4G/LTE - RRC

RRC States

At high level view, LTE RRC has only two status (RRC Connected and RRC Idle). It may sound too simple comparing to other Radio Access Technology, but the detailed processes undergoing in these two status is not as simple as you might think. Following illustration is based on 36.331 4.2.1 UE states and state transitions including inter RAT.

As an engineer, you need to have ability to see both forest and the individual trees in the forest. This illustration is important but only a picture of forest. To see the individual trees in the forest, you need to look into almost every pages in this site. I recommend you to switch back and forth between this page and other pages related to each individual process.

Connected User (UE) vs Active User (UE)

I was asked of what is the difference between Connected UE and Active UE. However, I don't personally see the term Active User in any of 3GPP specification (Probably I missed something when I am reading the documents). However, from the context in which these two terms are used, I would define these terms as follows.

• Connected User : User (UE) that are in RRC Connected

• Active User : User (UE) that are in RRC Connected and undergoing User Data Transaction and you may include the UE that performing Signaling Messages (e.g, sending Measurement Report etc) as well.

Based on this definition, you may say Active User can be a kind of subset of Connected user meaning that the number of Active User in a specific moment (at a specific subframe) is highly likely to be smaller than the number of Connected User. It seems that the concept of Active User is often used in the context of estimating the capacity of a base station. In terms of base station point of view, UEs not transmitting/receiving any data (ether U plane or C plane data) would not put any load on it. So these UEs can almost be ignored for that specific moment.

RRC Status , SRB/DRB Establish and Connected/Active User

I think I need to add some comments on the inter-relations among the terms RRC Status and SRB/DRB assignment and Connected/Active User.

If you think of the protocol sequence of what UE goes through to get into 'RRC Connected Status' from 'RRC Idle', you would notice it always goes through 'RRC Connection Reconfiguration' and would/would not go through 'RRC Connection Setup' depending on whether it is the initial attach process or just to add additional bearer.

If you look further details of RRC Connection Setup and RRC Conection Reconfiguration message, you would notice that RRC Connection Setup is mainly for SRB establishment and RRC Connection Reconfiguration is mainly for setting up DRB setup (RRC Connection Reconfiguration can redefine SRB as well).

This implies that 'RRC Connected Status' indicates the status in which SRB and DRB is allocated for the UE.

Then, can I say the UE with SRB/DRB is always an Active User ?

Not Necessarily. SRB/DRB is more of a concept running through full stack (i.e, PHY to Core Network), but Active User is more for MAC/PHY concept. Even when a UE is assinged SRB/DRB, there can be a certain moment (Subframe) in which no MAC/PHY resource is allocated for it, meaning no PHY/MAC data transfer happens. In this case (i.e, No PHY/MAC data transfer), we can say that the UE is in RRC Connected Status but is not in the Active Status for that specific subframe. We can call it 'Active user' in another subfrme if it get granted for data transmission in that subframe.