UNITED STATES DEPARTMENT OF AGRICULTURE

**Rural Electrification Administration**

**BULLETIN 1751H-501**

**SUBJECT: [Fundamentals of ISDN]{.underline}**

**TO:** Telephone Borrowers

REA Telephone Staff

**EFFECTIVE DATE:** Date of Approval

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

**OFFICE OF PRIMARY INTEREST:** Transmission Branch, Telecommunications
Staff Division

**PREVIOUS INSTRUCTIONS:** None - this bulletin is new information.

**FILING INSTRUCTIONS:** Place this document in Section 1751H,
Transmission Design Bulletins.

**PURPOSE:** To provide basic information on the Integrated Services
Digital Network, or ISDN. This includes theory, fundamentals, equipment,
applications and deployment. The document assumes some familiarity with
digital telecommunications systems but no prior exposure to ISDN.

\_\_\_\_\_\_\_ [Gary C. Byrne]{.underline}\_\_\_\_\_\_\_\_\_\_\_
\_\_\_\_[04/12/91]{.underline}\_\_\_

Administrator Date

**FUNDAMENTALS OF ISDN**

**TABLE OF CONTENTS**

1\. INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

2\. GENERAL OVERVIEW OF ISDN . . . . . . . . . . . . . . . . . . . . . 4

3\. ISDN IN DETAIL . . . . . . . . . . . . . . . . . . . . . . . . . . 6

3.1 Purpose . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

3.2 Bit Rates & Channel Structure . . . . . . . . . . . . . . . . 6

3.3 Clear Channel Capability . . . . . . . . . . . . . . . . . . . 7

3.4 2B1Q Transmission Line Code . . . . . . . . . . . . . . . . . 7

3.5 ISDN Architecture & Functional Groups . . . . . . . . . . . . 9

3.6 OSI Layered Architecture . . . . . . . . . . . . . . . . . . . 11

3.7 Powering . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

3.8 DSL Activation/Deactivation . . . . . . . . . . . . . . . . . 13

3.9 Sealing Current . . . . . . . . . . . . . . . . . . . . . . . 13

3.10 U-Interface Frame Structure . . . . . . . . . . . . . . . . . 13

4\. ISDN PROVISIONING . . . . . . . . . . . . . . . . . . . . . . . . .
15

4.1 Loop Limitations . . . . . . . . . . . . . . . . . . . . . . . 15

4.2 Loop Extension Options . . . . . . . . . . . . . . . . . . . . 15

4.3 Overlay Networks . . . . . . . . . . . . . . . . . . . . . . . 17

4.4 Interference to Other Loop Transmission Systems . . . . . . . 18

4.5 Central Office Equipment Considerations . . . . . . . . . . . 19

5\. ISDN APPLICATIONS . . . . . . . . . . . . . . . . . . . . . . . . .
20

6\. RECOMMENDATIONS . . . . . . . . . . . . . . . . . . . . . . . . . .
22

**LIST OF FIGURES & TABLES**

FIGURE 1 - ISDN 2B+D Overall Configuration . . . . . . . . . . . . . . 5
FIGURE 2 - Comparison of AMI & 2B1Q Line Codes . . . . . . . . . . . . 8

FIGURE 3 - ISDN Interface Reference Model . . . . . . . . . . . . . . .
9

FIGURE 4 - U-Interface Frame Structure . . . . . . . . . . . . . . . .
14

FIGURE 5 - Options for Deploying ISDN . . . . . . . . . . . . . . . . .
16

FIGURE 6 - Example of ISDN Overlay Network . . . . . . . . . . . . . .
17

FIGURE 7 - Maximum Power Spectrum Density of 2B1Q Signal . . . . . . .
18

FIGURE 8 - BRI ISDN Applications . . . . . . . . . . . . . . . . . . .
21

TABLE 1 - ISDN Channel & Bit Rates . . . . . . . . . . . . . . . . . . 6

TABLE 2 - 2B1Q Symbol Blocks and Pulse Voltages. . . . . . . . . . . . 8

TABLE 3 - OSI Seven Layer Model. . . . . . . . . . . . . . . . . . . .
12

TABLE 4 - BRI Loop Limitations . . . . . . . . . . . . . . . . . . . .
15

SELECTED BIBLIOGRAPHY . . . . . . . . . . . . . . . . . . . . . . . . .
24

**INDEX:**

Digital Transmission

Integrated Services Digital Network

ISDN

**ABBREVIATIONS & DEFINITIONS**

The principal ISDN abbreviations and terms used in this document are
defined below. Other abbreviations and definitions related more to
general digital transmission are included in \"Digital Transmission
Terminology,\" located in Bulletin 1751H-403, *Digital Transmission
Fundamentals*.

**2B1Q (2 binary 1 quaternary):** Digital line code having four
information states used to obtain up to a 160 kb/s bandwidth over
non-loaded, paired-wire facilities.

**2B+D:** ISDN 144 kb/s basic rate access of two 64 kb/s channels and
one 16 kb/s channel.

**23B+D:** ISDN 1.544 Mb/s primary rate access of 23-64 kb/s channels
for voice or data use, and one 64 kb/s signaling channel.

**ANSI (American National Standards Institute):** Principal U.S.
standards setting organization.

**B-channel:** Designation for the 64 kb/s information-carrying channel
in both the BRI and the PRI forms of the ISDN.

**BRI (basic rate interface):** The 2B+D form of ISDN that provides two
64 kb/s B-channels and one 16 kb/s D-channel to a subscriber over a
single loop facility.

**CCITT (International Telegraph and Telephone Consultative
Committee):** International telecommunications standards setting body.

