Ready to converge? - Technology Information

VoDSL offers line consolidation and dynamic bandwidth allocation.

Over the past couple of years, telecommunications equipment vendors and industry analysts have focused a lot of attention on voice over IP (VoIP) as the defining application for network convergence. For all the hype surrounding Vole there has been little impact on monthly telephone bills. The few VoIP services offered today, primarily in international long distance, have yet to deliver the same high-quality voice available with POTS (plain old telephone service).

But a dark horse is emerging that will give network professionals a way to converge their voice and data networks without sacrificing voice quality and reliability. New voice-over-DSL (VoDSL) services promise to significantly lower monthly telephone bills by integrating voice and data on DSL access lines.

Until recently, DSL has been a data-only service, offered as a low-cost alternative to T1, Frame Relay, and ISDN for Internet access. Now DSL equipment vendors have given service providers a way to put multiple voice calls and data on a single copper pair. The key to VoDSL is quality of service (QoS). Providers of DSL services are betting that network professionals will switch from POTS to VoDSL if the service provides equivalent voice quality at a lower cost.

HOW VoDSL WORKS

ATM is the preferred transport protocol for VoDSL because of its rich QoS support and extensive traffic-management capabilities. ATM's ability to provide QoS for multiple virtual circuits (VCs) over a DSL local loop makes it possible to provision telephony, which is more sensitive to network delay than data, over the same DSL link that carries data traffic. An integrated DSL access concentrator and ATM switch (DSLAS) are located in the service provider's central office. The DSLAS supports QoS and a common bandwidth allocation scheme called weighted fair queuing (WFQ), both of which are necessary to provision voice over the DSL local loop. The DSLAS provides ATM switching and signaling at the point VoDSL traffic enters the DSL access network. This functionality ensures that data traffic does not utilize bandwidth required for voice calls. WFQ provides fair bandwidth allocation to parallel traffic flows. It guarantees that low bandwidth flows like voice always receive enough bandwidth, regardless of the behavior of other traffic flows. At the customer premise, DSL-based integrated access devices (IADs) perform the task of multiplexing voice and data traffic streams into ATM virtual circuits for transmission over a single DSL line. These devices can route or bridge Ethernet LAN traffic and support multiple voice calls simultaneously.

Ensuring QoS for multiple VCs on a DSL local loop requires that both the central office DSLAS and the IAD at the customer premise support the constant bit rate (CBR) and variable bit rate real-time (VBRrt) ATM classes of service. These classes of service are used to guarantee bandwidth for VCs carrying voice calls. Voice VCs are configured as either CBR or VBRrt connections, with limited cell loss and transit delay, in order to guarantee voice quality. Because VCs carrying voice calls receive a higher priority for bandwidth, the data connections on the same line do not interfere with the voice connections. Depending on the required performance level, dataVCs can be configured as CBR, variable bit rate (VBR), or unspecified bit rate (UBR) connections. UBR connections receive best-effort service only and lack QoS guarantees that control transmission characteristics like cell loss and delay. If there is no resource (bandwidth) available to transport UBR cells, they are discarded.

An enterprise that moves to VoDSL can implement a one-for-one replacement of voice and data lines. For example, a business with 10 telephone lines serving a 50-subscriber key telephone system or PBX, and using a Frame Relay, ISDN, or T1 line for Internet access, can replace all 11 lines with one symmetric digital subscriber line (SDSL) access line, which can provide bandwidth of up to 1.54 Mbps. ATM VCs on the SDSL line replace the 10 telephone lines and the Frame Relay, ISDN, or T1 line used for Internet access.

GETTING TO THE PSTN

A voice gateway in the central office is used to connect voice calls originating on the DSL access network to the public voice network. The gateway provides a bridge between the DSLAS and a circuit-based voice switch. The voice gateway uses a standard GR-303 interface over T1 facilities to connect to the voice switch. To the voice switch, the gateway appears as a digital-loop carrier (DLC) system. The principal function of the gateway is to reassemble VoDSL connections passed from the DSLAS into 64-kbps pulse-code-modulated (PCM) voice circuits and connect them to the voice switch. The gateway reverses this process for 64-kbps PCM voice circuits passed from the voice switch. PCM calls are converted into ATM VCs for presentation to the DSLAS. Because the voice VCs are CBR and VBRrt connections, each voice call is guaranteed bandwidth to ensure quality.

THE BENEFITS OF CONVERGENCE

Convergence of voice and data on DSL increases bandwidth efficiency. T1 service is channelized and requires bandwidth to be dedicated to voice lines even when they are not active. The result is that bandwidth cannot be used for other applications and sits idle until a call is made. Because DSL access networks are cell-based, VoDSL traffic consumes bandwidth on a dynamic, as-needed basis. VoDSL calls only consume bandwidth when a call is active. When VoDSL connections are inactive, the idle bandwidth is available for other applications, such as Internet access. This dynamic bandwidth allocation allows an enterprise to maximize the potential of each DSL connection, delivering to users the greatest number of telephone lines and highest possible data speeds.

VoDSL allows an enterprise to reduce the recurring costs of its network. By integrating multiple telephone lines and "always on" high-speed data connections over a single, low-cost copper pair, VoDSL creates a new economic model for enterprise networks. The convergence of voice and data on DSL increases network efficiency by optimizing bandwidth utilization and eliminating the need to manage separate networks.

Mitchell is director of product management for Promatory Communications, headquartered in Fremont, Calif.

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