This whitepaper focusses on different scenarios where Interworking Function will be required in order to provide seamless services to the users. The whitepaper also explores a specific use case in detail to provide a clear example of Interworking Function.
1. Introduction
With the arrival of 5G, the operators will have to upgrade their existing network to provide 5G services to the subscribers. It is unlikely that operators and service providers upgrade all their existing nodes to 5G immediately. Until then, they need to make sure that this does not impact services provided to 5G subscribers. In order to provide service continuity, operators can choose to deploy an Interworking Function which does the translation between the new 5G NFs and existing network elements.
2. IWF scenarios in 5G
This section provides brief insights on different IWF scenarios in 5G.
2.1. Non-Roaming scenario
Roaming services enables subscribers to avail service outside of their home network.
In the Local breakout roaming scenario the subscriber’s use the service of the Visited operator network and connect to the visited network’s PGW (Packet Data Network Gate Way) to access the internet services.
Consider a scenario where 5G subscriber User-A moves to an area where operator can provide only 4G services because the operator has not deployed 5G network yet.
User-A’s home operator has only 5G network deployed. In such a scenario, below interworking could be a possibility –
- MME and UDM Interaction:
In the visited network the MME will have to retrieve the subscriber’s profile from home UDM
- Interworking between v-PCRF (Diameter) communicating with home-PCF (Policy Control Function)(HTTP/2) to retrieve the subscriber’s policies.
Figure below depicts the interworking in the local breakout scenario –
Figure-1 : Role of IWF in 5G during OCS interaction
2.2. Roaming Scenario
Roaming services enables subscribers to avail service outside of their home network.
In the Local breakout roaming scenario the subscriber’s use the service of the Visited operator network and connect to the visited network’s PGW to access the internet services.
Consider a scenario where 5G subscriber User-A moves to an area where operator can provide only 4G services because operator has not deployed 5G network yet.
User-A’s home operator has only 5G network deployed. In such a scenario, below interworking could be a possibility –
- MME and UDM Interaction : In the visited network the MME will have to retrieve the subscriber’s profile from home UDM
- Interworking between v-PCRF (Diameter) communicating with home PCF (HTTP/2) to retrieve the subscriber’s policies.
Figure below depicts the interworking in the local breakout scenario –
Figure-2 : IWF in Local breakout scenario
2.3. Vo5G Scenario
User is establishing a Voice call in 5G network.
To provide a voice call service in the 5G network the operator may use the existing IMS network.
Below figure depicts the IMS architecture in 5G:
Figure-3
To establish voice call and get other IMS service in 5G, Interworking between the following will be required:
1. Application Server (Diameter Sh interface) and UDM (HTTP/2)
Application Servers supports services like call forwarding. The AS needs to know the subscribers information for the same. To fetch the subscriber’s information the AS will interact with 5G UDM.
2. Application server (Diameter Ro Interface) and CHF (HTTP/2)
The diameter protocol allows AS to issue charging events to a Charging function. As the Charging function is a 5G node, interworking between the two will be required. IWF will support translating the diameter protocol messages to HTTP/2
3. I/S-CSCF (Diameter Cx Interface) and UDM (HTTP/2)
3.1. I-CSCF must interact with the UDM to fetch the subscriber’s profile.This is required to check
a. whether a UE is allowed to roam or not
b. To know the identity(Name) of S-CSCF needs to be used to fulfill the call
3.2. S-CSCF must interact with UDM
a. authenticate the subscriber’s identity as well as the subscriber’s network
b. download subscription profile at S-CSCF.
c. to download the updated profile on S-CSCF if the profile is changed by administrator at UDM.
d. for administrative de-registration of a subscriber
Since the UDM being a 5G network function, an IWF will be required to translate the messages on Cx interface to HTTP/2.
4. P-CSCF (Diameter Rx Interface) and PCF (HTTP/2)
P-CSCF must interact with 5G PCF to get the PCC rules configured for the subscriber. PCF being a 5G network Function, Interworking between the two will be required.
2.4. SMS Scenario
SMS is one of the main means of messaging since the 2G, it will continue to have its impact in 5G as well. In 5G SMS delivery can happen via NAS and also via IMS network.
SMS over NAS:
The network function SMSF (Short Message Service Function) is responsible for delivering the SMS over NAS in 5G. A 5G device with SMS over NAS support can send and receive SMS via AMF which is connected with SMSF using HTTPv2 interface. An SMSF in the 5G network should be capable of connecting with the older generation nodes such as SMSC and HLR using MAP Protocol.
