Analysis of the Charging Protocols inthe All IP Network

Kati Lehtinen

16.04.2002

Supervisor: Professor Raimo Kantola

Instructor: M.Sc. Seppo Huotari (Nokia)

Table of Contents• Background• Main objectives of the Thesis• Methods used• The All IP Network• Charging in the All IP network• Diameter• GTP'• Use cases

• Postpaid

• Prepaid

• Conclusions and Results• Future Research

Background

• Mobile networks are in a transition phase

• Standardization of the Release 5 (All IPNetwork) is ongoing in 3GPP

• IP Multimedia Subsystem (IMS) is the main newfeature of this release

• Standardization of IMS charging wasstarted in the beginning of year 2002 andis targeted to be finished in June 2002

• Traditionally Intelligent Network (IN)concept has been used to provide prepaidservices for subscribers in mobile networks

Main Objectives of the Thesis

• To study what new characteristics will IMSbring to charging

• To study Diameter and GTP'

• To study whether these protocols could beused for charging in the All IP Network andhow

• Main focus on IMS and Diameter

Methods used

• Literature research

• Standardization follow-up

• Use cases

• Analysis of the protocols (functionalproperties needed for charging and thoseof both protocols identified and analyzed)

The All IP Network Architecture

OSA

SGSN

GGSN

CSCF

MGCF

IM-MGW

MSCGMSC

MGWMSC Server

RAN

HSS

SGW

BGCFSLF

MRFP

AS

SCP MRFC

All IP Network = 3GPP's Release 5 network

Subsystems

• The All IP network can be divided into twological parts: a core network and anaccess network (GERAN or UTRAN).

• The core network can be divided into four,separate subsystems.

• Circuit Switched Core Network (CS CN) Subsystem• consists of all the core network entities which

provide Circuit Switched services

• Packet Switched Core Network (PS CN) Subsystem• consists of all the core network entities which

provide packet switched connectivity services

• Services Subsystem• consists of all the entities providing capabilities to

support operator specific services

• IP Multimedia core network Subsystem (IMS)• consists of all the core network elements for

provision of IP multimedia and telephony services

IMS

• Provides IP-based telephony andmultimedia sessions on top of the RANand PS domains (although accessindependent)

• Service and session control of subscribedservices for a roaming subscriber is in thehome network.

• The Session Initiation Protocol (SIP) isused for session control. SIP is anapplication-layer control signaling protocolfor creating, modifying, and terminatingsessions with one or more participants(developed by the IETF).

Charging in the All IP Network (1)

• The purpose of charging is to collect dataon the network resource usage andservices to enable the billing of thesubscriber

• Charging layers• Access Layer: The charging for the usage of

bearer resources (e.g. GPRS access services)

• IP Multimedia Layer: The charging of servicesprovided by the IMS (e.g. multimedia sessions)

• Service and Application Layer: The charging ofservices provided by the service subsystem(e.g. games, location)

Charging in the All IP Network (2)

• Two separate charging architectures: off-line and on-line charging architecture

• Off-line charging architecture provides acharging process where charging informationdoes not affect, in real-time, the servicerendered

• On-line charging architecture provides acharging process where charging informationcan affect, in real-time, the service rendered andtherefore directly interacts with thesession/service control

Charging in the All IP Network (3)

• Main charging methods are postpaid andprepaid

• Can be based on on-line or off-line chargingmechanisms

• Postpaid: a charging method to pay for serviceafter service usage, usually by a bill everymonth

• Prepaid: a charging method to pay for a servicebefore the actual service usage. The money ispaid beforehand to a prepaid account, and thatmoney is then deducted from the accountaccording to the service usage.

Off-line charging architecture

S-CSCFS-CSCF

I-CSCFI-CSCF

ASAS

MGCFMGCF

BGCFBGCF

P-CSCFP-CSCF

GGSNGGSN

SGSNSGSN

BSBS

BSBSMRFCMRFC

Home(B)

Visited(B)

Rf

Ga

Rf

Rf

Rf

Rf

Rf

Charging information flow

CGFCGF

CCF

CGFCGF

CCF

Ga

S-CSCFS-CSCF

I-CSCFI-CSCF

ASAS

MGCFMGCF

BGCFBGCF

P-CSCFP-CSCF

GGSNGGSN

SGSNSGSN

BSBS

BSBS MRFCMRFC

Home(A)

Visited(A)

Rf

Ga

Rf

Rf

Rf

Rf

Rf

CGFCGF

CCF

CGFCGF

CCF

Ga

On-line charging architecture

P-CSCFP-CSCF

S-CSCFS-CSCF

Home (A)

I-CSCFI-CSCF

GGSNGGSN

ASAS

SGSNSGSN

MGCFMGCF

BGCFBGCF

OCF

CSE

SCF SCF

MRFCMRFC

CAP

Visited (A)

Ro

Ro

Ro

Rc

Charging information flow

Account

Account

P-CSCFP-CSCF

S-CSCFS-CSCF

Home (B)

I-CSCFI-CSCF

GGSNGGSN

ASAS

SGSNSGSN

MGCFMGCF

BGCFBGCF

OCF

CSE

SCF SCF

MRFCMRFC

CAP

Visited (B)

Ro

Ro

Ro

Rc

Account

Account

Realtime Rating System

Realtime Rating System

Diameter (1)

• An AAA (authentication, authorization, andaccounting) protocol defined by the IETF

• Not ready yet (expected to become anRFC during the summer 2002)

• Successor of RADIUS

• Session-based protocol

• The basic concept of Diameter is toprovide an extensible base protocol withfunctionality common to all supportedservices, while application-specificfunctionality can be provided through anextension mechanism

Diameter (2)

• The base protocol provides the followingfacilities:

• Delivery of attribute value pairs (AVPs)

• Capabilities negotiation

• Error notification

• Extensibility (through addition of newcommands and AVPs)

• Runs on TCP or SCTP, the latter is the preferredoption

Diameter (3)

• All data delivered by Diameter is in theform of an AVP. AVPs are used by thebase protocol to support the followingfeatures

• Transporting of user authentication information,for the purposes of enabling the Diameter serverto authenticate the user.

