ATM and DSL: A Perfect Combination of Elements

Greg Langdon
06/01/2000

Digital subscriber line and asynchronous transfer mode technologies go together like hydrogen and oxygen. While DSL turns copper phone lines into expressways for digital traffic, ATM (which has become DSL's standard Layer 2 transport protocol) allows different types of traffic to share the road. The union of these two elements has given rise to a third technology that's no infant in the industry--voice over ATM (VoATM).

Sometimes associated with VoIP, VoATM is a completely separate entity that possesses some important capabilities its Internet-related cousin has not yet grown into. In fact, VoATM provides the two things most important to any network that carries both data and voice traffic: the superior efficiency associated with data networks and the high-quality voice service users have come to expect whenever they pick up the phone.

Sharing the Wealth

It's not exactly news that DSL allows the copper twisted pair--which enters virtually every home and business in America--to carry data as well as voice traffic. DSL has effectively bridged the gap between the "haves" of high-speed data services--large companies that could afford dedicated T1/E1 lines--and the "have-nots"--everyone else. The technology allows small to medium-sized businesses and individual subscribers to use existing telephone wiring to establish an always-open, high-speed connection to the Internet or corporate WAN. And unlike cable modems, which require line sharing and may suffer from slowdowns or security breaches, each DSL user has a dedicated connection.

However, DSL alone does nothing to expand voice options. A splitter usually serves as a virtual median that keeps the two types of traffic completely separate, which means voice is still carried over the PSTN and reaps none of the efficiency benefits of DSL. If the customer wants more than one voice line, the phone company still has to come out and install another physical line.

Carrier Benefits of VoATM

Meeting Consumer Demand for Broadband Services

With VoATM, carriers can provide customers with the high-speed data services they crave, without breaking the bank. No additional wiring is needed at the customer site; no change to existing network architecture is required; and no serious ramp-up time is necessary. Installing the IAD at the customer site is simple, and the voice gateway is easily dropped into the hardware lineup at the CO.

Fulfilling Voice Quality Expectations

Customers may not realize they want or need high-quality voice service until they don't have it. With VoATM, customers never even notice the transition from a circuit-switched to a packet-switched network, which is just as it should be. And because VoATM supports QoS standards, carriers need not forego the revenue stream from CLASS features.

Service More Customers at Less Cost

If implementing DSL is like turning a one-lane dirt road into an interstate highway, VoATM enables the bandwidth highway to carry bumper-to-bumper traffic without the delays inherent in other packet-based protocols. By using bandwidth efficiently, carriers are able to support many more customers on the same physical infrastructure. This makes each copper plant more profitable, and it cuts down on capital expenditures.

Attract New Customers and Keep Existing Customers

Countless small to medium-sized businesses that could only dream of high-speed data access just a few years ago are now prime candidates for data services. A large, eager new market segment has been created, but carriers must act quickly to win market share. It's clear that a carrier's ability to offer a variety of new services at a reasonable cost will directly affect how successfully it courts these new potential customers.

Strengthen Customer Bonds Through Bundling

By packaging together multiple services--such as Internet access, virtual private networking and voice services--carriers can strengthen their relationships with existing customers. As they meet customer needs on a variety of fronts, carriers become indispensable. This is important since excessive customer churn has cost the telecommunications industry untold millions in recent years.

Bandwidth Efficiency

It quickly becomes apparent why ATM is the perfect complement to DSL. Designed from the ground up to integrate multiple services on one infrastructure, ATM makes it possible for voice and data to truly share the road--a road two dozen lanes wide in this case. In fact, VoATM allows a single twisted pair to carry up to 24 voice lines in addition to data traffic.

The ATM Adaptation Layer (AAL) allows for this bandwidth-sharing functionality. In essence, the AAL defines how voice and data traffic is changed into ATM cells and converts higher layers of services (such as TCP/IP) into a format appropriate for the ATM protocol layer. Two flavors of AAL--AAL 1 and AAL 2--are most often used for voice traffic.

