Modem Passthrough with NSE

The basic difference between the fax passthrough feature and the modem passthrough feature is whether the 2100 Hz answer tone from the answering fax modem contains phase reversals. If the answer tone contains phase reversals, the modem passthrough feature is engaged but an additional NSE message will be triggered. Devices with transmission rates faster than a typical fax machine, such as high-speed modems (using V.34 modulation speeds and higher) and Super G3 (SG3) faxes, send an ANSam tone...

Troubleshooting the Switchover Signaling

When troubleshooting fax and modem issues, it is important to confirm that the switchover from voice mode to fax modem relay or passthrough mode occurs in the proper manner. Recall that fax and modem calls on Cisco voice gateways always start off as a voice call initially and only after certain tones or flags are detected will a switchover happen. When this switchover does not happen, fax or modem calls almost always fail. Referring back to Figure 12-1, troubleshooting the switchover signaling...

Baudot Character

The original Baudot code was invented by Emile Baudot around 1870. Subsequent modifications adapted the code for teletypewriter use and altered it into its present-day form. The Baudot code used in today's text telephone protocols implements a character set that is defined by TIA EIA-825-A. This specification categorizes each character as a letter or a figure accessible by a shift function. Table 3-4 details the TIA EIA-825-A Baudot character set. Table 3-4 Baudot Character Set as Defined by...

Page Encoding

Therefore, before transmission, the page information is digitized by the fax device's scanner. First, the page is divided into horizontal lines known as scan lines. The scanner then moves across each line and based on the brightness level creates black and white picture elements (pixels). The typical scan line has 1728 pixels. The terms pixels per inch (ppi) and dots per inch (dpi) are often used interchangeably when discussing fax pages. Technically, however,...

Troubleshooting the Creation of the Fax Page Image

After the offramp MSPI process has successfully received the e-mail information, this information is passed to the DMSP process to be rendered into standard fax pages. The DMSP process can render both text and TIFF information from the incoming e-mail. Visibility into the DMSP process and its conversion of e-mail information to standard fax pages is quite limited. The debug that provides the best insight into DMSP is accessed via the command debug fax dmsp. This is the default DMSP debug...

Fax Pass Through with SIP Signaling

With SIP as the call signaling protocol, fax pass-through is very similar to H.323. SIP sets up a normal VoIP call, and then when the V.21 preamble is detected, SIP handles the transition to pass-through mode. Figure 4-10 illustrates fax pass-through with SIP as the call signaling protocol. Figure 4-10 Fax Pass-Through Call with SIP Signaling Protocol Figure 4-10 Fax Pass-Through Call with SIP Signaling Protocol Pass-through parameters for the upcoming session are established in the SIP INVITE...

T38 Fax Relay and Cisco Text Relay Configuration for SCCP

As mentioned throughout this chapter, SCCP fax, modem, and text features are configured globally under voice service voip. Example 10-4 shows an SCCP configuration for T.38 fax relay and Cisco text relay. In Example 10-4, the SCCP voice signaling protocol uses NSE-based T.38 fax relay, as shown by the keyword nse in the fax protocol t38 command. The force keyword within this same command line ensures that NSEs are always used even if NSE support has not been verified on the remote device during...

IOS Gateway Cisco Modem Relay Configuration for H323 SIP and SCCP

Cisco modem relay transports high-speed modem calls across IP networks using a Cisco proprietary relay protocol. The V.34 modulation along with V.42 error correction and V.42bis compression is officially supported. However, other high-speed modem modulations such as V.90 will work after they have been forced to train down by the Cisco modem relay feature. TIP Cisco modem relay should be used only between two Cisco gateways because the NSE transition and Cisco modem relay protocol itself are...

Baudot Protocol

The Baudot text protocol is the de facto standard for text telephone communication in North America and the protocol that will be focused on in this section. Other text protocol options such as ASCII and Turbo Code are available in North America, but they are optional features on some text telephones, and Turbo Code is a proprietary implementation. Additional text telephone protocols may be used in other countries (see Table 3-1). The best resource for technical details on these other text...

