The IP Multimedia Subsystem eta Ss e t pse tee ep 1 e

1
The IP Multimedia Sub-system ????? ?e?t?????a?
S??s? µe t?? ?p?????se? te???????e?
ep??????????(1? ????e??)

• G?a t? µ???µa
• ???t?a ?????? ?a??t?t??

2
?e??e??µe?a

• ??a?s?? e??????? te???????a? d??t???. ?? ßas????
  ????? e?sa????? t?? IMS te???????a?
• ?e????af? t?? IMS a???te?t?????? ?a? t??
  ßas???te??? d?µ???? t?? st???e???
• ?as???? a???? ?e?t?????a?

3

• ??a?s?? e??????? te???????a? d??t???. ?? ßas????
  ????? e?sa????? t?? IMS te???????a?

4
?as??? ?e?t??????? p??ß??µata s?µe????? d??t???
(1/2)

• ???µ??????e?a ?p??es???
• ???ß??µata d?a?e?t???????t?ta? ???? as?µßat?t?ta?
  p??t???????
• ???µ????e?? ????s? ?p??es??? (p.?. ? ????s?
  f???t???? ???se?? de? e??a? ? ?d?a se sta?e?? ?a?
  ????t? d??t?a)
• ?????? ???s?? p????? d?af??et???? te?µat????
• ??af??et???t?ta p???t?ta? ?p??es?a?
• ????p????t?ta d??t???

5
?as??? ?e?t??????? p??ß??µata s?µe????? d??t???
(2/2)

• ????t?s? t?? t?p?? te?µat???? ???st? ap? t?
  d??t?? p??sßas??
• ???µ??????e?a t?? p??t??????? t?? d??t???
  p??sßas??
• ????t?s? t?? ?p??es?a? ???st? ap? t?? t?p? t??
  d??t??? p??sßas??
• ?d??aµ?a ???p???s?? ep?????????? µeta?? d??t???
  p??sßas?? d?af??et???? t?p??
• ???ß??µata d?asf???s?? ?a? d?a?e???s?? p????
  µeta?? d??t??? a??µ???e???? t?p??

6
?? t?p?? t?? s???????? t??ep?????????? (1/3)

• ??af??et??? µ?sa µet?d?s??
• ???t?a ????t?? t??/??? (GSM, GPRS, UMTS..)
• ???t?a as??µat?? t??/??? (WiFi-IEEE802.11,
  WiMAX-IEEE802.16..)
• ???t?a ?pt???? t??/??? (WDM-SONET/SDH)
• ???t?a d???f?????? t??/???(V-SAT, DVB, DVB-RCS..)
• Sta?e?? d??t?a (xDSL, ISDN, PSTN, Ethernet..)
• ???t?a a?s??t???? (Event driven transducers)
• ???t?a d?af??et???? ta??t?t??
• Broadband traffic rates from few Mbps and higher
  (e.g.xDSL)
• Narrowband supported traffic rate is some Kbps
  (PSTN and ISDN)

7
?? t?p?? t?? s???????? t??ep?????????? (2/3)

• ???f? ????s??
• Circuit switched (µeta???? ?????µat??)
  Originally designed for time division multiple
  access (TDMA). Data packets are circulated in the
  network using fixed timeslots, over the so-called
  circuits, that is constant bit rate channels
  established between the communicating ends.
  Transmission is always synchronous. Examples
  ISDN, GSM and part of 3G UMTS (UTRAN interface).
• Packet switched (µeta???? pa??t??- IP d??t?a)
  In contrast to circuits in packet switched
  networks original packets are segmented in
  smaller packets and there is no pre-established
  circuit between the communicating ends. Packet
  switched networks are asynchronous and of
  variable bit rate. QoS, Routing and statistical
  multiplexing applies to packet switched networks.
  Examples ATM, X.25, IP, etc.

