UNITED STATES DEPARTMENT OF AGRICULTURE

**Rural Electrification Administration**

**BULLETIN 1751H-403**

**SUBJECT: [Digital Transmission Fundamentals]{.underline}**

**TO:** Telephone Borrowers

REA Telephone Staff

**EXPIRATION DATE:** Three years from the effective date.

**EFFECTIVE DATE:** Date of Approval

**OFFICE OF PRIMARY INTEREST:** Transmission Branch,

Telecommunications Staff Division

**PREVIOUS INSTRUCTIONS:** None - this bulletin replaces TE&CM 950 and
TE&CM 951.

**FILING INSTRUCTIONS:** Discard the following Telecommunications
Engineering & Construction Manual (TE&CMs) Sections:

> 1\) TE&CM 950 - Digital Transmission Systems
>
> 2\) TE&CM 951 - Digital Transmission Terminology

Replace the above TE&CM Sections with this bulletin.

**PURPOSE:** To provide basic information on digital transmission
theory, fundamentals and equipment to REA borrowers, consulting
engineers and other interested parties. This bulletin assumes little or
no prior knowledge of digital systems and is tutorial in nature. A
glossary of terms related to digital transmission appears in Section 9,
Digital Transmission Terminology.

\_\_\_\_\_\_\_[George E. Pratt]{.underline}\_\_\_\_\_\_\_\_
\_\_\_\_[07/02/91]{.underline}\_\_\_

Acting Administrator Date

**DIGITAL TRANSMISSION FUNDAMENTALS**

**TABLE OF CONTENTS**

1\. Introduction & History of Digital Communications . . . . . . . . 4

2\. Analog to Digital Signal Conversion. . . . . . . . . . . . . . . 5

3\. Digital Hierarchy. . . . . . . . . . . . . . . . . . . . . . . . 8

4\. Digital Framing. . . . . . . . . . . . . . . . . . . . . . . . . 9

5\. Signaling. . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

6\. Extended Superframe Format . . . . . . . . . . . . . . . . . . . 12

7\. Line Coding & Transmission Requirements. . . . . . . . . . . . . 13

8\. Overview of Digital Transmission Equipment . . . . . . . . . . . 15

8.1 Purpose . . . . . . . . . . . . . . . . . . . . . . . . . . 15

8.2 T-Carrier . . . . . . . . . . . . . . . . . . . . . . . . . 15

8.3 Lightwave . . . . . . . . . . . . . . . . . . . . . . . . . 18

8.4 Microwave Radio . . . . . . . . . . . . . . . . . . . . . . 19

9\. Digital Transmission Terminology . . . . . . . . . . . . . . . . 20

**FIGURES & TABLES**

Figure 1 - Pulse Amplitude Sampling of an Analog Waveform . . . . . 5

Figure 2 - µ255 Encoding Scale. . . . . . . . . . . . . . . . . . . 7

Figure 3 - Eight Bit Digital Word . . . . . . . . . . . . . . . . . 7

Figure 4 - Quantizing Error . . . . . . . . . . . . . . . . . . . . 8

Figure 5 - D4 Framing . . . . . . . . . . . . . . . . . . . . . . . 10

Figure 6 - D4 Superframe. . . . . . . . . . . . . . . . . . . . . . 10

Figure 7 - Robbed Bit Signaling . . . . . . . . . . . . . . . . . . 11

Figure 8 - ESF Format . . . . . . . . . . . . . . . . . . . . . . . 12

Figure 9 - Alternate Mark Inversion . . . . . . . . . . . . . . . . 13

Figure 10 - Spectral Energy of Bipolar vs. Unipolar Pulse Stream . . 14

Figure 11 - Overall Configuration of a T1 Span Line. . . . . . . . . 16

Figure 12 - Span Line Repeater . . . . . . . . . . . . . . . . . . . 16

Figure 13 - Central Office Span Line Equipment . . . . . . . . . . . 17

Figure 14 - Typical Lightwave System . . . . . . . . . . . . . . . . 18

Figure 15 - Typical Microwave Radio System . . . . . . . . . . . . . 19

Table 1 - Digital Hierarchy. . . . . . . . . . . . . . . . . . . . 9

**INDEX:**

Digital Transmission

Digital Carrier

Transmission, Digital

**ABBREVIATIONS & DEFINITIONS:**

The acronyms and abbreviations used in this document are explained
below. Definitions for these and other terms applicable to digital
transmission in general are in Section 9, Digital Transmission
Terminology.

**A/D**: Analog to Digital

**AMI**: Alternate Mark Inversion

**APS**: Automatic Protection Switch

**B8ZS**: Bipolar with 8 Zero Substitution

**BCD**: Binary Coded Decimal

**BPV**: Bipolar Violation

**b/s**: bits per second

**CO:** Central Office

**CPE**: Customer Premises Equipment

**D/A**: Digital to Analog

**DS0**: Digital Signal Level 0 (64 kb/s)

**DS1**: Digital Signal Level 1 (1.544 Mb/s)

**DS2**: Digital Signal Level 2 (6.312 Mb/s)

**DS3**: Digital Signal Level 3 (44.736 Mb/s)

**DSX**: Digital Signal Cross-connect

**ESF**: Extended Superframe Format

**F-Bit**: Framing Bit

**FEXT**: Far End Crosstalk

**Gb**: Gigabit (109 bits)

**Gb/s**: Gigabits per second

**Hz:** Hertz (cycles per second)

**IXC**: Interexchange Carrier

**kb**: kilobit (103 bits)

**kb/s**: kilobits per second

**kHz**: kilohertz

**km:** kilometer

**LEC**: Local Exchange Carrier

**LSB**: Least Significant Bit

**MHz**: Megahertz (106 Hz)

**modem**: modulator/demodulator

**MSB**: Most Significant Bit

**mux**: multiplexer

**NEXT**: Near End Crosstalk

**PAM**: Pulse Amplitude Modulation

**PCM**: Pulse Coded Modulation

**RF**: Radio Frequency

**S/N**: Signal to Noise Ratio

**SF**: Superframe

**SS7**: Signaling System No. 7

**T1**: Digital 4-wire metallic transmission at DS1

**TDM**: Time Division Multiplexing

**µsec**: microsecond (10-6 seconds)

**VF**: Voice Frequency

**ZBTSI**: Zero Byte Time Slot Interchange

**1. INTRODUCTION & HISTORY OF DIGITAL COMMUNICATIONS**

**1.1** The theory of transmitting voice signals via digital
transmission emerged in the late 1930s. Alec Reeves of the International
Telegraph and Telephone Corporation (ITT) in Europe developed this
principle in 1937 and termed it pulse code modulation, or PCM carrier.
However, appropriate technology did not then exist with the capability
that Reeves\' theory required.

**1.2** The first major technological breakthrough that paved the way
for actual PCM transmission was the development of the transistor by
researchers Bardeen, Brattain and Shockley of the Bell Telephone
Laboratories in 1947. The transistor permitted the type of high-speed
switching (without the excessive heat and the large size of vacuum
tubes) that Reeves\' theory required in order to convert and transmit
digitized voice signals accurately. Subsequent efforts by Jack Kilby of
Texas Instruments and Robert Noyce of Fairchild in the late 1950s
resulted in the later development of integrated circuits which provided
a more optimal packaging for the numerous transistors and other
components required for digital signal processing.

**1.3** In 1956 Bell Laboratories began the development of the first
commercial PCM system for telecommunications purposes. It was not until
1962 that a commercial digital transmission system was deployed to carry
actual telephone traffic at the Bell Telephone Company of Ohio in Akron.
Additional PCM development continued into the 1960s and 1970s. PCM
carrier was deployed initially for toll purposes but eventually saw use
in the exchange plant as well. The same PCM principles were later used
as the basis for digital central office (CO) switches.

**1.4** REA borrowers significantly began using digital PCM carrier
systems for interoffice trunk circuits in the late 1960s when this
equipment was placed on the *REA List of Materials*. Systems designed
for loop exchange use, presently known as digital loop carrier, became
available in the early 1970s and are seeing increasing use as borrowers
seek to improve quality, lower costs, shorten loop lengths and provision
plant capable of providing modern services.

**1.5** One very significant benefit arose from the use of PCM carrier
transmission. This was improved audio quality. Particularly for toll
connections, traditional analog voice frequency (VF) circuits produced
increasing noise as the distance increased, sometimes requiring callers
to shout or talk at high sound levels in order to be heard at the far
end. Digital transmission quality is generally insensitive to distance
in that the signals are not merely amplified along with ever increasing
circuit noise, but are regenerated at regular intervals. Other benefits
of digital over analog transmission include increased immunity to
interference, simpler electronic circuit design and lower maintenance
costs. All of these characteristic make digital transmission preferable
to analog transmission.

**2. ANALOG TO DIGITAL SIGNAL CONVERSION**

**2.1** The principals of PCM carrier largely revolve around the
conversion of the analog signals into digital signals, or the A/D
conversion. The process also operates in the reverse direction, known as
D/A, representing the conversion of the digital signals back into analog
signals.