IOS Gateway NSEBased Passthrough Configuration for MGCP

NSE-based passthrough configuration for MGCP uses completely different commands than SIP and H.323. For MGCP, there is no dial-peer configuration. It is entirely done in global configuration mode. Table 9-2 covers the available commands for an MGCP NSE-based passthrough configuration. These commands support both fax and modem passthrough. The same configuration should be on both the TGW and OGW. Table 9-2 Passthrough IOS Configuration Commands for MGCP Table 9-2 covers the available commands...

Call Control Protocol

The call control protocols used when transporting fax, modem, and text with Cisco voice gateways in IP networks are H.323, Session Initiation Protocol (SIP), Media Gateway Control Protocol (MGCP), and Skinny Client Control Protocol (SCCP). However, not all of these call control or voice signaling protocols support all the various passthrough and relay transport methods. Table 7-4 provides a quick overview of the transport methods supported by the H.323, SIP, MGCP, and SCCP voice signaling...

HPI Debugs for Cisco Modem Relay

The command debug voip hpi provides good information about a Cisco modem relay call, too. Just like with a fax relay call, you can view the message from IOS that programs the DSP. Example 12-58 shows the modemrelay_mode message as seen with the command debug voip hpi. Example 12-58 modemrelay_mode HPI Message from Host to DSP Apr 19 04 56 30.895 MR Role Called leg, MR DebugState 0x00000000, Latency 200(ms), Retries 12, XID Negotiation Enabled, dict_size 1024, StringLength 32, Compress direction...

Fallback

The fallback feature on Cisco IOS voice gateways provides a means for an alternate fax transport protocol to be used if the initial T.38 fax relay transport method fails to negotiate successfully. Fallback is only available with T.38 fax relay on H.323 and SIP voice gateways, and two different options are available. The fallback itself occurs seamlessly with either option, and in most cases the fax machines never realize that a fallback has even occurred. The first fallback option occurs by...

Fax Server T38 Integration with a Cisco Voice Gateway

The current trend with fax server deployments is to use the T.38 fax relay protocol for integrating the fax server directly into IP networks. Two primary integration models are possible when connecting a T.38-enabled fax server with Cisco equipment. The first model involves the fax server communicating directly with Cisco voice gateways, and the other model involves the addition of Unified CM to handle the call control signaling between the fax server and the voice gateways. In this section,...

Fax Passthrough with NSE

In this section, the information exchange and negotiations between a TGW and an OGW for an NSE-signaled fax passthrough call are covered. Figure 4-7 illustrates this message flow for a typical fax call. Figure 4-7 Fax Passthrough Call with NSE Signaling Figure 4-7 Fax Passthrough Call with NSE Signaling OGW Upspeed Codec and Switch to Passthrough Mode OGW Upspeed Codec and Switch to Passthrough Mode TGW Upspeed Codec and Switch to Passthrough Mode NOTE The NSE signaling flow shown in Figure 4-7...

IP Troubleshooting for NonIOS Gateways

The 6608, VG248, and ATA also have statistics and counters pertaining to the incoming RTP media stream. Although this information might not be quite as easy to access as it is on the IOS gateways, the information itself is almost identical. The best way to look at the DSP counters on the 6608 is by using the Dick Tracy utility. You can find a reference to a document on Cisco.com providing more information about Dick Tracy in the section Telephony Troubleshooting earlier in this chapter. The...

Summary and Best Practices

Although design information for basic VoIP networks is not hard to find, applying VoIP-specific design information to modulated communications rarely works. Planning for modulated communications such as fax, modem, and text communications in IP networks requires focused design information tailored to these technologies. This chapter provided the necessary design information required for implementing fax, modem, and text communications over IP. Organized into four distinct sections, a number of...

