NSEBased Switchover for T38 Fax Relay
Even though T.38 is a standards-based protocol for fax relay, it can use proprietary NSE messages for the switchover. The NSE-200 message signifies a T.38 switchover request. The NSE-201 message represents a T.38 switchover ACK (acknowledgment), and the NSE-202 message is a NACK (negative acknowledgment) and means that the T.38 switchover request has been declined. The most common reason for a gateway to send an NSE-202 NACK is when the gateway is not properly configured for NSE-based T.38 fax...
Brief History of Text Telephony
Teletypewriters or TTYs were originally used by businesses such as telephone and media companies to relay printed text. On the receiving end, electrical pulses over a line were converted to typed letters on a page. Many TTYs featured a keyboard and a printer and this allowed two-way typed communication between units. Early TTYs were massive machines weighing several hundred pounds, and some were even fittingly painted battleship gray. Composed of gears, clutches, levers, and other moving parts,...
Fax Server TDM Integration with a Cisco Voice Gateway
In the older, traditional fax server deployment model, fax servers were connected directly to PSTN circuits or conventional PBX devices. To maintain this same sort of connectivity in modern VoIP networks, many voice gateways can pass TDM timeslots from a digital PSTN connection to a fax server using either the TDM hairpin call or the drop-and-insert feature, as shown in Figure 8-3. Figure 8-3 Fax Server TDM Integration with a Cisco Voice Gateway Voice gateway routes calls to the fax server...
Relay Fundamentals
Compared to passthrough, the IP transport mechanism of relay is more complicated. During a relay call, the gateway plays a more active role in the communication occurring between the data devices. The voice gateway configured for relay must be able to understand and speak the protocols that are being used by the fax, modem, or text devices themselves. For example, when a fax relay call takes place, the voice gateways understand the T.30 and T.4 messaging that occurs between the fax machines....
Default Fax Relay Configuration for H323 and SIP
Example 10-1 shows the default fax relay configuration for H.323 and SIP. At this point, no fax relay commands have been configured on the voice gateway. However, even though there are not any visible fax-specific commands in the configuration, default commands are present, and a fax call can be handled by the gateway. Example 10-1 Default Fax Relay Configuration for H.323 and SIP voice service voip is not present in the configuration when its options do not differ from the defaults. Default...
Fax Send Configuration Commands for Offramp
A number of T.37 offramp commands are available that enable you to alter the fax send configuration parameters. These commands are not configured under a dial-peer but are global in nature. Many of these fax send parameters deal with the formatting and information displayed on the fax image page and the cover page. Other commands enable you to adjust the fax transmission speed and the information contained in the T.30 transmitting subscriber identification (TSI) message. For easier...
Passthrough Fundamentals
With only a few minor variations that are discussed at the end of this section, a passthrough call is treated the same as a VoIP call from a voice gateway perspective. The human voice sample that is processed by the gateway on a VoIP call is simply replaced with the modulated data used by faxes and modems. For both voice and passthrough calls, a process known as pulse code modulation (PCM) converts an analog signal to an equivalent digital representation. This digital signal is what is...
About the Technical Reviewers
Richard Collette is a senior project manager for Sagem-Interstar, where his primary focus is the development and troubleshooting of Voice over IP applications. He holds a bachelor's degree in electrical engineering from l' cole Polytechnique de Montr al. Richard has been involved in the design and development of fax applications for more than 15 years. John Combs is manager of Accessibility Safety Telecom for the Cisco Compliance & Certification department. He is responsible for ensuring...
Fax Call PCM Trace Analysis
Analyzing a basic fax call with a PCM trace is not too difficult. After both the audio streams for both directions are opened in Audacity, you can figure out what is happening. Figure 12-23 illustrates bidirectional PCM traces for a successful one-page fax call. Figure 12-23 PCM Trace of a Successful Fax Call Figure 12-23 PCM Trace of a Successful Fax Call In Figure 12-23, you will see that there are two smaller windows with waveforms within the Audacity window. Each of these smaller windows...
Brief History of Modems
Like several other core Internet and computer technologies, the modem was first developed in the 1950s for the Semi-Automatic Ground Environment (SAGE) air defense system. The modems were used to transmit military data over dedicated telephone lines between terminals at the various participating sites. As commercial computer use increased, so did the need for communications between them. AT& T manufactured the first commercial modem, known as the Bell 103, in 1962. The Bell 103 allowed...
Text Design Considerations
Text calls over IP can be transported in one of two ways by Cisco IOS voice gateways text over G.711 or Cisco text relay. The text over G.711 method is a manual passthrough configuration that uses the G.711 codec to transport the text tones across the IP network. Cisco text relay is a proprietary transport method that passes text characters out of band using special RTP payload types. Both text over G.711 and Cisco text relay and how they each work were covered in previous chapters. For more...
Advanced Troubleshooting for Cisco Text Relay
If you are already familiar with the general concepts of how a voice gateway feature such as Dual Tone Multi Frequency (DTMF) relay works, Cisco text relay is similar. In both cases, tones are taken from the incoming audio on the telephony side of the call and relayed across the IP network. On the other side, the tones are accurately replayed back into the audio stream out of the far end voice gateway's voice port. The main difference is that with DTMF relay these tones represent DTMF keys,...
