umts core networkilenia/course/13-umts-core.pdfnew air (radio) interface based on wcdma access...

UMTS UMTS CoreCore NetworkNetwork

V. Mancuso, I. Tinnirello

UMTS UMTS CoreCore NetworkNetwork

GSM/GPRS core networkRadio access networkBSS PS

TN

, ISD

N

BTS

BSC

MSC

VLR

GMSC

HLRMS

GSM/GPRS NetworkGSM/GPRS Network ArchitectureArchitecture


database

IP Backbone

Internet

PS

TN

, ISD

N

BTS

SGSNAuC

EIR

GGSN

PCU

Core network (GSM/GPRS-based)Radio access networkUTRAN

UE

Iu CS

IurUu

PS

TN

BS

RNC MSC

VLR

GMSC

HLR

Iub

3GPP Rel.’99 Network Architecture3GPP Rel.’99 Network Architecture


Iur

Iub

Uu

GnIu PS

database

IP Backbone

Internet

BS

RNC SGSNAuC

EIR

GGSN

Radio access networkUTRAN

UE IurUu

BS

RNCIub

2G => 3G MS => UE (User Equipment), often also called (user) terminal

New air (radio) interface based on WCDMA access technology

3GPP 3GPP RelRel.’99 Network Architecture.’99 Network Architecture


Iur

Iub

Uu

BS

RNCNew RAN architecture(Iur interface is available for soft handover,BSC => RNC)

Core network (GSM/GPRS-based)

Iu CS

PS

TNMSC

VLR

GMSC

HLR

Changes in the core network:

MSC is upgraded to 3G MSC

SGSN is upgraded to 3G

3GPP Rel.’99 Network Architecture3GPP Rel.’99 Network Architecture


GnIu PS

IP Backbone

Internet

SGSNAuC

EIR

GGSN

SGSN is upgraded to 3G SGSN

GMSC and GGSN remain the same

AuC is upgraded (more security features in 3G)

Circuit Switched (CS) core networkUTRAN

(UMTS Terrestrial Radio Access Network)

PS

TN

MSC Server

GMSC Server

SGW SGW

3GPP Rel.4 Network Architecture3GPP Rel.4 Network Architecture


PS

TN

New option in Rel.4:

GERAN

(GSM and EDGE Radio Access Network) PS core as in Rel.’99

SGW

MGW

SGW

MGW

Circuit Switched (CS) core network

PS

TN

MSC Server

GMSC Server

SGW SGW

MSC Server takes care of call control signalling

The user connections are set up via MGW (Media GateWay)

“Lower layer” protocol



PS

TN

PS core as in Rel.’99

SGW

MGW

SGW

MGW“Lower layer” protocol conversion in SGW (Signalling GateWay)

RANAP / ISUP

SS7 MTP IP Sigtran

CS core

PS

TN

MGW

HSSIMS (IP

Multimedia

UTRAN

(UMTS Terrestrial Radio Access Network)

New core network part:



SGSN GGSN

Intern

et

HSSMultimedia System)

PS core

GERAN

(GSM and EDGE Radio Access Network)

CS core PS

TN

Intern

et / oth

er IMS

HSS

The IMS can establish multimedia sessions (using IP transport) via PS core between UE and Internet (or another IMS)

IMS (IP Multimedia

MGW



SGSN GGSN

Intern

et / oth

er IMS

HSS

PS core

Call/session control using SIP (Session Initiating Protocol)

Interworking with the PSTN may be required for some time ...

Multimedia System)

Service provider Service provider

Content provider Content provider

New Service ConceptNew Service Concept


End user End user

Carrier provider

all want to make profit

OSA is being standardised, so that services provided by different service/content providers can be created and seamlessly integrated into the 3G network (this is the meaning of “open” architecture)

OSA means in practice:

OSA (Open Services Architecture/Access)OSA (Open Services Architecture/Access)


3G network

API API API

Service Creation Environment (SCE)API = Application

Programming Interface

(Standardised)

CAMEL (Customised Applications for Mobile network Enhanced Logic) is a set of “IN” (intelligent network) type functions and procedures that make operator-specific IN services available to subscribers who roam outside their home network.

