Charging for Mobile All-IP Telecommunications

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Charging for Mobile All-IP Charging for Mobile All-IP TelecommunicationsTelecommunications

Yi-Bing LinYi-Bing LinChair Professor and the Chair Professor and the

DeanDeanCollege of Computer College of Computer

ScienceScienceNational Chiao Tung National Chiao Tung

UniversityUniversityHsinchu, TaiwanHsinchu, Taiwan

[email protected]@csie.nctu.edu.tw

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OutlineOutlineIP Multimedia core network Subsystem (IMS)IP-based Online Charging System (OCS)Design of IMS Prepaid Application Server for SIP-based ServicesModeling OCS Credit Reservation ProcedureReducing OCS Re-authorization CostFuture Research Directions

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MotivationMotivation

Internet environment encourages global usage with flat-rate tariffs and low entry costs.

A major problem of the “flat-rate” tariffs is that such business model cannot justify the expensive equipment/operation investments of mobile services.

Introduction of the 3G mobile system has further driven the Internet into new markets to support mobile users.

To have cooperation with service/content provider, creating a killer environment has now become a key point in terms of competition.

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GSM supports circuit-switched telephony servicesMobile Switching Center (MSC)

UMTS supports real-time IP multimedia servicesGeneral Packet Radio Service (GPRS) IP Multimedia core network System (IMS)

Charging Management Evolution

UMTS/IMS Online ChargingUMTS/IMS Online Charging

Single Session Multi-Session

SS7-based(Circuit Switched)

IP-based(Packet Switched)

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IP Multimedia core network Subsystem (IMS)

UE

GPRS Network

Radio Access Network GGSN

MGCF

PSTN

BGCF

T-SGW

MGW

Application Server

signalingsignaling and data

CSCF

Online Charging System (OCS)

BGCF: Breakout Gateway Conrol Function CSCF: Call Session Control FunctionGGSN: Gateway GPRS Support Node HSS: Home Subscriber ServerI-CSCF: Interrogating CSCF MGCF: Media Gateway Control FunctionMGW: Media Gateway PSTN: Public Switched Telephone NetworkT-SGW: Transport Signaling Gateway UE: User Equipment

HSS

Gy

Ro

Ro

UMTS/IMS ArchitectureUMTS/IMS Architecture

ab

c

d

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Rr

BoCSCF

MGCF

MRFC

Application Server

WLAN

SGSN

ISC

Ro

BGCF: Breakout Gateway Control Function CAP: CAMEL Application Part CSCF: Call Session Control FunctionGSGN: Gateway GPRS Support Node IMS-GWF: IMS Gateway Function IMS: IP Multimedia SubsystemISC: IMS Service Control MGCF: Media Gateway Control Function MRFC: Media Resource Function ControllerSGSN: Serving GPRS Support Node TPF: Traffic Plane Function

GGSNTPF Gy

Wo

CAP

IMS-GWF

SessionBased

ChargingFunction(SBCF)

EventBased

ChargingFunction(EBCF)

ABMF

RF

CGF

RechargeServer

BillingSystem

Rc

Ga

ReTariffs

Database

Account Database

Onl ine Charging System (OCS)

NetworkNodes

UMTS Online Charging SystemUMTS Online Charging System

ABMF: Account Balance Management Function

CGF: Charging Gateway FunctionRF: Rating Function

CCR

CCA

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Support of IP-based Charging Support of IP-based Charging

MobileOperator Prepaid and Postpaid

Users Single BillingTransparent of the Price

Service/ContentProvider Dynamic Pricing policy

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[1] Multiple Service Charging between the Prepaid Application Server and the OCS[2] Online Credit Reservation between the CSCF and the OCS[3] Credit Re-authorization between the GGSN and the OCS

UE

GPRSNetwork


MGCF

PSTN

BGCF

T-SGW

MGW

Application Server

CSCF

Online Charging System (OCS)

HSS

Ro

RoGy

Research TopicsResearch Topics

[1][1]

[2][2]

[3][3]

Diameter Credit Control ProtocolCCR: Credit Control RequestCCA: Credit Control Answer

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[1][1] Design of IMS Prepaid Application Design of IMS Prepaid Application Server for SIP-based ServicesServer for SIP-based Services

S.-I. Sou, Y.-B. Lin, Q. Wu and J.-Y. Jeng, Modeling of Prepaid Mechanism of VoIP and Messaging Services. IEEE Transactions on Vehicular Technology, 56(3): 1434-1441, 2007. S.-I. Sou, Q. Wu, Y.-B. Lin and C.-H. Yeh, Prepaid Mechanism of VoIP and Messaging Services, IEEE International Conference on Information Technology Research and Education (ITRE), Hsinchu, Taiwan, 2005.S.-I. Sou, Q. Wu, Y.-B. Lin and W.-E. Chen, SIP-based VoIP Prepaid System on NTP VoIP Platform, Proceedings of the Taiwan Academic Network Conference (TANET), Taitung, Taiwan, 2004.

