AT&T 555-600-736 User Manual

Abstract

AT&T

MERLIN LEGEND™ COMMUNICATIONS SYSTEM

APPLICATION NOTES

MERLIN LEGEND™ COMMUNICATIONS SYSTEM

Applications Note On Basic Trunking Concepts

This Application Note describes the various types of trunks that link the MERLIN LEGEND Communications System with the telephone network. Operating characteristics of the various trunks are

shown to be a function of both the trunk type and the capabilities of the LEGEND system hardware and software. The Note is designed to help Account Executives and System Consultants, so a basic approach

to the subject is used. The concepts covered also apply to other customer premises switching equipment as well as the MERLIN LEGEND CS.

This Application Note is designed for use as a reference manual.

Refer to it each time you get involved with a MERLIN LEGEND system sale requiring a mixture of the various types of lines and trunks.

MERLIN is a registered trademark of AT&T. DIMENSION is registered trademarks of AT&T. MERLIN LEGEND is a trademark of AT&T. MERLIN MAIL is a trademark of AT&T. MLX-10, MLX-10D, MLX-10L MLX-28D are trademarks of AT&T.

ACCULINK is a trademark of AT&T. ACCUNET is a trademark of AT&T.

555-600-736

Issued January 1992

Jim Pastorius

Kevin Lyons Writers/Editors

Contributors: A. Cohen

D. Guerro V. Illuzzi R.G. Koppenheffer D. Margolis

S.W. Osborne H.T. Reeve M. Stevenson

B. Tannu C.A. White J. Webb

MERLIN LEGEND TRUNKING CONCEPTS

APPLICATION NOTE INDEX

Introduction

Tip & Ring Explained

Lines & Trunks

Loop-Start Trunks Operations

Potential Problems When to Use L/S

Ground-Start Trunks Operations

Potential Problems

When to Use G/S

Direct Inward Dialing Operations

Signaling Characteristics LEGEND Operation Administration When to Use DID

2 2

4 5 7

9 10 10

11 12 13 13 15

TIE Trunk Operations

Tandem TIE Trunk Operations

Transferring Calls Over TIE Trunks

TIE Signaling and Implementation

Off-Premises Stations

T-1 (DS1) Service

Data Communications Equipment

DS1 Facility Services

Hotel/Motel Trunks

LEGEND Line/Trunk Hardware

Administration

Considerations

References

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INTRODUCTION TO TRUNKING CONCEPTS

The way lines and trunks have been used to meet customers’ needs over past decades is rapidly changing. T-1, ISDN technology, fiber optics, and other advances in telecommunications are forcing us to rethink what lines and trunks should be used for a PBX or Key system. It is critical, however. that everyone involved in giving customers the most advanced, yet economical. system have a basic understanding of the concepts which are the foundations for sending and receiving telephone voice and data messages.

The goal of this Application Note is to explain the various types of lines and trunks which may connect to

the MERLIN LEGEND Communications System. The Note focuses on the concepts of each type of line or trunk. While it also covers major interactions between the LEGEND system and the various lines/trunks, it is not aimed at explaining everything needed to install, initialize. and maintain them on the system. This is fully covered in the LEGEND System Installation, Programming, and Maintenance Manual. Additional information on lines and trunks may be found in the LEGEND PBX and Key Systems Planning Guides, as well as the Systems Reference Manual.

TIP AND RING EXPLAINED

The terms “Tip” and “Ring” occur inevitably in any description of telephone lines and trunks.

These terms originated in the early days of telephony when telephone connections were made by an operator who inserted a plug into a jack. This plug was similar to the plug on a set of conventional stereo

headphones in that it had three conductors. These conductors were the “Tip,” corresponding to the tip of the plug. the “Sleeve,” the longest Conductor at the base of the plug, and a ring of metal between the Tip

and the Sleeve called “Ring.” The Sleeve was connected to a local electrical ground at the subscriber's premises and did not carry a signal

to the telephone company Central Office (CO). The Tip and Ring conductors each connected to a wire that

carried signals to the central office. The terms became solidly embedded in telephone jargon and are still used in modem, electronic switching

even though the actual “tips” and “rings” have long since disappeared. Today the terms Tip and Ring are used primarily as means for people in different places to identify precisely where each individual wire in each pair of wires (in a huge bundle of wires) needs to be connected. When a pair of wires is reserved for service as a trunk, one wire is designated Tip and the other wire is designated Ring.