CAMEL = IN technology + global mobility

CAMEL (2G & 3G)CAMEL (2G & 3G)


CAMEL Service Environment (CSE) is a logical entity in the subscriber’s home network which processes IN related procedures

CSE ≈ SCP in home network

The IMSThe IMS


The IP Multimedia Subsystem

Vincenzo Mancuso, PhD

The IMSThe IMS

�The Third generation networks aim to mergetwo most valuable resources in communicationtechnology, along with local PSTN networks

�Cellular Networks

�The Internet


�Use packet switching, IP

�The IP Multimedia Subsystem or IMS is the solution to integrate allthe services that the internet provides with the cellular and othernetworks

�Triple Play: coordination of voice, video and data

IMS ValueIMS Value--added servicesadded services

�IMS also can provide integratedservice to the user

�Third party developed services can be provided byoperators, thus developing value-added services

�Appropriate charging for multimedia sessions,


�Appropriate charging for multimedia sessions,according to the content and the service offered

IMS Service IntegrationIMS Service Integration

�Problem Statement: variousnetworks providing services have a“vertical infrastructure”

�no horizontal links between networks


�The challenge: to integrate thesenetworks and to create newapplications that would provideservice to next generationnetworks

Component #1: the media Component #1: the media

transporttransport

�After many other services, introduction of real time voice/video on Internet

�The 1st problem is easy: how to transmit voice/video?

IP packets


� RTP (Real-Time Protocol) is a protocol which transports an encoded multimedia stream as pieces, with a timestamp on each piece, and sends them using UDP/IP�Any encoding is permitted for voice/video with

a Codec: MP3, ACC, MPEG4, AMR…�Several streams (voice+video) can be sent in

parallel with the same timestamp�The timestamp is used by the receiver to play

the voice/video in a regular way for quality� RTP is the media transport

CodecRTPUDP

IP

Component #2: session initiationComponent #2: session initiation

� The 2nd problem is more difficult: how to initiate the call? how to know the IP-address of the called phone?

� It is necessary to have a table which translates a symbolic name “Betty” into an IP address

Registration of Betty’s phone and address


symbolic name “Betty” into an IP address

� Each time a terminal service is put on, it must notify its presence to update the table

� So, thanks to an access to this table, it is possible to make the address resolution : name -> IP address

� The control of the address table creates a tough competition…

Registration of John’s phone and address

Why IMS?Why IMS?

Solutions to make the address resolutionsSolutions to make the address resolutions

MSN, Yahoo, AOL have designed a calling architecture hyper-centralized: 1 table worldwide, only 1 operator

Skype promotes a hyper-decentralized architecture:

“The introduction and development of solution like Skype is highly significant


decentralized architecture: 1 table per terminal with peer-to-peer update between terminals

as it alters the notion of telecommunications operators by making the process fully electronic“ IDATE - ART, 2004

Operators dislike such approaches, so they push a way very similar to e-mail: centralization per domain. This approach is named “Internet Protocol Multimedia Subsystem”, IMS.

IMS User IMS User Identities Identities

[email protected]

Sip:[email protected]

tel:+17324567888


[email protected]

Sip:[email protected]

tel:+88028112347

Public User Identities

Private User IdentityIMS Subscriber

User IdentityUser Identity

� Private identity� Issued by home provider

� Used for AAA

� Saved on ISIM (not modifiable)

� Public identity� Normal SIP address (URI or TEL)

� Identifies the user publicly

� User has one or more identities

Private User Identity 1

Public User Identity 1



Implicitly Registered ID

Set 1



Set 2

Service Profile 1

Service Profile 2

Service Profile 3

IMS Subscription


� User has one or more identities

� Used for routing

� Can be grouped into implicit registration sets� If one of the set is registered then the others are as well

� At least one is stored on ISIM� In case no ISIM is provided

» Use a temporary identity derived from USIM during initial registration (derived from IMSI)

» PIDs are then provided by the S-CSCF in its reply to the registration


Set 3

Private User Identity 2



Service Profile 4

UICCUICC

Universal Integrated Circuit Card

� Used to store data, including authentication information

� Contains one or more applications� SMS

� Phonebook

� …

� SIM


� SIM

�GSM Subscriber Identity Module

� USIM

�UMTS SIM

� ISIM

� IMS SIM

� Applications are independent� SIM, USIM and ISIM can coexist on the same UICC

� … but SIM cannot be used for IMS access (for security reasons)

IP Multimedia SubsystemIP Multimedia Subsystem

A standardAn enabler

of newapplications

Next-gen network

architecture

SIPSIPIMSIMSIMSIMS

PushPushPushPush----2222----TalkTalkTalkTalk

Converged ServicesConverged ServicesConverged ServicesConverged ServicesRich CommunicationsRich CommunicationsRich CommunicationsRich Communications


ServiceServiceServiceService----layerlayerlayerlayer

ControlControlControlControl----layerlayerlayerlayer

TransportTransportTransportTransport----layerlayerlayerlayerIMS is access agnostic, cost reducing, and service IMS is access agnostic, cost reducing, and service IMS is access agnostic, cost reducing, and service IMS is access agnostic, cost reducing, and service

enhancingenhancingenhancingenhancingIMS gives the customer & carrier choiceIMS gives the customer & carrier choiceIMS gives the customer & carrier choiceIMS gives the customer & carrier choice

