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1 Introduction
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1.1 The Need for IMS (1/3)...........................
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1.1 The Need for IMS (1/3)
The new communication p...
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1.1 The Need for IMS (2/3)
This redefines applicat...
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1.1 The Need for IMS (3/3)
True integration of voi...
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1.2 UMTS Architecture RAN and CN (1/4)
The Radio A...
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1.2 UMTS Architecture RAN and CN (2/4)
The basis o...
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1.2 UMTS Architecture RAN and CN (3/4)
In its ulti...
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1.2 UMTS Architecture RAN and CN (4/4)
The UMTS Re...
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1.3 UMTS Architecture Planes (1/3)
Another concep...
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1.3 UMTS Architecture Planes (2/3)
The user plane...
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1.3 UMTS Architecture Planes (3/3)
The Control Pl...
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1.4 UMTS R99 Architecture (1/9)
The UMTS network ...
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1.4 UMTS R99 Architecture (2/9)
UMTS Release 99 i...
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1.4 UMTS R99 Architecture (3/9)
The new UMTS Terr...
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1.4 UMTS R99 Architecture (4/9)
The UMTS Core Net...
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1.4 UMTS R99 Architecture (5/9)
The Iu interface ...
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1.4 UMTS R99 Architecture (6/9)
Now let‘s have a ...
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1.4 UMTS R99 Architecture (7/9)
Apart from the ne...
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1.4 UMTS R99 Architecture (8/9)
As a consequence,...
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1.4 UMTS R99 Architecture (9/9)
On the control pl...
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1.5 UMTS R4 Architecture (1/2)
The purpose of Rel...
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1.5 UMTS R4 Architecture (2/2)
The MSC user plane...
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1.6 UMTS R5 Architecture (1/4)
The major changes ...
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1.6 UMTS R5 Architecture (2/4)
By introducing the...
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1.6 UMTS R5 Architecture (3/4)
IMS introduces new...
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1.6 UMTS R5 Architecture (4/4)
The latter require...
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1.7 UMTS R6 Aspects
During its ongoing specificat...
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1.8 UMTS R7 Aspects
Furthermore, 3GPP continues w...
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1.9 NGN Aspects
Earlier, we said that IMS is acce...
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1.10 Involved Standardization Bodies (1/10)
As we...
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1.10 Involved Standardization Bodies (2/10)
The I...
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1.10 Involved Standardization Bodies (3/10)
3GPP ...
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1.10 Involved Standardization Bodies (4/10)
In Ju...
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1.10 Involved Standardization Bodies (5/10)
OMA h...
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1.10 Involved Standardization Bodies (6/10)
There...
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1.10 Involved Standardization Bodies (7/10)
The T...
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1.10 Involved Standardization Bodies (8/10)
3GPP2...
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1.10 Involved Standardization Bodies (9/10)
Howev...
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1.10 Involved Standardization Bodies (10/10)
Addi...
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2 Entities & Functions
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2.1 Overview ........................................
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2.1 Overview
In the previous chapter, we presented...
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2.2 Proxy-Call Session Control Function (P-CSCF) (...
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2.2 Proxy-Call Session Control Function (P-CSCF) (...
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2.3 Serving-Call Session Control Function (S-CSCF)...
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2.4 Interrogation-Call Session Control Function (I...
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2.5 Home Subscriber Server (HSS) (1/6)
The Home Su...
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2.5 Home Subscriber Server (HSS) (2/6)
User identi...
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2.5 Home Subscriber Server (HSS) (3/6)
A special ...
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2.5 Home Subscriber Server (HSS) (4/6)
IMS access...
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2.5 Home Subscriber Server (HSS) (5/6)
There can ...
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2.5 Home Subscriber Server (HSS) (6/6)
There can ...
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2.6 Policy Decision Function (PDF) (1/2)
The Poli...
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2.6 Policy Decision Function (PDF) (2/2)
Service ...
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2.7 Multimedia Resource Function Controller (MRFC...
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2.8 Multimedia Resource Function Processor (MRFP)...
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2.9 Media Gateway Control Function (MGCF)
The Med...
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2.10 Signaling Gateway (SGW)
For the lower signal...
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2.11 Breakout Gateway Control Function (BGCF)
The...
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2.12 IMS Multimedia Gateway Function (IMS-MGW)
Th...
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2.13 Security Gateway (SEG)
To protect signaling ...
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2.14 SGSN and GGSN
Finally, we should not forget ...
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2.15 Application Servers (1/3)
So far we have gai...
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2.15 Application Servers (2/3)
The Application Pl...
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2.15 Application Servers (3/3)
Application can be...
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2.16 Summary (1/3)
As we come to the end of this ...
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2.16 Summary (2/3)
In the Control Plane we have a...
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2.16 Summary (3/3)
Finally, the Application Plane...
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3 NGN Aspects
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3.1 IMS and NGN: General Aspects (1/3)...............
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3.1 IMS and NGN: General Aspects (1/3)
Earlier, we...
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3.1 IMS and NGN: General Aspects (2/3)
NGN archite...
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3.1 IMS and NGN: General Aspects (3/3)
An NGN is d...
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3.2 Layers, Sub-systems and Functions
The NGN arch...
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3.3 Transport Layer
The transport layer is divided...
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3.4 Service Layer (1/5)
The service layer contains...
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3.4 Service Layer (2/5)
The term PSTN emulation is...
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3.4 Service Layer (3/5)
The term PSTN/ISDN simula...
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3.4 Service Layer (4/5)
The PSTN/ISDN Emulation S...
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3.4 Service Layer (5/5)
The service layer in NGN ...
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3.5 IMS and NGN: Network Entities (1/9)
NGN is la...
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3.5 IMS and NGN: Network Entities (2/9)
The P-CSC...
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3.5 IMS and NGN: Network Entities (3/9)
The I-CSC...
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3.5 IMS and NGN: Network Entities (4/9)
Common fu...
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3.5 IMS and NGN: Network Entities (5/9)
The UPSF ...
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3.5 IMS and NGN: Network Entities (6/9)
When seve...
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3.5 IMS and NGN: Network Entities (7/9)
The Inter...
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3.5 IMS and NGN: Network Entities (8/9)
NGN ident...
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3.5 IMS and NGN: Network Entities (9/9)
Finally, ...
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4 Reference Points
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4.1 IMS Reference Points: Overview...................
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4.1 IMS Reference Points: Overview
Let us now have...
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4.2 IMS Reference Points: Gm
The Gm interface conn...
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4.3 IMS Reference Points: Mw
The Mw interface then...
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4.4 IMS Reference Points: Cx
HSS permanently store...
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4.5 IMS Reference Points: Dx
When multiple and sep...
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4.6 IMS Reference Points: ISC
Let us now involve t...
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4.7 IMS Reference Points: Sh
An AS may need subscr...
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4.8 IMS Reference Points: Si
When the AS provides...
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4.9 IMS Reference Points: Dh
Again we might encou...
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4.10 IMS Reference Points: Ut
The Ut reference po...
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4.11 IMS Reference Points: Mm
For communicating w...
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4.12 IMS Reference Points: Mg
The Mg reference po...
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4.13 IMS Reference Points: Mi
When the S-CSCF dis...
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4.14 IMS Reference Points: Mj
In order to address...
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4.15 IMS Reference Points: Mk
If the destination ...
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4.16 IMS Reference Points: Mr
When the S-CSCF nee...
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4.17 IMS Reference Points: Mp
When the MRFC needs...
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4.18 IMS Reference Points: Mn
H.248 is also used ...
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4.19 IMS Reference Points: Go
In order to provide...
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4.20 IMS Reference Points: Gq
PDF information abo...
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5 Signaling Protocols
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5.1 Overview ........................................
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5.1 Overview
While discussing IMS reference points...
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5.2 Session Control Protocols
IMS protocols involv...
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5.3 Session Initiation Protocol (SIP)
IP is a text...
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5.4 Diameter
IETF’s DIAMETER has been selected as ...
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5.5 Common Open Policy Service (COPS)
A Common Ope...
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5.6 Media Gateway Control Protocol (MEGACO)
H.248 ...
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5.7 Real Time Transport Protocol (RTP)
RTP (Real-t...
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5.8 Real Time Transport Control Protocol (RTCP)
T...
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5.9 Resource Reservation Protocol (RSVP)
Only RSV...
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5.10 Session Description Protocol (SDP)
The text-...
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5.11 Domain Name Service (DNS)
Domain Name servic...
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5.12 Transport Layer Security (TLS)
Transport Lay...
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5.13 Internet Protocol Security (IPSec)
In contra...
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5.14 Dynamic Host Configuration Protocol for IPv6...
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5.15 XML Configuration Access Protocol (XCAP)
XML...
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5.16 Conference Policy Control Protocol (CPCP)
Th...
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5.17 Summary (1/2)
At the end of this description...
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5.17 Summary (2/2)
The second overview relates pr...
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6 Media Encoding and Transport
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6.1 Overview ........................................
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6.1 Overview
In this module we will have a closer ...
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6.2 Adaptive Multi-Rate Codec (1/4)
3GPP defined t...
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6.2 Adaptive Multi-Rate Codec (2/4)
AMR consists o...
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6.2 Adaptive Multi-Rate Codec (3/4)
The AMR modes ...
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6.2 Adaptive Multi-Rate Codec (4/4)
The AMR codec ...
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6.3 AMR-WB Codec (1/4)
3G networks using WCDMA acc...
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6.3 AMR-WB Codec (2/4)
When AMR is transported ove...
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6.3 AMR-WB Codec (3/4)
The AMR-WB codec as define...
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6.3 AMR-WB Codec (4/4)
AMR-WB consists of a famil...
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6.4 Video Codecs (1/2)
Video encoding relies on s...
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6.4 Video Codecs (2/2)
IMS video encoding formats...
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6.5 H.263 Codec (1/3)
H.263 evolved out of H.261,...
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6.5 H.263 Codec (2/3)
The luminance components is...
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6.5 H.263 Codec (3/3)
Image resolution formats us...
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6.6 H.261 Codec
ITU-T’s H.261 standard defines th...
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6.7 MPEG Codecs (1/3)
The MPEG video standards ar...
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6.7 MPEG Codecs (2/3)
MPEG-2 was developed to enc...
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6.7 MPEG Codecs (3/3)
Initially, MPEG-3 was desig...
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6.8 Text Encoding (1/3)
Text already consists of ...
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6.8 Text Encoding (2/3)
Instant messages convey a...
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6.8 Text Encoding (3/3)
Real-time text consists o...
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6.9 Save Media Transport (1/2)
For a safe media t...
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6.9 Save Media Transport (2/2)
Hence, the type of...
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6.10 Real-Time Transport Protocol (1/4)
Applicati...
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6.10 Real-Time Transport Protocol (2/4)
RTP’s maj...
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6.10 Real-Time Transport Protocol (3/4)
The recei...