During the process of ordering telephone service, a line assigner at the local operating company will reserve a pair of wires for each line or trunk ordered. Each individual wire in the bundle is identified by a unique color or combination of colors. Each trunk will have one color-coded wire assigned as the Tip lead and one color-coded wire assigned as the Ring lead of that particular trunk. For instance, a cable might contain one solid blue wire which would be reserved as the Ring lead on a given trunk, and one blue-and-white striped

wire which would be reserved as the corresponding Tip lead for that same trunk. The line assigner will then furnish this information to installers at the customer’s premises and technicians in the central office. In this way technicians can coordinate their installation work and make the proper connections between the customer’s telephone equipment and the operating company’s central office.

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LINES AND TRUNKS

Telephone lines and telephone trunks are facilities that carry voice or data communications. They are similar in form and function, and the two terms are usually treated as if they are interchangeable. The fundamental difference between a line and a trunk is that a line connects a station instrument to a switching system, and a trunk connects one switching system to another switching system.

The connection between your home phone and the telephone company’s central office is a line. The telephone facilities that serve key systems are also telephone lines, since a station in a mechanical key system accesses a telephone facility by pressing the specific button that corresponds to the exact facility desired. In a mechanical key system no switching takes place.

MERLIN LEGEND can be administered as a PBX, a telephone switching system that happens to be located on a customer’s premises.

facilities, none of which need to be permanently connected to that specific voice terminal, by dialing a code (such as “9”) and having the PBX select one facility from a group, or pool, of facilities. The voice connection from that station is then electronically switched to the selected facility.

Meanwhile, the facility that connects the voice terminal (station instrument) to the PBX is called a station line. In this example the voice terminal corresponds to your home phone, and the Legend system represents a scaled-down central office. Even though the station and the switching system are both in the same building, a facility that connects a station instrument with a switching system is a line.

A person using a LEGEND system telephone can access a wide variety of

Most of the facilities that connect the LEGEND system to the local central office are properly called trunks, but there are some misconceptions about this simple definition. Many PBXs, including MERLIN LEGEND, support Personal Lines. These facilities typically appear on a voice terminal button and pass transparently through the PBX, without being switched, to the central office. Selecting a personal line

button on a voice terminal and lifting the handset brings dial tone directly from the central office.

Historically, most telephone operating companies automatically engineered a trunk to better standards than a line in a process called "conditioning." The central office equipment and the cable path used for trunks had a meet higher standards for transmission quality than equipment used for line. Today most operating companies are removing this provision from tariffs, and if a PBX requires conditioned trunks they will

probably be available on an extra-cost basis.

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LOOP-START TRUNKS:

OPERATIONS

Loop-start facilities are the simplest and most common end-user facilities in the nation-wide telephone network. Loop-start facilities provide virtually no supervision between the central office and the customer premises equipment (CPE). For this reason, loop-start facilities are usually suited for use with telephone systems that provide human supervision. key systems with line status lights, or older PBXs. Loop-start facilities are generally not well suited for use with PBXs that provide mechanical or electronic supervision.

A loop-start line or trunk consists of two wires running from the Central Office to the Customer Premises

Equipment (CPE). For historical reasons these wires are called Tip and Ring. (The term Ring, when

applied to one of the leads in a line or trunk facility, does not refer to the ringing signal sent to announce an

incoming call.) The CO applies battery voltage to the Ring lead and connects the Tip lead to ground. These wires meet at a switch in the CPE that is normally open, and are bridged by ringing detection equipment (such as the bell in a single line set) which provides a.high electrical resistance.

Telephone equipment on a loop-start facility signals the central office that it needs the facility for an” outgoing call by closing this switch between the Tip and Ring leads. This may be as simple as lifting the

handset from the switchhook if the equipment is a single line set. When this switch is closed the resistance

between the 17p and Ring leads drops low enough to allow battery current to flow in a “loop” running from

the CO out to the CPE, and back to the CO. When the CO detects current flowing through the loop it

provides dial tone for an outgoing call.

Loop-start facilities are primarily intended for single line sets,

The central office signals an incoming call on loop-start facilities by sending a ringing signal on the line. These are the only two signals that are uniformly used by loop-start facilities.

