LTE attach procedure

Last two articles was about Offline and Online charging interfaces. So now, after we know "how the things are  are going" in charging domain, it's a good time to describe how UEs are able to being charged for services they receive.

Always it's good to start with a high level abstract picture, but not today.
Being honest, today I'm a little bit lazy and just don't want to draw network architecture myself. You can easily find that on Google image search engine.
Information below are part of 3GPP TS 23.401, I just copied them. Below is everything you need to know about Network Attach procedure in LTE with many references.

UE needs to register with the network to receive services that require registration. This registration is called Network Attachment. The always on IP connectivity for UE is enabled by establishing a default EPS bearer during Network Attachment procedure. Predefined PCC rules most common set in PGW will be applied for default bearer. Attach procedure may trigger one or multiple Dedicated Bearer Establishment procedures to establish dedicated EPS bearer for that UE.
During the Initial Attach procedure the Mobile Equipment Identity is obtained from the UE. The MME operator may check the Mobile Equipment Identity with EIR. At least in roaming situations, the MME should pass the ME Identity to the HSS, and, if a PGW outside of the VPLMN (Visited PLMN), should pass the ME Identity to the PGW.

The E-UTRAN Initial Attach procedure is used for Emergency Attach by UEs that need to perform emergency service but cannot gain normal services from the network. These UEs are in limited service state (more about this in 3GPP TS 23.122). Also UEs that had attached for normal services and do not have emergency bearers established and are camped on a cell in limited service state shall initiate the Attach procedures indicating that the attach is to receive emergency services. UEs that camp normally on a cell, i.e. UEs that are not in limited service state, should initiate normal initial attach when they are not already attached and shall initiate the UE Requested PDN Connectivity procedure to receive emergency EPS bearer services.

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.

Gx interface - sitting between PCRF and PCEF

After you are familiar with OFCS (Offline Charging System) and OCS (Online Charging System) time has come to be more up to date with interfaces that are being used to exchange signaling data. At first I will talk about Gx interface which is responsible for Offline Charging, but not only.

The Gx reference point is located between the Policy Control and Charging Rules Function (PCRF) and the Policy and Charging Enforcement Function (PCEF). The Gx reference point is used for provisioning and removal of Policy and Charging Control (PCC) rules from the PCRF to the PCEF and the transmission of traffic plane events from the PCEF to the PCRF. The Gx reference point can be used for charging control, policy control or both by applying AVPs relevant to the application.

As you probably know, it's always good to start from big picture. Here we go.

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

So at first, few information about..

Offline Charging System (OFCS) - introduction to Gx interface

Last time I tried to familiarize You with Online Charging System (OCS), with was a introduction to Gy based on a diameter. But before I will go to the diameter, and all those CCRs and CCAs just I wanted to talk about Offline Charging System.

What is Offline Charging System (OFCS)?
According to TS 32.240 and TS 32.299 3GPP documentation:
Offline charging is a process where charging information for network resource usage is collected concurrently with that resource usage. The charging information is then passed through a chain of logical charging functions, which are described below. At the end of this process, CDR files are generated by the network, which are then transferred to the network operator's Billing Domain for the purpose of subscriber billing and/or inter-operator accounting (or additional functions, e.g. statistics, at the operator’s discretion). The BD typically comprises post-processing systems such as the operator's billing system or billing mediation device.

Generally offline charging is a mechanism where charging information does not affect, in real-time, the service rendered.
As it's always good to start with a high level design picture, let's start.

Fig. 1. Offline Charging high level architecture

Where:

CTF: Charging Trigger Function

CDF: Charging Data Function

CGF: Charging Gateway Function

BD: Billing Domain. This may also be a billing system/ billing mediation device.

Online Charging System (OCS) - introduction to Gy interface

Previously I wrote about network elements and interfaces between them, now it's time to write about most important thing in a mobile network, from operator's side.
How to charge customers for the fact they are using mobiles to many activities, including web surfing. Today I'm going to present some informations about Online Charging System (OCS), how does it work, its call flow basics scenarios, and last but not least basic principles.

As always is good to know about what we will be talking about, so please have a look at high level architecture concept of online charging.
Ro is reference point from the Charging Trigger Function (CTF) to the Charging Data Function (CDF) and the Online Charging Function (OCF) respectively, and are intended for the transport of charging events. Ro reference point is equivalent to Gy interface.

Fig. 1. Online charging high level architecture

Where:
CTF: Charging Trigger Function
CDF: Charging Data Function
CGF: Charging Gateway Function
BD: Billing Domain. This may also be a billing mediation device / post-processing system.

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

Functions of main LTE packet core elements - MME, SGW, PGW

Now when I have to start writing, I'm looking deep into back of my skull, to figure out with what I have my first LTE-like problem. And you know what? I don't remember.
So maybe the best way of starting anything is to start (writing) from the beginning.
Let's have a closer look at three main packet core elements of LTE network.

MME - Mobility Management Entity
MME is the key control node for LTE access network. It is responsible for tracking and paging procedure including retransmissions, and also for idle mode of User Equipment (UE). MME is also involved in bearer activation and its deactivation procedures, to its task also belongs choosing the SGW for a UE in process of initial attach and when the intra-handover take place which involves Core Network (CN) node relocation.
MME is responsible for authenticating user towards the HSS, if user is roaming MME terminates S6a interface towards user's home HSS. All Non Access Stratum (NAS) signaling terminates at the MME point, which is also responsible for generation and allocation of temporary UE identities (GUTI). Among its duties is also authorization UE to Public Land Mobile Network (PLMN) and enforcing UE roaming restrictions if there are any. MME is also termination point of ciphering and integrity protection for NAS signaling. Lawful Interception (LI) of signaling could be also supported by MME entity. It also provides the control plane function for mobility between LTE and 2G/3G networks by the S3 interface( from SGSN to MME).

This is where it all start

For all you people who will come to this site (some day I hope), I will try to publish posts about LTE/4G technology, but not only.

Being totaly honest, my idea for this tech-blog is to create but mostly to gather materials that helped me achive level of knowledge on which I am now. Of course I will be puting links to sources other blogs or 3gpp docs. I wont be asking for permission, but I'm not a thief.


Why do I am doing it?
I started doing this after being send to Hungary to work  on EPC. I noticed that I have many free time, and would rather do something for myself and maybe for others who are always looking for info, rather than just drinking beer, or watching cat videos.

Oh, and one more thing. As you can see English is not my native language, so if any of you spot something disturbing in spelling, grammar or also will have doubts as to the substantive content please feel free to write them in comment or let me know about this by email.