**D-channel:** Designation for both the 16 kb/s and the 64 kb/s
signaling channels used in the BRI and PRI forms, respectively, of ISDN.

**DLC (digital loop carrier):** Device that digitally encodes and
multiplexes subscriber loop channels for more efficient transmission and
greater range from the central office.

**DSL (digital subscriber line):** A metallic subscriber loop over which
full-duplex voice and/or data is transmitted in digital format.

**ET (exchange termination):** Termination point located within the
central office where a DSL is connected to the switching network.

**ISDN:** Abbreviation for the Integrated Services Digital Network.

**LT (line termination):** Physical termination point of a DSL located
at the line frame of a central office switch, DLC or RT.

**NT (network termination):** General term used to refer to equipment
that terminates a subscriber line at the customer\'s premises.

**NT1 (network termination type 1):** Type of subscriber termination
that provides for the basic electrical/mechanical interface (OSI
Layer 1) between customer equipment and the DSL.

**NT2 (network termination type 2):** Type of subscriber termination
that provides for both the basic electrical/mechanical interface (OSI
Layer 1) and higher level OSI protocols between customer equipment and
the DSL.

**OSI (open systems interconnection):** Structured architecture designed
for information exchange between different communication or data
systems.

**PRI (primary rate interface):** The 23B+D form of ISDN access giving
the user 23-64 kb/s B-channels and one 64 kb/s D-channel for a total
bandwidth of 1.544 Mb/s.

**R-interface:** ISDN subscriber reference point located between the TE2
and a TA.

**RT (remote terminal):** Remotely-located terminal of a subscriber
carrier or concentrator system that is connected to the central office.

**S-interface:** ISDN reference point located at a subscriber\'s
premises between an NT2 and a TA; or, between an NT2 and a TE1.

**TA (terminal adapter)** Equipment that converts standard computer or
voice electrical interfaces to the ISDN S-interface or the T-interface.

**TE1 (terminal equipment type 1):** Voice or data terminal equipment
with the internal capability to directly connect without conversion to
the ISDN S-interface or the T-interface.

**TE2 (terminal equipment type 2):** Voice or data terminal equipment
that requires the use of a terminal adapter (TA) to connect to the ISDN
S-interface or the T-interface.

**T-interface:** ISDN subscriber reference point located between an NT1
and a NT2; or between an NT1 and a TE1.

**U-interface:** ISDN subscriber reference point located between the
network DSL and the NT.

**V-interface:** ISDN network reference point located at the CO between
the LT and the ET.

**1. INTRODUCTION**

**1.1** This bulletin provides REA borrowers, consulting engineers and
other interested parties with technical information on the fundamentals
of the Integrated Services Digital Network, generally referred to as
ISDN. Much of the information contained in this document is of a
descriptive nature which is intended to provide the reader with a basic
overview of the subject. The emphasis is on the Basic Rate Interface
(BRI) form of ISDN, known also as 2B+D. Some information is presented on
the Primary Rate Interface (PRI), referred to as 23B+D, for comparative
purposes. However, BRI is given considerably more treatment than PRI
because of its higher likelihood for loop applications, particularly in
rural environments. The key concepts of BRI ISDN are covered from both a
network and customer equipment perspective. Although customer equipment
is an item generally no longer directly provided by telephone companies,
the complexity of an ISDN network requires some understanding of what
lies beyond the subscriber side of the telephone company/customer
interface.

**1.2** The provisioning of BRI in the loop plant presents some
important challenges to local exchange carriers (LECs), such as reducing
loop length and eliminating loaded plant. In particular, proper
consideration must be given to existing and future analog station
carrier systems because of the potential for interference caused by the
ISDN BRI signal. Means of deploying ISDN through various loop
architectures are presented with a brief discussion of central office
equipment (COE) considerations. Some possible applications of ISDN for
different customer services are addressed. Recommendations for future
loop planning are offered in the final section.

**2. GENERAL OVERVIEW OF ISDN**

**2.1** ISDN is a recently developed method for communicating both data
and voice information over the public-switched telephone network. Using
ISDN, both data and voice information can be integrated and transmitted
end-to-end in digital, not analog, format. Unnecessary analog to digital
conversions are avoided. An intelligent overlay network provides
out-of-band signaling, performance monitoring and maintenance
capabilities. Moreover, ISDN provides for a set of common interfaces
between the network and the end user.

**2.2** In its most basic format, the digitized voice and data signals
are multiplexed into a single bitstream and then transmitted over a
single pair of wires to the telephone central office (CO) switch at a
rate of 160 kb/s. The facility over which this information is
transmitted is termed a digital subscriber line, or DSL. The 160 kb/s
bandwidth results from two 64 kb/s B-channels, one 16 kb/s D-channel and
16 kb/s of network overhead. At the CO, the two B-channels and the
D-channel are then separated and switched to their intended
destinations. Any of the two B-channels can be either circuit-switched
or packet-switched, depending upon customer requirements and network
capabilities; the D-channel can be only packet-switched. Figure 1
illustrates these concepts.

**2.3** ISDN at the 160 kb/s BRI rate could typically be used in a home
or small business to connect a single line with three customer devices -
a computer terminal, a FAX machine and a telephone, for example. ISDN in
this configuration provides multiple benefits: 1) a universal interface
that provides the capability for simultaneous operation of all three
devices over a single telephone line; 2) a corresponding subscriber rate
savings compared to purchase of three, separate lines; 3) the higher
quality of digital transmission as compared to analog; and 4) a
significant enhancement in bandwidth over what is available on an analog
loop. ISDN in this application is serving, in effect, as simply a
digital multiplexer.

**FIGURE 1**

**ISDN 2B+D BASIC CONFIGURATION**

(AVAILABLE ON HARD COPY ONLY.)

