Tag Archives: Online Charging

5G Online Charging with the Nchf_ConvergedCharging SBI

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There’s no such thing as a free lunch, and 5G is the same – services running through a 5G Standalone core need to be billed.

In 5G Core Networks, the SMF (Session Management Function) reaches out to the CHF (Charging Function) to perform online charging, via the Nchf_ConvergedCharging Service Based Interface (aka reference point).

Like in other generations of core mobile networks, Credit Control in 5G networks is based on 3 functions:
Requesting a quota for a subscriber from an online charging service, which if granted permits the subscriber to use a certain number of units (in this case data transferred in/out).
Just before those units are exhausted sending an update to request more units from the online charging service to allow the service to continue.
When the session has ended or or subscriber has disconnected, a termination to inform the online charging service to stop billing and refund any unused credit / units (data).

Initial Service Creation (ConvergedCharging_Create)

When the SMF needs to setup a session, (For example when the AMF sends the SMF a Nsmf_PDU_SessionCreate request), the CTF (Charging Trigger Function) built into the SMF sends a Nchf_ ConvergedCharging_Create (Initial, Quota Requested) to the Charging Function (CHF).

Because the Nchf_ConvergedCharging interface is a Service Based Interface this is carried over HTTP, in practice, this means the SMF sends a HTTP post to http://yourchargingfunction/Nchf_ConvergedCharging/v1/chargingdata/

Obviously there’s some additional information to be shared rather than just a HTTP post, so the HTTP post includes the ChargingDataRequest as the Request Body. If you’ve dealt with Diameter Credit Control you may be expecting the ChargingDataRequest information to be a huge jumble of nested AVPs, but it’s actually a fairly short list:

  • The subscriberIdentifier (SUPI) is included to identify the subscriber so the CHF knows which subscriber to charge
  • The nfConsumerIdentification identifies the SMF generating the request (The SBI Consumer)
  • The invocationTimeStamp and invocationSequenceNumber are both pretty self explanatory; the time the request is sent and the sequence number from the SBI consumer
  • The notifyUri identifies which URI should receive subsequent notifications from the CHF (For example if the CHF wants to terminate the session, the SMF to send that to)
  • The multipleUnitUsage defines the service-specific parameters for the quota being requested.
  • The triggers identifies the events that trigger the request

Of those each of the fields should be pretty self explanatory as to their purpose.
The multipleUnitUsage data is used like the Service Information AVP in Diameter based Credit Control, in that it defines the specifics of the service we’re requesting a quota for. Inside it contains a mandatory ratingGroup specifying which rating group the CHF should use, and optionally requestedUnit which can define either the amount of service units being requested (For us this is data in/out), or to tell the CHF units are needed. Typically this is used to define the amount of units to be requested.

On the amount of units requested we have a bit of a chicken-and-egg scenario; we don’t know how many units (In our case the units is transferred data in/out) to request, if we request too much we’ll take up all the customer’s credit, potentially prohibiting them from accessing other services, and not enough requested and we’ll constantly slam the CHF with requests for more credit.
In practice this value is somewhere between the two, and will vary quite a bit.

Based on the service details the SMF has put in the Nchf_ ConvergedCharging_Create request, the Charging Function (CHF) takes into account the subscriber’s current balance, credit control policies, etc, and uses this to determine if the Subscriber has the required balances to be granted a service, and if so, sends back a 201 CREATED response back to the Nchf_ConvergedCharging_Create request sent by the CTF inside the SMF.

This 201 CREATED response is again fairly clean and simple, the key information is in the multipleQuotaInformation which is nested within the ChargingDataResponse, which contains the finalUnitIndication defining the maximum units to be granted for the session, and the triggers to define when to check in with CHF again, for time, volume and quota thresholds.

And with that, the service is granted, the SMF can instruct the UPF to start allowing traffic through.