SIP

SIP SIPSIP

At HomeAt HomeAt HomeAt Home In the OfficeIn the OfficeIn the OfficeIn the OfficeOn the MoveOn the MoveOn the MoveOn the Move

IMSIMSIMSIMSIP NetworkIP NetworkIP NetworkIP Network

SIP

PushPushPushPush----2222----TalkTalkTalkTalkPushPushPushPush----2222----ViewViewViewView

PushPushPushPush----2222----XXXX

Presence, LocationPresence, LocationPresence, LocationPresence, Location----based Servicesbased Servicesbased Servicesbased Services

Interactive Services, Interactive Services, Interactive Services, Interactive Services, Interactive GamingInteractive GamingInteractive GamingInteractive Gaming

Games, Games, Games, Games, Music Music Music Music

DownloadsDownloadsDownloadsDownloads

Picture Picture Picture Picture Messaging, Messaging, Messaging, Messaging,

MM MessagingMM MessagingMM MessagingMM Messaging

VoiceVoiceVoiceVoice

Video ClipsVideo ClipsVideo ClipsVideo ClipsSports, NewsSports, NewsSports, NewsSports, News

EEEE----mailmailmailmailIMIMIMIM

Streaming Streaming Streaming Streaming Audio, VideoAudio, VideoAudio, VideoAudio, Video

IMSIMS


Goals & Tools

IMS goalsIMS goals

�Combine latest trends in tech

�Run fast, no time for standardization of services

�Mobile/Nomadic internet

�Create a platform for multimedia services


services

�… and their development

�Exploit/allow mobile packet-switching networks

�Not a mere circuit-switching replacement

IMS requirementsIMS requirements

�Support for establishing IP Multimedia Sessions� Audio, video, messaging…

�Support for mechanisms to negotiate QoS� distinguish users� operators want to control QoS


� operators want to control QoS

�Support for interworking� with Internet� with packet-switching networks

�Support for roaming� Home and visited network� Inter-operators roaming

�…

IMS requirements IMS requirements (cont’d)(cont’d)

�…�Support for user activity control

� policies imposed by the operator (general policies and per-user policies)� accomplish to service agreements

�Support for fast service creation� don’t require service standardization


� don’t require service standardization� Standardize service capabilities instead

�Support for multiple access� Not only GPRS, UMTS� IP is independent on lower protocol layers

Protocols in IMSProtocols in IMS

�3GPP reuses protocol developed by other standards development organizations� ETSI (European Telecommunications Standard Institute)� IETF (Internet Engineering Task Force)� ITU-T (International Telecommunications Union - Telecommunications)

�3GPP interacts with standards development organizations as for the


development organizations as for the development of existing and new protocols

�Protocols� RTP for media transport� Session control protocols� AAA protocols� Other

Session Control ProtocolSession Control Protocol

�SIP (Session Initiation Protocol, by IETF- RFC2821)�Protocol to establish and manage multimedia session

over IP�SIP borrows some design principles from SMTP and

HTTP


HTTP�Does not differentiate the user-to-network interface from

the network-to-network interface (unlike BICC and H.323)

�It follows the client-server model�Text-based protocol

�Easy to debug, extend and reuse for service building

AAA protocol: DIAMETERAAA protocol: DIAMETERone protocol for many interfacesone protocol for many interfaces

�Authorization� What service can be used?

�Authentication� Are you really who you say you

are?

�Accounting� € $ £

DiameterClient Application

DiameterServer Application

Session Management Session Management


Routing Management

Connection Management

Connection Management

Base Protocol Base Protocol

Routing Management

• DIAMETER– by IETF RFC3588– Evolution of RADIUS– Base protocol– Diameter applications

• Diameter applications used to customize/extend the base protocol for different interfaces, environments, and applications

• E.g.: interact with SIP session setup (Authorization and Authentication)• E.g.: interact with the billing subsystem to control accounting • E.g.: interact with routing entities

IMS ArchitectureIMS Architecture


New services for mobile New services for mobile

and fixed networksand fixed networks

� Open, standardised, operator friendly, NGN multimedia architecture for mobile and fixed services

� Based on SIP, DIAMETER and COPS controls

� Supports legal interception, localisation, PSTN interworking, etc.


