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The IP Multimedia Sub-system – Αρχές
Λειτουργίας –Σχέση με τις υπάρχουσες
τεχνολογίες επικοινωνιών
(1η Διάλεξη)
Για το μάθημα:
Δίκτυα Υψηλών Ταχυτήτων
Περιεχόμενα
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Πλαίσιο εξέλιξης τεχνολογίας δικτύων. Οι βασικοί λόγοι
εισαγωγής της IMS τεχνολογίας
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Περιγραφή της IMS αρχιτεκτονικής και των βασικότερων
δομικών της στοιχείων
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Βασικές αρχές λειτουργίας
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Πλαίσιο εξέλιξης τεχνολογίας δικτύων. Οι βασικοί λόγοι
εισαγωγής της IMS τεχνολογίας
Βασικά λειτουργικά προβλήματα
σημερινών δικτύων (1/2)
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Ανομοιογένεια υπηρεσιών
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Προβλήματα διαλειτουργικότητας λόγω ασυμβατότητας
πρωτοκόλλων
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Ανομοιογενή χρέωση υπηρεσιών (π.χ. η χρέωση φωνητικής
κλήσεως δεν είναι η ίδια σε σταθερά και κινητά δίκτυα)
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Ανάγκη χρήσης πολλών διαφορετικών τερματικών
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Διαφορετικότητα ποιότητας υπηρεσίας
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Πολυπλοκότητα δικτύων
Βασικά λειτουργικά προβλήματα
σημερινών δικτύων (2/2)
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Εξάρτηση του τύπου τερματικού χρήστη από το δίκτυο
πρόσβασης
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Ανομοιογένεια των πρωτοκόλλων των δικτύων πρόσβασης
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Εξάρτηση της υπηρεσίας χρήστη από τον τύπο του δικτύου
πρόσβασης
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Αδυναμία υλοποίησης επικοινωνιών μεταξύ δικτύων
πρόσβασης διαφορετικού τύπου
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Προβλήματα διασφάλισης και διαχείρησης πόρων μεταξύ
δικτύων ανομοιγενούς τύπου
Το τοπίο των σύγχρονων
τηλεπικοινωνιών (1/3)
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Διαφορετικά μέσα μετάδοσης
 Δίκτυα κινητών τηλ/νων (GSM, GPRS, UMTS..)
 Δίκτυα ασύρματων τηλ/νων (WiFi-IEEE802.11, WiMAXIEEE802.16..)
 Δίκτυα οπτικών τηλ/νων (WDM-SONET/SDH)
 Δίκτυα δορυφορικών τηλ/νων(V-SAT, DVB, DVB-RCS..)
 Σταθερά δίκτυα (xDSL, ISDN, PSTN, Ethernet..)
 Δίκτυα αισθητήρων (Event driven transducers)
Δίκτυα διαφορετικών ταχυτήτων:
 Broadband: traffic rates from few Mbps and higher
(e.g.xDSL)
 Narrowband: supported traffic rate is some Kbps (PSTN and
ISDN)
Το τοπίο των σύγχρονων
τηλεπικοινωνιών (2/3)
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Μορφή κίνησης
 Circuit switched (μεταγωγή κυκλώματος): 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 (μεταγωγή πακέτου- IP δίκτυα) : 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.
Το τοπίο των σύγχρονων
τηλεπικοινωνιών (3/3)
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What is the main driver in today’s 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 user’s service experience over any
access network.
 Accessibility of communication services, everywhere at any
time and over any user terminal.
Οι λόγοι εισαγωγής της ΙMS τεχνολογίας
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Η ανάγκη ομογενοποίησης των υπηρεσιών χρήστη μέσα σε ένα
κοινό πρωτόκολλο επικοινωνίας, το ΙΡ, μέσω του οποίο
υλοποιούνται και σηματοδοτικές διεργασίες αλλά και η
επικοινωνία χρήστη (data plane).
Χρήση ενός και μόνο μηχανισμού για την διασφάλιση και
διαχείριση πόρων πάνω από οποιοδήποτε δίτκυο πρόσβασης
Κοινή πολιτική χρέωσης ανεξάρτητα από την τύπο του δικτύου
πρόσβασης
Κοινοί μηχανισμοί αναγνώρισης χρήστη, παροχής υπηρεσίας
και ασφάλειας
Γενικευμένη διαλειτουρικότητα όλων των ΙΡ δικτύων
πρόσβασης, των τύπων τερματικού χρήστη, ακόμη και των
δικτύων μεταγωγής κυκλώματος
Τι είναι η IMS δικτυακή τεχνολογία;
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Από τεχνολογικής απόψεως είναι το «σημείο συνάντησης» των
παραδοσιακών τηλεπικοινωνιών και της τεχνολογίας
πληροφορίας .
Ένα ΙΜS δίκτυο είναι ένα «υπερκείμενου τύπου» δίκτυο το
οποίο δεν αναιρεί τις υπάρχουσες δικτυακές τεχνολογίες
πρόσβασηςαλλά τις εντάσσει στο πλαίσιο της μέσω της χρήσης
του ΙΡ πρωτοκόλλου.
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Περιγραφή της IMS αρχιτεκτονικής και των βασικότερων
δομικών της στοιχείων
General Architecture
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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)
Detailed architecture
IMS Requirements
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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
Description of IMS components (1/13)
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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.
Description of IMS components (2/13)
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Main components:
 SIP servers (CSCF)
 Databases (HSS, SLF)
 Application Servers (AS)
 MRF (divided in MRFC and MRFP)
 BGCFs
 MGW divided into SGW, MGCF, MGW
Description of IMS components (3/13)
- CSCF (Call/Session Control Function)
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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-).
Description of IMS components (4/13)
- P-CSCF (Proxy Call/Session Control Function)
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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
Description of IMS components (5/13)
- S-CSCF (Serving Call/Session Control
Function)
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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.
Description of IMS components (6/13)
- I-CSCF (Interrogating Call/Session Control
Function)
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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.
Description of IMS components (7/13)
- Databases
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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.
Description of IMS components (8/13)
- AS
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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.
Description of IMS components (9/13)
- AS
Description of IMS components (10/13)
- MRF
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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.
Description of IMS components (11/13)
- BGCF
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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.
Description of IMS components (12/13)
- IMS-ALG/TrGW
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IMS-Application Layer Gateway & Transition Gateway offer IPv4 <->
IPv6 interworking without terminal support.
IMS-ALG performs interworking of signaling (SIP messages)
TrGW performs interworking of data traffic (e.g. RTC streams)
Description of IMS components (13/13)
- MGW
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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.
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Βασικές αρχές λειτουργίας
Network organization (1/2)
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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).
Network organization (2/4)
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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).
Network organization (3/4)
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Path establishment concept
Network organization (4/4)
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IMS allows manifold exploitation opportunities
 IP-CANs, IMS, Services hosted on AS can be exploited by different actors
User identification in IMS (1/5)
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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. sip:[email protected]
 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:+1-212-555-0293
 TEL URIs can be transferred over the IMS network with SIP
messages:
 E.g sip:[email protected];user=phone
User identification in IMS (2/5)
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A private user identity called IMSI (IMS Subscribed Identity) of NAI
(Network Access Identifier) type with the format:
 [email protected]
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.
User identification in IMS (3/5)
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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.
User identification in IMS (4/5)
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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.
User identification in IMS (5/5)
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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
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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
Bibliography
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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