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What's The Difference Between T3, DS3 and OC3 Bandwidth?
What these acronyms mean and what they have to offer in speed and reliability.

By: John Shepler

One thing about the telecommunications industry is that there is no shortage of acronyms. How about T3, DS3 and OC3? Or T1, DS1, DS0 and DSL? You can tell these players, but you'll need the scorecard and here it is.

T3 vs DS3
In a practical sense T3 and DS3 are used interchangeably and mean the same type of service. T3 means trunk line, level 3 and DS3 means digital signal, level 3. They both refer to a 45 Mbps time division multiplexed synchronous digital voice or data service.

What is DS3?
If you want to get picky, DS3 refers to the digital signal service which could be provided on a T3 coaxial cable or a fiber optic cable. T3 is part of the T-carrier system and refers to the physical line, which is copper wire. You can get DS3 without the specific T3 wires, but not the other way around.

Compared to T1
T1 is the other popular T-carrier service. It is a 1.5 Mbps line traditionally provisioned on two pair of copper phone wires. Some providers are now using a single pair to transmit T1 using a more advance modulation scheme than was originally specified. At any rate, the digital signal level for T1 is called DS1.

What is DS0?
That's the lowest DS rate specified, which is 64 Kbps or just the right size to carry one telephone call or one dial-up modem connection. DS1 is made up by combining 24 DS0 channels. DS3 is made up of 28 DS1 channels or 672 DS0 channels.

Where Does DSL Fit?
DSL is something else entirely. It stands for Digital Subscriber Line and is a completely different format, although it typically runs at speeds similar to T1 lines. Go figure.

How About OC3?
Now where does OC3 fit into all of this? Actually OC3 is part of the optical carrier numbering scheme called SONET for Synchronous Optical NETwork. OC3 runs at 156 Mbps. So OC3 has nothing to do with T3, or does it?

The Common Thread of Timing
You see, the common theme that runs through all of these digital transmission schemes is the basic design of synchronous time division multiplexing based on the DS0 channel. Why? Because DS0 carries exactly 1 telephone call and that's the lowest common denominator for the telephone companies who designed the standards. Would you be surprised to learn that one OC3 can carry 3 DS3, 84 DS1, or 2,016 DS0 channels?

Copper or Fiber
The T1 or T3 service you order might be delivered on a dedicated copper wire circuit or it might be dropped off from a larger OC3 fiber optic cable. It depends on which hookup is most economical.

Still Confused?
Just remember T3 and DS3 refer to the same thing. OC3 is much faster, and more expensive.

Competition from Carrier Ethernet
In recent years, a newer technology called Carrier Ethernet has started replacing both T1 lines, DS3 and SONET OCx fiber. Ethernet over Copper (EoC) uses the same twisted pair wiring at T1 and is available from 2 to 45 Mbps. Ethernet over Fiber (EoF) is highly scalable in the range from 10 Mbps to 10 Gbps, with 100 Mbps and Gigabit Ethernet being very popular service levels. Ethernet is far more scalable than earlier technologies and enjoys a cost advantage as well.

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