Access Independent

MGW

3G mobile

PSTNPSTN

IP transport

IMS PlatformWLANWiMAX

DSLFibre

Ethernet

ApplicationsInternetInternet

Functions and nodesFunctions and nodes

�IMS standardizes functions

�IMS does NOT standardize nodes

�The IMS Architecture is a collection of functions linked by standardized interfaces


�A function can be implemented through one or more nodes

�Multiple functions can share a node

�Commonly a function per node

A standard architecture for A standard architecture for

service deliveryservice delivery

ASApplication

Layer

Session Control HSS

SIPSIP

DIAMETER

AS

SIPMRF


MGCFControlLayer

ConnectivityLayer PSTN/PLMNPSTN/PLMNPSTN/PLMNPSTN/PLMNIP Network

HSS

H.248

SIP

SIP

F

CPE

MGW

GGSN

Access to IMSAccess to IMS

The user can connect to an IMS network invariety of ways, all of which use the standard Internet Protocol (IP) for packet switching�e.g., IMS terminals can register directly

on an IMS network� e.g., mobile phones, personal digital assistants (PDAs) and computers


� e.g., mobile phones, personal digital assistants (PDAs) and computers� even when they are roaming in another network or country (the visited

network)� The only requirement is that they can use IPv6 (also IPv4 in early IMS)

and run SIP user agents

Other access examplesOther access examples

�Fixed access

- e.g., Digital Subscriber Line (DSL), cable modems, Ethernet

�Mobile access

� e.g., W-CDMA, CDMA2000, GSM, GPRS

�Wireless access


� e.g., WLAN, WiMAX

�Other phone systems like plain old telephone service (POTS -- the old analogue telephones) or PSTN, H.323 and non IMS-compatible VoIP systems, are supported through gateways

IMS functional elementsIMS functional elements


Infrastructure of SIP ProxiesInfrastructure of SIP Proxies(media do not traverse the IMS..)(media do not traverse the IMS..)


IMS Functional ElementsIMS Functional Elements

�Session Management (SIP)

�Routing

�Databases

�Network Interoperability Elements


�Services and Support Components

�Charging Components

Nodes/Functions in the IMSNodes/Functions in the IMS

�User databases� HSS (Home Subscriber Server)� SLF (Subscriber Location Function)

�SIP servers� CSCF (Call/Session Control Function)

�AS (Application Server)�MRF (Media Resource Function)


�MRF (Media Resource Function)� MRFC (MRF Controller)� MRFP (MRF Processor)

�BGCF (Breakout Gateway Control Function)�PSTN/CS gateways, decomposed into:

� SGW (Signaling Gateway)� MGCF (Media Gateway Controller Function)� MGW (Media Gateway)

�Charging collection functions

Databases (HSS,SLF)Databases (HSS,SLF)

�HSS is an evolution of the HLR (Home Location Register) of GSM

�Contains the user-related subscription data (e.g., location, authorization and authentication information)

�More than one HSS is possible


�If #HSS > 1 �� SLF required

�SLF maps users’ address to HSSs

�HSS and SLF use DIAMETER with an IMS-specific diameter application

Home Subscriber Server (HSS)Home Subscriber Server (HSS)

�Presence, Location and Profile

�End-User Identity

HSSHSS

Diameter


�Private and Public End-User Information

�Registration Information

�Service Initiation Information

�Subscriber Service Profile (SSP)

�Downloaded to CSCF at Registration

More on HSSMore on HSS

�HSS is a master user database that supports all the IMS network functions that actually handle communications

�contains the subscription-related information (user profiles)

�performs authentication and authorization of the user


�can provide information about the user's physical location

�is similar to the GSM Home Location Register (HLR) and Authentication Centre (AUC) together

CSCFCSCF

�SIP servers or proxies, collectively called Call Session Control Function (CSCF), are used to process SIP signaling packets/messages in the IMS

�P-CSCF (Proxy)

�I-CSCF (Interrogating)



�S-CSCF (Server)

Call/Session Call/Session Control Control FuncFunc. (. (CSCF)CSCF)

� CSCF – Processes SIP Signaling� P-CSCF

� First Point of User Contact (located in the visited domain)

� Authenticates user

II--CSCFCSCF

SS--CSCFCSCF

PP--CSCFCSCF

SIP

SIP

SIPSIP

SIP

Diameter


� Authenticates user� May Include Policy Functions

� S-CSCF� Central Node of Control Plane� Acts as Registrar for User (located in the home domain)

� Invokes Application Servers� Performs Primary Routing Function

� I-CSCF� Located at Edge of Administrative Domain (contact point for inter-domain messages)� Is the Ingress Network Point Defined in DNS� Shields Network Topology from External Networks

IMS Signaling PathIMS Signaling Path(1 domain, no roaming)(1 domain, no roaming)


IMS Signaling PathIMS Signaling Path(4 domain roaming, media goes directly)(4 domain roaming, media goes directly)

Irish User

Sweden User


Irish UserIn roaming

Sweden UserIn roaming

CSCFCSCF

�P-CSCF (Proxy)

�Outbound/Inbound proxy server

�First server contacted by the user

�Fixed while registered

�Security functions (integrity protection, IPSec,…)