To fetch the subscriber information from the 5G UDM, Interworking Function will be required withing SMSF, to interwork between MAP and HTTP/2 messages.
Figure-4
3. Specific Non-Roaming scenario use case
Say an operator has a 5G network and has legacy OCS. The operator does not wish to currently upgrade the OCS to 5G standards.
The legacy OCS supports Gy & Sy interface over Diameter protocol to interact with SMF and PCF respectively. These Diameter messages must be converted to HTTP/2 and vice-versa.
Interworking of OCS with various 5G NFs is explained below.
3.1. SMF Interaction with OCS
SMF interacts with OCS during one or more PDU session create/update/terminate. SMF will use both “Online Charging” or “Offline only charging” services of OCS. The interaction between SMF and OCS remains the same for all the procedures such as SSC (Session and Service Continuity) mode 1, SSC mode2 and SSC mode3 as defined in 3GPP TS 32.255.
Following messages/requests originating from SMF needs to be translated to Diameter:
- SMF sends “Charging Data Request” to OCS by invoking Nchf_ConvergedCharging_Create API, requesting to grant and authorize the service, and to allow the number of units the Resource URI used at SMF for sending HTTP POST request is:
http://{apiRoot}/nchf-convergedcharging/v1/chargingData
2. IWF Converts this HTTP/2 request to Diameter “Credit Control Request” and sends the request to OCS on Gy interface.
Figure-5
3.OCS responds with Credit Control Answer Message, which will be converted to HTTP/2 by IWF and forwarded to SMF.
Figure-6
3.2. Use case-2 - PCF Interaction with OCS
Whenever there is a change in the policy counter status of the subscriber, the PCF may wish to receive notifications.
The PCF may interact with the OCS to make PCC decisions based on spending limits. PCF will use the “Nch_SpendingLimitControl API” that enables PCF to access policy counter status information relating to the subscriber.
- IWF converts the HTTP PUT nchf-spendinglimitcontrol Request to Diameter Spending Limit Report on Sy interface.
Figure-7
2. IWF converts the diameter Spending Limit Report on Sy interface to HTTP PUT nchf- spendinglimitcontrol Response towards PCF.
Figure-8
Whenever there is a change in the policy counter status of the subscriber, the OCS will send RAR (Re-Auth-Request) messages towards the PCF. These messages also need to be translated.
3. IWF translates the diameter RAR messages initiated from the OCS to HTTP/2 and forward towards PCF.
Figure-9
4. IWF translates HTTP/2 message “http://{apiRoot}/nchf-chargingnotify/v1/response” to RAA “and forward towards PCF.
Figure-10
3.3. Use case 3 - A 2G/3G subscriber sending SMS to 5G user
When a 2G/3G subscriber sends the SMS towards a 5G subscriber who supports the SMS over NAS , SMSC needs to connect with the UDM to get the serving SMSF address of the 5G subscriber. Since the SMSC is in the legacy network it uses the MAP message for communication. An IWF in the edge of the network can help to convert the MAP messages from the SMSC to HTTP request and forward the same to the UDM.
Figure-11
- The SMSC receives the messages from the 2G/3G UE
- The SMSC sends the MAP Send Routing Info for SM Message towards the UDM of 5G subscriber.
- The IWF receives the MAP SRI_SM message from the SMSC and converts the Message to HTTP Request Get_smsf-3gpp-access and forwards the same to the UDM.
- The UDM process the “GET …/{ueId}/registrations/smsf-3gpp-access” HTTP Service request from IWF and send the details of SMSF in which the UE is registered in the 200 OK response. The IWF receives the same and store it.
- IWF needs to get the IMSI details of the subscriber also from the UDM. To fetch the same IWF initiate HTTP service request “Get ../{gpsi}/id-translation-result” towards the UDM. The’gpsi’ field carry the MSISDN of the subscriber.
- The UDM process the message and returns the SUPI of the subscriber in the 200 OK message. IWF process the HTTP response messages and extracts the IMSI of subscriber from the SUPI.
- IWF creates SS7 Send Routing Info response message and forwards the same to SMSC based on the information received from the UDM.
8 to 10.The SMSC forwards the message to the SMSF and SMSF will deliver the message towards the UE via AMF.
4. Conclusion
With Interworking Function, the operational costs of the service providers can be reduced and also the overhead of upgrading the existing working nodes can be avoided.
As operators continue to deploy 5G, many more interworking scenarios shall be identified. Studies are ongoing to understand these scenarios.