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6.10 Real-Time Transport Protocol (4/4)
t is obvi...
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6.11 Real-Time Transport Control Protocol (1/3)
R...
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6.11 Real-Time Transport Control Protocol (2/3)
I...
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6.11 Real-Time Transport Control Protocol (3/3)
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7 Procedures
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7.1 IMS Registration (1/13)..........................
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7.8 Call Release by PSTN (2/4) ......................
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7.1 IMS Registration (1/13)
Let us start with the ...
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7.1 IMS Registration (2/13)
At first, we will look...
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7.1 IMS Registration (3/13)
This first part of the...
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7.1 IMS Registration (4/13)
After the UE has disco...
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7.1 IMS Registration (5/13)
Now the P-CSCF realize...
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7.1 IMS Registration (6/13)
The P-CSCF determines ...
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7.1 IMS Registration (7/13)
The I-CSCF is the ent...
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7.1 IMS Registration (8/13)
The S-CSCF needs to a...
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7.1 IMS Registration (9/13)
The S-CSCF selects an...
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7.1 IMS Registration (10/13)
Upon reception, the ...
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7.1 IMS Registration (11/13)
Again a DNS query is...
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7.1 IMS Registration (12/13)
The S-CSCF checks th...
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7.1 IMS Registration (13/13)
The S-CSCF notifies ...
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7.2 IMS Session Scenarios (1/2)
IMS session contr...
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7.2 IMS Session Scenarios (2/2)
Let us have a clo...
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7.3 Mobile Originated Call (non-roaming) (1/10)
L...
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7.3 Mobile Originated Call (non-roaming) (2/10)
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7.3 Mobile Originated Call (non-roaming) (3/10)
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7.3 Mobile Originated Call (non-roaming) (4/10)
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  1. 1. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 1 of 40 1 Introduction
  2. 2. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 2 of 40 1.1 The Need for IMS (1/3)...................................................................3 1.1 The Need for IMS (2/3)...................................................................4 1.1 The Need for IMS (3/3)...................................................................5 1.2 UMTS Architecture RAN and CN (1/4) ...........................................6 1.2 UMTS Architecture RAN and CN (2/4) ...........................................7 1.2 UMTS Architecture RAN and CN (3/4) ...........................................8 1.2 UMTS Architecture RAN and CN (4/4) ...........................................9 1.3 UMTS Architecture Planes (1/3)...................................................10 1.3 UMTS Architecture Planes (2/3)...................................................11 1.3 UMTS Architecture Planes (3/3)...................................................12 1.4 UMTS R99 Architecture (1/9) .......................................................13 1.4 UMTS R99 Architecture (2/9) .......................................................14 1.4 UMTS R99 Architecture (3/9) .......................................................15 1.4 UMTS R99 Architecture (4/9) .......................................................16 1.4 UMTS R99 Architecture (5/9) .......................................................17 1.4 UMTS R99 Architecture (6/9) .......................................................18 1.4 UMTS R99 Architecture (7/9) .......................................................19 1.4 UMTS R99 Architecture (8/9) .......................................................20 1.4 UMTS R99 Architecture (9/9) .......................................................21 1.5 UMTS R4 Architecture (1/2) .........................................................22 1.5 UMTS R4 Architecture (2/2) .........................................................23 1.6 UMTS R5 Architecture (1/4) .........................................................24 1.6 UMTS R5 Architecture (2/4) .........................................................25 1.6 UMTS R5 Architecture (3/4) .........................................................26 1.6 UMTS R5 Architecture (4/4) .........................................................27 1.7 UMTS R6 Aspects........................................................................28 1.8 UMTS R7 Aspects........................................................................29 1.9 NGN Aspects................................................................................30 1.10 Involved Standardization Bodies (1/10) ......................................31 1.10 Involved Standardization Bodies (2/10) ......................................32 1.10 Involved Standardization Bodies (3/10) ......................................33 1.10 Involved Standardization Bodies (4/10) ......................................34 1.10 Involved Standardization Bodies (5/10) ......................................35 1.10 Involved Standardization Bodies (6/10) ......................................36 1.10 Involved Standardization Bodies (7/10) ......................................37 1.10 Involved Standardization Bodies (8/10) ......................................38 1.10 Involved Standardization Bodies (9/10) ......................................39 1.10 Involved Standardization Bodies (10/10) ....................................40
  3. 3. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 3 of 40 1.1 The Need for IMS (1/3) The new communication paradigm is about networking Internet Protocol (IP)-based mobile devices. These terminals have built-in cameras, large, high-precision displays and plentyful applications resources. They are always-on-always-connected application devices.
  4. 4. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 4 of 40 1.1 The Need for IMS (2/3) This redefines applications. Applications are no longer isolated entities exchanging information only with the user interface. The next generation of more exciting applications are peer- to- peer entities, which facilitate sharing: shared browsing, shared whiteboard, shared game experience and shared two-way radio session (i.e. push to talk).
  5. 5. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 5 of 40 1.1 The Need for IMS (3/3) True integration of voice and data services increases productivity and overall effectiveness while the development of innovative applications integrating voice, data and multimedia will create demands for new services. These will include, presence, multimedia chat, push to talk and conferencing. The ability to combine mobility and the IP-network will be crucial to service success in the future and will require more capable networks than the present 3G mobile networks.
  6. 6. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 6 of 40 1.2 UMTS Architecture RAN and CN (1/4) The Radio Access Network (RAN) and Core Network (CN) were concepts developed to overcome the problem of compatibility between the many fixed and wireless network types being developed throughout the world.
  7. 7. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 7 of 40 1.2 UMTS Architecture RAN and CN (2/4) The basis of the concept is that the RAN part of the fixed network architecture takes care of the radio aspects of the radio link for the mobile station. The aspects covered by the RAN include handover, power control, random access and radio bearer load control.
  8. 8. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 8 of 40 1.2 UMTS Architecture RAN and CN (3/4) In its ultimate form, there will be many different types of RAN (GERAN, UTRAN, UMA, WLAN) as well as many types of CN (IP, Narrow-band-ISDN, Broadband-ISDN, UMTS Core network, Public Data Networks).
  9. 9. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 9 of 40 1.2 UMTS Architecture RAN and CN (4/4) The UMTS Release 99 architecture will focus on the UTRAN connected to a GSM CN. I.e., the Network Subsystem NSS for circuit Switched (CS) services, and the GPRS network for Packet Switched (PS) services. Both parts together create the UMTS Core Network CN.
  10. 10. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 10 of 40 1.3 UMTS Architecture Planes (1/3) Another concept introduces the two logically independent planes - the control and user planes - used to separate control messages, i.e., signaling from user information flow,i.e., content between user equipment (UE) and the CN.
  11. 11. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 11 of 40 1.3 UMTS Architecture Planes (2/3) The user plane in the UMTS system, for instance, will transport all of the user data traffic between the UE and the CN. The user data can be any type of user data associated with a Circuit Switched (CS) or a Packet Switched (PS) connection, e.g., voice and e-mail or Web access data.
  12. 12. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 12 of 40 1.3 UMTS Architecture Planes (3/3) The Control Plane is used to pass control messages between the UTRAN / CN and the UE. The control messages are varied and include signaling messages which relate to functions like location updates, handover and call setup.
  13. 13. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 13 of 40 1.4 UMTS R99 Architecture (1/9) The UMTS network architecture is divided into three subsystems, with well defined interfaces: The Radio Access Network (RAN) - the Core Network (CN) – and the Operations Support Systems (OSS). The functions of each subsystem are similar to those of GSM. This makes it very easy for operators to upgrade their existing networks.
  14. 14. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 14 of 40 1.4 UMTS R99 Architecture (2/9) UMTS Release 99 introduces the UTRAN, the radio access network for UMTS, and provides all transmission and control functions needed for area wide radio coverage. The Air-Interface Uu connects the mobile user equipment UE to the UTRAN. The CN provides the main switching and subscriber handling functions and connects to the UTRAN via the Iu interface. The OSS is responsible for the management of the whole network. In the following steps we will introduce the network elements of the different subsystems and show the network structure will develop within future releases.
  15. 15. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 15 of 40 1.4 UMTS R99 Architecture (3/9) The new UMTS Terrestial Radio Access Network (UTRAN) consists of the base station Node-B and the RNC. The base station is responsible for the W-CDMA transmission on the air interface, and connects to the Radio Network Controller RNC via the Iub Interface The main tasks of RNC are radio resource management, mobility management and radio network supervision.
  16. 16. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 16 of 40 1.4 UMTS R99 Architecture (4/9) The UMTS Core Network consists of two logical independent domains: The CS circuit switched domain and the PS packet switched domain. Already familiar from GSM NSS and GPRS, the CS and the PS domain are based on the same main elements: CS consists mainly of MSC, VLR and Gateway-MSC, while PS is based on GPRS core network elements like the GGSN, also introducing a 3G SGSN. The CS and PS domains share HLR, AC and EIR as common network elements.
  17. 17. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 17 of 40 1.4 UMTS R99 Architecture (5/9) The Iu interface between the UTRAN and the CN is divided into two separate functional parts according to the services supported: For the circuit switched services the RNC is connected to a Media Gateway / MSC tandem via the Iu-CS interface.The Media Gateway functionality will be explained later. For the packet switched services the Iu-PS interface is used between RNC and SGSN. RNCs can also be interconnected via the Iur-interface.
  18. 18. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 18 of 40 1.4 UMTS R99 Architecture (6/9) Now let‘s have a look at the combination of the UMTS network with an existing GSM and GPRS structure. The 2G Base Station Subsystem BSS , consisting of BTS and BSC, is connected to the CS domain via the A interface towards the MSC, whereas the connection to the PS domain takes place via the Gb interface between the BSC and the 2G-SGSN.
  19. 19. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 19 of 40 1.4 UMTS R99 Architecture (7/9) Apart from the new W-CDMA structure on the air interface, there are two more major differences between GSM and UMTS networks: 1. In GSM, transmission is based on circuit switching of TDMA timeslots, but in UMTS transport takes place via the network ATM cell transmission. 2. 2. In 2G, transcoding is defined as a BSS functionality, whereas in UMTS it is part of the CS-domain of the CN.
  20. 20. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 20 of 40 1.4 UMTS R99 Architecture (8/9) As a consequence, with Rel. 99 a new network element is introduced in the CS-domain: the Multimedia Gateway. With the help of the MGW an existing MSC can be reused. This combination of MSC and new MGW is also called a 3G-MSC. The main function of the MGW is to provide UTRAN interworking functions for CS services towards MSC. At the user plane the MGW takes care of both UMTS transcoding functionality and the ATM/TDM conversion.
  21. 21. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 21 of 40 1.4 UMTS R99 Architecture (9/9) On the control plane the MGW is responsible for interface signalling conversion between the different protocols, BSSAP protocol stack defined on A-interface and RANAP signalling stack on the Iu-CS interface. In the same manner, in the PS domain a new ATM-based 3G SGSN takes responsible for the 3G packet oriented traffic on the Iu-PS interface, while timeslot based 2G SGSN is still in use.