Cisco Text Relay
Cisco text relay provides the functional equivalent of fax and modem relay for text telephones. The Baudot tones used by text devices are decoded and passed as characters across the IP network before being played back as Baudot signals once again on the far side. Cisco text relay is a proprietary solution that leverages portions of different specifications to implement a viable method for transporting text over IP. Table 5-5 details the multiple specifications that Cisco text relay uses. Table...
Sample Onramp Configuration
Example 11-3 combines many of the T.37 onramp commands discussed in the previous sections into a cohesive, working configuration. Comments are made for some of the commands to aid in understanding certain configuration sections, but for more detailed explanations refer back to the tables containing these commands. Example 11-3 T.37 Onramp Gateway Configuration hostname fax_2811 Output omitted for brevity Define the value to be used in the CSI message sent to the originating fax machine. Enable...
Troubleshooting the Offramp SMTP Connection
The offramp T.37 application is activated with the reception of a fax e-mail by the MSPI process. Without a successful fax mail reception, T.37 offramp fails from the beginning. Therefore, it is important to effectively troubleshoot the offramp SMTP connection and its MSPI process before looking at other offramp processes. If you just need to view SMTP and the messages exchanged between the mail server and the offramp gateway, use the debug fax mta command. Example 13-13 highlights the envelope...
Protocol Based Pass Through for
The other method used to trigger a transition from voice mode to passthrough mode is in the messaging of the VoIP call signaling protocol. Although the call signaling protocol's primary responsibility is to set up and tear down the VoIP call, it can also be used to transition to passthrough mode. At this point, a distinction needs to be made between the term passthrough and pass-through. The term passthrough is what has been used until now when referring to the VBD feature in general and the...
NSEBased Passthrough
When passthrough is configured on a voice gateway, it takes the modulated data from a fax, modem, or text device and transparently transports it in the media stream as PCM samples encapsulated in RTP. The terminating gateway (TGW) always switches to NSE-based passthrough mode first by detecting the appropriate tone from the answering modem or fax machine. This tone is the 2100 Hz CED from a standard fax machine or the 2100 Hz ANSam tone from a modem or SG3 fax machine. When the TGW detects this...
Baudot Modulation Details
In addition to standardizing the Baudot character set, EIA-TIA-825-A also details how these bits are modulated asynchronously across a PSTN network using character oriented FSK (Frequency Shift Keying). A binary 1 is represented by a 1400 Hz tone, and a 0 is encoded with a 1800 Hz tone. Each of these bits is transmitted for a duration of 22 ms for a signaling rate of 45.45 bps. For more detailed information on FSK modulation, see the section Frequency Shift Keying (FSK) in Chapter 1. NOTE Be...
Comparing Modem Passthrough and Cisco Modem Relay
Of all the transport options available for fax and modem communications, modem passthrough enjoys the most widespread support among the Cisco voice gateways. All the Cisco IOS voice gateways support modem passthrough and all the non-IOS voice gateways. From a Cisco voice gateway interoperability standpoint, modem passthrough is always safe to use. However, when it comes to third-party voice gateway integration, the proprietary NSE-based switchover of modem passthrough is not supported by other...
Dealing with Packet Loss
In an ideal world, packet loss would not exist in VoIP networks. However, this is not the case in the real world, and packet loss is present. Fortunately, fax relay provides a couple of options to deal with it. If possible, packet loss problems and other IP impairments should be resolved and not allowed to affect fax, modem, and text calls. Unfortunately, however, this is not always possible. In scenarios where communications must occur over the Internet or through a network where you have no...
Modified Huffman
MH is a combination of Huffman and Run Length Encoding types. Developed by David Huffman in 1952, Huffman coding specifies that short bit representations should be used for the most commonly occurring characters. So, the binary coding used to identify a character is inversely proportional to that character's frequency. Using the alphabet as an example for Huffman encoding, commonly used letters such as T and E would be assigned a smaller bit pattern compared to letters that are rarely used such...
VG248 Passthrough Configuration
The VG248 is a 48-port analog gateway that is controlled by Unified CM through the SCCP signaling protocol. It is commonly used to provide analog lines to fax machines and modems. The VG248 is different from most other Cisco voice gateways. The hardware and software architecture, supported features, and the user interface are unlike other platforms in the Cisco family of gateways. For example, the VG248 configuration is accomplished through an ASCII-like menu interface. The VG248 supports modem...
Advanced Troubleshooting for Passthrough
Cisco voice gateways support two types of passthrough. The first type uses an NSE-based switchover for faxes and modems, and it is configured using the modem passthrough command in IOS voice gateways. The second type of passthrough uses a protocol-based switchover, and it is referred to as pass-through because it is configured by the IOS command fax protocol pass-through. In addition, pass-through is only for faxes because it is triggered only by the fax specific V.21 flags. From a...