**2.4** The higher bandwidth form of ISDN, the PRI, operates at an
overall rate of 1.544 Mb/s. It contains 23 B-channels for information,
one D-channel for signaling and 8 kb/s of overhead for framing and
maintenance. Both the B and D-channels are 64 kb/s each. The PRI
transmission rate has greater capacity and would typically be used to
connect with PBXs or digital multiplexers which have the capability to
process both voice and data traffic from many lines.

**2.5** As with the lower rate BRI, the PRI would also offer the
benefits of integrated voice and data digital transmission. ISDN
applications in the PRI format by business customers extend into several
additional areas. These include provisioning of ANI (automatic number
identification) on signaling channels separate from those carrying
incoming calls so that the identity of the caller is immediately known
at the terminating end. This feature could be used to speed inquiries
and orders in, for example, a catalog order firm. Other uses would
include access to LANs and PC-to-PC communications, using both packet
and circuit-switched networks.

**3. ISDN IN DETAIL**

**3.1 Purpose**

**3.1.1** The purpose of the section is to provide further descriptive
detail on the various aspects of ISDN, beyond the overview presented in
Section 2. This section is organized around ten separate topics that
cover bit rates, channel structure, line codes, functional groups, etc.
ISDN provisioning and applications are addressed in later sections.

**3.2 Bit Rates** **& Channel Structure**

**3.2.1** ISDN operates at two different bit rates depending upon
whether one is referring to BRI or PRI ISDN. The BRI operates at the
lower digital transmission rate of 160 kb/s. This is a significantly
higher bit rate than what is typically used for digital transmission of
a single voice channel at the standard rate of 64 kb/s, or digital
signal level zero (DS0). The transmission rate of 160 kb/s coupled with
use of the 2B1Q line code results in a symbol or signaling rate of 80
kilobaud. (Line codes are discussed in further detail in Section 3.4.)

**3.2.2** Although 160 kb/s is the rate at which the BRI is furnished
over the local loop facilities, only 144 kb/s is available for the
customer \"payload.\" The remaining 16 kb/s of overhead is used for
telephone company network operating purposes, such as performance
monitoring, framing, provisioning and timing. This 16 kb/s channel is
sometimes referred to as the maintenance or M-channel. BRI ISDN can
accommodate two full duplex 64 kb/s channels (referred to in ISDN
parlance as Bearer or B-channels) and one 16 kb/s Delta, or D-channel,
used for customer signaling and data.

**3.2.3** The PRI ISDN operates at an overall bit rate of 1.544 Mb/s. It
contains 23 B-channels and one D-channel. The B-channels can either be
circuit switched voice or data. The D-channel is limited to signaling
purposes and packet-switched data. Of the 1.544 Mb/s transmission rate,
only 1.536 Mb/s is available for customer use or payload. The remaining
8 kb/s is used for network overhead, such as framing and maintenance.
The characteristics of both the BRI and PRI ISDNs are summarized in
Table 1.

**TABLE 1**

**ISDN CHANNEL & BIT RATES**

(AVAILABLE ON HARD COPY ONLY.)

**3.3 Clear Channel Capability**

**3.3.1** Both the BRI and the PRI forms of ISDN are designed for an
important capability for data communications. This is clear channel
capability. Clear channel capability refers to the ability to use the
full 64 kb/s bandwidth of a DS0 channel entirely for customer payload
without any restrictions on the ones or zeros content of data.
Traditionally, DS0 channels provided less than 64 kb/s bandwidth to the
user because of the need to maintain a minimum density of ones for
timing and synchronization purposes. Furthermore, the least significant
bit in every sixth frame was also robbed and used for signaling
purposes. The DS0 channel capacity was effectively narrowed from a data
perspective by 8 kb/s to 56 kb/s because of these two requirements.
While this limitation is not a problem for digitally-encoded voice
signals, it presents undesirable restrictions with digital data
transmission.

**3.3.2** The BRI ISDN concept consists of clear capability for all
three of the 2B+D channels. To satisfy signaling requirements, it
includes a separate signaling channel (the D-channel). This eliminates
the need to perform bit robbing for in-band signaling purposes. The
minimum ones density requirements can be addressed by using a digital
zero substitution code on interoffice transmission equipment. With a
code such as Binary 8 Zero Substitution (B8ZS) or the lesser-used Zero
Byte Time Slot Interchange (ZBTSI), timing can be properly maintained by
coding and subsequently decoding successive strings of zeroes. This
manipulation of the bitstream is transparent to the user and does not
restrict any use of the full 64 kb/s channel. Continuous strings of
zeros can, therefore, be reliably transmitted. Although clear channel
capability from an exchange perspective will be available when BRI ISDN
is deployed on a loop basis, the initial implementation of ISDN on
certain *interoffice* networks may not result in 64 kb/s capability
until SS7 is furnished and older terminal equipment has been replaced or
modified.

**3.4 2B1Q Transmission Line Code**

**3.4.1** A line code is the numeric scheme whereby digital information
is converted into pulses for transmission. A variety of line codes have
been developed for this purpose. The most common is the Alternate Mark
Inversion (AMI), which is used in digital T-1 carrier transmission. AMI
simply alternates the pulse polarity from positive to negative for every
digital one; zeroes are represented by a zero voltage level. Line codes
are designed to optimize the transmission of digital information over a
particular type of facility. Some considerations involved in selecting a
line code are spectral energy, near end crosstalk (NEXT), echo noise,
impulse noise, intersymbol interference and quantizing noise.