**2.2** The A/D conversion for telecommunications purposes is performed
by channel banks. The Western Electric Company, following the research
performed by Bell Labs, built the first channel banks in the U.S. around
1960. They were designated D1. Subsequent versions using improved
technology were designated D2, D3, D4 and D5. The latter two are
considered current technology. (Additional detail on channel banks is
located in REA Bulletin 1751H-405.)

**2.3** The A/D conversion consists of two separate steps: 1) the
sampling or measurement of the analog waveform level at periodic
intervals and 2) the conversion of these measurements into binary
numbers. In this manner the analog signal can be transmitted in digital
format and then converted back into an analog signal. Earlier digital
equipment performed these functions in two discrete steps; more modern
equipment performs the A/D and D/A conversions in a single operation.

**2.4** A graphical depiction of how the analog waveform is sampled and
converted into repetitive pulses is shown in Figure 1. This stage of the
A/D process is known as pulse amplitude modulation or PAM. Pulses of
varying magnitudes spaced at regular intervals represent the analog
signal. Both the sampling frequency and the height of the individual
pulses contain the characteristics of the analog waveform.

**FIGURE 1**

**PULSE AMPLITUDE SAMPLING OF AN ANALOG WAVEFORM**

> ![](media/image1.emf){width="5.459027777777778in"
> height="3.2215277777777778in"}

**2.5** The rate or frequency at which an analog signal is sampled (or,
how many PAM pulses are used) is critical to how much information is
retained from the analog signal in the A/D conversion process. A theorem
developed in the 1920s by H. Nyquist, a mathematician, shows that an
analog signal can be faithfully reconstructed if the analog signal is
sampled at a rate equal to or greater than two times the highest
frequency contained in the analog signal. This is known as the Nyquist
Theorem. For example, in order to represent a nominal 4 kHz bandwidth of
a telecommunications voice circuit accurately, the waveform must be
sampled at a rate of 8 kHz, or 8,000 times per second.

**2.6** Once the analog waveform has been sampled, the magnitude or
height of each pulse is determined; these are then converted from a
decimal-encoded measurement to a binary-encoded number. In other words,
the PAM-encoded voice signal is converted to a PCM-encoded voice signal.
It is exactly at this stage in the process where the actual A/D
conversion takes place, when the various pulse magnitudes are converted
from a Base 10 number to a Base 2 number.

**2.7** The height of each PAM pulse is electronically measured against
a form of \"yardstick\" with an unconventional scale. The yardstick has
a nonlinear scale, where the spaces between divisions varies depending
upon their location along the yardstick. This is in contrast to a
conventional ruler where the distance between inches is uniform,
regardless of whether one is examining inch one or inch twelve. The
reason for a nonlinear scale is that it affords better analog signal
representation while minimizing the amount of data that has to be
collected. Essentially, it results in less noise and distortion than use
of a linear encoding scale at the same information rate.

**2.8** The yardstick used to measure PAM pulses in North America and
Japan is known as µ255, or the Mu-law. This is because it has a total of
255 gradations or divisions along its scale for representing the PAM
pulses.[^1] To represent the positive portion of an analog waveform, 8
*segments* extend upward from the zero line. Each segment has within it
finer divisions of 16 equally spaced *steps*. Likewise, to represent the
negative portions of a waveform, there are an equal number of segments
and steps in the negative direction downward from the zero line. Thus, a
total of 16 segments exist. The segments and steps can be viewed as
somewhat comparable to the divisions of feet and inches, respectively,
on a yardstick *except* that the µ255 scale is nonlinear. Figure 2
depicts the µ255 scale.

**2.9** As Figure 2 shows, the µ255 scale has finer-spaced divisions at
the lower signal levels where most of the analog signal, and
consequently the PAM samples, reside. The coarser-spaced divisions are
at the higher signal levels (both positive and negative). The nonlinear
scale results in a compression of the signal which provides two
advantages: 1) it requires less bandwidth, and 2) it improves the
signal-to-noise ratio (S/N).

**FIGURE 2**

**µ255 ENCODING SCALE**

> ![](media/image2.emf){width="4.913194444444445in"
> height="3.064583333333333in"}

**2.10** The magnitude of each PAM pulse measured according to the
Mu-law is stored in binary form using an 8-bit digital word. As
mentioned, this is where the actual conversion occurs of the analog
signal into a digital format. The 8-bit word is shown in Figure 3.

**FIGURE 3**

**EIGHT BIT DIGITAL WORD**

**BIT #1 BIT #2 BIT #3 BIT #4 BIT #5 BIT #6 BIT #7 BIT #8**

POSITIVE\
OR THREE BIT [SEGMENT]{.underline} NUMBER FOUR BIT [STEP]{.underline}
NUMBER\
NEGATIVE

**2.11** The 8-bit word has three basic components. The first is located
at bit position #1, which indicates whether the PAM pulse (as well as
the analog signal) is positive or negative, represented by a 1 or 0,
respectively. The second component includes bit positions #2 through #4
to indicate the segment number (out of eight possibilities) where the
pulse lies. The remaining four bit positions specify the pulse magnitude
in further detail; they indicate the particular step out of 16 possible
steps within the noted segment.

**2.12** Given that no measurement system is entirely perfect, some
error is introduced into the digital representation of the analog
signal. This error is referred to as quantizing error and can result in
audible distortion, if excessive. In most instances the error is
negligible and not noticeable to the human ear. Quantizing errors occur
when the magnitude of the PAM pulse is imprecisely converted to a
numeric value. For example, should the height of the pulse lie between
two step positions on the µ255 scale, the signal will be read to the
nearest step position. The quantizing error is the difference between
the pulse\'s actual magnitude and its recorded magnitude. Figure 4 shows
several pulses located between step levels within a single segment, and
the quantizing errors that result from the difference between the actual
pulse heights and their recorded values.

**FIGURE 4**

**QUANTIZING ERROR**

> ![](media/image3.emf){width="5.895833333333333in"
> height="4.007638888888889in"}

**3. DIGITAL HIERARCHY**

**3.1** The Nyquist Theorem discussed in Section 2 states that a 4 kHz
analog VF signal needs to be sampled at a rate of 8,000 times per second
in order to be accurately represented. Given that each sample is
represented in binary form by an 8-bit word, this results in a bit rate
of 64,000 bits per second, or 64 kb/s, for each voice channel. A single
digitized voice channel at 64 kb/s is known as Digital Signal Level 0,
or DS0.

**3.2** Because digital or PCM transmission equipment evolved in North
America on the basis of a unit capacity of 24 voice channels or 24-DS0s,
higher order digital signals were constructed from that basic building
block.[^2] The 24-DS0s at 64 kb/s each when combined, or multiplexed,
gives a bit rate of 1,536,000 b/s, or 1.536 Mb/s. An additional 8,000
bits per second were required for organizational and timing purposes.
The result is that the multiplexed transmission of 24-DS0 channels
required an overall bit rate of 1,544,000 b/s, or 1.544 Mb/s. This bit
rate was denoted Digital Signal Level 1, or DS1. It is also referred to
as T1 when transmitted over metallic facilities. DS1s can be further
combined with other DS1s to give higher bit rate digital signals. The
structure of DS0, DS1 and higher order digital signals is known as the
digital hierarchy. It is represented below in Table 1.

**TABLE 1**

**DIGITAL HIERARCHY**

> **DIGITAL SIGNAL DIGITAL BIT RATE EQUIVALENT \# OF LEVEL VF (DS0)
> CIRCUITS**
>
> DS0 64 kb/s 1
>
> DS1 1.544 Mb/s 24
>
> DS2 6.312 Mb/s 96
>
> DS3 44.736 Mb/s 672
>
> DS4 274.17 Mb/s 4,032

**3.3** Multiplexers (sometimes called muxes) combine digital signals at
a lower bit rate from the digital hierarchy into digital signals at a
higher bit rate in the digital hierarchy. They are typically designated
as Mxy, where the x indicates the digital signal\'s incoming bit rate
and the y designates the digital signal\'s outgoing, or higher, bit
rate. For example, a commonly used multiplexer is designated M13. It
combines 28-DS[1]{.underline} signals into one DS[3]{.underline}
bitstream. Channel banks, in addition to performing A/D and D/A
conversions, also function as multiplexers, but generally only from the
DS0 rate to the DS1 rate.

**4. DIGITAL FRAMING**

**4.1** As Section 3 explains, multiple 64 kb/s voice channels are
multiplexed and transmitted at DS1 or higher bit rates. To facilitate
the conversion of each DS0 channel back into its analog form, the
multiplexing process must be carefully structured and synchronized so
that the information transmitted as channel 1 out of 24 channels will be
received as channel 1 and not channel 5 or channel 18, or even portions
of other channels. This organizational structure for the bitstream is
known as framing.

