 Softswitch T 38 Fax Relay Step 8 Step 9 Step 10 Troubleshooting Fax Relay W Enable T 38 Packet Redundancy The T 38 fax relay packet redundancy feature can be turned on by configuring the following command under the appropriate dial peers on both gateways gateway name config dial peer fax protocol t38 Ls redundancy X Hs redundancy Y where X gt 0 and Y 0 only make changes to Ls redundancy This feature can alleviate T 38 fax relay failures due to packet loss However T 38 packet redundancy significantly increases bandwidth usage and it is preferable to eliminate packet loss where possible Set the fax NSF command to all zeroes The fax NSF command can be helpful in troubleshooting the brands of fax machines that alter the NSF field during fax negotiation for proprietary encodings This command allows the gateway doing T 38 fax relay to override the settings made by fax machines that are trying to implement proprietary encodings Before the fax NSF command was available fax relay would fail
7. fails it is very likely that whatever is causing the fax calls to fail with T 38 fax relay is also causing the failures with this test The network may have a large amount of jitter or packet loss Step 7 Check for packet loss on the network The easiest and most accurate way to determine if there is packet loss is to do the following a Disable VAD on the packet network dial peers b Make a voice call between the same ports where the fax machines are connected Fax machines may be able to serve as ordinary phones or you might need to connect handsets to the fax ports c When the call is connected 1 Issue the show voice dsp command You can see in the output that one of the DSP channels has the configured codec loaded Usually the column TX RX PAK CNT shows that the transmit and receive packet counters are equal meaning no packets are being lost If the counters are not equal packets might be getting lost Type the show voice dsp command several times at 30 second intervals to determine if the difference increases If so then packets are being lost 2 Issue the show voice call summary command to see which port and timeslot if applicable is allocated to the voice call Type terminal monitor then issue the show voice call command with the voice port and timeslot if applicable specified to get the detailed DSP statistics In the DSP VOICE VP_ERROR STATISTICS section of the output look for the counters They are usually 0 or
8. 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 e FAX T38 CAMODE SUPP if set to Y the default value the CA controlled mode is supported for T38 fax relay e FAX INBAND_ SUPP if set to Y the default value the inband mode is supported for T38 fax relay e 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 Cisco BTS 10200 Softswitch T 38 Fax Relay External Interfaces W 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 MGA BCM H3A CNM API EstablishCx_ex PCMU T38 CRCX PCMU t38 RTP Cut thru H323 fast start SAI CCM OOBINFO REO ROQNT fxr t380 t NTFY fxr t38 start Service feature Ind fax start tart H245 Request mode t38 CNM API RemoteModifyCl_ex in
9. search since there could be a fax relay defect in the Cisco IOS Software release installed For example if T 38 fax relay is not working in a particular Cisco IOS Software release you can search for defects using the Bug Toolkit on CCO If a defect in a particular release causes all T 38 fax relay to fail then an upgrade is needed to a release in which this defect is no longer present Eliminate hardware faults In some cases it is easier to isolate the problem by excluding potential problem sources one by one You can do this by replacing different hardware parts and using alternative IP connections between the gateways When extra hardware is available the following steps can help Use different ports on the gateways If your configuration involves two gateways connected to PBXs or the PSTN with T1 or El and if you have the FXS ports available try to connect the fax machines directly to the FXS ports on the voice gateways This procedure will help further isolate the problem by excluding the possibility of the T1 or E1 cards failing problems on the telephony side or T1 or El clock synchronization or cable problems Cisco BTS 10200 Softswitch T 38 Fax Relay Support gy Cisco BTS 10200 Softswitch T 38 Fax Relay E Troubleshooting Fax Relay Try different hardware If you have another voice gateway with FXS ports available to you try to connect it directly with the Ethernet crossover cable to each of th
10. was a fax machine which one e What fax protocol events took place prior to the connection being terminated The following steps have been shown to resolve the majority of issues involving T 38 fax relay over packet networks Identify and isolate the problem The first step to take when you troubleshoot any fax relay issue is to reduce the problem to its simplest form Many issues arise in situations where multiple fax machines are not able to pass fax traffic It is easier to isolate two fax machines that are having problems and concentrate on a simple topology First determine how these two machines are connected to one another and resolve the issue between this pair In addition you should draw a complete picture of the network topology and determine how all the fax machines are interconnected Troubleshooting one issue at a time minimizes confusion and allows for methodical troubleshooting It is also possible that the solution for this problem will resolve other fax relay problems in the network Most fax relay problems result from poor packet network configuration or network design These lead to basic connectivity problems and physical line or packet loss and jitter problems After you have identified and isolated the problem the next steps relate to verifying the basic voice over packet network configuration and monitoring the health of the network Check basic network connectivity Basic fax connectivity issues can be the result
