T.38 and the future

索享科技DreamFax支持T.38协议下面是T.38 介绍和未来的展望

T.38 and the Future of Fax

The Emerging Standard for Real-Time Fax Over IP Networks

Fax Takes Its Place in the Changing Communications Landscape

Internet technology is creating a major shakeup in the way businesses worldwide communicate. The effects of this shakeup include a new Universal Messaging infrastructure that opens the possibility of transmitting voice, fax, and data using a single data network infrastructure. Powerful new solutions include Web-enabled call centers and help desks.

This communications shakeup is also sending tremors through the fax world. Today a high percentage of corporate communications expenses comes from the cost of transmitting fax over standard telephone lines. That picture is quickly changing. Transmitting fax using the data network infrastructure opens up many opportunities to enhance fax delivery and reduce expenses. Entrepreneurs envision a new world where corporations save millions of dollars a year by using existing data networks-such as the Internet and private Internet Protocol (IP) networks-as a global fax delivery network. This can include real-time fax, store-and-forward fax, or some combination of the two.

One tool that is crucial to building the new world of real-time fax over IP is the emerging International Telecommunications Union (ITU) T.38 standard for real-time fax over IP, which makes it possible for fax machines from different vendors to talk to each other over IP networks.

Intel will fax enable all its Intel® Dialogic® IP Link IP telephony products, which include densities from 4 to 60 ports on PCI and CompactPCI. This means a voice-over-IP solution in the field can evolve into a voice-and-fax-over-IP solution with a simple software upgrade. For more details, see "Intel Pioneers T.38 Fax Support" below.

Fax Over IP Saves Big

How Big Is Big?
A recent Pitney Bowes study estimates the average Fortune 500 company spends $37 million each year on telephone charges. Nearly 40% of those charges are the cost of sending fax messages.

The key benefit of sending fax over IP is that it saves tremendous amounts of money on long-distance charges. When a user sends a fax over IP, a fax machine or PC client transmits the p_w_picpath data as packets through an IP data network-the public Internet, a company intranet, or an external extranet-instead of over the public telephone network (PSTN). If fax-over-IP servers even need the PSTN, it is only for the local leg of a call. For a call to any location connected directly to the IP network, there is no telephone charge at all, not even a local charge. Thus, it is possible to send faxes to any other location inside the company's IP network literally for free. The vision of a worldwide network without long-distance or international tariffs is quickly moving closer to reality.

Fax Is Here to Stay

Facsimile, or fax, is a fundamental business technology. Some may argue that fax is dead because email is the killer application in enterprise communications. For many reasons, this is simply not true. Fax has proven itself as a reliable document delivery mechanism that companies and individuals will continue to use for many years. The estimated installed base of fax machines is 70 million worldwide, with still more sold each year. Globally, annual sales of fax machines reached more than 12 million units by 1995. Estimates were that sales of 20 million units per year worldwide occurred by 1999.

The most compelling reason for many people to use a regular fax machine is its simplicity. A fax machine is a quick, easy way to send either printed or handwritten notes. Fax is great for complex documents, since p_w_uploads are already opened and formatting and graphics are not issues. As messaging technology, fax is ideal for communicating over time zones-since a fax machine is typically turned on 24 hours a day, seven days a week. Because it is written, fax is often used to cross cultural and language barriers where spoken words can be easily misinterpreted. It is estimated that nearly 40% of the calls between the U.S. and Europe at any one time are fax calls, and that between the U.S. and Asia the figure approaches 50%.

Real-Time Improves on Store-and-Forward Fax

Why Fax Over IP Is Sometimes Better Than Email

A signed fax sent over IP is still a legal document.
Fax over IP is more secure than email.
Fax is great for complex documents and language translations.
90% of faxes go through non-PCs.
Formatting and graphics are preserved.
Attachments are already "opened."
You reach end points connected to the IP network and connected to phone circuits.