  22. 22. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 22 of 40 1.5 UMTS R4 Architecture (1/2) The purpose of Rel. 4 is to offer higher flexibility and better efficiency of transport resources in the core network. Thus the user plane and the control plane are strictly separated. As a consequence the network elements in the circuit switched domain 3G MSC, VLR and GMSC are replaced by MSC Server (MSS) that either provides IP or ATM backbone connectivity.
  23. 23. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 23 of 40 1.5 UMTS R4 Architecture (2/2) The MSC user plane switching functions are brought to the Multimedia Gateway for MSS which is responsible for bearer control. Thus, calls can be switched at MGW sites without being routed to the MSC server site. The MSC Server handles all control plane functions for CS-call control and mobility control parts from MSC and VLR. The Release 4 architecture allows a centralization of call control functions to relatively few MSC servers. In the interest of convergence with the packet switched domain the telephony core is based on an ATM and IP backbone.
  24. 24. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 24 of 40 1.6 UMTS R5 Architecture (1/4) The major changes in the Rel.5 network will be the IP Multimedia Subsystem enabling simpler service integration. UTRAN will be improved by the High Speed Uplink & Downlink Packet Access (HSUPA / HSDPA) for enhanced uplink data rates at a max. of 1.8Mbps and downlink data rates at a max. of 10Mbps. 2G BSC and 3G RNC will be directly connected by a new Iur-g Interface. The whole network will become an All-IP network, meaning IP transport in the core network as well as in the UTRAN, offering End-to-end IP services.
  25. 25. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 25 of 40 1.6 UMTS R5 Architecture (2/4) By introducing the IP Multimedia Subsystem (IMS) network operators can offer an universal all-IP backbone network that is able to support any kind of wireless and wire-line access networks. Thus, network operators can offer seamless services, operate all network subsystems more easily and benefit from an utmost flexibility in services creation and network extension.
  26. 26. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 26 of 40 1.6 UMTS R5 Architecture (3/4) IMS introduces new categories of server-based network components with dedicated functionality:  Session Management and routing servers, e.g., the Call Session Control Function  Data bases, e.g., the Home Subscriber Server which is an evolved HLR  Interworking Elements, e.g., Media Gateways  Support Entities  Charging Entities and  Application Servers We'll present all entities in detail later.
  27. 27. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 27 of 40 1.6 UMTS R5 Architecture (4/4) The latter requires an enhancement of our plane model that we discussed earlier. An Application Plane is added for bearer-independent content presentation and handling.
  28. 28. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 28 of 40 1.7 UMTS R6 Aspects During its ongoing specification work 3GPP has shifted some original UMTS R5 features to a later introduction date. R6 main features related to IMS will include:  Full interworking with circuit-switched networks, WLANs and other IP networks  Multiple registration  Emergency sessions  Usage of Public Key Infrastructure  Presence services, group management, conferencing
  29. 29. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 29 of 40 1.8 UMTS R7 Aspects Furthermore, 3GPP continues with R7 specification work including a set of IMS features like  Multiple Input Multiple Output antennas (MIMO)  Improvements to the Radio Interface, i.e., UMTS at 900 / 1,700 / 2,600MHz  PS domain and IMS impacts for the support of IMS Emergency calls  Location Services enhancements  Advanced Global Navigation Satellite System (A-GNSS) concept  System enhancements for fixed broadband access to IMS  WLAN 3GPP IP Access  Voice over IMS bearer related enhancements 3GPP work items are always subject to modification, this list provides the status as of April 2006.
  30. 30. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 30 of 40 1.9 NGN Aspects Earlier, we said that IMS is access network-independent, although 3GPP has focussed on making sure that the radio access networks are ready for IMS services. IMS services from fixed broadband networks such as Asymmetric Digital Subscriber Lines (ADSL) are also referred to as Next Generation Networks (NGN). We will look at VoIP-based NGN aspects in a later module.
  31. 31. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 31 of 40 1.10 Involved Standardization Bodies (1/10) As we have seen already, IMS was developed by 3GPP from UMTS network and service concepts. To achieve better global interoperability other major standardization bodies contribute to the IMS specifications. We will now have a look at some of them.
  32. 32. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 32 of 40 1.10 Involved Standardization Bodies (2/10) The Internet Engineering Task Force (IETF) is a standardization body that assumes the task of developing and evolving the Internet and its architecture, as well as ensuring its smooth and secure operation. The IETF is made up of network designers, academics, engineers and researchers from many companies. IETF participation does not require membership and is open to any individuals who share the same interests.
  33. 33. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 33 of 40 1.10 Involved Standardization Bodies (3/10) 3GPP and IETF work closely together. 3GPP adopts protocols developed at the IETF as needed (e.g. SIP, SDP, RTP, DIAMETER). 3GPP generates requirements for a specific problem and then contacts the IETF for a possible solution to its requirements. The IETF evaluates the 3GPP requirements and provides 3GPP with a protocol that satisfies those requirements.
  34. 34. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 34 of 40 1.10 Involved Standardization Bodies (4/10) In June 2002 the mobile industry set up a new, global organization called the Open Mobile Alliance (OMA). OMA has taken its place as the leading standardization organization for doing mobile specification work. OMA’s role is to specify different service enablers, such as digital rights management or push to talk over cellular service (PoC).
  35. 35. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 35 of 40 1.10 Involved Standardization Bodies (5/10) OMA has recognized that it is not beneficial for each service enabler to have its own mechanism for security, quality of service, charging, session management, etc. On the contrary, service enablers should be able to use an infrastructure like the IMS that provides these basic capabilities in a very efficient way.
  36. 36. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 36 of 40 1.10 Involved Standardization Bodies (6/10) Therefore, OMA and 3GPP will increase their cooperation in the future. OMA might gradually take overall responsibility for the invention and design of applications and services, while 3GPP continues to develop the core IMS.
  37. 37. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 37 of 40 1.10 Involved Standardization Bodies (7/10) The Third Generation Partnership Project 2 (3GPP2) is a collaborative project for developing 3G systems for the ANSI (American National Standards Institute) community. Like its sister project 3GPP, 3GPP2 cooperates with several important organization like ARIB (Association of Radio Industries and Businesses), CCSA (China Communications Standards Association), TIA (Telecommunications Industry Association) and market representation partners, e.g., CDMA Development Group and the IPv6 Forum.
  38. 38. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 38 of 40 1.10 Involved Standardization Bodies (8/10) 3GPP2’s role in IMS standardization lies in specifying IMS as part of the Multimedia Domain. Multimedia Domain and the CDMA2000 Access Network together form 3GPP2’s 3G All-IP Core Network. In turn, 3GPP contributes with its Release 5 IMS specifications.
  39. 39. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 39 of 40 1.10 Involved Standardization Bodies (9/10) However, there are differences between 3GPP IMS and 3GPP2 IMS Release 5 solutions due to different underlying packet and radio technology.
  40. 40. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 40 of 40 1.10 Involved Standardization Bodies (10/10) Additionally, both IMS approaches have defined further additions or limitations. The most significant differences are  IP version 4 is also supported in 3GPP2 IMS, whereas 3GPP IMS exclusively supports IP version 6  3GPP2 specifics no default codec.  There are differences in the charging solutions.  3GPP2 does not support a Universal Integrated Circuit Card which could contain IMS access parameters.  3GPP2 does not support Customized Applications for Mobile network Enhanced Logic-related functions.
  41. 41. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 1 of 29 2 Entities & Functions
  42. 42. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 2 of 29 2.1 Overview ........................................................................................3 2.2 Proxy-Call Session Control Function (P-CSCF) (1/2) .....................4 2.2 Proxy-Call Session Control Function (P-CSCF) (2/2) .....................5 2.3 Serving-Call Session Control Function (S-CSCF)...........................6 2.4 Interrogation-Call Session Control Function (I-CSCF) ....................7 2.5 Home Subscriber Server (HSS) (1/6) .............................................8 2.5 Home Subscriber Server (HSS) (2/6) .............................................9 2.5 Home Subscriber Server (HSS) (3/6) ...........................................10 2.5 Home Subscriber Server (HSS) (4/6) ...........................................11 2.5 Home Subscriber Server (HSS) (5/6) ...........................................12 2.5 Home Subscriber Server (HSS) (6/6) ...........................................13 2.6 Policy Decision Function (PDF) (1/2)............................................14 2.6 Policy Decision Function (PDF) (2/2)............................................15 2.7 Multimedia Resource Function Controller (MRFC) .......................16 2.8 Multimedia Resource Function Processor (MRFP).......................17 2.9 Media Gateway Control Function (MGCF)....................................18 2.10 Signaling Gateway (SGW)..........................................................19 2.11 Breakout Gateway Control Function (BGCF)..............................20 2.12 IMS Multimedia Gateway Function (IMS-MGW) .........................21 2.13 Security Gateway (SEG) ............................................................22 2.14 SGSN and GGSN.......................................................................23 2.15 Application Servers (1/3) ............................................................24 2.15 Application Servers (2/3) ............................................................25 2.15 Application Servers (3/3) ............................................................26 2.16 Summary (1/3)............................................................................27 2.16 Summary (2/3)............................................................................28 2.16 Summary (3/3)............................................................................29
  43. 43. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 3 of 29 2.1 Overview In the previous chapter, we presented the 6 main categories of server-based IMS network entities including:  Session Management and routing servers, e.g., a Call Session Control Function  Data bases, e.g., the Home Subscriber Server which is an evolved HLR  Interworking Elements, e.g., Media Gateways  Support Entities  Charging Entities and  Application Servers We will now have a closer look at each entity and its key functionality.
  44. 44. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 4 of 29 2.2 Proxy-Call Session Control Function (P-CSCF) (1/2) The Proxy Call Session Control Function (P-CSCF) is the first contact point for users within the IMS. All SIP signaling traffic from or to the UE go via the P-CSCF. It validates the request, forwards it to selected destinations and processes and forwards the response. An operators network can contain one or many P-CSCFs.
  45. 45. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 5 of 29 2.2 Proxy-Call Session Control Function (P-CSCF) (2/2) Furthermore, the Proxy Call Session Control Function (P-CSCF) provides IPSec or ESP (Encapsulating Security Payload) for SIP signaling and interacts with the PDF (Policy Decision Function) for media policing purposes. Finally, it contributes to the charging process by sending accounting-related info to the CCF (Charging Collection Function).