11. 200 Softswitch T 38 Fax Relay Support Onn Cisco BTS 10200 Softswitch T 38 Fax Relay Troubleshooting Fax Relay W Fax Analyzers Sometimes it 1s necessary to go beyond the debugging capabilities of the Cisco voice gateways to resolve fax relay problems Tools such as protocol analyzers and fax analyzers are used to see what is occurring during fax relay operation Fax analyzers such as the Genoa ChannelProbe FaxProbe by QualityLogic or the HP Telegra can be positioned between the fax device and the gateway to capture what is occurring Protocol analyzers such as Sniffer and Domino can also be helpful when you need to view the fax relay packets that are being exchanged between the gateways The ability to resolve a complex problem sometimes requires using a combination of test equipment an analyzer to capture the fax traffic at each fax machine and a protocol analyzer to capture the fax relay packets A single fax call is placed to reproduce the problem and then the information is captured from the attached devices for analysis Most fax analyzers have adequate help screens and documentation to help you determine what is happening The T 30 specification is also very helpful For the protocol analyzers decoding can be a little more difficult since sometimes the encodings are proprietary or the analyzer software does not have the specific decode needed If the protocol analyzer is not able to decode the frame the frame can be manual
12. Es 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 e 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 e 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 e 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 Cisco BTS 10200 Softswitch T 38 Fax Relay External Interfaces W External Interfaces This section contains information about the external operational management and administrative interfaces for the T 38 Fax Relay featur
13. Fax Relay Table 3 T 38 Fax Relay Tokens Token Values FAX RELAY CAP INBAND Possible only for G 711 and G 726 supported by MGX 8260 Specifies FAX Relay T38 CA FAX CA controlled issuing RequestMode relay in real time Option for this gateway This is the default value Cisco BTS 10200 Softswitch T 38 Fax Relay Support Ou Cisco BTS 10200 Softswitch T 38 Fax Relay H 323 Trunk Group Profile W H 323 Trunk Group Profile Command Types Examples Usage Guidelines Token Properties The H 323 Trunk Group Profile h323 tg profile table defines the characteristics of each H 323 trunk A h323 tg profile id must be created in this table before H 323 trunk group entries can be added Table Name H323 TG PROFILE Table Containment Area Call Agent Show add change and delete e show h323 tg profile id gwl cisco com e add h323 tg profile id ras tg ras n e change h323 tg profile id ras tg ras y e delete h323 tg profile id ras tg Primary Key Token s id Add Rules FK constraints If fax pref mode FAX T38 CAMODE then fax t38 camode supp Y If fax pref mode FAX INBAND then fax inband supp Y Change Rules FK constraints Delete Rules FK constraints Table 3 lists the token properties that were added to this command for T 38 Fax Relay Table 4 T 38 Fax Relay Tokens Token Values FAX T38 CAMODE SUPP Y N Default Y CA controlled T 38 fax relay in real time CA instructs GW when to sw
14. 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 e Introduction page 2 Fax Basics page 2 Fax Relay Basics page 3 Fax Modes page 4 e 1 38 Benefits page 4 e External Interfaces page 7 Billing Interface page 7 Alarms Events page 7 Measurements page 7 Provisioning page 8 e Related Features page 11 Supported Platforms page 11 e Supported Standards page 12 e Prerequisites page 13 e Configuration Tasks page 13 e Command Reference page 13 e Troubleshooting Fax Relay page 17 e Glossary page 24 e Copyright Notice page 27 Cisco IOS Release xx x x X Cisco BTS 10200 Softswitch T 38 Fax Relay HI 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 d
15. Interaction Manager SIP Session Initiation Protocol SNMP Simple Network Management Protocol SQL Structured Query Language SS7 Signaling System 7 SSTSS Slow Start To Slow Start Calls T 30 ITU T Recommendation for analog phone line Group 3 facsimile terminals T 38 Fax ITU T Recommendation T 38 describes the features necessary to transfer facsimile documents in real time between two standard Group 3 facsimile terminals over the Internet or other networks using IP protocols The recommendation allows the use of either TCP or UDP depending on the service environment TCS Terminal Capability Set Cisco BTS 10200 Softswitch T 38 Fax Relay Support gy Cisco BTS 10200 Softswitch T 38 Fax Relay E Glossary TDM Time Division Multiplexing TDP Trigger Detection Point TG Trunk Group VCM Voice Compression Module Cisco BTS 10200 Softswitch T 38 Fax Relay Support BEJE Cisco BTS 10200 Softswitch T 38 Fax Relay Copyright Notice W Copyright Notice THE SPECIFICATIONS AND INFORMATION REGARDING THE PRODUCTS IN THIS MANUAL ARE SUBJECT TO CHANGE WITHOUT NOTICE ALL STATEMENTS INFORMATION AND RECOMMENDATIONS IN THIS MANUAL ARE BELIEVED TO BE ACCURATE BUT ARE PRESENTED WITHOUT WARRANTY OF ANY KIND EXPRESS OR IMPLIED USERS MUST TAKE FULL RESPONSIBILITY FOR THEIR APPLICATION OF ANY PRODUCTS THE SOFTWARE LICENSE AND LIMITED WARRANTY FOR THE ACCOMPANYING PRODUCT ARE SET FORTH IN THE INFORMATION PACKE
