Broadband describes a number of different technologies that deliver digital
data to homes and businesses at speeds faster than today’s narrowband
technology, analog modems. Today, analog modems comprise the vast
majority of the world’s 40 million connections for homes and small
businesses. All of these modems use standard telephone lines as their
medium to transmit information. The goal of broadband technologies is to
overcome the current limitations of analog modems, namely:
- Bandwidth Limitation. Today’s fastest analog modems (V.90 standard) can transmit and receive data at only 56 Kilobits per second (Kbps). Moreover, line quality and line distances typically reduce bandwidth to 33 Kbps or less.
- Dial-Up Connection. An analog modem establishes a connection to another modem using standard telephone dial tones. This process is lengthy and prone to busy signals if the receiving modem is busy.
- Dedicated Line. When an analog modem uses a telephone line, the line cannot simultaneously be used for voice communication. For homes or businesses that frequently use the modem, a dedicated phone line for the modem is usually necessary.
Technology Overview
Broadband technologies can be classified as either one-way or two-way.
One-way technologies are capable of sending information to the end user at
very high speeds, but they rely on some other means (usually an analog
modem and a phone line) to receive information from the end user. Although
lacking a high-speed return path for information, one-way technologies are
often wireless and offer much easier deployment over large areas. In
contrast, two-way technologies can send and receive information at very high
speeds over the same medium but require a wiring infrastructure that
supports these technologies.
In this document, we focus on the two most prevalent two-way broadband
technologies, cable and Digital Subscriber Line (DSL), and the most prevalent
one-way broadband technology, satellite. For each of these technologies, the
following sections will provide a quick overview, a discussion of the
technology’s advantages and disadvantages, background on standards
development, and trends in the near future.
DSL Overview
DSL is a modem technology that uses digital coding techniques to transform
ordinary phone lines (also known as "twisted copper pairs") into high-speed
data transfer lines. While voice data is transmitted below frequencies of
4 Kilohertz (KHz), Internet data can be transmitted at 25 to 138 KHz upstream
(from client to Internet) and 139 to 1100 KHz downstream (from Internet to
client), on the same phone line.
DSL enables simultaneous high-speed Internet access and analog voice
service. As a result, DSL enables telephone companies to use the world's
nearly 750 million existing phone lines to deliver affordable, high-speed
access to the Internet, corporate networks, and online services.
DSL Interoperability : DMT or CAP
There are two technologies competing to be the modulation standard for
ADSL, carrierless mplitude and phase modulation (CAP) and discrete
multitone modulation (DMT). Both CAP and DMT are coding technologies that
allow ADSL to utilize more of copper's available frequency range. CAP and
DMT are not interoperable.
In 1994, the ANSI working group adopted DMT as the ADSL standard.
However, many DSL manufactures and providers continue to offer CAP
services. Although DMT is cited as superior to CAP by ANSI, DMT is a
relatively new technology, while CAP chipsets have been used for almost
twenty years. Consequently, it's believed that CAP technology actually has a
larger installed ADSL base than DMT. Until DMT catches up in deployments,
there will continue to be uncertainty in modulation standard.
Despite the existance of two modulations standards, great steps have been
made in DSL interoperability, with many vendors domonstrating that their
CPE will work with the Central Office equipment (DSLAMs) of another vendor,
or vice versa. Increased interoperability should help encourage retail sales of
DSL modems-customers will not haveto be concerned that DSL modem they
purchase in a store will not function with the DSL equipment installed at the
phone company's Central Office. A number of vendors have demonstrated
the interoperability of their ADSL equipments.
Types of DSL technologies
A number of DSL technologies have been developed to meet a full range of
customer needs. The most common is Asymmetric Digital Subscriber Line
(ADSL), which delivers greater downstream bit-rate transmission than
upstream. Conversely, Symmetric Digital Subscriber Line (SDSL) delivers
symmetric upstream and downstream bit-rate transmission between the
subscriber’s home and the CO. SDSL is deployed when customers, usually
Small-Office/Home-Office (SOHO) users, telecommuters, or small
businesses, need larger upstream bandwidth to upload files or handle
videoconferencing.
Other DSL technologies include High-bit-rate Digital Subscriber Line (HDSL),
which uses more than one twisted pair to deliver T1 speeds, and ISDN Digital
Subscriber Line (IDSL), which delivers data at 128 Kbps into an IDSL
"modem bank" connected to a router. IDSL is generally deployed when
ADSL is not viable because of the length of the customer loop. The highest
performance DSL technology available is Very high data rate Digital
Subscriber Line (VDSL), delivering bit-rate transmission from 12.9 to
52.8 Mbps with corresponding maximum reach ranging from 4,500 feet to
1,000 feet of 24-gauge twisted pair. VDSL may be deployed when extremely
high-bandwidth applications are necessary and may eventually be used for
Internet video services. The following is a summary of these technologies.
DSL Advantages
- Point-to-point dedicated connection: the DSL connection is point-to-point
- between the consumer and the telco and thus provides guaranteed
- bandwidth and more inherent security than the cable architecture.
- Always-on connection: instead of having to dial in to their ISP to establish
- a connection (such as with standard analog modems), consumers have a
- standby, ready-to-use, high-speed connection, much like a telephone line.
- Low-cost modems.
- Dual-mode modems
- High-speed access: access from 256 Kbps to 8 Mbps downstream and from 90 Kbps to 512 Kbps upstream.
- Simultaneous voice and data service: consumers can have a high-speed
- data connection while using voice/fax at the same time.
DSL Disadvantages
- Cost: DSL service is currently more expensive than cable modem service
- (approximately $10/month more).
- Slower rollout of services.
- Availability beyond 18,000 feet from the CO is problematic.
- Installations can be difficult.
taken from intel engineers documents
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