C

Complexity mode, 225 flex-complexity mode, 226 CAC (Call Admission Control), managing bandwidth consumption on VoIP networks, 213 CA-controlled T.38 fax relay, 321 call analysis, 34 call disconnect, 49 data compression, 48-49 error control, 45-48 ECM fax calls, 82-84 call analysis (fax) CED tone, 67 CFR messages, 73 CNG tone, 66 CSI messages, 70 DCN messages, 76 DCS messages, 71-72 DIS messages, 68 EOM messages, 75 EOP messages, 75 FTT messages, 73 MCF messages, 76 MPS messages, 75 NSF...

A

Abbreviations (text telephone communication), 116-117 accessing fax servers, 267 acoustic coupling, 113 acronyms (text telephone communication), 116-117 adaptive Huffman encoding, 48 ADP (Answer Detection Pattern), 47 advanced troubleshooting Cisco text relay, 506-510 fax relay, 487 high delay, 500, 502 NSF NSS messages, 499-500 packet loss, 488, 490 Super G3, 490-491 T.30 fax messaging, 491-496 modem relay, 503-506 passthrough, 485-486 AFE (analog front end), 9 Americans with Disabilities Act,...

Brief History of

Fax machines can trace their roots back to a patent issued in 1843 to the Scottish clockmaker Alexander Bain (1811-1877). Bain discovered a way to transmit a two-dimensional image as a series of electrical pulses across two wires. The genius of Bain's patent was the stylus mechanism used to scan the image. Building on his knowledge as a clockmaker, Bain invented a stylus that was an electrically conductive swinging pendulum. As the pendulum swung back and forth across a raised image on a copper...

Future Look at ITUT V152

Both NSE-based passthrough and protocol-based pass-through have their positives and negatives. NSE-based passthrough works for modem and faxes, but this solution is Cisco proprietary and excludes Cisco voice gateways from interoperating with other vendors. On the other hand, protocol-based pass-through offers third-party interoperability, but this is only an option for fax calls. Modem calls do not work with protocol-based pass-through. The ITU-T has a specification titled V.152 that details a...

Acoustic Coupling Versus Direct Connections

Because of telephone company restrictions, the very first text telephones used an acoustic coupler to connect to the PSTN. Until the laws were changed many years later, direct connections were not allowed into the telephone company network. Today's text telephones offer either acoustic coupling or direct connections or both. Figure 3-5 illustrates a text telephone with an acoustic coupler that is connected to a Cisco IP Phone. Figure 3-5 Text Telephone with an Acoustic Coupler Connected to a...

Advanced Troubleshooting for Modem Relay

Compared to fax relay, modem relay has limited troubleshooting resources on the IOS voice gateways themselves. Troubleshooting debug commands such as debug modem relay and its various options produce output that is useful only to developers familiar with the Cisco implementation of modem relay. This debug command output is not decodable by end users. Therefore, much of the troubleshooting for modem relay occurs on the modem endpoints themselves and through the viewing of statistical information...

Attacking the Problem

Fax, modem, and text problems in VoIP networks can range from simple issues to complex issues that may require a substantial amount of troubleshooting. However, with the appropriate troubleshooting methodology, you can attain more expedient resolutions. People do not always troubleshoot problems in the same exact way. Different strategies, points of view, and levels of experience are just some of the factors that determine a person's troubleshooting methodology. When it comes to troubleshooting...

Call Disconnect

A modem can commence a graceful disconnect at any time after data mode is entered. It can be done from the AT command interface (typically ATH), or a drop in DTR, or any other reason that makes the modem go on-hook in an orderly manner. Such a disconnect can be initiated by either the calling or the answering modem. A normal modem disconnect is typically done by sending a disconnect frame. The type of disconnect frame depends on the EC protocol that was negotiated. For V.42, a LAPM disconnect...

Catalyst Blade Fax Relay Configuration

The 6608 is a blade for the Catalyst 6000 6500 series of switches. This card is MGCP controlled by Unified CM and features eight T1 or E1 ports, each of which is treated as a separate gateway. The 6608 supports Cisco fax relay only, not modem relay or text relay. The 6608 is configured via the Unified CM graphical user interface (GUI). Figure 10-1 illustrates the default fax settings for a 6608 port. This particular screenshot is from Cisco Unified Call Manager (CUCM) Version 5.0(4), but all...

Catalyst Blade Passthrough Configuration

The 6608 is a line card for the Catalyst 6xxx series chassis that functions as a voice gateway. It has eight digital ports that can be used as a Primary Rate Interface (PRI T1 and E1) or channel-associated signaling (CAS T1 only). These digital ports are all MGCP controlled by Unified Communications Manager (CM), because that is the only signaling protocol the 6608 supports. TIP An interesting point to note is that each port on the 6608 is viewed as a separate gateway by Unified CM. Therefore,...

Configuring Relay 311

IOS Gateway Relay Configuration 311 Fax Relay 312 IOS Gateway Fax Relay Configuration for H.323, SIP, and SCCP 313 IOS Gateway Fax Relay Configuration for MGCP 320 Modem Relay 325 IOS Gateway Cisco Modem Relay Configuration for H.323, SIP, and SCCP 326 IOS Gateway Cisco Modem Relay Configuration for MGCP 329 Cisco Text Relay 332 IOS Example Configurations for Relay 334 Default Fax Relay Configuration for H.323 and SIP 334 Cisco Fax Relay and Modem Passthrough Configuration for H.323 and SIP 336...