Update (ConvergedCharging_Update)

Once the granted units / quota has been exhausted, the Update (ConvergedCharging_Update) request is used for requesting subsequent usage / quota units. For example our Subscriber has used up all the data initially allocated but is still consuming data, so the SMF sends a Nchf_ConvergedCharging_Update request to request more units, via another HTTP post, to the CHF, with the requested service unit in the request body in the form of ChargingDataRequest as we saw in the initial ConvergedCharging_Create.

If the subscriber still has credit and the CHF is OK to allow their service to continue, the CHF returns a 200 OK with the ChargingDataResponse, again, detailing the units to be granted.

This procedure repeats over and over as the subscriber uses their allocated units.

Release (ConvergedCharging_Release)

Eventually when our subscriber disconnects, the SMF will generate a Nchf_ConvergedCharging_Release request, detailing the data the subscriber used in the ChargingDataRequest in the body, to the CHF, so it can refund any unused credits.

The CHF sends back a 204 No Content response, and the procedure is completed.

More Info

If you’ve had experience in Diameter credit control, this simple procedure will be a breath of fresh air, it’s clean and easy to comprehend,
If you’d like to learn more the 3GPP specification docs on the topic are clear and comprehensible, I’d suggest:

  • TS 132 290 – Short overview of charging mechanisms
  • TS 132 291 – Specifics of the Nchf_ConvergedCharging interface
  • The common 3GPP charging architecture is specified in TS 32.240
  • TS 132 291 – Overview of components and SBIs inc Operations

Jaffa Cakes explain the nuances between Centralized vs Decentralized Online Charging in 3GPP Networks

EPC, Mobile Networks, SDM, , , ,

While reading through the 3GPP docs regarding Online Charging, there’s a concept that can be a tad confusing, and that’s the difference between Centralized and Non-Centralized Charging architectures.

The overall purpose of online charging is to answer that deceptively simple question of “does the user have enough credit for this action?”.

In order to answer that question, we need to perform rating and unit determination.

Rating

Rating is just converting connectivity credit units into monetary units.

If you go to the supermarket and they have boxes of Jaffa Cakes at $2.50 each, they have rated a box of Jaffa Cakes at $2.50.

1 Box of Jaffa Cakes rated at $2.50 per box

In a non-snack-cake context, such as 3GPP Online Charging, then we might be talking about data services, for example $1 per GB is a rate for data.
Or for a voice calls a cost per minute to call a destination, such as is $0.20 per minute for a local call.

Rating is just working out the cost per connectivity unit (Data or Minutes) into a monetary cost, based on the tariff to be applied to that subscriber.

Unit Determination

The other key piece of information we need is the unit determination which is the calculation of the number of non-monetary units the OCS will offer prior to starting a service, or during a service.

This is done after rating so we can take the amount of credit available to the subscriber and calculate the number of non-monetary units to be offered.

Converting Hard-Currency into Soft-Snacks

In our rating example we rated a box of Jaffa Cakes at $2.50 per box. If I have $10 I can go to the shops and buy 4x boxes of Jaffa cakes at $2.50 per box. The cashier will perform unit determination and determine that at $2.50 per box and my $10, I can have 4 boxes of Jaffa cakes.

Again, steering away from the metaphor of the hungry author, Unit Determination in a 3GPP context could be determining how many minutes of talk time to be granted.
Question: At $0.20 per minute to a destination, for a subscriber with a current credit of $20, how many minutes of talk time should they be granted?
Answer: 100 minutes ($20 divided by $0.20 per minute is 100 minutes).

Or to put this in a data perspective,
Question: Subscriber has $10 in Credit and data is rated at $1 per GB. How many GB of data should the subscriber be allowed to use?
Answer: 10GB.

Putting this Together

So now we understand rating (working out the conversion of connectivity units into monetary units) and unit determination (determining the number of non-monetary units to be granted for a given resource), let’s look at the the Centralized and Decentralized Online Charging.