It is important to note that these signals do not give advance warning of a change in switchhook state. The only signal that the CPE sends to the central office that it is seizing a loop-start facility is the act of going off-hook on that line. The signal from the central office that alerts the CPE to an incoming call is the

ringing signal received at the CPE at the same time

or slightly after

the call is connected to the facility.

Also, it should be noted, there is no defined method for a central office to terminate a call, so there is no

reliable way to force a disconnect when only one of the two parties on a call hangs up.

1. Actually, the proper term here is impedance, not resistance, but the effect is the same for all practical purposes, no current flows.

2. See The Problem of Glare in the next section.

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LOOP-START TRUNKS: POTENTIAL-PROBLEMS

The most common application for loop-start facilities is single line residential service. Many of the disadvantages of a loop-start trunk in a PBX environment can be inferred from the operation of a loop-start residential telephone line, like the one you have at home.

The Problem of Glare

Perhaps, at least once in your life, you’ve picked up your home phone to make a call. only to hear a startled voice saying “hello?” associated with loop-start facilities. This is due to the interaction of the extremely simple methods of

signaling that a line is being seized.

When a call for a given loop-start facility comes in to a central office it is connected to the proper facility, and the signal from the central office ringing generators (90 volt, 20 cycle alternating current) is super

imposed onto it. The central office ringing generators provide the ringing signal in a cycle of two-secondson (ringing), and four-seconds-off (silence). ringing signal to start at the reception of each individual call, so it is entirely possible for one call to be connected to a loop-start facility during the ringing period of the ringing cycle, and the next call to be connected during the silent period of the ringing cycle. An incoming call may be present on a loop-start facility for up to four seconds before ringing begins.

In a low traffic, single line, residential telephone situation this problem is uncommon, and sometimes it's even amusing. Even though the connection is made without warning in a residential situation there are only a few people that the incoming caller could be trying to reach. The problem, however, is more noticeable with higher traffic, multiline key systems. But since most key systems are relatively small and typically have close human supervision over line status and selection, these problems are usually identified and resolved quickly.

instead of dial tone. This is called glare. Glare is the most obvious problem

With a PBX, such as MERLIN LEGEND CS, the problem of glare becomes more serious and comples. Since ringing current is the only way that the central office can signal the PBX of an incoming call, and since the PBX typically asssumes that a facility is available until it receives the first cycle of ringing current, it's possible for the PBX to try to place an outgoing call on a loop-start trunk that has just been seized by the central office and is carrying an incoming call. The PBX thus connects, without waring, two parties who did not intend to reach each other.

A PBX trunk typically carries much higher levels of both incoming and outgoing traffic (more tails per

hour) than a residential telephone line, so glare is statistically more likely to occur on a PBX trunk.

Meanwhile, the ordinary station users on a PBX exercise little direct supervision over the individual trunks

in the system.

On the simplest LEGEND system, one without Automatic Route Selection (ARS), a station user would

typically lift the handset and hear dial tone from the PBX. The user would then dial an access code to a

trunk or group of trunks, and wait for dial tone from the CO. It is at this point that the user might suddenly

say “hello” to an unexpected incoming caller.

Features such as ARS magnify the problem. When the LEGEND system is administered for ARS the PBX

is a “slenderized” system. It holds the digits that are dialed, selects the appropriate trunk, and then goes off-

hook. LEGEND then waits two seconds on this trunk (under normal circumstances this is long enough to

obtain dial tone from the CO), and then dials the telephone number. Notice that LEGEND doesn’t actually

recognize dial tone, and it cannot recognize glare, it just waits two seconds and dials.

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The ARS feature also isolates the station user from the specific trunk, and even the group of trunks. that carries each outgoing call. Other features. such as Callback Queuing (LEGEND) or Busy-to-Idle (System

25) reminders, allow station users to seize trunks moments after a previous conversation concludes, and easily fast enough to beat a ringing signal on an incoming call.

The glare problem can also be compounded if the PBX customer has terminals or computers that place outgoing data calls without human supervision.

encounters glare it will probably disconnect (hanging up on the incoming caller), record a line failure. and

try again. The data user may become annoyed that a call attempt failed, but will never know about the rude reception that was just given to a potential customer.