**4.2** The basic form of standardized framing is known as D4 framing,
after the current technology of channel banks. Each frame contains 24
time slots, or 24-64 kb/s DS0 channels of 8-bit words. In order to
precisely identify the beginning of each frame, it is marked just after
time slot #1 with a framing or F-bit in the 193rd bit position.
Consequently, each frame consists of 193 bits (192 bits from 24 8-bit
words plus the F-bit). Figure 5 illustrates this organization. Because
each voice channel is sampled 8,000 times per second, the frame with 193
bits is transmitted in 1/8000 second, or 125 µsec (125 x 10-6 seconds).

**FIGURE 5**

**D4 FRAMING**

> ![](media/image4.emf){width="4.913194444444445in"
> height="3.1430555555555557in"}

**4.3** The basic D4 frame is further organized into what are known as
D4 superframes. A single superframe contains 12 standard frames, each
frame followed by its own F-bit. The superframe contains 2,316 bits
which result from 12 frames of 193 bits each. A D4 superframe is shown
in Figure 6.

**FIGURE 6**

**D4 SUPERFRAME**

> ![](media/image5.emf){width="4.913194444444445in"
> height="2.671527777777778in"}

**5. SIGNALING**

**5.1** Although a digital voice channel provides a complete
communications path, it does not offer a continuous metallic path from
the CO to the customer premises equipment (CPE) or telephone.
Consequently, some other means to provide signaling information is
required. Signaling information includes initiating ringing current for
the CPE and detecting off-hook/on-hook status for both dialing and busy
identification. To provide this information within each individual
digital voice channel, a small number of signaling bits are employed.
These bits, in effect, are \"robbed\" from those bits used to represent
the analog voice signal in digital form. Thus, this concept is known as
robbed bit signaling.

**5.2** Because the robbed bits are taken from the digital
representation of the analog signal in each channel, an effort was made
to minimize any degradation in voice quality. To do so, the bits with
the least amount of digital information are used. These bits are known
as the least significant bits, or LSB. The LSB is bit position #8 of the
8-bit word shown in Figure 3. Only two bits per channel per superframe
are used; one in the 6th frame and one in the 12th frame. This amounts
to 48 bits per 2,316 bit superframe. The 6th frame bit is called the
A-bit and the 12th frame bit is called the B-bit. These two bits (with
binary numbering) permit four different signaling states to be
represented. Figure 7 is a matrix illustration of the robbed bits.

**FIGURE 7**

**ROBBED BIT SIGNALING**

> ![](media/image6.emf){width="5.895833333333333in"
> height="3.8506944444444446in"}

**6. EXTENDED SUPERFRAME FORMAT**

**6.1** The superframe format (SF) that was developed for organizing a
DS1 bitstream was further improved in the early 1980s by AT&T in order
to provide additional bits for signaling and communication links.
Because channel bank technology at that time was more mature and
sophisticated, proper synchronization was possible with fewer framing
bits than were required with SF framing at the time of its development.
This freed up the unneeded framing bits for other purposes.

**6.2** The new DS1 framing format became known as the Extended
Superframe Format, or ESF. A single ESF frame contains 24 standard
frames, as compared with 12 frames for SF. This improvement results in
only 2,000 b/s needed for synchronization, whereas SF requires 8,000
b/s, giving a savings of 6,000 b/s for new or special features. These
features include performance monitoring, control and data links
transmitted outside the 24-DS0 channels. Figure 8 diagrams an ESF frame.

**FIGURE 8**

**ESF FORMAT**

> ![](media/image7.emf){width="4.913194444444445in"
> height="3.457638888888889in"}

**6.3** Although ESF was developed by one particular vendor, its
benefits have been widely recognized. Consequently, ESF framing has been
adopted and incorporated in the equipment of various manufacturers of
channel banks and customer T1 equipment. Many interexchange carriers
(IXCs) and local exchange carriers (LECs) have also adopted this format
for network interfaces. Two somewhat different versions of ESF have
evolved, however. The differences pertain to how performance monitoring
data is made available over the 6 kb/s communications path.[^3]

**7. LINE CODING & TRANSMISSION REQUIREMENTS**

**7.1** Line coding is the translation of the digital information in
binary format into a specific code, while retaining the PCM information
content, in order to optimize its transmission over a particular medium,
such as copper wire, optical fiber or radio waves. Because digital
telecommunications signals were originally designed to be transported
over metallic cable facilities, a line code was chosen that was
appropriate to that medium. The code selected was alternate mark
inversion, or AMI.

**7.2** AMI is a relatively simple code. It merely requires that the
ones pulses alternate in polarity from positive to negative. Zeros are
unaffected. This process also can be described as the conversion of
unipolar pulses to bipolar pulses. Figure 9 shows this conversion.

**FIGURE 9**

**ALTERNATE MARK INVERSION**

> ![](media/image8.emf){width="4.913194444444445in"
> height="3.2215277777777778in"}

**7.3** There are several reasons the AMI code is preferable to a
standard unipolar signal for transmission over metallic plant. First of
all, the conversion to bipolar causes the signal\'s spectral energy peak
to be approximately halved, from 1.544 Mhz to 772 kHz. Figure 10 shows
the change in frequency spectrum. The lowering of frequencies reduces
the signal attentuation caused by the cable plant.

**7.4** Secondly, the bipolar signal eliminates the DC voltage component
in the bitstream. This facilitates the separation of the high frequency
digital signal and the DC voltage which is applied to the copper pairs
for powering of repeaters. Lastly, the alternating positive and negative
pulses provide a convenient means for a rudimentary form of error
checking. Should a timing loss occur and a ones pulse be lost or gained,
equipment can recognize the error through the occurrence of two
consecutive *nonalternating* pulse polarities, otherwise known as
bipolar violations, or BPVs. Of course, if two consecutive pulses were
lost, or any even number of consecutive pulses, the AMI format would
still be preserved and no errors could be detected through observation
of BPVs.

**FIGURE 10**

**SPECTRAL ENERGY OF BIPOLAR VS. UNIPOLAR PULSE STREAM**

> ![](media/image9.emf){width="4.476388888888889in"
> height="3.2215277777777778in"}

**7.5** One other requirement of a digital bitstream is necessary for a
DS1 signal to be adequately transmitted on metallic plant. In order that
line repeaters and terminal equipment operate in synchronization, they
must receive frequent timing information from the digital signal. Timing
is obtained from ones pulses; it cannot be derived from zeros.
Consequently, two limitations have been placed on the number of zeros
that can be continuously transmitted. They are known as the ones\'
density requirements and consist of the following:

**Ones\' Density Requirements**

> 1\) No more than 15 consecutive zeros can be transmitted.
>
> 2\) A minimum of three ones must be present in every 24 bits.

**7.6** The ones\' density requirements pose no great problems as long
as the information being transmitted is of a voice frequency nature,
either pure

voice or VF-encoded data from a modem. This is because voiceband
signals, when encoded in digital format, inherently contain ones in
sufficient quantities and frequencies to meet these requirements. The
problem occurs with transmission of digital data because it legitimately
can contain long strings of zeros. For example, accounting records could
show months of inactivity with continuous zeros.

**7.7** The problem of zeros in digital data transmission can be
remedied in one of two ways by the terminal equipment. Traditionally, in
order to meet the ones\' density requirements, the zero in the 7th bit
position has been replaced with a one when the entire 8-bit word
contains all zeros. However, with the advent of common channel signaling
schemes and the opportunity to use the entire 64 kb/s channel for data,
it was no longer desirable to rob this bit position for ones\' density
purposes. To provide sufficient ones for synchronization, two different
zero encoding methods were developed as an alternative that effectively
hide the true zero content of the data being transmitted through
encoding.

**7.8** The most commonly-used encoding method is B8ZS, which stands for
Binary 8 with Zero Substitution. Of lesser use is the code, ZBTSI,
referring to Zero Byte Time Slot Interchange. Both codes effectively
suppress continuous zeros for transmission purposes and then restore the
zeros within the signal to their original content at the receiving end.
Brief definitions of both B8ZS and ZBTSI are provided in Section 9.

**8. OVERVIEW OF DIGITAL TRANSMISSION EQUIPMENT**

**8.1 Purpose**

**8.1.1** This section provides a brief, descriptive overview of various
equipment used to transmit digital information for interexchange
purposes. Although the emphasis in this section is on transmission over
metallic plant (referred to as a digital span line or T-carrier) many of
the principals can be applied to other technologies using digital
transmission techniques.

**8.1.2** This section also presents some information on newer
technologies that use digital transmission. These are lightwave (or
fiber optics) and digital microwave radio. They are included because of
their reliance on fundamental digital transmission techniques. Other
digital transmission technologies do exist in addition to T-carrier,
lightwave and digital radio. These include satellite and cellular
telephone, for example. However, the intent in solely presenting
information on those discussed above is simply to illustrate how the
fundamental digital transmission techniques may be applied. Other REA
bulletins provide more specific and more detailed information on other
technologies.

**8.2 T-Carrier**

**8.2.1** The overall configuration of a T1 span line system consists of
the CO equipment and the span line containing the cable and the
repeaters, as shown in Figure 11. Span *line* refers to the facilities
connecting two terminating points, such as between two COs. It includes
both the CO and line repeaters, the cable and other terminating
equipment. A span *section* is the facility between two repeaters. The
span section nearest to the CO is known as an end section. Digital
signals on a span line are transmitted in one direction on each pair of
copper wires. Thus, two pairs are required for a two-way digital
connection. Frequently, a spare span is provided for backup.

