SVKMs Narsee Monjee

Mukesh Patel School of Technology

Management & Engineering, NMIMS

A REPORT

ON

QUALITY OF SERVICE IN INTERNET PROTOCOL

MULTIMEDIA SUBSYSTEM

By

PRATIK CHOWDHARI

MBA-TECH (EXTC)

71208110034

RELIANCE COMMUNICATION, MUMBAI

A REPORT

ON

QUALITY OF SERVICE IN INTERNET PROTOCOL

MULTIMEDIA SUBSYSTEM

BY

PRATIK CHOWDHARI

MBA-TECH (EXTC)

71208110034

A report submitted in partial fulfillment of the

requirements of 5 years Integrated MBA (Tech) Program

of Mukesh Patel School of Technology Management &

Engineering, NMIMS

Abstract:

Internet Protocol Multimedia Subsystem is an architectural framework designed by

the 3GPP to provide various multimedia services to the users over the common

Internet based protocols. The IMS is an access independent architecture which

means that IMS services can be provided over any IP connectivity networks like

GPRS, EDGE, WLAN, x-Digital Subscriber Line.

Quality of Service is the performance of the network as seen by the user. The

Quality of Service considers several parameters such as reliability, throughput,

jitter, delay and the availability of the network.

The IMS network architecture is designed such that it overcomes the delays while

routing the signals from the source to the destination and also avoids loss of

packets.

Introduction:

The study of the project is divided in multiple parts.

1. Study of the IMS network architecture:

The study of the IMS network involves the various architectural requirements to

make the IMS networks and the components involved in the architecture.

Registration of a user equipment to the IMS network. Initiation, modification

and termination of a session.

2. Study of QoS in the IMS network:

This involves the study of the Quality of Service in the IMS network so as to

enhance the network performance from the users perspective.

MAIN TEXT:

The architectural requirements of the IMS network are:

1. IP Multimedia Session:

The IMS network provides various multimedia services to the users in a single

session. Thus the architecture should be designed such that the user can mix and

match various multimedia services in a single session without ending the

session. Eg: Suppose two users can start a session as a voice session and can

later on add a game or a video session to it.

2. IP Connectivity:

The basic requirement for a user equipment to connect to a IMS network is that

it should have IP connectivity to it. IMS supports both IPv4 and IPv6

connectivity but IPv6 is more prevalent since it does not have address shortage

in it. The user equipment can obtain IP connectivity from either the home

network (HN) or the visited network (VN). Thus even if the user is roaming in a

location where IMS network is not deployed, it can access IMS facilities by

obtaining the IP connectivity from the HN.

Fig 1.0: IMS connectivity options when user is roaming

The figure depicts the connectivity options while the user is roaming. The leftmost

side presents an option when the user obtains IP connectivity from the visited

network. This means that when considering the UMTS network the Radio Access

Network (RAN), the Serving GPRS support node (SGSN) and Gateway GRPS

support node (GGSN) are located in the visited network. The rightmost part

presents an option when the user obtains IP connectivity from home network. In

this case the Radio Access Network (RAN), the Serving GPRS support node

(SGSN) are located in the visited network while the Gateway GRPS support

node (GGSN) is located in the home network.

3. Quality of Service:

The quality of service is an important parameter to be considered as this parameter

determines the performance of the network. To have a better performance, the user

negotiates and expresses its QoS requirements with the IMS network during the

Session Initiation Protocol (SIP) session initiation or session modification. The

parameters negotiated are

a. Media type

b. Direction of media flow

c. Packet Size

d. Packet Transport Frequency

e. Bandwidth Adaptation

4. Charging Arrangements:

Charging a user is a must from the perspective of the operator and thus charging

arrangements should be provided in the IMS architecture so that it can provide

Call Detail Record (CDRs) to the billing system. The IMS architecture supports

both online as well as offline charging facilities.

a. Online Charging:

This is similar to prepaid account wherein the IMS network first checks whether

the user has enough credits to continue a particular service. If the user wishes to

proceed with a particular service and does not have enough credits in its account,

then the IMS network does not route the call to the destination and return an error

message saying not enough credits to carry out a particular application. If the user

has exhausted the credits during a session, then the IMS network disrupts the

service.

b. Offline Charging:

This is similar to a postpaid account where the CDRs are collected over a monthly

cycle and then the operator posts a bill to the user at the end of the month.

5. Security Arrangements:

Security is a very important parameter to be considered while designing the IMS

network. The IMS has its own authentication and authorization procedure to be

followed along with the normal GPRS security arrangements. Thus the IMS

network is designed in such a way that the users are first authenticated before they

can access the IMS services.