**3.4.2** The line code that has been selected for transmission of the
BRI ISDN signal in the U.S. is known as 2B1Q, or 2 Binary 1 Quaternary.
2B1Q means that two bits of digital information are represented by a
four-level, or quaternary, line signal. Digital information *is not*
represented on a single bit basis, as with AMI, but in blocks of two
bits. Because the block code has four possible logical states, the two
bit blocks are known as quats. The 2B1Q code was standardized in the
U.S. by the American National Standards Institute (ANSI) standard for
telecommunications.[^1] This quaternary line code is compared
graphically to the AMI code for a given 10-bit bitstream in Figure 2.
Table 2 shows the 2B1Q code in terms of bits, blocks and pulse voltages.

**FIGURE 2**

**COMPARISON OF AMI & 2B1Q LINE CODES**

(AVAILABLE ON HARD COPY ONLY)

**TABLE 2**

**2B1Q SYMBOL BLOCKS & PULSE VOLTAGES**

(AVAILABLE ON HARD COPY ONLY)

**3.4.3** Prior to the adoption of an American standard line code for
transmission of the BRI over a digital subscriber line (DSL), a variety
of line codes were evaluated for use. 2B1Q was found to be superior in
meeting the requirements of typical loops in North America. Other signal
processing techniques are used along with this code to further enhance
digital transmission over loop facilities. They include scrambling, echo
cancellation and adaptive equalization.

**3.4.4** The standardized line code for BRI ISDN has its peak spectral
energy at an approximate frequency of 40 kHz, as compared to 80 kHz for
an AMI code. The lower frequency, or spectral components, minimize loss
over the largely inductive loop plant. The baud rate (or symbol rate) of
2B1Q is 80 kilobaud. This results from the use of one symbol per two
bits of data times the 160 kb/s bit rate (1/2 x 160 kb/s). The use of
2B1Q over copper loop facilities is designed to produce a bit error rate
(BER) of less than 10-7.

**3.5 ISDN Architecture & Functional Groups**

**3.5.1** BRI ISDN consists of a number of reference points and
functional groups between the CO and the subscriber\'s terminal
equipment that are used for information transfer over the network. The
functional groups are used to define the specific functions that are
performed between each reference or access point. The reference points,
located between functional groups, are where the ISDN signals are
defined. This section will describe the principal components of this
network, their functions and their respective reference points. The
discussion will center around Figure 3.

**FIGURE 3**

**ISDN INTERFACE REFERENCE MODEL**

(AVAILABLE ON HARD COPY ONLY)

**3.5.2** As a general overview of the BRI interface, the two-wire DSL
operates bidirectionally between the CO and the subscriber\'s premises
at a rate of 160 kb/s. At the subscriber end, a network termination (NT)
device converts the two-wire facility to four-wire to provide a passive
bus-type connection to subscriber terminal equipment.

**3.5.3** More specifically and as illustrated in Figure 3, a BRI ISDN
begins at the CO where the DSL is affixed to the Line Termination (LT).
The LT is essentially equivalent to the line card of the switch.[^2] The
actual termination of a DSL at the CO end is defined at the Exchange
Termination (ET), which is the connection to the switching network. It
is at this point that access to either packet-switched or
circuit-switched networks is provided. The reference point between the
LT and the ET (physically located at the backplane of the CO or RT line
card frame) is known as the V-interface, shown at the extreme left of
Figure 3.

**3.5.4** At the CO, the LT contains a master digital transceiver that
is largely a mirror image of a slave digital transceiver located at the
subscriber\'s end of the DSL. The subscriber\'s transceiver is contained
within the NT. Both the LT and NT transceivers contain multiplexers and
demultiplexers to combine and separate the 2B+D channels. The digital
transceivers also perform a number of important signal processing
capabilities to ensure accurate transmission of the digital signal over
loop facilities.

**3.5.5** To deal with reflections at cable plant impedance
discontinuities (such as bridged taps or gauge changes) and to provide
for bidirectional transmission, the receivers contain a hybrid with echo
canceling capability. This feature removes echoes and crosstalk by
comparing the transmitted and received bitstreams and canceling
undesirable echoed signals, resulting in the overall elimination of the
undesirable components and restoration of the original signal.
Additionally, the receivers contain a post adaptive equalizer that
assesses the impedance characteristics of different loop facilities and
adjusts for the expected loop distortion factors. It then reshapes the
incoming digital pulses to their original form based upon the expected
distortion. Scrambling (temporary rearrangement of a bitstream to avoid
repetitive patterns) is also employed as a further measure to reduce
crosstalk within the paired cable. In summary, several relatively
sophisticated signal processing techniques are used to ensure accurate
delivery of the 160 kb/s signal.

**3.5.6** Figure 3 shows the DSL extending from the CO to the
subscriber\'s premises and terminating at the NT. The interface or
reference point between the DSL and the NT is termed the U-interface.
Physically, it is located by mutual agreement between the LEC and the
subscriber.

**3.5.7** The basic rate ISDN reference model provides for the division
of the NT into two separate functional components, known as NT1 and NT2.
(NT can refer to an NT1 or an NT2, or both.) An NT1 provides minimal
functionality by establishing and maintaining an OSI (Open Systems
Interconnection) Layer 1 connection, which involves only the basic
physical/electrical connection and related communication protocols
between each end of the DSL. An NT2 is optional and is used only when
higher layer OSI functions (Layers 1, 2 and 3) are required. (OSI Layers
are discussed in the following section, 3.6.) Only NT1s will be used in
most BRI applications; NT2s will see use in higher capacity
installations, particularly where the higher bandwidth PRI form of ISDN
is required.