**FIGURE 11**

**OVERALL CONFIGURATION OF A T1 SPAN LINE**

> ![](media/image10.emf){width="4.913194444444445in" height="2.75in"}

**8.2.2** The line repeater provides an essential contribution to the
operation of a span line system. Without repeaters, the DS1 digital
signal can propagate a distance of only 1.5 km or so before attenuation
begins to diminish the readability of the signal. As Figure 12 shows,
the repeater does not merely amplify, as with analog VF system
repeaters, it actually interprets the deteriorated, yet readable,
incoming digital signal and regenerates an entirely new, outgoing
digital signal. It is this regeneration, and not amplification, that
minimizes the introduction of noise and which characterizes digital
signal transmission as superior to analog.

**FIGURE 12**

**SPAN LINE REPEATER**

> ![](media/image11.emf){width="4.476388888888889in" height="2.75in"}

**8.2.3** The CO portion of a T1 span line contains a number of
additional components integral to the reliable operation of the digital
transmission system. As shown in Figure 13, the first component on the
far left is the channel bank. Although not directly part of the span
line system, the conversion of analog signals into digital signals by
the channel bank is frequently where digital transmission begins. (For
circuit-switched lines and trunks, most current digital switches
directly accept a DS1 level digital signal, eliminating the need for
channel banks.)

**8.2.4** Connected to the channel bank is a digital signal
cross-connect, or DSX. The DSX-1 refers to a cross-connect at the DS1
rate. This equipment facilitates the testing, access and mixing of
certain types of traffic at the DS1 level onto various span line
systems. On the far side of the channel bank is the automatic protection
switch, or APS. The APS monitors the status of each span line and
switches the traffic to a spare span line in the event of a failure or
excessive errors. To interconnect the various span lines, a second DSX-1
appears next to the span line terminating equipment. On the far side of
the second cross-connect is the span line terminating equipment, which
includes repeaters and line powering components.

**FIGURE 13**

**CENTRAL OFFICE SPAN LINE EQUIPMENT**

> ![](media/image12.emf){width="5.459027777777778in"
> height="3.772222222222222in"}

**8.3 Lightwave**

**8.3.1** Digital transmission via lightwave systems is similar to that
of span lines. One important distinction, however, is that optical
digital signals do not attenuate as rapidly as electrical digital
signals. Consequently, repeater use is minimized. In fact, many
lightwave installations do not require any repeaters even though the
distance may be on the order of 50 km to 75 km.

**8.3.2** In addition, the bit rate of lightwave systems is typically
many times higher than for DS1 span line transmission. Today, it is
common to transmit optical digital signals at nominal bit rates ranging
from 150 Mb/s to 600 Mb/s, the latter having a capacity of 12-DS3s or
8,064 DS0s. As with span lines, it is common to use one-way transmission
with a separate system providing for each direction of transmission.
Frequently, many systems are installed with spare fiber pairs and
terminal electronics to increase reliability.

**8.3.3** Because lightwave transmission systems have greater capacity
and operate at a much higher digital bit rates than do metallic span
lines, the role of multiplexers is much greater. A particular lightwave
transmission system may use a channel bank to provide DS1 channels, but
then use several additional multiplexers to construct a 150 Mb/s
electrical signal. This electrical signal is then converted into an
optical signal by the optical transceiver. A typical installation is
shown in Figure 14. (Detailed information on lightwave systems is in REA
Bulletin 1751H-601.)

**FIGURE 14**

**TYPICAL LIGHTWAVE SYSTEM**

> ![](media/image13.emf){width="5.459027777777778in"
> height="3.1430555555555557in"}

**8.4 Microwave Radio**

**8.4.1** Another digital transmission system that is in common use
today is digital, point-to-point microwave radio. A typical system
configuration with its fundamental components is shown in Figure 15. In
many respects this is similar to the lightwave system just discussed.
Microwave is capable of much higher bit rates than a T1 span line
system, but less than for lightwave. The capacity generally ranges from
one DS1 to several DS3s per radio channel, depending upon the actual
radio frequency (RF) used. As with lightwave, multiplexer use is also
prevalent. Frequently, as with T1 and lightwave, backup systems are
installed to increase reliability. Repeater spacings for microwave radio
vary widely, ranging from roughly 5 km to 50 km. The repeater spacings
are dependant upon a host of factors, including, the RF band used,
expected rain intensity and environmental/terrain considerations.

**FIGURE 15**

**TYPICAL MICROWAVE RADIO SYSTEM**

> ![](media/image14.emf){width="5.022222222222222in"
> height="3.536111111111111in"}

**9. DIGITAL TRANSMISSION TERMINOLOGY**

**9.1** The definitions in this section are intended for a wide range of
readers, from those relatively new to digital techniques to those having
considerable experience. Consequently, the definitions provided may not
be as precise as those given in other more academic or scientific
references. A rudimentary knowledge of general telephony is assumed.

**9.2** The reader is also advised to consult REA Bulletin 1751H-601,
*Lightwave Fundamentals, Systems, and Application*, for terms related
primarily to lightwave systems; and REA Bulletin 1751H-501,
*Fundamentals of ISDN*, for terms specific to ISDN.

**9.3** Terms used within certain definitions that are separately
defined in this glossary have been denoted by the use of italic print,
with the exception of several, frequently used terms.

**9.4** Acronyms are generally defined where the completely spelled-out
term is located, except where the usage of the acronym is viewed as more
common.

ëëëëëëëëëëëëëëëëëëëëëëëëëëëëëëëëëëëëëëëëëëëëëëëëëëëëëëëëëëëëë

**2B1Q** Abbreviation for 2 *Binary* 1 *Quaternary*; a digital *line
code* having four information states; used in digital subscriber loop
applications to obtain up to a 160 kb/s *bandwidth* for *ISDN* and other
services over non-loaded, paired-wire facilities.

ëëëëëëëëëëëë **A** ëëëëëëëëëëëë

**A/D** Abbreviation for Analog-to-Digital; a process that converts
analog signals to digital signals.

**add/drop multiplexer** See *drop & insert***.**

**address** Destination or storage location of a data element in a
transmission or processing system.

**ADPCM** Abbreviation for Adaptive Differential Pulse Code Modulation;
a 4-bit, digital signal encoding technique that bases its *quantizing*
thresholds upon the actual levels of the input signal, thereby reducing
*quantizing noise*; commonly used to encode 44 voice *channels* in place
of the usual 24 *channels* in a *DS1* signal to provide *bandwidth*
improvements over *differential pulse code modulation (DPCM)*.

**aliasing** The occurrence of distortion in a digitally-constructed
analog signal that results from insufficient digital *sampling* of the
original analog waveform; for voice signals, can be remedied by removal
of higher frequencies with an anti-aliasing filter.

**alternate mark inversion (AMI)** Digital *line code* where pulses,
representing a given state, alternate in polarity from positive to
negative.

**AMI** See ***alternate mark inversion***.

**ANSI** Abbreviation for American National Standards Institute; a U.S.
standards organization which publishes a variety of technical standards,
including those for telecommunications.

**analog** Continuously varying signal.

**APS** See *automatic protection switch***.**

**ARQ** Abbreviation for automatic request for repeat; error correcting
procedure used in data transmission to retransmit data when errors are
detected.

**asynchronous** Not *synchronous*; a process or signal that is not
operating at a constant or uniform rate with respect to a second signal
or process; opposite of *synchronous***.**

**automatic protection switch (APS)** Automated transmission facility
switching system used to switch traffic to alternative *channels* or
facilities in the event of a failure or excessive errors.

ëëëëëëëëëëëë **B** ëëëëëëëëëëëë

**B3ZS** Abbreviation for *Bipolar* with 3 *Zero Substitution*; a *zero
substitution code* commonly used at a *DS3* transmission rate that, upon
the occurrence of three consecutive zeros in a digital *bitstream*, will
insert (prior to transmission) a predetermined code to cause a *bipolar
violation*. At the receiving end this code will be recognized and the
stream of zeros will be properly restored into the *bitstream*.

**B6ZS** Abbreviation for *Bipolar* with 6 *Zero Substitution*; a *zero
substitution code* commonly used at a *DS2* transmission rate, that upon
the occurrence of six consecutive zeros in a digital *bitstream*, will
insert (prior to transmission) a predetermined code to cause a *bipolar
violation*. At the receiving end this code will be recognized and the
original stream of zeros will be properly restored into the *bitstream*.

**B8ZS** Abbreviation for *Bipolar* with 8 *Zero Substitution*; a *zero
substitution code* commonly used at *DS1* and *DS1C* transmission rates
that, upon the occurrence of eight consecutive zeros in a digital
*bitstream*, will insert (prior to transmission) a predetermined code to
cause two bipolar violations. At the receiving end this code will be
recognized and the stream of zeros will be properly restored into the
*bitstream*.

