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Feature Article
June 2001


Voice Over DSL -- Beyond Access Gateways


Voice over Digital Subscriber Line (VoDSL) deployment is well underway -- and although these are early days, research firm TeleChoice, Inc., estimates that ILECs and CLECs will deploy more than one million voice-enabled DSL lines by 2003. For those service providers beginning down the VoDSL path, today's solutions leverage existing DSL and PSTN infrastructure.

This article explores the current state of VoDSL and the next steps in VoDSL evolution, including the current methods of integration with Local End Offices (Class 5 switches), early examples of Local End Offices replacement, and the migration toward next-generation network architecture.

VoDSL Today
Market pressures have never been greater on traditional ISPs and CLECs. Having enjoyed generous access to capital in recent years -- and having invested massively on broadband deployments -- most are not yet realizing adequate return on investment -- and thus face financial distress. VoDSL can provide a major boost to service providers' profitability, because it is riding the wave of market trends. Despite having reached an inflection point, probably in 2000, where the amount of worldwide data traffic exceeded worldwide voice traffic, voice still pays the bills. In fact, according to the Gartner Group, voice will still account for more than 75 percent of the total data and fixed telephony market in 2003! Voice is the "killer application" -- and bundling voice with data access is a critical service provider survival strategy.

VoDSL provides all the capability of DSL -- and more. Specifically, while DSL is restricted to carrying data packets to and from a customer premise to the central office over standard, existing copper wire (and then on to the ISP), VoDSL allows that same bandwidth to support both voice and data, on an as-needed basis. For example, where, with DSL a single analog voice line can typically carry up to 1.5 Mbps of data traffic, with VoDSL that same 1.5 Mbps of bandwidth can now support as many as 16 voice channels, as well as substantial data traffic.

Importantly, this enhanced ability to support voice as well as data requires no major infrastructure changes, relying on the same basic architectural components as DSL -- a DSLAM (digital subscriber line access multiplexer) in the Central Office (CO) as well as the local end office operated by the incumbent or the challengers. Over time, DSLAM vendors have added Quality of Service (QoS) parameters to their equipment, not only to properly handle both voice and data packets, but to provide multiple classes of services for data traffic to enforce Service Level Agreements (SLA). This was relatively easy to do because most DSLAMs are based on ATM -- and ATM QoS is standards-based and well-understood. To achieve the benefits of VoDSL, the only requirements are a new Integrated Access Device (IAD), and a VoDSL gateway for converting packetized voice to TDM signals.

Note that the infrastructure between the IAD and the VoDSL gateways is all based on ATM. The customer premise equipment is a VoDSL IAD, containing all the intelligence needed to multiplex voice and data traffic and to allocate bandwidth accordingly, with voice always taking preference. Typically, from 8-16 voices lines are connected to the IAD. The PCs, typically on a 10Base T LAN, are connected through a hub to the IAD. For the voice side of the IAD, it will packetize the information using ATM AAL2 formats, as well as provide functions for signalling, ringing, echo cancellation, and voice compression. On the data side, the IAD encapsulates the traffic using ATM AAL5 formats and provides functions such as firewalling, routing, DHCP, etc.

Multiple DSL loops are aggregated into the DSLAM. At the point where they exit the DSLAM, the voice and data ATM packets are routed to a regional packet network and into the cloud. There, network switches separate the streams, sending the data to broadband remote access servers (B-RAS: also called a subscriber management system) and the voice to a VoDSL gateway, also called an access gateway.

The VoDSL gateway converts the packets coming from the DSLAM to TDM so that they can be transferred over T1/E1 lines (using GR-303 in North America and V5.2 in most of the rest of the world) and into existing CO switches. Note that because the VoDSL gateway converts ATM voice packets to the TDM format, it really is a VoATM gateway.

Today's VoDSL access gateways require these four basic technology enablers:

  • ATM-to-TDM conversion;
  • Echo cancellation with at least 64ms tails;
  • Compression, typically ADPCM; and
  • Trunking (T1 interfaces with GR-303 protocol or E1 interfaces running V5.2 protocol).

AAL2 and Loop Emulation Service
The recent advent of ATM Adaptation Layer 2 (AAL2), a set of standards developed by the International Telecommunications Union (ITU) and the ATM Forum, make it possible for compressed voice to ride alongside data and multimedia streams on an ATM infrastructure with all ATM's classic benefits: Scalability, mature standards, and dexterous handling of QoS.

In addition to the fact that most DSL networks are ATM-based today, ATM AAL2 is the perfect candidate for VoDSL, because it provides the following capabilities:

  • Multiple users channels on a single ATM virtual circuit.
  • Bandwidth efficient transmission of low-rate, short and variable packets in delay sensitive applications.
  • Support for both constant bit rate (CBR) and variable bit rate (VBR) classes of service for network traffic optimization. The VBR class, with statistical multiplexing, enables the treatment of compressed voice, silence detection/suppression, and idle channel removal.
  • Variable length payload across multiple cells for bandwidth efficiency.

