The most common type of subscriber telephone line is the analog loop start line. This service makes use of a single pair of twisted copper wires to provide one telephone channel. This type of phone line allows analog voice and dtmf (dual-tone modulation frequency) signals to travel between the telephone and the telephone company`s central office. When multiple lines are required, it often makes sense to use a T1 subscriber line instead of multiple analog loop start lines. With T1 you get up to 24 virtual phone lines or "channels" on a single physical line made up of two pairs of twisted copper wires. This arrangement is possible because T1 lines use digital signals instead of analog signals, which are used by the common loop start line. Digital signals are multiplexed over time slots, allowing multiple channels to share the same physical line.
T1 is an AT&T term for a digital carrier facility used to transmit a DS-1 formatted digital signal at 1.554 megabits per second. T1 transmission uses a bipolar return-to-zero alternate mark inversion line coding scheme to keep the DC carrier component from saturating the line. Although some in our industry consider T1 signaling obsolete, a lot of equipment operates at the "T1 rate," and such signals are either combined for transmission via faster circuits or demultiplexed into 64-kilobit-per-second circuits for distribution to individual subscribers.
T1 signals can be transported on unshielded twisted-pair telephone lines. The transmitted signal consists of peripheral interchange programs, or pips, of a few hundred nanoseconds in width--each inverted with respect to the one preceding. At the sending end, the signal is 1 volt, and as received, greater than 0.01V, requiring repeaters about every 6000 feet.
The information is contained in the timing of the signals, not the polarity. When a long sequence of bits in the transmitted information would cause no pip to be sent, "bit stuffing" is used so the receiving apparatus will not lose track of the sending clock.
A T1 circuit requires two twisted-pair lines, one for each direction. Some newer equipment uses the two lines at half the T1 rate and in full-duplex mode; the sent and received signals are separated at each end by components collectively called a "hybrid." Although this technique requires more-sophisticated equipment and lowers the line length, an advantage is that half the sent and half the received information is mixed on any one line, making low-tech wiretaps less of a threat.
There are three clear advantages to using a T1 line over analog loop start or DID (direct inward dialing) lines: cost, inbound routing flexibility, and channel density. T1 becomes more cost-effective than multiple analog loop start lines at a certain number of lines. The specific number of lines where T1 becomes less costly depends upon the volume of calls you make over the lines and how the T1 is tariffed (priced) by your phone company or long-distance provider. A T1 line can support fewer than 24 channels, which means you don`t pay for all 24 channels if you need fewer. This is referred to as "fractional T1." Your local phone company or your long-distance provider can help you determine the cost-effectiveness of T1 for your requirements. Make sure the phone company details all the options and costs, including installation and special equipment such as a channel service unit, which most phone companies require that you place between the T1 network and your system.
Without question, the most important reason users move to T1 is the cost savings. Case study after case study shows typical paybacks of fewer than six months, cost reductions of 25% to 35%, and up to $300,000 of savings in five years. Currently, the threshold for changing to T1 lines could be any of the following:
- $5000 per month in long-distance billing from one location
- eight or more leased-line modem connections
- four or more dedicated or switched 56-kilobit-per- second connections
- four to eight off-premises extension connections
- bandwidth requirements above 56 kbits/sec and below a full T1 for local area network or video services.
The bottom line? If you meet any of these criteria and you are not using T1 services today, then you are simply losing money.
Donna Ballast is a communications analyst at the University of Texas at Austin and a bicsi reg-istered communications distribution designer (rcdd). Questions can be sent to her at Cabling Installation & Maintenance or at PO Drawer 7580, the University
of Texas, Austin, TX 78713; tel: (512) 471-0112, fax: (512) 471-8883, e-mail: [email protected].