DSL Voice and Data Environment Changing Fast
posted 19th March 2001

Twentieth Century voice networks relied on analog lines and trunks as well as digital T-1 circuits to serve small and medium business subscribers. Still in wide use today, these technologies offer providers and their customers a Hobson's choice between low efficiency and high cost -- analog lines are inefficient and T-1 circuits are expensive. The inefficiency and expense of this 20th Century analog/digital transport network are compounded by its requirements for separate connections to deliver multiple voice lines and end-user data services. The advent of DSL has substantially improved providers' options. DSL lets providers use a single physical connection to carry both voice and data traffic. VoDSL lets providers further improve efficiency and reduce costs for voice service by replacing edge access and core transport components of the legacy voice network with elements that offer lower capital expense and lower recurring costs.

But even as it takes hold, VoDSL technology is evolving. This continuing evolution makes picking the right equipment one of the most challenging decisions for carriers deploying VoDSL service. And on one important level, the decision boils down to a choice between centralized, circuit-switched and distributed, packet-based (Internet) platforms. Initial VoDSL solutions serve as remote extensions to Class 5 Time Division Multiplexing (TDM) switches and offer telephony services to Plain Old Telephone Service (POTS) subscribers. One side of the DLC connects to telephone subscribers' analog POTS loops, with one copper loop required for each voice connection. The other side connects to a centralized, Class 5 switch using the GR-303 interface.

Early VoDSL solutions have used GR-303 gateways to enhance the capabilities of local loops. GR-303 gateways convert packet voice, transported using either IP packets or AAL2 Asynchronous Transfer Mode (ATM over fibre, now used by PlaNet from KCCS) cell encapsulations, to circuit-switched voice suitable for handoff to the legacy Time Division Multiplexing (TDM) network. Working in conjunction with packet switches for Internet traffic, they transform local loops into broadband pipes that enable delivery of multiple voice connections and high-speed data over a single copper pair. Despite their advantages, GR-303 gateways are hobbled by a singular disadvantage -- they depend on expensive, centrally located Class 5 switches to supply actual voice routing and calling features. But all roads for voice over DSL need not lead to a costly central switch. Next-generation, 21st Century VoDSL solutions rely on much less costly alternatives -- softswitches and trunk gateways -- to do the same work.

The Next Step Softswitches and trunk gateways use advanced signaling protocols like the Media Gateway Control Protocol (MGCP) to provide voice routing and calling features that have been traditionally provided by Class 5 switches. Running on standard server hardware or fault-tolerant computers, softswitches provide call control, administration and custom calling features for packet voice network devices. Trunk gateways are central office (CO) hardware platforms for internetworking between packet voice networks and the Public Switched Telephone Network (PSTN). Used together as a distributed switching system, softswitches and trunk gateways perform call control and routing for subscriber telephony traffic placed between packet voice endpoints or between packet and circuit-switched subscribers. Softswitches and trunk gateways take the features and functionality of the GR303 gateway and Class 5 switch and partition them among multiple devices that can be distributed throughout the carrier network at optimum points, and properly sized for each point of presence.

The distributed, client-server architecture of these next-generation solutions contain the following components: Integrated Access Devices (IADs) and media gateways at customer premises act as intelligent MGCP clients delivering line-side features like call waiting and call transfer. Softswitch MGCP call agents in carrier regional offices act as servers, controlling all signaling, feature administration, billing and call routing for both line-side features and network trunk interactions. A carrier can use a single call agent to control multiple trunk gateways. This offers significant cost advantages over the centralized approach, where all calls must pass through a large voice switch, and carriers must replicate signaling intelligence at every end office. Trunk gateways in regional offices convert between circuit and packet voice and multiplex packet voice to and from PSTN TDM trunks under the direction of the softswitch call agent.

The cost advantages of softswitches and trunk gateways are compelling. Carriers who opt for these solutions now or who plan to migrate to them in the future need to augment them with the right IADs and DSLAMs. At customer premises, carriers should deploy IADs that are compatible with circuit-switched and packet voice technologies. A number of leading CPE vendors are developing and marketing products that interoperate with both GR-303 gateways and MGCP packet voice platforms. Carriers provisioning voice and broadband services over DSL can choose between two kinds of DSLAMs today -- ATM-based and IP-based. Designed to operate with ATM and SONET transport technologies still prevalent in the network core, ATM-based DSLAMs work with fixed-size, 53-byte cells that are indistinguishable from each other with respect to source, destination and type of service. Because ATM-based DSLAMs cannot make these distinctions, DSL service providers relying on them must provision separate end-to-end Permanent Virtual Circuits (PVCs) for each kind of traffic, and for each subscriber.

This can lead to staggering provisioning and scaling problems as carriers sign up more and more subscribers for multiple, concurrent services, including VoDSL. IP-optimized DSLAMs work with whole Internet packets. Unlike ATM-based DSLAMs, they can see the packets' datagrams, which lets them tell the difference between different kinds of traffic and identify where the traffic is coming from and where it's going. Because IP-optimized DSLAMs can make these kinds of distinctions, DSL service providers who standardize on them can aggregate traffic from different subscribers on shared paths through the network

An Example

To understand the advantage of IP over ATM, consider a hypothetical DSL service provider who signs up 100 subscribers for a full menu of services, including Quality of Service (QoS) VoDSL. To provision 100 subscribers with an ATM-based DSLAM, this provider must set up and manage 100 PVCs (permanent virtual circuit, the bits that ATM fibre circuits are divided into with bandwidth from half to two Megabytes of data) -- one PVC for each subscriber. In contrast, a DSL service provider who standardizes on DSLAMs with IP service intelligence can send all voice subscribers' traffic down one low-delay path through the entire network. Based on any combination of header information at layers 2, 3, 4 and 7, IP-optimized DSLAMs prioritize traffic and keep traffic from different services and subscribers compartmentalized and secure, and -- perhaps most significantly -- aggregate traffic. Where the ATM-centric provider must set up 100 end-to-end PVCs to provision 100 subscribers, the IP-based provider can provision the same number of calls with only one aggregated network path. Because they can make intelligent traffic-forwarding and queuing decisions at the network's edge and exponentially reduce the number of circuits a carrier must manage, DSLAMs with IP service intelligence offer an optimal solution for delivering VoDSL over legacy GR-303 gateway/Class 5 circuit-switch networking architecture.

And because they are IP-optimized, they can migrate to the all-IP environment of next-generation softswitches and trunk gateways. ATM-based DSLAMs, with their reliance on cell transport technology, simply won't go there. The potential market for VoDSL is huge. Millions of small and medium businesses are hungry for cost-competitive, high-value voice services such as PBX, private voice networks, toll bypass and small office/home office virtual private networks for teleworkers. This market represents a tremendous revenue and earnings opportunity for carriers who can cost-effectively bundle voice services with a range of high-speed data services over DSL. And in my opionion, carriers who standardize on distributed IP-optimized solutions today will own the VoDSL market tomorrow. .. ..