Centralized Rating

In Centralized Rating the CTF (Our P-GW or S-CSCF) only talk about non-monetary units.
There’s no talk of money, just of the connectivity units used.

The CTFs don’t know the rating information, they have no idea how much 1GB of data costs to transfer in terms of $$$.

For the CTF in the P-GW/PCEF this means it talks to the OCS in terms of data units (data In/out), not money.

For the CTF in the S-CSCF this means it only ever talks to the OCS in voice units (minutes of talk time), not money.

This means our rates only need to exist in the OCS, not in the CTF in the other network elements. They just talk about units they need.

De-Centralized Rating

In De-Centralized Rating the CTF performs the unit conversion from money into connectivity units.
This means the OCS and CTF talk about Money, with the CTF determining from that amount of money granted, what the subscriber can do with that money.

This means the CTF in the S-CSCF needs to have a rating table for all the destinations to determine the cost per minute for a call to a destination.

And the CTF in the P-GW/PCEF has to know the cost per octet transferred across the network for the subscriber.

In previous generations of mobile networks it may have been desirable to perform decentralized rating, as you can spread the load of calculating our the pricing, however today Centralized is the most common way to approach this, as ensuring the correct rates are in each network element is a headache.

Centralized Unit Determination

In Centralized Unit Determination the CTF tells the OCS the type of service in the Credit Control Request (Requested Service Units), and the OCS determines the number of non-monetary units of a certain service the subscriber can consume.

The CTF doesn’t request a value, just tells the OCS the service being requested and subscriber, and the OCS works out the values.

For example, the S-CSCF specifies in the Credit Control Request the destination the caller wishes to reach, and the OCS replies with the amount of talk time it will grant.

Or for a subscriber wishing to use data, the P-GW/PCEF sends a Credit Control Request specifying the service is data, and the OCS responds with how much data the subscriber is entitled to use.

De-Centralized Unit Determination

In De-Centralized Unit Determination, the CTF determines how many units are required to start the service, and requests these units from the OCS in the Credit Control Request.

For a data service,the CTF in the P-GW would determine how many data units it is requesting for a subscriber, and then request that many units from the OCS.

For a voice call a S-CSCF may request an initial call duration, of say 5 minutes, from the OCS. So it provides the information about the destination and the request for 300 seconds of talk time.

Session Charging with Unit Reservation (SCUR)

Arguably the most common online charging scenario is Session Charging with Unit Reservation (SCUR).

SCUR relies on reserving an amount of funds from the subscriber’s balance, so no other services can those funds and translating that into connectivity units (minutes of talk time or data in/out based on the Requested Session Unit) at the start of the session, and then subsequent requests to debit the reserved amount and reserve a new amount, until all the credit is used.

This uses centralized Unit Determination and centralized Rating.

Let’s take a look at how this would look for the CTF in a P-GW/PCEF performing online charging for a subscriber wishing to use data:

  1. Session Request: The subscriber has attached to the network and is requesting service.
  2. The CTF built into the P-GW/PCEF sends a Credit Control Request: Initial Request (As this subscriber has just attached) to the OCS, with Requested Service Units (RSU) of data in/out to the OCS.
  3. The OCS performs rating and unit determination, and according to it’s credit risk policies, and a whole lot of other factors, comes back with an amount of data the subscriber can use, and reserves the amount from the account.
    (It’s worth noting at this point that this is not necessarily all of the subscriber’s credit in the form of data, just an amount the OCS is willing to allocate. More data can be requested once this allocated data is used up.)
  4. The OCS sends a Credit Control Answer back to our P-GW/PCEF. This contains the Granted Service Unit (GSU), in our case the GSU is data so defines much data up/down the user can transfer. It also may include a Validity Time (VT), which is the number of seconds the Credit Control Answer is valid for, after it’s expired another Credit Control Request must be sent by the CTF.
  5. Our P-GW/PCEF processes this, starts measuring the data used by the subscriber for reporting later, and sets a timer for the Validity Time to send another CCR at that point.
    At this stage, our subscriber is able to start using data.
  1. Some time later, either when all the data allocated in the Granted Service Units has been consumed, or when the Validity Time has expired, the CTF in the P-GW/PCEF sends another Credit Control Request: Update, and again includes the RSU (Requested Service Units) as data in/out, and also a USU (Used Service Units) specifying how much data the subscriber has used since the first Credit Control Answer.
  2. The OCS receives this information. It compares the Used Session Units to the Granted Session Units from earlier, and with this is able to determine how much data the subscriber has actually used, and therefore how much credit that equates to, and debit that amount from the account.
    With this information the OCS can reserve more funds and allocate another GSU (Granted Session Unit) if the subscriber has the required balance. If the subscriber only has a small amount of credit left the FUI (Final Unit Indication AVP) is set to determine this is all the subscriber has left in credit, and if this is exhausted to end the session, rather than sending another Credit Control Request.
  3. The Credit Control Answer with new GSU and the FUI is sent back to the P-GW/PCEF
  4. The P-GW/PCEF allows the session to continue, again monitoring used traffic against the GSU (Granted Session Units).
  1. Once the subscriber has used all the data in the Granted Session Units, and as the last CCA included the Final Unit Indicator, the CTF in the P-GW/PCEF knows it can’t just request more credit in the form of a CCR Update, so cuts of the subscribers’s session.
  2. The P-GW/PCEF then sends a Credit Control Request: Termination Request with the final Used Service Units to the OCS.
  3. The OCS debits the used service units from the subscriber’s balance, and refunds any unused credit reservation.
  4. The OCS sends back a Credit Control Answer which may include the CI value for Credit Information, to denote the cost information which may be passed to the subscriber if required.
Credit Control Request / Answer call flow in IMS Charging

Basics of EPC/LTE Online Charging (OCS)

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Early on as subscriber trunk dialing and automated time-based charging was introduced to phone networks, engineers were faced with a problem from Payphones.

Previously a call had been a fixed price, once the caller put in their coins, if they put in enough coins, they could dial and stay on the line as long as they wanted.

But as the length of calls began to be metered, it means if I put $3 of coins into the payphone, and make a call to a destination that costs $1 per minute, then I should only be allowed to have a 3 minute long phone call, and the call should be cutoff before the 4th minute, as I would have used all my available credit.

Conversely if I put $3 into the Payphone and only call a $1 per minute destination for 2 minutes, I should get $1 refunded at the end of my call.

We see the exact same problem with prepaid subscribers on IMS Networks, and it’s solved in much the same way.

In LTE/EPC Networks, Diameter is used for all our credit control, with all online charging based on the Ro interface. So let’s take a look at how this works and what goes on.

Generic 3GPP Online Charging Architecture

3GPP defines a generic 3GPP Online charging architecture, that’s used by IMS for Credit Control of prepaid subscribers, but also for prepaid metering of data usage, other volume based flows, as well as event-based charging like SMS and MMS.

Network functions that handle chargeable services (like the data transferred through a P-GW or calls through a S-CSCF) contain a Charging Trigger Function (CTF) (While reading the specifications, you may be left thinking that the Charging Trigger Function is a separate entity, but more often than not, the CTF is built into the network element as an interface).

The CTF is a Diameter application that generates requests to the Online Charging Function (OCF) to be granted resources for the session / call / data flow, the subscriber wants to use, prior to granting them the service.

So network elements that need to charge for services in realtime contain a Charging Trigger Function (CTF) which in turn talks to an Online Charging Function (OCF) which typically is part of an Online Charging System (AKA OCS).

For example when a subscriber turns on their phone and a GTP session is setup on the P-GW/PCEF, but before data is allowed to flow through it, a Diameter “Credit Control Request” is generated by the Charging Trigger Function (CTF) in the P-GW/PCEF, which is sent to our Online Charging Server (OCS).

