1. Cellular COMMUNICATIONS
12. IMS

2. Existing Telecommunications
Services
Transport
& Access
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

3. Emerging Telecommunications
Access
Transport
Services
Internet 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
Services and access technologies only need to interface to the common transport layer (IP)
Source: ASTAP05/WS-IP&NGN/13

4. Access and Service Independence
video
data
voice
dsl
wi-fi
cable
video
data
voice
dsl
wi-fi
cable
Internet Protocol
IP provides a common interface for access and services
One point of interface simplifies the introduction of new devices and services.
Each service must be integrated to a specific access technology.
With many services converging it becomes complex to integrate single access.
Source: ASTAP05/WS-IP&NGN/13

5. What is IMS?
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 Access Network Independence
P-CSCF
I-CSCF
MRF
MGW
MGCF
IMS
S-CSCF
SIP Application
Servers
HSS
DSL/Cable Modem
DSLAM/CMTS
CDMA 2000
WLAN
Corporate
MSC(Server)
RNC
SGSN
GGSN
IMS is designed to be applicable to the evolution of all types of networks. The major wireless carriers have committed to IMS as their next generations network. All of the U.S. major wireline carriers have embraced IMS in their evolution path. Cable network companies have not embraced IMS as of yet since Cable Labs has just defined IP-based network topology that was pre-IMS. It is thought that as cable networks evolve, they too will embrace IMS.
One major advantage of IMS’s commonality is that carriers from each discipline can purchase equipment based upon the same standards, thereby potentially decreasing the cost to provide duplicate networks for different media services.
BSC CN
UMTS/GPRS
MGW

7. Why IMS? The Voice over IP: the impulse
IP packets
Codec
RTP
UDP IP
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

8. The control of the address table creates a tough competition…
Why IMS? Transmission of voice/video is ok, … but transmission to which IP-address?
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

9. Why IMS? Solutions 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: 1 table per terminal with peer- to-peer update between terminals
Operators dislike such approaches, so they push a way very similar to centralization per domain. This approach is named “Internet Protocol Multimedia Subsystem”, IMS.

10. 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
Any transmission protocol. e.g., RTP
Any codec, voice/video
SIP messages are small texts

11. 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.

12. SIP Message Types Requests – sent from client to server INVITE ACK BYE
CANCEL
OPTIONS
REGISTER
INFO

13. SIP Message Types (Contd.)
Responses – sent from server to the client
Success
Redirection
Forwarding
Request failure
Server failure
Global failure

14. Courtesy – The RADVISION SIP Whitepaper

15. SIP Session Establishment and Call Termination
From the RADVISION whitepaper on SIP

16. John is calling Betty – introducing the HSS and the S-CSCF
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 register their name/IP to the HSS
Serving Call Session Control Function
Home Subscriber Server
SIP
S-CSCF
SIP
Changes the SIP message replacing “Betty” by its IP address found in the HSS

17. 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

18. 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 transcoding
S-CSCF
SIP
MRFC
The Media Resource Function (MRF) provides media related functions such as media manipulation (e.g. voice stream mixing) and playing of tones and announcements.
Each MRF is further divided into a Media Resource Function Controller (MRFC) and a Media Resource Function Processor (MRFP).
MGCP, H248 Megaco
RTP (codec: G729)
RTP (codec: AMR)
MRFP

19. 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
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
HSS …
S-CSCF (consumers)
A Proxy-CSCF (P-CSCF) is a 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).
An Interrogating-CSCF (I-CSCF) is another SIP function located at the edge of an administrative domain. Its IP address is published in the Domain Name System (DNS) of the domain (using NAPTR and SRV type of DNS records), so that remote servers can find it, and use it as a forwarding point (e.g. registering) for SIP packets to this domain.
P-CSCF
I-CSCF
S-CSCF (enterprises)
Visited Network
Home Network

20. Legacy Call control (SS7)
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
Megaco (officially H.248) is an implementation of theMedia Gateway Control Protocol architecture[1] for controlling media gateways in Internet Protocol (IP) networks and the public switched telephone network(PSTN).
RTP
Phone transmission
Internet
MGW
PSTN/PLMN network