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SIP-based Prepaid Application SIP-based Prepaid Application ServerServer

PSTNTerminating

Switch

SMS-IWMSC

MMS Relay Server

UE1

CSCF PAS

MGCF

MGW

OCS

CSCF: Call Session Control Function GPRS: General Packet Radio ServiceIWMSC: Interworking Mobile Switching Center MGW: Media GatewayMGCF: Media Gateway Control Function MMS: Multimedia Messaging ServiceOCS: Online Charging System PAS: Prepaid Application ServerPSTN: Public Switched Telephone Network SMS: Short Message ServiceT-SGW: Transport Signaling Gateway UE: User EquipmentUTRAN: UMTS Terrestrial Radio Access Network

Phone2

IP-MessageGateway

T-SGW

UTRAN GPRS

MessagiMessaging ng ServiceServicess

IMS-PSTN IMS-PSTN CallCall

•Handles multiple prepaid services simultaneouslyHandles multiple prepaid services simultaneously•Has more flexible credit controlHas more flexible credit control

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Prepaid Messaging DeliveryPrepaid Messaging Delivery

SMS: Short Message ServiceCCR: Credit Control RequestCCA: Credit Control Answer

200 OK

200 OK

SMS(short message)

200 OK

MESSAGE MESSAGE

MESSAGE

IP-Message GatewayUE1 CSCF OCS MS2

CCR (EVENT_REQUEST)

CCA (EVENT_REQUEST)

PAS

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IMS-PSTN Prepaid Call SetupIMS-PSTN Prepaid Call Setup

IAM: Initial Address MessageANM: Answer Message

CCR: Credit Control RequestCCA: Credit Control Answer

UE1 CSCF PAS OCS MGCF/MGW

INVITE (subsession 1)

CCR (INITIAL_REQUEST)CCA (INITIAL_REQUEST)


T-SGW

200 OK (subsession 2)




ACK (subsession 1) ACK (subsession 2)

IAMANM

IP IAM

IP ANM

Terminating Switch / Phone2

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Prepaid Call Force-Prepaid Call Force-TerminationTermination

REL: Release MessageRLC: Release Complete

RTP Stream

BYE (subsession 1) BYE (subsession 2)

RELRLC

200 OK (subsession 1) 200 OK (subsession 2)

CCR (TERMINATE_REQUEST)CCA (TERMINATE_REQUEST)

IP REL

IP RLC

UE1 CSCF PAS OCS MGCF/MGW T-SGW

Terminating Switch / Phone2

CCR: Credit Control RequestCCA: Credit Control Answer

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PAS Charging PolicyPAS Charging PolicySending out a message during an IMS call may result in insufficient credit left for this ongoing call. A threshold XT is set to protect the in-progress IMS-to-PSTN call when admitting a prepaid message delivery:

x*: the remaining credit left when a message arrives Tm: the charge for a prepaid instant message delivery

x* -Tm ≧ XT Send immediatelyx* -Tm< XT Delay until the call is finished

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PPUFT UFT :: the unnecessary force-termination probability of an in-progress call (i.e., x>tc and x*-Tm < t3-t2)

The prepaid call completes

tcThe prepaid call arrives

Time

A prepaid message arrives

x*

Prepaid authorized timer expires

t1 t2 t3 t4

x

Analytic Output MeasuresAnalytic Output Measures

(Tm)

Message delivery

delay is 0

Message delivery delay

is t3-t2

Assuming 1 credit unit = 1 time unit

EE[[ttdd]:]: the expected delay for the message services

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Input ParametersInput Parameters

X: the amount of the initial prepaid creditm: the arrival rate of the instant messages

tc: the prepaid call holding time

1/: the expected call holding time x: the remaining credit left when a call starts

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Analytic Results [1/2]Analytic Results [1/2]