  46. 46. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 6 of 29 2.3 Serving-Call Session Control Function (S-CSCF) The Serving CSCF (S-CSCF) is the brain of the IMS. An operator’s network can include multiple S-CSCFs with different functionalities. Key functions include:  Handling registration requests and user de-registration when needed  Mutual authentication between the user and the network  Download of user information and service-related data from the HSS  Routing of mobile-terminating traffic to the P-CSCF and mobile- originated traffic to the I-CSCF, the Breakout Gateway Control Function (BGCF) or the application server (AS).  Translation of standard telephone numbers according to ITU‘s E.164 numbering scheme into a SIP universal resource identifier (URI). A DNS (domain name system) translation mechanism is used for this purpose, also referred to as ENUM service.  Media policing, a process that checks the content of the user payload to find out whether it contains media types or codecs, which are not allowed for a user  Accounting-related information on the Charging Collection Function (CCF) for offline charging purposes and to the Online Charging System (OCS) for online charging purposes. To summarize here are all S-CSCF functions at a glance. Just click on the text to see the film again.
  47. 47. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 7 of 29 2.4 Interrogation-Call Session Control Function (I-CSCF) The Interrogating-CSCF (I-CSCF) is the contact point within an operator’s network for all connections made to a subscriber of that particular network operator. The functions performed by the I-CSCF are:  Contact with the HSS to obtain the name of the S-CSCF that is serving a user, and S- CSCF assignment  Forwarding of SIP requests or responses to the S-CSCF  Provisioning of accounting-related information to the CCF  Provisioning of a hiding functionality. An optional integrated Topology Hiding Inter- network Gateway (THIG) can be used to hide the configuration, capacity and topology of the network from outside an operator’s network. For scalability and redundancy reasons an operator’s network may contain multiple I-CSCFs.
  48. 48. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 8 of 29 2.5 Home Subscriber Server (HSS) (1/6) The Home Subscriber Server (HSS) is the main data storage for all IMS subscriber and service-related data. The data stored in the HSS includes user identities, registration information, access parameters and service triggering information.
  49. 49. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 9 of 29 2.5 Home Subscriber Server (HSS) (2/6) User identities consist of two types: private and public user identities. The private user identity is a user ID that is assigned by the home network operator and is used for purposes like registration and authorization. It can be compared to the International Mobile Subscriber Identity (IMSI) in GSM networks. The public user identity is the ID that other users can use for requesting communication with the end user. It serves a similar purpose as the TMSI, Temporary Mobile Subscriber Identity, in GSM.
  50. 50. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 10 of 29 2.5 Home Subscriber Server (HSS) (3/6) A special mechanism called the Subscription Locator Function (SLF) is implemented within I- CSCF, S-CSCF and Application Servers. When separately addressable HSSs have been installed within the network, it is used as a resolution mechanism to find the proper address of the HSS that holds the subscriber data for a given user identity.
  51. 51. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 11 of 29 2.5 Home Subscriber Server (HSS) (4/6) IMS access parameters are used to set up sessions and include parameters like user authentication, roaming authorization and allocated S-CSCF names. Service-triggering information enables SIP service execution. The HSS also provides user- specific requirements for S-CSCF capabilities. This information is used by the I-CSCF to select the most suitable S-CSCF for a user.
  52. 52. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 12 of 29 2.5 Home Subscriber Server (HSS) (5/6) There can be more than one HSS in a home network depending on the number of mobile subscribers, the equipment capacity and the organization of the network. Communication between different HSS functions is not standardized.
  53. 53. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 13 of 29 2.5 Home Subscriber Server (HSS) (6/6) There can be more than one HSS in a home network depending on the number of mobile subscribers, the equipment capacity and the organization of the network. Communication between different HSS functions is not standardized.
  54. 54. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 14 of 29 2.6 Policy Decision Function (PDF) (1/2) The Policy Decision Function (PDF) makes policy decisions based on session and media- related information obtained from the P-CSCF. In Release 5 it is an integral part of the P- CSCF, it acts as a policy decision point for Service Based Local Policy control.
  55. 55. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 15 of 29 2.6 Policy Decision Function (PDF) (2/2) Service Based Local Policy (SBLP) provides parameters for:  Session identification, e.g., IP addresses, port numbers, bandwidths, etc..  Session authorization to PDF and GGSN depending on the requested bearer, e.g., Packet Data Protocol or PDP context  Session maintenance (PDP context modification or re-establishment)  Session charging by passing an IMS-charging identifier to the GGSN and a GPRS- charging identifier to the P-CSCF
  56. 56. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 16 of 29 2.7 Multimedia Resource Function Controller (MRFC) The Multimedia Resource Function Controller (MRFC) is needed to support bearer-related services, such as conferencing, announcements to a user or bearer transcoding. The MRFC interprets SIP signaling received via S-CSCF and uses the Media Gateway Control Protocol (MEGACO) or H.248 instructions to control the Multimedia Resource Function Processor (MRFP). For charging purchasing, the MRFC is able to send accounting information to the CCF and OCS.
  57. 57. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 17 of 29 2.8 Multimedia Resource Function Processor (MRFP) The Multimedia Resource Function Processor (MRFP) allocates user-plane resources as instructed by the MRFC.
  58. 58. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 18 of 29 2.9 Media Gateway Control Function (MGCF) The Media Gateway Control Function (MGCF) connects IMS communication to legacy CS users. All incoming CS call control signaling, i.e., ISDN User Part (ISUP) or the Bearer Independent Call Control (BICC) messages are converted into SIP and vice versa.
  59. 59. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 19 of 29 2.10 Signaling Gateway (SGW) For the lower signaling layers, a Signaling Gateway (SGW) converts standard SS7 MTP or IP-based SCTP (Stream Contol Transmission Protocol) messages into the TCP or UDP format and forwards them to the MGCF.
  60. 60. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 20 of 29 2.11 Breakout Gateway Control Function (BGCF) The Breakout Gateway Control Function (BGCF) determines where a signaling breakout to the CS domain occurs. If the breakout occurs in the same network, the BGCF selects an MGCF in the same network to convert SIP signaling into ISUP/BICC signaling to the CS domain. If the breakout occurs in another network, the BGCF selects another BGCF in a different network. Then the BGCF selects its own MGCF to convert SIP signaling into ISUP/BICC signaling to the CS domain.
  61. 61. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 21 of 29 2.12 IMS Multimedia Gateway Function (IMS-MGW) The MGCF also controls an IMS user plane entity for the breakout to legacy CS networks: It's called the IMS Multimedia Gateway Function (IMS- MGW). It terminates the bearer channels from the CS networks and media streams from the backbone network and converts them accordingly. This includes transcoding and signal processing for the user plane when needed. Furthermore, the IMS-MGW provides tones and announcements to CS users. Obviously the MGCF must be able to report account information to the CCF.
  62. 62. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 22 of 29 2.13 Security Gateway (SEG) To protect signaling traffic between different security domains, traffic will pass through a security gateway (SEG). In many cases, a security domain will be identical to a network operated by one network operator. According to the operator’s security policy the SEG may be defined for interaction towards all reachable security domain destinations or it can be defined for only a subset of all those destinations.
  63. 63. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 23 of 29 2.14 SGSN and GGSN Finally, we should not forget the standard GPRS network elements like SGSN and GGSN. In IMS they used in the same way for UE mobility management, session management and interconnection to other IP networks.
  64. 64. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 24 of 29 2.15 Application Servers (1/3) So far we have gained a broad overview of IMS entities related to user plane and control plane functions. As we learned in the first module, IMS provides a third layer that caters for application that provides value-added multimedia services in the IMS.
  65. 65. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 25 of 29 2.15 Application Servers (2/3) The Application Plane does not need to belong to the IMS operator. Hence, independent value-added service providers may contribute to the mobile business with multimedia presence, conference or messaging services. SIP Application Servers host these services based on three basic functionalities:  Processing and impact an incoming SIP session received from the IMS  Originating SIP requests  Sending accounting information to the CCF and the OCS.
  66. 66. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 26 of 29 2.15 Application Servers (3/3) Application can be provided in three different ways:  as core SIP services that can directly interact with the S-CSCF  using the Open Service Architecture philosophy and an appropriate Application Programming Interface for IMS interworking or  using the legacy Intelligent Network approach with CAMEL services developed with a CAMEL Service Environment. To connect it to the SIP-based IMS a special IMS Service Switching Function is implemented using the CAMEL Application Part for internal signaling.
  67. 67. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 27 of 29 2.16 Summary (1/3) As we come to the end of this module let us now try to put all entities together for a full overview of the IMS architecture. In the User Plane we have 3 elements that provides connectivity to the different wireless and wire-line access networks: SGSN, GGSN and IMS- MGW. The MRFP allocates the necessary IMS-internal resources.
  68. 68. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 28 of 29 2.16 Summary (2/3) In the Control Plane we have a number of signaling-relevant entities like HSS, the 3 CSCFs with PDF and THIG, MRFC, MGFC and SGW, BGCF and SEG. The Charging Collection Function (CCF) and the Online Charging System (OCS) complete the list of network elements.
  69. 69. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 29 of 29 2.16 Summary (3/3) Finally, the Application Plane hosts Application Servers for value-added multimedia services. Remember, all these components are not necessarily stand-alone entities. Moreover, these elements are functions or processes that can be implemented on common server platforms.
  70. 70. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 1 of 21 3 NGN Aspects
  71. 71. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 2 of 21 3.1 IMS and NGN: General Aspects (1/3).............................................3 3.1 IMS and NGN: General Aspects (2/3).............................................4 3.1 IMS and NGN: General Aspects (3/3).............................................5 3.2 Layers, Sub-systems and Functions...............................................6 3.3 Transport Layer..............................................................................7 3.4 Service Layer (1/5) .........................................................................8 3.4 Service Layer (2/5) .........................................................................9 3.4 Service Layer (3/5).......................................................................10 3.4 Service Layer (4/5) .......................................................................11 3.4 Service Layer (5/5) .......................................................................12 3.5 IMS and NGN: Network Entities (1/9) ...........................................13 3.5 IMS and NGN: Network Entities (2/9) ...........................................14 3.5 IMS and NGN: Network Entities (3/9) ...........................................15 3.5 IMS and NGN: Network Entities (4/9) ...........................................16 3.5 IMS and NGN: Network Entities (5/9) ...........................................17 3.5 IMS and NGN: Network Entities (6/9) ...........................................18 3.5 IMS and NGN: Network Entities (7/9) ...........................................19 3.5 IMS and NGN: Network Entities (8/9) ...........................................20 3.5 IMS and NGN: Network Entities (9/9) ...........................................21
  72. 72. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 3 of 21 3.1 IMS and NGN: General Aspects (1/3) Earlier, we said that IMS is access network-independent, although 3GPP and 3GPP2 have focussed on their radio access networks being ready to accept IMS services. Fixed broadband networks using Digital Subscriber Lines (xDSL) will also offer IMS services. They will be referred to as Next Generation Networks (NGN). We will now talk about VoIP-based NGN aspects with the focus on architectural details.
  73. 73. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 4 of 21 3.1 IMS and NGN: General Aspects (2/3) NGN architecture has a different layered approach we met with IMS. In IMS we considered UE interaction with 3 planes representing application, control and user aspects.