�Security functions (integrity protection, IPSec,…)

�Authenticates the user and extend the authentication to other nodes within IMS

�Compress/decompress SIP messages

�Generates charging info

PP--CSFC detailsCSFC details

� SIP proxy that is the first point of contact for the IMS terminal

� It can be located either in the visited network (in full IMS networks) or in the home network (when the visited network isn't IMS compliant yet)

� The terminal discovers its P-CSCF with either DHCP

� or it is assigned in the PDP Context (GPRS)� It is assigned to an IMS terminal during registration, and

does not change for the duration of the registration


does not change for the duration of the registration � It sits on the path of all signaling messages, and can

inspect every message � It authenticates the user and establishes an IPsec security

association with the IMS terminal

� Other nodes trust the P-CSCF, and do not have to authenticate the user again � It can also compress and decompress SIP messages� it may include a Policy Decision Function (PDF), which

authorizes media plane resources� it also generates charging records

PDF for PDF for QoSQoS in IMSin IMS(intermediary between the application(intermediary between the application--level QOS and level QOS and

the 3G networkthe 3G network--level level QoSQoS))


CSCFCSCF


�SIP proxy at the edge of a domain

�Advertised by DNS

�Interface to HSS and SLF for routing purposes


�Optional: encryption of sensitive information about the domain (THIG: topology hiding inter-network gateway)

II--CSCF detailsCSCF details

� It is another SIP function located at the edge of an administrative domain

� Its IP address is published in the DNS of the domain� remote servers can find it, and use it as a forwarding point (e.g. registering) for

SIP packets to this domain� The I-CSCF queries the HSS using DIAMETER to

retrieve the user location� then it routes the SIP request to its assigned S-CSCF


� then it routes the SIP request to its assigned S-CSCF� Up to Release 6 it can also be used to hide the

internal network from the outside world (encrypting part of the SIP message)� in which case it's called a THIG (Topology Hiding Inter-network Gateway)� From Release 7 onwards this "entry point" function is removed from the I-

CSCF and is now part of the IBCF (Interconnection Border Control Function) which is also a firewall and a nat.

CSCFCSCF

�S-CSCF (Server)

�SIP server with session control functions

�SIP registrar (maintains a mapping between user location and public user identity)

�SIP routing


�SIP routing

�PEP

�Always located in the home network

�Interfaced with the HSS (e.g., to download the user’s profile)

SS--CSCF detailsCSCF details

� It is the central node of the signaling plane� SIP server + session control

� It is always located in the home network� It uses DIAMETER to the HSS to download and

upload user profiles� All necessary information is loaded from the HSS.

� It handles SIP registrations� bind the user location and the SIP address

� It sits on the path of all signaling messages, and can inspect every message


� It sits on the path of all signaling messages, and can inspect every message

� It decides to which application server(s) the SIP message will be forwarded, in order to provide their services

� It provides routing services, typically using Electronic Numbering (ENUM) lookups

� There can be multiple S-CSCFs in the network for load distribution and high availability reasons

� It's the HSS that assigns the S-CSCF to a user, when it's queried by the I-CSCF.

Application Server (AS)Application Server (AS)

� Contains Call Related Application Logic

� Facilitates a Service Creation Environment

ASASASASASAS

SIPDiameter


� Facilitates a Service Creation Environment

� Queried by S-CSCF in Real Time to Execute Logic

� Filters can be applied at S-CSCF in order to inspect the SIP messages and decide whether involve or not the AS

� Generally Specialized for Each Service

� May Provide Gateway to Legacy Applications (e.g. AIN)

� Can Behave as a SIP Proxy or Terminal (and in this case receive the media!)

ASAS

�Three different AS types� SIP AS

�native IMS application server� OSA-SCS (Open Service Access – Service Capability Server)

�Interface to Open Service Application (AS) framework application server (developed by 3GPP for UMTS)

� IM-SSF (IP Multimedia Service Switching Function)�Reuse/interface with CAMEL (GSM)


�Reuse/interface with CAMEL (GSM)�Gateway for GSM Service Control Function

�An AS can be located in the home network or in an external third-party network� If located in the home network, it can query the HSS with the DIAMETER

Sh interface (for a SIP-AS) or the Mobile Application Part (MAP) interface (for IM-SSF).

AS AS asas a SIP Proxya SIP Proxy(e.g. find a taxi)(e.g. find a taxi)


AS AS asas a SIP Terminala SIP Terminal(e.g. web server: receive media data!)(e.g. web server: receive media data!)


