By J Schipper
The term "broadband" is generally used to refer to data
transmission which sends multiple pieces of data
to boost the rate of transmission. In the setting up networks,
the term is used to describe methods that allow two or more
signals to share the same transmission.
Digital Subscriber Line, or DSL service, is broadband because
it uses a shared pair of wires to transmit digital information
on one channel while another is used to transmit sound. DSL
lines are used in the customer interface to a local telephone
network. The download speed of DSL can vary from 128 kilobits
per second (kbit/s) to 24,000 kbit/s depending on DSL
technology and service implemented. Downloading is faster than
uploading for ADSL, and identical for SDSL.
DSL is the most commonly used alternative to cable modems in
Europe and North America. The older ADSL standards have a data
speed of approximately 8 Mbits/s over about a 2 kilometer
radius (1-1/4 miles), whereas the new ADSL2+ can transmit up
24 Mbits/s. Of course, actual performance depends how far
customers are located from the central office, since the
further away they are, the less bandwidth will be available.
DSL was first used in 1988 by Bellcore to carry a digital
signal over unused frequencies on twisted pair of cables
connecting the central phone company office with customer
premises. Their engineers discovered that digital data could
be sent along phone lines without interfering with normal
telephone services. Existing local phone companies did not
promote DSL aggressively, because is was not as profitable as
installing extra phone lines for customers who wanted
simultaneous voice phone and internet access. However, this
changed in the 1990s when cable television companies began to
offer broadband internet access, and the telephone companies
realized that most of their customers would prefer the new
faster service. Phone companies quickly began to implement
service in an effort to keep their customer base.
When the public telephone system was designed, its inventors
were not planning for future internet access; they simply
wanted to transmit voice messages. The phone system passes
audio over the lines at 300 to 3,400 Hz, the range required
human speech to be clearly intelligible. Any signal above
Hz must be filtered by a voice phone to prevent distortion of
the audio message. Frequency used to transmit telephone calls
is known as commercial bandwidth.
However, the system is capable of sending transmissions over
the existing equipment both below and above the 300 to 3,400
range. Depending on the length and quality of the phone
the upper limit can be tens of megahertz.
Data transmission channels are created as 4312.5 Hz wide
channels starting between 10 and 100 kHz, depending on system
configuration, skipping over the audio phone transmission
and then continuing up to frequencies as high as 1.1 MHz for
ADSL. The available channels are then bonded into a pair of
virtual circuits, one for downstream traffic and another for
upstream. The quality of transmission is constantly checked
DSL transceivers, and channels are added or removed from
depending on how much functional bandwidth is available on a
given line. It is cheaper to develop new technologies to
exploit existing cables than it is to install extra lines,
all forms of DSL employ highly complex digital signal
to maximize the usage of available wiring.
The subscriber end of the connection consists of a DSL modem
which converts the digital output from a computer into a
voltage signal and vice versa.
Wireless Internet is accessed over cellular towers. This is a
contentious subject because it creates more electromagnetic
pollution in an environment already overburdened with
electronic devices and their associated EM output. The cities
of Toronto, San Fransisco and Philedelphia have all adopted
ambitious plans to have publicly-funded Wireless access
throughout the municipal area; meanwhile, Ontario's Lakehead
University has banned the technology on its campus (at least
areas where cable connections are available) due to fears of
radiation-induced cancer. However, to date no definitive study
exists to prove or disprove a correlation between low-level
radiation from EMF sources and illness.
Analog modems which operate at speeds over 600 bit/s are often
also called broadband. broadband. They reach high effective
transmission rates by employing multiple channels, though the
rate of each channel is limited to 600 baud. For example, a
2400 bit/s modem is made up of four 600 baud channels. This is
fundamentally different from the operation of a baseband
transmission such as Ethernet, which fills its entire
with one type of signal.
Each individual channel taking part in a multiplexing function
is by definition narrowband, whereas the entire set of
used simultaneously for the same communication are defined as
Broadband is usually faster than narrowband, and therefor the
term is often used to refer to any fast mode of transmission,
regardless of its technical method of data transmission. This
is not accurate, because many slower devices, such as analog
modems above 600 bit/s, are technically broadband because of
the way in which it is constructed; while Ethernet, which is
much faster, is actually narrowband.
To avoid this confusion, the International Telecommunication
Union Standardization Sector has recommended that broadband be
defined as a transmission capacity that is faster than primary
rate ISDN, at 1.5 to 2 Mbit/s. Currently, devices and services
with speeds of 256 kbit/s and higher are sold as "broadband."
About The Author: J Schipper loves Broadband.