UNITED STATES DEPARTMENT OF AGRICULTURE

Rural Electrification Administration

**BULLETIN 1751E-319**

**SUBJECT:** **[Interoffice Trunking and Signaling]{.underline}**

**TO:** All Telephone Borrowers

REA Telephone Staff

**EFFECTIVE DATE:** Date of Approval

**EXPIRATION DATE:** Seven years from effective date

**OFFICE OF PRIMARY INTEREST:** Central Office Equipment Branch,
Telecommunications Standards Division

**PREVIOUS INSTRUCTIONS:** This bulletin replaces TE&CM Section 319,
Issue No. 4, dated October 1978.

**FILING INSTRUCTIONS:** Discard REA Telecommunications Engineering and
Construction Manual (TE&CM) Section 319, Interoffice Trunking and
Signaling, Issue 4, dated October 1978, and replace it with this
bulletin. File with 7 CFR 1751 and on REANET.

**PURPOSE:** To provide basic technical information regarding the
principles of operation of interoffice trunk circuits and the most
commonly used trunking and signaling technologies. The material in this
bulletin is intended to be advisory in nature.

Robert Peters 11/16/93

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Acting Administrator Date

**TABLE OF CONTENTS**

1\.
General\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...3

2\. Basic
Information\...\...\...\...\...\...\...\...\...\...\...\...\.....3

3\. Trunk
Signaling\...\...\...\...\...\...\...\...\...\...\...\...\...\....4

4\. Loop
Signaling\...\...\...\...\...\...\...\...\...\...\...\...\...\.....4

5\. Polar-Duplex (DX)
Signaling\...\...\...\...\...\...\...\...\...\....6

6\. Multifrequency (MF) Signaling\...\...\...\...\...\...\...\...\....11

7\. Digitally Encoded
Signaling\...\...\...\...\...\...\...\...\...\...11

8\. Common Channel
Signaling\...\...\...\...\...\...\...\...\...\...\...12

**FIGURES**

Figure 1 Reverse Battery Supervision\...\...\...\...\...\...\...\.....4

Figure 2 Battery and Ground Pulsing\...\...\...\...\...\...\...\...\...5

Figure 3a E & M Signaling - Type I Interface\...\...\...\...\...\....7

Figure 3b E & M Signaling - Type II Interface\...\...\...\...\...\...8

Figure 4a Signal Lead Extension -Trunk Circuit

to Signaling Unit\...\...\...\...\...\...\...\...\...\...\.....10

Figure 4b Signal Lead Extension -Between Signaling Units\...\...10

Figure 5 Out-of-Band SS7 Signaling\...\...\...\...\...\...\...\...\...12

Figure 6 SS7 Link
Relationship\...\...\...\...\...\...\...\...\...\....14

Figure 7a Associated
Signaling\...\...\...\...\...\...\...\...\...\.....16

Figure 7b Quasi-associated Signaling\...\...\...\...\...\...\...\.....16

INDEX:

Trunking, Interoffice

Signaling

**ABBREVIATIONS and ACRONYMS**

ABS Alternate Billing Service

AC Alternating Current

AMA Automatic Message Accounting

CCITT International Telegraph and Telephone Consultative

Committee

CLASS Custom Local Area Signaling Service

DP Dial Pulse

DX Duplex

E/O Electrical/Optical

EMX Signal Lead Extension

Hz Hertz

kb/s Kilobits per second

LIDB Line Information Data Base

LSSGR LATA Switching Systems Generic Requirements

MF Multifrequency

PCM Pulse Code Modulation

REA Rural Electrification Administration

RST Remote Switching Terminal

SB Signal Battery

SCP Service Control Point

SG Signal Ground

SP Signal Point

SS7 Signaling System 7

SSP Service Switching Point

STP Signal Transfer Point

TE&CM Telecommunications Engineering and Construction Manual

**1. GENERAL**

**1.1** This bulletin is intended to provide REA borrowers and other
interested parties with technical information for use in the design and
construction of REA borrowers\' telephone systems. It discusses, in
particular, the principles of operation of interoffice trunk circuits.
It does not discuss the signaling between a Host office and an RST. For
the purpose of this bulletin, an interoffice trunk is defined as \"the
facility used to interconnect two switching centers for talking and
signaling purposes.\"

**1.2** This bulletin contains information on the most commonly used
trunking and signaling technologies.

**2. BASIC INFORMATION**

**2.1** The trunks described in Sections 3 through 7 of this bulletin
are voice, PCM and in-band data communication channels between two
switching centers and are referred to as interoffice trunks. They carry
data and audio transmission and supervisory signals in both directions
and routing and control information in one or both directions.