**3.5.8** The NT1 is similar to network channel terminating equipment,
or NCTE. As previously mentioned, it is a slave transceiver which
operates under the control of the LT master transceiver located at the
CO. The NT1 performs the conversion from the U-interface to the
T-interface, which is used only for direct connection to subscriber or
customer premises equipment (CPE) that is ISDN-compatible. The
T-interface is a 4-wire interface and operates at a bit rate of 192 kb/s
in each direction to facilitate the connection of multiple CPE devices
in a bus configuration.

**3.5.9** The reference point between the NT1 and the optional NT2 is
also termed the T-interface. The NT2 interfaces directly to ISDN
terminal equipment. This interface is known as the S-interface. It is
essentially equivalent to the four wire T-interface. Because of the
commonalties between the S and T-interfaces, sometimes both reference
points are collectively termed the S/T-interface. NT2 devices can
include appropriately-equipped PBXs, LANs (local area networks), ACDs
(automatic call distributors), multiplexers and terminal controllers.

**3.5.10** ISDN-compatible CPE is referred to as Terminal Equipment Type
1, or TE1. Non ISDN-compatible equipment is known as Terminal Equipment
Type 2, or TE2. If TE2 is to be used with ISDN, it requires a conversion
between the CPE and the T-interface of the NT1. This conversion is
performed by a device known as Terminal Adapter, or TA. TAs can either
be standalone devices or add-on component boards for existing data or
voice terminal equipment. Certain TAs have the capability to transform,
for example, an IEEE RS-232 data port of a personal computer and a
standard two-wire voice frequency interface into the four-wire 192 kb/s
T-interface for direct connection to the NT1. The interface point
between the TA and the TE2 is referred to as to the R-interface.

**3.5.11** In terms of ownership, it is expected that an LEC would
provide the DSL network facilities up to the U-interface, *excluding*
the NT1. The subscriber would be responsible for providing and
maintaining the NT1 and the NT2 (if needed), any TAs and the TE1 or TE2
CPE devices.

**3.6 OSI Layer Protocols**

**3.6.1** The OSI or Open Systems Interconnection reference model is
integral to the information transfer aspects of ISDN. This section will
generally describe the OSI structure and its role for ISDN.

**3.6.2** The OSI model was developed by the International Standards
Organization (ISO) to provide a common, yet flexible, framework to
facilitate the transfer of message, signaling and management information
in different data communications systems. It has since been adopted for
use in telecommunications.

**3.6.3** The basis for the OSI model is a seven layer hierarchy of
information transfer operations. Each layer is defined in terms of its
specific functions. The means of providing these functions within each
layer are not specifically defined; they are left up to individual
designers or users. The important aspect with respect to OSI
standardization is that the defined functions are performed by each
layer and that the proper information is passed between layers. This
flexibility is the essence of the OSI model and is what makes it
desirable from an information transfer perspective among different
equipment or systems. Table 3 shows the OSI model and briefly describes
the function of each of the seven layers.

**TABLE 3**

**OSI SEVEN LAYER ARCHITECTURAL MODEL**

(AVAILABLE ON HARD COPY ONLY.)

**3.6.4** Each OSI layer uses the services and information provided by
the lower layers in conjunction with its own functions; it then provides
further processing and furnishes information through its upper interface
to a higher level layer, where it is used to perform that layer\'s
functions.

**3.6.5** For the BRI ISDN, the network provides, at a maximum, only the
three lowest layers of the OSI model. These layers (numbered 1 through
3) are referred to as the lower layer functions; layers 4 through 7 are
referred to as the higher layer functions. Most BRI applications will
only use the Layer 1 services.

**3.7 Powering**

**3.7.1** In theory, power for BRI ISDN service to the NT can be
provided either over the network or locally by the subscriber. It is
anticipated, however, that in the U.S. the NT end of a DSL will be
locally powered using commercial power sources with a battery backup,
both provided by the subscriber. A low power consumption mode at the NT
is provided internationally through CCITT (International Telegraph and
Telephone Consultative Committee) Recommendation I.430 (ISDN
User-Network Interfaces: Layer 1). But due to an apparent industry
preference for customer powering in the U.S., this feature was not
adopted in ANSI T1.605.[^3]

**3.8 DSL Activation/Deactivation**

**3.8.1** A DSL may be operated in one of two modes: continuous
activation or activation upon call request. In the U.S., it is expected
that most DSL facilities will be continuously activated from time of
system turn-up. The reasoning is that the expected delay and complexity
involved in startup on a request basis would be unsatisfactory to a
subscriber, in spite of the potential NT power consumption savings that
could result from the deactivated mode.

**3.9 Sealing Current**

**3.9.1** In standard analog, circuit-switched POTS (plain old telephone
service) loops, a steady state DC current flows through the subscriber
pair when initiated by an off-hook action at the CPE. The current is
used for signaling purposes (to detect an off-hook condition by the CO
switch) and to provide a limited amount of network-supplied power to
operate CPE. A side benefit of this current flow is that it causes the
DC resistance of the loop to be minimized at splice joints. Without
occasional current flow, these joints tend to oxidize producing a
significant increase in loop resistance, leading to resistive
unbalances. This can cause increased circuit noise. In fact, for special
service circuits not requiring DC current for signaling, a common
practice is to apply a DC current around 10 milliamperes to ensure
minimum resistance through the splice points in the loop. This is called
a sealing current.

**3.9.2** With BRI ISDN, signaling is performed digitally over the
D-channel and, in general, all powering will be local. As a result, no
DC current flow will occur in the DSL and, therefore, splicing
resistance will not be minimized. This will be a problem for long loops
in particular. No specific solution has yet been developed where loop
resistance becomes excessive. (See Section 4.1 for loop resistance
recommendations.) One option is to use a DC termination on the NT1 that
will permit a continuous loop current to be supplied from the LT at the
CO. However, this is not desirable from a CO battery capacity and energy
cost perspective. Application of a pulsed DC current from the CO is a
second solution since splice resistance drops rapidly following a
current pulse and maintains its minimum value for some time following
the termination of current flow.