  74. 74. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 5 of 21 3.1 IMS and NGN: General Aspects (3/3) An NGN is divided into two main layers, namely the service layer and the transport layer. Each layer is composed of a number of subsystems and a number of common functions.
  75. 75. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 6 of 21 3.2 Layers, Sub-systems and Functions The NGN architecture allows for any distribution of the elements and the subsystems in different networks. As such, it provides for an access network, a visited network, and a home network, each one providing a different type of service. The transport layer provides the layer 2 connectivity, IP connectivity, and transport control.
  76. 76. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 7 of 21 3.3 Transport Layer The transport layer is divided further into the Network Attachment Subsystem (NASS), the Resource and Administration Control Subsystem (RACS), and a number of common transfer functions. NASS is responsible for supplying the terminal with configuration parameters, e.g. IP addresses, authentication at the IP- layer, authorization of network access and access configuration based on users’ profiles. It's also the location manager at the IP layer. RACS provides resource management, gate control functionality, policy enforcement, and admission control based on user profiles. Common transfer functions incorporate a number of functional elements, e.g., media gateways and border gateways. They are used and controlled by the different NASS or RACS entities.
  77. 77. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 8 of 21 3.4 Service Layer (1/5) The service layer contains a number of subsystems that provide the platform for enabling services to the user according to the concepts of PSTN/ISDN emulation and PSTN/ISDN simulation. Let us have a look at these two service concepts.
  78. 78. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 9 of 21 3.4 Service Layer (2/5) The term PSTN emulation is used to refer to an NGN that implements the same services as todays PSTN and ISDN networks. Users will have exactly the same services as in legacy networks, and will keep using their existing phnones. They will not be aware that an NGN is acutally delivering the service.
  79. 79. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 10 of 21 3.4 Service Layer (3/5) The term PSTN/ISDN simulation is used to refer to an NGN that provides services compatible with the PSTN/ISDN, but which need not be necessarily exactly the same. The concept indicates a replacement of the PSTN/ISDN by NGN, and of legacy terminals by better performing user equipment.
  80. 80. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 11 of 21 3.4 Service Layer (4/5) The PSTN/ISDN Emulation Subsystem (PES) implements the PSTN/ISDN emulation concept. PES is typically implemented as a monolithic softswitch. The core IMS that we discussed earlier implements the PSTN/ISDN simulation concept. Besides legacy speech services the core IMS enables SIP-based multimedia services to NGN terminals, e.g., presence, instant messaging, etc.
  81. 81. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 12 of 21 3.4 Service Layer (5/5) The service layer in NGN also provides for the existence of applications, typically implemented in Application Server Functions (ASF). A number of common functions provide functional services to several subsystems. This is the case of the User Profile Server Function (UPSF), a database that contains user- specific information, like the HSS for the IMS.
  82. 82. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 13 of 21 3.5 IMS and NGN: Network Entities (1/9) NGN is largely based on the 3GPP IMS specifications, but it considers only SIP network elements such as CSCFs, BGCF, MGCF, and MRFC. Applicatin Servers, MRFP, MGW, user databases, etc., are considered part of the common functions or of the transport layer subsystems.
  83. 83. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 14 of 21 3.5 IMS and NGN: Network Entities (2/9) The P-CSCF contains an IMS Application Level Gateway (IMS-ALG) that provides control for the network address and port translator functions, these are located in the Transition Gateway (TrGW). IPv4-IPv6 translation is sometimes necessary, e.g., when the local customer network implements a private IPv4 addressing scheme.
  84. 84. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 15 of 21 3.5 IMS and NGN: Network Entities (3/9) The I-CSCF, S-CSCF, BGFC, and MRFC are the same as in IMS. The MGCF in NGN keeps the same functionality as in the IMS, but it has additional functions that provide appopriate interworking with circuit-switched networks beyond the basic call.
  85. 85. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 16 of 21 3.5 IMS and NGN: Network Entities (4/9) Common functions represent NGN architecture elements, like the User Profile Server Function (UPSF), the Subscription Locator Functino (SLF), the Interconnection Border Control Function (IBCF), the Interworking Function (IWF), and Application Server Functions (ASFs).
  86. 86. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 17 of 21 3.5 IMS and NGN: Network Entities (5/9) The UPSF in NGN is similar to the HSS in IMS. Since HSS is an evolution of the GSM HLR it would also include HLR/ AUC pair to provide mobility management. As this is not required in NGN, the UPSF is limited to the IMS-specific parts of the HSS.
  87. 87. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 18 of 21 3.5 IMS and NGN: Network Entities (6/9) When several UPSFs are used in NGN a Subscription Locator Function (SLF) is available as in IMS. It identifies the UPSF to be addressed by the Service Layer entities. The Interconnection Border Control Function (IBCF) is a new functional entity that acts as a separator between two different IMS domains. It performs IP version interworking and inserts an Interworking Function (IWF) in the communication path, if needed. In addition the IBCF may hide some SIP headers that an operator may consider dangerours to expose externally.
  88. 88. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 19 of 21 3.5 IMS and NGN: Network Entities (7/9) The Interworking Function (IWF) provides interworking between SIP and other protocols, e.g., H.323. Application Server Functions (ASFs) execute IMS services, such as presence, instant messaging, video conferencing or document collaboration.
  89. 89. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 20 of 21 3.5 IMS and NGN: Network Entities (8/9) NGN identitfies two types of ASFs. ASF Type 1 may interact with some Transport Layer functionality when providing a service to the user. ASF Type 2 merely relies on the call control protocol to provide the service and is equivalentfunctionally to the AS in the IMS.
  90. 90. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 21 of 21 3.5 IMS and NGN: Network Entities (9/9) Finally, a Charging Collection Function (CCF) has the same function in NGN, as in IMS.
  91. 91. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 1 of 22 4 Reference Points
  92. 92. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 2 of 22 4.1 IMS Reference Points: Overview....................................................3 4.2 IMS Reference Points: Gm.............................................................4 4.3 IMS Reference Points: Mw .............................................................5 4.4 IMS Reference Points: Cx ..............................................................6 4.5 IMS Reference Points: Dx ..............................................................7 4.6 IMS Reference Points: ISC.............................................................8 4.7 IMS Reference Points: Sh ..............................................................9 4.8 IMS Reference Points: Si .............................................................10 4.9 IMS Reference Points: Dh ............................................................11 4.10 IMS Reference Points: Ut ...........................................................12 4.11 IMS Reference Points: Mm.........................................................13 4.12 IMS Reference Points: Mg..........................................................14 4.13 IMS Reference Points: Mi...........................................................15 4.14 IMS Reference Points: Mj...........................................................16 4.15 IMS Reference Points: Mk..........................................................17 4.16 IMS Reference Points: Mr ..........................................................18 4.17 IMS Reference Points: Mp..........................................................19 4.18 IMS Reference Points: Mn..........................................................20 4.19 IMS Reference Points: Go..........................................................21 4.20 IMS Reference Points: Gq..........................................................22
  93. 93. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 3 of 22 4.1 IMS Reference Points: Overview Let us now have a look at the interfaces of the various IMS elements to better understand their functionality. Because the network structure is so complex, we will set some limitations for clarity:  We will describe interfaces to Charging Functions and Security Gateway separately  We will not differentiate between different types of Application Servers nor show the interfaces between Application and User Plane.
  94. 94. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 4 of 22 4.2 IMS Reference Points: Gm The Gm interface connects the UE to the IMS. Please note: Whatever RAN is used, Gm traverses it transparently! Gm carries all SIP signaling messages between the UE and the IMS counterpart which is the P-CSCF. 3 main Gm procedures can be identified:  Registration / De-registration  Session control and  Transactions, i.e., message exchange without previous dialog definition between the entities.
  95. 95. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 5 of 22 4.3 IMS Reference Points: Mw The Mw interface then takes over the responsibility for the procedures. P-CSCF, I-CSCF and S-CSCF process Registration / De-registration, session control and transaction messages respectively.
  96. 96. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 6 of 22 4.4 IMS Reference Points: Cx HSS permanently stores subscriber and service data that is needed by the I-CSCF and the S-CSCF when the user registers or receives sessions. Therefore, there must be a reference point (Cx) between the HSS and these two CSCFs. In contrast to the previously mentioned interfaces Cx uses the Diameter protocol. This is an evolution of the familiar RADIUS protocol (Remote Authentication Dial-In User Service) and will be described later. Cx procedures can be divided into three main categories:
  97. 97. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 7 of 22 4.5 IMS Reference Points: Dx When multiple and separately addressable HSSs have been deployed in a network, I-CSCF or S-CSCF will need help to know which HSS to contact. This is the task of the SLF, a fuction integrated to either I-CSCF or S-CSCF. A dedicated Dx interface is defined and is always used in conjunction with the Cx reference point. It also works using the DIAMETER protocol. As the SLF knows which HSS will host the requested subscriber data it informs the requesting network element to re-direct the Cx messages to the appropriate HSS.
  98. 98. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 8 of 22 4.6 IMS Reference Points: ISC Let us now involve the Application Plane with an AS that hosts and executes services, e.g., presence, messaging and session forwarding. An interface is needed to send and receive SIP messages between the I-CSCF, S-CSCF and the AS. This reference point is called the IMS Service Control (ISC) interface. The selected protocol is SIP. ISC procedures can be triggered by either the CSCFs or the AS.
  99. 99. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 9 of 22 4.7 IMS Reference Points: Sh An AS may need subscriber data or need to know which S-CSCF to address. This information is stored in the HSS. Hence, a reference point between the HSS and the AS must be established: it's the Sh interface, again using the DIAMETER protocol. Sh procedures are divided into two main groups:  data handling and  subscription notification The HSS maintains a list of ASs per user which are allowed to obtain or store data.
  100. 100. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 10 of 22 4.8 IMS Reference Points: Si When the AS provides the service using CAMEL it is the AS-inherent IM-SSF that communicates with the HSS. Instead of Sh interface, the Si reference point is used to transport CAMEL subscription information including triggers from the HSS to the IM-SSF. The protocol used is SS7 Mobile Application Part (MAP).
  101. 101. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 11 of 22 4.9 IMS Reference Points: Dh Again we might encounter the problem of multiple HSSs in the network where the AS cannot know which HSS it needs to contact. Once more the SLF will help out being contacted first via the Dh reference point. As Cx and Dx reference points are always used in conjunction Dh and Sh are always used together by DIAMETER messages. To get an HSS address, the AS sends the Sh request which is aim for the HSS to the SLF. On receipt of the HSS address from the SLF, the AS sends the Sh request to the HSS.
  102. 102. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 12 of 22 4.10 IMS Reference Points: Ut The Ut reference point links the UE and the AS. It enables users to surely manage and configure their network services-related information hosted on an AS. HTTP is used for Ut message transfer, hence any protocol chosen for an application that makes use of Ut must be based on HTTP.