16. LCD screen at the completion of the fax negotiation phase It is unlikely that the fax machines could complete negotiation if the fax codec had not been successfully downloaded On the other hand if no remote fax machine ID is displayed further debugging in this area is appropriate There are two ways to ensure the gateways detect the fax transmission and successfully load the fax codec e Issue the debug vtsp all command and the debug voip ccapi inout call trace Voice Telephony Service Provider VTSP is an architecture that defines the interface between the Cisco call control and a DSP end point connected to standard telephony equipment such as a PBX fax or central office via analog or digital interfaces The debug vtsp all output does not contain a lot of information from a T 30 protocol perspective but it does provide useful state information from the router such as Confirmation that the gateway entered fax mode Fax rate and codec used Confirmation that the terminating gateway received a fax tone from the terminating fax machine e Issue the show voice trace command Show commands use less resources than debug commands and are preferable in production networks Cisco BTS 10200 Softswitch T 38 Fax Relay Support gy Cisco BTS 10200 Softswitch T 38 Fax Relay HM Troubleshooting Fax Relay Step 6 Disable T 38 fax relay and change codec for passthrough In the previous troubleshooting steps you established that vo
17. MODE then fax t38 camode supp Y If fax pref mode FAX INBAND then fax inband supp Y Change Rules none Delete Rules id does not exist in any mgw mgw profile id Token Properties Table 3 lists the token properties that were added to this command for T 38 Fax Relay Table 5 T 38 Fax Relay Tokens Token Values FAX T38 CAMODE SUPP Y N Default Y CA controlled T 38 fax relay in real time CA instructs GW when to switch to T 38 mode FAX INBAND SUPP Y N Default Y Specifies whether inband mode of fax relay is supported FAX PREF MODE FAX T38 CAMODE This is the default value Designates preferred mode of FAX INBAND Possible only for G 711 and G 726 T 38 fax relay for this gateway supported by MGX 8260 Cisco BTS 10200 Softswitch T 38 Fax Relay Support 16 Cisco BTS 10200 Softswitch T 38 Fax Relay Troubleshooting Fax Relay W Troubleshooting Fax Relay Step 1 Step 2 The purpose of this section is to provide basic troubleshooting procedures for anyone trying to resolve Cisco BTS 10200 Softswitch T 38 fax relay issues The technical intricacies of faxing and fax relay are not covered in detail but you will get a basic understanding and be able to perform troubleshooting for a majority of common fax relay issues It is important to know the following information e Phase in which the fax transmission error occurred e Whether the gateways or fax machines terminated the connection and if it
18. Relay E Troubleshooting Fax Relay It is not always required that you have identical configurations on the outgoing and incoming VoIP dial peers When you have a fax relay problem though make sure you have a dedicated incoming VoIP dial peer on the terminating gateway and that its configuration matches the configuration of the outgoing VoIP dial peer on the originating gateway Another method you can use to check dial peer matching is to issue the debug voip ccapi inout command The debug output from this command will show an ssaSetupPeer message that lists all the dial peers matching the called number A ccCallSetupRequest message follows with the outbound peer option indicating the outgoing VoIP dial peer selected When multiple VoIP dial peers are configured for the same destination it is possible that the initial call setup could fail and another dial peer tried In this case another ccCallSetupRequest will appear in the debug On the terminating voice gateway the first line of the debug voip ccapi inout call trace will be acc_api_call_Setup_ind message with a peer_tag option that refers to the incoming VoIP dial peer on the terminating gateway Incorrectly configured dial peers on one or both sides After you confirm that the correct dial peer is being matched in this case dial peer 100 for the originating gateway and dial peer 400 for the terminating router confirm in the configuration that the dial peer is configured correctl