If a terminal seizes a line for an outgoing call and

Automated Attendant Ghost Calls and Loop-Start Facilities

Several customers with loop-start facilities and the Automated Attendant have reported problems with ghost calls. The human attendant who handles overflow calls from the Automated Attendant will notice

that often, when the phone rings there will be no one there. or occasionally the attendant will pick up the

ringing phone and hear dial tone. The problem is caused by the fact that the Central Office does not send a reliable signal to force a

disconnect at the telephone terminal when only one party on a loop-start line call hangs up. Until the second party hangs up, the loop-start line is considered in use by the CO, and can not be used to receive a new call. This happens sometimes with ground-start trunks as well, since there is usually 20-40 seconds for

front end disconnect to pass through.

Consider the case of an automobile dealer that uses the Automated Attendant to direct calls to the new car

sales, used car sales, parts, and service departments. During the course of the day, the alder will receive several calls from people who, for one reason or another, hang up while waiting for the Automated Attendant.

Since there is no positive disconnect on loop-start facilities, (the CO does not signal the set to hangup), the central office will continue to hold up the connection to the PBX. When this “connection” reaches the

Automated Attendant, the usual list of dial-selectable destinations will be recited, but since there is no longer anyone on the call there will be no response. The Automated Attendant will time out and, in the

absence of a positive response, it will assume that it is dealing with a caller who has a rotary dial phone. It

will then forward the abandoned call to the human backup attendant. This person's phone will ring, but there won't be anyone there. The condition can also give the perception that the caller was cutoff.

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LOOP-START TRUNKS: WHEN TO USE LOOP-START TRUNKS

There are two times when you consider the use of loop-start facilities: when you facilities and when you

must

use loop-start facilities.

should

use loop-start

Proper Loop-Start Facility Applications

If loop-start facilities are prone to problems such as glare why are they still in use? Most of the problems associated with loop-start facilities only come into play with automated PBXs.

Loop-start facilities are perfectly acceptable for use with single-line telephones, key systems, and manual (cordboard or switchboard) attendant operated PBXS.3 Some PBXs, especially those that cannot be administered by the customer, can operate properly with with loop-start facilities for one-way incoming and one-way outgoing trunks, that must be handled by an attendant.

Besides, the cost of converting all of the loop-start customer and CO equipment currently installed to

ground-start operation would be an incremental expense.

Note:

Loop-start trunks

mode operation. Ground-start trunks are

are not recommended

always

for use with the MERLIN LEGEND configured for PBX

preferred.

Unavoidable Loop-Start Facility Applications

One situation that will require the use of loop-start facilities on the LEGEND system would be the connection of Centrex lines to the PBX. In some regions, Centrex is also called Essex or Centron lines. Most operating companies will provide Centrex service only on loop-start facilities. Some operating companies may consider converting the lines to ground-start facilities as an extra-cost special assembly, but this is at the operating company’s option. Currently, the LEGEND system does not support the use of GS Centrex.

Consider the case of a state government. In the early seventies the state linked all departments, large and small, in one Centrex system in order to control inter-departmental message unit expenses. The individual

departments terminate their Centrex lines in large, non-uniform key systems.

Now, departments like Motor Vehicles and the State Police, have grown to the size of small companies and occupy their own buildings. The Centrex is no longer capable of providing efficient internal communications for the large departments, and traffic demands from large departments are swamping service to smaller departments. The state would like to keep Centrex for convenient interdepartmental calling without message unit charges. The larger departments, however, need PBXs like the LEGEND system to take the load off the Centrex and for better internal communications. The local operating company will only provide Centrex service on loop-start facilities. In this case, some loop-start Centrex

lines will terminate in the Legend system. Glare may occur on the Centrex lines. In general,

facilities are always preferred.

3. Some true manual PBXs are probably still present in the field. but you are not likely to encounter one during the installation of a modern PBX. On a true manual PBX all calls, both incoming and outgoing, must be handled by an attendant. there is no way to dial a code such as “9” for an outside line. h is this close supervision by the attendant that makes loop-start facilities acceptable for use with manual PBXs.

ground-start

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Cost

Unfortunately, the most common reason for using loop-start trunks is the least valid. This reason is cost. If a customer currently has a key system, service at that location is undoubtedly supplied by loop-start lines.

