Legacy Selective
Router Gateways
Terry Reese
NENA NG9-1-1 Architecture Evolution
Subcommittee Chair
Senior Consultant, Ericsson
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Legacy Selective Router Gateways
 The
Selective Router (SR)/E9-1-1
tandem is a critical element in
today’s Emergency Services Network
infrastructure
 Provides
the tandem switching of 9-1-1
calls
 Controls delivery of the voice call with ANI
to the Public Safety Answering Point (PSAP)
 Provides Selective Routing, Speed Calling,
Selective Transfer, Fixed Transfer, and
certain maintenance functions for each
PSAP
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 On
the incoming/ingress side, the SR
interconnects with wireline end office
switches and Mobile Switching
Centers (MSCs) via dedicated SS7 or
MF trunk groups
 On the outgoing/egress side, the SR
interconnects with PSAPs via
Traditional or Enhanced MF trunk
groups
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Legacy Selective Router Gateways
Evolution
toward NG9-1-1 has
been motivated by:
The
need to support VoIP callers and NG
PSAPs
The desire for better alignment of the
Emergency Services architecture with
the overall evolution of
telecommunications networks to IPbased architectures
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 An
NG9-1-1 solution has 3 major
components:
 Originating
networks
 Emergency Services Networks
 PSAPs
 Different
components will evolve at
different times/rates
 Gateway systems are needed to
facilitate transition to an end-to-end
NG9-1-1 solution
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 Transition
to NG9-1-1 may include
support for the interconnection of
legacy Selective Routers (SRs) with a
NG Emergency Services Network
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Legacy Selective Router Gateways
 Consider
where:
a transitional scenario
Emergency
Services Network has
evolved to an i3 ESInet on which i3
Functional Elements reside
Originating network continues to
interconnect with legacy SRs and/or
PSAPs continue to be served by a
legacy SR
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Legacy Selective Router Gateways
 Legacy
Selective Router Gateway
(LSRG) provides gateway
functionality between legacy SRs and
the i3 ESInet
LSRG
may be located on the ingress side
and/or the egress side of an i3 ESInet
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Legacy Selective Router Gateways
 Example
#1: Originating network served
by SR; PSAP served by ESInet
Signal Path
Location Retrieval
MPC/
GMLC
Legacy
Originating
Network
Routing Query
ALI
SRDB
ECRF
ECRF
ESRP
VPC
Legacy SR
i2/VSP
Network
SS7
SIP
Legacy
SR
Gateway
ALI – Automatic Location Identification
ECRF – Emergency Call Routing Function
ESRP – Emergency Services Routing Proxy
GMLC – Gateway Mobile Location Center
MPC – Mobile Positioning Center
PSAP – Public Safety Answering Point
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SIP – Session Initiation Protocol
SR – Selective Router
SRDB – Selective Routing Database
SS7 – Signaling System No. 7
VPC – VoIP Positioning Center
VSP VoIP Service Provider
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PSAPs
F l o r i d a
Legacy Selective Router Gateways
 The
LSRG looks to the legacy SR like
another legacy SR
Use
tandem-to-tandem routing
mechanisms to deliver emergency calls
to LSRG
NENA
03-003 - SS7 trunk interface with
Selective Routing at the 2nd tandem
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 What
does the LSRG do when it
receives an emergency call from a
legacy SR?
Interworks
incoming SS7 signaling to
outgoing SIP signaling
Routes the call to the ESInet
Needs
to obtain location information that
can be used to route the call to/through
the ESInet
Supports
delivery of display/dispatch
location to the PSAP
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Legacy Selective Router Gateways
 Routing
Location vs. Display/Dispatch
Location
The
LSRG will query the ALI system to
obtain location
ALI
may need to query an MPC/GMLC in a
wireless network or a VPC in an i2 VSP
network to obtain location
Location obtained from the ALI system will
be used for display/dispatch purposes
Location obtain from an ALI system may not
be suitable for routing purposes
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 Routing
Location vs. Display/Dispatch
Location (cont.)
Routing
Location must reflect
agreements between PSAPs to take
certain types of calls on their behalf
For
example, PSAP B may agree to handle
wireless calls that originate in PSAP A’s
serving area on behalf of PSAP A.
Location
returned by the ALI system may
not reflect such arrangements
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 How
does the LSRG obtain a Routing
Location?
For
calls from fixed or nomadic users, the
routing location and the
display/dispatch location are the same
Location
information returned by the ALI
system (possibly after querying a VPC) can
be used for both routing and display
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 How
does the LSRG obtain a Routing
Location? (cont.)
 For
calls from mobile users, the routing
location is obtained based on preprovisioned mappings of the ESRK or ESRD
to a routing location that, when used to
query the ECRF, will cause the call to be
routed to the desired primary PSAP
E.g.,
a point within the desired primary PSAP’s
serving area, or a polygon that describes the
serving area of the desired primary PSAP
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 What
does the LSRG do when it receives
an emergency call from a legacy SR?
(cont.)
 The
LSRG uses the routing location to query
the ECRF for routing info
 The LSRG interworks incoming SS7 signaling
to outgoing SIP signaling and delivers the
call to the i3 ESInet with callback and
location information
Location-by-value
(typically for fixed or
nomadic callers)
Location-by-reference (typically for mobile
callers)
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Legacy Selective Router Gateways
 What
does the LSRG do when it
receives an emergency call from a
legacy SR? (cont.)
If
the location information delivered to
the i3 ESInet with the call is location-byreference:
The
LSRG will provide routing location in
response to location de-reference requests
from ESRPs
The LSRG will provide display/dispatch
location in response to location dereference requests from i3 PSAPs
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 Example