Example of Filter RuleExample of Filter Rule(at S(at S--CSCF)CSCF)

�User A wants to divert all calls from boss to an answering machine (i.e. the AS)

(method=INVITE) AND

(P-Asserted-Identity = [email protected]) AND

(Session Case = Terminating) ->message is addressed to AS


AS

Multimedia Resource Function (MRF)Multimedia Resource Function (MRF)

SIP

MSMS MSMS

MRFCMRFC


�Offers Services Such as Conferencing

�MRFC – SIP User Interface toward S-CSCF

�MRFP – Controls the Media Server (MS)

MRFMRF

� The MRF (Media Resource Function) provides a source of media in the home network

� It provides media related functions such as media manipulation (e.g. voice stream mixing, media trans-coding) and playing of tones and announcements.

� Each MRF is further divided into� Media Resource Function Controller (MRFC) – signaling plane


� Media Resource Function Controller (MRFC) – signaling plane

� Media Resource Function Processor (MRFP) – media plane

� The MRFC acts as a SIP User Agent to the S-CSCF, and controls the MRFP with a H.248 interface

� The MRFP is a media plane node that implements all media-related functions

Multiparty CallsMultiparty Calls(MRF as a special AS)(MRF as a special AS)


REFER:Ask other

to join

Multiparty CallMultiparty Call


BGCFBGCF

�The Breakout Gateway Control Function is a SIP server that includes routing functionality based on telephone numbers

�used when calling from the IMS to a phone in a circuit switched network

�e.g., IMS to Public Switched Telephone Network (PSTN) or to


�e.g., IMS to Public Switched Telephone Network (PSTN) or to Public Land Mobile Network (PLMN)

�BGCF routes to the appropriate (remote) circuit switching domain

�BGCF selects the (local) PSTN/CS gateway

PSTN GatewayPSTN Gateway

MGCFMGCF

H.248

ISUP

BGCFBGCFSIP

SIP

TDM

SGWSGW

SIP


� BGCF – Routes to Gateway Based Upon Telephone Number

� MGCF – Controlling Function for SGW and MGW

� SGW – Provides Signaling Conversion Between SIP and ISUP

� MGW – Provides Conversion between RTP and TDM

MGWMGWTDM

Home Network Home Network -- Functional ElementsFunctional Elements

HSSHSSDNSDNS

ENUMENUM

Home Network

ASASASASASAS

Home Subscriber Server• Centralized DB• HLR successor• User profile• Filter criteria (sent to S-CSCF)

• Which applications• Which conditions

Application Servers• Push-to-talk• Instant messaging• Telephony AS• 3rd party or IMS VendorDomain Name Server

SIP

SIP

SIP

Diameter

MRFCMRFC

Media GatewayControl Function• Interfaces to PSTN/PLMN by

• Converting SIP <-> ISUP• Interworking RTP to circuit

Media Resource Function Controller• Pooling of Media servers (e.g. conference)

PP--CSCFCSCFSIP

UA/UE


VisitedVisitedNetworkNetwork

II--CSCFCSCF SS--CSCFCSCFPP--CSCFCSCF

MGCFMGCF

MGWMGWPSTNPSTN

H.248

ISUPBGCFBGCF

SIP

SS7SS7

SIPSIP

SIP

SIPSIP

SIP

RTP TDM

Call SessionControl Function• SIP registration • SIP session setup

MSMS MSMS

MRFCMRFC • Interworking RTP to circuit• H.248 control of MGW

Breakout Gateway Control Function• Selects network (MGCF or other BGCF)in which PSTN/ PLMN breakout is to occur

Proxy CSCF• 1st contact point for UA• QoS• Routes to S-CSCF

Interrogating CSCF• Entry point for incoming calls• Determines S-CSCF for Subscribers• Hides network topology

Serving CSCF• Registrar• Session control• Application Interface

SIP

UA/UE

NetworkNetwork--toto--Network ConnectivityNetwork Connectivity

HSSHSSDNSDNS

ENUMENUM

SIP

ASASASASASAS

BackboneBackbone

AccessAccessSIP

SIP SIP

SIP

SIP

DiameterRTP

RTP

MRFCMRFC

PP--CSCFCSCF SIP

SIP

UA/UE


II--CSCFCSCF SS--CSCFCSCF

VisitedNetwork

Home Network

P/SP/S--CSCFCSCFBackboneBackbone

PacketPacketNetworkNetwork

MGCFMGCF

MGWMGWPSTNPSTN

H.248

ISUPBGCFBGCF

SIP

SIP

SS7SS7

SIPSIP

SIP

SIPSIP

RTP TDM

MSMS MSMS

MRFCMRFC

Proxy/Serving CSCF•Manages call origination•Selects destination network• Routes to I-CSCF

Interrogating CSCF• Entry point for incoming calls• Determines S-CSCF for Subscribers• Hides network topology

