What's The Difference Between
DSL and T1?
DSL and T1 lines are both digital
services, but they are intended for different applications.
By: John Shepler
Digital is digital, right? If so, then
there really isn't any big difference between DSL and T1 lines,
Actually there is quite a bit of difference,
but there are also similarities. You can get always-on Internet
access on both a DSL line and a T1 line. T1 lines download at
1.5 Mbps. DSL may also be that fast. You can use either with
your VoIP telephone adaptor. So why would you spend ten times
as much for a T1 line as you would to get DSL?
The answer is that there's digital and
then there's digital. Digital line services come in many flavors
and the each have their advantages and disadvantages.
Take DSL for instance. When you think about
getting a DSL line for under $50 a month, what you are thinking
of is residential DSL service that is technically called ADSL
or asymmetrical Digital Subscriber Line. The asymmetrical part
means that upload and download speeds are different. The reasoning
is that most Internet users need fast download speeds for web
browsing, but can live with lower upload speeds for sending email,
typing in web addresses, filling out forms, etc. You might get
1.5 Mbps download but 128, 256 or 384 Kbps upload speed. There
is a symmetrical type of DSL service with the same upload and
download speed, but that is usually sold to businesses at a higher
T1 is based on a different technology,
although both T1 and DSL are digital lines. T1 is a synchronous
time division multiplexed system, meaning that it is not only
on all the time, but it is sending something all the time and
expecting to receive something all the time. That something might
be no more than framing bits to keep both ends of the connection
synchronized to define the individual time slots. That is not
really important for Internet access, but does allow T1 lines
to carry standard switched telephone calls as well as VoIP phone.
T1 can carry 24 telephone lines to support a business PBX system.
Each phone call occupies a well defined time slot in the T1 transmission.
T1 is also symmetrical. That means that
the download speed is 1.5 Mbps and the upload speed is 1.5 Mbps.
That synchronized line speed never varies and it doesn't matter
how far you are from the phone company office. It's always 1.5
Mbps. DSL on the other hand, uses a Discrete Multitone Modulation
(DMT) technology that adapts itself to the condition of the telephone
line it shares. The speed you get depends on how far away you
are from the equipment providing the DSL service, and the amount
of noise and interference on the line. The closer you are, the
higher your line speed. If you are more than a few miles away,
you may not be able to get DSL at all. Just about everyone can
get T1 service installed, but the cost goes up as you get more
and more remote.
The other difference between DSL and T1
is not technical. It is in the commitment to availability of
the service. T1 lines come with a Service Level Agreement or
SLA that provides compensation for outages. DSL is a "best
effort" service. That doesn't mean DSL is inherently unreliable,
but if there is an outage then T1 with its SLA gets the highest
level of attention.
So, why would you buy one service over
the other? For residential use, DSL provided on your phone line
or cable Internet service provided by the local Cable TV company
give you very good Internet broadband service at a reasonable
price. Businesses might choose business grade DSL if their need
is fairly light Internet access or running a free WiFi hotspot.
But if full-time Internet access is key
to your business or if you plan on running VoIP telephones or
standard switched telephones using a PBX system, T1 is really
what you need. T1 lines can also be set up as dedicated point
to point connections between business locations. Granted, T1
prices are 8X to 10X what you'll pay for DSL. But that's still
reasonable as a dependable digital lifeline for most medium and
even small businesses.
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