Network Systems DesignLine | VDSL2 meets rising demands for upstream network bandwidth

November 14, 2005

VDSL2 meets rising demands for upstream network bandwidth

The limits of upstream bandwidth in broadband access are painfully apparent when uploading rich media content, The challenge is to deliver significantly higher downstream and upstream bandwidth to enable such services as IPTV without spending billions of dollars on forklift upgrades or FTTH deployment. As fiber is pushed out farther into access networks, VDSL2 extends fiber-level bandwidth from the node, curb or building/basement through copper access lines. See how.

By Piyush Sevalia, Ikanos Communications

Business and residential subscribers are constantly demanding more bandwidth. Historically, this demand has been in the downstream direction. Now, however, users are accumulating and generating large amounts of their own content, and with it a greater appetite for upstream bandwidth. Telecommuting, home networks, P2P applications, videoconferencing, monitoring and surveillance, and interactive gaming, are just a few of the emerging residential applications that will continue to drive upstream bandwidth.

To effectively enhance revenues and stave off competitive pressures, carriers are announcing such services as IPTV, video-on-demand (VoD), and interactive gaming. While carriers are experiencing static or declining wireline revenues, they are also under pressure from satellite and cable providers, venturing into wireline and wireless offerings with new technologies such as Voice over IP (VoIP).

The challenge then, is to deliver significantly higher downstream and upstream bandwidth to enable IPTV and other services, without spending billions of dollars on forklift upgrades or fiber-to-the-home deployments. In this article, we'll examine the role of VDSL2 technology in addressing this challenge.

Data consumers are becoming data producers
At the core of the debate over next-generation broadband access is the notion of increasing bandwidth in both directions. Business users preferred symmetrical access services such as T1 and SDSL for years because the speed of upstream access to partners, customers, or remote employees, has been just as important as the speed of downstream access. Today, consumer broadband needs are changing for the same reason.

Historically, most carrier customers were consumers of content, browsing Web sites, downloading music and photos, or accessing files on corporate LANs. With the rise of digital music and photography, however, consumers increasingly want to share content with others. The limits of upstream bandwidth with legacy DSL technologies have become painfully apparent when consumers want to upload rich media content (See Table 1).

Table 1. Upload timeframes for digital media with typical ADSLx upstream bandwidth levels.

As carriers plan for rollouts of triple play services, most of the focus is on the amount of downstream bandwidth required. However, at a time when consumers are increasingly becoming producers, upstream bandwidth should also be considered (See Table 2).

Table 2. Bandwidth requirements for next-generation services. *Assumes available compression technologies are in use

Why VDSL?
As vendors attempt to deliver carrier equipment offering higher interactive bandwidth over existing copper in support of new services, Very High Bit Rate DSL (VDSL) is the only technology that meets requirements. Proven for carrier deployments in Asia over the past three years, VDSL technology was accepted as an international ITU standard (VDSL2 – G.993.2) in May 2005, and carriers in Europe and North America have indicated their intention to use it.

VDSL2 is Discrete Multitone-based (DMT) as are ADSL2 and ADSL, the two most widely used legacy DSL standards. As a result, VDSL2-based systems can be cost-effectively designed to support ADSL2 and ADSL, hence higher-bandwidth VDSL2 can be deployed without disrupting existing subscriber services. Able to provide up to 100 Mbps bandwidth in both upstream and downstream directions however, the VDSL2 standard offers a variety of operating profiles that support line cards for multiple applications. Acknowledging the carrier trend toward pushing fiber out farther into access networks, VDSL2 extends fiber-level bandwidth from the node, curb or building/basement through copper access lines.

VDSL2 specifications
VDSL2 offers a quantum leap over the amount of frequency available in previous versions of ADSL, along with more frequency bands, a wider selection of power levels, and a consequential increase in the number of implementation variables and potential applications. (See Table 3.)

Click here for Table 3

When compared against the frequency ranges available for ADSL2+, for example, the VDSL2 standard provides for a broader and more flexible breakdown of frequency ranges for upstream and downstream bandwidth (See Figure 1). While the lower, longer-range frequencies are fairly well divided between upstream and downstream bandwidth, there is much more spectrum allocated to upstream bandwidth. The total amount of frequency for downstream use is 12.51 MHz, while the amount of frequency for upstream use is 16.85 MHz.

