Service transparency could be considered the linchpin in the move away from delivering voice, fax, modem and other data services over Internet protocol (IP) infrastructure. Clearly IP equipment provides a nice cost savings for carriers, but to be an effective solution, this same equipment should effectively handle voice, video, data, and a host of other streams. That requires the IP equipment to transparent handle any protocol as it travels across a line card, backplane, or chassis design.
Clearly, the hope for transparency lies in the work that standards bodies, such as the International Telecommunications Union (ITU) and European Telecommunications Standards Institute (ETSI). One group, Study Group 16 in the ITU, plays particular importance with respect to service transparency. This study group experts on various different media and physical transport methods meet to provide means for delivering service transparency in an IP network.
This article provides insights into some of the development efforts being developed in Study Group 16. Specifically, the article will look at the standards efforts surrounding delivering fax and modem services over IP as well as call discrimination specifications for VoIP networks.
Developments in Fax over IP
As the initial movement toward using VoIP was launched in the mid-Nineties, the fax community responded by launching efforts for standardization of fax-over-IP. Two approaches were taken: 1) The IETF and the ITU collaborated on a series of RFCs and the T.37 specification, which collectively define store and forward fax operations over Internet email. 2) The ITU worked on a specification for real time fax-over-IP, which resulted in the T.38 recommendation, approved initially in 1998.
In the time since, T.38 has gained popularity as the preferred method for sending faxes over a voice oriented IP network. T.38 offers reductions in the required bandwidth for IP fax transport compared to other methods and preserves important distinctions in fax state that can enable providers to fine tune their implementations for increased reliability and connectivity with the existing Group 3 fax installed base.
There has also been additional standards work conducted in order to ensure that T.38 will be usable in conjunction with the most popular standards for the session control of VoIP. This has resulted in a series of Annexes to T.38 that define how to operate in coordination with the H.323, SIP, and H.248/Megaco protocols.
There is one additional enhancement to the T.38 recommendation that is still under review in ITU Study Group 16. T.38 procedures do not currently provide IP transport methods for the latest generation of fax modems that support V.34 half-duplex operation at fax data rates of up to 33.6 kbps. Many of the fax machines and fax boards installed in recent years support the enhanced performance offered by V.34 fax, so there is a market need to extend T.38 to enable transport of these faster fax sessions over IP networks.
However, these V.34 fax modems share the use of the same pre-session negotiation approaches used by V.34 and V.90 data modems. Therefore, it is now likely that the extensions for incorporating V.34 fax support within T.38 will need to be coordinated somewhat with the pending standards for modem-over-IP.
Developments in Modem-over-IP
While the 90s prompted work on the VoIP and fax-over-IP fronts, little work was done to bring standardization to modem-over-IP sessions. There was some activity undertaken by vendors to support transport of data sessions over IP, but all of this activity was done on a proprietary basis.
However, during the process of standardizing the Megaco/H.248 protocol, there was a call for experts in the various media areas to coordinate their efforts to ensure that Megaco sessions could comfortably support not only voice operations, but other types of multimedia. When some early drafts of modem support within Megaco proved unsatisfactory in their content, ITU Study Group 16 launched the new project, dubbed modem-over-IP (MoIP), to help standardize the transport of modem-initiated data traffic over IP networks.
Some observers may question the relevance of this work in the era of the cable modem and xDSL. However, dialup modem traffic is still the predominant mode of access to the Internet and will need to be supported by telcos and service providers at the edge of the converged network for many years to come. There are also a number of legacy modem applications which are used in various vertical applications such as point of sale, e-commerce, electronic data interchange and so on. Hence, there is great interest among various gateway suppliers and component vendors in this work.
The MoIP participants in the ITU-T are now gaining consensus on some key elements of the protocols required to do this work. Two main directions will be undertaken:
1. Voice Band Data This is a method will which take data modem traffic and transport it over IP networks using voice band codecs such G.711 and G.726. It provides a fairly simple approach to moving data traffic over IP and enables the handling of a variety of legacy V-series modem traffic. This method tends to be bandwidth intensive, but does not require a gateway to demodulate modem data in order to transport it. The basic concept is consistent with methods that various vendors have used on a private (i.e. non-standard) basis.
2. Modem Relay This method will demodulate the incoming modem data and convert it into digital form for transport over IP networks. It promises to be more bandwidth-efficient and more reliable than voice band data.
Conceptually, the modem relay approach there are many similarities to the approach that has been specified for transporting fax over IP in T.38. However, it also is much more complex to implement than the voice band data method and requires fairly detailed levels of data parsing support that can vary widely for different types of modulation.
