An Introduction to T1 and Its Role in Meeting Growing Business Needs.

Business communication networks are growing at an ever-increasing rate because of the pressing need for more and more information. To meet these demands, the ideal network must provide end-to-end digital service and support voice, data, image and control applications. One way to meet these requirements is to implement digital networkds based on 1.544-Mb/s digital facilities. Today's challenge is to understand these facilities.

These facilities were invented to solve a problem in the telephone industry. The goal was to develop a means of sending multiple voice channels over existing cable paris. The desire was to have a robust digital system with regenerators that could replace load coils in manholdes that were spaced approximately one mile apart. This manhole spacing, the type of cables in place, the need to encode voice and the technology available were the constraints that led to 1.544 Mb/s being adopted.

The first systems were made available in 1962. Since then, there have been improvements in voice encoding equipment (channel banks) that converts voice signlas into a T1 sginal, and high-level multiplexers have been developed that can transmit multiple T1 signlas. The standard multiplexer, the number of T1 lines and the transmission rate of each multiplexer are shown in the chart on the opposite page. The multiplexing levels between T1 and T4 are called the North American Hierarchy. It should be noted that in formal telephone terminology T1 is called DS1.

Today, these systems have gained wide acceptance. There are more than 120 million miles of T1 facilities in service. The network is equipped to switch individual voice channels on a T1 basis, and a variety of vendors are providing PBXs that can interface directly with a T1 facility. How T1 Can Be Provided

There are a variety of ways to provide T1.

* Twisted Pair--In the telephone industry, the primary method of transmitting T1 is over twisted pair. One pair is used for the transmit direction and one pair is used for the receive direction. A T1 signal can be transmitted for over a mile before a repeater is required to regenerate the signal. By using repeaters, a T1 signal can be transmitted for more than 100 miles.

For private communications networks, twisted pair can be used to tramsmit T1 signals around a campus or between floors of a building.

* 18 and 23-GHz Radios--These radios offer an ideal approach for transmitting one to four T1 lines across a street or a campus. They are inexpensive and reliable.

* Digital Radios--These radios are similar to the 18 and 23-GHz radios, but they operate in the common carrier and industrial microwave bands. They are usd to carry large numbers of T1 lines and are expensive.

* Coaxial Cable Modems--These modems connect T1 to a signal that can be transmitted over coaxial cables that are used to transmit CATV signals.

* Fiber-Optic Systems--These systems can transmit 1.544 Mb/s directly over fiber. Also, through the use of multiplexers, many T1 signals can be transmitted over fiber.

* Common Carriers--T1 systems can also be leased from common carriers such as AT&T, regional Bell operating companies or other telcos, Western Union, SBS and American Satellite. Both terrestrial and satellite services are aailable. The names of AT&T's T1 offerings have been changed (see Figure 2).

Fundamentally, these services offer a 1.544-Mb/s terrestrial and satellite transmission path between customer premises, from a customer premiset o a telephone company central office, for access to a Central Office Multiplexing Service Function or for connection to a high-speed switched digital service. Offers Very low Error Rate

These basic transport services offer customers large-capacity transmission capabilities on a full-time basis for multiple uses. They are designed to provide an average performance that equals or exceeds 95 percent error-free seconds of transmission measured over a 24-hour period. The service availability objective is 99.7 percent measured over one year.

The framing organization of a T1 facility includes the framing bit (one bit on each 193 bits), data bits (192 bits organized into 24 eight-bit words) and superframe (a repeating sequence of 12 frames that contain 12 synchronizing bits used for frame synchronization and for identifying voice-frequency (VF) signaling information).

Signal statistics include at least one "1" in 15 bits and at least three "1's" in 24 bits.

The VF signaling method for T1 uses time-sharing of the least-significant bit in the sixth and 12th frames.

In telephone terminology, the T1 facility is also called a DS1 facility. T1/DS1 is the standard method of interconnecting digital communications systems within the telephone industry and North America. There are more than 120 million miles of T1 transmission facilities. The line operates at a rate of 1.544 Mb/s, which allows 24 64-kb/s channels to be multiplexed. The 64-kb/s rate is compatible with generating toll-quality voice. One additional bit is used for frame synchronization. How Rate Is Derived

The 1.544 Mb/s is derived as follows:

* The 24 telephone lines are sampled 8,000 times per second.

