1. Introduction…………………………………………………………………………1

2. Overview Of Enhanced 911 Service……………………………………………….. 1

Figure 1 – E911 System Overview………………………………………… 2

3. E911 Interfaces………………………………………………………………………3

Figure 2 – Format Of Text Portion Of Response From E911 Data Base ….. 10





1. Introduction

1.1 This Technical Reference is published by BellSouth to provide a description of the interfaces
between a BellSouth Enhanced 911 (E911) tandem office and an E911 Public Safety Answering
Point (PSAP) and between the BellSouth E911 database system and a PSAP. These interfaces
provide the network interaction with customer premises equipment that provide E911 PSAP

1.2 Reason for Reissue

This document has been reissued to remove references to 1AESS™ tandem offices, change the format of the information sent by the E911 tandem to the PSAP, and change the format of the text portion of the response from the E911 database.

2. Overview Of Enhanced 911 Service

The number 911 is the 3-digit telephone number that has been designated by the FCC for public use throughout the United States to report an emergency and/or request emergency assistance. The number 911 is intended as a nationwide universal telephone number which provides the public with direct access to a public safety answering point (PSAP). A PSAP is an agency or facility which is designated and authorized to receive and respond to emergency calls requiring one or more public services such as police, fire, and/or ambulance services. Any one agency or a group of agencies may be designated as a PSAP. One or more attendants are located at a PSAP facility to receive and handle emergency calls in accordance with local requirements.

With E911 service, a BellSouth switching office is arranged to serve as an E911 tandem for all 911 calls from other local offices in the 911 service area and as a local office for 911 calls originated by customers served by the E911 tandem office. Calls may be routed by this E911 tandem office to one or more PSAPs in the 911 service area selectively. Calling party number identification is passed via the trunk facility to the PSAP location for display at the PSAP answering position. Additional information associated with the calling party number as stored in the BellSouth E911 database can also be delivered to the PSAP answering position. This information is queried real time from the database over dedicated data links by the customer premises control equipment.

This document describes the interfaces between an individual PSAP and the BellSouth E911 tandem and between the PSAP and the database system. An overview of a typical E911 system is given in Figure 1.

Figure 1 – E911 System Overview

3. E911 Interfaces

3.1 Trunk Interface

Two wire, 1-way, outgoing trunks arranged for loop-reverse battery operation are provided in the central office and connected to the PSAP through a standard local loop. The E911 tandem trunks will be terminated at the network interface point at the customers premises in standard RJ21X jacks.

The electrical characteristics and signaling states used by trunks of the above type that may be used by the E911 tandem office are given in the Signaling section of Telcordia’s, LSSGR: Signaling for Analog Interfaces [3] document. Additional information on local loop characteristics can be found in the documents ANSI T1.414 – Network to Customer Installation Interfaces – Enhanced 911 Analog Voicegrade PSAP Access Using Loop Reverse-Battery Signaling [1] and Lightning and 60-Hz Disturbances at the Bell Operating Company Network Interface [2].

(a) Calling Number Identification

The telephone number of the station calling into the PSAP is delivered to the PSAP equipment using standard Automatic Number Identification (ANI) techniques over the E911 tandem trunks. Operation of ANI number delivery is described in TR-TSY-000350, E9-1-1 Public Safety Answering Point Interface Between a 1/1A ESS Switch and Customer Premise Equipment [4]. Typically the number delivered to the PSAP is displayed at the answering position by the customer premises equipment. The PSAP is also required to formulate a request for associated data from the E911 database on this number. See section 3.2 for a description of this activity.

Once the PSAP detects that the E911 tandem has seized an idle trunk by the presence of an off-hook condition, the PSAP is expected to send a start pulsing signal (250 ± 50 ms wink) within 4 seconds. If the wink signal is not received within 4 seconds, the E911 tandem will retry once on a different E911 trunk after placing the original trunk on a maintenance list.

The E911 tandem will begin outpulsing the calling number as multifrequency (MF) tones within 4 seconds of receipt of the wink signal. If no calling number is received or if distorted tones are received, the PSAP should assume ANI failure has occurred and proceed to return audible ringing to the calling party and signal an attendant to answer. Although no calling information would be available to present to the attendant in this case, the customer provided equipment shall send the appropriate ANI failure digits to the E911 database system.

