SIP in 3G

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. Internet Services Subsystem RNC BSC IP 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. External IP networks and other IMS networks Applications & Services Legacy mobile signalling network R-SGW SCP P/I/S-CSCF Sc Ms PSTN/ Legacy /External Mh MRF BGCF Mc Mm S-CSCF Mm Cx HSS Mw Cx Gc I-CSCF Mi Gi Mk Mw BGCF Mg Go Mw GGSN P-CSCF Mj T-SGW MGCF Gi Mc MGW 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

10. 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

11. 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

12. 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 <-> 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

13. Roaming model User A A’s visited network Required onregistration,optional on sessiion establish A’s home network 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 B’s visited network B’s 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 user’s service priveleges. Responsible for selecting access to home network application server (service platform) and for providing access to that server

14. 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 SLF HSS AS Cx Cx Dx Gm Mw Mw UA P-CSCF I-CSCF S-CSCF Mg Mg BCGF MGCF Mc MGW SIP in interfaces

15. 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

16. 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

17. Registeration

18. Mobile initiated call setup 1-22: Session description exchange 23-31: Resource reservation 32- 43: Alerting 44-52: Answering the call

19. Example of INVITE message

20. 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

21. 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

22. 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