19. See the Command Reference section for configuration examples Command Reference This section documents new or modified Cisco BTS 10200 Softswitch commands associated with the T 38 Fax Relay feature All other commands used with this feature are documented in the Cisco BTS 10200 Softswitch Release 3 2 3 3 Command Reference Guide or in the Cisco user documentation for the gateway s you are using New Commands There are no new commands associated with the T 38 Fax Relay feature Modified Commands This section documents modified commands associated with the T 38 Fax Relay feature H 323 Gateway The H 323 Gateway h323 gw table defines the capabilities of each H 323 protocol gateway There can be four instances of an H 323 gateway running on the Call Agent at any one time Table Name H323 GW Table Containment Area Call Agent Command Types Show add change and delete Examples e show h323 gw id gw1 cisco com e add h323 gw id gw1 cisco com fax relay cap t38 ca fax Cisco BTS 10200 Softswitch T 38 Fax Relay Support a Cisco BTS 10200 Softswitch T 38 Fax Relay E H 323 Gateway e change h323 gw id gwl cisco com fax relay cap t38 ca fax e delete h323 gw id gwl cisco com Usage Guidelines _ Primary Key Token s id Add Rule status OOS Change Rule status OOS Delete Rules FK constraints status must be OOS Token Properties Table 3 lists the token properties that were added to this command for T 38
20. 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 MGWSs 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 a Cisco BTS 10200 Softswitch T 38 Fax Relay HI 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 e 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 gatewa
21. T THAT SHIPPED WIT H THE PRODUCT AND ARE INCORPORATED HEREIN BY THIS REFERENCE IF YOU ARE UNABLE TO LOCATE THE SOFTWARE LICENSE OR LIMITED WARRANTY CONTACT YOUR CISCO REPRESENTATIVE FOR A COPY The Cisco implementation of TCP header compression is an adaptation of a program developed by the University of California Berkeley UCB as part of UCB s public domain version of the UNIX operating system All rights reserved Copyright 1981 Regents of the University of California NOTWITHSTANDING ANY OTHER WARRANTY HEREIN ALL DOCUMENT FILES AND SOFTWARE OF THESE SUPPLIERS ARE PROVIDED AS IS WITH ALL FAULTS CISCO AND THE ABOVE NAMED SUPPLIERS DISCLAIM ALL WARRANTIES EXPRESSED OR IMPLIED INCLUDING WITHOUT LIMITATION THOSE OF MERCHANTABILITY FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT OR ARISING FROM A COURSE OF DEALING USAGE OR TRADE PRACTICE IN NO EVENT SHALL CISCO OR ITS SUPPLIERS BE LIABLE FOR ANY INDIRECT SPECIAL CONSEQUENTIAL OR INCIDENTAL DAMAGES INCLUDING WITHOUT LIMITATION LOST PROFITS OR LOSS OR DAMAGE TO DATA ARISING OUT OF THE USE OR INABILITY TO USE THIS MANUAL EVEN IF CISCO OR ITS SUPPLIERS HAVE BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGES CCVP the Cisco logo and the Cisco Square Bridge logo are trademarks of Cisco Systems Inc Changing the Way We Work Live Play and Learn is a service mark of Cisco Systems Inc and Access Registrar Aironet BPX Catalyst CCDA CCDP CCIE CCIP CCNA CCNP CCSP
22. ase refer to the online release notes or if supported Cisco Feature Navigator Supported Standards The T 38 Fax Relay feature is compliant with the following ITU standards Standards ITU T 4 Standardization of Group 3 facsimile terminals for document transmission e ITU T 30 Procedures for document facsimile transmission in the general switched telephone network e ITU T 38 Procedures for real time Group 3 facsimile communication over IP networks The T 38 Fax Relay feature utilizes the following Management Interface Bases MIBs MIBs e SNMP MIB v2c To obtain lists of supported MIBs by platform and Cisco IOS release and to download MIB modules go to the Cisco MIB website on Cisco com at the following URL http www cisco com public sw center netmgmt cmtk mibs shtml RFCs e to be determined Cisco BTS 10200 Softswitch T 38 Fax Relay Support ea Cisco BTS 10200 Softswitch T 38 Fax Relay Prerequisites E Prerequisites The T 38 Fax Relay feature requires that the following prerequisites be completed prior to sending or receiving fax transmissions e to be supplied Configuration Tasks The T 38 Fax Relay feature requires that the following configuration tasks be completed prior to sending or receiving fax transmissions Each task in the list is identified as either required or optional e Configure the H 323 Gateway e Configure the H 323 Trunk Group profile e Configure the Media Gateway profile
23. atures 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 e Only partial support of MGCP fax package fax start fax stop fax failure events only e 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 e CA controlled mode feature cannot function for H 323 to H 323 calls transiting the CA 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 acommon preferred mode which is also present among the common capabilities that mode is chosen Ifno common preferred T 38 mode is available then one of the common T 38 mode capabilities is chosen
24. controller T1 El and 1 0 will vary command several times during a fax call to see if the number of errors increases The most common problem with slips is timing synchronization resulting in clocking errors In packet voice networks confirming that the router is clocking from the line is usually sufficient If it isn t ensure that the clock source line command is entered at the controller level However in VoATM or TDM networks where a clocking hierarchy is established and the gateways might need to pass clock signals through the network additional considerations need to be made Check fax interface type On some Cisco media gateways including the Cisco 3660 5300 5350 5400 and 5800 the gateway defaults to fax interface type modem The fax interface type modem global configuration command forces fax calls to a modem usually for T 37 Store and Forward fax and not to a DSP For T 38 fax relay to work the fax call must be sent to a DSP which means it must be configured by issuing the fax interface type t38 command Make sure you reload the Cisco media gateway otherwise the fax interface type t38 command won t take effect Fax calls will fail on some Cisco media gateways with T 38 fax relay so this is an important command to check This problem is seen when a gateway is upgraded to Cisco IOS Release 12 2 or later Make sure that the fax codec is loaded during the fax call Each fax machine displays the remote fax machine ID on its
