SIP%20in%203G

1
SIP in 3G

• HUT S-38.130 Spring 2001
• Tuomo Sipilä
• Nokia Research Center

2
SIP in 3G Content

• Background
• 3GPP R5 architecture
• Packet Core Network
• IP Multimedia Subsystem
• Requreiments
• Architecture
• SIP protocol in 3G
• 3G SIP requirements
• Problems
• Conclusions

3
Background

• 3G is known as UMTS in Europe, as IMT-2000 in
  Japan
• The standarisation work for IP based multimedia
  started in Autumn 1999 based on input from 3G.IP
• Targets to standardise the required enhancements
  for the 3G network so that
• IP telephony and multimedia can be provided with
  equal user perceived quality as with the current
  mobile network services
• 3G network can function fully based on packet and
  IP connections (without traditional circuit
  switched domain)
• IP multimedia would in the future provide via IP
  a wider and more flexible service set than the
  current networks
• SIP was selected as the signalling protocol for
  IP Multimedia in Spring 2000

4
3GPP Rel5 system architecture

• Radio Access Network Domain (RAN) For radio
  access WCDMA based UTRAN or GSM/EDGE based GERAN
• Circuit Switched Core Network Domain (CS CN) for
  Circuit switched services
• Packet Switched Core Network Subsystem (PS CN)
  for provision of PS connectivity services
• IP Multimedia Core Network Subsystem (IMSS) for
  the IP base multimedia services, IPv6 based
  system !
• Service Subsystem for operator specific services
  (e.g. IN and OSA)
• Subsystem independent evolution and access
  independency is the principle

NOTE Not all interfaces are shown !
5
PS CN Architecture

• Key issues
• Normally Terminal activated the PDP contexts
  (between GGSN and UE)
• Four QoS classes defined for packet connection
• Primary PDP context activation issue IP address
  to the terminal
• Secondary PDP context new flow with new QoS with
  same IP address
• Traffic Flow Template Filters the IP flows to
  the right PDP context
• Gi and Go interface towards IP Multimedia
  Subsystem (Go for policy control)

HSS
UTRAN
CAP over SS7/IP
Gr
Gc
Iu
Gi/Go
Gn
Gn
SGSN
GGSN
GERAN
Iu
PS CN domain
6
3G QoS Classes in Packet Core network
7
PDP Context activation
8
IM SS architecture

• Gi interface from GGSN to external networks is
  not shown in the figure

9
Requirements for IMSS

• at least equal end-to-end QoS for voice as in
  circuit switched (AMR Codec based) wireless
  systems
• equal privacy, security or authentication as in
  GPRS and circuit switched services
• QoS negotiation possibility for IP sessions and
  media components by both ends
• access independence i.e. the IP Multimedia
  network and protocols evolve independently of
  radio access (WCDMA, EDGE/GSM/GPRS, WLAN etc)
• applications shall not be standardised
• IP policy control possible i.e the operators
  shall have the means to control which IP flows
  use the real-time QoS bearers
• automated roaming with the services in home and
  visited network
• hide the operator network topology from users and
  home/visited network
• the resources shall be made available before the
  destination alerts
• adressing with SIP URL or E.164 number
• procedures for incoming and outgoing calls,
  emergency calls, presentation of originator
  identity, negotiation, accepting or rejecting
  incoming sessions., suspending, resuming or
  modifying the sessions
• user shall have the choice to select which
  session components reject or accept

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Network Elements (1/3)

• HSS (Home Subscriber Server)
• User Identification, Numbering and addressing
  information.
• User Security information Network access control
  information for authentication and authorization
• User Location information at inter-system level
  HSS handles the user registration, and stores
  inter-system location information, etc.
• The User profile (services, service specific
  information)
• P-CSCF (Proxy Call State Control Function)
• First contact point for UE within IM CN subsystem
  forwards messages to S-CSCF
• Is like proxy or user agent in RFC 2543 (SIP)
• Is discovered using DHCP during registration or
  the address is sent with PDP context activation
• May perform number analysis (e.g., detect local
  service numbers)
• Detect and forward emergency calls
• Call monitoring and logging (e.g., billing
  verification)
• Authorization of resource usage

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Network Elements (2/3)

• S-CSCF (Serving Call State Control Function)
• Maintains call state required to provide call
  related services
• Interacts with Services Subsystem
• Controls MRF
• Monitors sessions for billing purposes
• I-CSCF (Interrogating Call State Control
  Function)
• "is the contact point within an operator's
  network for all connections destined to a
  subscriber of that network operator, or a roaming
  subscriber currently located within that network
  operator's service area"
• can be reagarded as a firewall
• Routes SIP requests from another networks to
  S-CSCF and MGCF
• May hide service provider's network topology
• Selects S-CSCF during registration

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Network Elements (3/3)

• MGCF (Media Gateway Control Function)
• Protocol conversion between ISUP and SIP
• Routes incoming calls to appropriate CSCF
• Controls MGW resources
• MGW (Media Gateway)
• Transcoding between PSTN and 3G voice codecs
• Termination of SCN bearer channels
• Termination of RTP streams
• T-SGW (Transport Signalling Gateway)
• Maps call related signalling from/to PSTN/PLMN on
  an IP bearer
• Provides PSTN/PLMN lt-gt IP transport level
  address mapping
• MRF (Multimedia Resource Function)
• Performs multiparty call and multi media
  conferencing functions
• BGCF (Breakout Gateway control function )
• selects the network in which the PSTN
  interworking should occur
• selects the MGCF which will perform the
  interworking