How Fax Works

Phases of a Fax Call 62 Message Format Overview 63 Analyzing a Basic Fax Call 65 CNG Tone 66 CED Tone 67 DIS, NSF, and CSI Messages 68 DCS and TSI Messages 71 TCF, CFR, and FTT Messages 73 MPS, EOP, EOM, MCF, RTP, RTN, and DCN Messages 75 Other T.30 Messages 77 Understanding Error Correction Mode 81 ECM Call Analysis 82 PPS and PPR 84 Important G3 Timers 86 Super G3 Faxing 88 Comparison of SG3 and G3 89 Super G3 Call Analysis 89 Modified Huffman 92 Modified READ 97 Modified Modified READ 103

How Text Telephony Works 107

A Brief History of Text Telephony 107 Carrier Based Versus Carrierless Protocols 111 ITU-T Recommendation V.18 112 Acoustic Coupling Versus Direct Connections 113 Originating and Receiving Text Telephone Calls 114 Baudot Character Set 121 Baudot Modulation Details 123 Summary 124 Part II IP Solutions and Design 127 NSE-Based Passthrough 137 Fax Passthrough with NSE 139 Modem Passthrough with NSE 141 Protocol-Based Pass-Through for Fax 143 Fax Pass-Through with H.323 Signaling 144 Fax...

Configuring Passthrough 287

IOS Gateway Passthrough Configuration 288 IOS Gateway NSE-Based Passthrough Configuration 289 IOS Gateway NSE-Based Passthrough Configuration for H.323, SIP, and SCCP 289 IOS Gateway NSE-Based Passthrough Configuration for MGCP 292 IOS Gateway Protocol-Based Pass-Through Configuration 293 IOS Gateway Text over G.711 Configuration 295 6608 Catalyst Blade Passthrough Configuration 295

Checking the Basics

Before executing detailed onramp or offramp troubleshooting techniques, some basic troubleshooting items must be checked. Although confirming some of these items might seem obvious and somewhat simplistic, you can save a lot of frustration and time down the road if you pay due diligence to these possible issues in the beginning. Table 13-1 highlights some basic items that you should check or validate before proceeding with more in-depth troubleshooting. Because Table 13-1 contains only...

Checking the Condition of Originating and Terminating Devices

A variety of issues can occur on the fax, modem, or text devices themselves and cause failures. These issues range from fax machines not having any paper to modems having been accidentally unplugged. Although these issues are not always common, catching them early can save you some frustration and even embarrassment down the road. Table 12-1 highlights some common conditions and tests for determining the status of fax, modem, and text endpoints. Table 12-1 Checking the Condition of Fax, Modem,...

Circuit Type Circuit Name Circuit Function

Data TxD Data transmitted by the DTE to the DCE. Table 1-6 RS-232 Circuits and Their Function (Continued) Table 1-6 RS-232 Circuits and Their Function (Continued) Data received by the DTE from the DCE. Raised by the DTE when it is ready to get access to the DCE link. The modem will not dial unless it sees DTR Raised by the DCE when it is powered up and in such a state where the communications channel is available for transmission reception. The DTE will not request...

Cisco Text Relay

Cisco text relay allows the modulated signals from text telephones to be transported reliably and efficiently over IP. These signals typically represent the alphanumeric characters making up a conversation between text telephone users. NOTE For additional information on text telephony, see Chapter 3, How Text Telephony Works, and for more information on Cisco text relay and how it works, see the section Cisco Text Relay in Chapter 5. The configuration of Cisco text relay is relatively simple...

Command Syntax Conventions

The conventions used to present command syntax in this book are the same conventions used in the IOS Command Reference. The Command Reference describes these conventions as follows Boldface indicates commands and keywords that are entered literally as shown. In actual configuration examples and output (not general command syntax), boldface indicates commands that are manually input by the user (such as a show command). Italic indicates arguments for which you supply actual values. Vertical bars...

Configuring Passthrough

A passthrough call is handled in much the same manner as a regular VoIP call from a voice gateway perspective. There are a few events and minor changes that a Cisco voice gateway undergoes in preparation for a passthrough call. The terminating gateway always uses tone detection to go into passthrough mode. After that, the terminating Cisco voice gateway can use three ways to signal the switchover to passthrough to the originating gateway. These methods are as follows Named Telephony Events...

Configuring Relay

The configuration of modem, fax, and text relay is more involved than the passthrough configurations discussed in Chapter 9, Configuring Passthrough. More configuration options and additional parameters can be modified. In the case of Cisco IOS gateways, this in turn leads to a larger number of command-line interface (CLI) commands to manage. The relay configuration options for the voice signaling protocols of H.323, Session Initiation Protocol (SIP), Skinny Client Control Protocol (SCCP), and...

Configuring T37 Offramp

T.37 offramp faxing provides for the conversion of e-mails to G3 fax calls, the reverse of what is accomplished by the T.37 onramp function. Just as with onramp, a mixture of global configuration commands and dial-peer specific commands are necessary for generating a working T.37 offramp configuration. Table 11-10 simplifies the configuration of T.37 offramp to a few steps. Each step in the table references a section of this chapter where more information may be obtained for that configuration...

Configuring T37 Storeand Forward

Recommendation T.37 store-and-forward fax allows for the transport of fax information using e-mail. This replaces the real-time nature of the fax transaction with the convenience of sending and receiving faxes directly from your favorite e-mail application. Only Cisco IOS gateways support T.37. Non-IOS gateways such as the 6608, VG224, and ATA, which were discussed previously in the passthrough and relay configuration chapters, are not discussed here because of their lack of support for the...