Hairpin Calls
A hairpin call occurs when a standard inbound telephony call is simply routed back out another telephony interface on the same voice gateway. A VoIP component is not present for this sort of call. This type of call is also commonly referred to as a POTS-to-POTS call, TDM switching, or a TDM hairpin call. A scenario involving a hairpin call is illustrated in Figure 7-4. In this figure, a Cisco voice gateway is connected to the PSTN by a digital T1 or E1 circuit. Voice calls are routed via VoIP...
NSEBased Switchover for Modem Passthrough
S DSP d VoIP payload 0x64 ssrc 0x1EF2 sequence 0x1FBB Jan 10 22 01 58.463 Jan 10 22 01 58.471 Pt 100 Evt 192 Pkt 00 00 00 < Snd> > > s DSP d VoIP payload 0x64 ssrc 0x1EF2 sequence 0x1FBD Jan 10 22 01 58.471 Jan 10 22 01 58.483 Pt 100 Evt 192 Pkt 00 00 00 < Snd> > > s VoIP d DSP payload 0x64 ssrc 0x9A20101 sequence 0x18C2 timestamp 0xADA80F0A Jan 10 22 01 58.483 < < < Rcv> Pt 100 Evt 192 Pkt 00 00 00 Example 12-27 debug voip rtp session named-event Command Output for a...
Message Format Overview
With the exception of certain signals that are nothing but single frequency tones, the T.30 messages used in communication between two fax devices consist of binary coded data. From a high-level overview, this can simply be viewed as a preamble followed by the binary data itself in the form of High-Level Data Link Control (HDLC) frame(s) as shown in Figure 2-6. Figure 2-6 Primary Components of a T.30 Fax Message A G3 fax preamble occurs every time new information is sent in either direction....
Fax Server T38 Integration with Unified CM
The main disadvantage to sharing voice gateway resources between a fax server and Unified CM as discussed in the previous section is that separate fax and VoIP networks are being used. Although this is the easiest and quickest way to integrate a fax server into a Unified CM network, it is not always the most efficient from a management perspective. For example, separate dial plans must be maintained for the fax server network and the VoIP network. Although this sort of deployment model might be...
Phases of a Fax Call
To simplify the fax messaging process further, T.30 breaks down a fax call into five distinct phases. Recognizing these phases assists you in further understanding the messaging that occurs between two fax devices. Figure 2-5 summarizes these fax messaging phases. Figure 2-5 Phases of a T.30 Fax Call Figure 2-5 Phases of a T.30 Fax Call Off-hook, Dialing, Ringing, Answering, CNG and CED Tones Fax Terminal Identification, Capabilities Exchanged and Set, Training Transmission of Pages, Line...
HPI Debugs for Fax Relay
Viewing the HPI messages for a Cisco or T.38 fax relay call allows you firsthand access to what is being seen by the DSP and the communications between the DSP and IOS during a call. If the DSP is not detecting V.21 fax flags or being programmed correctly, problems may result. Example 12-54 shows the DSP notifying IOS that a fax tone has been detected. This is a message that is typically seen on the terminating voice gateway, which can then trigger an NSE-based, RTP PT-based, or protocol-based...
Protocol Based Fax Pass Through and T38 Switchovers for H323
The H.323 voice signaling protocol can handle both fax pass-through and T.38 fax relay within its protocol stack. Both of these fax transport methods are triggered by the detection of V.21 fax flags on the terminating gateway, and both use the H.245 Request Mode message for initializing the switchover. The H.245 Request Mode message specifies the new media stream parameters for the fax call. This in turn leads to the opening of logical channels for the new fax media stream and the closing of...
IOS Gateway Protocol Based Pass Through Configuration
Protocol-based pass-through uses the actual messaging of the signaling protocol to signal a switchover from voice mode to pass-through mode. H.323 and SIP are the only voice signaling protocols that support protocol-based pass-through. MGCP used to support protocol-based pass-through in older versions of IOS, but it is no longer supported in more current versions. NOTE The support for pass-through using the MGCP protocol stack was dropped because of protocol conflicts and reliability concerns....
VG248 Fax Relay Configuration
The VG248 is an analog gateway consisting of 48 FXS ports controlled directly by Unified CM using the SCCP protocol. Since code version 1.3(1), the VG248 has supported both Cisco fax relay and NSE-based T.38 fax relay. Neither Cisco modem relay nor Cisco text relay is supported by the VG248. There are only two main screens on the VG248 where the fax relay configuration parameters are set. One screen sets global attributes for fax relay, and the other screen is attached to each individual port...
Bandwidth
Bandwidth consumption of passthrough and relay calls is one of the most overlooked aspects of VoIP network design, and it can have a major impact on network capacity planning. Often, the VoIP network is designed with only traditional VoIP calls in mind. Modulated traffic such as faxes is often overlooked completely or the improper assumption is made that modulated communications and voice traffic can be accounted for in the same manner. All passthrough and relay calls start out as voice calls...
Debugging Best Practices
As you delve deeper into troubleshooting fax, modem, and text problems, you will eventually need to enable debug commands on the Cisco gateways and Unified CM, if it is present, to view certain pieces of information. However, enabling debugs in an incorrect manner can cause instability in those devices and impact their performance in a negative way. Therefore, it is highly recommended that you follow the best practices and tips in this section whenever it is necessary to enable debugs. NOTE...