21. IMS basic components UE Visited Network Home Network SLF AS HSS P-CSCF
CSCF – Call State Control Functions P – Proxy I – Interrogating S – Serving UE – User Equipment SLF- Subscriber Location Function HSS- Home Subscriber Server AS – Application Server
SLF
HSS AS
Diameter Protocol
SIP Protocol UE
P-CSCF
I-CSCF
S-CSCF
GGSN
Gateway GPRS Support Node
Session control services
Registration
AS interaction
Charging etc.
First Point of Contact
Privacy Control & QoS Authorisation
Local Services: Emergency & Local Numbering
Access Point to Network
Hides Topology & Configuration
Diameter is a AAA protocol, a type of computer networking protocol for authentication, authorization and accounting
he Subscriber Location Function, or SLF is an entity within an IP Multimedia Subsystemthat provides information about the Home Subscriber Server (HSS) that is associated with a particular user profile. It is generally implemented using a database. If the home domain contains more than one HSS, I-CSCF and S-CSCF will communicate with SLF and find the appropriate HSS based on user profile. CSCF communicates with the SLF using diameter Dx interface and the application server communicates with the SLF using Dh interface.
Visited
Network
Home
Network
Source: ASTAP05-WP.IP&NGN-08_ETSI

22. Simplified IMS architecture

23. Standardisation Overview 3GPP / TISPAN IMS Architectural Overview
This is only a logical
(functional) architecture,
not a physical one.
Application
(SIP AS,
OSA AS,
CAMEL SE)
OSA SCS
IM SSF
SIP AS AS
HSS
‘IMS Data’
SLF
CSCF
HLR/AuC (‘CS/PS’)
IMS Session Signalling
BGCF
IMS User Plane Data
S-CSCF
I-CSCF
IPv4 based Signalling
IPv4 User Plane Data
MGCF
P-CSCF
BAS UE
DSLAM
3gpp R7 / TISPAN R1…
SPDF/ A-RACF
NASS
MRF
SGW
IMS GW
The complete solution for the support of IP multimedia applications consists of terminals, IP-Connectivity Access Networks (IP-CAN), and the specific functional elements of the IMS subsystem.
In the 3GPP IMS specifications GPRS/UMTS is only one example of an IP-Connectivity Access Networks. 3GPP defines the link (QoS, charging, etc) between IMS and several IP-CANs:
3GPP R5 -> only GPRS/UMTS access
3GPP R6 -> also WLAN
3GPP R7 (in co-operation with TISPAN R1) -> DSL access
The IP-CAN maintains the IP connection while the user moves and hides these moves from the IMS subsystem. Each IP-CAN exposes one anchor point (GGSN, BRAS, WLAN PDG) towards the IMS.
PDG = WLAN Packet Data Gateway (IP Edge, WLAN tunnel endpoint)
WAG = WLAN Wireless Access Gateway (Intermediate tunnel point aggregating WLAN tunnels between hotspots and mobile operator)
SGSN = Serving GPRS Serving Node (Intermediate GPRS tunnel node, involved in mobility/authentication etc)
GGSN = Gateway GPRS Serving Node (IP Edge, puts external IP packets to/from terminal on/from GPRS tunnel from/to the Backbone)
The IMS specific functional elements are: CSCFs (SIP session control servers), HSS/SLF (user database), AS (Applications), PDF (QoS/Charging enforcer), BGCF/MGCF/SGW (interworking with legacy circuit switched networks), IMS-ALG/Tr-GW (interworking between IPv6 and IPv4 networks), MRF (conferencing control etc).
3GPP & TISPAN standards only define a “logical / functional” architecture, not a “physical” one. Manufacturers can each chose how to combine the 3GPP functional blocks into physical products.
CS Networks
(PSTN, CS PLMN)
PDF
MRFC
ALG
WLAN
PDG UE
WLAN WAG
3gpp R6
MRFP
TrGW
IMS-MGW
IPv4 PDN
(IPv4 Network) BB
(IP v4/
IPv6)
GGSN
PEF BG
IPv6 PDN
(IPv6 Network) UE
RAN
SGSN
3gpp R5

24. 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. Home Subscriber Server (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
The HSS is the central repository for user-related information. In wireless networks it is the evolution of the HLR. The HSS contains all the user-related subscription data required to handle multimedia sessions. These data include, among other items, location information (not the physical location), security information (including authentication and authorization), user profile information (including the services that the user is subscribed to) and the S-CSCF that is allocated to the user.
A network may contain more that one HSS in the case the number of subscribers is too high to be handled by a single HSS. All data related to a particular user are stored within a single HSS. The HSS is typically implemented using a redundant configuration.