Fax Modulations

Fax devices use multiple modulation schemes for communication. Many of these modulation schemes may seem familiar because they are used by modems. You can learn more about the concept of modulation, and read a discussion about common modem modulation types in the section Modulation in Chapter 1, How Modems Work. The ITU-T Recommendation V.21 is one of the first modulation standards created, and it is used for all the messaging that occurs in a G3 fax transmission. Operating at 300 bps, the low...

Troubleshooting the Offramp Telephony Interface

The offramp telephony interface is responsible for taking the fax pages generated by the DMSP process and transmitting them out the gateway's voice port to the destination fax machine. The main process involved in the offramp telephony interface from the T.37 perspective is FMSP. The FMSP process communicates with the destination fax machine using the ITU-T T.30 protocol, which was covered in detail throughout Chapter 2. The best technique for troubleshooting the T.30 messaging between the FMSP...

Secure RTP

Defined in IETF RFC 3711, Secure Real-Time Transport Protocol (SRTP) provides for encryption of the RTP protocol used by VoIP. Without SRTP, VoIP conversations can be easily captured and listened to with a simple packet-capture device or software program. SRTP encrypts the VoIP conversations so that they are protected from unauthorized eavesdropping. One of the main benefits of the SRTP encryption scheme is that only the RTP payload is encrypted. Therefore, secure tunnels for the media do not...

Fax Server Redundancy and Failover

To provide a truly fault-tolerant fax server solution, multiple fax servers can be deployed in a redundant manner. This then allows network elements such as voice gateways, Unified CM, or even gatekeepers to fail over to a backup fax server when the primary fax server is no longer available. With a fault-tolerant solution, a fax server going down or losing connectivity to the IP network does not result in a total loss of an organization's fax processing ability. Providing redundancy and...

Future Look at ITUT T38 V1501 and V151

The three most promising standards for fax, modem, and text relay are T.38, V.150.1, and V.151, respectively. Although V.150.1 and V.151 have yet to be widely adopted compared to T.38, all of these standards offer a glimpse into the future of transporting modulated data over an IP network. T.38 has been increasingly deployed in recent years as the de facto fax relay solution. With this increased deployment, T.38 has continued to evolve and offer new features and functionality. Two of the more...

Checking the Modem Endpoints

Modems are capable of a variety of configuration options, and some of these options can cause the modem calls to fail over a modem relay connection. For example, Cisco modem relay supports only the V.34 modulation. Any modem communication that is not capable of V.34 will fail to use Cisco modem relay as its transport through the VoIP network. Assuming that the information in the rest of the chapter did not resolve your Cisco modem relay problem, you should look at the modem endpoints to ensure...

VCO Voice Carry Over

As its name implies, VCO is a text telephone option that allows a party to speak his responses while reading information that is directed to him. The CA in a VCO scenario acts as the ears for the disabled person and translates the voice directed to that person into text. Figure 3-8 demonstrates VCO with a CA. Figure 3-8 Text Telephone Call Using VCO Figure 3-8 Text Telephone Call Using VCO Some text devices have built-in VCO features that allow the user to pick up the handset to speak while...

Unified CM Integration

The Unified CM product is the heart of most Cisco IP telephony deployments. Subsequently, its support of fax is a common design concern. After all, to fully migrate a legacy voice infrastructure over to IP, Unified CM must be able to handle both voice and fax communications. In the past, fax support on Unified CM has lagged behind the fax capabilities of the Cisco voice gateways. This is one of the reasons for the implementation of Cisco proprietary NSE packets for handling the fax switchover....

Acknowledgments

David Hanes First, I would like to thank Gonzalo Salgueiro for co-authoring this book with me. I can honestly say that this book would never have happened without him. Gonzalo's diligence in ensuring that this book achieved the highest level of excellence and accuracy was unparalleled. His meticulous attention to detail and long hours late at night working in the lab and reviewing chapters required many sacrifices and time away from his family. In addition to being my co-author in the writing...

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

Weitbrecht Modem

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...

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...

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...

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...

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...

Cisco Gateway Dscp

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...

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...

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...

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

Udptl Packet Structure Example

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...