25. e which include the following e Billing Interface e Alarms Events e Measurements e Provisioning Billing Interface Three new fields have been added to the Call Data Blocks CDBs to support the T 38 fax relay feature e 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 Voice amp Fax 3 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 e 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 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 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 measurem
26. e 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 e 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 e 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 e 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 Ifa 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 restric
27. e to an end of image message sequence by the receiving end It indicates that the image was received with less than five percent of lines in error It is normally followed by the DCN disconnect message 7 Ifthere is no MCF message the transmission was not completed successfully It may suggest a high error rate caused by one of the following e Digital line errors clocking cabling e VoIP packet loss queueing prioritization fragmentation compression e Hardware fault e Cisco IOS DSPW incompatibility rarely occurs 8 Ifthe DIS or DCS messages are retransmitted several times it may be that they are not passed across the VoX connection correctly or only passed in one direction a software or configuration problem 9 If training repeats multiple times each time at a lower speed and then the transmission fails it may be that the fax codec was not loaded and the voice gateways handle the fax transmission as a normal voice conversation again a configuration or software issue Trace information may look different in different versions of Cisco BTS 10200 Softswitch software but you should be able to understand whether the fax codec was loaded correctly and which fax protocol was used by the output following the FAX keyword If the fax codecs were not loaded correctly there is probably a software problem Make sure that the latest release is used and if there is still a problem open a case with the Cisco TAC Cisco BTS 10
28. e voice gateways and send a fax using the fax machine connected to the FXS port This procedure will help determine if there are problems in the packet network such as queuing fragmentation or prioritization e Use debug commands on the gateways to determine what is happening Describing the details of debug outputs is beyond the scope of this document but described below are the basic transactions that are commonly seen when debugging T 38 fax relay For more in depth information on T 30 messaging or for information on messages that are not listed below see the T 30 specification 1 The fax transmission starts with the exchange of DIS DCS CSIDIS TSILDCS messages e DIS is the initial message stating the capabilities of the answering end The accompanying CSI frame has the phone number e DCS defines the transmit parameters and starts an image transfer sequence The accompanying TSI frame has the phone number 2 The fax machines go into training mode and can make several attempts to agree on a transmission speed For instance the fax machines may first train to a speed of 9600 bps fail and then train to a speed of 7200 bps 3 Successful training is followed by a CFR message 4 Transmission starts following the CFR message 5 If there is a high number of errors a good fax analyzer will detect them If the terminating fax machine considers the error rate too high it will terminate the connection 6 MCF is the normal respons
29. ents refer to the Cisco BTS 10200 Softswitch Operations Guide Cisco BTS 10200 Softswitch T 38 Fax Relay Support a Cisco BTS 10200 Softswitch T 38 Fax Relay E External Interfaces Provisioning 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 e 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 e FAX T38 CAMODE SUPP if set to Y CA controlled mode is supported for T38 fax relay e FAX INBAND_ SUPP if set to Y the inband mode is supported for T38 fax relay e 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
30. etails 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 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