8
?? t?p?? t?? s???????? t??ep?????????? (3/3)

• What is the main driver in todays communication
  networks evolution
• Migration of communications into the frame of the
  IP protocol.
• Consolidation of user services beyond the simple
  voice communication model, also incorporating
  other media such as video, audio and data.
• Provision of communication services over the
  Internet infrastructure.
• Homogenization of users service experience over
  any access network.
• Accessibility of communication services,
  everywhere at any time and over any user terminal.

9
?? ????? e?sa????? t?? ?MS te???????a?

• ? a????? ?µ??e??p???s?? t?? ?p??es??? ???st? µ?sa
  se ??a ????? p??t?????? ep????????a?, t? ??, µ?s?
  t?? ?p??? ???p?????ta? ?a? s?µat?d?t????
  d?e??as?e? a??? ?a? ? ep????????a ???st? (data
  plane).
• ???s? e??? ?a? µ??? µ??a??sµ?? ??a t??
  d?asf???s? ?a? d?a?e???s? p???? p??? ap?
  ?p???d?p?te d?t??? p??sßas??
• ????? p???t??? ????s?? a?e???t?ta ap? t?? t?p?
  t?? d??t??? p??sßas??
• ?????? µ??a??sµ?? a?a?????s?? ???st?, pa?????
  ?p??es?a? ?a? asf??e?a?
• Ge???e?µ??? d?a?e?t??????t?ta ???? t?? ?? d??t???
  p??sßas??, t?? t?p?? te?µat???? ???st?, a??µ? ?a?
  t?? d??t??? µeta????? ?????µat??

10
?? e??a? ? IMS d??t?a?? te???????a

• ?p? te?????????? ap??e?? e??a? t? s?µe??
  s????t?s?? t?? pa?ad?s?a??? t??ep?????????? ?a?
  t?? te???????a? p????f???a? .
• ??a ??S d??t?? e??a? ??a ?pe??e?µe??? t?p??
  d??t?? t? ?p??? de? a?a??e? t?? ?p?????se?
  d??t?a??? te???????e? p??sßas??a??? t?? e?t?sse?
  st? p?a?s?? t?? µ?s? t?? ???s?? t?? ??
  p??t???????.

11

• ?e????af? t?? IMS a???te?t?????? ?a? t??
  ßas???te??? d?µ???? t?? st???e???

12
General Architecture

• IMS is built around the SIP protocol
• Combines access/backbone networks under a common
  service management frame
• Backward compatible (smoothly cooperates with IP
  non-IMS and circuit switched networks)

13
Detailed architecture
14
IMS Requirements

• Requirements set by operators
• Allow multimedia services provisioning over any
  packet switched network type, mainly targeting
  mixed communication scenarios e.g. mobile-fixed
  networks.
• Create a common platform for services
  provisioning
• Allow users to create and use their own services,
  as it happens today over the Internet.
• Requirements set by standardization bodies
• Transfer control of user call from the underlying
  network to the IMS platform using IMS sessions
• Use of uniform QoS mechanisms for dynamic
  bandwidth allocation
• Support of network roaming and handover

15
Description of IMS components (1/13)

• From logical point of view IMS is a collection of
  logical blocks interconnected to each other and
  with the underlying network through logical
  interfaces.
• The main core of the IMS is the session control
  protocol realized with the SIP protocol. SIP is
  easy in use and smoothly cooperates with internet
  protocols such as the HTTP. Also SIP is
  modifiable in terms of supported messages and
  procedures
• With the exception of the Media Gateways, all IMS
  components are realized using the SIP protocol.

16
Description of IMS components (2/13)

• Main components
• SIP servers (CSCF)
• Databases (HSS, SLF)
• Application Servers (AS)
• MRF (divided in MRFC and MRFP)
• BGCFs
• MGW divided into SGW, MGCF, MGW

17
Description of IMS components (3/13)- CSCF
(Call/Session Control Function)

• The CSCF is an essential node in IMS.
• It is built using the SIP protocol.
• Depending on the provided functionality there are
  three CSCF types
• P-CSCF (Proxy-),
• I-CSCF (Interrogating-),
• S-CSCF (Serving-).