Fax can be transmitted over IP in two ways: real time or store and forward. New IP fax solutions will replace the shortcomings of store-and-forward solutions with real-time fax that delivers everything users like about using their tried and true fax machine.

Real-time fax over IP works exactly like a regular fax call. The two fax machines synch up and send data over a local telephone connection, with an IP leg between the two local connections. If the fax is busy, the caller gets a busy signal and the user has the option to retry sending later or to revert to store-and-forward mode as a transportation mechanism. Store-and-forward-mode fax over IP works like email. The caller sends the fax message to a server, which sends it to another server, which sends the fax to the recipient. Eventually, the caller is notified whether the fax made it.

Many of today's Internet fax services offer only store-and-forward fax. These services bridge the user's computer terminal with a fax gateway somewhere in the world. This is an extension of traditional LAN fax technology, where a fax gateway at the customer premise allows documents created on client PCs or workstations to be faxed through a common gateway.

The drawback of store-and-forward fax is that it does not give users three of the fundamental advantages of fax technology:

Guaranteed delivery. LAN fax services treat fax transmissions like email instead of regular fax messages.The fax is forwarded until its final delivery at the remote site-without any guarantee of delivery.
Easy, familiar operation. Store-and-forward fax delivery assumes most corporate fax traffic is generated at the client PC. Although plenty of fax users do like sending from their PC, 90% of faxes still go through old, familiar fax machines.
Immediate satisfaction. Store-and-forward technology fails to recognize that fax machines offer users a psychological satisfaction. As the paper goes into the machine, it is easy to visualize it printing on the remote side. This satisfaction and user perception are very powerful and should never be underestimated when designing a fax solution.

Figure 1. The number of installed fax machines is growing-and will continue to grow.

In short, fax is a prevalent method of immediate document distribution, worldwide. Therefore, real-time fax over IP-giving users the same satisfaction they get from their reliable old fax machine, along with the cost savings of transmitting over IP-will bring a revolution to the fax world.

T.38 Emerges as the New Fax Standard

To become an integral part of the Universal Messaging infrastructure, real-time IP fax must be based on accepted industry standards. Without a standard for real-time fax, terminals around the world would not be able to talk to each other-meaning a user could only send faxes to people with gateways of the same type.

The T.38 Standard

The standard for real-time fax over IP is T.38, ratified on June 18, 1998, by Study Group 8 of the International Telecommunications Union (ITU). The ITU recommendation, entitled "T.38: Procedures for Real-Time Group 3 Facsimile Communication Over IP Networks," defines the procedures to be applied to allow Group 3 facsimile transmission between terminals, where a portion of the transmission path between terminals includes-besides the PSTN or ISDN-an IP network such as the Internet.

Find It on the Web
The ITU is an international organization that allows governments and the private sector to coordinate global telecommunications networks and services through standards, international regulations and treaties, and worldwide telecommunications development. For a detailed description of the format for T.38 fax packets, see Appendix A or visit the ITU Web site at http://www.itu.org to purchase a copy of the T.38 recommendation.

With its longstanding commitment to open industry standards, Dialogic was one of the companies responsible for developing the T.38 standard.

Figure 2. Public fax service providers: outsourced fax delivery from the corporate network.

Figure 3 shows the architectural framework of the T.38 recommendation. A traditional Group 3 facsimile terminal is connected to a gateway, emitting a facsimile through an IP network to a receiving gateway. The receiving gateway makes a PSTN call to the receiving Group 3 facsimile equipment on the other side of the network. Once the PSTN calls are established on both ends, the two Group 3 terminals are virtually linked. The terminals establish the T.30 session and negotiate capabilities for fax functions like positive page confirmation and minimal call elongation. Another scenario, defined in the T.38 recommendation, is a connection on one or both ends of the transmission to a fax-enabled device like a PC, which is directly connected to an IP network.