**2.2** The external transmission medium between two switching centers
may be metallic, optical fiber, or radio. This medium is terminated at
each switching center on an interface unit which is designed to match
the impedance of the switching center to the impedance of the
transmission medium. Optical fiber is terminated to an
Electrical/Optical (E/O) Converter prior to impedance matching.

**2.3** Analog trunk circuits historically consisted of discrete
components such as relays, resistors and capacitors. Since the
introduction of digital switching centers, direct digital trunk
terminations have become increasingly popular. In fact, analog trunk
circuits in a digital office are becoming nothing more than interface
units that convert analog speech and signaling information to or from a
digital signaling format.

**2.4** A direct digital interface of an interoffice trunk to a digital
switching center is often referred to as a trunk circuit. In fact, in a
direct digital interface a \"trunk circuit\" is only internal switch
operations that perform the functions of a trunk circuit.

**2.5** Whether digital or analog, the basic function of the trunk
circuit is the same. It connects the switching center to the external
interoffice trunk facility and provides the controls for the voice,
holding and signaling circuits. This includes changing the type of
signals transmitted or received to the type required by the connecting
equipment.

**3. TRUNK SIGNALING**

**3.1** The term \"trunk signaling\" refers to the means used to
transmit trunk seizure information, receiver \"on-hook\" and
\"off-hook\" information (supervision) and routing and control
information over an interoffice trunk. The following sections briefly
describe the most commonly used types of trunk signaling.

**4. LOOP SIGNALING**

**4.1** **[Reverse-Battery Signaling]{.underline}** Loop signaling over
interoffice trunks usually involves a pair of wires over which seizure
is effected through closure of a loop, dial pulsing through interruption
of the loop closure and supervision (on-hook/off-hook) by battery
reversal on the tip and ring. This type of signaling is called
reverse-battery and is the preferred and most widely used loop signaling
method (See Figure 1). When this type of trunk is terminated to an
electromechanical type of trunk circuit, it is limited to about 1200
ohms of loop resistance.

**Figure 1**

**Reverse Battery Supervision**

**Available in Hard Copy Only**

**4.2** **[Loop-Seizure Battery and Ground Pulsing]{.underline}**
Another method of operation known as \"loop-seizure battery and ground
pulsing\" was devised to extend the loop limits to about 2000 ohms and
with special equipment such as pulse correction to about 3000 ohms.
Battery and ground pulsing is accomplished by having battery and ground
at both ends of the loop in a series aiding arrangement during the dial
pulse signaling mode. This doubles the voltage available for signaling,
thereby extending the dialing and supervision range. Figure 2 shows
battery and ground pulsing operation.

**Figure 2**

**Battery and Ground Pulsing**

**Available in Hard Copy Only**

**4.3** In a digital office when either loop pulsing or battery and
ground pulsing type trunks are terminated to an electronic type trunk
circuit, the dialing range is increased even farther. Typically, the
loop resistance limit is 2000 ohms and 4500 ohms, respectively. This is
due to the sensitivity of an electronic trunk as opposed to
electromechanical.

**4.4** Supervision, the passing of \"on-hook\" and \"off-hook\"
information from one office to the other, is normally accomplished in
either type of trunk by reverse battery. As the name implies, battery
and ground are reversed on the tip and ring to change the signal to the
distant office from \"on-hook\" to \"off-hook\" or vice versa.