18
Description of IMS components (4/13)- P-CSCF
(Proxy Call/Session Control Function)

• P-CSCF Is the first point of contact between the
  IMS terminal and the IMS network. P-CSCF acts as
  an outbound/inbound SIP proxy server. This means
  that all requests initiated or destined to the
  IMS terminal traverse P-CSCF. P-CSCF forwards SIP
  requests and responses in the appropriate
  direction.
• Functions
• Network identification point (all underlying
  access networks are assigned an individual
  P-CSCF)
• QoS Enforcement logic

19
Description of IMS components (5/13)- S-CSCF
(Serving Call/Session Control Function)

• S-CSCF Is the central node of the signaling
  plane. It recognises incoming from the P-CSCF SIP
  messages and responds accordingly.
• Functions
• Session Control
• User registration and authentication point
• Interfaces the HSS for the purpose of retrieving
  user profiles, access privileges and charging
  records.

20
Description of IMS components (6/13)- I-CSCF
(Interrogating Call/Session Control Function)

• I-CSCF Is located at the edge of IMS networks
  and has role of SIP proxy.
• Functions
• It is listed in the DNS and is used by SIP
  servers (in the case of IMS the S-CSCF) to route
  SIP messages to the next hop, that is the S-CSCF
  residing outside of the home IMS network.

21
Description of IMS components (7/13)- Databases

• There are two database types HSS and SLF.
• The Home Subscriber Server (HSS) is the central
  repository for user-related information.
• The HSS is the equivalent of the HLR of the GSM
  networks.
• Data hosted on HSS include
• User identities
• Security information (authentication/authorisation
  )
• User profile
• An SLF (Subscriber Location Functions) is used to
  resolve to which HSS an IMS user belongs when the
  IMS network has more than one HSS.

22
Description of IMS components (8/13)- AS

• The Application Server (AS) is a server that
  hosts part of the service logic.
• The AS does not necessarily host the full service
  logic but at least the part of it that is
  responsible for session control (virtual channels
  establishment and QoS set up).
• There are 3 AS types
• SIP AS when the service is native SIP service
• OSA-SCS (Open Service Access-Service Capability
  Server) when the service runs in an OSA
  environment
• IM-SSF (IM-Service Switching Function) Used for
  hosting CAMEL services of mobile networks over
  IMS platforms.

23
Description of IMS components (9/13)- AS
24
Description of IMS components (10/13)- MRF

• The MRF (Media Resource Function) provides
  sources of media upon user request, e.g. playback
  of recorded announcements, mixing of media
  streams, transcoding of media streams, etc.
• MRF is divided in MRFC (Media Resource Function
  Control) and MRFP (Media Resource Function
  Processor).
• MRFC provides a SIP interface to allow S-SCSF to
  control the resources of the MRFP.
• MRFP implements all media related functions e.g.
  playback and mixing of media.

25
Description of IMS components (11/13)- BGCF

• The Breakout Gateway Control Function (BGCF)
  performs routing of SIP messages in the case of
  mixed calls between packet switched (IMS) and
  circuit switched (non-IMS) networks.

26
Description of IMS components (12/13)-
IMS-ALG/TrGW

• IMS-Application Layer Gateway Transition
  Gateway offer IPv4 lt-gt IPv6 interworking without
  terminal support.
• IMS-ALG performs interworking of signaling (SIP
  messages)
• TrGW performs interworking of data traffic (e.g.
  RTC streams)

27
Description of IMS components (13/13)- MGW

• A Media GateWay (MGW) performs media transcoding
  between CS (Circuit Switched) Networks and the
  IMS network. E.g. translation of ATM packets into
  ISDN frames and vice versa.
• To function properly MGW requires a signaling
  function to perform corresponding interworking of
  the signaling traffic. This function is provided
  by the MGCF in cooperation with the BGCF.

28

• ?as???? a???? ?e?t?????a?

