UMTS Core Network

UMTS Core Network

V. Mancuso, I. Tinnirello

GSM/GPRS Network Architecture Radio access network BSS

GSM/GPRS core network

VLR

BTS MS

GMSC

BSC

HLR

PCU

AuC SGSN

EIR

BTS IP Backbone

GGSN

database

Internet V. Mancuso, I. Tinnirello

PSTN, ISDN

MSC

3GPP Rel.’99 Network Architecture Radio access network UTRAN RNC

Iu CS

GMSC

VLR

BS UE

MSC

HLR

Iur

Uu

AuC Iub

RNC

BS

SGSN

EIR

Iu PS Gn IP Backbone

GGSN

database


PSTN

Iub

Core network (GSM/GPRS-based)

3GPP Rel Rel.’99 .’99 Network Architecture Radio access network UTRAN Iub

RNC

BS UE

Iur

Uu Iub

RNC

BS


2G => 3G MS => UE (User Equipment), often also called (user) terminal New air (radio) interface based on WCDMA access technology New RAN architecture (Iur interface is available for soft handover, BSC => RNC)

3GPP Rel.’99 Network Architecture Core network (GSM/GPRS-based)

MSC is upgraded to 3G MSC

Iu CS

MSC

GMSC

VLR

SGSN is upgraded to 3G SGSN GMSC and GGSN remain the same AuC is upgraded (more security features in 3G)

HLR AuC SGSN

EIR

Iu PS Gn IP Backbone

GGSN


PSTN

Changes in the core network:

3GPP Rel.4 Network Architecture UTRAN (UMTS Terrestrial Radio Access Network)

Circuit Switched (CS) core network MSC Server

GMSC Server

New option in Rel.4: GERAN (GSM and EDGE Radio Access Network)


MGW

SGW MGW

PS core as in Rel.’99

PSTN

SGW

3GPP Rel.4 Network Architecture MSC Server takes care of call control signalling The user connections are set up via MGW (Media GateWay)

RANAP / ISUP SS7 MTP

IP Sigtran


MSC Server

GMSC Server

SGW MGW

core

SGW MGW

PS core as in Rel.’99

PSTN

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

Circuit Switched (CS) network

3GPP Rel.5 Network Architecture UTRAN (UMTS Terrestrial Radio Access Network)

CS core

SGSN

GGSN

PS core


HSS

Internet

GERAN (GSM and EDGE Radio Access Network)

IMS (IP Multimedia System)

MGW

PSTN

New core network part:

3GPP Rel.5 Network Architecture

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


MGW

IMS (IP Multimedia System)

SGSN

HSS

GGSN

PS core

Internet / other IMS

Call/session control using SIP (Session Initiating Protocol)

CS core PSTN

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

New Service Concept Content provider

Content provider

Service provider

Service provider

Carrier provider

End user


End user

all want to make profit

OSA (Open Services Architecture/Access) 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: Service Creation Environment (SCE) API

API 3G network


API

API = Application Programming Interface (Standardised)

CAMEL (2G & 3G) 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 Service Environment (CSE) is a logical entity in the subscriber’s home network which processes IN related procedures CSE ≈ SCP in home network


The IMS The IP Multimedia Subsystem Vincenzo Mancuso, PhD


The IMS The Third generation networks aim to merge two most valuable resources in communication technology, along with local PSTN networks Cellular Networks The Internet Use packet switching, IP The IP Multimedia Subsystem or IMS is the solution to integrate all the services that the internet provides with the cellular and other networks Triple Play: coordination of voice, video and data


IMS ValueValue-added services IMS also can provide integrated service to the user Third party developed services can be provided by operators, thus developing value-added services Appropriate charging for multimedia sessions, according to the content and the service offered


IMS Service Integration Problem Statement: various networks providing services have a “vertical infrastructure” no horizontal links between networks The challenge: to integrate these networks and to create new applications that would provide service to next generation networks V. Mancuso, I. Tinnirello

Component #1: the media transport After many other services, introduction of real time voice/video on Internet The 1st problem is easy: how to transmit voice/video? IP packets