SMTP Commands and Sample Sessions
The easiest way to grasp how SMTP works is to look at a basic SMTP session. Example 6-1 details how a simple e-mail is transmitted between two mail devices using SMTP. 220 smtp-outbound.cisco.com ESMTP Sendmail 8.12.10 8.12.6 Fri, 20 Oct 2006 250 smtp-outbound.cisco.com Hello 192.168.1.1 , pleased to meet you MAIL FROM < testuser cisco.com> 250 2.1.0 testuser cisco.com Sender ok RCPT TO < myfriend example.com> 250 2.1.5 myfriend example.com Recipient ok 354 Enter mail, end with . on a...
Troubleshooting the Onramp SMTP Connection
In Figure 13-1, the MSPI process is responsible for establishing and maintaining the SMTP connection to the mail server. In addition, after the TIFF is created by DMSP, it is passed to MSPI for transport as an e-mail attachment. From a troubleshooting standpoint, the MSPI process can be looked at directly with debug fax mspi. Also, you can look strictly at the SMTP commands that are exchanged during the session with debug fax mta. Although a number of different options exist for the debug fax...
Troubleshooting the Cisco Fax Relay Switchover
Cisco fax relay uses a unique switchover method composed of RTP payload types. Like NSE-based switchovers, the Cisco fax relay switchover uses the existing RTP media stream and bypasses the voice signaling protocol. So, it does not matter whether the voice signaling protocol is H.323, SIP, MGCP, or SCCP when it comes to a Cisco fax relay switchover. For IOS gateways, the debug command for viewing NSE switchovers is also the command to use for Cisco fax relay switchovers debug voip rtp session...
Telephony Troubleshooting
Although checking the POTS call leg using the show call active voice brief command and its variants is important in telephony troubleshooting, other commands exist that provide additional information. The most important of these IOS commands is show controllers t1le1 . Voice gateways have a variety of analog and digital telephony interfaces that are used to connect to the PSTN or directly to fax, modem, and text devices. However, digital interfaces are responsible for more problems with fax and...
IOS Gateway Cisco Modem Relay Configuration for MGCP
The configuration of Cisco modem relay for the MGCP voice signaling protocol is similar to that of H.323, SIP, and SCCP. The main difference is that with MGCP, the command syntaxes differ a little. The main command that enables Cisco modem relay for MGCP is mgcp modem relay voip mode nse. Although this command still requires at least one additional argument, this core command enables Cisco modem relay and specifies a switchover using NSE packets. In conjunction with the gw-controlled argument,...
T37 Storeand Forward
As discussed in the previous chapter, T.37 store-and-forward fax provides a conversion between faxes and e-mail. This is a unique process for handling fax communications, and it allows T.37 to serve as an alternative transport method to fax passthrough and relay. The ability to send and receive faxes directly from an e-mail client is the main allure of T.37. Without T.37, a typical fax scenario could be similar to the following 2 Go retrieve it from a printer. 3 Walk the document over to an...
Timing and Synchronization
Timing and synchronization on the voice gateway are more critical for fax, modem, and text communications than for voice. Even seemingly minor timing problems can cause fax, modem, and text calls to fail. The clocking on a voice gateway's digital interfaces and the lack of clock synchronization between DSPs on the originating and terminating gateways are two areas that can present timing and synchronization problems. Of the two, achieving error-free clocking on the voice gateway's digital...
Troubleshooting the Onramp Telephony Interface
The onramp telephony interface is composed of the physical voice port that connects to the fax machine along with the associated FMSP T.37 process. In addition, the raw T.30 fax messaging can be viewed on this telephony interface, too, if you want to troubleshoot the actual fax connection itself. When a fax machine initiates a call to the onramp telephony interface, this interface acts exactly as a terminating fax machine. All the T.30 messaging is terminated by this interface, and it is not...
Troubleshooting the TIFF Image Creation
The process responsible for the conversion of the incoming fax page into a TIFF image is DMSP. Unfortunately, the debugs for DMSP do not provide a lot of visibility into what is actually occurring in the TIFF creation process. Most of the information presented by the debug fax dmsp command relates to the bridging of the DMSP process with FMSP and MSPI. Example 13-5 highlights the relevant messages from the debug fax dmsp command when DMSP bridges to its peer processes. Example 13-5 debug fax...
IOS Gateway Fax Relay Configuration for MGCP
When MGCP is the voice signaling protocol, a new set of IOS CLI commands must be used for configuration. From a syntax perspective, you will notice that these commands share most of the keywords found in the H.323, SIP, and SCCP fax relay command set. MGCP signaling makes use of the concepts of call agent (CA) controlled signaling and gateway-controlled signaling when it comes to transitioning to fax relay. Because Cisco fax relay can operate only in a gateway-controlled manner, these concepts...
Call Legs in IOS Gateways
One of the most fundamental concepts in routing a call through an IOS voice gateway is that of a call leg. The notion of a call leg is inexorably tied to that of a dial-peer. In the case of a VoIP call, two types of dial-peers are used POTS dial-peers and VoIP dial-peers. A POTS dial-peer configures the characteristics of its corresponding telephony interface and ties a dial string to a specific voice port on the local gateway. Correspondingly, a VoIP dial-peer sets the attributes of the IP...
