What's The Difference Between
DS1 and T1?
Understand the fine technical
differences so you know what digital service to order.
By: John Shepler
T1 lines are becoming ubiquitous for business
digital connections. T1 phone lines replace up to 24 standard
telephone lines in one digital trunk line. T1 Internet service
gives you 1.5 Mbps bandwidth both upload and download. You can
even combine the two with T1 Integrated line service that allocates
part of your T1 line for telephone voice circuits and part for
Internet data. So, then, what should you order? Is it a T1 line
or DS1 service?
DS1? Where did that come from? Is DS1 a
better or worse choice than T1? What's the difference between
DS1 and T1 anyway?
Relax. In colloquial terms, DS1 and T1
mean the same thing. There is a very fine point of technical
difference between the two. You can get DS1 service without a
T1 line but if you have a T1 line, you've got DS1.
DS1 is a digital signal level that is part
of a standardized hierarchy or building blocks of digital services
for TDM (Time Division Multiplexing) digital lines. A DS1 is
a DS1 regardless of whether it is delivered on a T1 line, a microwave
relay, a T3 line or a fiber optic carrier.
Actually it all starts at the bottom with
DS0. DS0 is the bandwidth you need to transmit one digitized
telephone call using the legacy telephone standard for PCM or
Pulse Code Modulation. It's an 8 bit channel transmitted at 8
Kbps or a total of 64 Kbps. That's the gold standard for "toll
quality" voice. That same channel can also be used to carry
64 Kbps of data rather than a telephone call. The data bits might
represent a dial-up modem connection, a portion of a broadband
Internet service, a portion of an Ethernet signal, a small portion
of a video transmission, or signaling and Caller ID information
to support a call center.
A T1 line multiplexes or aggregates 24
of these DS0 channels into a DS1 with a total bandwidth of 1.536
Mbps. But a T1 line runs at 1.544 Mbps. What's missing? The extra
8 Kbps represents the framing bit that T1 adds to each group
of 24 DS0 channels so that the CSU or channel service unit at
the other end of the line knows where to demultiplex the frame
of 24 DS0s. With synchronous lines such as T1, framing bits are
needed to keep the sending and receiving ends tightly synchronized.
Remember that most people will associate
DS1 with T1 and that's normally how you would get DS1 service.
If you needed a lot more bandwidth, you might order DS3 service
that would be delivered on a T3 line. DS3 is the equivalent of
28 DS1s. A DS1 service might be separated from a higher speed
service using an add/drop multiplexer. For instance, an OC3 fiber
optic carrier can deliver 3 DS3 services or 84 DS1 services or
2,016 DS0 channels. Because they are all constructed according
to the rules of the same digital signal hierarchy, you can multiplex
and demultiplex these signals at will.
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