* The voltage of these sampled analog voice channels is quantified into a number between 0 and 255, and is then transmitted as an eight-digit code.

* There aer 24 numbers generated, representing 24 phone conversations.

* Each number is eight bits of digital information in length.

* So, 24 times 8 plus an added coded digit equals 193. Therefore, 193 times 8,000 samples per second equals 1,544,000 bits per second (1.544 Mb/s).

This section discusses the characteristics of the T1 signal (Figure 3). Time is divided into 648 nanosecond clock periods, which may or may not contain a pulse. When a pulse occurs, it occupies only 50 percent of the clock period and has a polarity opposite to the polarity of the last pulse transmitted.

The signal is described as a "1.544-megabit bipolar 50-percent pulse train." There are 1,544,000 time slots per second and each slot carries one binary digit (bit) of information that has the value 1 if the pulse is present, 0 is the pulse is absent. The 50 percent refers to the pulse-duty factor, and bipolar refers to the alternating character of the pulses. Pulses Not Polarity

Note that the binary information is carried by the presence or absence of the pulses and not by their polarity. Bipolar transmission is used because the bipolar mode eliminates any DC component and it concentrates the energy content of the bit stream at one half of the bit rate (722 kHz). Nevertheless, the power spectrum of this signal covers a broad frequency range with significant energy components extending from 20 kHz to 1 MHz. The T1 bipolar signal can be transmitted approximately one mile over 24-gauge twisted pair. To send signals more than a mile requires a repeater, which is normally powered over the line and regenerates the signal; that is, it detects ones and zeros, recovers clock and generates a retimed signal.

Because of regenerative repeaters, T1 systems can operate successfully over long distances through very poor transmission facilities. Ideally, the signal delivered by the final repeater to the receiving terminal is identical to that which left the transmitting terminal, regardless of the number of intervening repeaters. In practice, line lengths is limited by the gradual accumulation o timing jitter in a long string of repeaters. Most-Widely Used

There are a variety of AT&T frame formats (DID, D2 and D3/D4 mode 3). However, only the D3/D4 Mode 3 format will be discussed, since it is by far the most-widely used. The D3/D4 Mode 3 bit stream is organized into superframes, frames and words as shown in Figure 4.

Each superframe consists of 2 frames, while each frame contains one synchronizing bit so that the receiving terminal can decode, demultiplex and distribute the incoming bit streams properly. Thus, each superframe contains synchronizing bits that form a 12-bit sequence (identified as framing bits in Figure 4).

This sequence is used for frame synchronization and to mark frame numbers 6 and 12, which contain signaling bits. This 12-bit sequence can be subdivided into two sequences: BF.sub.t (framing) adn BF.sub.s (signaling framing). BF.sub.tS are odd-numbered framing bits, and BF.sub.ss are even numbered.

Note that the BF, bit alternates (1-0-1-0) every other frame. The receiver can easily locate this unique sequence in the incoming bit stream to maintain or regain frame. Note also that frame 6 occurs when the BF, bit is a 1 preceded by three BF.sub.s zeros, and that frame 12 occurs when the BF.sub.s bit is a 0 preceded by three BF.sub.s 1s. The receiver can readily detect this sequence to identify these two frames.

Each frame consists of 24 channel words (one for each channel) plus the BF.sub.t or BF.sub.s framing bit. Eacch channel is sampled 8,000 times per second. Note that the channels are sampled in numerically increasing sequence (D3 compatible) and that each framing bit occurs after channel 24 and before channel 1.

Each channel word consists of eight bits (called B1 to B8). The eight bits of each channel word may represent PCM voice (if associated with a VF module ) or digital data (if associated with a data module). If voice, the signaling must be transmitted, as well as the channel voice samples. This is done by time sharing the least-significant bit (B8) between voice and signaling as shown in Figure 4. The B8 bits of each voice channel carry voice sample information for five frames, followed by one frame of signaling information. This is followed by five more frames of B8 voice and another frame of B8 signaling. This sequence is repeated every 12 frames. A Reliable Medium

In summary, T1 is a reliable, proven transmission medium with a well-defined interface. When equipment is compatible with this interface, it can be handled by the telephone network. At present, compatibility means a D3/D4 Mode 3 frame format, a system whose information is divided into eight-bit words.

Future articles will discuss telephone channel banks, their evolution into voice/data multiplexers and future network enhancers.