**B-channel** Designation for the 64 kb/s information-carrying *channel*
in the Basic Rate Interface and the Primary Rate Interface, both of the
Integrated Services Digital Network; see *ISDN***.**

**BISDN** Abbreviation for Broadband Integrated Services Digital
Network; a general designation for high *bandwidth* network capability
provided at a nominal *DS3 bit rate* of 45 Mb/s or higher; see *ISDN*.

**bandwidth** Measure of the spectral frequency that a system is able to
process or transmit; in digital systems, it is proportional to the *bit
rate*.

**baseband** Source frequency band that contains voice, data or video
information that is used in a *modulation* process.

**BCD** See *binary coded decimal*.

**BER** See *bit error rate***.**

**binary** Numeric system of only two states, such as 0 or 1.

**binary coded decimal (BCD)**   *Binary*-encoded form of a decimal
number.

**bipolar** Of two polarities, positive and negative. Digital signals on
T1 facilities are typically transmitted in a *bipolar format*.

**bipolar violation (BPV)** Occurrence of two consecutive pulses of the
same polarity in a *bipolar* digital *bitstream*; not consistent with
the *bipolar* standard of alternating positive and negative pulses; see
*bipolar*.

**bit** Abbreviation for *binary* digit; the smallest unit of
information in a digital system.

**bit compression multiplexer** Device that converts multiplexed and
compressed digital signals, such as *ADPCM*, to standard *PCM* format
and vice versa.

**bit error rate (BER)** Ratio of the number of digital bits that are in
error for a given transmission or processing system to the total number
of bits transmitted during a given time interval; typically expressed in
units of errors per one million bits transmitted.

**bit interleaving** Technique for combining multiple digital *channels*
on a bit-by-bit basis in the same digital *bitstream***.**

**bit rate** Speed of a digital *bitstream*. Generally represented in
units of either bits per second (b/s), kilobits per second (kb/s),
megabits per second (Mb/s), or gigabits per second (Gb/s).

**bit stuffing** Method used for maintaining synchronization in a
digitally multiplexed signal where additional pulses of ones are added;
also referred to as pulse stuffing.

**bitstream** Series of uninterrupted, sequential bits.

**block** Grouping of digital bits that are transmitted or processed as
a unit.

**block error rate (BLER)** Ratio of the number of data *blocks* in
error that are transmitted or processed over a given time interval to
the total number of *blocks* transmitted; typically expressed in units
of errored blocks per one million *blocks* transmitted.

**BPV** See *bipolar violation*.

**b/s** Abbreviation for bits per second; also designated as bps.

**buffer** Temporary storage (memory) location for digital information,
typically used in interfacing two devices.

**bus** Common pathway for the flow of digital information; also
referred to as a highway.

**byte** Collection of *binary* digits processed or stored as a unit;
for telecommunications, a byte is generally composed of 8-bits; see
***octet***.

**byte interleaving** Technique for combining multiple digital signals
on a byte-by-byte basis in the same digital *bitstream*.

ëëëëëëëëëëëë **C** ëëëëëëëëëëëë

**carrier** High frequency signal used to transport a second lower
frequency signal by using a specific technique of incorporating the
second signal; see *digital loop carrier, modulation, subscriber
carrier*.

**CCIS** See *common channel interoffice signaling***.**

**CCITT** Abbreviation from the French language for the International
Telegraph and Telephone Consultative Committee; an international
telecommunications standards setting organization which operates under
the auspices of the International Telecommunications Union, a
specialized agency of the United Nations.

**central office terminal (COT)** Electronic central office component of
a *concentrator* or *subscriber carrier* system connected to a *remote
terminal* by a digital transmission facility; interfaces the digital
transmission facility to the switch on an analog line basis by
performing *D/A* and *A/D* signal conversions; see *remote terminal*.

**channel** Subdivision of a digitally-encoded and multiplexed signal
representing sufficient *bandwidth* for transmission or processing of an
individual voice, data or video signal.

**channel bank** Electronic terminal equipment that digitally encodes
and time division multiplexes analog signals; also performs the reverse
process by demultiplexing and decoding digital signals into analog
signals; processing is generally performed on 24 *channel* groups, or
multiples thereof; see ***D*.**

**check bit** Bit used for error checking in a digital signal; a *parity
bit***.**

**check parity** Technique for detecting digital signal errors in which
an extra bit is added and then verified at the receiving end.

**clear channel capability (CCC)** Feature available to a digital
bitstream that provides a full 64 kb/s of user available *bandwidth* (as
opposed to 56 kb/s) and any combination of zeros and ones in each *DS0*
*channel*. To provide this capability, the signaling and supervisory
information are transmitted over a separate *channel* and either the
*B8ZS* or *ZBTSI* *zero substitution code* is used.

**clear channel signal** See *clear channel capability***.**

**clock** Device that generates periodic signals at a uniform rate for
unit or system synchronization.

**codec** Abbreviation for coder/decoder; a device that converts analog
signals into *binary*-coded digital signals, and vice versa.

**common channel interoffice signaling (CCIS)** Interoffice transmission
signaling network that transmits signaling information in digital format
on circuits separate from the message channels.

**companding law** Defined standard for the nonlinear (or nonuniform)
compression and expansion of analog signal levels.

**compandor** Abbreviation for compressor/expandor; device that
compresses the amplitude range of analog signals prior to signal
transmission or processing and then expands the analog signal at the
receiving end to restore the original amplitude range; improves the
*signal-to-noise ratio*.

**concentration ratio** Ratio of the number of connected subscriber
loops to the available transmission *channels* or paths in a line
concentrator or central office switch (typically 6:1 or 4:1); used to
take advantage of the diversity of telephone traffic; see
*concentrator*.

**concentrator** Switching device, generally located at a distance from
a central office, that connects a larger number of subscriber loops to a
smaller number of transmission paths to the central office switch;
results in increased efficiency for use of outside plant cable
facilities; see *concentration ratio***.**

**continuously variable slope delta modulation (CVSDM)** A digital
*modulation* technique that samples (generally at 32,000 times per
second) the slope or the rate of change, rather than the amplitude, of
an analog signal prior to digital encoding.

**crash point** Point in the operation of a digital transmission or
processing system in which certain operating parameters are beyond their
acceptable range, thus causing the failure of the system.

**cross-connect** Mechanical or electronic device that permits the
interconnection of separate *channels* or circuits; see *digital*
*cross-connect*.

**crosstalk** Form of interference due to electromagnetic coupling from
one circuit or *channel* to another; can occur within a single system or
between systems.

**CVSDM** See *continuously variable slope delta modulation*.

**cyclic redundancy check (CRC)** Error checking procedure used in
digital transmission systems in which mathematical calculations are
performed on the originating signal and the results transmitted with the
outgoing *bitstream*. At the receiving end, the same calculations are
performed and the results are compared to those from the transmitted
end.

ëëëëëëëëëëëë **D** ëëëëëëëëëëëë

**D** Refers to one or more of a series of *channel banks* used for
digital and analog signal conversion, including D1, D1A, D1B, D1C, D1D,
D2, D3, D4 and D5; the higher alphanumeric reference indicates units
with increased capacity, reduced size and other improvements.

**D/A** Abbreviation for Digital-to-Analog; a conversion process by
which digital signals are converted to analog signals.

**D-channel** Abbreviation for the 16 kb/s signaling *channel* of the
Basic Rate Interface, and for the 64 kb/s signaling *channel* of the
Primary Rate Interface, both of the Integrated Services Digital Network;
see ***ISDN*.**

**data skew** Distortion of a digital *bitstream* resulting from lack of
proper synchronization between two terminals on a transmission system.

**dB** Abbreviation for decibel; a logarithmic measure of the ratio of
two signals, either on a voltage or power basis.

**DDS** Abbreviation for Digital Data Service; a Bell System term that
refers to a tariffed subscriber service for transmitting data in digital
format.

**decoder** Device that decodes a digital signal, generally into analog
form.

**degraded minute (DM)** One-minute interval during which the *bit error
rate* exceeds one bit error per one million bits transmitted, or other
predetermined thresholds.

**delta modulation** See *differential pulse code modulation*.

**demodulation** Technique of recreating or extracting an original
*baseband* waveform from the information contained in a *carrier* signal
that has been modulated with the *baseband* information.

**demultiplex** Process of extracting an individual *channel* from a
single composite signal made up of two or more channels previously
combined through either *time division multiplexing* or *frequency
division multiplexing* techniques.

**DI** See *drop & insert***.**

**differential pulse code modulation (DPCM)** Digital *modulation*
technique in which the analog signal is *sampled* and *quantized*
(generally employing four bits per sample) based upon the differences in
signal levels over time, rather than on absolute levels; results in a
reduction in *bandwidth* over standard *pulse code modulation*; also
known as delta modulation.

**differential phase shift keying (DPSK)** *Modulation* technique in
which the digital baseband signal is represented by the changes in the
phase of the carrier signal; see *phase shift keying***.**

**digital** Device or process that uses discrete numeric states for
signal representation, as opposed to use of a continuously varying or
analog signal.