Figure 1. ADSL2+ vs. VDSL2 frequency ranges

Thus, while ADSLx allows very little flexibility in terms of applications (CPE devices and DSLAMs), VDSL2 allows designers to use it for CPE, in-building terminals, fiber to the curb (FTTC) or fiber to the node (FTTN) line cards, and other applications. Designers simply use appropriate profile and spectrum allocation based on the relative needs for upstream and downstream bandwidth, the application, and the distance to be covered.

VDSL deployment examples
The deployment flexibility offered by VDSL2 enables its use in line cards that support many applications, including CO-to-subscriber, FTTN, FTTB (MDU/MTU), and FTTC. For example, FTTB applications in which carriers bring fiber to the building basement can use the 30MHz VDSL2 profile to deliver 100 Mbps of symmetrical bandwidth over existing twisted pair wiring in the building over distances up to 300 meters. This application eliminates the need for building owners to upgrade in-building wiring, thus enabling faster service availability at a much lower cost.

Another example is FTTN, which is a key part of North American carrier deployment strategies such as SBC's Project Lightspeed. FTTN brings fiber within 5,000 feet of subscriber premises. In this application, designers use a 12 MHz VDSL2 profile to deliver 25 Mbps downstream and 5 Mbps upstream bandwidth. When subscribers are closer to the node (within 3,500 feet, for example), carriers can offer higher levels of bandwidth in both directions. In FTTN applications, VDSL2 allows carriers the flexibility to upgrade service capabilities without necessarily requiring CPE upgrades by the subscriber—those who wish to upgrade their service can obtain new modems from the carrier and access faster service provided by the new VDSL2 connection, while those who wish to remain on the old ADSLx service may continue to do so.

By supporting ADSLx interoperability, VDSL2 systems allow carriers to upgrade each consumer at the consumer's convenience. In addition, carriers can upgrade a remote terminal on one truck roll by populating it only with VDSL2 line cards.

Selecting a chipset manufacturer
When selecting VDSL2 chipsets for implementation in network systems or line cards, design engineers should consider a variety of factors that significantly impact time to market as well as a product's capital and operating costs for carriers. Assuming that chipset manufacturers are compliant with the various VDSL2 standards, this section lists key considerations.

The reason for VDSL2 standard's higher bandwidth specification is to ensure that carriers can offer higher value services such as triple play. Usually, these services are extremely dependent on the higher performance, and more importantly, on the stability of the underlying physical layer system. A few CRC errors or line disconnects will completely destroy the video viewing experience of a consumer and defeat the purpose of deploying VDSL2 for the carrier.

A patient consumer may request that the carrier fix the problem, causing provider truck rolls, reconfigurations, equipment shipments, or even potentially withdrawing the service from the consumer, all of which are expensive. An impatient consumer will just move to cable or satellite services, which are field-tested and proven.

Using a higher amount of spectrum for VDSL2 has its own challenges for the chipset manufacturer. To obtain the maximum performance at the best stability out of the system, the manufacturer must:

Experience, as always, helps solve the challenges, and ensures that a customer receives the best possible experience. A chipset manufacturer with extensive VDSL-DMT development and deployment experience will offer a system with very high performance, and one that is stable and reliable under various deployment environments, ensuring the fastest time to market (and revenue) for the OEM.

Deployment experience also ensures that existing infrastructure in specific countries (such as TLAN in Korea and J-ISDN in Japan) does not interfere with new deployments.

Products optimized for profiles
Manufacturers often prefer to source chipsets from one vendor across multiple versions of a product. Designers can reduce design time by buying parts from vendors that offer products for multiple VDSL2 applications and profiles, since they will be able to use a common software platform for all applications.