Converting the lines to ground-start trunks may involve a longer installation intend, and may incur additional charges from the operating company. As a result, many loop-start lines are simply re-used as

loop-start trunks when the key system is replaced by a PBX.

However, if a MERLIN LEGEND CS is used, the customer has the flexibility of using either Loop Start or Ground Start lines/trunks. This advantage not only eliminates out-of-service conditions, as would be required if new replacement lines had to be installed, but can be a major cost-break by using the more economical Loop Start Modules. Also, the combined GS/LS board is a key advantage of the LEGEND system in facilitating the coordination with the telephone company in converting from LS to GS. It’s not necessary to provide a loop-start module, and then have to replace it with a ground-start board should the

lines be converted.

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GROUND-START TRUNKS: OPERATIONS

Ground-start facilities were specifically introduced to solve the problems that PBXs encounter on loop-start

trunks. A ground-start facility provides an immediate signal when it is seized and it provides a positive

signal when one party disconnects. When a ground-start facility is idle the CO provides battery voltage on the Ring lead, but the CPE does not

provide a ground on the Ring lead, and no current flows. Meanwhile, the CPE (LEGEND) monitors the Tip lead for ground at the CO, but the CO does not provide a ground on the Tip lead when the trunk is idle.

When the CO needs to seize a facility for an incoming call it selects an idle trunk, makes sure that the CPE

has not applied ground to the Ring lead (no current is flowing), and the CO applies ground to the Tip lead. The ground connection on the Tip lead completes an electrical path, current begins to flow, and the CPE recognizes immediately that the facility has been seized for an incoming call. The CO also super imposes

the ringing generators onto the facility, but ringing may not occur at once. The significant operational difference here is that, unlike the signal from the ringing generators, the ground

signal on the Tip lead is synchronized with the start of the incoming call. The CPE knows

the facility is not available for outgoing traffic, even if it does not begin to ring for several seconds. Ground-start facilities also provide a positive indication of a disconnection. When the distant party goes

on-hook the CO removes battery from the Tip and Ring leads. Ground-start CPE (LEGEND) is designed to recognize this disconnect signal and remove its party from the facility. (This prevents the party from waiting for dial to to return, and prevent a toll restricted station from by-passing the restrictions.

Nearly round-start

obtain a positive disconnect signal and prevent a user from making multiple calls on a single coin. Also,

some unattended data equipment uses ground-start lines so the modem can determine when to start dialing after going off-hook. Most modem equipment uses loop-start lines and alternate systems, such as a dialtone detector in the modem, instead of ground-start lines.

all

ground-start facilities are trunks, connecting PBXs with COs. There are only a few cases where

lines

might be encountered. Some pay phones, especially older ones, use ground-start lines to

immediately

that

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GROUND-START TRUNKS:

POTENTIAL PROBLEMS

There are very few problems that can occur when properly functioning ground-start trunks serve a properly

administered PBX, since modem PBXs and ground-start trunks were literally made for each other. This Application Note does not cover the kinds of problems that can occur when a trunk develops a problem and no longer functions properly. Suffice it to say that a malfunctioning ground-start trunk may behave just as poorly as a malfunctioning loop-start trunk, but a properly functioning ground-start trunk will always outperform a properly functioning loop-start trunk.

It’s possible that when a large key system customer upgrades to a PBX, the conversion from loop-start lines to ground-start trunks may extend the installation interval. It is still always best to convert, and accept the one-time inconvenience, in order to enjoy a much better grade of service over the long haul.

The most common installation problems involved with ground-start trunks are reversed Tip and Ring connections and improper grounding of the PBX.

GROUND-START TRUNKS: WHEN TO USE GROUND-START TRUNKS

Ground-start trunks should be used for a MERLIN LEGEND Communications System when configured as a PBX.

They offer the following major benefits:

The Tip ground seizure signal virtually eliminates the possibility that a two-way trunk can be seized

1. from both ends simultaneously.

The “window of opportunity” for glare is reduced from four seconds to a small fraction of a second.

2. Ground-start operation also provides a positive indication of network disconnect when programmed

at the CO for Calling Party Control, allowing a PBX to disconnect a station when the outside party on a call hangs up.

The possibility of a user circumventing toll restrictions is eliminated.

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