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Asymmetric DSL vs. Symmetric DSL
xDSL is a new digital service that uses high-bandwidth
to bring voice, video and data to your business. It is also used for fast,
"always on" access to the Internet. It has many useful applications for SMBs.
xDSL is the acronym for the general category of Digital Subscriber Line
service, with its many flavors. Each flavor of xDSL has its own
characteristics and special features.
Asymmetric Digital Subscriber Line (ADSL) and Symmetric Digital Subscriber
Line (SDSL) are two of the many flavors of xDSL. ADSL is sometimes referred
to as Asynchronous, or full-rate DSL. SDSL is sometimes referred to as
Synchronous DSL. Both ADSL and SDSL offer unique features that can provide
an excellent telecom solution for your business. Choosing one over the other
will depend upon the particular needs of your business. Look below to
determine which service is most suitable for your small to medium sized
business.
What is ADSL?
Asymmetric Digital Subscriber Line (ADSL) is currently the most popular
flavor of DSL. It is widely deployed and used by many home consumers and
SMBs. ADSL is called "asymmetric" because most of its two-way bandwidth is
devoted to the downstream direction that sends data to the user. Only a
small portion of bandwidth is available for upstreaming. Most Internet
users, and especially those who use graphics- or multi-media intensive Web
data, need lots of downstream bandwidth. For these applications, user
requests and responses are small and require little upstream bandwidth.
Using ADSL, up to 8 Mbps of data can be sent downstream and up to 640 Kbps
are available for upstreaming. The high downstream bandwidth means that your
telephone line will be able to bring large data files, video, audio, and 3-D
images to your computer or hooked-in TV set. And, since it supports Plain
Old Telephone Service (POTS), a small portion of the downstream bandwidth
can be devoted to voice, rather than data, so you can make phone calls
without having to use another separate line.
What is SDSL? Single-Line, or Synchronous, Digital Subscriber Line (SDSL) supports
symmetric service. A single line can carry anywhere from 160 Kbps to as much
as 1.5 Mbps in each direction equally. SDSL is often used in wideband
digital transmissions for carrying large amounts of data in both directions
within a business site.
SDSL is symmetrical (upstreaming and downstreaming). For this reason, the
maximum data rate is lower than that for ADSL. It can carry large amounts of
data in both directions. SDSL does not support POTS. It is data-intensive,
rather than being voice-intensive.
G-Lite ADSL G-Lite ADSL is a "lighter " version of full-rate ADSL. It can deliver up
to 1.5 Mbps downstream and up to 384 kbps upstream. A splitter is not
required for installation. It is intended for home consumers, small
businesses and remote offices.
How ADSL and SDSL Work
The terms "downstreaming" and "upstreaming" are key to understanding what
ADSL and SDSL can do for your business. . Downstreaming denotes downloading
( such as downloading large files), while upstreaming denotes uploading
(such as uploading large files). Both of these terms refer to the direction,
or destination, of data flow, flowing either from the Internet to your
computer or network (downstreaming), or from your computer or network to the
Internet (upstreaming). Downstream data rate refers to the speed this data
travels from the Internet (or other remote network) to your local computer
or network. Upstream data rate refers to the speed this data travels from
your local computer or network to the Internet or other remote network.
ADSL service works quite differently from SDSL.The downstreaming data rates
for ADSL are much faster than the upstreaming data rates. What this means is
that the path used for downloading files can carry more data than the path
used for upstreaming, and it can carry this data at a much faster rate.
Using ADSL technology, you may be able to download a file as fast as you can
when using a premium service such as T-1. However, since the upstreaming
rates are much slower, the service cannot accommodate uploading large files
with the same quickness. For SMBs that frequently download large files but
have minimal need for downstreaming, ADSL is an excellent telecom solution.
SDSL is an excellent solution for businesses that require fast speeds for
both upstreaming and downstreaming, but do not need voice service. It uses
both paths for data capacity and data rates for both directions are equally
fast. Speeds are less than those of ADSL, but both uploading and downloading
large files can be accomplished with the same speed.