**5. POLAR-DUPLEX (DX) SIGNALING**

**5.1** DX type signaling will provide more reliable service, especially
over long trunk loops of up to 5000 ohms and where large and varying
differences in earth potentials are encountered. The talking circuit
also has a much better balance against AC induction noise. The signaling
circuit uses the same conductors (2 wire or 4 wire) as the talking
circuit and no filters are required to separate the signaling
frequencies from the voice frequencies.

**5.2** Originally, E&M lead signaling consisted of two leads between
the trunk circuit and the signaling circuit: the M lead which
transmitted signals from the trunk equipment to the signaling equipment
and the E lead which received signals from the signaling equipment. This
original E&M lead signaling arrangement as shown in Figure 3a is called
a Type I interface. Because only one lead was used in each direction for
signaling with a common ground return, the signaling leads had a much
greater susceptibility to noise than balanced circuits. Although this
type of circuit was acceptable in electromechanical systems, it is not
acceptable in the newer electronic systems. Consequently, a new method,
using a pair of leads in each direction over interoffice 4-wire
facilities, was developed to improve signaling and reduce noise
influence. This Type II interface is shown in Figure 3b.

**5.2.1 Type I Interface** - This is the original 2-wire E&M signaling
arrangement and signaling was accomplished by the trunk circuit applying
local -48V battery to the M lead to indicate an off-hook condition and
by applying a local ground to indicate an on-hook condition to the
signaling circuit. The battery was connected through a current limiting
device for fuse and circuit protection. Signaling in the reverse
direction (signaling circuit to trunk circuit) was over the E lead and
was accomplished by applying local ground to the E lead to indicate an
off-hook condition and an open to indicate an on-hook condition. The E
lead sensor in the trunk circuit was biased with -48 V battery so that
full sensor voltage was present on the E lead during an on-hook
condition.

**Figure 3a**

**E & M Signaling - Type I Interface**

**Available in Hard Copy Only**

**5.2.2 Type II Interface** - The Type II interface is a 4 wire facility
(E, M, SB (Signal Battery), and SG (Signal Ground)), fully looped
arrangement. It was developed to eliminate some unacceptable elements
found in the Type I interface. Signaling is accomplished from the trunk
circuit to the signaling circuit by applying -48V battery on the SB lead
of the signaling circuit which is connected to the trunk circuit and
looped back to ground in the signaling circuit on the M lead. The trunk
circuit applies a closure to these leads to indicate an off-hook
condition and an open to indicate an on-hook condition. In the Type II
interface, the current limiting device is on the SB lead for fuse and
circuit protection. Signaling in the reverse direction is accomplished
by connecting ground to the SG lead to the signaling circuit and looped
back to -48 volt battery in the trunk circuit on the E lead. The
signaling circuit applies a closure to these leads to indicate an
off-hook condition and an open to indicate an on-hook condition. The E
lead sensor in the trunk circuit should be biased with -48 V battery so
that full sensor voltage is present on the E lead during an on-hook
condition.

**Figure 3b**

**E & M Signaling - Type II Interface**

**Available in Hard Copy Only**

**5.2.3** Any trunk circuit or signaling circuit can be directly
connected if they have the same interface. However, if a Type I
interface connects to a Type II interface, a Type I to Type II
conversion circuit is required.

**5.3** DX signaling offers the following advantages:

> a\. Low pulse distortion;
>
> b\. Simultaneous signaling in both directions (duplex operation);
>
> c\. Ground potential compensation;
>
> d\. Reduced noise created by AC induction;
>
> e\. Operation through negative impedance type voice frequency
> repeaters; and
>
> f\. No filters to separate the signaling frequencies from the voice
> frequencies.

**5.4 [Signal Lead Extension Equipment]{.underline}**

**5.4.1** It is possible to extend the normal operating limits of E&M
signaling by the use of a signal lead extension set. This circuit
converts signals from signaling circuit E lead conditions to signaling
circuit M lead conditions.