**digital COE** Abbreviation for digital central office equipment;
generally an electronic, stored-program, common controlled switching
system that performs the temporary interconnection of line and trunk
circuits in a time domain rather than through the physical, metallic or
electrical connection of circuits.

**digital cross-connect system (DCS)** Software-controlled electronic
device that permits direct access to and interconnection of individual
*DS0* and *DS1* subrate *channels* contained in a higher-order
multiplexed digital *bitstream* without having to *demultiplex* the
entire higher order signal; some devices also permit the direct
interconnection of *DS3* signals; used to *groom* and route circuits by
similar use and/or destination.

**digital hierarchy** Organizational structure upon which multiplexed
digital signals of various bit rates are constructed; in the U.S., one
*DS3* of 44.736 Mb/s comprises either: 1) 672 *DS0*s of 64 kb/s each or,
2) seven *DS2*s of 6.312 Mb/s each, or 3) 28 *DS1*s of 1.544 Mb/s each;
see *DS1***,** *DS2***,** *DS3***.**

**digital loop carrier (DLC)** Device that digitally encodes and
*multiplexes* subscriber loop *channels* into a *DS1* signal or higher
for more efficient transmission or extended range; may or may not have
*concentrator* capability; see *integrated digital loop carrier***,**
*universal digital loop carrier, pair gain***.**

**digital pad** Device that alters a digitally-encoded *bitstream* to
attenuate the resulting analog signal.

**digital radio** Type of radio system that transmits and receives
digitally-encoded signals; used for both interexchange and subscriber
loop applications.

**digitizing** Conversion of an analog signal into digital format; see
*A/D, encoder*.

**digroup** Grouping of digitally-encoded and multiplexed 24 *channels*
operating at a nominal rate of 1.544 Mb/s, or *DS1*.

**DLC** See *digital loop carrier*.

**DPCM** See *differential pulse code modulation*.

**DPSK** See *differential phase shift keying*.

**drop & insert (DI)** Device or process that both extracts and inserts
lower level digital *channels* from and to a higher order digital
*bitstream* without the need to demultiplex the entire higher order
signal; also may have analog-to-digital and digital-to-analog conversion
capability.

**DS0** Abbreviation for Digital Signal Level 0 in the *digital
hierarchy*; a digitally-encoded 64 kb/s *channel*; often has a usable
capacity of only 56 kb/s due to signaling and overhead information
needs; the entire 64 kb/s bandwidth is available only if *clear channel
capability (CCC)* is employed.

**DS1** Abbreviation for Digital Signal Level 1 in the *digital
hierarchy*; a digitally-encoded signal at a nominal rate of 1.544 Mb/s;
often multiplexed with 24 *DS0* channels; equivalent to one
*digroup***.**

**DS1C** Modified version of the *DS1* digital transmission rate,
consisting of a digitally-encoded signal at a nominal rate of 3.152
Mb/s; often multiplexed with 48 *DS0* channels.

**DS2** Abbreviation for Digital Signal Level 2 in the *digital
hierarchy*; a digitally-encoded signal at a nominal rate of 6.312 Mb/s;
often multiplexed with either four *DS1* or 96 *DS0* channels.

**DS3** Abbreviation for Digital Signal Level 3 in the *digital
hierarchy*; a digitally-encoded signal at a nominal rate of 44.736 Mb/s;
often multiplexed with either seven *DS2*, 28 *DS1* or 672 *DS0
channels***.**

**DSX** Abbreviation for digital signal *cross-connect*; often used in
the form DSXm, where the m is either a 1, 2 or 3 referring to a DS1, DS2
or DS3 digital signal, respectively; see *cross-connect*.

**duobinary encoding** Digital *bitstream* encoding scheme of four, as
opposed to two, information states in which the information rate, or
*bit rate*, can be doubled.

**duplex** Exchange of data or voice information between two devices or
systems in both directions simultaneously; sometimes referred to as full
duplex; see *half-duplex***.**

ëëëëëëëëëëëë **E** ëëëëëëëëëëëë

**EFS** See *error free seconds*.

**encoder** Device that converts the measured values obtained from the
*sampling* and *quantizing* of an analog signal to a digital code.

**encoding law** Specifically defined means for constructing the various
nonlinear (or nonuniform) amplitude levels used for *sampling* and
*quantizing* an analog signal into a digital signal; in the U.S. and
Japan the *Mu-law* is standard, in Europe and much of the rest of the
world, the A-law is standard.

**engineering loss** Worst case design cable loss used in the
engineering of digital *span* *lines*.

**error code** Code used to announce the presence of an error in the
transmission or processing of a digital signal.

**error correction** Automatic system that automatically performs a
correction on transmitted or processed information upon the detection of
errors.

**error detection** Automatic system that detects the presence of errors
in transmitted or processed information; detection is generally followed
by either retransmission or *error correction* of the corrupted
information.

**error free second (EFS)** One-second interval during which zero bits
are in error.

**errored second (ES)** One-second interval during which one or more
bits are in error.

**error rate** See *bit error rate, block error rate, residual error
rate***.**

**ES** See *errored second*.

**ESF** See *extended superframe format*.

**extended superframe format (ESF)** Improved digital *bitstream*
framing scheme for *DS1* transmission that provides 24 frames (in lieu
of the 12 frames used in the *superframe* format) and additional
communication paths for performance monitoring and other supervisory
functions; framing scheme used in D4 and D5 *channel banks*; see
*frame***,** *superframe***.**

ëëëëëëëëëëëë **F** ëëëëëëëëëëëë

**FDM** See *frequency division multiplexing*.

**FEXT** Abbreviation for far end crosstalk; form of electromagnetic
noise interference between two transmission systems operating in the
same direction where the source of the interference is at the far end.

**FEC** See *forward error correction*.

**forward error correction** Error correcting procedure used in data
transmission where corrections on erred data are performed at the
receive end.

**frame** Repetitive organizational grouping of digital *words* from all
available *channels* in a digital *bitstream*, containing *baseband*,
signaling and timing information; e.g., for the D4 frame format, a frame
consists of 24 8-bit timeslots and one framing bit, operating at 1.544
Mb/s.

**framing bit** **(F-bit)** Bit used to synchronize digital *frames* by
marking the divisions between *frames*; located at the 193rd bit
position in each frame.

**frequency division multiplexing (FDM)** Technique for modulating
*carrier* signals where the *baseband* information to be transmitted is
placed in separate and adjacent carrier frequencies, and then combined
to constitute a much wider frequency spectrum.

**frequency shift keying (FSK)** Digital *modulation* technique that
shifts the *carrier* signal\'s frequency to different finite values
within a predetermined range and in accordance with the characteristics
of the digital *baseband* signal to be transmitted or processed.

**FSK** See *frequency shift keying.*

ëëëëëëëëëëëë **G** ëëëëëëëëëëëë

**Gb** See *gigabit*.

**Gb/s** Abbreviation for gigabits per second; one billion (1 x 109)
bits per second; also designated Gbs.

**gigabit (Gb)** One billion (1 x 109) bits.

**groom** Arrange and group like circuits or *channels* according to
their respective characteristics or destinations to use transmission or
switching equipment more efficiently.

ëëëëëëëëëëëë **H** ëëëëëëëëëëëë

**half-duplex** Exchange of data or voice information between two
devices or systems in either direction, but in only one direction at a
time; see **duplex.**

**handshaking** Exchange of signaling, synchronization or control
information between two devices or systems following the establishment
of an interconnection.

**hubbing** Centralization of network functions or traffic at a single
location.

ëëëëëëëëëëëë **I** ëëëëëëëëëëëë

**IDLC** See *integrated digital loop carrier*.

**impulse noise** Noise having random waveform characteristics of short
duration and rapid change; disturbs the accuracy of digital information
transmitted by a device or system.

**integrated digital network** Digital network of subscriber loop,
switching and interoffice trunk facilities that are all
digitally-interfaced; results in continuous and uninterrupted digital
transmission paths; also can refer to the integration of switching and
transmission equipment.

**integrated digital loop carrier (IDLC)** *Digital loop carrier* system
that has a direct, digital interface to a digital central office switch;
eliminates the need to perform analog-to-digital signal conversion and
multiplexing of loop circuits at the switch through a *central office
terminal*; see *digital loop carrier***,** *universal digital loop
carrier***.**

**integrated services digital network** See *ISDN*.

**interface** Location or device that connects two, generally differing
systems; a shared boundary.

**interleaving** Combination of two or more digital signals into a
single digital *bitstream*.

**interrogation** Action, by one device or system, of requesting
information from another device or system.

**ISDN** Abbreviation for the Integrated Services Digital Network;
advanced digital subscriber loop transmission concept that provides
multiple *channels* of *baseband* and signaling information for the
simultaneous transmission of voice, data and other special services over
a single facility. The Basic Rate Interface contains a total user
*bandwidth* of 144 kb/s, consisting of two 64 *kb/s B-channels* and one
16 *kb/s D-channel* for signaling or other data. The Primary Rate
Interface contains a user *bandwidth* of 1.544 Mb/s, consisting of
23-*DS0* 64 kb/s *B-channels* and one 64 kb/s *D-channel* for signaling;
ISDN also exists in a less specifically defined broadband format (BISDN)
at rates of *DS3* and above for high *bandwidth* services.