Confirming the Configuration

For fax, modem, and text calls to work successfully, the gateways on either side and Unified CM, if present, must be properly configured. However, for a variety of reasons, the gateway or Unified CM can be misconfigured or its configuration can inadvertently get changed. Often, configuration changes are made for the installation of new equipment or to address another VoIP-related problem. Later on, these changes, especially when they involve dial-peers, are found to have unintentionally...

Contents

Introduction xxiii Part I Laying the Groundwork 3 External Versus Internal Modems 10 Hardware Versus Software Modems 10 Fax Modems 12 Terminal-to-Modem Communication 14 DTE and DCE 15 RS-232 Signaling 15 Asynchronous Framing 19 User Interface 20 Modem-to-Modem Communication 26 Modulation 26 Frequency Shift Keying (FSK) 28 Phase Shift Keying (PSK) 29 Amplitude Modulation (AM) 30 Quadrature Amplitude Modulation (QAM) 31 Trellis Coded Modulation (TCM) 32 Modulation Standards 33 Modem Call Analysis...

Contents at a Glance

Chapter 3 How Text Telephony Works 107 Part II IP Solutions and Design 127 Chapter 6 T.37 Store-and-Forward Fax 189 Chapter 7 Design Guide for Fax, Modem, and Text 207 Chapter 9 Configuring Passthrough 287 Chapter 10 Configuring Relay 311 Chapter 11 Configuring T.37 Store-and-Forward Fax 351 Chapter 12 Troubleshooting Passthrough and Relay 379 Chapter 13 Troubleshooting T.37 Store-and-Forward Fax 525 Index 562

Conversation Conventions

Over the years, text users have adopted certain conversation conventions and other shortcuts to make text telephone communication more efficient. Some of these conventions were brought about by the half-duplex nature of the Baudot protocol itself, whereas others have been created to save time and speed up the pace of a typed conversation. The two most notable conventions or initialisms are GA (go ahead) and SK (stop keying). GA is used when a user has finished typing to notify the other party...

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Data Mode

The part of the call analyzed in this section is data mode. This is where Phase IV negotiations have completed and the procedures and protocols to begin data transmission occur. At this point of the call, several things can happen. Among them are the possible occurrences of speedshifts retrains as an adaptive tool that the modulation protocol uses as a means of coping with the varying line conditions after the initial training. This is also the part of the call that will involve negotiation of...

Debugging Modem Relay

Because the modem relay debugs in IOS gateways are meant for developer-level decoding, the main troubleshooting command to use for modem relay problems is the show modem relay statistics command. This command contains a number of options, but the most useful are phy, which provides physical layer statistics, and pkt, which lets you view HDLC framing information. The command show modem relay statistics phy provides a glimpse at the physical or modulation layer of the modem relay call. Example...

Design Guide for Fax Modem and Text

In the design and planning stage of many VoIP networks, accounting for modulated communications, such as faxes, modems, and text telephony devices is often omitted or forgotten. Unfortunately, because of some of the unique characteristics of transporting modulated communications over IP and certain gateway and protocol interoperability issues, this can lead to problems later during network implementation. This chapter provides the design and planning information necessary to ensure that a...

E

EC (error correction) protocols, 178 ECM (Error Correction Mode), 81-82, 488 call analysis, 82, 84 PPR messages, 84-86 PPS messages, 84-86 ECM considerations for VoIP networks with fax, modem and text capability, 233-235 EHLO command, 193 EIA-TIA-825-A specification, FSK, 123-124 enabling T.37 store-and-forward fax, 351 encoding schemes, 59, 92 MH, 92-96 MMR, 103-104 MR, 97, 100-103 endpoints, secure modem relay, 254-255 ENVID parameter (DSN messages), 197 EOM (end of message) messages, 75 EOP...

Enabling T37 Storeand Forward

Whether you are configuring onramp, offramp, or both, T.37 first needs to be enabled globally on the voice gateway. This is accomplished using the configuration command fax interface-type fax-mail, as detailed in Table 11-1. Table 11-1 fax interface-type fax-mail IOS Configuration Command Enables T.37 functionality on the gateway and directs the gateway to use voice digital signal processors (DSPs) to process T.37 fax store-and-forward data. Note Enabling or disabling this command requires a...

External Versus Internal Modems

An external modem physically resides outside the computer and has its own chassis, power supply, front-panel indicator LEDs, and so on. Also, it is connected to the computer with a cable that generally connects to the serial interface on a COM port. Internal modems reside in the computer, typically in a PCI or ISA slot, and usually create a virtual COM port. Table 1-1 provides a quick comparison of external versus internal modems. Table 1-1 External and Internal Modem Comparison

F

Fallback considerations for VoIP networks with fax, modem and text capability, 239-241 fax call analysis, 65 CED tone, 67 CFR messages, 73 CNG tone, 66 CSI messages, 70 DCN messages, 76 DCS messages, 71-72 DIS messages, 68 EOM messages, 75 EOP messages, 75 FTT messages, 73 MCF messages, 76 MPS messages, 75 NSF messages, 69 RTN messages, 76 RTP messages, 76 TCF messages, 73 TSI messages, 71-72 ECM, 81-82 call analysis, 82-84 PPR messages, 84-86 PPS messages, 84-86 encoding schemes, 59, 92 MH,...