Modulation
The analog signals used in the transmission of modulated data are simply sinusoidal waveforms, and the primary components of any waveform are the amplitude, the frequency, and the phase. Figure 1-10 helps explain these concepts visually. The amplitude is the magnitude of the wave. In the diagram, Wave 1 has twice the amplitude of Wave 2. The frequency is the inverse of the wavelength and thus is the number of oscillations occurring in a period. Note that in Figure 1-10 Wave 1 has half the...
Call Setup
The V.34 initial handshaking procedure between an originating modem and a terminating modem can be broken down into the four primary phases shown in Figure 1-18. Figure 1-18 V.34 Modem Call Setup Procedure V.8 Messaging - Disable Echo Cancellers, Exchange Capabilities, Determine Call Modulation Line Characterization, Determination of Carrier Frequency, Symbol Rate, and Round Trip Delay Half Duplex Equalizer and Echo Canceller Training, Digital Impairment Learning (V.90) Full Duplex Training and...
Handling High Delay
Fax machines, like most other modulated communication devices, were designed for PSTN environments where the delay time between when a message is sent and when it is received is minimal. Nonetheless, fax machines tend to have quite a high tolerance for delay. Even with the introduction of IP networks and their inherently much higher delay times compared to the PSTN, delay does not usually affect fax transmissions. Only certain high-delay scenarios, such as satellite hops or multiple VoIP...
Comparing Fax Passthrough and Fax Relay
One of the most common design questions about transporting fax over IP has to do with selecting passthrough or relay as the transport method. The resounding question from a network design perspective is which one is better Unfortunately, the answer to this question is usually not simple and depends on a number of factors, most of which have already been discussed in this chapter. Therefore, the best way to decide between a fax passthrough or relay implementation is to consider the differences...
QoS
QoS is the measure of transmission quality and service availability for a network. A sufficient level of QoS must be ensured for the real-time traffic of fax, modem, and text otherwise, these communications will not be reliable. The transmission quality aspect of QoS is determined by the impairment factors of packet loss, delay, and jitter. Table 7-5 defines these factors while also commenting on how they impact fax, modem, and text traffic compared to VoIP. As discussed in Table 7-5, a...
Capturing PCM Traces
You can obtain a PCM trace in multiple ways. Some ways will be better than others depending on the situation, tools available, and expertise of the person obtaining the capture. Table 12-15 highlights the most commonly used PCM capture methods and notes their specific advantages and disadvantages. Works for analog and digital voice ports Provides PCM traces from three different reference points within the DSP Allows for the capture of multiple calls at the same time that must be decoded by a...
Protocol Based Fax Pass Through and T38 Switchovers for SIP
Like H.323, the SIP voice signaling protocol can also support both fax pass-through and T.38 fax relay switchovers. The SIP equivalent to the H.245 Request Mode message is known as a re-INVITE. The SIP re-INVITE message is responsible for transitioning the voice call to either fax pass-through or T.38 fax relay. Within the SIP re-INVITE message are SDP parameters that define the new media stream for the fax call. These SDP parameters and the re-INVITE message itself are best viewed with the...
Resource Utilization
Fax, modem, and text calls and their different transport methods may impact the resources of a voice gateway differently. In some cases, this can lead to a need for more DSP resources, and in other cases this can lead to a need for more bandwidth on an interface. Understanding how fax, modem, and text calls can impact the resources on a Cisco voice gateway is an important design concept. Properly planning the resource use of a voice gateway in the beginning can prevent problems later when...
MTA Configuration Commands for Onramp
To send fax images over e-mail, an onramp gateway must be able to interact via SMTP with the MTA. Therefore, MTA-specific configuration commands are necessary for defining the different SMTP headers and other parameters associated with an e-mail. Table 11-9 lists the MTA commands for onramp and describes their function. Table 11-9 MTA Configuration Commands for Onramp Fax mta send filename string date This command specifies a particular filename for the string specifies the filename of the...
Loss Planning
The primary purpose of loss planning in a traditional telephony environment is to maintain the voice signal at an optimal level throughout the network. This ensures the best voice quality from a user perspective and mitigates other voice-impacting impairments such as echo. Loss planning in voice networks can take on another layer of complexity with voice gateways being inserted into the path of traditional telephony links or being part of an IP PBX solution. This has led to the development of...
IOS Gateway Fax Relay Configuration for H323 SIP and SCCP
The IOS command set for fax relay with H.323, SIP, and SCCP is relatively the same. Identical CLI command syntaxes can be used with each of these voice signaling protocols to achieve the same configuration results. The H.323 and SIP protocols take this same command set a step further, and the whole fax relay configuration itself, from the command placement in the configuration to supported relay features, is identical. In fact, it is even possible to switch between the H.323 and SIP signaling...