31. fax analyzer traces connected in the monitor mode to the originating and terminating fax machines if available e Results of the troubleshooting and debugs already performed if available Cisco BTS 10200 Softswitch T 38 Fax Relay Support E Cisco BTS 10200 Softswitch T 38 Fax Relay WE Glossary Glossary AIN Advanced Intelligent Networking ANI Automatic Number Identification ANI is a service that provides the receiver of a telephone call with the number of the calling phone The service is often provided by sending the digital tone multi frequency DTMF tones with the call Users of ANI can screen callers with this information ANM Announcement API Application Programming Interface ATM Asynchronous Transfer Mode BCM Basic Call Module CA Call Agent CALLP Call Processing CAS Channel Associated Signaling CDB Call Detail Block CFNA Call Forwarding No Answer CLC Close Logical Channel CLCA Close Logical Channel Acknowledge CNAM Calling Name CNM Connection Manager CTX Centrex DA Directory Assistance DAS Data Acquisition System DSP digital signal processor ECS Enmpty Capability Set EDP Event Detection Point EMG Emergency 911 Call EMS Element Management System EPOM Extensible Provisioning and Operations Manager FCP Feature Control Protocol FS Feature Server FTP File Transfer Protocol GUI Graphical User Interface H 235 H 235 provides security for
32. for these brands of fax machines Typically the fax NSF command is used to set the NSF field to all zeroes which forces a standard fax negotiation from both sides Using this command has been successful with certain brands such as Harris and Lanier and it is recommended when T 38 fax relay is failing When all else fails If the preceding troubleshooting steps did not resolve the T 38 fax relay issue the problem may require more advanced troubleshooting Listed below are additional steps to try before you open a case with the Cisco Technical Assistance Center TAC Learn the brands and models of the fax machines that are failing and investigate those brands and models for known issues Sometimes there are defect reports that address known problems for certain brands of fax machines These defects are not in the Cisco IOS Software they are known incompatibilities with the fax machines proprietary fax signaling protocol when these fax devices are used on each side of a connection The workaround is to disable the proprietary protocol on the fax machines or to disable T 38 fax relay and use a higher bandwidth codec e Use search tools to look for known fax problems in the Cisco IOS Software release where the problem is occurring In the previous step searches were made for a specific fax brand in the hope of identifying a known issue between a certain fax brand and the Cisco T 38 fax relay code The next step is to perform a generic
33. ge 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 Da Cisco BTS 10200 Softswitch T 38 Fax Relay Introduction W 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 amp 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 co
34. ic set of features and which features are supported in a specific Cisco IOS image You can search by feature or release Under the release section you can compare releases side by side to display both the features unique to each software release and the features in common To access Cisco Feature Navigator you must have an account on Cisco com If you have forgotten or lost your account information send a blank e mail to cco locksmith cisco com An automatic check verifies that your e mail address is registered with Cisco com If the check is successful account details with a new random password is e mailed to you Qualified users can establish an account on Cisco com by following the directions at http www cisco com register Cisco Feature Navigator is updated regularly when major Cisco IOS software releases and technology releases occur For the most current information go to the Cisco Feature Navigator home page at the following URL http www cisco com go fn Availability of Cisco IOS Software Images for Media Gateways Support for the features available in the Cisco BTS 10200 Softswitch Release 3 3 software is dependent on the availability of the appropriate Cisco IOS software images for the supported platforms media gateways IOS software images for some Cisco platforms may be deferred delayed or changed without notice For updated information about platform support and availability of software images for each Cisco IOS software rele
35. ice calls work faxes work through PSTN and all digital interfaces in the fax relay path are free from errors This step determines whether faxes can go through with T 38 fax relay disabled Under the dial peers enter the following commands on both media gateways gateway name config voice port 2 0 15 gateway name config voiceport no echo cancel enable gateway name config dial p voice 3 gateway name config dial peer fax rate disable gateway name config dial peer codec g71llulaw gateway name config dial peer no vad These commands disable fax relay disable echo cancellation and force the call to use a high bandwidth codec without voice activity detection VAD The gateway samples the fax tones like a normal voice call With the high bandwidth codec G 711 the most precise sample possible is captured and the tone replayed on the other side is as accurate as possible Be aware that the G 711 is a 64 kbps bandwidth codec so each call consumes up to 80 kbps when the additional transport protocol overhead is added If this test is positive two things have been accomplished First if per call bandwidth consumption is not a major issue for the network there is now a potential fax passthrough workaround for the fax relay problem e Secondly and more significantly if bandwidth consumption is an issue the problem has been isolated to the T 38 fax relay software and you should open a case with the Cisco TAC If this test