Fax Components

Figure 2-2 Simple View of a Fax Machine AMMMM mwiiw fl WWWV fl WWWV fl VWWW Present trends are showing a decline in standalone fax machines. Fax functionality today is being packaged in all-in-one devices that also include a scanner, printer, and copier. As illustrated in Figure 2-2, combining a fax capability with these other functions is a natural fit.

Fax Messaging

If you've spent any time at all around fax machines, you've probably heard the tones and chirping sounds that a fax machine makes when it starts talking to another fax device. Or you may have heard the short beeping tone that plays when a fax machine mistakenly dials a nonfax number. Sometimes this fax beeping tone is even left in voice mail as a message. These sounds that are heard emanating from fax machines all have specific purposes and they need to be understood, especially for anyone...

Fax Modem and Text for IP Telephony

All rights reserved. No part of this book may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or by any information storage and retrieval system, without written permission from the publisher, except for the inclusion of brief quotations in a review. Printed in the United States of America First Printing June 2008 Library of Congress Cataloging-in-Publication data is on file. ISBN-13...

Fax Modems

Another type of modem relevant to the discussions in this book is the fax modem. This type of modem is nothing more than a modem that runs software that enables it to transmit documents to a fax machine or another fax modem. Most modems sold since the early 1990s contain fax modem functionality. Fax modems, like regular modems, can be either internal or external. Fax modems have become popular because of certain advantages they offer over regular fax machines. One advantage is that fax modems...

Fax Pass Through with H323 Signaling

H.323 fax pass-through occurs when a VoIP call is set up using the H.323 protocol stack, and then H.323 messages are used to transition the call to pass-through mode. Figure 4-9 illustrates pass-through using the H.323 call signaling protocol. Initially, the VoIP call is established using the H.323 signaling protocol. When the terminating fax machine answers the call it plays a 2100 Hz CED tone. However, unlike NSE-based passthrough, this tone does not trigger a pass-through switchover....

Fax Receive Configuration Command for Onramp

The fax receive called-subscriber command is the only command used in configuring the fax receive parameters for T.37 onramp. Table 11-8 covers this command and its function. Table 11-8 fax receive called-subscriber IOS Configuration Command fax receive called-subscriber d 1 string Configured under the POTS dial-peer, this command defines the called subscriber number that is sent to the originating fax machine via the T.30 called subscriber identification (CSI) message. For more information on...

Fax Relay

Cisco voice gateways are capable of two different fax relay implementations T.38 and Cisco fax relay. Both of these fax relay protocols accomplish the same thing from a functional perspective relaying a fax call across an IP network. TIP Cisco gateways only support G3 fax calls with either T.38 or Cisco fax relay. To properly handle the higher speeds of SG3 calls, modem passthrough must be used. Further discussion of this topic is in the section Super G3 in Chapter 7, Design Guide for Fax,...

Fax Relay Data Rate

One of the most common problems with fax relay is the complaint from users that it takes too long to send faxes. This problem stems from the fact that, by default, fax relay calls on Cisco IOS gateways will not exceed the bandwidth being used by the voice codec. Furthermore, the default codec for VoIP dial-peers on Cisco voice gateways is G.729, an 8000 bps codec. This results in Cisco voice gateways exhibiting the default behavior of not allowing fax relay calls to exceed 8000 bps. Under this...

Fax Server Integration Solutions

Fax servers may be integrated with Cisco voice gateways and Unified CM in a number of different ways. However, practically all fax server integrations fall under one of the three integration methods or solutions discussed in this section. The first fax server solution covered is the direct connection model where the fax server communicates directly with the PSTN through a telephony interface, such as a T1 or E1 digital circuit. A Cisco voice gateway is not technically required for this...

Fax Servers

Traditionally, fax servers were computers equipped with one or more fax modems connected to the public switched telephone network (PSTN). The function of the fax server was to accept incoming faxes and pass them on to users electronically or to store the fax documents locally. Conversely, the fax server also received documents from users, converted them into faxes, and transmitted them to other fax machines. Because fax servers provide secure, automated, and efficient handling of fax documents,...

Feedback Information

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Further Reading

The authors recommend the following resources for more information. Cisco.com The Cisco website is one of the best resources for additional documents related to fax, modem, and text technologies and IP telephony in general. Usually the easiest way to find a document is to use the web page's search feature. Other useful links on Cisco.com include the following For design related documents, see http www.cisco.com go srnd. For Unified Communications product information, refer to http www.cisco.com...

General Passthrough and Relay Design Considerations

A number of design considerations must be looked at when designing VoIP networks that will successfully handle modulated communications such as fax, modem, and text. Not taking into account these design considerations from the beginning can cause problems later upon implementation. This section focuses on passthrough and relay design considerations in a general sense, meaning that the information in this section applies equally to faxes, modems, and text devices. These considerations are...

Hardware Versus Software Modems

The architecture of a modem, discussed earlier, determines whether a modem is classified as a hardware modem or a software modem. A hardware modem is one that has hardware that handles all the data pump and controller functions on its own. On the other hand, a software modem is one that offloads one or both of those responsibilities to the host computer. NOTE Although WinModem is a USR brand of modem, it has become a popular term used to refer to software modems. The two major types of software...

