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Internet Telephony (VoIP)
Henning Schulzrinne Dept. of Computer Science Columbia University Fall 2003


new Internet services: ³telephone´, ³radio´, ³television´ why Internet telephony? why not already? Internet telephony modalities components needed:
± ± ± ± ±

audio coding data transport quality of service ± resource reservation signaling PSTN interworking: gateway location, number translation

Name confusion

Commonly used interchangeably:
± ± ±

Internet telephony Voice-over-IP (VoIP) IP telephony (IPtel)


Also: VoP (any of ATM, IP, MPLS) Some reserve Internet telephony for transmission across the (public) Internet Transmission of telephone services over IP-based packet switched networks Also includes video and other media, not just voice

New Internet services


tougher: replacing dedicated electronic media vs. new modes (web, email) distribution media (radio, TV): hard to beat one antenna tower for millions of $30 receivers typewriter model of development radio, TV, telephone: a (protocol) convergence?

The phone works ± why bother with VoIP
user perspective
variable compression: tin can to broadcast quality no need for dedicated lines security through encryption caller & talker identification better user interface (more than 12 keys, visual feedback, semantic rather than stimulus) no local access fees (but dropping to 1c/min for PSTN) adding video, application sharing is easy

carrier perspective
better codecs + silence suppression ± packet header overhead = maybe reduced bandwidth shared facilities simplify management, redundancy advanced services cheaper bit switching fax as data rather than voiceband data (14.4 kb/s)

Emergency Calling

911 in North America, 112 in Europe, others elsewhere First implemented 1968 in US, now roughly 95% of US population Basic 911 service: route emergency call to nearest emergency call center (public safety answering point ± PSAP) Later, enhanced 911 (E-9-1-1) for selective routing and conveying caller location information to PSAP Roughly, 150 million 911 calls per year (2000)
± ± ±

45 million wireless Phase I conveys call back number + Pseudo-ANI (cell face identifier) to PSAP Phase II provides caller location (e.g., via GPS or TOA)

For wireless: Phase I and Phase II

Wireless 911 Phase II - TDOA


Wireless 911 Phase II - EOTD


Wireless 911 Phase II
Accuracy Handset-based Network-based 67% 50m 100m 95% 150m 300m


Example: Sprint PCS and Nextel use GPS Implementation just starting VolP has similar problems as wireless:

devices change ³network attachment point´

E9-1-1 Call flow elements - wireline
End office ES Trunks

E9-1-1 Tandem w/SRDB

EM Trunks

Public Safety Answering Point

Loop Acces s Contr ol ie DLC Syste m

The Local Loop Recent Change Links

PSAP ALI Data Links


SCP GATEWAY (Firewall)


Update Links Service Providers

ALI Database Elements

ALI/SR DBASE 9 6 2 5 E9-1-1 Tandem w/SRDB
Public Safety Answering Point

PDE 3 4 MPC E2





#9 is only applicable in a CAS-Hybrid architecture, such as BellSouth¶s WLS911 Solution


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