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Most of today's xDSL transmission standards are proprietary. Each vendor implements its own data transmission method pursuant to the ANSI T1.413, Issue 2 ADSL standard, which does not specify how to encode data, but hems vendors into listed noise and frequency parameters. If you subscribe to xDSL service, you receive an xDSL router or modem that matches the head-end equipment in the central office. G.Lite was designed to standardize the transmission parameters of xDSL, letting consumers choose the DSL modem and get access from remote data jacks, such as those in hotels and airports.

ADSL provides for data rates as high as 8 Mbps downstream and up to 1.5 Mbps upstream. ADSL's asymmetric nature--transmit and receive rates are not identical--supposedly lets local carriers offer competitive ADSL prices without destroying existing T1/ leased-line market share. Many telephone companies have been slow to respond to the ADSL market demands, however, likely for fear it will erode their existing (read: higher-priced) T1 markets.

xDSL and G.Lite are limited to a maximum distance of 18,000 feet from the central office (CO). Approximately 70 percent to 80 percent of U.S. homes are within a three-mile radius of a CO, so a user's odds of getting xDSL or G.Lite service are good. However, some telcos have installed load coils that strip out high-frequency noise from longer copper runs. These load coils improve the quality of analog voice signals operating below the 4-KHz frequency band. xDSL technologies usually operate over these same copper wires, but at much higher frequencies. Load coils effectively filter out the xDSL signal, and also affect ISDN installations. Typically, carriers will remove load coils for ISDN installations, but stories concerning ornery refusals to remove the coils for xDSL are multiplying. In either case, a backup plan is critical for delivering home access to users ineligible for xDSL service.

While the full xDSL standard specifies downstream data rates up to 8 Mbps, the G.Lite standard limits data rates to approximately 1.5 Mbps downstream (and some proposals limit it to 1 Mbps downstream) and roughly 768 Kbps upstream. This limitation helps the carrier extend the reach of xDSL to a full 18,000 feet. Longer copper runs are subject to more electrical interference, which can degrade data performance. By limiting data rates to lower speeds, the G.Lite standards body hopes more consumers will be eligible for G.Lite DSL service.

Going Backward With G.Lite
The G.Lite standards body also is working to maintain backward compatibility with the ANSI T1.413, Issue 2 ADSL standard. Many carriers are rolling out xDSL systems, leaving the working committee for G.Lite with the difficult task of choosing the implementation with which G.Lite will interoperate. The two major DSL implementation methods are FDM (Frequency Division Multiplexing), which is the most widely deployed standard, and Echo Cancellation. FDM technology uses two separate frequency bands for transmit and receive channels, making it impossible for signals to collide as they would on a shared Ethernet network. Echo Cancellation lets both local and remote sides transmit and receive on the same frequency at the same time. Because both transmission and reception happen on the same frequency band, the signals combine on the wire. Sophisticated DSPs (digital signal processors) filter out the transmit or receive part of the signal. This filtering requires careful timing and synchronization with the far end of the copper wire, but has the potential for higher throughput and longer distances. Of the two, FDM is significantly more cost-effective; the processing power necessary to perform echo cancellation adds significant cost to a DSL router. Because G.Lite is designed for the consumer market, we anticipate the standard will use FDM encoding to deliver its data across copper.

Full-rate xDSL operates over the same piece of copper as your phone line, so consumers can have a single phone line supporting a local voice and simultaneous data line. To achieve this, a splitter must be installed at each end of the copper wire. The splitter separates the xDSL high-frequency signal from the low-frequency analog voice signal. Without a splitter, you must run a dedicated xDSL line to the customer premises (a line with no voice traffic). With or without a splitter, the local phone company must visit the customer site to install the splitter or second line. These so-called "truck rolls" typically cost the xDSL provider $100 to $200 per trip, significantly contributing to the installation costs.

In contrast, G.Lite is a splitterless technology. All the equipment necessary to split the voice and data signals is built into the G.Lite modem, resulting in significantly reduced costs. However, splitterless technology has drawbacks. Each unterminated wire in the customer's home (open phone jacks and some low-quality telephones) acts like a gigantic antenna, picking up unwanted high-frequency noise. This interference can have a significant impact on DSL splitterless operation. To counter this, the UAWG (Universal ADSL Working Group) recommends that high-frequency filters be installed on each phone jack in the house. The low-cost snap-in filters prevent high-frequency noise and simplify G.Lite installation.

Because xDSL is terminated at the CO, it's considered an always-on technology--the end user will have on-demand Internet access without having to dial up. However, always-on Internet connections present security risks. Depending on the service provider, home-office users will have an IP address directly on the Internet or possibly a direct link into the office network.