Buzz
You may be familiar with DSL, but have you discovered its faster counterpart, VDSL? deepblue4you / Getty ImagesThe need for high-speed internet has surged in recent years. With the rise in home computer usage and the growth of home networks, the demand for broadband (high-speed) connections has steadily increased. Two key technologies, cable modems and Asymmetric Digital Subscriber Line (ADSL), currently dominate the market.
Though both cable modems and ADSL offer speeds far beyond a 56K modem, they still fall short when it comes to supporting home services like digital television and video-on-demand.
Another DSL technology, known as very high bit-rate DSL (VDSL), is considered by many to be the next evolution in delivering a comprehensive home communication and entertainment system. Companies like U.S. West (now part of Qwest) are already providing VDSL services in select regions. VDSL offers remarkable bandwidth, with speeds reaching up to 52 megabits per second (Mbps). When compared to the 8 to 10 Mbps of ADSL or cable modems, the shift from current broadband tech to VDSL could be as transformative as the leap from 56K modems to broadband. As VDSL becomes more widespread, expect the integrated service packages to cost less than the total of current separate services.
In this article, you'll discover VDSL technology, its significance, and how it stacks up against other DSL technologies. But before diving into that, let's review the fundamental concepts of DSL.
In the United States, a typical telephone installation consists of a pair of copper wires installed by the phone company in your home. These wires provide more than enough bandwidth to transmit data alongside voice calls. Voice calls only use a small portion of the wire's capacity, and DSL takes advantage of the remaining bandwidth to transmit data without interfering with voice conversations.
Standard telephone service limits the frequencies that switches, phones, and other equipment can handle. Human speech in normal conversation occupies a frequency range of 400 to 3,400 Hertz (cycles per second). However, the wires themselves can accommodate frequencies as high as several million Hertz. Modern digital equipment can safely use a far larger portion of the phone line's capacity, and DSL takes full advantage of this.
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In the upcoming section, we'll explore ADSL.
ADSL
An ADSL network diagramADSL operates using two key devices: one at the customer's location and one at the provider's end:
- Transceiver - Located at the customer's site, this device often offers additional services beyond DSL.
- DSL access multiplexer (DSLAM) - This device, used by the DSL provider, handles incoming customer connections.
Most residential users refer to their DSL transceiver as a DSL modem. However, engineers from the telephone company or ISP typically call it an ATU-R, which stands for ADSL Transceiver Unit - Remote. Regardless of the name, the transceiver is the device that connects the data from the user's computer or network to the DSL line. It can connect to a customer’s equipment in various ways, though most home setups use Universal Serial Bus (USB) or 10BaseT Ethernet
The DSLAM at the service provider’s end is what truly makes DSL function. A DSLAM aggregates connections from multiple customers and combines them into a single, high-capacity link to the Internet. These devices are flexible, supporting various types of DSL and offering additional features like routing and dynamic IP address assignment. For more details on ADSL, see How DSL Works.
DSL technology is sensitive to distance: as the distance increases, signal quality and connection speeds decrease. ADSL has a maximum range of 18,000 feet (5,460 m) between the modem and the DSLAM, although many service providers set even lower limits for speed and service quality reasons. At the maximum range, speeds can fall far below the advertised rates, while customers located closer to the central office or DSL termination point may enjoy near-maximum speeds, with potential for even faster speeds in the future.
You might wonder why distance is a limitation for DSL but not for voice telephone calls. The key lies in small amplifiers known as loading coils, used by telephone companies to boost voice signals. These coils are incompatible with DSL signals, as the amplifier interferes with the data integrity. If there’s a voice coil in the loop between your phone and the telephone company's central office, DSL service won’t be available. There are also several other factors that may prevent you from receiving ADSL:
- Bridge taps - These are extensions in the line that service other customers, affecting your connection.
- Fiber-optic cables - If part of your telephone circuit uses fiber-optic cables, ADSL signals can’t pass through the analog-to-digital-to-analog conversion process that occurs.
- Distance - Even if you know the location of your central office (though, the phone companies typically don’t share this), checking a map won’t necessarily help. The signal may take a convoluted route, making the distance farther than it seems.
Fiber-optic cables, a key factor disrupting ADSL, are actually the technology that makes VDSL possible. In the upcoming section, you'll discover why.
VDSL Speed
A fiber optic cable
Photo courtesy of CorningVDSL operates over the same copper wires in your phone line as ADSL, with a few key differences. VDSL offers extraordinary speeds, reaching up to 52 Mbps downstream (to your home) and 16 Mbps upstream (from your home). This is a significant improvement over ADSL, which offers a maximum of 8 Mbps downstream and 800 Kbps upstream. However, VDSL's remarkable speed comes with a limitation: it can only function over copper lines for a short distance, around 4,000 feet (1,200 m).