Home and Visited NetworksHome and Visited Networks

�Inherited from 2G, 3G

�Most of IMS nodes and functions are located in the Home network

�P-SCFC can be either in the home or visited network� If GPRS access �P-CSCF in the same network of the GGSN


� If GPRS access �P-CSCF in the same network of the GGSN�simplifies the operator’s management�allows roaming to non-IMS GPRS CANs (Connectivity Access networks)�Drawback: trombone effect when GGSN is in the home network (GGSN is

in the media plane)�In a long term perspective the P-CSCF will be located in the visited

network

�Media servers can be out of both

IMS architectureIMS architecture


Examples

John is calling BettyJohn is calling BettyIIntroducing ntroducing the HSS and the Sthe HSS and the S--CSCFCSCF

� The HSS is the table user/address

� The S-CSCF is a SIP proxy which works on messages to provide users (consumers, enterprises) with calling services including registration being a mediation SIP2DIAMETER

HSS

When the phones get connected they


SIP S-CSCF

get connected they register their name/IP to the HSS

Changes the SIP message replacing “Betty” by its IP address found in the HSS

SIP

John is calling a taxi to meet Betty John is calling a taxi to meet Betty Introducing Introducing the Application Server (AS)the Application Server (AS)

� In addition to the basic name/address translation, the S-CSCF routes SIP messages to:� The network of Betty, if different

� The applications such as: Push-To-Talk, Instant Messaging, Advance Call Control, Voice/video mailbox, nearest Taxi… running on AS, a SIP proxy application server


S-CSCF

AS

…nearest Taxi application (location, fleet…)

Changes the SIP message replacing “taxi” by the IP address of the nearest available taxi

John’s and Betty’s phones do John’s and Betty’s phones do not support not support a common a common

voice encodingvoice encodingIntroducing the MRFC and MRFPIntroducing the MRFC and MRFP

� Intercepting the SIP “invite” message, the S-CSCF/AS detects a non compatibility between the codecs of the phones : it forwards it to the MRFC (a SIP proxy).

� The MRFC adjusts the SIP messages in order to orient the RTP flow to the MRFP (a RTP proxy), for trans-coding


RTP flow to the MRFP (a RTP proxy), for trans-coding

MRFC

MRFP

MGCP, H248 Megaco

RTP (codec: AMR)

SIPS-CSCF

RTP (codec: G729)

During its travel John is calling BettyDuring its travel John is calling Betty

The The operator has made a segmentation of its services offeroperator has made a segmentation of its services offerIIntroducing ntroducing the the PP--CSCF, ICSCF, I--CSCFCSCF

� The P-CSCF is the 1st SIP proxy seen by the terminal

� It controls the bearer plan via COPS protocol

� It adjusts the SIP message (e.g., compression) and forwards it to the I-CSCF of the home network

� The operator may have several S-CSCFs (e.g., offer segmentation)

� So it introduces, the I-CSCF SIP proxy as the entry point of its network


network

� also used as the entry point for calls from other operators

Home Network

Visited Network

P-CSCF

… S-CSCF (consumers)

S-CSCF (enterprises)I-CSCF

HSS

John is calling Betty who hasJohn is calling Betty who has

a legacy phonea legacy phoneIntroducing the MGCF and the MGWIntroducing the MGCF and the MGW

� At the border of the IMS network with the phone network, an adaptation is necessary.

� The MGCF handles the control for the 2 worlds and drives the MGW (Media gateway)

� … controls circuits and MGW much like a VoIP softswitch


MGCF

MGW

H248 MGCP,Megaco

SIPLegacy Call control (SS7)

Phone transmission

…

Internet PSTN/PLMN network

RTP

IMS Communication FocusIMS Communication FocusRoaming IMS Mobile Users Roaming IMS Mobile Users

Access Network A

P-CSCFCSIP / SDP

S-CSCFA

P-CSCFD

Service Platform A(ASA)

S-CSCFBPDF

Gm

SIP / SDPSIP / SDP

Service Platform B(ASB)

PDF

Serving Network AServing Network B

Access Network B


SGSN GGSN GGSN SGSN

UEBUEA

SIP/SDPinviting Go SIP/SDP

IP Backbone Network

Data- Path

PDP ContextSessionlevel(SIP/SDP signalling)Bearer level(PDP context activation / modification / Release)Interactionbetweensession andbearer level(COPS)

Gm Gm

PDP Context

I-CSCF (between P-CSCF and S-CSCF) not shown for simplicity

[email protected]

Go

A Typical Example of an IMS CallA Typical Example of an IMS CallNetwork XNetwork X Network YNetwork Y