Super G3 Faxing
Defined in ITU-T Recommendation T.30 Annex F, the Super G3 fax classification is a highspeed alternative to a G3 fax call. Whereas G3 has a maximum page transmission speed of 14.4 Kbps, Super G3 can transmit at speeds up to 33.6 Kbps using the V.34 modulation. Consequently, Super G3 faxing is also called V.34 faxing. The V.34 modulation is covered in detail in the section Modem Call Analysis in Chapter 1. Table 2-5 shows the key differences between SG3 and G3. Many of these differences occur...
Cisco PCM Capture Tool
The Cisco PCM capture tool requires you to enable a feature known as the HPI logger. The HPI logger will then stream the DSP trace information to a file. The steps for enabling the HPI logger on Cisco IOS gateways are as follows Step 1 Configure the HPI capture buffer size using the IOS configuration command voice hpi capture buffer size, where size refers to the capture buffer size. This capture buffer is used internally for the DSP messages, and a typical value is 1000000. Step 2 Configure...
Loading the TCL Scripts
The IOS voice gateway requires a Tool Command Language (TCL) script to know how to handle an incoming onramp or offramp fax call. Unlike some scripts that are bundled with the IOS software itself, the onramp and offramp TCL fax scripts are typically downloaded from the Cisco website and placed on the voice gateway's flash memory. You can find the location of the latest TCL scripts for T.37 onramp and offramp faxing at the following website. Note that access to these scripts requires a valid...
Configuring T37 Onramp
Configuring T.37 onramp allows fax calls coming into the voice port of an IOS voice gateway to be converted to an e-mail attachment. The IOS gateway handles the conversion process, whereby the incoming, standard fax page is changed into a TIFF file and attached to an outgoing e-mail. Dial-peers are still used to route the onramp call through the voice gateway, but there are some configuration differences compared to the dial-peer configuration used for a fax relay or fax passthrough call....
T37 Onramp Troubleshooting
Because T.37 onramp converts incoming fax calls to e-mails, troubleshooting onramp requires a thorough understanding of both the ITU-T T.30 protocol and SMTP. The T.30 protocol and its messages are discussed in detail throughout Chapter 2, How Fax Works, and SMTP is covered in the section SMTP Overview of Chapter 6. To better visualize the T.37 onramp process that occurs in a Cisco voice gateway, consult Figure 13-1. This figure illustrates how an incoming fax call comes in through the...
Dial Peer Configuration for Onramp
Just like a passthrough or relay call through a Cisco IOS gateway, T.37 onramp must also match an inbound and outbound dial-peer. In the case of T.37 onramp, the inbound dial-peer will always be a POTS peer, and the outbound dial-peer will be an MMoIP peer. From a configuration perspective, the inbound onramp POTS dial-peer is configured the same as any other inbound POTS dial-peer for a VoIP call with the addition of the service command. Table 11-4 covers this command and its function. Table...
Dial Peer Configuration for Offramp
As with any other standard VoIP call going through a Cisco voice gateway, T.37 offramp requires an inbound and outbound dial-peer. On the inbound side for the connection to the mail server, an MMoIP dial-peer is configured. On the outbound leg, a POTS dial-peer is needed. Unlike T.37 onramp, which requires T.37-specific commands on both dial-peers, T.37 offramp has only T.37-specific commands for the inbound MMoIP dial-peer. The offramp POTS dial-peer is just configured normally to route the...
Group Classifications
In the beginning, facsimile communication was achieved through proprietary solutions with a dedicated use, such as sending and receiving pictures for newspapers. With the evolution of fax groupings pertaining to varying standards, fax messaging has grown rapidly, and interoperability is no longer a major issue. An international standards body called the International Telecommunications Union Standardization Sector (ITU-T) recognized the need for establishing transmission standards that could be...
T37 Offramp Troubleshooting
The T.37 offramp application converts e-mails containing text and TIFF attachments to fax calls. This conversion process depends on the offramp gateway successfully receiving the fax e-mail over an SMTP connection with the TIFF file in the proper format. Only then can a normal fax call be generated from the offramp gateway's voice port to the terminating fax device. Figure 13-2 illustrates the IOS processes involved in a Cisco offramp gateway. Each of these processes have a dedicated function...
ATA Passthrough Configuration
The Cisco ATA (Analog Telephone Adaptor) only offers passthrough as a fax transport solution. Neither of the two ATA models have support for fax relay. There are no current or future plans to add such functionality. Although the ATA does not officially support modems when it is in passthrough mode, many customers have had success with modem calls over an ATA. NOTE The two currently available ATA models are the ATA 186 and the ATA 188. They are the same except for the fact that the ATA 186 has a...
IP Troubleshooting for IOS Gateways
For Cisco IOS voice gateways, the best commands for viewing the incoming IP media stream statistics are the commands show call active voice brief and show voice call port, where the port is the voice port associated with the media stream in question. The command show call active voice brief was already covered in the section Viewing Call Legs earlier in this chapter, so this section focuses on the command show voice call port. Much of the same information is displayed in the output of either of...
IP Troubleshooting Using Packet Captures
The best way to troubleshoot the IP portion of a fax modem text call is with a packet capture. Packet captures provide an unbiased, third-party view of what is occurring on the IP network. In addition, you can observe not only the incoming media stream, but also the outgoing one to ensure that the gateway is properly transmitting packets as it should. NOTE Although there is an abundance of packet-capturing software available, the easiest to acquire and use is probably Wireshark. The Wireshark...