Distance Requirements
Both ADSL and SDSL are both distance sensitive. The closer your business is
located from the CO, the faster your speed and the higher your bandwidth.
For ADSL, your business must be located within 18,000 feet from the
telephone company's central office (CO) to get service. For SDSL, you can be
located as far away as approximately 23,000 feet.
Speed and Bandwidth
Speed and bandwidth are very important considerations when choosing a
service. Speed refers to data transmission rate. Bandwidth is the capacity
to carry data. Both are measured in bits per second. Speed and bandwidth
determine the amount of data that can be transmitted, and how fast the data
can be download and uploaded. Example: A 72 MB file downloaded at 128 Kbps
will take about 10 minutes; if the same file is downloaded at 1.5 Mbps, it
will only take about 48 seconds.
ADSL's data rates are 640 Kbps upstream and up to 8 Mbps downstream,
depending upon your distance from the CO. At 9,000 feet from the CO, you can
get as much as 8 Mbps; at 18,000 feet, you can get about 1.5 Mbps.
SDSL has more business bandwidth flexibility than ADSL. You can select the
symmetric bandwidth that you desire from your service provider. The service
can be packaged in a wide range of bandwidth configurations, including 144
Kbps, 160 Kbps, 192 Kbps, 384 Kbps, 416 Kbps, 768 Kbps, 1 Mbps, and 1.5
Mbps. Remember, the greater your distance from the CO, the lower the speed
you can attain, regardless of its availability. Check with your service
provider for the maximum speed you can get.
Installation: Wiring
ADSL and SDSL are easily installed on the ordinary twisted-pair copper wire
currently behind the walls at your business. In some cases, the installation
may require a different jack, a splitter and, occasionally, new wiring. The
telephone company can complete the wiring for you, you can do it yourself,
or you can hire an independent contractor.
Internet Access
Both ADSL and SDSL services provide an "always on" connection to the
Internet. Once you install service, the connection is never interrupted.
If speedy access is very important to your business, and you don't require
the bi-directional features associated with SDSL, ADSL is an excellent
business solution. Using ADSL with its downstreaming speeds from 1.5 Mbps to
8 Mbps, data downloaded from the Internet will appear to pop-up almost
instantly on your computer.
SDSL is also an excellent choice for Internet access. The downstreaming and
upstreaming data rates of 144 Kbps to 1.5 are very good and yield good
results.
Applications
ADSL is used by home consumers and many small business owners who mainly
want to download files. It is used for carrying voice, data and video at
fast rates. You can download large data files, streaming video, audio, and
multimedia from the Internet very fast. But, generally, you cannot upload
much more than e-mail messages.
SDSL is an excellent telecom solution for many SMBs. Using SDSL, you can
upload and download large files equally fast. You can also run web and
e-mail servers. It is also a cost-effective solution for connecting a small
LAN to a LAN or a small LAN to a WAN.
Required Equipment
For both ADSL and SDSL, a POTS splitter is required. G-Lite ADSL does not
require a splitter. A DSL modem and an Ethernet card are also required for
installation.
Unlike your analog telephone line, your xDSL line does not draw its current
from your local service provider central office. You'll need an
Uninterruptible Power Supply (UPS) to ensure continuous service in the event
of a power failure.
xDSL use requires that your computer have a Peripheral Component
Interconnect (PCI) slot for installing the PCI. PCI adaptor cards may also
be necessary to combine an Ethernet interface to your PC with DSL modem
functions.
Cost and Value Considerations
ADSL and SDSL are very affordable and cost-effective services based upon a
flat rate. Since cost is not based upon usage, you can use these services
any time for as long as you want without incurring any usage charges. They
are easily and inexpensively installed.
ADSL is generally best for SMBs where downloading files is most important.
This is because of its larger data rate downstream. This means that the
typical user will be able to download faster than they can send data.
SDSL is intended for businesses. The symmetrical features provide
versatility and some options that are not available with ADSL. It also
provides more flexibility than ADSL.
