The leading technology for delivering higher bidirectional bandwidth is VDSL (Very-high-bit-rate Digital Subscriber Line), which has been standardized by the ITU-T as the VDSL2 standard. VDSL2 was designed to take full advantage of operators' existing broadband infrastructure by increasing the capacity of fiber optic cable to the node, access point, or building. VDSL2 can provide 100 Mbps of symmetrical bandwidth, thus bringing operator services on par with desktop LAN switching.
Operators worldwide are experiencing great success with their initial VDSL services. As a result, most of these operators are planning to add new or enhanced services that can generate additional sales, potentially as an upgrade to existing (and increasingly inadequate) services.

The Need for Speed:
Users, operators, and equipment vendors have a long history of underestimating speed requirements to keep costs down. But abrupt upgrades are disruptive and expensive.

Let's consider what has happened on the local loop over the past two decades (see Figure 1). Data communications on the Public Switched Telephone Network (PSTN) began with 300-baud modems, but these were soon replaced by modems supporting 1200 bps, then 2400 bps, and then 9600 bps. A major shift followed with the arrival of the 14.4 Kbps modem, and then the truly 'fast' 28K modem. After the move to 33.6 kbps, the modem reached its full potential with 56K technology. But even at this point, the asymmetric V.90 standard was quickly superseded by the symmetric V.92 version.
Despite these advances, users wanted more. The Integrated Services Digital Network (ISDN) was a major step forward for the industry, but the arrival of Digital Subscriber Line (DSL) technology began to erode ISDN's market share. Some versions of DSL even began to supplant traditional T/E operator services, especially T1 and E1. Asymmetric DSL (ADSL) became the most popular choice among users. But even ADSL is far from slowing the pace of change: from ADSL to ADSL2, then to ADSL2+, all with the corresponding reinvestment in infrastructure.
Current broadband access agreements complicate this upward trend. The different versions of ADSL provide asymmetric bandwidth. This shortcoming is becoming increasingly evident in the market.

Business subscribers were the first to recognize the limitations of ADSL. The reason is that business applications require symmetrical bandwidth to deliver content. Organizations consume and generate a significant amount of content and require adequate bandwidth in both directions. Where fiber optic cabling is available in such facilities, professionals often revert to DS-3 or Fractional DS-3 services. And where only copper lines are available, operators have found suitable solutions in T/E1 multiplexing or DSL services to meet bandwidth demands.
Residential subscribers now have the same concerns. As a large amount of subscriber content is routed from home to the internet, the balance is shifting as consumers generate their own content. The residential applications listed below will require increased bandwidth in the upstream direction:

- Home Networks: The home network is starting to resemble that of small businesses with multiple client PCs and a shared server. Different family members, all with internet access, require increased bandwidth in both directions to ensure an acceptable internet experience.
- Remote Work: There has been a dramatic increase in the number of people working remotely full-time or part-time. Creating an 'office-like' work environment at home requires adequate symmetrical bandwidth for uploading content such as presentations and spreadsheets.
- Peer-to-Peer (P2P) Applications: Given how traffic flows, P2P is driving consumer broadband usage worldwide, and this also creates bottlenecks. Asymmetrical bandwidth is simply insufficient for many of these needs from both the client and server perspectives. A study by the University of Washington, shown in Figure 2, concluded that P2P bandwidth dominates internet bandwidth and contributes to its traffic 'peak' figures. According to the study, 24% of internet users who use P2P consume more than 90% of the bandwidth. Upload bandwidth is also much higher than download bandwidth because users typically share audio and video files, which are much larger than data files. Figure 3 shows a comparison of the traffic types studied.