HCO Hearing Carry Over

During a normal text telephone conversation, both parties read and type as their means of communication. HCO offers those who can hear but maybe not speak the option to listen to the other party but type responses instead of speaking them. HCO is usually implemented with a TRS, but it also works between two text telephones. Figure 3-7 illustrates a text call with HCO involving a TRS. Figure 3-7 Text Telephone Call Using HCO Figure 3-7 Text Telephone Call Using HCO HCO is beneficial to all...

How Modems Work

Although analog modem technology stood on its own for many years in public switched telephone network (PSTN) environments, the rapid evolution of IP telephony (IPT) is now requiring that modem communications work successfully over IP networks. However, before discussing this complicated convergence of modems and IP, it is important to first attain a solid foundation in basic analog modem operation and communication. This chapter addresses the basics of analog modem technology and prepares you...

How Text Telephony Works

Text telephony provides a way for those with hearing and speech disabilities to interface with others using the public switched telephone network (PSTN). Users can type their conversations to one another instead of depending on hearing and speaking for communication. This makes text telephony an integral part of the growing trend of providing accessibility in IP telephony (IPT) environments to all users. Accessibility in many cases is federally mandated here in the United States by legislation...

How This Book Is Organized

This book is logically laid out with critical, fundamental concepts defined at the beginning in Chapters 1 to 6. Later chapters build upon these concepts to assist you with network design, configuration, and troubleshooting. Once the initial fundamental chapters are covered in the first two sections, the remaining chapters do not have to be read in any particular order even though the listed chapter sequence is what we believe to be the most beneficial for learning the subject matter. The...

HPI Debugs for Passthrough

The modem passthrough and fax pass-through equivalent to the fax relay fax_mode message and the Cisco modem relay modemrelay_mode message is the voice_mode message. This message details the activities associated with a passthrough switchover, including the codec upspeed if necessary and the disabling of Voice Activity Detection (VAD). Example 12-61 highlights a voice_mode message with a Packet Id 73 for a modem passthrough call. Example 12-61 voice_mode HPI Message from Host to DSP Apr 19 04 56...

Important G3 Timers

Table 2-4 defines the basic T.30 message timers for G3 faxing. These timers are used in error recovery and to make sure that the fax transaction never gets stuck in a hung state. In calling scenarios involving international connections or satellite hops, fax calls may experience a delay that is high enough to cause certain timers to expire. Amount of time a calling fax machine waits for an answer from the terminating side. Usually a CED or DIS stops this timer. Amount of time a fax device...

Info

The purpose of the V.42 detection phase is to determine whether the remote side supports LAPM error correcting. This phase is undergone by modems that trained to an older modulation that does not support V.8. There is a well-defined bit pattern, known as the Originator Detection Pattern (ODP), that the LAPM originating modem sends to announce that it can do LAPM. If the answering modem is V.42 capable, it responds with another bit pattern known as the Answer Detection Pattern (ADP) indicating...

IOS Example Configurations for Relay

Looking at configuration samples is a good way to grasp how the relay commands that have just been discussed form a proper fax, modem, or text relay configuration. In addition to relay examples, passthrough examples are mixed in, too, to illustrate how these two transport methods can be configured together to handle different types of modulated data. The sample configurations in this section assume that the IOS gateway has already been properly configured with the selected voice signaling...

IOS Gateway NSEBased Passthrough Configuration

The IOS configuration commands for NSE-based passthrough are identical for H.323, SIP, and Skinny Client Control Protocol (SCCP). Regardless of which of these voice signaling protocols are used, the configuration syntax will be pretty much the same. For this reason, they are discussed together. The commands for configuring NSE-based passthrough for Media Gateway Control Protocol (MGCP) are totally distinctive. Both the syntax of the commands and the way they are implemented are completely...

IOS Gateway NSEBased Passthrough Configuration for H323 SIP and SCCP

For H.323, SIP, and SCCP, only a single configuration command is used to enable both fax and modem NSE-based passthrough. This configuration command is modem passthrough nse. Table 9-1 shows the syntax of the command and summarizes the various arguments of this command and the function of each of one. Table 9-1 modem passthrough IOS Configuration Command Table 9-1 modem passthrough IOS Configuration Command modem passthrough system 1 nse payload-type number codec codec-type redundancy...

IOS Gateway Passthrough Configuration

Passthrough configurations for fax, modem, and text are available on IOS gateways. This section covers the necessary commands and configuration steps to properly implement each of them. NOTE The commands covered in this section are based on what is available in Cisco IOS Software Release 12.4(9)T1. Although some commands might have a different syntax or be unavailable in other IOS versions, the majority of the commands are applicable to any IOS version. In addition, be aware that the default...

IOS Gateway Relay Configuration

The majority of Cisco voice gateways today use Cisco IOS Software. All of these IOS gateways share similar commands and configuration steps across a variety of different hardware platforms. NOTE The commands covered in this section are based on what is available in Cisco IOS Software Release 12.4(9)T1. Although some commands might have a different syntax or be unavailable in other IOS versions, the majority of the commands are applicable to any IOS version. In addition, be aware that the...

IOS Gateway Text over G711 Configuration

Before the release of the Cisco text relay feature, the only way to transport text over an IP network was over a traditional G.711 voice session. This passthrough-like transport method of carrying text packets over a high bit rate codec is referred to as text over G.711. Note that text over G.711 is much more rudimentary than the other passthrough methods that were previously discussed. There is no tone detection, no NSE messaging, and no codec upspeeding that takes place. However, text over...