29
Network organization (1/2)

• IMS borrows the concept of mobile networks of
  home and visited network.
• Definition of home network A user is
  requesting/accessing a service, being hosted by
  the same IMS network that holds his identity and
  profile details (e.g. credentials, access
  privileges, etc).
• Definition of visited network The user is
  requesting/accessing a service, being hosted by a
  IMS network different from the one that holds his
  identity and profile details.
• An IMS network is defined by the presence of a
  single S-CSCF.
• As in the mobile communications world, the
  concept of changing IMS networks, while using a
  service, is called roaming. Roaming is performed
  transparently to the access network type (fixed,
  mobile, wireless).
• Access networks are distinguished in IP-CANs (IP-
  Connectivity Access Network).
• There can be many IP-CANs of any type (UMTS,
  WiFi, xDSL) connected to a single IMS (the host
  IMS).

30
Network organization (2/4)

• Each IP-CAN is connected to the IMS core network
  via a single P-CSCF.
• P-CSCF is responsible for
• IP-CAN addressing,
• QoS enforcement on the path between the IP-CAN
  and the core network,
• Service proxy between the IMS network and the
  user (UE).

31
Network organization (3/4)

• Path establishment concept

32
Network organization (4/4)

• IMS allows manifold exploitation opportunities
• IP-CANs, IMS, Services hosted on AS can be
  exploited by different actors

33
User identification in IMS (1/5)

• Two identities are used
• A public one (a SIP URI or a TEL URI) used as
  user contact information (called IMPU).
• SIP URI is used as the source or destination
  address for SIP messages routing
• E.g. sipfirst.last_at_operator.com
• TEL URI is used to identify an IMS from external
  CS networks (e.g. PSTN). The TEL URI is indeed
  the equivalent of PSTN number number
  corresponding to an IMS when being called from an
  ISDN or PSTN network.
• E.g. tel[phone]
• TEL URIs can be transferred over the IMS network
  with SIP messages
• E.g sip1-212-555-0293_at_operator.comuserphone

34
User identification in IMS (2/5)

• A private user identity called IMSI (IMS
  Subscribed Identity) of NAI (Network Access
  Identifier) type with the format
• username_at_operator.com
• Private user identities are not used for SIP
  messages routing but for subscription
  identification and authentication purposes.
• The private user identity plays the same role as
  the IMSI in mobile networks. Is call IMPI.
• It need not be known by the user.
• It is stored in the terminal on a smart card.

35
User identification in IMS (3/5)

• Relation between private and public identities
• A user should have only one private identity but
  many public identities
• HSS stores the collection of the private and the
  public identities.
• S-CSCF has the logic of identities correlation.

36
User identification in IMS (4/5)

• Migration from SIM and USIM to ISIM
• Management of private identities seamlessly to
  the user is performed on mobile phones through
  the use of an embedded smart card, called UICC
  (Universal Integrated Circuit Card).
• In past technology GSM phones there was an
  application associated with management of IMSI
  called SIM (Subscriber Identity Management),
  providing functions
• collection of subscription info,
• authentication keys,
• authentication triplets.

37
User identification in IMS (5/5)

• In UMTS, USIM (Universal Subscriber Identity
  Management) is an application similar to SIM
  providing more enhanced IMSI management such as
• Storage of messages
• Payment methods
• Subscriber information
• Authentication information
• For IMS ISIM (IMS Services Identity Module) is an
  enhancement of USIM applicable for all terminal
  types (not only mobiles). ISIM provides
  applications for managing the IMSI (private user
  identity), offering functions such as
• Terminal configuration
• User information
• Authentication information
• Storage of application configurations

38
Bibliography

• 3GPP TS 23.002 "Network Architecture".
• 3GPP Technical Specification 23.228. (2006). IP
  Multimedia Subsystem (IMS). 3rd Partnership
  Project. www.3gpp.org
• 3GPP TS 23.221 "Architectural Requirements".
• 3GPP. Characteristics of the IP Multimedia
  Service Identity Module (ISIM) application. TS
  31.103. www.3gpp.org
• 3GPP. Characteristics of the USIM application. TS
  31.102. www.3gpp.org