26. Application Server (AS)
Diameter
SIP
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)
Application Servers host and execute services. Depending upon the application they may operate as a SIP Redirect Server, Proxy, User Agent or Back to Back User Agent (B2BUA).
There may be three categories of AS: SIP AS, OSA-CS and IM-SSF.
SIP AS – This is a native Application server that hosts and executes IMS services based upon SIP. New IMS applications will be developed in the SIP AS.
OSA-SCS (Open Services Access – Service Capability Server) – This Application Server provides an interface to the OSA framework Applications Server. In essence it provides a gateway function.
IM-SSF (IMS Service Switching Function) – This is a specialized Applications Server that alls reuse of CAMEL (Customized Applications for Mobile network Enhanced Logic) services. These services are specific to GSM and European networks. A similar function will provide access to U.S. Advanced Intelligent Network (AIN) services.

27. Call/Session Control Function (CSCF)
Diameter
P-CSCF
I-CSCF
SIP
S-CSCF
SIP
SIP
SIP
SIP
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
In general CSCF provide the SIP routing logic in the IMS network.
P-CSCF – The P-CSCF is the first point of contact between the IMS terminal and the network . All signaling from/to the IMS terminal go through the P-CSCF. Thje P-CSCF is allocated to the IMS terminal during registration and provides functions such as security, authentication, and the correctness of the SIP requests. The P-CSCF may include a Policy Decision Function (PDF) that authorizes media plane resources and manages Quality of Service over the media plane.
I-CSCF – The is a SIP Proxy located at the edge of an administrative domain. The address of the I-CSCF is listed in the DNS records of the domain. When a SIP server follows SIP procedures to find the next SIP hop for a particular message the SIP server obtains the address of an I-CSCF of the destination domain.
S-CSCF – The S-CSCF is the central note of the IMS signaling plain. It acts as a registrar in that when the IMS terminal registers the S-CSCF obtains SSP information from the HSS. All signaling passes through a S-CSCF. The S-CSCF inspects every SIP message and determines whether the SIP signaling should visit one or more Application Servers. Those ASs would potentially provide a service to the user.

28. PSTN (Circuit Switched) Gateway
SGW
ISUP
SIP
SIP
BGCF
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
IMS networks must be able to deliver calls to and receive calls from the PSTN. In order to do this there is a need to interwork signaling (e.g. SIP to ISUP) and bearer channels (e.g. RTP to TDM).
BGCF – The BGCF provides routing functionality based on telephone numbers. The BGCF is only used in a circuit switched network, such as the PSTN. Its basic functions are 1) select an appropriate network where interworking with the circuit switched (CS) domain is to occur or 2) select an appropriate PSTN/CS gateway (i.e. MGCF).
MGCF – The MGCF is the central node of the PSTN/CS gateway. It implements a state machine that does protocol conversion and maps SIP to ISUP. It also controls the resources of the Media Gateway.
SGW – The Signaling Gateway performs the lower layer protocol conversion. In this presentation it is assumed part of the MGCF.
MGW – The Media Gateway interfaces to the media plane of the CS network. One side the MGW is able to send and receive IMS media over RTP and on the other side the MGW uses one or more PCM time slots to connect to the CS network.

29. Multimedia Resource Function (MRF)
MRFC
SIP MS MS
Offers Services Such as Conferencing
MRFC – SIP User Interface toward S-CSCF
MRFP – Controls the Media Server (MS)
The Media Resource Function provides a source of media in the IMS network. This may be the ability to play announcements, mix media streams (for conferencing), transcode between different codecs, and do any sort of media analysis.