Analyzing PCM Traces
After a PCM trace file has been obtained from one of the three methods outlined in the previous section, the file must be processed so that it can be seen and heard. A number of available audio programs can accomplish this task. Some of the more popular programs are Adobe Audition (formerly CoolEdit), Audacity, and Goldwave. The examples in this section use the free, open source Audacity program, but any of the others work just fine, too. You can download Audacity from http...
Overview of T37 Storeand Forward
Approved by the ITU-T in 1998, the T.37 specification is a conglomerate of standards that address different facets of the conversion process from fax to e-mail and vice versa. The major standards referenced in T.37 are detailed in Table 6-1. G3 fax standards that T.37 gateways must implement to converse directly Covers SMTP and Extended SMTP (ESMTP) procedures for relaying mail Defines MIME (Multipurpose Internet Mail Extensions) and how it works Details Internet fax procedures based on...
Secure Modem Relay
Secure modem relay may also be referred to as secure communication between STE endpoints. This transport method is different from Cisco modem relay, and it is specifically designed for transporting the specific V.32 or V.34 modulations used by secure telephone devices. These devices pass encrypted voice using these modulations in an effort to prevent eavesdropping. NOTE This section provides only a general overview of secure modem relay because of the unique, focused market segment that...
Understanding Error Correction Mode
Initially found on more expensive fax machines, ECM is a feature that is found increasingly more often on today's fax devices. Defined by ITU-T Recommendation T.30 Annex A, ECM ensures a much higher level of fax page data integrity than what is consistently achieved on non-ECM G3 fax transactions. Whereas normal G3 fax transactions typically present a fax with acceptable quality, ECM usually guarantees near perfect quality. The ECM feature is negotiated at the beginning of a fax call during the...
Protocol Based T38 Switchover for MGCP
The MGCP voice signaling protocol supports T.38 fax relay, but unlike H.323 and SIP does not support fax pass-through on Cisco voice gateways. Therefore, only the protocol-based T.38 fax relay switchover can be troubleshot for MGCP. The FXR package on Cisco voice gateways is the MGCP package necessary for T.38 fax relay support. This package name is sent to the CA (call agent) in the response to the MGCP AUEP (Audit Endpoint) message. If the FXR package is not sent to the call agent in the AUEP...
Debugging T30 Fax Messaging
The most frequently used tool for troubleshooting fax relay problems is the IOS command debug fax relay t30 all-level-1. This command displays the T.30 messaging that is passing through the Cisco voice gateway from the perspective of the gateway's DSP. Output that is equivalent to this IOS command can be found on Cisco non-IOS gateways using the procedures discussed at the end of this section. Before using this command, you should have already determined that the Cisco voice gateways are...
Analyzing T38 Fax Relay Packet Captures
Packet capture programs such as the freely available Wireshark program can decode T.38 fax relay and even graphically display the fax messaging that is being transported by T.38. For more information about Wireshark and how to acquire this software program, refer back to the section IP Troubleshooting in this chapter. TIP Sometimes the T.38 fax relay information in Wireshark is not displayed properly by default, and this usually occurs when NSEs are used for the switchover. If the T.38 packets...
Cisco Fax Relay and Modem Passthrough Configuration for H323 and SIP
When fax and modem calls are occurring through the same gateway, it is possible to handle them using the same transport method or different methods protocols may be used. For example, with the command modem passthrough nse for SIP and H.323 gateways, all fax and modem calls can be handled using voice-band data (VBD) and the G.711 codec. However, fax and modem calls can also be handled through different means, too. Various passthrough and relay schemes can be used to handle fax and modem calls...
Dsp Hpi Troubleshooting
The interaction between a DSP involved in a fax, modem, or text call and the operating system software of the Cisco voice gateway can be quite complex. In the case of IOS gateways, the software architecture is such that there are multiple call processing and control layers present just between the voice signaling protocol and the DSP itself. Each of these layers within IOS has specific functions that are integral to ensuring proper call handling. Figure 12-16 provides a high-level overview of...
Modem Call PCM Trace Analysis
Analyzing PCM traces of modem calls is more difficult than fax calls. This difficulty has to do with the more complex negotiations and full duplex nature of most modem calls. However, some basic analysis can still take place to help you troubleshoot modem problems. After a modem PCM trace has been loaded into an audio program such as Audacity, you will see the modem negotiation at the beginning of the call. Depending on the modulation and supported negotiation procedures, this negotiation can...
Analyzing a Basic Fax Call
Although the phases of a fax call provide a general overview of how fax calls work, the next step is to look into the actual T.30 messages and tones that make up these phases. Analyzing a basic call is the easiest way to get acquainted with fax signals. Figure 2-9 illustrates an example of a typical, two-page G3 fax transaction. In Figure 2-9, the fax machine on the left is originating a call to the fax machine on the right. The CNG and CED signals are simple tones that occur in the very...