- Videoconferencing: As people have become accustomed to digital quality, many videoconferencing solutions are not ready for consumer use due to their mediocre image and sound quality. Adequate bidirectional bandwidth is the only way for operators to overcome this obstacle and offer high-definition television (HDTV) quality videoconferencing services.
- Multimedia Messaging (MMS): MMS and other forms of instant messaging are standard applications today, but in the future, they will need to be more robust to support video. Inexpensive or integrated cameras allow users to easily send video emails and video messages or have instant video chats if they have sufficient bandwidth.
- Video monitoring/Surveillance: Inexpensive webcams allow users to perform checks from remote locations. To achieve decent video quality, the upload speed must be dynamically partitioned and high enough to support such applications.
- Content creation and publication: Blogs and vlogs are gaining popularity as consumers become internet publishers. The trend toward multimedia-rich content and full multimedia productions is increasing the demand for upload bandwidth.

- Interactive games: Home PCs owe much of their popularity to games and 'educational entertainment' applications. As gamers seek new competitions worldwide, they need additional bandwidth to ensure uninterrupted gameplay.
- Remote desktop control: Many applications take advantage of remote control capabilities between PCs. The complexity of PCs now also allows for providing this access to Technical Support Teams. Without bidirectional bandwidth, this capability can be very slow and inefficient.

These emerging, bandwidth-intensive applications, coupled with competitive pressures from cable and satellite providers, are forcing operators to rethink their strategies. Novel technologies, especially hybrid fiber coaxial (HFC) and wireless broadband networks, threaten to undermine the inherent strategic advantage of copper/fiber infrastructure operators. What operators need is a robust, bidirectional broadband solution that can be cost-effective for both business and residential subscribers and that will stand the test of time.

Bidirectional Broadband with VDSL2:
Most operators are rapidly deploying fiber in their networks, as new or advanced services require enormous amounts of bandwidth. Fiber offers virtually unlimited broadband potential and, as a result, is a robust investment that will stand the test of time. However, bringing fiber to every individual subscriber is difficult to justify financially, even under the weight of increasing competition. Other factors complicating fiber deployment include trenching in streets, drilling through walls, and setup times of up to two hours, all of which are inconveniences for the consumer and result in a longer payback period.
One DSL technology designed to allow operators to enjoy all the benefits in a building without fiber is VDSL2. VDSL2 technology delivers bidirectional bandwidth over conventional unshielded twisted-pair cabling (see Figure 4). Of all the available DSL technologies, VDSL2 is simply the fastest, delivering up to 100 Mbps both downstream and upstream.
This data transmission speed (100 Mbps) is significant. 100 Mbps switching is currently the predominant choice for desktop connectivity in LANs. The ability to deliver the same 100 Mbps service speed for network access represents a game-changer. For this reason, it will be a long time before the potential of VDSL2 is exhausted.
VDSL2 can deliver ADSL2+ connectivity to all subscribers and achieves its maximum performance level for those subscribers closest to the operator's central office (CO) or remote terminal (RT). With this powerful transmission speed/range profile, operators have greater flexibility to offer a full suite of interactive services to offices and residences near the CO/RT and basic internet connectivity to consumers at a greater distance. VDSL2 solutions are available with full-featured DSL access multiplexers (DSLAMs) or as remote gateways/concentrators that can be deployed as either COs or RTs. The basic customer premises equipment (CPE) is typically a single-port gateway or modem that incorporates a DSL transceiver.

VDSL2 Today... and Tomorrow.
Operators around the world are successfully deploying VDSL. Among these operators is
AT&T Inc., which is just one provider that has capitalized on existing copper infrastructure. For its U-verse deployment, AT&T is building fiber to the node (FTTN) and using VDSL2 to turbocharge existing copper subscriber loops entering homes. AT&T estimates that this architecture costs only about $360 per user to deploy, almost five times less than the cost of a full building with Verizon fiber.
Verizon and NTT are extensively using VDSL2 in a hybrid approach to deliver broadband services to multi-family dwellings (MDUs) such as apartment complexes or high-rise buildings. In this context, they are using VDSL2 as the last-mile technology because deploying fiber in space-constrained areas is very
complex.