Codec RTP UDP IP

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 V. Mancuso, I. Tinnirello

Component #2: session initiation

Registration of Betty’s phone and address

The 2nd problem is more difficult: how to initiate the call? how to know the IPaddress of the called phone? It is necessary to have a table which translates a 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

Registration of John’s phone and address

The control of the address table creates a tough competition… V. Mancuso, I. Tinnirello

Why IMS? Solutions to make the address resolutions MSN, Yahoo, AOL have designed a calling architecture hypercentralized: 1 table worldwide, only 1 operator Skype promotes a hyperdecentralized architecture: 1 table per terminal with peer-to-peer update between terminals

“The introduction and development of solution like Skype is highly significant 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. V. Mancuso, I. Tinnirello

IMS User Identities Sip:[email protected]

tel:+17324567888 [email protected] m

IMS Subscriber

Private User Identity

Sip:[email protected]

tel:+88028112347 Public User Identities


User Identity Private identity Issued by home provider Used for AAA Saved on ISIM (not modifiable)

Public User Identity 1

Private User Identity 1

Public identity

IMS Normal SIP address (URI or TEL) Subscription Identifies the user publicly 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


»


Private User Identity 2



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


Implicitly Registered ID Set 1

Service Profile 2


In case no ISIM is provided »

Service Profile 1



Service Profile 3

Service Profile 4

UICC Universal Integrated Circuit Card Used to store data, including authentication information Contains one or more applications

SMS Phonebook … 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) V. Mancuso, I. Tinnirello

IP Multimedia Subsystem An enabler of new applications

Next-gen network architecture

A standard

IMS

SIP

Video Clips Sports, News

IP Network SIP

ServiceService-layer ControlControl-layer TransportTransport-layer

At Home

SIP

On the Move

Converged Services Rich Communications

SIP

SIP

SIP

In the Office

IMS is access agnostic, cost reducing, and service enhancing IMS gives the customer & carrier choice


Voice

PushPush-2-Talk Push--2-View Push PushPush-2-X

Location-Presence, Location based Services

Picture Messaging, MM Messaging

Streaming Audio, Video

E-mail IM Games, Music Downloads

Interactive Services, Interactive Gaming

IMS

Goals & Tools


IMS goals Combine latest trends in tech Run fast, no time for standardization of services Mobile/Nomadic internet Create a platform for multimedia services … and their development Exploit/allow mobile packetswitching networks Not a mere circuit-switching replacement V. Mancuso, I. Tinnirello

IMS requirements Support for establishing IP Multimedia Sessions Audio, video, messaging…

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

Support for interworking with Internet with packet-switching networks

Support for roaming Home and visited network Inter-operators roaming

… V. Mancuso, I. Tinnirello

IMS requirements (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 Standardize service capabilities instead

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


Protocols 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 of existing and new protocols Protocols RTP for media transport Session control protocols AAA protocols Other V. Mancuso, I. Tinnirello

Session 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 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 V. Mancuso, I. Tinnirello

AAA protocol: DIAMETER one protocol for many interfaces Authorization What service can be used?

Diameter Client Application

Diameter Server Application

Authentication Are you really who you say you are?

Accounting

Session Management

Session Management

Routing Management

Routing Management

Connection Management

Connection Management

Base Protocol

Base Protocol

€$£ •

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 Architecture


New services for mobile and 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 Applications

Internet

3G mobile WLAN WiMAX DSL Fibre Ethernet

IMS Platform

PSTN IP transport


MGW

Functions 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 service delivery Application Layer

AS

AS SIP

DIAMETER

Session Control Layer

SIP SIP

HSS

MR F

MGCF

MGW

Connectivity Layer

H.248

SIP

IP Network

GGS N

PSTN/PLMN

CPE V. Mancuso, I. Tinnirello

Access to IMS The user can connect to an IMS network in variety 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 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 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 V. Mancuso, I. Tinnirello

IMS functional elements


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


IMS Functional Elements Session Management (SIP) Routing Databases Network Interoperability Elements Services and Support Components Charging Components


Nodes/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) 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 V. Mancuso, I. Tinnirello

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 IMSspecific diameter application