• Transporting of service specific authorizationinformation, between client and servers,allowing the peers to decide whether a user'saccess request should be granted.

• Exchanging resource usage information, whichmay be used for accounting purposes, capacityplanning, etc.

• Relaying, proxying and redirecting of Diametermessages through a server hierarchy.

GTP' (1)

• A CDR transport protocol defined by 3GPP

• Designed based on GPRS TunnelingProtocol (GTP) which is used for packetdata tunneling in the backbone network

• Runs on TCP or UDP

• Used for CDR transport between theGSNs (SGSN and GGSN) and the CGF

SGSN GGSN

CGF

GTP' GTP'

BillingSystem

(FTAM/FTP)

GTP' (2)

• GTP’ contains the following functions:• CDR-transfer mechanisms between the GSNs

and the CGF.

• Re-direction of CDR transfer to another CGF.

• Ability to detect communication failures betweenthe communicating peers

• Ability of a CDR handling node to advertize topeers about its CDR transfer capability (e.g.,after a period of service downtime).

• Ability to prevent duplicate CDRs that mightarise during redundancy operations.

Postpaid with GTP' (CDR transfer)UE NE Accounting Server

3. DRTR(SEND_RECORD [SN1])

4. DRTA(REQUEST_ACCEPTED [SN1])

1. Service Request

2. Service Allowance

8. Service finished

5. DRTR(SEND_RECORD [SN2])

6. DRTR(SEND_RECORD [SN3])

7. DRTA(REQUEST_ACCEPTED [SN2, SN3])

9. DRTR(SEND_RECORD [SN4])

10. DRTA(REQUEST_ACCEPTED [SN4])

Postpaid with Diameter (CDR transfer)UE NE Accounting Server

3. ACR(EVENT_RECORD)

4. ACA(EVENT_RECORD)

1. Service Request

2. Service Allowance

7. Service finished

5. ACR(EVENT_RECORD)

6. ACA(EVENT_RECORD)

8. ACR(EVENT_RECORD)

9. ACA(EVENT_RECORD)

Prepaid with Diameter (continuousservice)

UE NE Accounting Server

2. ACR(START_RECORD)

3. ACA(START_RECORD)

4. Service allowance

1. Service request

5. ACR(INTERIM_RECORD)

10. ACR(STOP_RECORD)

6. ACA(INTERIM_RECORD)

11. ACA(STOP_RECORD)

...

refund/deduct money

Credit reservation

7. ACR(INTERIM_RECORD)

8. ACA(INTERIM_RECORD)

9. Service finished/final unitindication

Credit reservation

refund/deduct money

Credit reservation

refund/deduct money

Prepaid with Diameter (one-timeservice)

UE NE Accounting Server

2. ACR(EVENT_RECORD)

3. ACA(EVENT_RECORD)

4. Service allowance

1. Service request

Conclusions and Results (1)

• Two long-term trends for IMS chargingwere identified:

• The direction seems to be towards a real-timenetwork. Prepaid is becoming increasinglypopular and a real-time network offers bettertools for control against frauds.

• Interworking with the "Internet world" is seen asa long term objective for the All IP network. Thisalso creates new, profitable business models.This trend should be taken into account alsowhen designing the charging architecture.

• One notable risk related to roamingidentified:

• If the GGSN is chosen from the visited network,the home network has no way to be sure howmuch user plane traffic is transferred throughthe visited network.

Conclusions and Results (2)

• Charging in IMS can be effectively realizedwith a charging protocol

• Diameter• Well suited for on-line charging

• GTP'• Well suited for off-line charging

• On-line and off-line architectures should becombined

• Diameter used for both interfaces

Future research

• Performance analysis

• Inter-network charging• Security aspects

• Usage of authentication and authorization partsof Diameter

• Integrating IETF's AAA concept and All IPnetwork

• The new Diameter accounting application

• Interworking of SIP session control and theDiameter accounting application

AbbreviationsAAA Authentication, Authorization, and Accounting

AS Application Server

AVP Attribute Value Pair

BGCF Breakout Gateway Control Function

BS Billing System

CAP CAMEL Application Part

CCF Charging Collection Function

CDR Charging Data Record

CGF Charging Gateway Function

CN Core Network

CS Circuit Switched

CSCF Call Session Control Function

FTAM File Transfer, Access, and Management

FTP File Transfer Protocol

GERAN GSM/EDGE Radio Access Network

GGSN Gateway GPRS Support Node

GMSC Gateway Mobile Switching Center

GPRS General Packet Radio Service

GTP' GPRS Tunnelling Protocol (extended for accounting)

HSS Home Subscriber Server

IETF The Internet Engineering Task Force

IMS IP Multimedia core network Subsystem

IN Intelligent Network

MGCF Media Gateway Control Function

MGW Media Gateway

MRFC Multimedia Resource Function Controller

MRFP Multimedia Resource Function Processor

OCF On-line Charging Function

OSA Open Service Architecture

PS Packet Switched

RADIUS Remote Authentication Dial In User Service protocol

RAN Radio Access Network

SCP Service Control Point

SCTP Stream Control Transmission Protocol

SGSN Serving GPRS Support Node

SGW Signaling Gateway function

SIP Session Initiation Protocol

SLF Subscription Locator Function

UDP User Datagram Protocol

UTRAN Universal Terrestrial Radio Access Network

Questions?