Some people or companies use a DS1 line, while some decide on a DS3. While a DS3 Line is a great idea for big businesses and fast internet communication, DS1 is good for other reasons.  DS1 signals are often used in order to connect equipment within a facility. In this case, a low-level signal called the DSX1 is used. DSX refers to a digital signal cross connect, and it is a patch panel allowing easy interconnection. When a DS1 leaves the building, it becomes a T1 and is referred to as a span.

Then the signal is boosted to a higher level and superimposed on a DC voltage, enabling repeaters in the field to be powered from the span itself. Repeaters are placed every few thousand feet, to clean up and strengthen the signal.  DS3 signals are almost exclusively used within buildings.

This is a result of the fact that a T3 circuit can only go about 600 feet between repeaters. When a customer orders a DS3, they usually get a SONET circuit run into the building and a multiplexer mounted in a big cabinet. The DS3 is delivered in its familiar form, two coax cables with BNC connectors on the ends. 

Keep in mind that the T-carrier system traditionally uses in-band signaling or bit robbing, resulting in lower transmission rates than the E-carrier system. This resulted in many US ISDN installations only having an effective data rate of 56 kbit/s over a nominal 64 kbit/s channel, which depends on the framing format used, and almost all systems are now capable of transmitting a "clear" 64 kbit/s channel, despite the failure of providers to sell such services. 

Also, in telecommunications, T-carrier is the generic designator for any of several digitally multiplexed telecommunications carrier systems originally developed by Bell Labs and used in North The most common legacy of this whole system is the line rate designations. A "T1" now seems to mean any data circuit that runs at the original 1.544 Mbit/s line rate.

Originally, the T1 format carried 24 pulse-code modulated, time-division multiplexed speech signals each encoded in 64 kbit/s streams, leaving 8 kbit/s of framing information which facilitates the synchronization and demultiplexing at the receiver. T2 and T3 circuit channels carry multiple T1 channels multiplexed, resulting in transmission rates of up to 44.736 Mbit/s.

The 1.544 Mbit/s rate was chosen because tests done by AT&T Long Lines were done underground, and cable vault manholes were physically 6600 feet apart, and so the optimum rate was chosen empirically--the capacity was increased until the failure rate was unacceptable, then reduced. Given that the highest frequency at which voice communications occurs is at 4000 Hz, when converting analog voice to digital data, at least double that frequency for the sample rate.

Because each T1 frame contains 1 byte of voice data for each of the 24 channels, that system needs then 8000 frames per second to maintain those 24 simultaneous voice channels. Since each frame of a T1 is 193 bits in length (24 channels x 8 bits per channel + 1 control bit = 193 bits), 8000 frames per second is multiplied by 193 bits to yield a transfer rate of 1.544 Mbit/s (8000 x 193 = 1544000).

The basic unit of the T-carrier system is the DS0, which has a transmission rate of 64 kbit/s, and is commonly used for one voice circuit. 

The E-carrier system, where 'E' stands for European, is incompatible with the T-carrier and is used just about everywhere else in the world besides North America and Japan.