Digital Subscriber Lines

This section examines the final high-speed Internet access area, DSLs. DSL network components and architecture are presented and contrasted with cable modem networks for delivering high-speed digital communications to residential subscribers. DSL is just a pair of modems running over existing telephone wiring in a broader range of frequencies than is used for voice communications alone. Thus, DSL signifies a modem pair, and not a type of telephone line. This means that two modems attached to a line create a DSL. When a telephone company buys xDSL, it buys modems and attaches them to the lines they already own. Thus, DSL is really a new type of modem and not a line.

Employing the modem's DSL service provides a dedicated digital circuit from your home to the telephone company CO, over the existing analog telephone line. DSL overlays the existing networks so it also provides a separate channel for voice phone communication. This means voice and fax calls are carried at the same time high-speed data is flowing across the line. DSL uses the frequency spectrum between 0KHz-4KHz for analog voice, and 4KHz-2.2MHz for data. xDSL is a generic acronym for a family of dedicated services. The "x" designates:

Service Type

Down Speeds

Up Speeds

ADSL (Asymmetric Digital Subscriber Line)

1.544 to 8 Mbps

64 Kbps to 1.544 Mbps

ADSL (Lite)

1 Mbps

512 Kbps

HDSL (High-bit-rate Digital Subscriber Line)

1.5/2.048 Mbps

1.5/2.048 Mbps (4-wire)

SDSL (Single-line Digital Subscriber Line)

1.5/2.048 Mbps

1.5/2.048 Mbps (2-wire)

VDSL (Very-high-bit-rate Digital Subscriber Line)

13 Mbps - 52 Mbps

1.5 Mbps- 2.3 Mbps

IDSL (ISDN Digital Subscriber Line)

128 Kbps

128 Kbps

RADSL (Rate-Adaptive Digital Subscriber Line)

1.544 to 8 Mbps

64 Kbps to 1.544 Mbps

UDSL (Universal Digital Subscriber Line) (Also called "splitter-less" DSL, as it doesn't require

1.0 Mbps a splitter)

300 Kbps

DSLs are the telephone company's answer to cable modems. DSL technology provides high-speed data transmission and a single telephone voice channel over existing local loop wiring. In this manner, they preserve the telephone company's investment in its wiring infrastructure while answering the need for high-speed Internet access.

DSL requires equipment to be installed at both the customer premise and in the telephone company CO. This equipment uses analog transmission over a wider range of frequencies than the standard 4,000-cycle voice channel combined with sophisticated modem encoding and decoding algorithms to provide transmission speeds typically ranging between 640 Kbps to 13 Mbps.

DSL equipment is limited to operating within 18,000 feet of the telephone company CO. Higher speeds require that the customer premise be within 12,000 feet of the telephone company Class 5 CO.

DSL equipment connects to a telephone at the customer premise as well as directly to the PC using (typically) Ethernet. Similarly, the telephone voice channel is split at the telephone company CO and routed into the branch exchange switch, while the PC connection is routed to a LAN. This telephone company CO LAN is then connected via high-speed digital channels to the Internet.

DSL availability and rates vary. For a 128 Kbps/384 Kbps channel, Pac-Tel charges $89 per month. If the speed is increased to 384 Kbps/1.5 Mbps, the cost increases to $279 per month. These costs are substantially higher than cable modem costs. At one time, Verizon was offering Infospeed DSL service with a top speed of 680 Kbps/ 7.1 Mbps for a monthly fee of $189 per month. It currently has limited availability.

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