With international approval of the VDSL2 (G.993.2) standard in May 2005, telephone carriers now have the opportunity to upgrade both upstream and downstream bandwidth to their subscribers and to thereby offer valuable new services such as IPTV, interactive gaming, peer-to-peer file sharing and videoconferencing. VDSL2 chipsets can be designed to deliver up to 100Mbps of symmetrical bandwidth to each subscriber. In addition, these chipsets incorporate previous functionality outlined in the ADSL (G.992.1), ADSL2+ (G.992.3 and G.992.5) and VDSL (G.993.1) standards, making VDSL2 a multi-mode solution for a broad range of service needs. Asian and European carriers are already operating VDSL-based services, and U.S. carriers such as SBC and BellSouth have indicated that VDSL2 will be part of their fiber-to-the-curb (FTTC) and fiber-to-the-node (FTTN) deployment plans.

In most cases, migrating to VDSL2 is a simple matter of swapping out ADSL line cards for VDSL2 line cards or inserting new line cards in empty chassis slots. VDSL2 chipsets support multi-mode operation (ADSL, ADSL2+, VDSL and VDSL2) so it is easy to deploy without disrupting service to existing customers. Carriers can install VDSL2 line cards without requiring any changes to customer premise equipment (CPE), so customers with existing ADSL modems will continue to receive the same level of service they had been getting.

However, VDSL2 offers the flexibility to migrate easily to higher-bandwidth services as customers want it. In these cases, carriers could mail out VDSL2-compatible modems or residential gateways, and customers could self-install the devices. Once the device is connected, the VDSL2 line card serving that customer automatically recognizes the higher capabilities and delivers a notice of new services available to that customer.

Deployment Topology Scenarios

VDSL2 delivers better service with simpler operations across the board. Here are four typical scenarios:

In central office DSLAMs, carriers will replace multiple versions of ADSL, ADSL2+ and VDSL line cards with one standard VDSL2 card. VDSL2 supports all current DSL-DMT services, and it eases inventory and maintenance issues because carriers can support all services with only one line card instead of several. For customers up to 5000-8000 feet from the DSLAM or Mini-RAM, VDSL2 allows carriers to offer better upstream and downstream bandwidth than ever before.

Fiber to the Node (FTTN) or Remote Terminal (RT) topologies are the "sweet spots" for VDSL2, and will probably account for the lion's share of deployments. Greater than 80 percent of remote terminals worldwide are within 5000 feet of the customer, and at that range, VDSL2 can offer up to 25Mbps downstream and 5Mbps upstream, delivering enough bandwidth to support 2 HDTVs, videoconferencing, VoIP, and higher-speed data services. Belgacom, SBC and other major carriers will deliver most of their "quadruple-play" services through FTTN deployments in the near future.

In 2007 and beyond, as the network fiber infrastructure moves closer to the home, many carriers will deliver most of their "quadruple-play" services through Fiber to the Curb (FTTC) deployments. In this topology, the VDSL2 line cards are deployed in neighborhood nodes that are within 1000-2000 feet of the customer and serve a smaller number of customers from each node. At this distance, VDSL2 can deliver up to 50Mbps downstream and 30 Mbps upstream, offering even more interactive bandwidth for services that have already been mentioned and for new services yet to be dreamed up.

For Fiber to the Premises (FTTP) or Fiber to the Home (FTTH) applications such as multiple tenant/dwelling units (MTU/MDUs), upgrading the building's service terminal in the basement from ADSL to VDSL2 will allow carriers and building owners to upgrade tenants to 100Mbps of symmetrical bandwidth service over existing telephone wiring. Extensive MTU and MDU deployments of VDSL have been taking place in Japan and Korea over the last three years.

Considerations

As with any new technology, there are several variables to consider when deploying VDSL2, and carriers should understand these in order to maximize their success.

Chipset design – In the MTU and MDU deployment topology, carriers should look for VDSL2 solutions using chipsets that can deliver the maximum performance of 100Mbps symmetrical bandwidth.  History has proven that our appetites for bandwidth will continue to grow with new availability of services, and carriers should insist on full bandwidth in their VDSL2 solutions in the MTU/MDU topology.

In Exchange and FTTN/RT deployments, the number of customers and power consumption requirements may push for a slightly lower performance product that delivers performance up to 60Mbps downstream and 30Mbps upstream to optimize for these topologies. These chipsets will maximize the port density and number of ports while minimizing the power consumption.

Access platforms – The Digital Subscriber Line Access Multiplexers (DSLAMs) that deliver service in Central Office and FTTN/RT topologies in today's networks may not have the backplane capacity to support a full population of VDSL2 line cards. Since a VDSL2 line card delivers higher upstream as well as downstream bandwidth, its aggregate bandwidth usage is much higher than the usage for an ADSLx card. VDSL2 applications which envision 25Mbps/5Mbps service (downstream and upstream, respectively) should not exceed the backplane capacity of most existing DSLAMs. In higher-speed service deployments, however, DSLAM upgrades may be necessary, depending on the percentage of subscribers using the higher-level offerings.  Next generation IP DSLAMs have taken this into consideration and will be able to support all possible VDSL2 rates.

Another consideration with access platforms is that heavy VDSL2 usage may require upgrading DSLAM uplinks back to the remote terminal or central office. However, major carriers have spent the past ten or more years deploying fiber to their FTTN / RT access platforms, so sufficient fiber is often already installed to support this upgrade.

CPE – North American carriers learned some painful lessons during the early days of DSL by deploying several different types of CPE (DSL modems), which complicated installation, inventory, support, and compatibility issues. For example, carriers once offered PCI-based internal modems for desktop PC users and external modems for laptop users, while other carriers offered more than one brand of modem. More recently, North American carriers have settled on external modems and are consolidating around one to three brands and models within each deployment.

When rolling out VDSL2 services, however, carriers will want to think beyond traditional xDSL modems. Since new services will incorporate video and voice functionality, it will be necessary to include Ethernet-based service gateways and VoIP adapters. The carriers will need to control the equipment further into the home to guarantee reliability and Quality of Service (QoS) that are enabled in gateways. 

To make this deployment and servicing possible, carriers should consider deploying full-service residential or business gateways that handle voice, video and data in one device. Vendors are now bringing these devices to market.  Due to the control and increased capabilities of these CPE, it will likely drive the number of vendors that are deployed by a carrier to one or perhaps two vendors.  In fact, the rapid deployment of VDSL2-based services can be seen as a primary driver for the burgeoning full-service residential gateway industry.

The other trend occurring with new VDSL2 CPE is their capability to also operate as ADSL, ADSL2+ and VDSL devices.  With this capability in the CPE, it is advisable for the carrier to start deploying these CPE as soon as possible so that when the infrastructure is in place for VDSL2, they can deliver new services to these customers without having to disturb the customer with a new CPE device.

With its backward compatibility and significantly higher service capabilities, VDSL2 allows carriers to rapidly deliver next-generation services while leveraging their investments in millions of existing connections. By carefully weighing a few key considerations, VDSL2 migration can be a seamless and highly profitable undertaking.

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