Once the ANI digits are received, the customer provided equipment shall return audible ringing to the 911 calling party at the same time as attendant positions are alerted of the incoming call by the customer provided system. Once an attendant answers, the customer provided equipment should remove audible ringing and cut the incoming call through to the position. At the same time, the customer provided equipment database request message is sent to the BellSouth E911 database system. This system will return information, relating to the calling telephone number, suitable for display on a terminal at the attendant position answering the call. The customer provided equipment is responsible for decoding the ANI MF data and formatting the ASCII encoded request to the BellSouth E911 database system. This database interface is described in section 3.2 of this document.

The format of the information sent by the E911 tandem will be one of the following:



8 digit ANI


10 digit ANI


Two 10 digit ANI


(calling party’s number) (dialed number or pseudo ANI)

The KP and ST are standard MF signals. The calling number is given by the NXX-XXXX. For 8 digit ANI, the “A” is coded as in Table 1 and represents the numbering plan area (NPA) from which the 911 call originated in those cases when the 911 service area includes two or more NPAs. In cases where special attention or handling may be required, the 4, 5, 6 and 7 digits are used. Examples of such cases are a 911 call originated via a foreign exchange (FX) line serving a station which is not physically located in the 911 service area or a telephone number which does not have an emergency service number (ESN) assigned. Other cases for the special handling digits may be defined by local practices.

The “II” represents encoded information indicating whether the calling line display device (ANI display) should remain steady or flash, or if the call is a test call. II digits will be used as follows:

II Digit Meaning

Steady ANI display

44 Flashing ANI display
48 Test Call







NPD 0 (Lowest Numbered NPA)


NPD 1 (Second Numbered NPA)


NPD 2 (Third Numbered NPA)


NPD 3 (Highest Numbered NPA)


NPD 0 (Special Handling Required)


NPD 1 (Special Handling Required)


NPD 2 (Special Handling Required)


NPD 3 (Special Handling Required)


Maintenance Test Call


Not Used

ANI Failures

If the E911 tandem office is unable to present a valid calling number, one of the following is sent to the PSAP:


8 digit ANI


10 digit ANI


Two 10 digit ANI with good

CPN, bad pseudo-ANI


(good CPN) (bad pseudo-ANI)

Two 10 digit ANI with bad CPN, good pseudo-ANI


(bad CPN) (good pseudo-ANI)

Two 10 digit ANI with bad CPN and bad pseudo-ANI

NPA-911-0TTT NPA-911-0TTT

(bad CPN) (bad pseudo-ANI)

These formats are sent due to ANI failures between the end office and the E911
tandem, or possibly a 911 call received via a message trunk from outside the
local tandem area. The digits TTT indicate the Emergency Service Central
Office (ESCO) number associated with the originating office to tandem trunk.

Anonymous Calls
0-911-0000 (8 digits) or 000-911-0000 (10 digits) is sent when an anonymous call is made to a PSAP. An anonymous call is a 7 or 10 digit call (non-911) to the directory number of a PSAP. ANI identification is not available for such calls. A PSAP may be assigned a nondialable number to preclude such calls.

(b) Call Transfer Interface

Transfer of an incoming call to an alternate PSAP is available as a standardservice. The primary
PSAP may implement transfer to a secondary PSAP using the standard Call Transferand Speed
Calling features of the E911 tandem office. These features are implemented byswitch hook
flashes and abbreviated dialed digits issued by the customer provided equipmentto the E911
tandem office trunks. The answering position may be provided a single buttoninterface to these
features by the customer premises control equipment if so arranged or they maybe manually
exercised by a standard station set at the answering position.

Normal call transfer sequences are used to initiate call transfer. The PSAPequipment should send
an on-hook flash signal of 500 ± 50 ms to the E911 tandem office. Normally theE911 tandem
will seize a three port conference circuit and a DTMF (Dual ToneMulti-Frequency) receiver will
be attached to the trunk to receive the transfer digits and dial tone will bereturned to the PSAP
initiating the transfer. If these facilities are not available in the E911tandem due to load
conditions, the flash signal will be ignored. After detecting dial tone, thedesired transfer
destination should then be sent by the PSAP as DTMF pulses conveying either theSpeed Calling
digits or regular directory number of the destination.

Three types of Call Transfer may be provided: selective, fixed and manual.