**5.4.2** Signal Lead Extension (EMX) equipment is similar to DX
signaling equipment. In fact, one end of the circuit can be DX
equipment. The other end, called the \"intermediate end,\" has a
signaling circuit which is designed to work into another signaling unit
rather than a trunk circuit. A block diagram of this arrangement is
shown in Figure 4a. The other signaling unit is often part of a carrier
terminal, although it could be any kind of E and M signaling unit. Also,
both ends of the physical circuit could be EMX units. A block diagram of
this arrangement is shown in Figure 4b. An example of this would be a
trunk group consisting of microwave, cable pairs and finally carrier
along the trunk route.

**Figure 4a**

**Signal Lead Extension -**

**Trunk Circuit to Signaling Unit**

**Available in Hard Copy Only**

**Figure 4b**

**Signal Lead Extension -**

**Between Signaling Units**

**Available in Hard Copy Only**

**6. MULTIFREQUENCY (MF) SIGNALING**

**6.1** Multifrequency pulsing is accomplished by the use of
multifrequency senders and receivers in the switching systems. Each
digit 1 through 0 is represented by a single pulse that consists of a
combination of two out of five frequencies. These frequencies are 700,
900, 1100, 1300 and 1500 Hz. An additional frequency, 1700 Hz, is used
in combination with the five previously mentioned frequencies in a two
out of six combination to provide Automatic Message Accounting (AMA)
format functions (KP, ST, etc.). Multifrequency receivers interpret the
pulses and pass this information to other equipment to establish
connections through the switch. Because multifrequency signaling
transmits numerical and control information only, additional signaling
equipment such as DX, or loop, is required for supervision.

**6.2** Multifrequency pulsing has several advantages such as speed,
accuracy and range. Also, because multifrequency signaling requires less
holding time per call, a small number of multifrequency senders and
receivers can function as common equipment for a large number of trunks.

**6.3** Multifrequency signaling pulses are transmitted over regular
voice channels, and, because they are in the voice band, can pass
through entire facilities, hybrid repeaters and changes in types of
facilities as easily as speech.

**7. DIGITALLY ENCODED SIGNALING**

**7.1** Pulse Code Modulation (PCM) is the most widely used digital
encoding system in telephony.

**7.2** The economic and technical advantages of digitally encoded,
integrated transmission and switching are significant. The economic
advantages in using a direct digital interface are largely savings
brought about by the elimination of trunk circuits in the switching
center. This is somewhat offset by added complexity of the digital
switch for things such as digital attenuation and special control
software. On the technical side, the voice could be encoded at or near
the calling subscriber\'s telephone set and decoded at or near the
called subscriber. The transmission quality of the circuit would depend
almost entirely on the characteristics of a single encoder and decoder.
The digitally encoded signal is not significantly affected by the
digital transmission and switching paths. Voice quality will remain good
as long as there is only one (or few) analog to digital conversions in
the transmission path. Data transmission is also enhanced through the
use of integrated transmission and switching. The digital bit stream
provides for wideband data (56 kb/s per voice channel plus signaling)
and is more immune to interference than analog techniques.

**7.3** For a detailed, technical explanation of digital trunking and
signaling, refer to the following REA Telephone Engineering and
Construction Manual (TE&CM) Section, REA Bulletins, and Bell
Communications Research document:

a\. TE&CM Section 954 Digital Terminals and Multiplexers

(Planned redesignation as Bulletin)

b\. Bulletin 1751H-403 Digital Transmission Fundamentals

c\. Bulletin 1751H-405 Digital Span Line Systems

d\. Bulletin 1751H-601 Lightwave Fundamentals, Systems, and

Applications

e\. LSSGR TR-TSY-000506 (Section 6) Signaling

**8. COMMON CHANNEL SIGNALING**

**8.1** Signaling System 7 (SS7) is the most recent, and widely used,
common channel signaling system in use in the United States today. A
future bulletin will be issued addressing SS7 in its entirety, including
planning and economic considerations. SS7 is the North American version
of the CCITT No. 7 common channel signaling system. Unlike in-band
signaling that passes voice and signaling over the same transmission
path, SS7 uses separate data links (out-of-band) to pass routing,
control, and supervisory information back and forth between other
processor controlled nodes. The data link speed in most cases is 56
kb/s. This is about 800 times faster than MF signaling (5600 digits per
second as opposed to 7 digits per second using MF signaling). SS7
signaling replaces conventional multifrequency (MF) or dial pulse (DP)
signaling. Figure 5 shows a block diagram depicting SS7 signaling.