The pmf of the Poisson messaging arrivals during tc is

Let fx(x) be the density function of the remaining credit x left when a prepaid call arrives. The UFT probability for XT =0 is derived as

dxdtxftfT

txtNP cxcc

X

x

x

tm

ccUFT

c

)()()(Pr0 0

cmtn

cmc e

ntntN

!)(])(Pr[

mmm

Tx

i

i

j

jmmiTxxX

x jiTxee

/

0 0

))((

0 !)])([(11

dxxf x )(

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Analytic Results [2/2]Analytic Results [2/2] For XT ≥ X , the expected unnecessary delay for message delivery is derived as

]|[2]|[][

2

mcc

mccd TtxtE

TtxtEtE

])()1([

)(])(211[ 2

dxxfxeTX

dxxfxxeTX

x

X

Tx

xm

x

X

Tx

xm

m

m

Mean Residual Life for tc isE[tc

2]/2E[tc]

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Effects of the Inter-message Effects of the Inter-message Arrival Time Arrival Time (X = 25E[tc] and E[tc]=4Tm)

PUFT increases as m increases. - more message deliveries are likely to occur during an in-progress call

E[td] is insignificantly affected by m. - messages are random observation points of prepaid call holding intervals

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Effects of the Variance of Call Effects of the Variance of Call Holding TimeHolding Time (X = 25E[tc] and 1/λm = 0.5E[tc])

The performance of both PUFT and E[td] degrades as the variance Vc of the call holding time increases.

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Effects of the Initial Prepaid Effects of the Initial Prepaid Credit AmountCredit Amount

(1/λm = 0.5E[tc])

Both PUFT and E[td] decrease as X increases. - more credit units left for a prepaid call- more likely that there is enough credit for both services

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SummarySummaryWe developed an IMS prepaid application server to handle prepaid calls and messaging services in UMTS/IMS.

This application server can be used to accommodate existing Internet services in the mobile environment.

A threshold XT is set to protect the in-progress prepaid call when admitting a prepaid message delivery.The XT value should be appropriately selected to adapt to various traffic patterns.

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[2][2] Modeling Online Credit Modeling Online Credit Reservation Procedure in OCSReservation Procedure in OCS

S.-I. Sou, H.-N. Hung, Y.-B. Lin, N.-F. Peng, and J.-Y. Jeng, Modeling Credit Reservation Procedure for UMTS Online Charging System. IEEE Transactions on Wireless Communications, 6(11): 4129-4135, 2007.

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Previous approach (in research topic [1] and before)Previous approach (in research topic [1] and before) - Real-time deduction in the service control point

New ApproachNew Approach - Diameter Credit Reservation at the OCS

UE

GPRS Network


MGCF

PSTN

BGCF

T-SGW

MGW

Application ServerOnline Charging

System (OCS)

HSS

Ro

IP Multimedia core network Subsystem (IMS)

CSCF

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Online Charging System

IMS CSCF

CCR (INITIAL_REQUEST)

CCA (INITIAL_REQUEST)

Reserve Units Operation

i

CCR (UPDATE_REQUEST)

CCA (UPDATE_REQUEST)

Reserve Units and Debit Units Operation

Diameter Credit Reservation Diameter Credit Reservation ProcedureProcedure

For a type i session, each time the OCS grants i credit units to the session.

iWhen these credit units are consumed, the OCS grants next i credit units to the session.CCR (TERMINATE_REQUEST)

CCA (TERMINATE_REQUEST)

Debit Units Operation

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RTCR MechanismRTCR Mechanism

We propose a Recharge Threshold-based Credit Reservation (RTCR) mechanism. In RTCR, when the balance of the user account at the OCS is below a recharge threshold Cmin,, the mechanism

reminds the user to refill the prepaid account by sending a Recharge Warning, andrejects new session requests

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RTCR Mechanism RTCR Mechanism (cont.)(cont.)

Cmin is set too smallnot enough credit is leftuser satisfaction is degraded

Cmin is set too largenew session requests are unnecessarily rejectedthe user is frequently asked to refill the account

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Output MeasuresOutput MeasuresE[Ni]: the expected number of the reserve credit message exchanges executed during a type-i session.

The larger the E[Ni] value, the higher the credit control message overhead.

Pc: the completion probability that all in-progress sessions are finished.

The larger the Pc value, the better the user satisfaction.

E[Cd]: the expected unused credit units left after all existing sessions are finished.

Cd ≥ 0 if all sessions are normally finished. Cd = 0, otherwise.