**isochronous** Relationship between two signals in which the
differences between their phase angles remains constant.

**IXC** Abbreviation for interexchange carrier; organization that
transports long distance telecommunications traffic across *LATA*
boundaries, both intrastate and interstate.

ëëëëëëëëëëëë **J** ëëëëëëëëëëëë

**jitter** Instability or variability of a **signal** at a relatively
rapid rate; results in pulse fluctuations in frequency, amplitude,
duration and phase.

ëëëëëëëëëëëë **K** ëëëëëëëëëëëë

**kb** See *kilobit*.

**kb/s** Abbreviation for *kilobits* per second; one thousand (1 x 103)
bits per second; also designated as kbs.

**kilobit (kb)** One thousand (1 x 103) bits.

ëëëëëëëëëëëë **L** ëëëëëëëëëëëë

**LATA** Abbreviation for Local Access and Transport Area;
specifically-defined geographic areas in the U.S. established by
regulators to separate telecommunications transport markets between
local exchange carriers (*LECs*) and interexchange carriers (*IXCs*);
*LECs* provide service [within]{.underline} a LATA; *IXCs* provide
service [between]{.underline} LATAs.

**least significant bit (LSB)** Bit that contains the least amount of
information; used in *bit* *robbing* for the transmission of signaling
information.

**LEC** Abbreviation for local exchange carrier; organization that
transports local telecommunications traffic within a *LATA*.

**line code** Specific technique of encoding digital signals for optimal
transmission or processing through a particular medium; see *AMI, 2B1Q,
non-return-to-zero.*

**linear encoding** Any analog-to-digital encoding scheme that uses
linearly-constructed (uniform) amplitude thresholds for *sampling* and
*quantizing* analog samples.

**linear predictive coding (LPC)** Analog-to-digital encoding scheme
where *bandwidth* savings are realized through use of an algorithm which
relies upon a linear or uniform function to predict the analog signal\'s
behavior.

**loss** Amount of signal attenuation resulting from the transmission or
processing of a given device or system.

**LPC** See *linear predictive coding***.**

**LSB** See *least significant bit***.**

ëëëëëëëëëëëë **M** ëëëëëëëëëëëë

**M** Abbreviation for a digital multiplexer; often used in the form
Mxy, that denotes multiplexing from the *DS*[x]{.underline} level to the
*DS*[y]{.underline} level; also used in the format M44 and M24, denoting
multiplexing of 44 or 24 voice *channels* into a *DS1* signal.

**Mb** See megabit.

**master clock** Highly accurate *clock* which produces timing signals
for other subtending clocks or other systems.

**Mb/s** Abbreviation for *megabits* per second; one million (1 x 106)
bits per second; also designated as Mbs.

**megabit (Mb)** One million (1 x 106) bits.

**modem** Abbreviation for modulator/demodulator; device that converts
digital data into a voice frequency analog signal, and vice versa, for
transmission over standard telephone lines.

**modulation** Process by which a *carrier* signal is varied in one
fashion or another to represent or carry another signal (*baseband*) for
purposes of transmission; see *carrier*.

**most significant bit (MSB)** Bit that contains the most amount of
information in a digital *word*; see *least significant bit.*

**MSB** See *most significant bit*.

**Mu-law** Nonlinear, 16-segment, analog-to-digital *encoding law* used
in N. America.

**multiplex** Process of combining two or more individual signals or
*channels* into a single composite signal that can be subsequently
*demultiplexed* for retrieval of the original individual signals. Either
time or frequency division multiplexing methods are used.

**multiplexer** Device that *multiplexes***.**

**mux** Abbreviation for *multiplexer*.

ëëëëëëëëëëëë **N** ëëëëëëëëëëëë

**NEXT** Abbreviation for near end crosstalk; a form of electromagnetic
noise interference between two transmission systems operating in
opposite directions where the source of the interference is at the near
end.

**node** Meeting point for two or more lines and/or paths in a network.

**non-return-to-zero (NRZ)** Digital *line code* in which the two
logical states are represented by positive and negative pulses, with no
zero or neutral state.

**nonlinear encoding** Any analog-to-digital encoding scheme that uses
nonlinear or nonuniform amplitude thresholds for *sampling* and
*quantizing* analog signals.

**NRZ** See *non-return-to-zero*.

**Nyquist Interval** Maximum time interval at which an analog signal can
be *sampled* for conversion into a digital signal and still allow the
analog signal to be fully and accurately reconstructed; equal to two
times the reciprocal of the highest frequency contained in the analog
signal; see *Nyquist Theorem***.**

**Nyquist Theorem** Mathematical theorem that demonstrates that an
analog signal can be fully reconstructed from digital data if the analog
signal is sampled at a rate equal to or greater than two times the
highest frequency in the original signal.

ëëëëëëëëëëëë **O** ëëëëëëëëëëëë

**octet** Group of eight *binary* digits, known as an 8-bit *word*,
which is processed as a unit; equivalent to a *byte*.

**OSI** Abbreviation for Open Systems Interconnection; structured
architecture designed for information exchange between different
communication or data systems

**overhead bit** Any digital bit that does not have voice or data
information content.

ëëëëëëëëëëëë **P** ëëëëëëëëëëëë

**packet** Collection of digital information, including signaling data,
into a discrete package of a specified format which is then individually
transmitted and switched to its ultimate destination; see *packet
network*.

**packet assembler-disassembler (PAD)** Electronic device that both
assembles data and information into *packet* format to prepare it for
switching and transmission on a *packet network*, and performs the
disassembly of the *packets* transmitted to it into the original data
format; see *packet network*.

**packet network** Digital network designed to accommodate the
transmission of individual short information messages called *packets*
where the individual *channels* are only occupied for sufficient time to
complete the transmission of each *packet*; see *packet
assembler-disassembler*, *packet switch*.

**packet switch** Digital switch designed to process and route
individual short messages called *packets* onto a packet network; see
*packet* *network, packet assembler disassembler*.

**pair gain** Process by which the *channels* of two or more subscriber
loop pairs are *multiplexed* together by either digital or analog
techniques and then transmitted on facilities of lesser capacity than
otherwise would be required if the same group of signals were
transmitted individually on one pair of wires per channel; permits
greater efficiencies in use of outside plant cable facilities.

**PAM** See *pulse amplitude modulation*.

**parallel transmission** Process by which all of the bits of each
digital *word* are transmitted simultaneously along separate paths;
opposite of *serial* *transmission***.**

**parity bit** Bit added in a *error detection* scheme to make the total
number of logical one bits either an even or odd number; see *parity
check***.**

**parity check** Method of *error detection* used in digital systems in
which an additional bit is added, where necessary, to each digital
*word* in order to make the total number of logical one bits either an
even or odd number. At the receive end, absence of the proper parity is
an indicator of errored data.

**PCM** Abbreviation for Pulse Code Modulation; a *modulation* technique
in which the amplitude of an analog signal is *sampled* at regular
intervals and converted to numerical values, which are generally
represented in digital form as an 8-bit *word*. The resulting numerical
values can then be used to reconstruct the analog waveform following
signal transmission or processing.

**phase shift keying (PSK)** *Modulation* technique for digital
transmission in which the phase of the *carrier* frequency is varied
within a predetermined range in accordance with the characteristics of
the digital *baseband* signal to be transmitted.

**plesiochronous** Frequency relationship between two or more signals
that are at nominally the same frequency, but are not exactly equal;
within some specified limits of each other; almost *synchronous***.**

**port** Point of access in a device to provide for the input or output
of information.

**protocol** Predetermined set of rules for how information will be
exchanged in a given system or equipment.

**pseudorandom signal** Signal that appears to be random but is actually
repetitive when examined over a sufficiently long period of time; often
used for testing of digital systems.

**PSK** See *phase shift keying*.

**pulse** Signal characterized by relatively rapid rise and decay times.

**pulse amplitude modulation (PAM)** *Modulation* technique for digital
transmission in which the amplitude of the *carrier* pulses is varied
within predetermined limits in accordance with the characteristic of the
digital *baseband* signal to be transmitted.

**pulse code modulation** See *PCM***.**

**pulse interval** Amount of time between two pulses, measured from the
same points on each pulse.

**pulse stuffing** See *bit stuffing***.**

**pulse width modulation (PWM)** *Modulation* technique in which the
pulse width of the *carrier* signal is varied within predetermined
limits in accordance with the *baseband* signal to be transmitted or
processed.

**PWM** See *pulse width modulation*.

ëëëëëëëëëëëë **Q** ëëëëëëëëëëëë

**QAM** See *quadrature amplitude modulation***.**

**QPSK** See *quadrature phase shift keying***.**

**QRS** Abbreviation for *Quasi-Random Signal*; see *pseudorandom
signal***.**

**QRSS** See *quasi-random signal source***.**

**quadrature amplitude modulation (QAM)** *Modulation* technique in
which the *baseband* signal is digitally represented through four
different amplitude states of the *carrier* signal; a variation of
*amplitude* *modulation*; used in *digital* *radio* systems.