IP Troubleshooting

Properly troubleshooting the IP call leg is critical for fax, modem, and text telephony calls. Because these modulated communications were designed strictly for the PSTN, transporting them over IP adds additional challenges. Making sure the IP portion of the call path is operating as reliably and efficiently as possible is necessary for ensuring that fax, modem, and text calls are successful. Although the previously discussed show call active voice brief command is helpful for detecting...

ITUT Recommendation V18

The V.18 specification issued by the ITU is a dual standard designed to address some of the issues with text telephone communication. While defining interoperability processes among the major text telephone protocols, V.18 also details a completely new text protocol. As illustrated by the disparate text protocols in Table 3-1, interoperability is a major issue among text phone users, especially on international calls. V.18 addresses these interoperability issues by detailing how multiple text...

M

Manually replicating SMTP sessions, 553 Marsters, James C., 109 MCF (message confirmation) messages, 76 MDN (message disposition notification) messages, 198-200 configuring for MMoIP dial peers, 359 Disposition field, 200-201 Notification field, 201 messages T.30, 77-81 T.30 fax, 63-64 MFD (Multi Function Device), 267 MFP (Multi Function Printer Product Peripheral), 267 MGCP (Media Gateway Control Protocol) Cisco modem relay, configuring, 329-331 fax relay, configuring on Cisco IOS gateways,...

Modem Architecture

Modems allow for communication between computers in much the same way a telephone allows for communication between humans. Fundamentally, an analog modem converts the digital signals from a computer to analog signals that are transmitted over voice-grade access to the PSTN. Figure 1-3 shows a high-level view of the architecture of an analog modem. Figure 1-3 Analog Modem Architecture Figure 1-3 Analog Modem Architecture S S Analog Signal n_ru Digital Signal Modem architecture can generally be...

Modem Call Analysis

This section analyzes a call in its entirety. For the purpose of this discussion, the modem call is broken down into three component parts The first part deals with the call setup and training sequence between modems. The second part covers the protocols and procedures associated with data transmission, including speedshifts, retrains, error control, and data compression. The third, and final part, discusses the call disconnect sequence. Keep in mind that this subdivision of a modem call into...

Modem Design Considerations

Similar to fax, modem communications have the option of both passthrough and relay transport methods. The passthrough option for modems is simply named modem pass-through, and it shares this same syntax when it is configured on Cisco IOS gateways. Modem passthrough is also applicable to fax calls and has already been discussed throughout the previous section, Fax Design Considerations. The technical intricacies of modem passthrough and its NSE-based switchover are discussed in the section Modem...

Modem Relay

Two types of modem relay are available for Cisco gateways Cisco modem relay and modem relay for secure communications between STE (Secure Terminal Equipment) endpoints, which is referred to as secure modem relay for the sake of brevity. Both of these modem relay types are Cisco proprietary implementations, although some aspects of secure modem relay are based on ITU-T Recommendation V.150.1. Cisco modem relay is designed to carry V.34 modem traffic efficiently across IP. The voice signaling...

Modemto Modem Communication

Modulation is the most fundamental aspect of modem communications. It the means by which the binary digital data from the DTE link is encoded onto an analog signal that is sent over the PSTN. Different protocols describe the different types of modulations, and one of these protocols must be successfully negotiated between each modem on a point-to-point link for any communication to be possible. In this section, the various modulation schemes and how they correlate with the various ITU-T...

Modulation Standards

The preceding discussion focused on the theoretical nature of the different methods of modulation. Over time, public and proprietary standards have been defined based upon the different modulation schemes. These modulation standards can generically be broken down into two categories analog modem modulation and digital modem modulation. The primary difference between the two is the carrier used. Analog modulation uses an analog carrier, whereas digital modulation uses a digital carrier. Table...

N

Non-IOS gateways, troubleshooting IP problems, 419-423 Notification field (MDN messages), 201 NOTIFY parameter (DSN messages), 196-197 NSE messages, 179-181 NSE-based passthrough, 137-143 configuring, 289-293 Event IDs, 139 NSE-based switchover for Cisco modem relay, troubleshooting, 434-436 for modem passthrough, troubleshooting, 430-434 for T.38 fax relay, 167-169, troubleshooting, 436-438 validating, 438-445 NSE-based T.38 fax switchover with CUCM, 245-247 NSF (nonstandard facilities)...

NSEBased Switchover for Cisco Modem Relay

Cisco modem relay requires that a switchover to modem passthrough be completed before a switchover to Cisco modem relay occurs. Therefore, the output from the command debug voip rtp session named-event for a Cisco modem relay call always begins with the display of the NSE-192 and NSE-193 messages seen with a modem passthrough call. In addition to seeing the modem passthrough-specific NSE messages, Cisco modem relay uses NSE-199 and NSE-203 messages. The NSE-199 message is used by the Cisco...

Originate Side Session AnswerSide Session

The OK response signals the originate DCE's AT parser's ability to accept command input. ATD1234 Modem goes offhook, hears dial tone, transmits DTMF, and waits to hear answerback tone (ABT). The modem gets a fast busy. This time the call goes through and the PSTN presents ring voltage to the answer modem. The answer DCE transmits this on the AT interface it also toggles the RS-232 RI signal. Normally an answer modem will automatically answer upon incoming ring, but in this case the answering...

