requirements_and_architecture_for_emergency_calling----Page:4
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Core problems PSTN: approximate routing often works same switch based on cell tower based on caller number PSTN: relatively few, regionally-limited telecom providers (carriers) IP: carrier = bobs-bakery.com IP: no such approximations (usually) application layer (e.g., SIP) has no clue as to location L1—L3 may know about location (at least approximately), but don’t know about emergency calls |