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n: the number of types of session-based IMS servicesi: the inter-session arrival rate of type-i service

th,i: the session holding time of type-i service

1/i: the expected value of th,i

i: the amount of credit units that the OCS grants in each

reserve credit message for a type-i sessionCmin: the recharge threshold in RTCR

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Derivation for Derivation for EE[[NNii]]

The expected number of the reserve credit message exchanges executed during a type-i session

When th,i is Exponential distributed, we have

...]3Pr[]2Pr[]Pr[1][ ,,, iihiihiihi tttNE

1

, ]Pr[1j

iih jt

iieNE i

1

1][

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Derivation for Derivation for PPcc and and EE[[CCdd] (] (nn=1) =1) [1/2][1/2]For n=1, at t1, the total unused credit is ,

where .1

~0

~minC

t1 Cmin+~ Time

A reserve credit request arrives

t2

The session completes

tr,1

(1) Reserve 1

(2) Remain minmin CC 1

~

(3) Send Recharge Warning

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Derivation for Derivation for PPcc and and EE[[CCdd] (] (nn=1) =1) [2/2][2/2]The completion probability is derived as

The unused credit is derived as

]Pr[~

1, minrc CtP

1

~

~

1,~

0

~

1,

~

0 1,1, )()(

ddtftf r

C

t rrmin

r

~

1,

~

1,1,0 0 1,

~)()()(][ ~

1~

~

1,

ddtftftCCE rrr

C

t rmindmin

r

1

1 21

)(11

111

eeeC

minC

min

11

11

11

)(11

ee minC

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Derivation for Derivation for PPcc and and EE[[CCdd] (] (nn=2) =2) [1/2][1/2]Case I. One active type-1 session

(with residual holding time tr,1)Case II. One active type-2 session

(with residual holding time is tr,2)Case III. Two active sessions

(with residual holding times tr,1 and tr,2)

For sufficiently small i, the completion probability Pc is derived as

]Pr[]Pr[]Pr[ 2,1,2,1, minrrIIIminrIIminrIc CttPCtPCtPP

21

21212121

1221 21

minminminmin

CCCC eeee

212121 2

1

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Derivation for Derivation for PPcc and and EE[[CCdd] (] (nn=2) =2) [2/2][2/2]

For sufficiently small i, the expected unused credit E[Cd] is derived as

2

21

1

21

212121

)1()1(2

1 21

minmin CC

mineeC

)()1()1(2

2121

22

2121

12

minmin CC ee

}]0,[max{}]0,[max{][ 2,1, rminIIrminId tCEPtCEPCE

}]0,[max{ 2,1, rrminIII ttCEP

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Effects of the Granted Credit Effects of the Granted Credit UnitsUnits

E[Ni] is not affected by the threshold Cmin and the number of services types.

E[Ni] decreases as i increases.- The session holding time is Exponential distributed with parameter i. - We observe i ≥ 2.5/i, E[Ni] ≈1.

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Effects of the Recharge Effects of the Recharge ThresholdThreshold (n = 2, 1 = 1 and 2 = 2 = 21 )

Pc increases as the threshold Cmin increases. E[Cd] increases as the threshold Cmin increases.

- more unused credit units are available in the prepaid account.

1/: the expected session holding time

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Effects of the Number of Session Effects of the Number of Session TypesTypes (Cmin = 6/, i = i1 and i = i1)

When the recharge warning is sent, the number of simultaneous in-progress service sessions increases as n increases.

Two conflicting effects are observed:- more credit units will be consumed in these sessions - the “net” unused credit units that have granted to the sessions increase

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SummarySummaryWe proposed the threshold-based credit reservation mechanism.Analytic and simulation models are developed to investigate Pc and E[Cd].

Based on our study, the operation can choose appropriate parameters i and Cmin for various traffic conditions.

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[3][3] Reducing Credit Re-authorization Cost Reducing Credit Re-authorization Cost

S.-I. Sou, Y.-B. Lin and J.-Y. Jeng, Reducing Credit Re-authorization Cost in UMTS Online Charging System. Accepted and to appear in IEEE Transactions on Wireless Communications.

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Online Charging for GPRS SessionsOnline Charging for GPRS Sessions

The ABMF keeps the subscriber’s account data and controls the account balance.

In a telecom network, the ABMF and the SBCF may physically reside at different (and possibly remote) locations. The message exchanges in the Rc interface may be expensive.

