911 services: wireline,
wireless and VoIP
Prof. Henning Schulzrinne
Dept. of Computer Science
Columbia University, New York
FCC Solutions Summit
March 18, 2004
Overview
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E911 for wireline
E911 for wireless (Phase II)
Short-term 911 service for VoIP
Differences between PSTN and VoIP
Objectives
Internet standardization efforts
Components of emergency calling
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Three core components that need to
be replicated – everything else are
implementation details
1.
2.
identifying emergency calls (“911”)
determining the right PSAP for current
caller location
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3.
coarse-grained location
currently, ALI and MSAG
deliver caller location to PSAP
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fine-grained location
E911 for wireline
CAMA or SS7
555-1234
313 Main St
CO Switch
LEC
network
PSAP #1
CAMA or PRI
delivers ANI
(555-1234)
Tandem Switch
(911 Selective Router)
555-1234
 PSAP #1
MSAG
ANI: 555-1234
 313 Main
ALI
100-500 Main Street  ESN 1789
private
data link
555-1234  PSAP #1, 313 Main St
provisioned
updates
verify address
validity
Wireless 911
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Phase I (April 1998)
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Route all call to the
appropriate PSAP based
on call sector
Provide cell/sector
location data to PSAP
Provide call back number
to PSAP
Phase II (October 2001)
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Phase I + latitude and
longitude
67%
95%
handset
50m
150m
network
100m
300m
Wireless 911: Phase 2
MSC
pANI
(ESRD or ESRK)
ISUP
r s
i l
t
r
i
y N et
LEC
selective router
Wireless Tower
PDE
MPC/SCP
E2
S
A-GPS, UTDOA, …
t
w .a p
dynamic
updates
.c o
ESRK = unique for call
ESRD = unique for location
ALI
ESRK or ESRD 
coordinates
callback number
Problems with existing 911
system
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1970s technology:
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gets complicated if multiple providers
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CAMA trunks induce long call setup delays
limited in ability to transfer information (10 digits)
ILEC vs. CLEC
multiple wireless providers
tied to ILEC rate centers and other PSTN
routing artifacts
hard to move PSAPs on short notice (e.g.,
emergency evacuation)
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can’t just plug into any network termination
PSTN vs. Internet Telephony
PSTN:
Signaling & Media
Internet
telephony:
Signaling & Media
China
Signaling
Signaling
Media
Belgian customer,
currently visiting US
Australia
How does VoIP differ from
landline and wireless PSTN?
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Telephone companies are no
longer needed
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new location, but same identifier
there are still carriers for DSL and
cable “IP dial tone”
but unaware of type of data carried
(voice, web, IM, …)
VSP may be in another state or
country
Corporations and universities
don’t have email carriers, either
Yahoo

voice service provider
MCI
All devices are nomadic
ISP
(IP)
NYSERNET
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dark fiber
provider
(λ)
(TCP, RTP, SIP)
The role of phone numbers and
identifiers
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Wireline  line, device, subscriber & location
Wireless  device, but not location
VoIP (phone number and URIs):
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mostly identifies person, not device
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multiple devices located in different states can share the
same number
however, may not have a phone number
if it does, area code may be from different state
than customer billing address
multiple devices
device can move, while number stays the same
not related to ISP
Why is VoIP ≠ wireless?
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VoIP devices may not have phone numbers as lookup keys
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e.g., sip:hgs@cs.columbia.edu
Location information for devices is civil, not
longitude/latitude
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e.g., service address for VSPs
GPS not available (nor functional) on indoor devices
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Cell phones don’t work in our building…
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plus, accuracy of 50 m (67%) or 150 m spans many buildings…
no floor information
so A-GPS is unlikely to work there, either
Plus, wireless E911 complexity due to old signaling
mechanism
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expensive and complicated to connect to multiple wireless
operators
proposals to use IP-based solutions
50m
Objectives for IP-based 911
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International
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Multimedia
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integrity, privacy and confidentiality, protection against denial-of-service attacks
Technology-independent
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users can test operation without tying up operator resources
Secure
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easily re-route calls to any number of backup PSAPs
Testable
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re-use standard protocols (SIP, DNS, DHCP, HTTP, XML, …)
avoid repeat of CAMA trunks
Resilient
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integrate alternate modalities such as text (TDD) and video (sign language)
COTS
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devices must work anywhere
independent of local emergency number
international roaming
do not depend on (e.g.,) specific wireless or link technology
Pro-competitive
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does not require carriers or gatekeepers
Three stages to VoIP 911
when
deployable
?
use 10digit
admin.
number?
mobility
caller
location
conveyed
to PSAP?
PSAP
modification
ALI (DB)
modification
new services
I1
now
allowed
stationary
no
no
no
none
I2
December
2004
no
stationary
nomadic
yes
no (10-digit)
yes
none
I3
specified
by late
2004
no
stationary
nomadic
mobile
yes
IP-enabled
replaced by
DNS
GNP
multimedia
international
calls
Example I1 solution #1
SS7
CLEC #1
Example: VoxPath
End Office
Switch
LAN
IP Phone
Customer
Signaling
Gateway
PSAP
Softswitch
Broadband
Network
Media
Gateway
End Office
Switch
Selective
Router
1
2
3
4
5
6
7
8
9
*
8
#
Call Taker
POP
CLEC #2
Mark Lewis
I1 Solution #2
Level3
Customer
Softswitch
Dedicated
911 trunks
PSTN
E911
Tandem
End
Users
Public Internet
or
Private IP
Network
E911
Tandem
IP Phones
ALI
DB
PBX
(emergency lines)
Level 3 911
Softswitch
Network
ACD
PSAP
Mark Lewis
Possible I2 architecture
IP
INVITE sos
PSTN
MG
Selective
Router
ISUP
CAMA
ESRK, DN  loc
ALI
SIP
PUBLISH
ALIFE
E2+
ESP
PAM
Local
ALI
National
Voice N/Ws
Emergency Services N/W
based on slide
by Martin Dawson
IETF I3 standardization efforts
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IETF = Internet Engineering Task Force =
international open standardization body
“911”
sip:sos@
include
civil and/or geo
911  sos
112  sos
sip:psap@leonia.nj.gov
provide location
(civil or geo)
DHCP
cn=us, a1=nj, a2=bergen
Conclusion
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Existing 911 system closely tied to PSTN
history
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number as universal identifier
close affiliation with PSTN switches
incremental, constrained evolution
VoIP offers opportunity to increase
robustness and decrease costs
Initial international and US standardization
efforts in progress
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IETF and NENA collaboration combines 911 and
Internet expertise