The secret behind VDSL is the replacement of many telephone company main lines with fiber-optic cables. Many providers are planning Fiber to the Curb (FTTC), which means replacing all copper lines up to the point where your phone line connects at your home. At a minimum, most companies aim to implement Fiber to the Neighborhood (FTTN), which involves running fiber-optic cables to the main junction box of a neighborhood, rather than all the way to each individual home.
By installing a VDSL transceiver in your home and a VDSL gateway in the junction box, the distance restriction is efficiently bypassed. The gateway handles the analog-digital-analog conversion issue that prevents ADSL from working over fiber-optic lines. It converts the data from the transceiver into light pulses that can be transmitted through the fiber-optic network to the central office, where the data is routed to its intended destination. When data is sent back to your computer, the VDSL gateway converts the signal from the fiber-optic cable and sends it to the transceiver. This process occurs millions of times per second!
ADSL and VDSL are just two examples of DSL technology. On the next page, you'll find a comparison chart that outlines the different variations and how they measure up against one another.
Comparing DSL Types
Mytour.comDSL technology comes in many forms. In fact, there are so many variations that the term xDSL is often used, where x represents any of the DSL types, in general discussions about DSL.
- Asymmetric DSL (ADSL) - This type is referred to as 'asymmetric' because the download speed is higher than the upload speed. This configuration is used because the majority of Internet users typically download more content than they upload.
- High bit-rate DSL (HDSL) - This version offers transfer speeds similar to a T1 line (about 1.5 Mbps), with the unique feature of transmitting and receiving data at the same speed. However, it requires two lines, separate from your regular phone line.
- ISDN DSL (ISDL) - Primarily intended for users of the Integrated Services Digital Network (ISDN), ISDL operates slower than most other DSL types, with a fixed speed of 144 Kbps in both directions. The benefit for ISDN users is that they can continue using their existing equipment, though the actual speed improvement is typically limited to just 16 Kbps, as ISDN runs at 128 Kbps.
- Multirate Symmetric DSL (MSDSL) - This variant of Symmetric DSL supports multiple transfer rates. The transfer rate is determined by the service provider, generally based on the pricing or service level.
- Rate Adaptive DSL (RADSL) - A popular form of ADSL, RADSL enables the modem to automatically adjust the connection speed based on the line's length and quality.
- Symmetric DSL (SDSL) - Similar to HDSL, this version sends and receives data at the same speed. It also requires a separate line, but only one line, as opposed to the two needed for HDSL.
- Very high bit-rate DSL (VDSL) - Known for its extremely fast speeds, VDSL is asymmetric, but it only works over a short distance using standard copper phone wiring.
- Voice-over DSL (VoDSL) - A type of IP telephony, VoDSL enables multiple phone lines to be consolidated into a single line, which also supports data transmission.
The following chart compares the various types of DSL technologies:
As demonstrated, VDSL offers a significant performance enhancement over all other DSL variants. However, for VDSL to become widely accessible, it must first be standardized.
VDSL Standard: DMT
Discrete MultiTone divides the available frequency spectrum into 247 distinct 4-KHz channels.After a prolonged standards conflict between the VDSL Alliance—comprising Alcatel, Texas Instruments, and other partners, which endorsed VDSL through a carrier system called Discrete MultiTone (DMT)—and the VDSL Coalition, headed by Lucent and Broadcom, proposing a carrier system based on Quadrature Amplitude Modulation (QAM) and Carrierless Amplitude Phase (CAP), DMT emerged as the victor. As reported by equipment manufacturers, most ADSL devices today utilize DMT technology.
DMT works by splitting signals into 247 distinct channels, each occupying 4 kilohertz (KHz, or 1,000 cycles per second). You can imagine this as the phone company dividing your copper line into 247 individual 4-KHz channels, each connected to a modem. Essentially, you have 247 modems working simultaneously to communicate with your computer!
Each channel is continuously monitored, and if the quality of a channel deteriorates, the signal is moved to a better one. This dynamic system ensures that the signal always finds the most suitable channels for both transmission and reception. Additionally, some of the lower channels (starting at around 8 KHz) are designated as bidirectional channels, handling both upstream and downstream data. The process of managing and optimizing the information flow across all 247 channels makes DMT more intricate to implement than other carrier technologies, but it also provides greater flexibility over lines with varying levels of quality.
