Bootstrapping server function (BSF) in LTE

Recently I faced question about role of Bootstrapping Server Function (BSF) function in the LTE network, this made me thinking and digging. On the end I decided to share this woit you in case anyone has ever wondered about that. So what is the role of BSF in LTE?


The BSF queries the Subscriber Locator Function (SLF) over Dz interface, which is Diameter based,  to get the name of the HSS containing the required subscriber specific data.

It's similar to Zh or Zh' (Z-h-prim), dont remember it right now, interface used by BSF to connect to HLR to fetch authentication information.



Check the 3GPP 33.220 for more details http://www.3gpp.org/ftp/specs/archive/33_series/33.220/

DIAMETER based interfaces in EPC, UMTS and IMS

Recently I've seen question regarding on which interface in LTE or UMTS the DIAMETER protocol is used.

Basically DIAMETER is an Authentication, Authorization & Accounting (AAA)  protocol. That is why any of you will see DIAMETER used for these functions.

                    S6a - Authentication, more in TS 29.272
                    Gy - Prepaid charging, more in TS 23.203, TS 32.299;
                    Gz - Postpaid charging;
                    Gx - QoS/Policy, more in TS 29.211, TS 29.212;
                    Rf - Charging, more in TS 32.299;
                    Ro - Charging, more in TS 32.299;
                    Rx - QoS/Policy, more in TS 29.214;
                    S6d - Authentication;
                    S9 - QoS/Policy;
                    Sh - Subscriber Profile;
                    Cx - Subscriber Profile;
                    e2 - Location.
But also few others specific to IMS

The Sy interface - between PCRF and OCS

Now I have more work to do then I had and it's all focused on GGSN node part and additional equipment not covered by 3GPP specs.Few months ago, in comments some of you asked me to look deeper into the Sy interface. Have to admit that hadn't have much time lately and.. let's be honest, some kind of a mood to research, put together and publish new articles.

Back to the point where we should start.
     

The Sy interface overview

The Sy reference point is defined between the PCRF and the OCS. Big picture of the relationships between functional entities are shown in Fig 1. below.

Fig. 1. Sy overview

As you may (even should) notice on Fig 1. above are simoultaneously SPR and UDR. Those entities work separetly, that means if there is SPR with Sp interface, there is no UDR with Ud interface in the network. Those were put one one figure to save space. Cojunction or means there is the one OR the other. When UDC architecture is used, the SPR with Sp interface, whenever mentioned in this document, is replaced by UDR and Ud.

Gy interface - sitting between OCS and PCEF

Last time I tried to cover Gx interface topic, with many details such as Diameter based PCC rules flow, short description on PCEF and PCRF functions, and Policy Charging and Control (PCC) itself. Every thing what I just mentioned you can find here. Almost whole article is based on information mentioned earlier. I will put links to every section if needed.

Gy is between Online Charging System (OCS) and PCEF. In most cases PCEF is based inside PDN GW (Packed Data Network Gateway) or just short PGW.

Gy interface allows online credit control for service data flow based charging. 

As always, today is also good way to start with a quite high level abstract figure. So.. please enjoy.

Fig. 1. Gy reference point at the Policy Charging and Control (PCC) architecture

The basic structure follows a mechanism where the online client (in OCS represented by CTF) requests resource allocation and reports credit control information to the Online Charging System (OCS). As described earlier charging could be based on different Charging Scenarios.

Interfaces and their protocol stacks

After familiarising with main network elements, time has come to better know interfaces between those elements.
Interfaces are allowing MME, SGW and PGW cooperating with other network elements( e.g. HSS or PCRF).
Each one of them is built in standardised way described by 3GPP.org. Each interface described here is taken from 23.401 3GPP.org documentation.
Please keep in mind, that the documentation is (sometimes) bigger than we need it to be, so not every aspect of interfaces is described here.

Lets start with big picture of situation in which user is not roaming.

Fig.1. Non-roaming architecture by 3GPP