In the T.38 architecture, fax signals are demodulated at the voice/ fax gateway and sent over the IP network as IP fax packets using either TCP or UDP, depending on the service environment. Figures 4 and 5 show the packet structure for both transport mechanisms. TCP provides 100% error-free transmission, but the downside is reduced real-time performance. Routers discard TCP packets first when congestion occurs. UDP has good real-time performance, since routers do not discard datagrams as readily as TCP packets. However, the downside of UDP is a reduced quality of service. It is possible for datagrams to get lost, arrive out of order, or to be duplicates. At the other end of the IP network, the remote gateway decodes IP fax packets and regenerates fax signals. Modulation methods that can be used with the T.38 format are V.21 Channel 2, V.27ter 2400, V.27ter 4800, V.29 7200, V.29 9600, V.17 7200, V.17 9600, V.17 12000, and V.17 14400.

Figure 3. The T.38 system comprises Group 3 fax equipment (G3FE) devices, Internet-aware facsimile (IAF) devices, and T.38 gateways.

Figure 4. High-level IFC/TCP/IP packet structure.

Figure 5. High-level UDPTL/IP packet structure.

H.323 and ITU T.38

One key benefit of T.38 is its ties to the H.323 protocol. Anyone involved with IP-based voice is familiar with H.323, the approved standard for video and audio call setup over IP networks. H.323 has become the recognized standard for initiating an IP-based telephone call, endorsed by Microsoft*, Intel, and Netscape* as the protocol of choice. H.323 provides a format for selecting a coder or algorithm to encode (and thereby compress) the voice data stream as it travels across an IP network. T.38 is basically no different from any other voice encoding standard such as G.723 or G.711. The T.38 protocol has already been earmarked for inclusion into the H.323 protocol as the fax coder of choice.

Figure 6. The H.323 gateway supports T.38 fax coder.

Note that the T.38 protocol itself operates directly between the emitting and receiving gateways. Call setup and other communication between the gateways and facsimile terminals and/or other devices is outside the scope of the T.38 protocol. In the case of Intel® Dialogic® IP telephony and fax gateways, the call setup is established using the H.323 procedures for voice calls first. As the default, a voice coder is selected. As soon as the gateway detects fax signals, the gateways switch on the fly to fax operation using the T.38 coder. If the fax signal is interrupted, the gateways switch back to voice mode without breaking the connection.

Annex B of the ITU T.38 recommendation describes how H.323 can be used for call establishment procedures of T.38 fax transmissions and gives more guidelines for real-time facsimile over H.323. Note that Annex B is outside the scope of the T.38 recommendation itself. It is therefore possible to be T.38 compliant without following all the guidelines from a particular annex. The H.323 Revision 2 will also add a description of real-time facsimile over H.323 as an annex to the standard.

The possibility of using T.38 in combination with H.323 will be a key success factor for the adoption and deployment of the T.38 standard. As the world deploys Internet voice gateways to augment the existing PSTN/TDM infrastructure, fax traffic will be handled in the same manner as voice traffic.

Intel Pioneers T.38 Support

Intel began developing T.38-compliant voice and fax gateways even before the recommendation was ratified by the ITU, knowing that gateways that can seamlessly support PSTN-to-IP voice and fax will be in high demand. In the spring of 1998, Intel's former Dialogic division was the first company to demonstrate an early version of T.38 real-time fax over IP. The first product to be compliant with the official T.38 standard was the Intel® Dialogic® IP Link, Release 3. That software release enabled previously released IP Link boards to process real-time fax over IP with a simple firmware download. Support for ITU recommendation T.38 is implemented as a coder, similar to a voice coder (It converts PCM data to and from ASN.1 coded Internet fax packets).

