Digital signal 1 (DS1, also known as a T1) is a T-carrier signaling scheme devised by Bell Labs. It is a widely used standard in telecommunications to transmit voice and data between devices. A DS1 circuit is made up of twenty-four 8-bit channels (a.k.a. timeslots and DS0's), each channel being a 64 kbit/s DS0 multiplexed pseudo-circuit. A DS1 is also a full-duplex circuit, meaning you can (in theory) send 1.536 Mbit/s and receive 1.536 Mbit/s simultaneously. A total of 1.536 Mbit/s of bandwidth is achieved by sampling each of the twenty-four 8-bit DS0's 8000 times per second. Before the jump in Internet traffic in the mid 1990's, DS1's were found almost exclusively in the telephone company central office as a means to transport voice traffic between locations. DS1's have been and still are the primary way cellular phone carriers connect their central office switches (MSC's) to the cell sites deployed throughout a city.
Today, companies often use an entire DS1 for Internet traffic, giving you 1.544 million bits per second of connectivity (actually, it's 1.536 Mbit/s; the other 8 kbit/s goes to framing overhead.) However, if you so desire, you can order the DS1 as a channelized circuit and reserve channels for non-data (i.e., voice) traffic.
DS1 signals are frequently used to connect equipment within a facility. In this case, a low-level signal (6 volts peak-to-peak differential) called the DSX1 is used. DSX refers to a digital signal crossconnect, and it is essentially a patch panel allowing easy interconnection. When a DS1 leaves the building, it becomes a T1 and is referred to as a span. 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.
http://www.data-connect.com/DS1_DS3.htm
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