Gateway Interoperability Considerations
Because of the various methods for transporting fax calls over IP, the interoperability of different voice gateways must be considered when creating a network design. Table 7-12 provides a quick summary of the different fax transport methods that are available for fax. The technical details of these methods have already been discussed in Chapters 4 and 5. From a design perspective, these different transport methods for fax highlighted in Table 7-12 require due diligence in verifying that a...
Validating NSE Switchover Support
If the output from the command debug voip rtp session named-event shows no NSE switchover messages or messages in only one direction, the most common reason for this problem is that the NSE switchover ability was not confirmed first within the voice signaling protocol. Although the voice signaling or call control protocol does not handle the actual switchover, in some cases it does confirm ahead of time that each voice gateway supports an NSE-based switchover. By the gateways validating NSE...
Modem Relay
Modem relay is functionally equivalent to fax relay except that modems are the end devices rather than fax machines. Voice gateways demodulate and modulate the modem signals as they enter and exit the IP network while the actual modem data is relayed across IP using special modem relay protocols. From a Cisco voice gateway perspective, there are multiple modem relay implementations. Two of these implementations are Cisco proprietary, and although the other is standards based, it is not...
T38 Fax Relay
T.38 is a fax relay standard defined by the ITU-T. Because it is a standard, T.38 is now the predominant choice for fax relay scenarios, especially in networks where multivendor interoperability is necessary. Cisco voice gateways fully support the original 1998 version of the T.38 specification. Although later versions of T.38 introduce such features as Real-Time Protocol (RTP) encapsulation and support for SG3 V.34 faxing, these are not discussed in this chapter. Only the aspects of T.38...
Sample Offramp Configuration
The configuration of T.37 offramp on Cisco IOS voice gateways is not a difficult task, especially if you follow the quick reference configuration steps in Table 11-10. Following those steps can assist you in creating a working T.37 offramp configuration like the one shown in Example 11-4. Example 11-4 T.37 Offramp Gateway Configuration hostname fax_2851 Output omitted for brevity Define the TSI value to be sent by the offramp gateway to the terminating fax machine. fax send...
MTA Configuration Commands for Offramp
The MTA T.37 offramp commands configure the connection between the offramp gateway and the mail server. Table 11-15 defines these commands and their functions. Table 11-15 MTA Configuration Commands for Offramp Fax mta receive generate mdn I permanenterror Specifies a hostname to be used as an SMTP alias for the offramp gateway. Up to 10 different aliases can be configured. The gateway will accept an incoming e-mail as long as it matches one of the configured aliases. Note If this command is...
T38 Fax Relay and Modem Passthrough Configuration for MGCP
A common scenario for handling regular faxes along with modems or SG3 fax devices at the same time is to use a fax relay protocol for the fax traffic and modem passthrough for the modem or SG3 fax traffic. Although this scenario can be configured for the H.323, SIP, MGCP, or SCCP voice signaling protocols using T.38 or Cisco fax relay, Example 10-5 shows how this scenario is configured specifically for an MGCP voice gateway running NSE-based T.38 fax relay and modem passthrough. Example 10-5...
Cisco Fax Relay
Cisco fax relay is a proprietary fax relay implementation on Cisco voice gateways. Developed before the T.38 fax relay standard, Cisco fax relay provides the same basic functionality as T.38, but there are differences with regard to packet formats and the mechanisms used for switching into fax relay mode. Just like T.38, Cisco fax relay demodulates all incoming fax signals and passes the fax tones and HDLC frames across the IP network using data packets. At the far end, these data packets are...
User Interface
The standard method used for an async DTE to control its DCE (for example, modem) is through a command-line interface (CLI) protocol called the AT interface. AT stands for ATtention each command line sent by the DTE is prefixed with AT, which serves to get the CLI's attention. As discussed earlier in the history section, the AT interface was introduced by Hayes Microcomputer Products (now a brand of Zoom Telephonics) in 1981. It has existed as an evolving de facto standard, with many...
Error Correction Mode
The Error Correction Mode (ECM) feature provides a means for fax machines to ensure error-free page transmissions. This feature is optional, and not all fax devices support ECM. Even on fax machines that do support ECM, it can usually be disabled. The ECM feature can be of critical importance for faxed information, especially contracts and legal documents. Without the ECM feature enabled, a small percentage of scan-line errors can occur without causing a complete call failure. This in turn may...
Fax Detect Script
Cisco IOS gateways can run TCL scripts for handling a wide variety of both voice and fax features. Common TCL scripts for fax include the onramp and offramp scripts that are used when configuring T.37 store-and-forward fax. Another TCL script that is available from the Cisco website is the fax detect script. Registered users can download this script at The fax detect TCL script allows for Cisco IOS voice gateways to provide a single number reach capability for voice and fax calls. One telephone...
T38 Fax Relay Cisco Modem Relay and Cisco Text Relay Configuration for H323 and SIP
Different modulation types can each be configured with their own relay protocol. A fax modulation may use Cisco fax relay or T.38, modems can use Cisco modem relay or modem relay for secure communications, and text communications can use Cisco text relay. Each traffic type has its own unique relay transport methods. Example 10-3 illustrates a configuration where fax, modem, and text traffic are each assigned an appropriate relay transport method. T.38 fax relay is chosen for the fax traffic,...