Real-time fax, compliant with the ITU T.38 standard, will be available for all boards in the Intel® Dialogic® IP telephony product family. To fax-enable products in the field, you will need only a simple software upgrade. With this simple upgrade, your IP telephony gateway will become a voice-and-fax-over-IP solution. The DSP chips on the IP Link board have the unique ability to handle multiple coders at the same time. That means some of the channels handled by a single DSP can be voice calls and others can be fax transmissions. It is also possible to switch automatically between voice and fax without breaking the connection. For example, if a fax transmission comes in during a voice call, the system automatically switches to fax mode. The user does not need to hang up. After the fax transmission is completed, the connection still exists and the user can continue to talk. Naturally, it is also possible to use only the fax-over IP (or voice-over-IP) capabilities of an Intel® Dialogic® IP telephony board. However, when you decide to add voice (or fax), the capability is already there.

Real-Time IP Fax With T.38:
The Future of Fax

The T.38 standard is crucial to building the new world of real-time fax over IP. Without such a standard, fax machines from different vendors would not be able to talk to each other.

Real-time fax over IP-used instead of or in combination with store-and-forward fax-gives users the same satisfaction they get from their fax machine, along with the cost savings of transmitting over IP. The result is a true revolution in the fax world.

Appendix A

T.38 Packets

There are two different T.38 packet elements:

Type elements
Data elements

Type elements describe the packet's function: whether the packets that follow contain signaling information (T30_INDICATOR) or actual data (T30_DATA). If both sides of the fax transmission do not recognize the type element, it and the related data element are ignored. (See the table below for valid entries for the type elements.) These values reflect signaling methods, modem modulation training methods, and data modulation methods supported by T.38 real-time fax recommendation.

Data elements contain the data from the PSTN connections and some indication of the data format. The data element is a structure containing one or more fields. Each field has two parts: The first indicates the field type and the second part contains the field data. Multiple fields can appear in a single data element. The example below shows two HDLC (Facsimile UDP Transport Layer protocol) frames arranged in a single data element. (Data transport headers are not shown). The TYPE Element signals that V.21 fax modulation is used by the sending fax machine. For more details about the T.38 Fax Packet Format please refer to the ITU T.38 recommendation.

Table 1. T.38 Fax Packet Format

Type Element Data Element V.21 Data	Field type	HDLC- Data	FCSOK	HDLC- Data	FCS-OK-Sig- End
	Field part description	First HDLC frame	Indicates end of HDLC frame and more data to follow	Second HDLC frame	Indicates end of HDLC frame and end of HDLC data

Appendix B
Glossary

Emitting gateway The IFP peer, which initiates IFT service for a calling G3FE. It initiates a TCP or UDP connection to a receiving gateway to begin an IFT session.

G3FE Group 3 Facsimile Equipment. Any entity which presents a communications interface conforming to Recommendation T.30, T.4, and, optionally, T.6. A G3FE may be a traditional Group 3 facsimile machine, an application with a T.30 protocol engine, or any of the other possibilities mentioned in the network model for IP facsimile.

IFP Internet Facsimile Protocol.

IFT Internet Facsimile Transfer.

IP Internet Protocol.

ITU Recommendation H.323 (1996) Visual telephone systems and quality of service.

ITU Recommendation T.30 (1996) Procedures for document facsimile transmission in the General Switched Telephone Network

Receiving gateway IFP peer that accepts a TCP or UDP connection from an emitting gateway, providing IFT service to a called G3FE.

TCP Transmission Control Protocol. A session-based, confirmed delivery service.

UDP User Datagram Protocol. Datagram service, non-confirmed delivery.

Appendix C
Bibliography

International Telecommunications Union. "Procedures for Real-Time Group Facsimile Communication Over IP Networks." International Telecommunications Union (ITU) Recommendation T.38, June 1998.

Kahan, Russ. "You Can Save Money Faxing Over IP." Computer Telephony, March 1998: pages 124126.

Shaw, Steve. "T.38, Emerging Standard for Real-Time Fax Over IP Networks." Telecommunications, August 1998.

*All company names, products, and services mentioned in this directory are the trademarks or registered trademarks of their respective owners.

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转载于:https://blog.51cto.com/zhuzhengchao/560942

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