**3.10 U-Interface Frame Structure**

**3.10.1** The digital bitstream between the CO and the subscriber for
the BRI U-interface has a rate of 160 kb/s and uses the 2B1Q line code,
previously described in Section 3.4. The organization or frame structure
for this segment of an ISDN connection is based upon a frame that
contains 240 bits, or 120 quaternary symbols[^4] (quats). This frame
passes in a nominal time period of 1.5 milliseconds (ms), giving the 160
kb/s bit rate. The frames are further organized into superframes, which
each contain eight standard frames. The superframes are 12 ms in
duration. The frame and superframe are depicted in Figure 4.

**FIGURE 4**

**U-INTERFACE FRAME STRUCTURE**

(AVAILABLE ON HARD COPY ONLY.)

**3.10.2** As Figure 4 shows, each frame contains 9 quats for
synchronization, 108 quats for the 2B+D user data and 3 quats for
maintenance or overhead. The overhead bits are sometimes referred to as
M bits; they are organized on a superframe basis, giving 24 quats or 48
bits per superframe for a variety of functions. These include: an
embedded operations channel[^5] (EOC) at 2 kb/s that utilizes 24 bits, a
cyclic redundancy check[^6] (CRC) using 12 bits, and far end block error
(FEBE) detection by the CRC using one bit. The remaining eleven bits
within each superframe are for miscellaneous overhead and status
functions (such as powering and activation); some are reserved for
future overhead use.

**4. ISDN PROVISIONING**

**4.1 Loop Limitations**

**4.1.1** Because of the relatively high bit rate used by the BRI for
loop purposes, loop limitations have been developed to ensure the
integrity of the transmitted data pulses. These requirements[^7] were
designed so that approximately 99% of the twisted pair, *non-loaded*
loops in North America could be used for BRI ISDN provisioning without
modification. Loaded loops do not comply because they do not permit
adequate transmission of the digital signal. This is because the load
coils act as low pass filters having an upper cutoff around 3 kHz,
significantly attenuating the 2B1Q signal, whose spectral energy peaks
near 40 kHz. The principal loop specifications are noted in Table 4.

**TABLE 4**

**BRI LOOP LIMITATIONS**

(AVAILABLE ON HARD COPY ONLY.)

**4.2 Loop Extension Options**

**4.2.1** As Table 4 indicates, the ISDN BRI can only be reliably
offered on metallic loops with a nominal length of 5.5 km (18 kf) or
less and without loading coils. Because many rural areas contain
subscribers outside this region, additional methods must be employed to
satisfactorily offer ISDN at a greater range. As with analog loops,
electronic devices can also be used with ISDN to permit significant
extension of a loop\'s range. Several options are depicted in Figure 5.

**4.2.2** One option in Figure 5 includes digital line repeaters that
would regenerate the 2B1Q signal over the DSL, similar in function to T1
span line equipment. These are technically feasible and are not
precluded by ISDN technology plans. However, it is not expected that
they will be

**FIGURE 5**

**OPTIONS FOR DEPLOYING ISDN**

(AVAILABLE ON HARD COPY ONLY.)

widely used by LECs, except in applications where economics suggest that
they are the only viable alternative. DSL repeaters could also be used
by certain subscribers as CPE to extend the range from the U-interface
to the NT1 in campus-type environments where the plant is owned and
maintained by the customer. Consequently, the deployment of electronic
devices such as digital loop carrier (DLC), concentrator or remote
terminals (RTs) is viewed as the primary option available, where
economically justified, to facilitate widespread provisioning of BRI
ISDN. It should also be noted that the transmission systems (whether
T-carrier or lightwave) used to connect the DLCs or RTs to the CO will
require 3 DS0 channels *per DSL* to provide for transport of the entire
2B+D signal.

**4.2.3** Electronic devices used to extend the range of the 2B1Q signal
cannot be of the same type used in conventional analog plant; they must
be ISDN-compatible and be designed to operate and interface with an
ISDN-compatible CO switch. DLC and RT plant of this type are beginning
to emerge now that ISDN standards are becoming finalized.

**4.3 Overlay Networks**

**4.3.1** As ISDN is initially provisioned, many exchanges (particularly
smaller, rural exchanges) will not have a sufficient number of
subscribers to justify the upgrade of the CO equipment and the outside
plant facilities to ISDN compatibility within that exchange. For
example, the CO switch technology serving that exchange may not even
have the capability to be equipped to offer ISDN, and switch replacement
at that time would cause a significant economic penalty. Or, the small
number of subscribers desiring ISDN may not justify the hardware and
software additions that are necessary for an ISDN-capable switch. In
spite of these considerations, options exist that would permit BRI ISDN
to be deployed in these situation. Figure 6 depicts an option known as
overlay networks.

**FIGURE 6**

**EXAMPLE OF ISDN OVERLAY NETWORK**

(AVAILABLE ON HARD COPY ONLY.)

**4.3.2** Figure 6 shows the deployment of ISDN through the extension of
facilities from an IDSN-equipped exchange to serve portions of a nearby,
*non* ISDN-equipped exchange. The use of either DLCs or RTs served from
the ISDN exchange makes this deployment possible. This solution is
similar to that utilized for DSL loop extension where ISDN facilities
are needed beyond their maximum metallic range, as discussed in the
section 4.2.

