CELLULAR COMMUNICATIONS

12. IMS

2 | IMS in I&R | January

2007

Existing Telecommunications

Benefits: Worked well for stand-alone systems

Challenges: Many Networks = High Operational and

Interworking Costs Slow to introduce new services Users require different devices for different services

Difficult to integrate new services or technologies

Services

Transport & Access

3 | IMS in I&R | January

2007

Emerging Telecommunications

Services and access technologies only need to interface to the common transport layer (IP)

Access

Transport

ServicesInternet Protocol

Benefits: Rapid Service Deployment = New Service Revenues Allow continued growth of the network Flexible architecture for future growth and new

technologies Allows for competition at individual layers

Challenges: Legacy policy frameworks are challenged by the emerging

telecommunications model throughout the world

Source: ASTAP05/WS-IP&NGN/13

4

Access and Service Independence

Each service must be integrated to a specific access technology.

With many services converging it becomes complex to integrate single access.

IP provides a common interface for access and services

One point of interface simplifies the introduction of new devices and services.

video data voice

dsl wi-fi cable

Internet Protocol

video data voice

dsl wi-fi cable

Source: ASTAP05/WS-IP&NGN/13

What is IMS?5 | IMS in I&R

| January 2007

The “Internet Protocol Multimedia Subsystem” (IMS) has been created for the 3G networks and is now considered to be the standard for fix and mobile Internet-based telephony by Operators.

Developed in 3GPP (R5,R6,R7) since2001 but now adopted by 3GPP2(MMD in 2003) ETSI/TISPAN R1(2005) & R2 (on going)

The protocols come from the IETF: RTP for media SIP for signaling/address

resolution

6 | IMS in I&R | January

2007

RNCMSC(Server)

SGSNGGSN

CNMGW

BSC

UMTS/GPRS

WLAN

Corporate

P-CSCF

I-CSCFMRF

MGW

MGCF

IMS

S-CSCF

SIP ApplicationServersSIP Application

Servers

HSS

CDMA 2000DSL/Cable Modem

DSLAM/CMTS

IMS Access Network Independence

Why IMS?The Voice over IP: the impulse

7 | IMS in I&R | January

2007

After many other services, introduction of voice/video calls on Internet

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

IP packets

CodecRTPUDPIP

Why IMS? Transmission of voice/video is ok, … but transmission to which IP-address?

8 | IMS in I&R | January

2007

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

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 Betty’s phone and address

Registration of John’s phone and address

Why IMS? Solutions to make the address resolutions

9 | IMS in I&R | January

2007

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

Skype promotes a hyper-decentralized architecture: 1 table per terminal with peer-to-peer update between terminals

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.

Why IMS? SIP (IETF) is replacing H323 signaling (ITU)

The Session Initiation Protocol (SIP) performs 1 function: signaling.

SIP covers the registration for address resolution of users/terminals as well as the call handling

In fact, Internet Telephony call processing is working on the text of SIP messages to prepare the RTP voice/video flows.

SIP is naturally integrated in Internet as an increment to existing services

DNS “url” naming, like e-mail Any transmission protocol. e.g., RTP Any codec, voice/video SIP messages are small texts

10 | IMS in I&R | January

2007

Introduction

SIP is the core protocol for initiating, managing and terminating sessions in the Internet

These sessions may be text, voice, video or a combination of these

SIP sessions involve one or more participants and can use unicast or multicast communication.

SIP Message Types

Requests – sent from client to server INVITE ACK BYE CANCEL OPTIONS REGISTER INFO

SIP Message Types (Contd.)

Responses – sent from server to the client Success Redirection Forwarding Request failure Server failure Global failure

Courtesy – The RADVISION SIP Whitepaper

SIP Session Establishment and Call Termination

From the RADVISION whitepaper on SIP

John is calling Betty – introducing the HSS and the S-CSCF

16 | IMS in I&R | January

2007

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

HSS

S-CSCF

When the phones 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 – introducing the Application Server (AS)

17 | IMS in I&R | January

2007

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 not support a common voice encoding – introducing the MRFC and MRFP

18 | IMS in I&R | January

2007

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 transcoding

MRFC

MRFP

MGCP, H248 Megaco

RTP (codec: AMR)