**Figure 5**

**Out-of-Band SS7 Signaling**

**Available in Hard Copy Only**

**8.2** The SS7 network is comprised of several components. These are
Signal Points/Service Switching Points (SP/SSP), Signal Transfer Points
(STP), Service Control Points (SCP) and signaling links. The functions
of these are described in the following paragraphs.

**8.2.1 Signal Point (SP)** - This is the basic requirement to interface
the SS7 network for SS7 service. It will allow interoffice circuit
related services such as Custom Local Area Signaling Service (CLASS),
trunk signaling, and ISDN Transport Services. It must be equipped with
the required hardware and software for any of these services. It does
not provide access to an SCP data base for query/response based
services.

**8.2.2 Service Switching Point (SSP)** - This is a software/hardware
addition to an SP to allow access to an SCP for query/response based
services of the SS7 network.

**8.2.3 Signal Transfer Point (STP)** - This is a packet switch that
transports and concentrates messages between other SS7 switches. No
messages originate or terminate in an STP. It also directs messages from
an SSP to the appropriate SCP for query/response requests and then sends
the retrieved information back to the SSP and/or other SPs/SSPs for call
completion based on the response from the data base.

**8.2.4 Service Control Point (SCP)** - This is a storage point for data
base information in the SS7 network, such as A800 Data Base, Alternate
Billing Service / Line Information Data Bases (ABS/LIDB), etc.

**8.3** The SS7 network consists of the following two modes of
operation:

**8.3.1 Query/Response mode (non-circuit related)** - An SCP accesses
various data bases to get call information in response to a query from
an SSP via the STP. It then sends the information back to the SSP via an
STP.

**8.3.2 Interoffice Trunk Signaling mode (circuit related)** - This
provides a message channel for call set-up and take-down. It is also
used to furnish CLASS between other switching centers that are connected
to the SS7 network.

**8.4** The SS7 network is connected together by a series of six 56 kb/s
digital data links identified by the letters A through F. The
relationship of these links is shown in Figure 6. These links are
normally installed in pairs (2 links) or quads (4 links) between various
network nodes in the hierarchy as described in the following paragraphs.

**Figure 6**

**SS7 Link Relationship**

**Available in Hard Copy Only**

**8.4.1** **A-links** - Access links, provisioned in pairs, connect
SP/SSP switches to STPs. They are also used to connect STPs to SCPs.

**8.4.2** **B-links** - Bridge links, provisioned in quads, connect one
mated pair of STPs to another mated pair at the same level in the
hierarchy (e.g., local to local).

**8.4.3** **C-links** - Cross links, provisioned in pairs, connect one
STP to another STP to form a mated pair.

**8.4.4** **D-links** - Diagonal links, provisioned in quads, connect
one mated pair of STPs at one level of the network hierarchy with
another mated pair of STPs at a different level of the network hierarchy
(e.g., local to regional).

**8.4.5** **E-links** - Extended links, provisioned in pairs, connect an
SP/SSP to a remote pair of STPs. This provides added protection in the
event the home STP fails.

**8.4.6** **F-links** - Fully Associated links, provisioned in pairs,
directly connect one SP/SSP with another SP/SSP. This is called
associated signaling.

**8.5** There are two methods of communicating in the SS7 network:
associated signaling and quasi-associated signaling. In associated
signaling as depicted in Figure 7a, messages are sent from a signal
point directly to another signal point and do not pass through other
SPs/SSPs. In quasi-associated signaling as shown in Figure 7b, messages
are sent from one SP/SSP through intermediate STPs to other SPs/SSPs or
data bases.

**Figure 7a**

**Associated Signaling**

**Available in Hard Copy Only**

**Figure 7b**

**Quasi-associated Signaling**

**Available in Hard Copy Only**