**4.3.3** The overlay concept may even be extended one step further. If
the 5.5 km maximum loop limitation does not permit reaching the desired
subscriber locations from the overlay network as in Figure 6, electronic
loop extension may be employed as previously discussed. This generally
requires that the RT and its associated DLC both have ISDN capabilities.

**4.4 Interference to Other Loop Transmission Systems**

**4.4.1** The 2B1Q signal used for transmission of the BRI ISDN is of a
relatively high power level and broad bandwidth. Figure 7 shows the
maximum spectral parameters, in terms of dBm per Hz, as defined by
current standards. While this signal is not expected to cause
interference to voice frequency signals, it may cause problems with
analog station carrier systems in certain applications. This is due to
the overlap and interaction of the station carrier frequencies with that
of the frequencies resulting from use of the DSL line code.

**FIGURE 7**

**MAXIMUM POWER SPECTRUM DENSITY OF 2B1Q SIGNAL**

(AVAILABLE ON HARD COPY ONLY.)

**4.4.2** Analog station carrier systems typically operate in the
frequency band range from 8 kHz to 160 kHz. The peak energy of the 2B1Q
signal is at 40 kHz with the upper reaches of its fundamental and
harmonic components extending beyond 40 kHz. With the information
provided in Figure 7, over a 6 kHz bandwidth (typical for station
carrier) it can be calculated that the maximum power from the 2B1Q
signal can be as high as approximately +8 dBm, or roughly 6
milliwatts.[^8] Crosstalk coupling (specifically, NEXT), if both systems
are contained within the same cable at sufficient proximities, could
cause the high output 2B1Q signal to interfere with the lower level
receive signal of the station carrier.

**4.4.3** Preliminary calculations suggest that placement of both the
analog carrier and the ISDN 2B1Q systems within the same small cable, or
within the same cable binder groups of larger cables, will likely result
in intolerable noise conditions on the station carrier. Depending on
cable size and NEXT characteristics, placement of the systems in
non-adjacent binder groups of larger cables may provide acceptable
operating conditions. Use of screened cable would likely present no
problems with noise interference to the station carrier.

**4.4.4** While laboratory testing is complete, practical operating
experience has yet to be gained to learn the exact extent and under what
conditions that DSL and station carrier systems can co-exist within a
single cable. Certain newer analog carrier systems may be compatible
with the 2B1Q signal. As BRI ISDN, or equivalent digital loop
transmission schemes, begin to be deployed on rural networks, more
information will become available and it is expected that more specific
operating recommendations will then be furnished.

**4.5 Central Office Equipment Considerations**

**4.5.1** In addition to numerous transmission considerations, provision
of BRI ISDN within an exchange or service area requires that significant
enhancements be made at the CO switch. This section will briefly
highlight BRI ISDN from a switching systems perspective in order to
round out the discussion. It should be noted that differences exist
between the BRI and PRI forms of ISDN in terms of the needed CO
additions and configurations. Details on the central office aspects of
ISDN provisioning will be left to other, more CO-specific documents.

**4.5.2** To offer ISDN CO switch capability, several additions are
required. First, the switch must be digital and, from a software
perspective, the generic software residing in the switch must be
replaced or upgraded to one having BRI ISDN capability. Some, but not
all, CO vendors currently offer BRI ISDN software and hardware upgrades,
or have announced release dates for such products in the near future.

**4.5.3** Secondly, the analog line equipment serving those subscribers
desiring ISDN must be replaced with those designed for 2B1Q operation
and other functions needed for DSL operation. Fundamental differences
exist between the standard analog line card and the DSL card in terms of
signaling, supervision, battery, test, etc. Some switch vendors have
previously produced \"ISDN\" line cards that employed a nonstandard and
proprietary line code known as alternate mark inversion, or AMI. Since
that time, the 2B1Q code has been officially adopted in the U.S. as the
standard basic rate line code through ANSI Telecommunications Standard
T1.601-1988. AMI-type line cards are not compatible with the 2B1Q
standard used on NT (network termination) equipment.

**4.5.4** Because the BRI ISDN contains the ability to access either
packet-switched or circuit-switched networks, capability for this
feature needs to be provided. Hardware additions such as a packet
switching unit or packet handler are integral to these functions.

**4.5.5** If it is desired to offer ISDN services beyond the exchange in
which the ISDN subscribers reside, then Signaling System #7 (SS7) also
will need to be installed at the ISDN serving office. This is necessary
because ISDN operates on a common channel signaling basis whereby
signaling information for trunking purposes is transmitted on separate
channels, unlike conventional trunks that utilize in-band signaling.
Implementation of SS7 will additionally require compatible software,
hardware and SS7 communication links to other switching offices.

**5. ISDN APPLICATIONS**

**5.1** This section will briefly describe some potential applications
for which BRI ISDN technology may be used. Although these applications
can be largely characterized as being the responsibility of an ISDN
subscriber, a general understanding of basic subscriber uses is
considered beneficial to the network provider.

**5.2** As discussed in Section 2, the BRI form of ISDN can be portrayed
as simply a form of digital multiplexing, or pair gain. This view will
likely be shared by many ISDN subscribers in response to this new
transport technology. They will see an ISDN DSL as an opportunity to
simultaneously connect to a standardized interface and operate several
data or voice communication devices with only one telephone line.
Subscribers will also expect to realize economic benefits commensurate
with the use of this single DSL as compared to several, separate voice
and data subscriber lines.

**5.3** The specific characteristics of a subscriber\'s application
will, in large part, depend upon to what extent the communication
devices or CPE and their associated communications software are directly
ISDN-compatible. If they are not, use of TAs will be required. CPE that
is compatible may be

connected directly to the 4-wire S/T-interface. Non ISDN-compatible
devices must be connected using TAs, which provide an interface from
standard data or voice outputs to the S/T-interface. The configurations
in Figure 8 show two possible applications with voice, facsimile and
personal computer terminal equipment. The uppermost figure depicts a
single TA having both voice and data inputs, thereby facilitating the
use of the two non-ISDN terminals.