Home Subscriber Server (HSS) HSS Diameter

Presence, Location and Profile End-User Identity Private and Public End-User Information Registration Information Service Initiation Information Subscriber Service Profile (SSP) Downloaded to CSCF at Registration


More 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


CSCF 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 Control Func. Func. ((CSCF) CSCF) Diameter

SIP

PCSCF

ICSCF

SIP

SIP

SCSCF

SIP

SIP

CSCF – Processes SIP Signaling P-CSCF First Point of User Contact (located in the visited domain) 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 Path (1 domain, no roaming)


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

Sweden User

Irish User

Sweden User In roaming

Irish User In roaming


CSCF P-CSCF (Proxy) Outbound/Inbound proxy server First server contacted by the user Fixed while registered Security functions (integrity protection, IPSec,…) Authenticates the user and extend the authentication to other nodes within IMS Compress/decompress SIP messages Generates charging info


P-CSFC 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 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 V. Mancuso, I. Tinnirello

PDF for QoS in IMS (intermediary between the applicationapplication-level QOS and the 3G networknetwork-level QoS) QoS)


CSCF I-CSCF (Interrogating) 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)


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


CSCF S-CSCF (Server) SIP server with session control functions SIP registrar (maintains a mapping between user location and public user identity) SIP routing PEP Always located in the home network Interfaced with the HSS (e.g., to download the user’s profile) V. Mancuso, I. Tinnirello

S-CSCF 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 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. V. Mancuso, I. Tinnirello

Application Server (AS) AS AS AS Diameter SIP

Contains Call Related Application Logic 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!) V. Mancuso, I. Tinnirello

AS 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) 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). V. Mancuso, I. Tinnirello

AS as a SIP Proxy (e.g. find a taxi)


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


Example of Filter Rule (at SS-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


Multimedia Resource Function (MRF)

SIP

MRFC

MS

MS

Offers Services Such as Conferencing MRFC – SIP User Interface toward S-CSCF MRFP – Controls the Media Server (MS)


MRF 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 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 Calls (MRF as a special AS)

REFER: Ask other to join


Multiparty Call


Multiparty Call


BGCF 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 Public Land Mobile Network (PLMN) BGCF routes to the appropriate (remote) circuit switching domain BGCF selects the (local) PSTN/CS gateway


PSTN Gateway SGW ISUP

SIP

BGCF

SIP SIP

MGCF H.248

MGW

TDM

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 V. Mancuso, I. Tinnirello

Home Network - Functional Elements Home Subscriber Server

Domain Name 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 Vendor

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

Home Network DNS ENUM

AS AS AS

HSS

P-CSCF

Media Gateway Control Function

Diameter SIP

P-CSCF UA/UE

I-CSCF

SIP

S-CSCF

SIP

SIP

MRFC MS

SIP

SIP

UA/UE

SIP

MS

• Interfaces to PSTN/PLMN by • Converting SIP ISUP • Interworking RTP to circuit • H.248 control of MGW

SIP

BGCF

SIP

MGCF ISUP

SIP

Call Session Control Function

H.248

• SIP registration • SIP session setup

RTP

MGW

SS7 TDM

Serving CSCF

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

Visited Network

• Registrar • Session control • Application Interface

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


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

PSTN

Network--toNetwork to-Network Connectivity

SIP

UA/UE Access

DNS ENUM

RTP RTP

Visited Network

HSS Diameter

SIP

Backbone Packet Network

AS AS AS SIP

P/SP/S-CSCF

SIP

I-CSCF

SIP

S-CSCF

P-CSCF SIP SIP

SIP

MRFC MS

SIP

SIP

MS

SIP

BGCF

SIP

MGCF ISUP

RTP

Home Network

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


H.248

MGW

SS7 TDM

PSTN

Home 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 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 V. Mancuso, I. Tinnirello

IMS architecture Examples


John is calling Betty Introducing the HSS and the S S--CSCF

HSS When the phones get connected they register their name/IP to the HSS

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

SIP

S-CSCF

SIP

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


John is calling a taxi to meet Betty Introducing 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

… nearest Taxi application (location, fleet…)

AS

S-CSCF Changes the SIP message replacing “taxi” by the IP address of the nearest available taxi V. Mancuso, I. Tinnirello