In addition to support for the two modes of data transport, there are also a number of common elements that are being reviewed for inclusion within the pending MoIP recommendation. One of these common elements is in the area of call discrimination, which will have implications that extend beyond MoIP itself. Let's take a look at call discrimination further.
Call Discrimination's Impact
Call discrimination is a key area of standardization needed to enable service transparency for various types of media. The purpose of call discrimination is to enable a gateway, pair of gateways, or softswitch system to analyze the tone and data content of a voice stream and to use decision rules that can accurately identify the type of call. Thus, call discrimination is quite important for VoIP gateways that also need to carry other common types of calls such as fax, data modem, or text telephone.
Call discrimination provides the triggers that will allow a gateway or media gateway controller to decide the type of call and communicate this information to the other gateway involved in a call. In practice, the efforts to distinguish among different types of calls within media gateways have taken a back seat in standards efforts when compared to the definitions of the media transport itself. However, the more recent development of the standards for handling media in softswitches has identified and helped drive a need to provide standard solutions for call discrimination. Simply stated, unless there is a way to identify the type of media call, there is no way for a gateway to realize that it needs to switch into a special mode to handle a non-voice call, such as a fax.
Call discrimination is also important in avoiding unpleasant user experiences. For example, suppose that a call has been initiated from a regular touchtone phone and then reaches a VoIP gateway, for transport over an IP network. Networking systems have traditionally employed digital signal processors (DSPs) to detect fax calls by listening on the line for tones being generated by a calling fax device. Upon hearing a sound that seems to match the characteristics of a fax calling tone, some network devices will assume that the call is a fax and hence switch the call over to a line that contains a fax device.
The problem with this approach is that it does not take into account the possibility that the intended called party was not really a fax device and that the caller actually wanted to hold a voice conversation, but instead is now being subjected to annoying fax modem signals.
Recent efforts in the fax-over-IP arena have centered on developing call discrimination engineers that take prevent the gateway from switching a voice mode to a fax mode unless there is a clear indication on the receiving side (via analysis of tones and signals) that a fax session is being requested (Figure 1). So, in our example above, the user's session would not be switched over into a fax mode unless the proper sequence of fax signals has been detected by the receiving gateway.
Click here for Figure 1
Figure 1: Using a call discrimination engine, designers can establish equipment designs that will one turn voice channels over to fax mode when a clear indication arrives on the receiving side of the link.
There has been some effort to backfill procedures in the standard protocols used for VoIP and the various types of other media transport in order to tackle the call discrimination issue. For example, call discrimination work accomplished in ITU Study Group 16 during the past two years has been incorporated in standards such as H.323, H.248, and T.38. The net result has been to provide guidance on how gateways can detect fax or text telephone traffic from within a voice stream and then shift the gateways into the proper mode of operation.
However, since there have not been any standards for transporting modem data over IP, the call discrimination approaches to date have not fully taken into account the possibility that a voice call is really a data call. As a result, there were several contributions submitted to recent ITU Study Group 16 meetings with ideas on how to standardize techniques for call discrimination that will work for VoIP, fax-over-IP, and MoIP.
One of the key approaches that is being proposed is to develop an in-band method that will enable gateways to exchange information about the results of call discrimination activities and to issue commands between gateways when switching from one mode of operation to another (for example, from VoIP into voice band data). There have also been initial steps taken to allow a related set of characteristics to be negotiated in an H.323 session, via extensions to the H.245 protocol.
In time, there will also be a need to consider MoIP-enabled call discrimination in a softswitch context (i.e. for Megaco or MGCP), but there have been no contributions received yet on how to do this. Annex F of H.248 provides a call type discrimination package, but it does not yet take into account the new variations of operation that MoIP will make possible.
ITU-T Study Group 16 is a central point for the standardization of transporting multimedia streams over converged networks, notably IP. Through work on the voice, fax, modem, and call discrimination fronts, this group will help drive the development of IP equipment that transparently deliver multimedia services to end users.
About the Author
James Rafferty is a senior product manager at Brooktrout. James contributed to the recently approved Annex F of the ITU-T H.248 Media Gateway Protocol and participates in related work in the Megaco working group of the IETF. he also contributed to the ITU-T T.38 Annexes, which support SIP and and H.248. James holds a B.S. and a Master of Engineering degrees in Management Engineering from Rensselaer Polytechnic Institute in Troy, New York. He can be reached at firstname.lastname@example.org.