  103. 103. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 13 of 22 4.11 IMS Reference Points: Mm For communicating with other IP multimedia networks, an interface is needed between both networks. The Mm reference point allows I-CSCF to receive a session request from another SIP server or terminal. Similarly, the S-CSCF uses the Mm reference point to forward IMS UE-originated requests to other multimedia networks.
  104. 104. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 14 of 22 4.12 IMS Reference Points: Mg The Mg reference point links the circuit-switched edge function, i.e., the Media Gateway Control Function, to the I-CSCF. This interface allows MGCF to forward incoming session signaling from any CS domain to the I-CSCF. MGCF is responsible for converting incoming SS7 ISUP or Bearer Independent Call Control (BICC) signaling to SIP. This is the protocol used on Mg.
  105. 105. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 15 of 22 4.13 IMS Reference Points: Mi When the S-CSCF discovers that a session needs to be routed to a CS domain it uses the Mi interface to forward the session to a Breakout Gateway Control Function, this in turn knows how to route the signaling to the destination circuit-switched network.
  106. 106. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 16 of 22 4.14 IMS Reference Points: Mj In order to address the destination CS network BGCF forwards the session to MGCF via the Mj reference point using SIP. Again MGCF converts SIP into SS7 ISUP or BICC.
  107. 107. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 17 of 22 4.15 IMS Reference Points: Mk If the destination CS network has to be reached via another IMS the Mk interface between the two BGCFs involved takes care of signaling message transfer using SIP.
  108. 108. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 18 of 22 4.16 IMS Reference Points: Mr When the S-CSCF needs to activate bearer-related services, e.g., an announcement, it activates the Media Resource Function Controller (MRFC) via the SIP Mr interface.
  109. 109. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 19 of 22 4.17 IMS Reference Points: Mp When the MRFC needs to control media streams it addresses the Media Resource Function Processor (MRFP) within the User Plane using ITU’s H.248 protocol. This might be the case if connections for conference media must be established or released.
  110. 110. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 20 of 22 4.18 IMS Reference Points: Mn H.248 is also used for signaling between MRFC and the IMS Media Gateway for general user plane resource management.
  111. 111. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 21 of 22 4.19 IMS Reference Points: Go In order to provide the GGSN with the allowed service characteristics for a particular user session, the Policy Decision Function submits the needed QoS level information via the Go reference point. Charging correlation information can also be added. The protocol used here is the Common Open Policy Service (COPS) protocol.
  112. 112. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 22 of 22 4.20 IMS Reference Points: Gq PDF information about session parameters is also needed in the network element that acts as the primary contact point of the UE. That is the P-CSCF. The Gq reference point caters for this task, using the Diameter protocol.
  113. 113. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 1 of 20 5 Signaling Protocols
  114. 114. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 2 of 20 5.1 Overview ........................................................................................3 5.2 Session Control Protocols ..............................................................4 5.3 Session Initiation Protocol (SIP) .....................................................5 5.4 Diameter.........................................................................................6 5.5 Common Open Policy Service (COPS) ..........................................7 5.6 Media Gateway Control Protocol (MEGACO).................................8 5.7 Real Time Transport Protocol (RTP) ..............................................9 5.8 Real Time Transport Control Protocol (RTCP) .............................10 5.9 Resource Reservation Protocol (RSVP).......................................11 5.10 Session Description Protocol (SDP) ...........................................12 5.11 Domain Name Service (DNS).....................................................13 5.12 Transport Layer Security (TLS) ..................................................14 5.13 Internet Protocol Security (IPSec) ..............................................15 5.14 Dynamic Host Configuration Protocol for IPv6 (DHCPv6)...........16 5.15 XML Configuration Access Protocol (XCAP) ..............................17 5.16 Conference Policy Control Protocol (CPCP)...............................18 5.17 Summary (1/2)............................................................................19 5.17 Summary (2/2)............................................................................20
  115. 115. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 3 of 20 5.1 Overview While discussing IMS reference points we mentioned some of the protocols used. Let us now get a more complete overview of those protocols used within and between the IMS planes.
  116. 116. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 4 of 20 5.2 Session Control Protocols IMS protocols involved in session control are all based on IP. Bearer Independent Call Control, developed by ITU under the Q.1901 standard and used in UMTS Rel. 99 networks was an initial candidate for a Session Control Protocol. Another candidate was ITU's H.323 used in legacy VoIP installations. Finally, IETF’s SIP (Session Initiation Protocol) was chosen as the future-proof solution for signaling between IMS network entities.
  117. 117. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 5 of 20 5.3 Session Initiation Protocol (SIP) IP is a text-based protocol using the same end-to-end working principles as the successful Simple Mail Transfer Protocol (SMTP) and Hypertext Transfer Protocol (HTTP). It is easy to extend and debug and services can be built easily using HTTP frameworks, e.g., CGI (Common Gateway Interface) and Java applets. It is the standard protocol for signaling messages exchanged between IMS entities within the control plane and between the application, control and user planes.
  118. 118. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 6 of 20 5.4 Diameter IETF’s DIAMETER has been selected as the IMS protocol for Authentication, Authorization and Accounting. It is an evolution of the Remote Access Dial-up User Service that is used to establish a fixed network connection with a standard Internet service provider. DIAMETER uses a set of “applications” for specific interactions with other protocols, e.g., with SIP for session set-up or credit control accounting. It's used for signaling message exchange  on Cx / Dx interfaces between CSCFs and HSS /SLF  on Sh interface between HSS and ASs  on Gq interface between P-CSCF and PDF
  119. 119. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 7 of 20 5.5 Common Open Policy Service (COPS) A Common Open Policy Service protocol is used for policy administration, configuration and enforcement. For these purposes, IETF designed COPS to use a simple query / response model between clients (users) and policy servers. It is used on the Go interface between PDF and GGSN.
  120. 120. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 8 of 20 5.6 Media Gateway Control Protocol (MEGACO) H.248 or Media gateway Control protocol (MeGaCo) was developed jointly by IETF and ITU. It provides standard capabilities to control media resources in the IMS User Plane, especially on the Mp interface between the MRFC and the MRFP, and on the Mn interface between MGCF and IM-MGW.
  121. 121. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 9 of 20 5.7 Real Time Transport Protocol (RTP) RTP (Real-time Transport Protocol) is optimized for IMS media transport, i.e., audio and video signals for a proper end-to-end delivery. RTP identifies the codec used, ensures sequence numbering, time stamping and delivery monitoring. RTP does not support any Quality of Service mechanisms. It is mainly used between User Plane entities.
  122. 122. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 10 of 20 5.8 Real Time Transport Control Protocol (RTCP) This is where RTCP (Real-time Transport Control Protocol) comes in. It ensures Quality of Service monitoring and provides information about media session participants. For this purpose, RTCP packets are inserted periodically into an RTP bit stream.
  123. 123. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 11 of 20 5.9 Resource Reservation Protocol (RSVP) Only RSVP (Resource Reservation Protocol) ensures defined Quality of Service levels for media transport. It is based on COPS and used on the Go interface between PDF and GGSN.
  124. 124. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 12 of 20 5.10 Session Description Protocol (SDP) The text-based Session Description Protocol (SDP) is part of the SIP application layer used to describe multimedia sessions between two enduser entities. Reception capabilities, media formats and reception address / port are announced during the session set-up process or while the session is in progress.
  125. 125. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 13 of 20 5.11 Domain Name Service (DNS) Domain Name services (DNS) is a legacy database that stores alphanumeric addresses and their corresponding IP addresses (a.O.). It has existed for a long time in public TCP/IP networks like the Internet and is used in the same way in IMSs. Divided into n-level domains, the DNS can resolve IP addresses and even pinpoints to a dedicated device in a company’s department in a specific country.
  126. 126. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 14 of 20 5.12 Transport Layer Security (TLS) Transport Layer Security (TLS) provides confidentiality and data integrity between two end points on a reliable transport layer such as TCP. It is split into two sub-layers: TLS Record protocol uses symmetric key cryptography for ciphering and a keyed message authentication checksum. It encapsulates the second sub-layer called TLS Handshake Protocol (a.o.) that, in turn, carries the client-server authentication.
  127. 127. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 15 of 20 5.13 Internet Protocol Security (IPSec) In contrast to TLS, Internet Protocol Security provides encryption between two infrastructure entities to cypher upper layers. It is available in two versions, IPv4 and IPv6. IPSec transport mode only offers limited protection to IP headers whereas in IPSec tunnel mode the entire IP datagram is protected.
  128. 128. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 16 of 20 5.14 Dynamic Host Configuration Protocol for IPv6 (DHCPv6) Dynamic Host Configuration Protocol for IPv6 is a client-server protocol that allows for device configuration including management information. A dynamically assigned address is issued per session by a DHCP server. In addition, it allows SIP clients to identify their outbound SIP servers by a SIP server domain name list or by a list of 128 bit IPv6 addresses.
  129. 129. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 17 of 20 5.15 XML Configuration Access Protocol (XCAP) XML Configuration Access Protocol uses HTTP to upload and read information set by users to enable the network to provide services to the user, e.g., messaging, presence or push-to- talk over cellular. The XCAP server forwards the user settings to the corresponding application server. XCAP is used on Ut interface between UE and ASs and between ASs.
  130. 130. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 18 of 20 5.16 Conference Policy Control Protocol (CPCP) The Conference Policy Control Protocol can be seen as a specific XCAP type used by IMS users to manipulate rules for an IMS conference. The Conference Policy Server defines the conference lifespan, the parties which are included or excluded and the conference parties’ roles and responsibilities.
  131. 131. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 19 of 20 5.17 Summary (1/2) At the end of this description of signaling protocols’ let us now put things together in two different ways. The first way shows how the different protocols are built upon the lower layers of the IMS signaling protocol stack. The Physical Layer and Data Link Layer are provided by the wireless access media, e.g., GPRS, W-CDMA or WLAN. IPv4 or IPv6 take care of Network layer functions. DHCP, DNS, RTP and RTCP rely on the UDP for Transport Layer functions. TCP is used by SIP, TLS and HTTP and, under certain circumstances, RTP and RTCP as well. In most cases, RSVP is directly supported by the IP layer, under some circumstances it resides upon the UDP. RTP, RTCP and RSVP provide services for Media Encoding whereas SIP is used by SDP and XCAP and CPCP by HTTP capabilities.
  132. 132. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 20 of 20 5.17 Summary (2/2) The second overview relates protocol used to the IMS network entities involved. SIP is the most used protocol between the P-, S- and I-CSCFs. BGCFs use it to communicate with S- CSCF and MGCF, P-CSCF with the UE. The S-CSCF addresses AS and MRFC using SIP. DIAMETER is used between AS and HSS, P-CSCF and PDF, I-CSCF, S-CSCF and HSS. H.248 or MeGaCo is used by MRFC to control the MRFP and IMS-MGW. COPS is only found between PDF and the GGSN. Finally, HTTP, TLS, XCAP and CPCP are used between the UE and the AS.