#2: Originating network served
by ESInet; PSAP served by Legacy SR
i3
PSAP
LIS
Signal Path
Location Retrieval
Routing Query
CIDB
i3 Compliant
Originating
Network
ECRF
Legacy
Originating
Network
SS7
Signaling
ESRP
SIP
D e v e l o p m e n t
Legacy
PSAP
SIP
Legacy
Network
Gateway
Location
Database
Legacy
SR
Gateway
ECRF
C o n f e r e n c e
E2+/PAM
Interface
ALI Interface
MPC/
GMLC
ALI – Automatic Location Identification
CIDB – Call Information Database
ECRF – Emergency Call Routing Function
ESRP – Emergency Services Routing Proxy
GMLC – Gateway Mobile Location Center
N E N A
Legacy SR
ALI
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LIS – Location Information Server
MPC – Mobile Positioning Center
PSAP – Public Safety Answering Point
SIP – Session Initiation Protocol
SR – Selective Router
SS7 – Signaling System No. 7
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 When
an emergency call arrives at
an LSRG from an ESRP in an i3 ESInet,
the incoming SIP signaling will contain
Callback
information
Location (by-reference or by-value)
The SIP address of the target PSAP
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 What
does the LSRG do when it
receives an emergency call from an
ESRP in an i3 ESInet?
Routes
the call to the legacy SR
Provides
information to the SR in a format
that it understands (i.e., 10-digit keys)
Interworks
incoming SIP signaling to
outgoing SS7 signaling
Supports delivery of display/dispatch
location to the legacy PSAP
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Legacy Selective Router Gateways
 Call Routing/Delivery to Legacy SR
 LSRG must map the SIP address of the
target PSAP to a PSAP Directory Number
 LSRG uses the PSAP DN to select the
outgoing trunk group to SR
 The LSRG signals the call to the SR as if it
were another legacy SR
NENA
 The
03-003 – Direct Routing at 2nd Tandem
LSRG must create a 10-digit location
key (pANI) and associate it with received
location information; passes the pANI to the
legacy SR via SS7 signaling
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 What
does the SR do when it receives
an emergency call from an LSRG?
Routes
the call to the target PSAP based
on the PSAP DN
First
queries the SRDB with the pANI
provided in incoming signaling to obtain
ESN information for potential future use
(e.g., for Selective Transfer)
Delivers
call
N E N A
the pANI to the PSAP with the
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 Support for Location Delivery to PSAP
 The PSAP uses the pANI to query the ALI
system
 The ALI system uses the pANI to steer the
location query to the LSRG, as if it were an
MPC/GMLC (e.g., using an E2 query)
 The LSRG returns the location information
associated with the pANI in an E2 response
If
the location information received by the
LSRG from the ESRP was a location-byreference, it must first de-reference the
location-by-reference to get location-by-value
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 What
about the non-location
information provided in the E2
response (e.g., service provider, class
of service)?
The
LSRG may obtain Additional Call
Data from the body of the received SIP
message (if provided), or it may need to
query a Call Information Database
(CIDB) to obtain it
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 The
LSRG will also play a role in the
transfer of emergency calls between
legacy PSAPs that are served by SRs and
PSAPs that are served by ESInets
 Transfer
from PSAP served by SR to PSAP
served by ESInet
 Transfer from PSAP served by ESInet to PSAP
served by legacy SR
 Bridged/transferred
based on location
calls are not routed
 Use
NENA 03-003 – Direct Routing/Transfer
at 2nd Tandem
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 Transfer
from PSAP served by SR to PSAP
served by ESInet
 SR
determines destination PSAP address based
on digits dialed by PSAP or internal translations
 SR routes call over tandem-to-tandem trunks to
LSRG
 LSRG queries ALI system using ANI/Calling
Number to obtain display/dispatch location
 Call is routed via ESRP to destination PSAP
based on PSAP address
 Call is delivered to the destination PSAP with
callback and location information
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 Transfer
from PSAP served by ESInet to
PSAP served by legacy SR
 Transferred
call is routed via a conference
bridge in the ESInet to the LSRG based on PSAP
address
 LSRG receives location and callback
information with the transferred call
 LSRG caches location and creates a pANI
which gets delivered to the destination PSAP via
the legacy SR
 The PSAP queries the ALI system with the pANI,
and the ALI steers the query to the LSRG
 The LSRG delivers location and non-location
information via the ALI system to the PSAP
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Status of LSRG Standard
The NENA i3 Architecture WG is in the
process of developing a standard for
the LSRG
Last
worked on in April 2014; work deferred
so that WG can focus on All Committee
Review comments on i3 standard
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Status of LSRG
Open issues:
Standard
Draft
LSRG standard currently models the
signaling between LSRG and legacy SR
after the wireless Wireline Compatibility
approach (i.e., call is delivered with pANI
only)
Should the standard also allow for
arrangements in which both callback
number and pANI are delivered by the
LSRG to the legacy SR (i.e., wireless NCAS
approach)?
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Open
 Draft
issues: (cont.)
LSRG standard includes a section for call
flows; need contributions to complete this section
N E N A

Initial Call, Originating Network on ESInet, Primary PSAP on SR

Initial Call, Originating Network on SR, Primary PSAP on ESInet

Transfer, Originating Network on ESInet, Primary PSAP on SR, Secondary
PSAP on ESInet

Transfer, Originating Network on ESInet, Primary PSAP on SR, Secondary
PSAP on SR

Transfer, Originating Network on ESInet, Primary PSAP on ESInet,
Secondary PSAP on SR

Transfer, Originating Network on SR, Primary PSAP on SR, Secondary PSAP
on ESInet

Transfer, Originating Network on SR, Primary PSAP on ESInet, Secondary
PSAP on ESInet

Transfer, Originating Network on SR, Primary PSAP on ESInet, Secondary
PSAP on SR
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Questions?
Comments?
Volunteers?
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