SIPS-CSCF

RTP (codec: G729)

During its travel John is calling Betty – introducing the P-CSCF The operator has made a segmentation of its services offer – introducing the I-CSCF

19 | IMS in I&R | January

2007

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

Home NetworkVisited Network

P-CSCF

… S-CSCF (consumers)

S-CSCF (enterprises)I-CSCF

HSS

John is calling Betty who has a legacy phone – introducing the MGCF and the MGW20 | IMS in

I&R | January 2007

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

21 |

IMS in I&R | January

2007

VisitedNetwork

UE

HomeNetwork

ASSLFHSS

GGSN

S-CSCFP-CSCF I-CSCF

Session control services

Registration

AS interaction

Charging etc.

Access Point to Network

Hides Topology &

Configuration

First Point of Contact

Privacy Control & QoS Authorisation

Local Services: Emergency &

Local Numbering

Diameter Protocol

SIP Protocol

IMS basic components

CSCF – Call State Control FunctionsP – Proxy

I – InterrogatingS – Serving

UE – User EquipmentSLF- Subscriber Location

FunctionHSS- Home Subscriber Server

AS – Application Server

Gateway GPRS

Support Node

Source: ASTAP05-WP.IP&NGN-08_ETSI

Simplified IMS architecture22 | IMS in

I&R | January 2007

Standardisation Overview 3GPP / TISPAN IMS Architectural Overview

23 | IMS in I&R | January

2007

This is only a logical(functional) architecture,

not a physical one.

IMS-MGW

UE

IPv6 PDN(IPv6 Network)

MGCF

PDF

I-CSCF

S-CSCF

BGCF

Application(SIP AS,OSA AS,

CAMEL SE)

MRFC

MRFP

MRF

BB(IP v4/IPv6)

P-CSCF

SGW

OSA SCSIM

SSF SIP AS

AS

RAN

BG

IMS Session Signalling

IMS User Plane Data

HSS

‘IMS Data’ SLF

ALG

TrGW

IMS GW

IPv4 PDN(IPv4 Network)

IPv4 based Signalling

IPv4 User Plane Data

PEF

CSCF

HLR/AuC (‘CS/PS’)

3gpp R5

WLANPDG

UE

WLAN WAG

3gpp R6GGSN

SGSN

CS Networks(PSTN, CS PLMN)BAS

UE

DSLAM

3gpp R7 / TISPAN R1…

SPDF/ A-RACFNASS

24 |

IMS in I&R | January

2007

FUNCTIONAL ELEMENTS DESCRIPTIONS

Home Subscriber Server (HSS) Application Server (AS) Call Session Control Function (CSCF) Breakout Gateway Control Function (BGCF) Media Gateway Function (MGW) Media Gateway Control Function (MGCF) Multimedia Resource Function Controller

(MRFC) Multimedia Resource Function Processor

(MRFP)

25 |

IMS in I&R | January

2007

Home Subscriber Server (HSS)

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

HSS

Diameter

26 |

IMS in I&R | January

2007

Application Server (AS)

Contains Call Related Application Logic

Facilitates a Service Creation Environment

Queried by S-CSCF in Real Time to Execute Logic

Generally Specialized for Each Service

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

ASASAS

SIPDiameter

27 |

IMS in I&R | January

2007

Call/Session Control Function (CSCF)

CSCF – Processes SIP Signaling P-CSCF

First Point of User Contact Authenticates user May Include Policy Functions

C-CSCF Central Node of Control Plane Acts as Registar for User (Downloads SSP from HSS) Invokes Application Servers Performs Primary Routing Function

I-CSCF Located at Edge of Administrative Domain Is the Ingress Network Point Defined in DNS Shields Network Topology from External Networks

I-CSCFS-

CSCFP-

CSCFSIP

SIP

SIPSIP

SIP

Diameter

28 |

IMS in I&R | January

2007

PSTN (Circuit Switched) Gateway

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

MGCF

MGW

H.248

ISUP

BGCFSIP

SIP

TDM

SGW

SIP

29 |

IMS in I&R | January

2007

Multimedia Resource Function (MRF)

Offers Services Such as Conferencing

MRFC – SIP User Interface toward S-CSCF

MRFP – Controls the Media Server (MS)

SIP

MS MS

MRFC