Originating and Receiving Text Telephone Calls

Call initiation and termination procedures for a text telephone are different than for a normal phone. The main difference is the reliance on visual cues compared to the typical auditory ones. The following steps illustrate the placement of a typical text telephone call Step 2 For text phones with an acoustic coupler, place the handset on the coupler. For phones with a direct connection, make sure that the text phone is properly connected. Step 3 Dial the telephone number. Most numbers can be...

Packet Capture PCM Extraction

Extracting PCM from a packet capture is easily accomplished through software programs such as Wireshark. The main criterion for extracting PCM from a packet capture is that the RTP packets contain the G.711 -law or G.711 a-law codec. In addition, passthrough redundancy for these codecs should be disabled before making the packet capture because the RFC 2198 packet format usually causes a poor PCM extraction. The RTP analysis feature within Wireshark can be used to extract the PCM from a G.711...

Passthrough

When the public switched telephone network (PSTN) was initially constructed, voice communication was the primary goal. However, as data communications such as fax, modem, and text became more important, they also were made to work over the PSTN using special protocols and transport methods. Today, with VoIP taking the place of the PSTN, voice communication is still the primary objective, and specific protocols and procedures are again needed to transport fax, modem, and text communications. One...

Passthrough and Relay Troubleshooting

In this chapter's first section, Attacking the Problem, a troubleshooting methodology for fax, modem, and text problems was presented. Figure 12-1 graphically demonstrates this methodology and breaks it down into its components. With all the other components being addressed in previous sections, the last one in this troubleshooting methodology, passthrough and relay troubleshooting, is discussed in this section. Some of the techniques covered in this section are advanced, primarily because the...

PCM Traces for Fax and Modem

PCM traces are audio captures of a fax modem conversation. In the same manner that a VoIP conversation can be recorded, fax and modem calls can be recorded, too. These recordings or PCM traces are almost always required when resolving the most complex fax and modem problems. Capturing and analyzing PCM traces is not for everyone because of the advanced nature of this troubleshooting technique. In many cases, only engineers with quite a bit of fax modem knowledge and experience will use PCM...

Protocol Based Switchovers and Unified CM

As mentioned in the section Unified CM Integrations in Chapter 7, when Unified CM is involved in the call routing for a voice network, it plays an active role in protocol-based switchovers. Whereas NSE-based and PT-based switchovers occur without the knowledge of Unified CM, protocol-based switchovers depend on Unified CM to participate in relaying the switchover messages and transitioning the media stream from voice to fax. Because of Unified CM's direct involvement in protocol-based...

Q

QAM (Quadrature Amplitude Modulation), 31 QoS considerations for VoIP networks with fax, modem and text capability, 215-218 dial peer configuration, 218 LLQ configuration, 219-221 quantization, 131 redundancy considerations for VoIP networks with fax, modem and text capability, 221-223 relay for H.323, SIP, and SCCP, 326-328 for MGCP, 329-331 Cisco text relay, configuring, 332-333 comparing with passthrough, 249-250 configuring on Cisco IOS gateways, 311 DSP functions, 153 troubleshooting,...

Redundancy

In the context of passthrough and relay, redundancy is the concept of sending multiple copies of the same data segment. The reasoning behind the redundancy concept is that if a packet is lost or significantly delayed another packet carrying the same information will still arrive at the destination in a timely manner. This ensures that the integrity of the data connection remains intact even though packet loss or significant delay is occurring. Passthrough calls are notorious for being very...

Sg3

Cisco voice gateways configured for either T.38 or Cisco fax relay can have problems with Super G3 (SG3) fax devices. SG3 fax devices implement a different negotiation procedure and modulation that is not supported by Cisco gateways using fax relay as the transport method. Often, an SG3 problem will present itself as simply fax call failures between the SG3 fax machine and one or more other fax machines. Occasionally, the problem may even be intermittent. Although SG3 interoperability problems...

SMTP Overview

As mentioned in the previous section and as illustrated in Figure 6-1, the T.37 gateway has the ability to communicate directly with G3 fax devices using the ITU-T T.30 fax protocol, which is covered in detail in Chapter 2. Similarly, the T.37 gateway also communicates natively with the mail server via the SMTP protocol. SMTP is one of the fundamental elements necessary for understanding and effectively troubleshooting T.37. Therefore, this section discusses some SMTP basics before tackling the...

Specifications and Standards

Because all modern mainstream fax devices adhere to the G3 classification, the standards that make up G3 are the most relevant for discussion in this book. The three main ITU-T recommendations that commonly define G3 are ITU-T T.30, ITU-T T.4, and ITU-T T.6. The T.30 specification describes how fax devices communicate with one another, and T.4 and T.6 define how page information is encoded for transmission. Figure 2-3 illustrates the use of the T.30, T.4, and T.6 protocols during a fax...

Standards and Specifications

There are a number of different text telephone protocols in the world today because each country defines text telephone standards for use on their own telephony networks. Unfortunately, this has led to many incompatibilities and interoperability issues on international text telephone calls. Table 3-1 shows the common text phone protocols in use by some countries today. Table 3-1 Common Text Phone Protocols United States, Canada, Ireland, Iceland, South Africa, and some usage in the United...

Summary

Over the past 150 years, the evolution of fax technology has culminated in the G3 fax classification. Practically all fax devices today adhere to this standard. Detailed in ITU-T Recommendations T.30 and T.4, G3 contains specific phases, messages, and page encodings that govern the communication between fax devices in a normal PSTN environment. Optional features such as ECM and Super G3 can enhance the speed and quality of G3 faxes. This chapter provided core information concerning the...