Gy

Account Balance

& Management

Function(ABMF)

Account

RatingFunction

Tariff

CGF BillingSystem

Online Charging SystemRc

Re

Ga

GPRSNetwork

GGSN

SessionBased

ChargingFunction(SBCF)

UE

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Credit Re-authorization (Basic Credit Re-authorization (Basic Scheme)Scheme)GGSN SBCF ABMF

CCR (INITIAL_REQUEST, QoS i)

ABMF Request

Rating Function


Tariff Request (QoS i)

Tariff Response

ABMF Response

A QoS change occurs

CCR (UPDATE_REQUEST, QoS j)

ABMF Request


Tariff Request (QoS j)

Tariff Response

ABMF Response

i: the number of credit units charged for a time unit for class i session : the time units granted in each credit reservation

i

j

iu

CCR (TERMINATE_REQUEST)


ABMF Request

ABMF Response

Reservei

Debitiu

Reservej

Debit j(u)ju

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If iu≥j skips ABMF message and grants iu credit units. Otherwise, executes ABMF message and grants j credit units

Credit Re-authorization (Threshold-Credit Re-authorization (Threshold-based Scheme)based Scheme)GGSN SBCF ABMF

CCR (INITIAL_REQUEST, QoS i)

ABMF Request

Rating Function


Tariff Request (QoS i)

Tariff Response

ABMF Response

A QoS change occurs

CCR (UPDATE_REQUEST, QoS j)

ABMF Request


Tariff Request (QoS j)

Tariff Response

ABMF Response

CCR (TERMINATE_REQUEST)


ABMF Request

ABMF Response

j

iu

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N: the number of the QoS classes that may change in a GPRS sessioni: the number of credit units charged for every time unit in a class i session : the threshold used by the SBCF to determine whether to interact with the ABMF or not: the time units granted to the GPRS session in each credit reservation u: the time units left at the end of the previous sub-session 1/s: the expected holding time of a sub-sessionP0: the probability that the GPRS session terminates at the end of a sub-session

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Analytic Modeling: Timing Analytic Modeling: Timing DiagramDiagram

n-th subsession starts

t1 tn

Time

xn

tn+1A QoS change occurs

A GPRS session starts

A GPRS session terminates

n+1-st subsession starts

The balance check occurs

tB

td

1cntnt

ABMF

dBt t

ABMF

ABMF ABMF

ABMF

After a random time, an in-progress session either terminates with probability P0, orswitches to another QoS class with probability (1- P0)/(N-1).

Exponential random variable with mean 1/s

Consume C credit in this period

How much have I spend on this

session?

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Output MeasuresOutput Measures

M: the expected number of ABMF message exchanges for a GPRS session. The smaller the M value, the lower the ABMF message overhead.C: the expected undebit credit units when a balance check occurs during an in-progress session. The smaller the C value, the more accurate the account balance reported by the OCS.We use (MB, CB) and (MT , CT) to represent the output measures of the Basic scheme and the Threshold-based scheme, respectively.

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Output Measures for the Basic Output Measures for the Basic SchemeSchemeWhen is Exponential distributed with , the expected

number of ABMF exchanges in a session for the basic scheme:

When a balance check occurs, the expected undebit credit units are:

N

i is

B NC

1

11

s

snB P

xEP

M

00

][11

Expected # of subsessionsExpected # of ABMF messages in a subsession

Expected credit units consumed in a time unit

The time period from the last updated ABMF

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Effects of the Threshold Effects of the Threshold ParameterParameter

The basic scheme is not affected by δ. For the threshold-based scheme, MT increases and CT decreases as δ increases.

- the performance of the threshold-based scheme is similar to that of the basic scheme when δ≥2.5.

(N=2, α2=2α1 and P0=0.01)

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SummarySummary

We proposed a threshold-based scheme with parameter to reduce the traffic signaling for the OCS credit re-authorization procedure. The threshold parameter increases

the number of ABMF message exchanges (MT ) increases the undebit credit units (CT) decreases

Combining the results from our previous work [2], the mobile operator can select the appropriate and values for various traffic conditions.

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Future DirectionsFuture DirectionsPerformance of the Tariff Switch Mechanism

During a service session, the tariff information may be changed when a specified event occurs (i.e., a tariff switch is reached). It is important to set the tariff switch time appropriately such that the OCS can handle all the requests without delaying the service continuity.

Policy and Charging Control IntegrationWhen the QoS policy control and the content-based charging functionalities are used as separate mechanisms, it will increase the interworking cost between the network nodes. Through the Policy and Charging Control (PCC), integration of QoS policy and charging rules can be realized in the IMS network or other bearer network (e.g., WLAN, WiMAX). The design of the PCC architecture to support roaming among heterogeneous wireless network is for further study.

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Q & A

Documents

Charging for Mobile All-IP Telecommunications