**FIGURE 8**

**BRI ISDN APPLICATIONS**

(AVAILABLE ON HARD COPY ONLY.)

**6. RECOMMENDATIONS**

**6.1** Subscriber demand for BRI ISDN at this time appears to be rather
limited. Although several dozen, well publicized trials are currently
underway nationally, many of these trials are with large organizations
using PRI ISDN. It is particularly difficult to determine when BRI ISDN
may see interest, application and growth in rural areas. In spite of
this uncertainty, many REA borrowers may want to consider planning for
their loop networks and CO equipment now for the capability of providing
BRI ISDN to their subscribers in the future.

**6.2** Because of the strict loop limitations over which ISDN operates,
minimizing loop length in anticipation of BRI offerings is a primary
consideration. The desire to be well positioned for future ISDN
provisioning should, of course, be balanced with the economic
considerations that may accompany such positioning. Positioning now for
BRI ISDN could translate into increased up-front costs in certain
instances, although in some cases this cost penalty may be small. For
example, DLC or RT installations can now be used to minimize loop
lengths in preparation for future ISDN deployment. These designs should
be compared to the traditional all copper physical loops to determine
cost differences. In any event, each situation must be individually
assessed. It is possible that higher up-front costs initially can
ultimately translate into lower life-cycle costs.

**6.3** The following loop plant recommendations are offered to those
LECs wishing to plan for ultimate provisioning of BRI ISDN.

> 1\) **Adopt a maximum nominal loop length** **[planning
> goal]{.underline} of 5.5 km (18 kf).**
>
> *Note - the nominal 5.5 km maximum length is affected by gauge and
> bridged tap considerations. The determining factors are the maximum
> resistance and maximum loss values as shown in Table 4.*
>
> Where currently economically justified, strategically place DLC/RT
> units to permit loops to meet this length limitation. Where placement
> of DLC/RTs cannot be currently justified, locate serving area
> interface (SAI) splice points so that *future* placement of DLC/RTs
> will permit loops to meet this maximum length requirement.
>
> 2\) **Strive to eventually eliminate all loaded loops**.
>
> Limit use of load coils by using the methods described in 1), above.
>
> 3\) **Strive to eventually eliminate all loop extension equipment**.
>
> Limit maximum DC loop resistance to 1300 ohms by using the methods
> described in 1), above.
>
> 4\) **Minimize use and length of bridged taps.**
>
> Bridged taps should be avoided to the maximum practical extent. Where
> elimination is not possible, bridged taps should be limited to a
> maximum of 1.5 km, total.
>
> 5\) **Deploy any new analog station carrier systems with discretion.**
>
> Future provision of BRI ISDN could either render this plant obsolete
> and/or create operational problems if the carrier system is not
> designed to be compatible with the 2B1Q signal.
>
> **SELECTED BIBLIOGRAPHY**

1\) American National Standards Institute (ANSI), *Integrated Services
Digital Network (ISDN) - Basic Access Interface for Use on Metallic
Loops for Application on the Network Side of the NT (Layer 1
Specification)*, ANSI T1.601-1988.

2\) American National Standards Institute (ANSI), *Integrated Services
Digital Network (ISDN) - Basic Access Interface for S and T Reference
Points (Layer 1 Specification)*, ANSI T1.605-1989.

3\) Bell Communications Research, *ISDN Basic Access, Transport Systems
Requirements*, TR-TSY-000397, Issue 1, October 1988.

4\) Bell Communications Research, *ISDN Basic Access, Digital Subscriber
Lines*, TR-TSY-000393, May 1988.

5\) CCITT Red Book, Volume III, Fascicle III.5, Series I
Recommendations, *Integrated Services Digital Network (ISDN)*,
International Telecommunications Union, Geneva, 1985.

6\) \"Line Codes for Digital Subscriber Lines,\" Joseph W. Lechleider,
*IEEE Communications Magazine*, September 1989.

7\) *The Status and Evolution of ISDN in the United States*, James L.
Neigh, Proceedings of the International Switching Symposium, May-June,
1990, Vol. II, p. 45, Stockholm, Sweden.

[^1]: ^1^ ANSI Standard No. T1.601-1988, *ISDN Basic Access Interface
    for Use on Metallic Loops for Application to the Network Side of the
    NT*.

[^2]: ^2^ The LT may also be located at any form of remote terminal (RT)
    extension of the CO, such as a digital loop carrier system,
    concentrator terminal or remote switching terminal.

[^3]: ^3^ ANSI Standard No. T1.605-1989, *ISDN Basic Access Interface
    for S and T Reference Points (Layer 1 Specification).*

[^4]: ^4^ Each quaternary symbol, or quat, for the 2B1Q line code (i.e.
    +3, +1, -1 and -3) consists of two bits, as shown in Table 2.

[^5]: ^5^ The embedded operations channel (EOC), which is part of the 16
    kb/s M-channel, supports communication needs between the LT at the
    CO and the NT at the subscriber.

[^6]: ^6^ The cyclic redundancy check (CRC) is an error checking
    procedure that performs mathematical calculations on transmitted and
    received signals.

[^7]: ^7^ As specified in ANSI Telecommunications Standard ANSI
    T1.601-1988.

[^8]: ^?^ -30 dBm equals 10-6 watts/Hz. Over a 6 kHz spectrum the
    composite energy is 6000 Hz x .001 milliwatts per Hz, equaling 6 mW.
    This equates to roughly +8 dBm.