AAL 1 handles constant bit rate (CBR) traffic, such as voice and video, by allocating fixed amounts of bandwidth to each connection. This prevents interruptions or delays, but stakes out an entire section of bandwidth for each CBR connection, even when the bandwidth is not fully used.

AAL 2, on the other hand, slips data traffic into the tiny silences that riddle any phone conversation, ensuring that every bit of available bandwidth is occupied. By identifying different classes of traffic, AAL 2 is able to give voice traffic priority over less time-sensitive data traffic. It then uses statistical multiplexing to allocate bandwidth in real time over a single ATM permanent virtual circuit (PVC).

Preserving Infrastructure

Just as VoATM prevents existing bandwidth capacity from going to waste, it makes efficient use of existing network infrastructure. The only new piece of equipment required at the customer site is a next-generation IAD. This device, which replaces a standard DSL access box, is connected to the customer's standard telephone equipment and to a PC or LAN. It combines voice and data traffic and sends it over the copper plant to the communications provider, where a voice gateway at the CO splits out the voice and data components. No further retrofitting is required.

Customer Benefits of VoATM

Business Quality Voice

Customers don't need to give up high-quality voice to enjoy the benefits of high-speed data access.

New Communications Services

Previously the exclusive domain of large companies with deep pockets, an array of high-speed services--including Internet access, streaming video, multimedia applications and voice--are now within reach of every mom and pop shop.

More Voice Lines at Less Cost

Many small to medium-sized businesses and residential users previously made do with one or two voice lines, simply because it cost too much to install more. With VoATM, the cost per line is far less since a single twisted pair can carry as many voice lines as subscribers need, in addition to data services.

One-Stop Shopping

Service bundling by carriers means customers have a single point of contact for support and service. There's no need to involve multiple providers to resolve a communications issue or to modify services. Better yet, the customer need not wait for a house call to implement a service change. No hardware upgrade is involved.

Safeguarding Voice Quality

If efficiency is the first order of business for data networks, transmission quality has always been of paramount concern on SONETs. While a file transfer is unaffected by a microsecond's delay between data packets, the human ear does not forgive even the tiniest distortion of an audible message. People expect a tele- phone conversation to sound the same as a face-to-face chat.

In this regard, VoATM is superior to VoIP. Although often touted as an economical way to offer voice service over existing Internet connections, IP's large, variable-length packets are better at transmitting data than voice. Because the large packets fill relatively slowly, and because IP's standard quality-control method is simply to re-send lost packets, phone conversations often suffer from signal loss and echo.

Some corporations with private IP networks have compensated for VoIP's data-oriented design by implementing proprietary protocols. But these solutions are effective only when both ends of the connection use the same hardware and software. The public Internet is a long way from implementing standards that effectively address voice quality issues.

In contrast, ATM is an ideal medium for voice as well as data. Because ATM uses small, uniform packets, a VoATM transmission is sent at a rate approximately 10 times faster than a voice transmission on an IP network. Even more important than speed, however, is ATM's excellence in the area of traffic management and QoS.

While IP currently treats every transmission the same whether it is an e-mail message or a two-way videoconference, ATM was designed to differentiate between high- and low-priority traffic. Since voice is time sensitive, it is always given higher priority, which means callers don't experience degraded service when the network is busy. Carriers are able to guarantee the "toll- quality" voice connections that users have come to expect.

The AAL 2 is actually responsible for VoATM's ability to blend great network efficiency with guaranteed quality. By including content information in each ATM cell, it can prioritize individual packets. This allows voice and data packets to share bandwidth, which is dynamically adjusted based on the traffic flow at any given moment. For instance, when someone takes a breath during a phone conversation, more bandwidth is turned over to data transport; when the conversation resumes, the bandwidth is reclaimed by higher priority voice packets.

Greg Langdon is vice president of marketing for Efficient Networks Inc. (www.efficient.com). He can be reached at glangdon@efficient.com.

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