36. itch to T 38 mode FAX INBAND SUPP Y N Default N FAX PREF MODE FAX T38 CAMODE This is the default value Designates preferred mode of FAX INBAND Possible only for G 711 and G 726 supported by T 38 fax relay for this gateway MGX 8260 Cisco BTS 10200 Softswitch T 38 Fax Relay Support gy Cisco BTS 10200 Softswitch T 38 Fax Relay HI Media Gateway Profile Media Gateway Profile The Media Gateway Profile mgw profile 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 An ID must be created in this table before entries can be added to the Media Gateway table Several tokens have values that can be overwritten after the Call Agent queries the Media Gateway for supported capabilities If the Media Gateway returns a value different from the value originally provisioned the returned value automatically replaces the originally provisioned value Table Name MGW PROFILE Table Containment Area Call Agent Command Types Show add change and delete Examples e show h323 mgw profile id resgw2000 e add h323 mgw profile id resgw2000 fax inband supp N fax pref mode fax t38 camode e change h323 mgw profile id resgw2000 fax inband supp N e delete h323 wgw profile id resgw2000 Usage Guidelines _ Primary Key Token s id Add Rules If fax pref mode FA X T38 CA
37. le for these problems and it makes sense to disable cRTP when fax relay problems occur and to check whether a Cisco IOS Software upgrade is an appropriate course of action Ensure that the VC Ware and Cisco IOS Software are compatible On Cisco AS5300 series voice gateways the Cisco IOS software might be incompatible with the VC Ware software Fax connectivity problems across the PSTN Hi Cisco BTS 10200 Softswitch T 38 Fax Relay Support Cisco BTS 10200 Softswitch T 38 Fax Relay Step 3 Step 4 Step 5 Troubleshooting Fax Relay W If voice calls work in both directions but fax calls are failing in at least one direction check that normal faxing between these two machines works across the PSTN In other words ensure the fax machines successfully transmit faxes to each other using the PSTN without traversing the packet network If they do not one or both fax machines may have problems that need to be addressed before you consider fax relay problems Check for slips and other errors on the interfaces If any T1 or E1 digital connections are used by the gateways performing T 38 fax relay make sure that they are error free T 38 fax relay is very sensitive to errors on digital interfaces especially slips The errors may not be noticeable on voice calls but they can cause fax calls to fail The T1 or E1 controllers at both the originating and terminating gateways should be error free If errors occur repeat the show
38. ly decoded using the specification With T 38 fax relay a Cisco proprietary format is used for the fax relay packets With fax analyzer and protocol analyzer information you should be able to resolve most T 38 fax relay problems Few fax relay problems reach this point and when they do escalation and other resources should already be involved for further assistance Opening a TAC Case If this document has not enabled you to isolate and resolve the problem open a case with the Cisco Technical Assistance Center TAC and provide the following information e Network topology description PDF Visio or Microsoft PowerPoint format e The fax machines used including vendor and model information The history of the problem Useful information includes whether the implementation is new or an established network that worked well and then failed If the network was established what changed before the problem occurred Is the problem intermittent Can the problem be reproduced and if so what are the steps to reproduce the problem Output of the show tech command from both fax gateways and all the routers on the IP path and relevant information for the active non Cisco network equipment e A pair of the call traces with the following debug flags enabled debug voip ccapi inout debug vtsp all debug isdn q931 if the ISDN or Q Sig is involved e A pair of the show voice call and show voice dsp outputs e A pair of the
39. o 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 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 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 Hi Cisco BTS 10200 Softswitch T 38 Fax Relay Support Cisco BTS 10200 Softswitch T 38 Fax Relay Related Features W Related Features The T 38 Fax Relay feature is directly related to the H 323 features introduced in Release 3 1 and 3 2 amp Note For more information on the H 323 features refer to the Cisco BTS 10200 Softswitch System Description Related Documents This Release 3 2 T 38 Fax Relay Feature Module should be used in conjunction with the following Cisco BTS 10200 Softswitch Release 3 1 and 3 2 documents e Cisco BTS 10200 Softswitch Physical and Network Site Surveys and Data Sheets Cisco BTS 10200 Softswitch Cabling Procedures Cisco BTS 10200 Softswitch Release Notes for Release 3 2 e Cisco BTS 10200 Softswitch System Description Cisco BTS 10200 Softswitch Application