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Roaming model
User A
As visited network
As home network
Required onregistration,optional on sessiion
establish
S-CSCF
P-CSCF
I-CSCF
I-CSCF
Optional
User B
I-CSCF
I-CSCF
P-CSCF
Required onregistration,optional on sessiion
establish
S-CSCF
Bs visited network
Bs home network
- P-CSCF - Proxy CSCF (Call Session Control
Function). The terminals point of contact in the
visited network after registration. - I-CSCF -
Interrogating-CSCF. Responsible for finding the
S-CSCF at registration. May also perform hiding
of the S-CSCF network architecture. - S-CSCF -
Serving-CSCF. Responsible for identifying users
service priveleges. Responsible for selecting
access to home network application server
(service platform) and for providing access to
that server
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SIP in interfaces

• SIP in IMSS interface
• Gm P-CSCF - UE
• Mw P-CSCF - S-CSCF and P-CSCF - I-CSCF
• Mm S/I-CSCF - external IP networks other IMS
  networks
• Mg S-CSCF - BCGF
• Mk BCGF - external IP networks other IMS
  networks
• SIP is used to interface the Application
  servers
• S-CSCF- SIP Application server
• S-CSCF- Camel Server
• S-CSCF-OSA Service Server

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Current 3GPP SIP procedures

• Local P-CSCF discovery
• Either using DHCP or carrying address in the PDP
  context
• S-CSCF assignment and cancel
• S-CSCF registration
• S-CSCF re-registration
• S-CSCF de-registration (UE or network initiated)
• Call establishment procedures separated for
• Mobile origination roaming, home and PSTN
• Mobile termination roaming, home and PSTN
• S-CSCF/MGCF - S-CSCF/MGCF between and within
  operators, PSTN in the same and different network
• Routing information interrogation
• Session release, Session hold and resume
• Anonymous session establishment
• Codec and media flow negotiation (Initial and
  changes)
• Called ID procedures
• Session redirect, Session Transfer

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Some requirement solutions

• Key issues
• A) Mobile terminated calls
• 1) have network initiated PDP Context activation
  (required static IP address)
• discussion ongoing on push services
• options 1 a new element to link the IMSI with
  the dynamic IP address allocation
• option 2 use SMS to trigger PDP activation in
  the terminal
• 2) provide an always on PDP context (signalling
  PDP context)
• the P-CSCF address to the terminal
• either during the PDP context activation or
• after PDP activation with DHCP procedures, then
  with DNS to find the IP address
• both options possible with current specs
• B)avoid alerting before the resources are
  available
• 2 phase call setup
• C) Should SIP use a signalling channel on Radio
  interface ?
• If yes the capabilities needs to be limited and
  message compression used
• will limite the usage of SIP to signalling
  protocol only

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Registeration
18
Mobile initiated call setup
1-22 Session description exchange
23-31 Resource reservation
32- 43 Alerting
44-52 Answering the call
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Example of INVITE message
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SIP protocol requirements in 3GPP

• addition of routing PATH header to the SIP
  messages to record the signalling path from
  P-CSCF to S-CSCF
• location information in the INVITE message to
  carry the location of the terminal (for instance
  Cell ID)
• emergency call type is needed to indicate the
  type of emergency call i.e. is it police,
  ambulance etc.
• filtering of routing information in the IM SS
  before the SIP message is sent to the terminal to
  hide the network topology from terminal
• refresh mechanism inside IM SS
• Network-initiated de-registration
• 183 Session Progress provisional response for
  INVITE to ensure that the altering is not
  generated before PDP contexts for session are
  activated
• Reliability of provisional responses - PRACK
  method to acknowledge the 183 message
• Usage of session timers to keep the SIP session
  alive
• Indication of resource reservation status - COMET
  method
• Security for privacy
• Extensions for caller preferences and callee
  capabilities
• Media authorisation token for the Policy Control
  function to authorise the PDP context with SIP
  connection in the UE

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Problems

• architecture complexity
• call establishment delay problems due to the
  signalling taking place on multiple levels (RAN,
  PS CN, IMSS).
• establishing a call there will be 6 round trip
  times (RTT) end to end on SIP level PDP context
  reservations
• guarantees of QoS
• Several elements and several IP based interfaces
• lengthy standardisation time
• suitability of the SIP protocol for the radio
  interface, long character based messages,
  compression needed
• IETF and 3GPP standardisation co-operation
• Terminal complexity

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Conclusions 3GPP specifics for SIP

• the architecture of the IMSS is defined based on
  3G model (home and visited), messages run always
  via S-CSCF
• Registration is mandatory
• The CSCFs interrogate the SIP and SDP flows
  either actively modifying the messages or reading
  the data, also the I-CSCF hides the names of CSCF
  behind it
• Codec negotiations in 3GPP do not allow different
  codecs in different directions
• in 3G networks there is a separation of UNI and
  NNI interface
• due to radio and packet core functionality there
  are some change proposals to the SIP and SDP
• due to the P-CSCF - S-CSCF interface and the 3G
  roaming mode there are some requirements to the
  SIP and SDP protocols
• in 3G SIP is used also to interface the
  application development elements, they set
  requirements for SIP and SDP protocols

• THUS
• SIP is suitable for 3G if the problems (call
  delays, SIP length, QoS) can be solved
• Specification work shall take still some time
• 3G and SIP should provide enhaced and rich
  services NOT be ONLY the replacement for CS