Super G3

Super G3 (SG3) or V.34 faxing uses different modulations and signaling than a normal G3 fax call. Rarely is this a problem, however, because SG3 is backward compatible with the ubiquitous G3 fax standard. If either the originating or terminating fax device does not support SG3, the fax transmission falls back to a normal G3 fax call. For more information about the technical details of SG3, see the section Super G3 Faxing in Chapter 2. Cisco voice gateways do not support SG3 fax transmissions...

T37 Offramp

Offramp functions in the exact reverse way that onramp does. An offramp gateway is responsible for accepting an e-mail and converting it to a standard fax format that is subsequently delivered to a standard G3 fax machine. Figure 6-3 traces through the process of an offramp fax call. The offramp gateway communicates natively via the SMTP protocol with the mail server. The mail server or MTA forwards the e-mail over that SMTP connection as described in the SMTP Commands and Sample Sessions...

T37 Onramp

By integrating e-mail with standard G3 fax, T.37 store-and-forward fax allows an added dimension of flexibility and convenience that is not possible with fax relay or fax passthrough. Faxes can only be sent and received to and from standard fax devices in the case of fax relay and fax passthrough. In addition to that functionality, T.37 allows receiving faxes as e-mail and sending e-mail that can be delivered as faxes. A T.37 call is naturally segmented into two completely independent parts...

T37 Storeand Forward

The ITU-T Recommendation T.37 offers an additional means of transporting fax transmissions beyond the methods of passthrough and relay that have been discussed in the previous two chapters. Often referred to as store-and-forward fax the T.37 specification details a process for integrating e-mail with fax communications. T.37 ensures that faxes arrive to users as e-mail, and it also allows users to transmit faxes by simply sending an e-mail. This chapter first gives a general overview of T.37...

Telephony and IP Troubleshooting

Ensuring the integrity of both the telephony and IP segments of a fax, modem, or text call is critical. Problems or errors in these areas can have more of a negative impact on fax, modem, and text calls than they would on a regular voice call. Because fax, modem, and text calls actually contain specific data that cannot be altered during its transport, these calls are more susceptible to telephony and IP problems. Voice calls may experience some degradation from certain network impairments, and...

Terminalto Modem Communication

This section deals with the protocols typically used on the asynchronous serial link between the host and the modem. First, you are introduced to the concept of data terminal equipment (DTE) and data circuit-terminating equipment (DCE). Then, the communications link between terminal and modem is divided into three layers. From the bottom up, they are as follows RS-232 physical layer This defines the mechanical, electrical, and hardware signaling used on the terminal-to-modem cable. Async...

Testing with PSTN Calls

Faxes, modems, and text devices were originally designed to function in a standard PSTN environment. Their migration to VoIP networks poses additional challenges such as delay, jitter, and packet drops that may negatively impact their success. Therefore, it is critical to ensure that a problematic fax, modem, or text device works over the PSTN first before expecting it to work over an IP network. Figure 12-3 illustrates a fax machine being moved from its VoIP network connection to a PSTN...

Testing with Voice Calls

After performing the checks in Table 12-1, the next step in checking basic functionality is to place a normal voice call over the fax, modem, or text connection. With many fax and text devices, placing this voice call is quite simple because a handset is built in to the device itself. When dealing with modems or fax text devices without built-in handsets, you can use a regular analog phone. Figure 12-2 illustrates a modem over IP topology where the modems have been disconnected and regular...

Text over G711

Text telephony poses a major challenge to both NSE-based and protocol-based passthrough mechanisms. There is not a common tone implemented by the different text telephony protocols that can be focused on to trigger a passthrough switchover. So, to pass text telephone protocols in a passthrough mode, a manual version of passthrough must be configured. This is referred to as text over G.711. With text over G.711, dedicated voice connections (using dial-peers on IOS gateways) are configured...

Text Telephone Operation

Because of the unfamiliarity with text telephones for most people, the basic operation and function of a text phone needs to be discussed. The terminology, conversation conventions, and even the connection options are much different from a typical phone. Figure 3-4 illustrates a modern-day text telephone. Figure 3-4 A Text Telephone from Ultratec Figure 3-4 A Text Telephone from Ultratec For example, because words are being typed rather than spoken, some shortcuts and syntax conventions are...

Text Telephone Relay Services

While text telephones provide access to the telephone network for the hearing and speech impaired, both parties must have a text device for a conversation to take place. So, what happens when a text user needs to make a doctor's appointment, call the plumber, or even contact a friend or relative who does not have a text telephone These situations require the use of a TRS (telecommunications relay service). Many countries provide a nationwide TRS system for text telephone users. In the United...

Text Telephone Terminology

In the beginning TTY just referred to a mechanical teletypewriter, as mentioned previously. Today, however, a TTY also means any device that can replicate the function of a teletypewriter and provide a disabled person access to the telephone network. The use of the term TTY to represent a modern-day text telephone is widely used in North America and Australia. The term TDD (telecommunication device for the deaf) can also refer to a text telephone. TDD was initially used to differentiate the...

Third Party Test Tools

Third-party tools are available that can capture the PCM directly from voice ports. One of the more commonly used tools is the T1 E1 Call Capture and Analysis Software that comes on test sets from GL Communications. You can find more information about this software at the GL Communications website, http www.gl.com. In most cases, the GL Communications unit is bridged into the T1 E1 link that is connected directly onto the voice gateway. PCM traces can be made automatically whenever a call is...