  133. 133. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 1 of 32 6 Media Encoding and Transport
  134. 134. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 2 of 32 6.1 Overview ........................................................................................3 6.2 Adaptive Multi-Rate Codec (1/4).....................................................4 6.2 Adaptive Multi-Rate Codec (2/4).....................................................5 6.2 Adaptive Multi-Rate Codec (3/4).....................................................6 6.2 Adaptive Multi-Rate Codec (4/4).....................................................7 6.3 AMR-WB Codec (1/4).....................................................................8 6.3 AMR-WB Codec (2/4).....................................................................9 6.3 AMR-WB Codec (3/4)...................................................................10 6.3 AMR-WB Codec (4/4)...................................................................11 6.4 Video Codecs (1/2).......................................................................12 6.4 Video Codecs (2/2).......................................................................13 6.5 H.263 Codec (1/3)........................................................................14 6.5 H.263 Codec (2/3)........................................................................15 6.5 H.263 Codec (3/3)........................................................................16 6.6 H.261 Codec ................................................................................17 6.7 MPEG Codecs (1/3) .....................................................................18 6.7 MPEG Codecs (2/3) .....................................................................19 6.7 MPEG Codecs (3/3) .....................................................................20 6.8 Text Encoding (1/3) ......................................................................21 6.8 Text Encoding (2/3) ......................................................................22 6.8 Text Encoding (3/3) ......................................................................23 6.9 Save Media Transport (1/2)..........................................................24 6.9 Save Media Transport (2/2)..........................................................25 6.10 Real-Time Transport Protocol (1/4) ............................................26 6.10 Real-Time Transport Protocol (2/4) ............................................27 6.10 Real-Time Transport Protocol (3/4) ............................................28 6.10 Real-Time Transport Protocol (4/4) ............................................29 6.11 Real-Time Transport Control Protocol (1/3)................................30 6.11 Real-Time Transport Control Protocol (2/3)................................31 6.11 Real-Time Transport Control Protocol (3/3)................................32
  135. 135. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 3 of 32 6.1 Overview In this module we will have a closer look how IMS media, i.e., speech, video and text are encoded to assure a high quality when transmitted over the air interface.
  136. 136. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 4 of 32 6.2 Adaptive Multi-Rate Codec (1/4) 3GPP defined two speech codecs for IMS terminals: AMR (Adaptive Multi-Rate) speech codec is the mandatory speech codec. It co-operates with legacy GSM terminals using Enhanced Full-Rate speech codecs. Then we have the Adaptive Multi-Rate Wide-Band speech codec for those UEs supporting wide-band services.
  137. 137. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 5 of 32 6.2 Adaptive Multi-Rate Codec (2/4) AMR consists of eight different codecs, each with a different bandwidth. Their different bandwidths are 12.2, 10.2, 7.95, 7.40, 6.70, 5.90, 5.15, and 4.75 kbps. These are also referred to as AMR modes. The 12.2, 7.40, and 6.70 kbps AMR modes are also known as GSM-EFR (Enhanced Full Rate) codecs.
  138. 138. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 6 of 32 6.2 Adaptive Multi-Rate Codec (3/4) The AMR modes were designed initially to be used in GSM networks that provide a fixed rate for circuit-switched voice calls. This rate is split into channel coding and speech coding. When radio quality is poor due to high air interface interference the terminals use low- bandwidth AMR modes, e.g., 4.75 kbps well protected by a large channel coding element. When the air interface interference is low terminals use high AMR bandwidth modes with 12.2 kbps and only a few bits for channel coding.
  139. 139. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 7 of 32 6.2 Adaptive Multi-Rate Codec (4/4) The AMR codec itself is able to switch modes on a frame-by-frame basis. That is, one 20 ms speech frame can be encoded using one particular AMR mode and the next speech frame using another one.
  140. 140. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 8 of 32 6.3 AMR-WB Codec (1/4) 3G networks using WCDMA access do not use AMR modes in the same way as GSM networks. WCDMA uses fast power control and does not perform mode adaptation at the channel-encoding level. WCDMA networks use low-bandwidth modes to gain capacity when many users make voice calls at the same time.
  141. 141. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 9 of 32 6.3 AMR-WB Codec (2/4) When AMR is transported over a packet-oriented path the overhead introduced by the RTP, UDP and IP headers is fairly large. Thus, it only allows for low-bit rate speech coding. IP header compression can be used to gain some extra payload capacity to use high-bandwidth AMR modes.
  142. 142. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 10 of 32 6.3 AMR-WB Codec (3/4) The AMR-WB codec as defined in 3GPP Technical Specification 26.190 encodes voice using 16,000 samples per second, instead of 8,000 samples per second used by AMR. This higher sampling frequency allows AMR-WB to encode a wider range of frequencies. Consequently, AMR-WB encodes speech with higher quality than the codecs we described earlier.
  143. 143. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 11 of 32 6.3 AMR-WB Codec (4/4) AMR-WB consists of a family of codecs which encode audio using the following bandwidths: 23.85,23.05, 19.85, 18.25, 15.85,14.25, 12.65, 8.85, and 6.60 kbit/s. AMR-WB is the mandatory codec for 3GPP IMS terminals that provide wideband services. Thus, IMS networks provide better speech quality due to higher bit rates per speech sample for both high and low air interface interference situations.
  144. 144. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 12 of 32 6.4 Video Codecs (1/2) Video encoding relies on single image encoding. Video transmission uses a set of encoded still images taken with a short interval between them. If the subsequent presentation of the still image is fast enough the human eye perceives this succession of images as a moving image. A standard sequence used in TV systems issues 25 pictures per sec, i.e., each 40ms a new picture is presented.
  145. 145. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 13 of 32 6.4 Video Codecs (2/2) IMS video encoding formats are well defined by international standardization bodies. Examples include: ITU’s H.263 which is mandatory for IMS or H.261 an additional option. MPEG (Motion Picture Experts Group) developed several standards that can also be used for IMS services.
  146. 146. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 14 of 32 6.5 H.263 Codec (1/3) H.263 evolved out of H.261, the previous ITU standard for video compression and the MPEG-1 and MPEG-2 standards. H.263 was developed for the ITU-T H.324 multimedia framework used with low bit rate communications. It supports five formats to encode images: Sub-QCIF (Quarter Common Intermediate Format), QCIF, CIF, 4CIF and 16CIF. All of these formats encode the color of the pixels using a luminance component and two chrominance components, but support different resolutions.
  147. 147. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 15 of 32 6.5 H.263 Codec (2/3) The luminance components is the black-and-white component of the image and defines how dark or light each pixel is. The chrominance components provide the color of a pixel in relation to the colors red and blue.
  148. 148. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 16 of 32 6.5 H.263 Codec (3/3) Image resolution formats used by H.263 are related to the four defined formats and vary in resolution. Luminance resolution can vary between 128 x 96 pixels and 1408 x 1152 pixels. The chrominance resolution varies between 64 x 48 pixels and 702 x 576 pixels. Please note the 2:1 relationship between the figures. This results from the fact that the human eye is more sensitive to luminance information than to chrominance information.
  149. 149. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 17 of 32 6.6 H.261 Codec ITU-T’s H.261 standard defines the video codec used by the H.320 video-teleconferencing framework. This codec was designed to be used over ISDN lines and therefore, produces bandwidths that are multiples of 64kbps ranging from 64 to 1984 kbps. The data rate of the coding algorithm was designed to be able to operate between 40 kbps and 2 Mbps. The standard supports CIF and QCIF video frames with luminance resolutions of 352x288 and 176x144 respectively. The chrominance resolutions are a 176x144 and 88x72. It also has a backward-compatible trick for sending still picture graphics with 704x576 luminance resolution. This was added in a later revision in 1994.
  150. 150. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 18 of 32 6.7 MPEG Codecs (1/3) The MPEG video standards are used for both media storage and for videoconferencing. MPEG-1 was developed to encode audio and video at rates about 15Mbps. It also includes the popular Layer 3 (MP3) audio compression format.
  151. 151. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 19 of 32 6.7 MPEG Codecs (2/3) MPEG-2 was developed to encode audio and video at rates between 4 and 80 Mbps which is higher than MPEG-1, also providing higher quality. While the VCD (Video CD) format is based on MPEG-1, DVD (Digital Video Disk), DVB (Digital Video Broadcasting) and some digital satellite and cable television systems are all based on MPEG-2.
  152. 152. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 20 of 32 6.7 MPEG Codecs (3/3) Initially, MPEG-3 was designed for High-definition TV, but it was abandoned when it was discovered that MPEG-2 with extensions was sufficient for HDTV. MPEG-4 expands MPEG- 1 to support video / audio "objects", 3D content, low bitrate encoding and Digital Rights Management. The DivX and XviD formats are based on MPEG-4.
  153. 153. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 21 of 32 6.8 Text Encoding (1/3) Text already consists of digital information, so there is no need to perform the analog-to- digital or digital-to-analog conversions which are needed for audio and video. There are two types of textcommunication: Instant messages and real-time text.
  154. 154. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 22 of 32 6.8 Text Encoding (2/3) Instant messages convey a whole message, such as: "How are you?" That is, the sender types the message, edits it if necessary, and sends it. This way the receiver only gets the final version of the message.
  155. 155. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 23 of 32 6.8 Text Encoding (3/3) Real-time text consists of transferring keystrokes instead of text. If the sender writes something and then deletes it to write something else, the receiver will see how these changes are performed. That is, the receiver gets letters and commands (e.g., carriage returns or delete characters) one by one as they are typed. The most common format for real-time text is ITU-T’s Recommendation T.140.
  156. 156. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 24 of 32 6.9 Save Media Transport (1/2) For a safe media transport, the critical question is whether or not the payload data can tolerate a certain amount of packet loss or not. Audio and video messages will loose quality dramatically if packets are lost whereas web browsing or instant messaging are more robust applications.
  157. 157. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 25 of 32 6.9 Save Media Transport (2/2) Hence, the type of media should choose within IMS depending on the question whether it is going to be transported on reliable connections like TCP or unreliable ones like UDP. As UDP is widely used to transport all kinds of media in IP networks some additional measures must be taken to ensure a proper media transport. The IETF has specified the Real-time Transport Protocol and its sister protocol, the Real-time Transport Control Protocol to reside upon UDP for this purpose.
  158. 158. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 26 of 32 6.10 Real-Time Transport Protocol (1/4) Applications using RTP are less sensitive to packet loss, but typically very sensitive to delays, so UDP is a better choice than TCP for such applications. Services provided by RTP include:  Payload-type identification - Indication of what kind of content is being carried  Sequence numbering - PDU sequence number  Time stamping - presentation time of the content being carried in the PDU  Delivery monitoring - packet can still delivered out of order RTP does not provide mechanisms to ensure timely delivery.  Nor does it give any Quality of Service (QoS) guarantees.  These things must be provided by some other mechanism.