Selective Call Transfer – activated by receipt of a *1X speed calling sequencefrom the
PSAP equipment, where X is 1 to 6. The E911 tandem will selectively route thecall to
the desired type (specified by the “X” code) of secondary PSAPdesignated for the calling
party’s jurisdiction. The association of various PSAPs with selective SpeedCalling codes
should be done as part of the installation process for the E911 system.

Fixed Call Transfer – activated by receipt of *NX speed calling sequences,where N is 2
to 4 and X is 0 to 9. Only single direct translation to a prespecified directorynumber will
be performed by the E911 tandem in this case.

Manual Call Transfer – the PSAP attendant manually performs the on-hook flash
sequence and then directly dials either the Speed Calling Code as specified inthe Fixed
Call Transfer or the directory number of the destination.

(c) Trunk Supervision of Disconnect

The E911 tandem maintains supervision of the off-hook status in bothdirections. To drop an
incoming call, the PSAP equipment must present an on-hook condition greater than1.2 seconds.
The E911 tandem will disconnect the call in both directions, sending on-hook tothe PSAP, and
idling the incoming trunk. This prevents an incoming call that remains off-hookfrom tying up the
PSAP trunk.

If a three party call has been established by the call transfer feature, thefollowing disconnect
supervision applies:

Primary PSAP

(1) Receipt of a flash signal (500 ± 50 ms on-hook) from the initial PSAP in a3 party call
setup will release the added party and maintain a 2 party connection between thecalling
party and the primary PSAP.

(2) Receipt of an on-hook signal longer than the minimum disconnect timinginterval (1.2
seconds) from the initial PSAP will disconnect that PSAP from the call andmaintain the
connection between the calling party and the added party.

Added Party

If an added party sends an on-hook signal for longer than 16 seconds, it will be disconnected but the original PSAP will remain connected to the calling party.

If the added party returns off-hook before a 16 second interval has expired, it will be returned to the 3 party connection.

If the primary PSAP sends a flash signal during the 16 second interval after the added party has sent an on-hook signal, the added party is disconnected immediately. The primary PSAP remains connected to the calling party.

If one of the original parties disconnects before the added party’s disconnect timing interval expires, that party is immediately released. However, the E911 tandem office continues the timing interval and will reconnect the added party to the party remaining off-hook if the added party returns off-hook within the interval.

Calling Party

Upon detection of the calling party disconnect, the calling party is released. The primary PSAP and the added party remain connected until either disconnects or the primary PSAP releases the added party.

(d) Night Service / Alternate PSAP

All calls destined for a PSAP may be automatically directed to an alternate PSAPor other
answering location. This feature is available for each PSAP. A single directorynumber is
assigned in the E911 tandem office for alternate routing and is used in any ofthe conditions where
an alternate is needed. These conditions could be traffic busy, night service,power loss, or other
failure at the PSAP.

Whenever calls are routed to an alternate PSAP, the ANI information of thecalling party is also
made available to the alternate PSAP. The E911 tandem presents the calling partynumber to the
secondary PSAP using the same mechanism described in section (a) as used for aprimary PSAP.

Alternate or night service is activated from the PSAP by key activation of acontrol lead to the
E911 tandem office. The network interface required for this separate controllead is given in
Telcordia Technical Reference TR-TSY-000349 [5].

(e) Tones Used

Standard tones (dial, busy, reorder, and audible ringing) are provided by theE911 tandem office
for attendant transfer calls. Interrupted high tone (120 ipm) is also providedwhen speed dialing
transfer is attempted, but not subscribed to by the PSAP.

(f) Trunk Maintenance Test Calls

From time to time the telephone company will place a test call to verify theintegrity of the trunk
circuit to the customer’s premises. This call will be identified by the specialANI code given in
Table 1 (KP-8-ST). The PSAP shall connect the trunk so identified to a permanentbusy tone
(continuous 60 ipm tone). This must be done within 20 seconds after receipt ofthe wink start
pulse, otherwise, the E911 tandem office would consider the trunk test afailure. The E911
tandem will disconnect approximately 5 seconds after receiving the 60 ipm tone.The PSAP
should then immediately release the trunk.

3.2 Database Interface

BellSouth provides a centralized database including information for eachdirectory number used
by wireline subscribers to the network. This information usually contains callerlocation and other
information useful for a public safety response to emergency situations where acall is placed from
a phone near the site of the emergency. A unique protocol is used over redundantdata links
between a PSAP and the BellSouth ALI Computers to retrieve this information. ThePSAP
equipment is responsible for formatting a request for ALI per this protocol andplacing it on the
data links. The returned information is available to the PSAP equipment todisplay on a terminal
at an attendant position or in some other manner per the design of the equipmentprovider.