Loop Emulation Service (LES) is a specialized subset of AAL2 based on ITU-T specification 1.366.2 "SSCS for Narrowband Services over AAL2." In a nutshell, LES supports derived POTS and ISDN services, specifying management of AAL2 channels, signalling details (both channel-associate signalling [CAS] and common channel signalling [CCS]), and in-band management for the IAD. LES is now the industry standard for open communications between IADs and VoDSL gateways.

A Step Toward the Future
Today, voice and data convergence has been achieved in the access part of the network while maintaining service transparency and leveraging all the goodness of the PSTN -- well developed customer care, robust management and billing, as well as the existing infrastructure (both DSLAMs and Local End Office switches).

The next wave of convergence is beginning to hit the Class 5 switches, in the form of entry-level local exchange switching solutions. An interesting example is the Broadband Voice Platform (BVP). You can think of this as a collapsed Class 5 switch with a VoDSL interface. The BVP provides a subset of Class 5 switch functionality, including SS7 signalling. It is not targeted at replacing the room-size Class 5 switch, nor supporting all the Class 5 functionality, but is an alternative for more focused deployments or simply for operators that may not want to incur the capital expenses associated with a big Class 5 switch.

Over and above the classical VoDSL gateways, the BVP is required to support tone detection and generation, as well as the ability to support SS7 signalling into the PSTN.

Now, let's consider convergence at the local end office -- and the evolution toward end-to-end packet voice.

The Next Generation Network
Worldwide markets are driving the evolution to packet voice -- and it is the impetus for major investment. Whether the ultimate goal of end-to-end packet voice will be met by ATM or IP or both remains to be seen, but at this stage ATM is superior to IP in its handling of end-to-end Quality of Service -- and thus will be a key player in this migration. Regardless of the end state, with as many as 2 billion circuit switched (wireline and wireless) phones worldwide by 2004 (Coleago Consulting Ltd. 2000) we will be working with hybrid networks for many years to come.

To address the requirements of hybrid networks, one might consider building a super switch to provide all the functions of a Class 5 and handle packet voice and continue to control all call processing for the myriad calling features that exist today. However, this approach would recreate the limitations of today's Class 5 switch: Lack of scalability, inflexibility, high costs, and perhaps most importantly, prohibitively long time to market for new services.

A fundamental aspect of the next generation network architecture is the softswitch architecture. It consists in taking the functionality of the classical Central Office Switch (call control, signalling, feature control, etc.) and exploding it into various platforms that perform more dedicated functions. The key here is that decomposition provides open interfaces between these essential network functions. This evolution can be compared to the evolution of the computing environment: Much like the movement from mainframes and dumb terminals to today's client-server architectures, the softswitch approach decentralizes the control of the network, separating the call processing function from physical switching function, while connecting these via a standard protocol, mostly either a media gateway control protocol (MGCP) or media gateway control (Megaco, ITU standard H.248). Megaco is presently the culmination of a plethora of standards that have emerged in the recent years. Megaco is presently being standardized in the ITU under H.248.

The key logical elements of the decomposed architecture are:

  • Media gateway, which performs the conversion of one medium (POTS lines, cable, DSL, 3G Wireless) onto another packet-based medium (ATM or IP or both).
  • Media gateway controller, (also known as a softswitch), which performs the call processing function.
  • Signalling gateway, which provides "translation" of signalling messages between the PSTN and the packet network. In an ATM network, this may take the form of SS7 to Q.BICC protocol conversion. In an IP-based network, this may be SS7 to SIP.

The typical switching fabric function that is at the heart of any TDM switch becomes the packet network cloud, which is populated with switches directing packets towards their destination. The typical end office functionality then becomes distributed and virtual, providing carriers a lot more deployment agility.

The media gateway is the switching/bearer transport platform -- it is the hardware on the edge of the network that typically converts packet-to-TDM conversion under the control of a softswitch. Media gateways (MG) will be the junctions that provide a path between switched and packet networks for such media as voice and fax. Media gateways can either reside at the very edges of the public network at the customer premises or in a central office.

The media gateway controller is the name of the softswitch in the Megaco scheme. Media gateway controllers (MGC) will serve as the brains of the operation because they will control many media gateways.

The typical softswitch device provides call control and routing functions for network voice and data traffic. It also serves as a platform for services-creation capability and is scalable. It also supports multiple applications and SS7 capability. It is not embedded with the media gateway, but stands alone, typically in a generic platform like a Sun Netra server.

The signalling gateway provides the linkage between the PSTN and the signalling methodology in the decomposed architecture. Today, this takes many forms, but the work is encompassed in the IETF Sigtran workgroup. Another component, a signalling gateway, will play an analogous role in capturing requests for address and enhanced service information by enabling retrieval via IP from existing SS7-enabled networks and their databases of subscriber-related information. In turn, the relevant address information is passed from a signalling gateway to an MGC.