  159. 159. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 27 of 32 6.10 Real-Time Transport Protocol (2/4) RTP’s major benefit is to allow receivers to play out media at a proper pace even if the IP network does not keep the time relationship of the transported data. As we know IP networks can produce jitter, that allows the jitter signal to arrive earlier than the original signal. Thus, another scheme is needed by the receiver to re-establish a certain information order - this is provided by the RTP timestamps.
  160. 160. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 28 of 32 6.10 Real-Time Transport Protocol (3/4) The receiver stores all incoming data in a buffer according to their timestamps and then starts playing them. If a certain packet is still missing because of delay an interpolation of the present signal is played instead. It might even be a simple replay of the whole sample. If the packet arrives afterwards it is discarded.
  161. 161. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 29 of 32 6.10 Real-Time Transport Protocol (4/4) t is obvious that the receiver should not wait too long before starting to play, nor should it start too early. Field trials have proven that a majority of bits arrive 50ms after they were sent. A waiting time of approx. 100ms should be a good interval before packets are played.
  162. 162. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 30 of 32 6.11 Real-Time Transport Control Protocol (1/3) RTCP, which stands for Real-time Transport Control Protocol, provides out-of-band control information for RTP flow. It partners RTP in the delivery and packaging of multimedia data, but does not transport any data itself.
  163. 163. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 31 of 32 6.11 Real-Time Transport Control Protocol (2/3) It is used periodically to transmit control packets to participants in a streaming multimedia session. The primary function of RTCP is to provide feedback on the quality of service being provided by RTP.
  164. 164. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 32 of 32 6.11 Real-Time Transport Control Protocol (3/3) RTCP gathers statistics on a media connection and information such as the bytes sent, the packets sent, lost packets, jitter, and round trip delay. An application can use this information to increase the quality of service perhaps by limiting flow, or using a low compression codec instead of a high compression one.
  165. 165. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 1 of 49 7 Procedures
  166. 166. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 2 of 49 7.1 IMS Registration (1/13)...................................................................4 7.1 IMS Registration (2/13)...................................................................5 7.1 IMS Registration (3/13)...................................................................6 7.1 IMS Registration (4/13)...................................................................7 7.1 IMS Registration (5/13)...................................................................8 7.1 IMS Registration (6/13)...................................................................9 7.1 IMS Registration (7/13).................................................................10 7.1 IMS Registration (8/13).................................................................11 7.1 IMS Registration (9/13).................................................................12 7.1 IMS Registration (10/13)...............................................................13 7.1 IMS Registration (11/13)...............................................................14 7.1 IMS Registration (12/13)...............................................................15 7.1 IMS Registration (13/13)...............................................................16 7.2 IMS Session Scenarios (1/2) ........................................................17 7.2 IMS Session Scenarios (2/2) ........................................................18 7.3 Mobile Originated Call (non-roaming) (1/10).................................19 7.3 Mobile Originated Call (non-roaming) (2/10).................................20 7.3 Mobile Originated Call (non-roaming) (3/10).................................21 7.3 Mobile Originated Call (non-roaming) (4/10).................................22 7.3 Mobile Originated Call (non-roaming) (5/10).................................23 7.3 Mobile Originated Call (non-roaming) (6/10).................................24 7.3 Mobile Originated Call (non-roaming) (7/10).................................25 7.3 Mobile Originated Call (non-roaming) (8/10).................................26 7.3 Mobile Originated Call (non-roaming) (9/10).................................27 7.3 Mobile Originated Call (non-roaming) (10/10)...............................28 7.4 Mobile Terminated Call (roaming) (1/2) ........................................29 7.4 Mobile Terminated Call (roaming) (2/2) ........................................30 7.5 PSTN-Originated Call (1/6)...........................................................31 7.5 PSTN-Originated Call (2/6)...........................................................32 7.5 PSTN-Originated Call (3/6)...........................................................33 7.5 PSTN-Originated Call (4/6)...........................................................34 7.5 PSTN-Originated Call (5/6)...........................................................35 7.5 PSTN-Originated Call (6/6)...........................................................36 7.6 PSTN-Terminated Call (1/2) .........................................................37 7.6 PSTN-Terminated Call (2/2) .........................................................38 7.7 Call Release by Mobile (1/5).........................................................39 7.7 Call Release by Mobile (2/5).........................................................40 7.7 Call Release by Mobile (3/5).........................................................41 7.7 Call Release by Mobile (4/5).........................................................42 7.7 Call Release by Mobile (5/5).........................................................43 7.8 Call Release by PSTN (1/4) .........................................................44
  167. 167. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 3 of 49 7.8 Call Release by PSTN (2/4) .........................................................45 7.8 Call Release by PSTN (3/4) .........................................................46 7.8 Call Release by PSTN (4/4) .........................................................47 7.9 Addition of Media Resources (1/2) ...............................................48 7.9 Addition of Media Resources (2/2) ...............................................49
  168. 168. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 4 of 49 7.1 IMS Registration (1/13) Let us start with the presentation of an IMS registration procedure.
  169. 169. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 5 of 49 7.1 IMS Registration (2/13) At first, we will look at the entire procedure which consists of 22 major steps. In this sample scenario we will assume that the UE performs the registration process in a roaming situation.
  170. 170. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 6 of 49 7.1 IMS Registration (3/13) This first part of the registration procedure represents the GPRS specific stages, i.e., PDP Context Activation. An IP address will be allocated by the GGSN which can be used as the host address for the duration of the PDP context.. Next, the UE must discover which P-CSCF to address in the visited network. Normally, the UE obtains the IP address of the P-CSCF during the PDP Context Activation procedure as the GGSN has a list of available P-CSCFs. The UE might also resolve the IP address using GGSN’s DHCP and Domain Name Service, if able to do so.
  171. 171. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 7 of 49 7.1 IMS Registration (4/13) After the UE has discovered the P-CSCF address, it can now start to construct the initial Register request. This SIP register request contains the SIP Universal Resource Identifier (URI), i.e. tom@home.de. It also contains the UE‘s IP address and the P-CSCF‘s IP address.
  172. 172. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 8 of 49 7.1 IMS Registration (5/13) Now the P-CSCF realizes that the request URI points to Tom‘s home network. So the P- CSCF acts as an outgoing proxy and has to locate the I-CSCF in the home network. To achieve this, a DNS lookup using the request URI is performed that will return the address of the I-CSCF.
  173. 173. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 9 of 49 7.1 IMS Registration (6/13) The P-CSCF determines which message transport type to take: Either UDP, TCP or secure TCP. In this example the P-CSCF selects UDP and forwards the Register request to the I- CSCF in the home network using UDP and port 5060.
  174. 174. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 10 of 49 7.1 IMS Registration (7/13) The I-CSCF is the entry point to Tom‘s home network. It receives every Register Request sent from Tom‘s UE. To determine the serving S-CSCF the I-CSCF queries the HSS which is assigned to the registering user. Once the appropriate S-SCSF has been found, the I-CSCF forwards the SIP Register to the S-SCSF.
  175. 175. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 11 of 49 7.1 IMS Registration (8/13) The S-CSCF needs to authenticate the UE and so it requests an Authentication Vector from the HSS. This request procedure is based on the Diameter protocol on the Cx interface between HSS and S-CSCF. The HSS returns a set of Authentication vectors, each containing the familiar parameters of the respective GSM scenario.
  176. 176. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 12 of 49 7.1 IMS Registration (9/13) The S-CSCF selects an Authentication Vector from the received set and forwards the parameters needed back to the UE using a “401 UNAUTHORIZED” response message.
  177. 177. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 13 of 49 7.1 IMS Registration (10/13) Upon reception, the UE extracts the relevant authentication parameters for the authentication response calculation. Furthermore, it calculates the numerous keys used for ciphering. Lastly, if all have passed, the UE returns the authentication response in a standard REGISTER request.
  178. 178. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 14 of 49 7.1 IMS Registration (11/13) Again a DNS query is performed at the P-CSCF level to locate the I-CSCF to forward the REGISTER request containing the authentication response. The I-CSCF interrogates the HSS to locate the S-CSCF to forward the REGISTER request message. In this case, the HSS returns the S-CSCF name that was selected previously in step 5.
  179. 179. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 15 of 49 7.1 IMS Registration (12/13) The S-CSCF checks the authentication response received from the UE against its own calculated value. lf the two values match the UE has been successfully authenticated. Registration on the S-CSCF can now take place.
  180. 180. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 16 of 49 7.1 IMS Registration (13/13) The S-CSCF notifies the HSS that the UE registration was successful. In turn, the HSS provides the S-CSCF with the complete user profile. The S-CSCF returns a SIP “200 OK” response to the I-CSCF which is then sent to the P-CSCF and then to the UE.
  181. 181. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 17 of 49 7.2 IMS Session Scenarios (1/2) IMS session control involves all 3 known planes, i.e., User Plane, Control Plane and Application Plane. For simplicity we will focus on session control procedures on the Control Plane level.
  182. 182. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 18 of 49 7.2 IMS Session Scenarios (2/2) Let us have a closer look at 7 typical IMS session scenarios:  Mobile Originated Call in a non-roaming situation  Mobile Terminated Call while one UE is roaming  PSTN Originated Call  PSTN Terminated Call  Call Release by Mobile  Call release by PSTN  and, finally, a sample multimedia session flow integrating a second media to an existing call.
  183. 183. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 19 of 49 7.3 Mobile Originated Call (non-roaming) (1/10) Let us start with a Mobile Originated Call in a non-roaming situation. To initiate an IMS call a SIP INVITE message is used. The INVITE message contains a number of important parameters, e.g.,  the Uniform Resource Identifiers (URI) of the calling party and  the URI of the called party  QoS parameters  Media information it is forwarded to the P-CSCF of the originating party.
  184. 184. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 20 of 49 7.3 Mobile Originated Call (non-roaming) (2/10) Next, a ”100 Trying” SIP message is returned to UE-1 to indicate a provisional response. P- CSCF1 adds some routing information to the INVITE message before forwarding it to S- CSCF1.
  185. 185. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 21 of 49 7.3 Mobile Originated Call (non-roaming) (3/10) S-CSCF1 acknowledges receipt of the INVITE message and evaluates the Media Criteria for the calling party that were received from the HSS with the service profile. They define the "triggers" that will cause the S-CSCF1 to send the message to the appropriate Application Server (AS).
  186. 186. IMS Basics, Version 1.1e  T.O.P. BusinessInteractive GmbH Page 22 of 49 7.3 Mobile Originated Call (non-roaming) (4/10) S-CSCF1 forwards the INVITE message to the I-CSCF. This is always located at the edge of the home network for the terminating party. I-CSCF responds with a “100 Trying” message. I- CSCF will now try to locate UE2. To do this, I-CSCF sends a Cx-Location-Request message to the HSS. The HSS responds with the address of the S-CSCF that is currently serving UE2 (S-CSCF-2) and forwards the INVITE message accordingly.

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