(a) Physical Interface

Access to the BellSouth centralized E911 database is provided by asynchronousprivate line
modem links to duplicated ALI Computers. These computers are located indifferent BellSouth
Centers for redundancy and diversity. Two links are required for reliability;they are functionally
identical. The ALI Computers receive the ALI request message from the PSAP,query the
database and return the stored information to the PSAP over the data link. Theactual network
interface is that of a private line digital data service. Digital facilitiesshall be ordered with
secondary channel. This feature is necessary in order to deploy an end-to-enddiagnostic and
configuration capability from the master end (BellSouth Center) withoutdisruptive consequences.
The characteristics of the equipment used to modulate these data links are:

2400 baud digital Channel Service Unit/Data Service Unit (CSU/DSU)
10 bit data character
1 start bit
8 data bits (8th bit ignored)
1 stop bit
Full Duplex
Continuous Carrier

For reliability, an individual ALI request message is expected to be receivedfrom the PSAP over
both links simultaneously by the duplicated ALI Computers. Responses from thedatabase system
are presented over one of the data links only. Both links must be monitored bythe PSAP for
responses since they are randomly chosen by the ALI Computers. If one data linkis out of
service, only the good link will be chosen by the ALI Computers for responses.

(b) Link Protocol

The protocol used over the data links consists of a request message sent fromthe PSAP to the ALI
Computers and the response message from the database to the PSAP. In addition, astatus
message or “heartbeat” is expected from the PSAP during idle periods.The ALI Computers will
respond with an “ACK” to the PSAP when a heartbeat is received.

(1) Request Message Format

Requests from the PSAP for additional data contained in the ALI databasemaintained by
BellSouth shall consist of 13 or 15 ASCII encoded digits terminated by acarriage return.
The message format for a request is given in Table 2.








Three digits identifying the caller’s area code


One digit given by the tandem office to identify the caller’s area code


Three digit Exchange prefix for caller’s serving office


Four digits identifying the Caller’s Directory Number or wireless pseudo-ANI


Two digit PSAP position answering call or a sequence number


Two digit Incoming trunk number over which call was received, range
00-94 decimal for automatic lookup and 95-99 decimal for special lookup.


One digit, that when added to the sum of the previous twelve digits, causes the sum to be evenly divisible by 8


CR – Carriage return, hex 0D, signals end of request

Example: “0555121201020<CR>” would be transmitted for 555-1212 on position 01
trunk 02 for 14 character request messages. Note the check digit is 0 since the sum equals 24 which is evenly divisible by 8. Digit position 1 is transmitted on the data link first in sequence.

“404555121201020<CR>” would be transmitted for 404-555-1212 on position 01 trunk 02 for 16 character request messages. Note the check digit is 0 since the sum equals 32 which is evenly divisible by 8.

NOTE: A request of all zeroes, “0000000000000<CR>” would be interpreted as indication that repair is needed on the PSAP ANI equipment. Upon occurrence of a large number of such messages, the BellSouth E911 Maintenance Center would notify responsible PSAP maintenance personnel.

ALI Response Message Format

The ALI Computers provide an immediate response to a request message with either an ASCII ACK (hex 06) or NAK (hex 15) character. If the negative acknowledgement, NAK, is received, the request message should be retransmitted only once by the PSAP.

The ALI response message must use the format described in Table 3. The ALI response message is delimited by the ASCII STX (hex 02) and ETX (hex 03) characters. Response messages are presented over one of the two data links randomly chosen.




Character Position



STX – Hex 02, indicates start of message.


TYPE – One digit message type designation as described below.


POS – Two digit position/sequence number.


Text – Up to 320 characters of text formatted from the ALI database.
The format of the text portion of the message is shown in
Figure 2.
Longitude/latitude (X/Y coordinates) will be placed in
the text portion of the message when the PSAP requests Wireless
Phase 2 compliance and the wireless carrier provides coordinates.
The longitude/latitude fields will contain spaces for wireline calls
and wireless calls when coordinates are not available. PSAP
CAD/Mapping software should validate the contents of the
longitude/latitude fields prior to using. These optional fields will
not be included in the text unless requested by the PSAP. In
this case the ETX immediately follows the ESN text data.
Figure 2 note 2.