P-CSCF

I-CSCFI-CSCF

S-CSCF

S-CSCF

HSS

HSS

ASAS


User B

DSL/Cable ModemDSL/Cable Modem

DSLAM/CMTS

RNC

GGSN

Network Z (UMTS/GPRS)Network Z (UMTS/GPRS)

User A

SGSN GRXGRXP-CSCF

The Signaling Plane The Signaling Plane


SIP functionalitySIP functionality

� SIP’s main goal is to deliver a session descriptor to a user at his/her/its location

� A session descriptor contains the information needed for a remote user to join the session� IP address and port � Codecs� …

� SDP (Session Description Protocol)� A (possible) text-based way to describe the media session


� A (possible) text-based way to describe the media session

SDPSDP

Session-level

version

user

subject

Start time

user IP


Media-level

G711µ-law

H.261 codec

Start time

Stream direction

SIP entitiesSIP entities

�Registrar

�User Agent� SIP endpoints handled by users (also automatically, based on user-

defined rules)

�Proxy server� SIP routers


� SIP routers

� Usually is co-located with the registrar, and always uses the information on the registrar (through a location server) to route the calls

�Forking proxy� Parallel or serial forking in the message routing

�Redirect server� For routing, but no message delivery

SIP transactionsSIP transactionsTransaction structure

� A request from the client

� Zero or more provisional responses from the server

� A final response from the server

Each message begins with a start line

� Request line (in a request)


� e.g.: INVITE sip:[email protected] SIP/2.0

� Status line (in an answer)

� e.g.: SIP/2.0 180 Ringing

Start lineOne or more headers (name:value)

---------EMPTY LINE ---------Message body (OPTIONAL)

SIP message format

Method Request URI Protocol version

Protocol versionStatuscode

Humancode

SIP methodsSIP methods

ACK ack the final response for INVITEBYE terminate a sessionCANCEL cancel a pending requestINFO transport PSTN telephony signalingINVITE establish a sessionNOTIFY notify a UA about a particular eventOPTIONS queries a server about capabilities


OPTIONS queries a server about capabilitiesPRACK ack for provisional responsePUBLISH upload UA’s info to a serverREGISTER map the current URI and location with

the public URISUBSCRIBE request to be notified about an eventUPDATE update session characteristicsMESSAGE instant message in the message bodyREFER instruct a server to send a request

SIP in the IMSSIP in the IMS

�In principle, no difference with the public Internet

�In practice, IMS imposes some constraints

�Wireless access

�Security


�QoS

�Roaming

�3GPP/IETF defines a profile of utilizationof SIP in the IMS environment

�E.g., RFC 3261 (redefinition of SIP)

�RFC 3665 and 3666 (Best Current Practice)

PrerequisitesPrerequisitesTwoTwo--phase registrationphase registration

Inbound/Outbound

Offline subscription

Login to theIP access net


Inbound/OutboundFixed Proxy

Online registration

IP access net

DHCP orLower levelmechanism

Entering the IP Connectivity Access Entering the IP Connectivity Access

NetworkNetwork�The access to GPRS includes the

assignment of servers and IP address


PDF Context Activation/DePDF Context Activation/De--ActivationActivation

�A PDP context is “pre-setup” for the SIP signaling

�Different PDP contexts are then activated for the media flows (e.g. according to the agreed codec)

�When the session ends, all PDP contexts are


�When the session ends, all PDP contexts are deactivated

PP--CSCF discoveryCSCF discovery

� The IMS terminal has to discover the IP address of the P-CSCF

� The procedure can be

� stand alone (DHCP+DNS)

� integrated with the access to the IP (e.g., with the PDP context)


IMSIMS--level registrationlevel registration

�The user requests authorization to use the IMS

�The IMS authenticate and eventually authorize the user

�SIP REGISTER is mandatory


�DIAMETER is used by the CSCF to contact the HSS

�For security reasons, the user is challenged to show its identity � this require two SIP REGISTER messages

IMSIMS--level registrationlevel registration

UA

A: U

ser A

uth

. An

s.M

AA

: Mu

ltile

dia

Au

th.A

ns

SAA

:Ser

verA

ssig

nm

ent A

ns.

•The HSS tells the I-CSCF whether a S-CSCF was already allocated to the user.•If not, I-CSCF will choose based on capabilities notified by HSS in the UAA

Withoutauthentication

DL User Auth vector


UA

R: U

ser A

uth

. Req

MA

R:M

ulti

med

iaA

uth

.Req

SAR

:Ser

verA

ssig

nm

ent R

eq.

Challenge the User

DL User Profile

authenticationvector included

Basic Session SetupBasic Session Setup


Session Session

setupsetup

Many nodes and


Many nodes and many messages involved in the setup

Different planes/layers•media•signaling•CAN•core

Session setup (Session setup (cont’edcont’ed))


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