The “Credit Control Answer” back from the OCS indicates the subscriber has the balance needed to use data services, and specifies how much data up and down the subscriber has been granted to use.

The P-GW/PCEF grants service to the subscriber for the specified amount of units, and the subscriber can start using data.

This is a simplified example – Decentralized vs Centralized Rating and Unit Determination enter into this, session reservation, etc.

The interface between our Charging Trigger Functions (CTF) and the Online Charging Functions (OCF), is the Ro interface, which is a Diameter based interface, and is common not just for online charging for data usage, IMS Credit Control, MMS, value added services, etc.

3GPP define a reference online-charging interface, the Ro interface, and all the application-specific interfaces, like the Gy for billing data usage, build on top of the Ro interface spec.

Basic Credit Control Request / Credit Control Answer Process

This example will look at a VoLTE call over IMS.

When a subscriber sends an INVITE, the Charging Trigger Function baked in our S-CSCF sends a Diameter “Credit Control Request” (CCR) to our Online Charging Function, with the type INITIAL, meaning this is the first CCR for this session.

The CCR contains the Service Information AVP. It’s this little AVP that is where the majority of the magic happens, as it defines what the service the subscriber is requesting. The main difference between the multitude of online charging interfaces in EPC networks, is just what the service the customer is requesting, and the specifics of that service.

For this example it’s a voice call, so this Service Information AVP contains a “IMS-Information” AVP. This AVP defines all the parameters for a IMS phone call to be online charged, for a voice call, this is the called-party, calling party, SDP (for differentiating between voice / video, etc.).

It’s the contents of this Service Information AVP the OCS uses to make decision on if service should be granted or not, and how many service units to be granted. (If Centralized Rating and Unit Determination is used, we’ll cover that in another post)
The actual logic, relating to this decision is typically based on the the rating and tariffing, credit control profiles, etc, and is outside the scope of the interface, but in short, the OCS will make a yes/no decision about if the subscriber should be granted access to the particular service, and if yes, then how many minutes / Bytes / Events should be granted.

In the received Credit Control Answer is received back from our OCS, and the Granted-Service-Unit AVP is analysed by the S-CSCF.
For a voice call, the service units will be time. This tells the S-CSCF how long the call can go on before the S-CSCF will need to send another Credit Control Request, for the purposes of this example we’ll imagine the returned value is 600 seconds / 10 minutes.

The S-CSCF will then grant service, the subscriber can start their voice call, and start the countdown of the time granted by the OCS.

As our chatty subscriber stays on their call, the S-CSCF approaches the limit of the Granted Service units from the OCS (Say 500 seconds used of the 600 seconds granted).
Before this limit is reached the S-CSCF’s CTF function sends another Credit Control Request with the type UPDATE_REQUEST. This allows the OCS to analyse the remaining balance of the subscriber and policies to tell the S-CSCF how long the call can continue to proceed for in the form of granted service units returned in the Credit Control Answer, which for our example can be 300 seconds.

Eventually, and before the second lot of granted units runs out, our subscriber ends the call, for a total talk time of 700 seconds.

But wait, the subscriber been granted 600 seconds for our INITIAL request, and a further 300 seconds in our UPDATE_REQUEST, for a total of 900 seconds, but the subscriber only used 700 seconds?

The S-CSCF sends a final Credit Control Request, this time with type TERMINATION_REQUEST and lets the OCS know via the Used-Service-Unit AVP, how many units the subscriber actually used (700 seconds), meaning the OCS will refund the balance for the gap of 200 seconds the subscriber didn’t use.

If this were the interface for online charging of data, we’d have the PS-Information AVP, or for online charging of SMS we’d have the SMS-Information, and so on.

The architecture and framework for how the charging works doesn’t change between a voice call, data traffic or messaging, just the particulars for the type of service we need to bill, as defined in the Service Information AVP, and the OCS making a decision on that based on if the subscriber should be granted service, and if yes, how many units of whatever type.