ETX – Hex 03, immediately follows text to indicate end of message.

TYPE Designations:

Type = 1 (hex 31) Data retrieved, only one path available.

Type = 2 (hex 32) Data retrieved, both paths operational (normal).

Type = 9 (hex 39) No address data found, text portion of message is of form

Type = 0,3-8 Not used

Note: If a problem is suspected in the received text, the PSAP may resend the request


<CR> 1 hex 0D

punctuation 1 ( hex 28

NPA 3 205

punctuation 1 ) hex 29

space 1 hex 32

NXX 3 456

punctuation 1 – hex 2D

Line Number 4 7890

space 3 hex 32

Time 5 14:25

space 4 hex 32

Date 5 08/12

<CR> 1 hex 0D

House Number 8 8600

House Number Suffix 4 -1/2

space 1 hex 32

Prefix Directional 2 NW

space 1 hex 32

Street Name 48 WALNUT ST

Community 18 BIRMINGHAM

State 2 AL

space 1 hex 32

ESN 3 056

space 1 hex 32

Class of Service 4 BUSN

<CR> 1 hex 0D

Customer Name 32 JOHN DOE INC

Location Information 20 BLG 3, FLR 10

space 1 hex 32

punctuation 2 P#

space 1 hex 32

Main NXX 3 454

punctuation 1 – hex 2D

Main Number 4 7891

<CR> 1 hex 0D

Additional Information 20 ALT# 205-454-7800 (see Note 1)

space 1 hex 32

punctuation 4 LEC:

Telco Company ID 5 BELSO

<CR> 1 hex 0D

<LF> 1 hex 0A

ESN Text (variable) 71 JEFFERSON COUNTY SO



Optional <CR> 1 hex 0D (see Note 2)

Optional Longitude/X Coordinate 11 +000.000000 or spaces (see Note 2)

Optional Space 1 hex 32 (see Note 2)

Optional Latitude/Y Coordinate 10 +00.000000 or spaces (see Note 2)


NOTE 1: This variable data field containsadditional information that may be helpful to the PSAP. One example is todisplay an ALTERNATE CALL-BACK NUMBER of the caller for interim numberportability or the CALL-BACK NUMBER for awireless call if this feature is available. This field will contain spaces if noadditional information is available. For wireless calls, the telephone number inthe first part of the response text may represent the psuedo-ANI (pANI) used toidentify the cell site or sector from where the wireless call originated. Cellsite/sector information will be populated in various data fields based on localagreements.

NOTE 2: These optional fields will notbe included in response message unless requested by a PSAP capable of processingX/Y. The ETX immediately follows the ESN Text field when these optional fieldsare not requested.

Figure 2 – Format of Text Portion of Response From E911 DataBase


Status Messages

During otherwise idle periods, the PSAP equipment shall place a “heartbeat” sequence on the data links at a minimum of once every two minutes of communications silence. Absence of data or a “heartbeat” on the data link from the PSAP for more that two minutes will be interpreted by the ALI Computers as a problem at the PSAP and reported to BellSouth maintenance personnel.

Upon notice of lack of a “heartbeat” from a PSAP, the BellSouth Maintenance Center will check the data links for possible trouble conditions. If the data links are found operational, the Maintenance Center will take action to notify responsible customer PSAP personnel of the failure.

The format of the “heartbeat” consists of the characters “H<CR>” (hex 48 and hex 0D).




“Network to Customer Installation Interfaces – Enhanced 911 Analog Voicegrade PSAP Access Using Loop Reverse-Battery Signaling”, ANSI T1.414, 1998.

“Lightning and 60-Hz Disturbances at the Bell Operating Company Network Interface”, TR-EOP-000001, Telcordia, Issue 2, June 1987.

“LSSGR: Signaling for Analog Interfaces”, GR-506-CORE, Telcordia, Issue 1, Revision 1, November 1996.

“LATA Switching Systems Generic Requirements (LSSGR)”, TR-TSY-000350, Telcordia, Issue 1, November 1987.

“Interface Between Miscellaneous Control Functions of Customer Premises Equipment and 1/1A ESS Centrex Switching Systems”, TR-TSY-000349, Telcordia, Issue 1, November 1986.

If you desire to order these Telcordia technical documents, please call (201)699-5800.