John’s and Betty’s phones do not support a common voice encoding Introducing 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

S-CSCF

SIP MRFC

MGCP, H248 Megaco

RTP (codec: G729) V. Mancuso, I. Tinnirello

MRFP

RTP (codec: AMR)

During its travel John is calling Betty The operator has made a segmentation of its services offer Introducing the P-CSCF, II-CSCF 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 also used as the entry point for calls from other operators

Visited Network

HSS

… P-CSCF

S-CSCF (consumers)

I-CSCF S-CSCF (enterprises)

Home Network V. Mancuso, I. Tinnirello

John is calling Betty who has a legacy phone Introducing 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 Legacy Call control (SS7)

SIP H248 MGCP, Megaco

… RTP

Phone transmission Internet


MGW

PSTN/PLMN network

IMS Communication Focus Roaming IMS Mobile Users Serving Network A Serving Network B Access Network A Service Platform B (ASB)

Service Platform A (ASA )

P-CSCFC PDF

Gm SIP/SDP inviting [email protected]

SIP / SDP

SIP / SDP

S-CSCFA

SIP / SDP

S-CSCFB

Access Network B

P-CSCFD PDF

Gm Go

Go

SGSN GGSN

Data- Path

SIP/SDP

GGSN SGSN

IP Backbone Network

UEA

UEB PDP Context

PDP Context

Sessionlevel(SIP/SDP signalling) Bearer level(PDPcontext activation / modification / Release) Interaction betweensession andbearer level(COPS) I-CSCF (between P-CSCF and S-CSCF) not shown for simplicity


A Typical Example of an IMS Call Network Y

Network X

AS

AS S-CSCF S-CSCF

HSS HSS

I-CSCF I-CSCF

P-CSCF

P-CSCF SGSN

DSL/Cable Modem

GRX

Network Z (UMTS/GPRS) GGSN

DSLAM/CMTS RNC

User A

User B


The Signaling Plane


SIP 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


SDP version user subject

user IP

Session-level

Start time G711µ-law Stream directio n

H.261 codec


Media-level

SIP entities Registrar User Agent SIP endpoints handled by users (also automatically, based on userdefined rules)

Proxy server 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 transactions Transaction 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) Method

Request URI

Protocol version

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

Status line (in an answer) e.g.: SIP/2.0 180 Ringing

Protocol version

Status Human code code


Start line One or more headers (name:value) ---------EMPTY LINE --------Message body (OPTIONAL) SIP message format

SIP methods ACK BYE CANCEL INFO INVITE NOTIFY OPTIONS PRACK PUBLISH REGISTER

ack the final response for INVITE terminate a session cancel a pending request transport PSTN telephony signaling establish a session notify a UA about a particular event queries a server about capabilities ack for provisional response upload UA’s info to a server map the current URI and location with the public URI SUBSCRIBE request to be notified about an event UPDATE update session characteristics MESSAGE instant message in the message body REFER instruct a server to send a request V. Mancuso, I. Tinnirello

SIP 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 utilization of SIP in the IMS environment E.g., RFC 3261 (redefinition of SIP) RFC 3665 and 3666 (Best Current Practice) V. Mancuso, I. Tinnirello

Prerequisites Two--phase registration Two Offline subscription

Login to the IP access net

Inbound/Outbound Fixed Proxy

DHCP or Lower level mechanism Online registration


Entering the IP Connectivity Access Network The access to GPRS includes the assignment of servers and IP address


PDF Context Activation/DeActivation/De-Activation 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 deactivated


P-CSCF 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)


IMS--level registration IMS 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


Without authentication DL User Auth vector

UAA: User Auth. Ans. MAA: Multiledia Auth.Ans SAA:Server Assignment Ans.

IMS--level registration IMS

•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

authentication vector included

DL User Profile


UAR: User Auth. Req MAR:Multimedia Auth.Req SAR:Server Assignment Req.

Challenge the User

Basic Session Setup


Session setup

Many nodes and many messages involved in the setup Different planes/layers •media •signaling •CAN •core V. Mancuso, I. Tinnirello

Session setup (cont’ed (cont’ed))


UMTS Core Network

UMTS Core Network

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