P-CSCF (Visited/Home):
`````````````````````
access,
compress SIP msgs, correctness

works with IMS terminals

authentication
asserts user identity
policy decision function
generates charging info

1st point of UE contact
which public identities registered

I-CSCF: - stateless
``````````
edge admin domain
DNS named

Diameter to HSS SLF (RFC 3588)
THIG
    encrypts # of servers, DNS names, capacity of CSCFs

S-CSCF:
`````````````
SIP server
REGISTRAR: IP AOR (PUblic User Identity)
Diameter to HSS
- download the authentication vectors of the user
- user profile (includes service profile == set of triggers to route message throughut  chain of AS)
- inform HSS that this S-CSCF is allocated for the user

TRANSLATION E.164 enum -
The system works by publishing a DNS zone, 'e164.org', that can be used by various Internet applications including SER, Gnome Meeting and Asterisk. The idea is to be able to map your phone number to an Email address, website, VoIP addresses, etc.
E164.org is a DNS zone that contains NAPTR records for phone numbers.


search=>e164.org
search=>e164.arpa

SIP:
sigcomp
subscribe-notify with throttling
SIP URI:
sip:+1-212-555-0293@operator.com;user=phone
tel:+1-212-555-0293
precondition [ QoS ]
sec-agree [security agreement]
Regular transactions - any except INVITE ACK CANCEL
CANCEL - r/t previous. Terminates prev transaction as ususal (final/ack)
Avoid fragmentation - NAT may drop segments (only 1st contains the port) -

REGISTRATION:
ISIM - IMS specific, contains PUBLIC/PRIVATE
USIM - previous one. During registration temporary URI/Private User Identity/Publics UI (in author. fileds) - derived from IMSI:
		MSIN@mcc.mnc.imsi.3gppnetwork.org
HSS marks which to use in subseq requests

URI for REGISTRATION can't be tel???
User Must subscribe to reg events (no coverage)

P-Access-Network-Info
P-Associated-URI - S-CSCF returns in 200 based on HSS MAA response
service route

Call set-up
`````````````
each node (except I-CSCF) inserts record-route, chooses a transport (P-CSCF has IPsec layer)
INVITE:

P-Preffered-Identity (because several Publlic UI) - security, charging, display to callee, trigger services

P-CSCF - > P-Asserted-Identity [default or one of preffered] - never to nontrusted
			P-Access-Network-Info: type GERAN, UTRAN => rates, bandwidth
									radio cell identity (optional - when available)
									- until last hop home network only, never in callee network

From/To/Call ID - nodes do not touch FROM is not policed =>any URI

contact - rather IP [if nor reverse DNS]

mandatory (required)/optional (supported)