Fig 1.1: IMS Security

6. Roaming:

IMS services should be available to the users irrespective of their geographical

location. Thus to make this happen the architecture should be such that its supports

roaming facilities. There can be two types of roaming facilities available:

a. GRPS Roaming:

GPRS roaming is a type of roaming to access the IMS facilities wherein the visited

network provides the RAN and SGSN whereas the home network provides the

GGSN and IMS facilities to the user.

b. IMS Roaming:

IMS roaming is a type of roaming to access the IMS facilities wherein the visited

network provides the IP connectivity i.e ( RAN, SGSN, GGSN) and the IMS

entry point only (P-CSCF) and the remaining IMS functionalities are provided by

the home network to the user.

7. Internetworking:

Internetworking means that the user equipment i.e an IP device can connect to a

circuit switched (CS) or a packet switched (PS) network. This facility should be

available as the IMS network is not deployed across the globe and not everyone

has access to IMS based services. This will help the IMS users to communicate

with PSTN, ISDN users as well.

IMS Architecture:

The IMS architecture is layered design architecture as it reduces the

interdependency of the layers on each other. This means that the transport

services are separated from the IMS signaling and session management services.

The layered design facilitates the addition of new access networks to the IMS

system. Eg: The 3GPP added WLAN support to the IMS network in the release 6

whereas the broadband access to the IMS network was added in the 3GPP release

7 of the IMS network. The following fig 1.3 shows the IMS architecture and its

layered design.

Fig 1.3: IMS Architecture

The various entities involved in the IMS architecture are:

1. Session Management and Routing ( CSCFs)

2. Databases (HSS, SLF)

3. Services (Application Server, MRFC, MRPC)

4. Internetworking (BGCF, MGCF, IMS-MGF, SGW)

5. Support Functions (PDF, SEG, THIG)

6. Charging entities

Session Management and Routing:

The SIP session management and routing within the IMS network is done by the

Call Session Control Function (CSCFs). There are three types of CSCFs and

each of them has been assigned particular tasks within the network, but the tasks

common to all the CSCFs are session management.

Three types of CSCFs:

1. Proxy Call Session Control Function (P-CSCF):

The P-CSCF is the first contact point for the users in the IMS network. Thus all the

traffic from the UE to the IMS network will pass through the P-CSCF and all the

terminating traffic from the IMS network to the destination UE will pass through

the P-CSCF. There are four tasks assigned to the P-CSCF:

a. SIP Signal Compression:

The SIP protocol is a text based protocol and thus it contains header, header

extensions and various other parameters which make the size of the SIP signal

larger. If the UE has indicated that it wants to receive SIP signaling messages

compressed then the P-CSCF compresses the SIP signal and sends to the

destination UE.

b. IPSec Security Association:

The P-CSCF takes care of the security and the integrity of the SIP signaling

messages that are transported through the IMS network. The P-CSCF and the UE

negotiate the security parameters during the registration of the UE and thus applies

this security information when the UE transmits some SIP signaling through the

IMS.

c. Interact with Policy Decision Function (PDF):

The P-CSCF interacts with the Policy Decision Function to exchange charging co-

relations with the GPRS network. Thus the P-CSCF sends its charging CDRs to

the GPRS network via the PDF and receives the charging CDRs from the GPRS

network via the PDF.

d. Detection of Emergency Sessions:

It is the role of the P-CSCF to detect an emergency session and to transfer the call

to the circuit switched (CS) networks as the IMS architecture release 7 does not

support emergency sessions. Thus when a user dials an emergency number then the

P-CSCF will transfer the call to the circuit switched networks as it detects that the

user has dialed a emergency number.

2. Interrogating Call Session Control Function (I-CSCF):

The tasks assigned to the I-CSCF are:

a. During the registration of a UE to the IMS network the I-CSCF will

determine the S-CSCF for that UE based on the capabilities specified by the

UE in the REGISTER request.

b. I-CSCF will route all the incoming requests to the S-CSCF of the user B to

which the call is destined.

c. I-CSCF will determine the next hop of the SIP signaling i.e the S-CSCF or

the Application Server.

d. The I-CSCF can provide Topology Hiding Inter-network Gateway Service

which helps the operator to hide the topology of the network from an outside

user.

3. Serving Call Session Control Function (S-CSCF):

The S-CSCF is the most important point of the IMS session management and

routing service. This handles all the routing information of the session, stores all

the service profiles of the users downloaded from the Home Subscriber Server

(HSS).

When a new user is trying to register to the IMS network, the request is sent to the

S-CSCF which will download the authentication data from the HSS and then

challenge the user based on the authentication data. The user responds with an

appropriate response to the challenge and then the S-CSCF verifies the response

and if correct it downloads the service profile of the user and registers it to the IMS

networks.

Fig 1.4: S-CSCF routing

If the UE uses MSISDN URL, then the S-CSCF will convert the MSISDN URL

to SIP URI (Universal Resource Identifier) as the IMS network does not route

requests further based on the MSISDN URL.