Like in client server architectures, there is not a 1-to-1 mapping between the softswitch's logical functions and the network topology. In particular, the media gateway control functions are distributed to provide scalability and flexibility.

Megaco/H.248 is an emerging standard that will enable voice, fax and multimedia calls to be switched between the public switched telephone network and emerging ATM or IP networks. It provides a comprehensive solution for the control of MGs by MCGs. As with earlier generations of media gateway control, Megaco is based on the principle that all call processing intelligence resides in the MGC. The MG provides only the capability to connect media streams under the control of the MGC, and to detect and transmit signalling associated with those media streams. Connections within the MG are created via Megaco signalling commands. The Megaco protocol is designed to be transport independent. Therefore, methodologies exist to transport the Megaco Protocol over TCP/IP or UDP/IP in IP-based networks, but there are good reasons to use an ATM-based transport such AAL2 to support remote MGs that operate with VoATM connections, as in the case of VoDSL.

The Megaco framework is a joint activity of the International Telecommunication Union and the Internet Engineering Task Force (IETF).

Let's take a look at how VoDSL will likely change as it migrates to this next gen architecture.

VoDSL in the Decomposed Architecture
As the PSTN evolves toward a softswitch architecture, we can expect to see VoDSL Access gateways evolving to support three key developments:

  • Support of signalling using Megaco/H.248, enabling the call control functions to be supported by a media gateway controller (and therefore eliminating the need for the classical Local end office switch);
  • Establishment of direct packet voice paths between IADs; and
  • Support for direct connections between IADs and trunking gateways.

Because the IAD takes certain types of inputs (e.g., POTS, LAN) and transforms them, by adapting the traffic using ATM adaptation layers, it can be seen as a media gateway. Signalling -- in the form of MCGP or Megaco/ H.428 -- performs the call control. Now, the VoDSL gateway has also been transformed into a media gateway.

Traffic coming out of this media gateway can be routed either on IP or ATM to the packet network (the NGN).

In this architecture, the IADs have the intelligence to send/receive info from the softswitch, so they could exchange signalling messages with it, but never actually route any signals to a TDM network -- voice packets could just go from one IAD to another. The benefit is this: The traffic is already in packets, and stays in packets, and doesn't incur the overhead (and consequent delay) of translation.

The distribution of the next-generation functions can take many forms. The general tendency is to evolve the access equipment to become multi-services nodes (DSLAMs, Digital loop carriers, etc.) supporting multiple types of media streams (packet and TDM), apply the proper class of service parameters and to work under the control of a MGC. One might tempted to call these platforms NGDLCs -- or Next Generation Digital Loop Carriers.

Of course, the VoDSL gateway will still be able to connect to the legacy Class 5 to keep the backward compatibility.

Other important areas demanding attention on the road to the converged network, including the migration of VoDSL, are complying with legal requirements for E-911 and making it possible for law enforcement agencies to "wiretap" conversations, even if they are all sliced up into packets.

In the U.S., the latter is referred to as CALEA readiness, after the U.S. Communications Assistance for Law Enforcement Act (CALEA). Enacted by Congress in 1994, CALEA establishes a September 2001 deadline requiring that telecommunications carriers' services and systems conform to standards that permit law enforcement officials to conduct legally authorized electronic surveillance. This capability involves the ability to duplicate, redirect, and fork media streams so that applications can redirect media streams to media servers for voice processing applications. (Note: Media streams redirection and forking also allows easier development of enhanced services such as calling card and call redirect or transfer applications.)

Other areas that may not be trivial in the pursuit of network convergence are the ability to bill actual services rendered as well as all the Operations, Maintenance, Administration, and Provisioning -- essentially all the carriers' back-office operations. These operations represent at least one-third of carriers' capital expenditures. Therefore, having technical solutions replicating the functionality of the PSTN solves only a small portion of the problem. The operational shift will be equally challenging, if not more than the convergence of voice and data traffic over packet networks.

Back to the Future
For many service providers today, VoDSL is an important technology that can provide an excellent source of revenue. Today's topology achieves convergence in the local access part of the network while leveraging existing network deployments: The ever ubiquitous copper pair and the PSTN network, as well as DSLAMs, which are seeing continuous new installations as well as being already deployed in thousands of central offices. Today's VoDSL implementation also leverages ATM, a well understood and developed set of standards that provide, among other key parameters, end-to-end class of service.

As we move towards the converged networks of the future, decomposing the functions of the central end-office into multiple, standards-based functions, VoDSL can evolve nicely to become a pure media gateway controlled by a media gateway controller, providing a smooth migration path.

While there are challenges in this migration, the industry momentum and the spirit of the Internet -- openness as opposed to proprietary protocols -- will prevail. It is just a matter of time.

Frederic Dickey is the Product Management Director of Natural MicroSystems' Network Access group. Natural MicroSystems is a technology leader supplying the world's major networking and communication equipment suppliers with the products and services required to create communications infrastructure, applications, and services faster and at lower costs than ever before.

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