**quadrature phase shift keying (QPSK)** *Modulation* technique in which
the *baseband* signal is digitally represented through four different
phase states of the *carrier* signal; a variation of *phase shift
keying*; used in *digital radio* systems.

**quantizing noise** Noise or distortion occurring as a result of an
analog-to-digital signal conversion; caused by inaccuracies from
*quantizing* *sampled* analog signal levels.

**quantizing** Process of converting the amplitude levels of an analog
signal sampled at discrete intervals to numeric values prior to digital
encoding.

**quasi-random signal (QRS)** See *pseudorandom signal*.

**quasi-random signal source (QRSS)** Test pattern of *pseudorandom*
digital signals used for testing the performance of digital transmission
systems.

**quaternary** Pertaining to four separate information states.

ëëëëëëëëëëëë **R** ëëëëëëëëëëëë

**random signal** Digital signal having no predetermined or regular
pattern over a long time period.

**regenerator** Device that samples a weak and/or distorted digital
signal, reconstructs it and then retransmits an identical new digital
signal for continued transmission or processing; also referred to as a
repeater.

**register** Device that temporarily receives and stores digital
information for later retrieval.

**remote terminal (RT)** Electronic remote component of a *concentrator*
or *subscriber carrier* system connected via a digital transmission
facility to a central office switch on a direct digital basis, or
through a *central office terminal*; performs *A/D* and *D/A* signal
conversion between subscriber lines and the digital transmission
facility; see *central office terminal*.

**repeater** See *regenerator***.**

**RER** See *residual error rate*.

**residual error rate (RER)** A ratio of the number of bits, blocks, or
other data units that continue to be in error after being acted upon by
*error correction* or *error detection* methods to the total number of
bits, blocks or other units that have been transmitted; see *bit error
rate*.

**return loss** Measure of the difference in attenuation between a
reflected signal as compared to the transmitted signal on two
interconnected networks; generally expressed in *dB*; a general measure
of the impedance mismatch between two networks.

**robbed bit signaling** Process where signaling bits are included in a
digital framing format by replacement of the *least significant bit* of
each *channel* in certain *frames* with a signaling bit.

ëëëëëëëëëëëë **S** ëëëëëëëëëëëë

**S/N** See *signal-to-noise ratio***.**

**sampling** Measurement of an analog signal at discrete intervals to
obtain its characteristics prior to digital encoding.

**sampling frequency** Frequency at which an analog signal is sampled or
measured prior to its conversion to a digital representation; see
*Nyquist Interval*.

**scrambling** Process of rearranging a digital *bitstream* in a
predetermined form to avoid consecutive zeros or repetitive patterns.
The *bitstream* can then be restored to its original form at the receive
end; also known as randomizing.

**screened cable** Outside plant cable specially constructed with
internal metallic shielding around groupings of paired wires to reduce
the *crosstalk* interference among different signals simultaneously
transmitted within the cable.

**serial transmission** Process by which the bits in a digital
*bitstream* are sequentially transmitted bit-by-bit along a single path;
opposite of the multiple paths used in *parallel transmission*.

**SES** See *severely errored seconds*.

**severely errored seconds (SES)** One-second interval in which the *bit
error rate* exceeds one bit error per 1000 bits transmitted, or other
predetermined thresholds.

**SF** See *superframe*.

**signal** Any electronic message of intelligence or control.

**signal-to-noise ratio (S/N)** Measure of the ratio of a desired signal
to an undesired signal, such as noise or adjacent *channel*
interference; generally expressed in *dB*.

**Signaling System No. 7 (SS7)** Digital, message-switched signaling
system that operates in separate out-of-band common *channels* using a
*packet network*; also referred to as CCITT No. 7 signaling.

**SONET** Abbreviation for Synchronous Optical Network; a new and
developing standard that provides for a common optical and electrical
*synchronous* interface for lightwave systems and equipment.

**span** Sum of all digital transmission facilities between two physical
locations, such as two central offices.

**span line** String of *regenerators* or *repeaters* between two points
on a digital transmission network.

**SS7** See *Signaling System No. 7*.

**start bit** Additional bit used to signal the beginning of a digital
*word* in *asynchronous* systems.

**statistical multiplexing** Form of multiplexing in which *channel*
assignments and *bandwidth* requirements are not dedicated but are
dynamically allocated based upon the actual transmission needs of input
devices at a given point in time; the device used to perform this
operation is sometimes referred to as a stat *mux*.

**stop bit** Additional bit used to signal the end of a digital *word*
in *asynchronous* systems.

**subscriber carrier** Electronic device that combines one or more
subscriber loop *channels* through either analog or digital
*multiplexing* on a *carrier* signal for transmission to the central
office; achieves *pair gain* benefits by reducing the need for outside
plant cable facilities; each subscriber circuit retains an independent
*channel* in that no line *concentration* is performed; see *digital
loop carrier***.**

**superframe (SF)** Repetitive organizational structure for a digital
*bitstream* consisting of 12 *frames*, each containing 24 *channels* of
8-bit *words* plus one framing bit, operating at the *DS1* rate of 1.544
Mb/s; the framing scheme used in D3 and some D4 *channel banks*; see
*frame, extended superframe format***.**

**synchronization bit** Bit used in a digital *bitstream* for timing or
synchronization purposes.

**synchronous** Process or transmission that is synchronized with a
common clocking system.

ëëëëëëëëëëëë **T** ëëëëëëëëëëëë

**T-carrier** Digitally multiplexed *carrier* system operating at the
*DS1* rate of 1.544 Mb/s; see *digital loop carrier***.**

**T1** Digital transmission system operating at a nominal rate of 1.544
Mb/s, or *DS1*, on metallic facilities.

**TDM** See ***time division multiplexing*.**

**ternary coding** Digital encoding scheme of three separate information
states generally obtained through use of positive and negative *bipolar*
pulses and zeros in the *bitstream*.

**time division multiplexing (TDM)** Any signal *multiplexing* process
in which multiple *channels* or signals share a common transmission
path, each for a very short time period.

**time slot interchange** Process of information interchange from one
time slot or *channel* to another on a time division multiplexed network
or device; used in *digital cross-connect systems* and *digital COE*
switches.

**topology** Physical organization and structure of a transmission
network.

**tracking** Measure of how accurately an analog signal is digitally
coded and decoded into its original analog state.

**transparent interface** Interconnection between two networks or
systems that permits the free exchange of information in both directions
without additional processing or modification.

**UDLC** See *universal digital loop carrier*.

ëëëëëëëëëëëë **U** ëëëëëëëëëëëë

**unipolar** Of one polarity, either positive or negative.

**universal digital loop carrier (UDLC)** *Digital loop carrier* system
capable of interfacing to any central office switch at the analog line
level; see *digital loop carrier, integrated digital loop carrier*.

ëëëëëëëëëëëë **V** ëëëëëëëëëëëë

**VF** Abbreviation for voice frequency; analog signal with a nominal 4
kHz bandwidth.

ëëëëëëëëëëëë **W** ëëëëëëëëëëëë

**wander** Instability or variability of a *signal* at a relatively slow
rate; results in pulse fluctuations in frequency, amplitude, duration
and phase.

**word** Group of bits processed as a unit that represents a single
quantity or instruction; for telecommunications, generally a collection
of 8-bits; often referred to as a digital *word* or *byte*.

ëëëëëëëëëëëë **Z** ëëëëëëëëëëë

**ZBTSI** Abbreviation for Zero Byte Time Slot Interchange; a digital
*zero substitution code* used to provide a 64 kb/s *clear channel* in a
*DS0* by replacing selected strings of eight consecutive zeros with an
address location of where the zeros are to be restored at the receiving
end.

**zero substitution code** Coding scheme used in digital transmission to
ensure accurate transmission and continuous synchronization through
devices that have zeros limitations; replaces long strings of
consecutive zeros with a pattern of ones having *bipolar violations*
(BPVs). At the receiving end, the BPVs are recognized and the original
string of consecutive zeros is restored; see *B3ZS, B6ZS, B8ZS*,
*ZBTSI*.

 

[^1]: With respect to international facilities, the rest of the world\'s
    telecommunication\'s facilities mostly use a similar, but different,
    14-segment encoding law. This is known as the A-law. International
    traffic from and to the U.S. must undergo a conversion from one to
    the other for proper decoding.

[^2]: Japan and Australia also use the 24-channel T1 format. In the
    remainder of the world, the 32-channel E1 format is used where the
    signaling and synchronization information is carried out of band in
    separate channels.

[^3]: The two different means for providing performance monitoring
    through ESF framing are addressed in: 1) AT&T Technical Publication
    \# 54016, *Requirements for Interfacing Digital Terminal Equipment
    for Services Employing the Extended Superframe Format*, and 2) ANSI
    Telecommunications Standard T1.403-1989, *Carrier-to-Customer
    Installation - DS1 Metallic Interface*.