CELLULAR COMMUNICATIONS
12. IMS
Existing Telecommunications
2
Services
Transport
& Access
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Benefits:
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Worked well for stand-alone systems
Challenges:
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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
Emerging Telecommunications
3
Services
Transport
Access
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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
Access and Service Independence
video
data
voice
video
data
voice
Internet Protocol
dsl


wi-fi
cable
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
dsl


wi-fi
cable
IP provides a common
interface for access and
services
One point of interface
simplifies the introduction
of new devices and
services.
4
What is IMS?
5 | IMS in I&R
| January
2007
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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 Internetbased 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
IMS Access Network Independence
6
SIP Application
Servers
DSL/Cable Modem
HSS
SIP Application
Servers
IMS
CDMA 2000
I-CSCF
MRF
DSLAM/CMTS
P-CSCF
MGCF
S-CSCF
MGW
WLAN
MSC(Server)
RNC
SGSN
GGSN
BSC
UMTS/GPRS
CN
MGW
Corporate
Why IMS?
The Voice over IP: the impulse
7 | IMS in I&R
| January
2007


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
Why IMS? Transmission of voice/video is ok, … but transmission to
which IP-address?
8 | IMS in I&R
| January
2007

Registration of Betty’s phone
and address
The 2nd problem is more difficult:
how to initiate the call? how to know
the IP-address of the called phone?


Registration of John’s phone
and address


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…
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 hyperdecentralized architecture: 1
table per terminal with peerto-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)
10 | IMS in
I&R | January
2007
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

Introduction
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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

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



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


HSS
When the phones get
connected they register
their name/IP to the HSS
SIP
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
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)
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
…
nearest Taxi application (location, fleet…)
AS
S-CSCF
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
S-CSCF
SIP
MRFC
MGCP, H248 Megaco
RTP (codec: G729)
RTP (codec: AMR)
MRFP
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



HSS
…
P-CSCF
S-CSCF (consumers)
I-CSCF
S-CSCF (enterprises)
Visited Network
Home Network
John is calling Betty who has a legacy phone –
introducing the MGCF and the MGW
20 | IMS in
I&R | January
2007
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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 basic components
21
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
AS
HSS
Diameter Protocol
SIP Protocol
P-CSCF
I-CSCF
S-CSCF
UE
GGSN
First Point of
Contact
Gateway GPRS
Support Node
Privacy Control &
QoS
Authorisation
Local Services:
Emergency &
Local Numbering
Source: ASTAP05-WP.IP&NGN-08_ETSI
Access Point to
Network
Hides Topology &
Configuration
Session control
services
Registration
AS interaction
Visited
Network
Home
Network
Charging etc.
Simplified IMS architecture
22 | IMS in
I&R | January
2007
Standardisation Overview
3GPP / TISPAN IMS Architectural Overview
This is only a logical
(functional) architecture,
not a physical one.
23 | IMS in
I&R | January
2007
Application
AS
HSS
(SIP AS,
OSA AS,
CAMEL SE)
OSA SCS
IM SSF
‘IMS Data’
SIP AS
SLF
HLR/AuC (‘CS/PS’)
CSCF
IMS Session Signalling
BGCF
IMS User Plane Data
S-CSCF
I-CSCF
IPv4 based Signalling
IPv4 User Plane Data
MGCF
P-CSCF
NASS
SPDF/ A-RACF
MRF
UE
DSLAM
3gpp R7 / TISPAN R1…
WLAN WAG
UE
BAS
PDF
RAN
3gpp R5
SGSN
ALG
MRFP
TrGW
SGW
CS Networks
(PSTN, CS PLMN)
IMS-MGW
IPv4 PDN
(IPv4 Network)
3gpp R6
UE
MRFC
WLAN
PDG
GGSN
IMS GW
PEF
BB
(IP v4/
IPv6)
BG
IPv6 PDN
(IPv6 Network)
FUNCTIONAL ELEMENTS DESCRIPTIONS
24
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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)
Home Subscriber Server (HSS)
HSS
25
Diameter
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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
Application Server (AS)
AS
AS
AS
26
Diameter
SIP
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
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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)
Call/Session Control Function (CSCF)
Diameter
27
P-CSCF
I-CSCF
SIP
S-CSCF
SIP
SIP
SIP

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CSCF – Processes SIP Signaling
P-CSCF
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First Point of User Contact
Authenticates user
May Include Policy Functions
C-CSCF

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SIP
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
PSTN (Circuit Switched) Gateway
SGW
28
ISUP
SIP
BGCF
SIP
SIP
MGCF
H.248
MGW
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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
Multimedia Resource Function (MRF)
29
SIP
MRFC
MS

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
MS
Offers Services Such as Conferencing
MRFC – SIP User Interface toward S-CSCF
MRFP – Controls the Media Server (MS)