It is the job of the S-CSCF to determine whether to route the call further in the

IMS network or breakout of the IMS network to the Circuit Switched Network. If

the S-CSCF finds that the user is trying to communicate with a circuit switched

network then it will route the call to the Breakout Gateway Control Function

(BGCF). The fig 1.4 shows the routing information of the S-CSCF.

DATABASES:

There are two databases deployed in the IMS network: the Home Subscriber

Server (HSS) and the Subscriber Locater Function (SLF).

Home Subscriber Server (HSS):

The Home Subscriber Server is the main database of the IMs network and it stores

all the services profiles and the service related information of the users registered

to the operator. The HSS stores the user identities of the users. The user identity

can be of two types:

a. Private User Identity:

This is provided by the operator to the user and is used for authorization and

authentication purposes. This identity is used by the user during the registration to

the IMS network.

b. Public User Identity:

This user identity is used to communicate with the other users over the IMS

network.

The HSS provides services related information or capabilities to the I-CSCF for

determining the S-CSCF for the user.

The S-CSCF downloads the authentication data as well as the service profile for

the user from the HSS.

The HSS along with the IMS facilities provides the Home Location Register/

Authentication Centre (HLR/AuC) for both packet switched and circuit switched

domain. This helps the subscribers access to the circuit switched and packet

switched networks as well.

Fig 1.5: Structure of HSS

Subscriber Locater Function (SLF):

The SLF is used when there are multiple HSS deployed by the operator in a single

region. Thus the SLF helps the S-CSCF determine the correct HSS in which the

service profile for a particular user is stored.

INTERNETWORKING:

Internetworking is required when the user in the IMS network wants to

communicate with a circuit switched network. Thus to facilitate this BGCF,

MGCF, SGW are used.

When a user tries to communicate to the circuit switched network, the S-CSCF will

route the call to the Breakout Gateway Control Function (BGCF). Thus the S-

CSCF determines when the request will break out from the IMS network. The

BGCF on receiving the request from the S-CSCF will determine whether the

breakout is happening in the same network or different network. If the break out

happens in the same network then the BGCF will route the signal to the Media

Gateway Control Function (MGCF). If the breakout happens to a different

network then the BGCF will route the signal to the BGCF in that particular

network.

The MGCF on receiving the signal performs protocol conversion as the protocol

used in IMS is SIP/IP and the protocol used in CS networks is SS7/MTP. This

converted protocol is then sent to the circuit switched network via the Signaling

Gateway (SGW). The SGW does not interpret the SIP/IP protocols and hence the

MGCF performs the protocol conversion.

Similarly all the requests from the circuit switched networks are sent to the MGCF

via the SGW which performs the necessary protocol conversion and sends it to the

BGCF.

REGISTER

An UE has to register to the IMS network in order to utilize the IMS facilities.

Once the user has registered to the IMS network, it is authenticated to use all the

IMS facilities which it is authorized to.

To register to the IMS network, the UE first has to determine the IP address of the

P-CSCF i.e. the first contact point of the IMS architecture. Once the UE

determines the IP address of the P-CSCF, it will send try and register to the

network.

The registration takes place in two phases:

Phase 1:

The UE sends a REGISTER request to the discovered P-CSCF which contains the

IP address of the UE, domain name of the I-CSCF. When this request reaches the

P-CSCF, it will resolve the IP address of the I-CSCF based on the domain name

provided in the REGISTER request and forward the request to the I-CSCF.

The I-CSCF will then select the S-CSCF based on the capabilities of the UE and

forward the request to the S-CSCF. Since the UE is not registered to the IMS

network, the S-CSCF will send an 401 Unauthorized code back to the UE via the

same path.

Phase 2:

The UE receives the 401 Unauthorized Code challenge and send back a response to

the challenge to the S-CSCF. The S-CSCF will then check the response, and if the

response was correct to the challenge made, the S-CSCF will contact the HSS to

download the user profile of the UE and register it to the IMS network and send

back a 200 OK response back to the UE.

ACRONYMS:

IMS- Internet Protocol Multimedia Subsystem

UE- User Equipment

P-CSCF- Proxy Call Session Control Function

I-CSCF- Interrogating Call Session Control Function

S-CSCF- Serving Call Session Control Function

HN- Home Network

VN- Visited Network

SIP- Session Initiation Protocol

IP- Internet Protocol

QoS- Quality of Service

CDR- Call Data Record

HSS- Home Subscriber Server

SLF- Subscription Locater Function

References:

1. THE IMS- IP Multimedia Concepts and Services- Miikka Poiksella and

Georg Mayer

2. http://en.wikipedia.org/wiki/IP_Multimedia_Subsystem