"How a T1 Line Works?"

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How a T1 line works is the most common question that people ask when they came to know about the enormous speed a T1 line offers. T1 is a digital service made available to the public in 1983. Prior to 1983, T1 was only used internally by the phone companies to inter-connect Central Offices (CO's).

T1 operates at a base band bit rate of 1.544 Mbps. Only 1.536 Mbps is accessible for payload data and the rest 8 Kbps is reserved for framing. The T1 is comprised of 24 DSOs, 64 kbps each. The t1 is transmitted as a time domain multiplexed group of DSOs. It is sometimes referred to as channels or timeslots.

T1 operates over special low capacitance shielded twisted pair cabling. In some cases unshielded twisted pair cabling is also used with all precautions to avoid crosstalk.

In lieu of high frequency of T1, repeaters are placed along the line at about every 6000 feet. These repeaters compensate for line loss and signal distortion.

A single T1 line comprises of 24 different voice channels or lines. Every T1 data stream is divided into segments called frames. Each frame has a framing bit. Hence the total capacity of one framing bit per frame plus twenty four channels in each frame and each have eight bit bytes. That makes it one hundred and ninety three bits per frame. The signal itself is a return to zero, bipolar pulse stream. The pulse alternate in polarity.

The 8 kbps framing format has two common standards available today. The newest standard is called extended super frame (ESF) and the earlier one is known as D4. Sometimes D4 is referred to as Super Frame (SF). In ESF there is a facility of additional error information (CRC6).

In addition to this it provides a 4 Kbps channel called, Facility Data Link, outside the 1.536 Mbps payload capacity. Due to FDL, phone companies are able to non- intrusively interrogate the customers terminating equipment for error statistics and line performance information to aid in predicting and preventing line outages and maintain the quality of service. On the other hand, D4 provides for only framing synchronization.

A T1 line between two locations is comprised of two twisted pair lines, one is for transmitting and the other is for receiving. T1 line runs from the customer site to the central office. These pairs are carefully chosen pairs within multi-pair cables already installed, repeaters are placed along the cable at predefined distances. These are powered by simplex current between the two pairs.

When the T1 enters the central office it is multiplexed up into a T3 or higher for transport to the to the remote location On the other hand serving central office take the T1 out of the higher bandwidth service and routs it into a twisted pair cable to the customer site. That is how a T1 line works.