There are many ways that a product can be optimized for a particular market segment. Chips designed and optimized for 12 MHz spectrum (profile 12a) will be very different than those optimized for 30 MHz spectrum (profile 30a). For example, the 30a profile requires processing of tones at 8 kHz tone spacing, which is twice the speed of the 4 kHz tone spacing used for profile 12a. Thus, the FFT/IFFT engine and other components must be architected to operate at twice the frequency for profile 30a when compared with profile 12a.

A line driver designed for 30 MHz operation will require linearity over the entire region, and operate at a maximum of 14.5 dBm (specified by the VDSL2 standard). Separately, a line driver designed for Profile 8a (8.5 MHz spectrum) must also support 20.5 dBm, which is a completely different requirement.

The tradeoffs are evident in VDSL2 chipset suppliers. Those chipset suppliers who claim to have a "universal product" that supports all profiles will usually consume much higher power per port, have much lower density, and may also have lower performance. A VDSL2 chipset supplier that has segmented product lines, which are optimized for different profiles, will have leading edge power consumption per port, the highest density, and the highest performance.

While it is true that OEMs would like to select a "universal" chipset that supports all deployment profiles, applications for the various profiles are completely different. The 30-MHz profile is typically deployed in the MxU environment where loop lengths are short and there is no co-deployment of ADSLx. MxU products are typically of the stackable pizza-box or small-port-count, line card based system, and have the benefit of easy expansion based on consumer demand. On the other hand, an 8.5 MHz profile is typically deployed from the central office, with long loops and co-deployment of ADSLx.

Consequently, central office products are typically large-port-count, line-card based designs. For the OEM, it is more important for the hardware and software architecture of the chipsets to be "universal" rather than the chipset itself. This approach ensures the maximum familiarity with products in the design-in phase (thus ensuring fastest time to market) without sacrificing optimum performance for specific applications.

Programmability
There are many country-specific requirements that make it beneficial for VDSL2 devices to support changes via software control, so that engineers can design one system or line card for use in several different markets. For example, in countries where ISDN is deployed, it is common to require that frequencies below 640 kHz are not used for VDSL transmission to avoid having the ISDN and VDSL interfere with each other.

Other programmable options include the kind of traffic that can be routed on the VDSL platform (ATM or IP), as well as a host of carrier-specific programmable parameters such as bandwidth, power down modes, and multi-mode operation (VDSL2, VDSL, ADSL2+, ADSL).

Density
Cost is a major factor for all consumer technologies, and VDSL2 falls in the same category. This is especially true of systems that are deployed in remote terminals. The total cost of a high-density line card may be higher initially, but is usually outweighed by the savings in eliminating multiple installation-truck-rolls. The higher the port density, the more consumers can be served from one card, and the lower the deployment cost per consumer.

System density is dependent not only on the architecture of the chipset, but also whether on a product is optimized for a particular profile.

Power Dissipation
Power consumption is always a concern in network systems, so devices that consume less power and dissipate less heat are always preferable. In addition, existing restrictions (due to NEBS compliance) on the amount of power that provided to a line card, will directly impact the density of the line card. Therefore, power consumption per port is a crucial factor in selecting the right chipset for this marketplace.

It is also expected that chipsets optimized for a particular deployment profile, rather than so-called "universal" chipsets that support all VDSL2 deployment profiles, will have the best per-port power consumption.

VDSL2: The right technology at the right time
VDSL2 technology has emerged at a particularly opportune time, when carriers' need for revenue enhancement coincides with subscribers' need for higher upstream bandwidth. With its ability to deliver far higher bandwidth in both directions, only VDSL2 has the performance and flexibility to meet carrier needs and to satisfy consumer service demands. By applying some simple criteria during their selection of VDSL2 solutions for networking products, network equipment manufacturers can deliver products that offer maximum performance with the shortest time to market.

About the Author
Piyush Sevalia is a Director of Product Marketing for Ikanos Communications where he is responsible for product management, positioning, and launch of the company's solutions. Mr. Sevalia earned a B.S. in electrical engineering from the University of Bombay in India, an M.S. in electrical engineering from the University of Michigan, and an MBA from the University of California, Berkeley. Piyush holds four patents. He frequently speaks at industry conferences, and has authored multiple technology papers and business articles. He can be reached at Psevalia@ikanos.com.