40. of the following factors e Normal voice connectivity problems Confirm that normal voice calls can be completed before investigating fax connectivity If there is no telephone attached to the fax machine unplug it and connect a regular telephone If normal voice calls do not connect the issue may be network related and you should troubleshoot the problem as a connectivity issue before fax troubleshooting Configuration problems related to dial peers such as Wrong dial peer being matched After ensuring that voice calls can be successfully completed in both directions through the network issue the show call active voice brief command and note the dial peers that are being matched with each voice call When you have VoIP trunks you should be able to see all the call legs with the show call active voice brief command Ensure that the configured dial peer is the peer that is being matched A common cause of fax relay problems is that the correctly configured dial peer is not the one being matched It is also common that there is no particular incoming VoIP dial peer configured on the terminating gateway and the Cisco IOS software selects the first appropriate including default VoIP dial peer as the incoming dial peer Hence the parameters for this incoming dial peer may not match those of the outbound dial peer on the originating gateway Cisco BTS 10200 Softswitch T 38 Fax Relay Support gy Cisco BTS 10200 Softswitch T 38 Fax
41. originating Signaling Adapter of the reception of 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 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 S Note 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 The Signaling Adapter upon receipt of the T 38 fax_start information invokes the CNM API 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 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 t
42. rt 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
43. st 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 A fax machine after generating a CED or hearing a CNG transmits a T 30 Digital Information Signal DIS as pa
44. the RAS signaling between H 323 endpoints and gatekeepers so that only duly authenticated and authorized endpoints are able to use Gatekeeper resources H 323 ITU T Recommendation for Visual Telephony System and equipment for local area networks which provide a non guaranteed quality of service H3A H 323 Gateway Adapter HCM High density Compression Module HNPA Home Numbering Plan Area Hi Cisco BTS 10200 Softswitch T 38 Fax Relay Support Cisco BTS 10200 Softswitch T 38 Fax Relay Glossary W INTRALATA Calls that occur within a single LATA INTERLATA Calls that cross LATA boundaries 1P Internet Protocol ISDN Integrated Services Digital Network ISUP ISDN User Part SS7 LATA Local Access Transport Area LNP Local Number Portability LRN Local Routing Number MCM Multimedia Conference Manager MGA Media Gateway Adapter MGCP Media Gateway Control Protocol MIB Management Information Base NANP North American Numbering Plan NPA Numbering Plan Area area code OLC Open Logical Channel OLCA Open Logical Channel Acknowledge PCS Personal Communications Service PIN Personal Identification Number POTS Plain Old Telephone Service PSTN Public Switched Telephone Network QOS Quality of Service RSVP Resource Reservation Protocol SAI Signaling Adapter Interface SCP Service Control Point SDP Session Description Protocol SIA Signaling Interface Adapter SIM Service
45. ting 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 e 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 Support 4 Cisco BTS 10200 Softswitch T 38 Fax Relay T 38 Benefits W 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 H3 A supported this capability only for IVR applications amp Note Any gateway that requires closure of both logical channels after sending ECS is not supported by the Cisco BTS 10200 Softswitch e 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 e Seamless integration of various fe
46. y for fax Some common errors to check for on both sides of the call are e Fax relay is disabled that is the fax rate disable command has been issued on the dial peer while a low bandwidth codec has been in use The dial peer on one gateway is configured for T 38 fax relay but the other gateway is configured for Cisco proprietary fax relay so the negotiation will fail e The default dial peer is used inbound on the terminating gateway and these parameters do not match with the parameters set for the outbound dial peer on the originating gateway Incorrect compand type The companding type for the US is p law for EMEA it is a law The show voice call command dispolays which value is currently configured If on a BRI or E1 port the companding type on the gateway does not match the one on the connected device calls will sometimes fail and sometimes connect When a connection is made the voice becomes heavily distorted the person becomes unrecognizable and a high volume low pitch noise level is heard Other basic connectivity problems not relating to dial peers including Cisco IOS Software incompatibilities on gateway pairs It is not always required that Cisco IOS Software releases on the gateways match but it is recommended to check releases for compatability when fax relay problems occur Compressed Real Time Transport Protocol CRTP problems There are several known problems associated with cRTP Fixes are availab
47. ys e pt uUse 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 tThis 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 PROFIL
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